Symposium on the testing and evaluation of materials for solid propellant rocket motor casings: February 2-3, 1959 (1959)

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-222- THE FRACTURE SENSITIVITY OF MATERIALS FOR HIGH-STRENGTH APPLICATIONS E P K l i e r , The C a t h o l i c U n i v e r s i t y of America C P G a z z a r a , Watertovm A r s e n a l L a b o r a t o r i e s A r t h u r MacDonald, C u r t i s s - W r i g h t C o r p o r a t i o n H i g h - s t r e n g t h m a t e r i a l s a r e s u b j e c t to b r i t t l e - t y p e f a i l u r e s , and to an i n c r e a s i n g degree as the s t r e n g t h l e v e l r i s e s above a v a l u e which appears to be c r i t i c a l for the d i f f e r e n t s t r u c t u r a l m a t e r i a l s T h i s c r i t i c a l s t r e s s l e v e l p o t e n t i a l l y v a r i e s a l s o w i t h the d i f f e r e n t s t r e s s systems to which these m a t e r i a l s may be exposed But for a g i v e n m a t e r i a l i t can be proposed that the f r a c t u r e behavior w i l l be determined by e i t h e r the presence of f l a w s , such a s notches and/or c r a c k s which a r e i n t r o d u c e d i n t o the metal e i t h e r a c c i d e n t a l l y or through manufacturing, or by the loads to which the s t r u c t u r e i s s u b j e c t i n s e r v i c e I n the p r e s e n t m a t e r i a l s e v a l u a t i o n program, i t was proposed t h a t the r e s p o n s e of a m a t e r i a l to f l a w s could be a t l e a s t i n p a r t e s t a b l i s h e d by t e s t - i n g a s m a l l n o t c h - t e n s i o n specimen, and s e r v i c e loads by use of a c i r c u l a r bulge t e s t The bulge specimens were to be t e s t e d m the absence o f i n t e n t i o n a l f l a w s and i n the presence of welds ^ EXPERIMENTAL PROCEDURE M a t e r i a l s I n a l l , a r e l a t i v e l y l a r g e number of m a t e r i a l s were examined The compositions and h e a t treatments a r e g i v e n I n Table I S c r e e n i n g o f these m a t e r i a l s was f i r s t e f f e c t e d i n the notch t e n s i l e t e s t , and t e s t s were then completed fo r the bulge specimen The m a t e r i a l s t e s t e d and the t e s t employed a r e g i v e n i n T a b l e I I and i n the f i g u r e s T e n s i o n and Notch T e n s i o n T e s t i n g Buttonhead specimens. F i g u r e 1, were loaded i n a s u i t a b l e a l i g n i n g f i x t u r e to minimize e c c e n t r i c i t y A minimum of one notched specimen was t e s t e d

-223- TABLE I HEAT TREATl-lENT CODE AND MATERIALS FOR BULGE TESTING Material Code Heat Treatment Vascojet 1000 1-V 1 - Z 1850°F/1 h r / A C , 950°F/2 h r / A C - t w i c e 1850°F/1 h r / A C , 1050"F/2 h r / A C - t w i c e 6Al-llV-Ti 6 6A 1IKX)°F/30 mln / W Q , 950*F/1^ h r / A C 11+00°F/30 mln/WQ PHI5-7M0 15F 15R 1T50''F/10 mln/cool t o - 100°F/8 h r , 950''F/1 h r / A C 1900''F/90 mln/, i t o C F / g O m i n / A C , 1050*F/90 mln / A C AISI lH35 li2 U l 16OO''F/30 m i n / A C , 1575'F/30 min/OQ, 700'F/2 h r / A C 1600''F/30 m i n / A C , 1575'F/30 min/OQ, 1HD0'*F/2 h r / A C MST 185 Annealed 18A Annealed 129 900°F/75 h r / A C B120VCA 128 800°F/75 h r / A C 12A Annealed

•224- 1 1"̂ t 500" I 1 300" ^ 0 0 4 0 R 1 212 Notch 0 0017 R (A) Notch Tensile Specimen 500" 1 ^ 0 0 4 0 R 200* I 5 1 8 300' (B) Snnooth Tensile Specimen Fig I Test Specimens

-225- flt each d i f f e r e n t t e s t c o n d i t i o n , w h i l e one t e n s i o n specimen x^as t e s t e d a t each temperature a t the minimum s t r a i n r a t e T e s t temperatures w i t h l i m i t e d exception were 75 and -100°F, w h i l e loading r a t e s corresponded to no-load cross-head movements of 0 2 and 2 0 i n c h e s per minute Bulge T e s t i n g Sheet specimens were loaded, as i n F i g u r e 2, a t room temperature I t was assumed t h a t nominally i d e a l deformation behavior obtained through the t e s t , so t h a t an adequate r a d i u s of c u r v a t u r e was obtained by use of the i n d i c a t e d r a d i u s gage S t r a i n measurements were attempted i n s e v e r a l d i f f e r e n t ways, with o n l y those obtained by s t r a i n gages being r e p o r t e d The bulge t e s t u n i t had been designed to t e s t r e l a t i v e l y l i g h t e r gage sh e e t than the m a t e r i a l s a v a i l a b l e , so i t was not p o s s i b l e to break a l l s p e c i - mens of v i r g i n s h e e t Welded s h e e t was, w i t h l i m i t e d e x c e p t i o n , broken d u r i n g the t e s t Data P r o c e s s i n g I t i s assumed t h a t the bulge specimens deform s p h e r i c a l l y over the c e n t e r p o r t i o n where measurements were made, t h a t they a r e e f f e c t i v e l y t h i n , and t h a t deformation i s i s o t r o p i c Then, the s t r e s s m the s h e e t i s gi v e n by p r o i l p r e s s u r e R = r a d i u s o f c u r v a t u r e - i n c h e s tj^ r i n s t a n t a n e o u s t h i c k n e s s A curve o f R/2t^ v e r s u s i n d i c a t o r d e f l e c t i o n i s g i v e n i n F i g u r e 3 I t w i l l be noted t h a t the s t r e s s - s t r a i n r e l a t i o n s f o r the bulge t e s t s a r e t r u e s t r e s s - s t r a i n c u r v e s i f the assumed i s o t r o p i c deformation o b t a i n s i n the t e s t

•226- Lever Catch Dial Indicator Fulcrum Lever Plunger Specimen Strain Gage Gasket Fig 2 Bulge Test Fixture and Measuring Gage

-227- 280, 260 240 220 200 CM a: 180 f60 140 120 100 0 043" Sheet Th ickness , , „ 10 20 30 40 50 60 70 Indicator Deflection - 10"' Inches Fig 3 R/2t V8f Indicator Deflection With Measuring Gage in Figure 2

-228- EXPE^IMENTAL RESULTS Ten s i o n T e s t s The notch t e n s i o n p r o p e r t i e s of the 5A1-2 5Sn-Titanium were examined a t l e n g t h , and the data i n F i g u r e 4 i n d i c a t e that t h i s a l l o y i s h i g h l y d u c t i l e as t e s t e d I n the notch t e n s i o n t e s t , as the s t r e n g t h l e v e l f o r a given duc- t i l e m a t e r i a l r i s e s , the notch s t r e n g t h w i l l tend i n i t i a l l y to r i s e and w i l l then b€ reduced When t h i s s t r e n g t h f a l l s below the t e n s i l e s t r e n g t h , the m a t e r i a l i s b r i t t l e The p r o p e r t i e s measured i n the s m a l l notch t e n s i o n s p e c i - men a r e not obtained when the specimen s i z e i s i n c r e a s e d , r a t h e r , reduced notch I toughness i s measured I t i s now r e c o g n i z e d t h a t l a r g e s h e e t p r o p e r t i e s are | dominated by the width dimension, r a t h e r than the t h i c k n e s s , and consequently, | s h e e t specimens must be c o n s i d e r e d as l a r g e specimens A m a t e r i a l , then, that i s not d u c t i l e i n the s m a l l notch t e n s i l e t e s t may be expected to be b r i t t l e i n the presence o f notches m wide s h e e t , i f the deformation h i s t o r y of the sheet does not s i g n i f i c a n t l y improve the notch p r o p e r t i e s Notch t e n s i l e data for Che a l l o y s s t u d i e s a r e presented i n F i g u r e s 5 to 11 I t i s e v i d e n t from these f i g u r e s t h a t most of the m a t e r i a l s , as examined, a r e e i t h e r notch s e n s i t i v e or a r e b o r d e r l i n e i n t h i s t e s t Bulge T e s t s The bulge t e s t s were completed so t h a t the true s t r e s s - s t r a i n c urve could be c a l c u l a t e d A t y p i c a l curve f o r a b r i t t l e specimen i s g i v e n i n F i g u r e , 12 I t i s e v i d e n t immediately that the e l a s t i c modulus i n d i c a t e d i n the bulge t e s t i s h i g h compared to t h a t measured m t e n s i o n T h i s i s m p a r t due to the r e l a t i o n s h i p e, - 1 (d -V6^ E E = e l a s t i c modulus e^ s strainiL<'2^ V r Poicson's r a t i o di z s t r e s s d i r e c t i o n 1 ^2 = s t r e s s d i r e c t i o n 2, k. 1

•229- 300 250 g • 150 c o i . 100 50 ; | — — A ' A a - — - ~ 6 2 0 002 O A Notch ® Smooth - 4 0 0 -300 -200 -100 0 Temperatjre - **F 100 Fig 4 The Tensile Properties of 5AI-2 5Sn-Ti

-230- Ti- l6V-Z5AI Vascojet 1000 4001 3oa 20d o o o t 0 c - • A _________ 16 \ A A 1 IZ MST 185- Annealed 1 2 0 0 0 2 O A Notch A Smooth AD=^' I8A :0 300 200 BI20VCA 100 > 9 — 129 Bl 20VCA I2A BI20VCA 128 -100 100 -log 400 300 200 Vascojet 1000 DO -100 100 -100 0 Temperature 100 *»F Figs 5-11 The Tensile and Notch Tensile Properties for the Indicated Materials and Heat Treatments (cf Table M

120 100 - 80 CO Q. O o O to (O 60 ^ 40 20 f / / / E = 2 0,000,000 psi 0 0001 0002 0003 0004 0005 Strain - in/ in Fig 12 Stress-Strain Curve (Bulge Test) for l6V-2 5AI-Ti Isj 0006 0007 0008

-232- from which i t i s e v i d e n t that the c r o s s s t r e s s l e a d s to a r e d u c t i o n i n elongation and an apparent 25 to 307 i n c r e a s e m the v a l u e o f E However, t h i s i s exactly c o r r e c t o n l y i f the s h e e t i s i s o t r o p i c , and the l a t t e r c o n d i t i o n i s g e n e r a l l y not met, e s p e c i a l l y f o r t i t a n i u m sheet A second comparison i s d i r e c t e d to the f r a c t u r e s t r a i n s Here the f r a c t u r e s t r a i n i n the bulge t e s t i s s i g n i f i c a n t l y l e s s than t h a t i n t e n s i o n , an o b s e r v a t i o n t h a t was f i r s t made by Ludwlk ( 1 ) , who co n s i d e r e d the r e d u c t i o n i n d u c t i l i t y thus i n d i c a t e d to be a g e n e r a l e f f e c t of combined s t r e s s e s (For the m a t e r i a l under i n v e s t i g a t i o n , however, i t would appear t h a t the apparent l a c k o f d u c t i l i t y m the she e t may be due to deformation h i s t o r y , as t h i s a l l o y i n the n o t c h t e n s i o n t e s t was moderately notch d u c t i l e ) I t i s now g e n e r a l l y r e c o g n i z e d t h a t Ludwik's a n a l y s i s i s i n c o r r e c t , and th a t the phenomenon under 5 d i s c u s s i o n i s b e s t c o n s i d e r e d by the use of a s u i t a b l e f a i l u r e theory, w i t h that a c c e p t e d a s h o l d i n g f o r d u c t i l e m e t a l s adapted a f t e r von Mises ( 2 ) T h i s f a i l u r e theory can be developed m s e v e r a l ways, but a l l methods r e q u i r e adjustment o f the flow s t r e n g t h envelope to c o i n c i d e w i t h the y i e l d s t r e n g t h measured m t e n s i o n When t h i s i s done, the y i e l d s u r f a c e v h i c h co- i n c i d e s w i t h the y i e l d s t r a i n d e f i n e d i n accordance w i t h the e f f e c t i v e s t r e s s - s t r a i n r e l a t i o n s h i p , v i z d e s c r i b e s an e l l i p s e which p a s s e s through the p o i n t s (d-^, 0 ) , (,<f^, <i-li , ( 0 , C^) I n a p r a c t i c a l sense t h i s means th a t the y i e l d s t r e n g t h i n combined s t r e s s f i e l d s must be measured a t s t r a i n v a l u e s which a r e a d j u s t e d , and m the c i r c u l a r bulge t e s t may be s e t e q u a l to 1/2 the s t r a i n v a l u e s i n t e n s i o n

-233- I n s i m p l e t e n s i o n , d e f o r m a t i o n r e s u l t s from the a c t i o n of a s i n g l e s t r e s s applied along the l e n g t h of the specimen The s t r e s s system i s then d s d, ^2 • ^3 • ^ consequent s t r a i n s a r e 6 = ^2 ^3 i s o t r o p i c sub- stance I n the c i r c u l a r bulge t e s t , the s t r e s s system becomes z *̂ 2 " (fj : 0, w h i l e 8 / 82 • ^3 Thus i n t e n s i o n becomes equal to i n the bulge t e s t , but of o p p o s i t e s i g n N u m e r i c a l l y , S i n t e n s i o n i s twice 6̂ ^ i n the bulge t e s t The a n a l y s i s developed above i s based on s e v e r a l assumptions, the most important of which i s the assumed i s o t r o p y o f flow p r o p e r t i e s T h i s i s o t r o p y in g e n e r a l does not o b t a i n , and d e v i a t i o n s have been d i r e c t l y measured f o r d u c t i l e metal ( 3 ) A n i s o t r o p y o f p r o p e r t i e s m the bUlge specimens can a r i s e from s e v e r a l sources I n the p r e s e n t work, w h i l e such a n i s o t r o p y i s known to e x i s t , i t has not been d i r e c t l y measured The presence of a n i s o t r o p y i n the bulge specimens i s immediately e v i d e n t from comparison of the modulus of e l a s t i c i t y measured i n tension and i n the bulge specimen Two examples were s e l e c t e d to i l l u s t r a t e t h i s p oint, and a r e p r e s e n t e d i n F i g u r e s 13 and 14 I n F i g u r e 13, the modulus of e l a s t i c i t y m the bulge specimen i s l e s s than the i d e a l v a l u e For t h i s c o n d i t i o n , a high y i e l d s t r e n g t h i s measured I n F i g u r e 14 the modulus o f e l a s t i c i t y m the bulge specimen i s g r e a t e r than the i d e a l v a l u e For t h i s c o n d i t i o n , a low y i e l d s t r e n g t h i s measured The e f f e c t s noted i n s t e e l s probably r e s u l t l a r g e l y from the a c t i o n o f r e s i d u a l s t r e s s e s i n the s h e e t For t i t a n i u m , t h e r e i s a l s o the l i k e l i h o o d of a super- imposed t e x t u r e e f f e c t The s e v e r a l f a c t o r s i n v o l v e d s t r o n g l y m i l i t a t e a g a i n s t the a c c u r a c y of y i e l d s t r e n g t h measurements made i n the bulge t e s t The y i e l d s t r e n g t h measurements p r e s e n t e d i n Table I I a r e o f f e r e d w i t h t h i s r e s e r v a t i o n s t r o n g l y emphasized

-234- 300 250 Q. 200 o I 150 to ICO 50 A O 1 Bulge Te Tensile 1 St "est / / — — / / y d / , / n /J // / 0 0 0 2 0 0 4 0 0 6 0 0 8 Strain - in/in 010 012 014 Fig 13 Comparative Stress-Strom Curves in the Bulge and Tensile Tests for Voscojet 1000, Heat Treotnnent IV (cf Table I)

•235- 3 0 0 250 200 i 0) 150 V) 100 50 A O 1 Bulge T Tensile 1 est Test A / 1 . i -A- 0 0 2 0 0 4 0 0 6 008 010 S t r a i n - in / in 012 014 016 Fig 14 Comparative Stress-Strain Curves in the Bulge and Tensile Tests for P H I 5 - 7 M o , Heat Treatment I5F (cf Table I)

TABLE I I THE YIELD MD FRACTUEE PROPERTIES PCJR THE INDICATED MATERIALS IN THE BULGE TEST MATERIAL SPECIMEN NO OFFSET YIELD x 1000 PSI FINAL PERCENT ELONGATION OBSERVED FRACTURE 0 01^ 0 02^ 0 1^ 0 2^ Vascojet 1000 ( f l u s h ground) 1-V (weld) 78 0 90 5 l42 0 181 0 1 0 2 0 none yascojet 1000 {reinforced) 1-V (weld) 126 0 150 0 198 0 — — — violent Vaseojet 1000 (reinforced) 1-V-A (weld) 121 0 135 0 176 0 192 0 — — none Vascojet 1000 (f l u s h ground) 1-Z (weld) ikO 0 155 0 230 0 — — — fragmentary- Vascojet 1000 (reinforced) 1-Z (weld) 132 0 11+2 0 — — — — explosive fragmentary Vascojet 1000 (reinforced) 1-Z-A (weld) 105 0 113 0 155 0 — — — peripheral 6Al-i+V-Ti 6 117 0 121 0 137 0 11̂ 5 5 1 5 0 peripheral 6Al-i+V-Ti 6A 109 0 l l U 0 129 5 136 5 2 0 1 5 none PH 15-7 Mo 15-F 176 0 — — — 0 0 explosive peripheral PH 15-7 Mo 15-R 86 0 98 0 131 0 145 0 k 0 — none A I S I 1+135 k2 121 0 126 0 139 0 153 0 1 5 2 5 none A I S I Ui35 k2A 99 0 105 0 128 0 139 0 5 0 3 0 none A I S I U135 kl 138 0 1U9 0 197 0 228 0 1 0 1 5 none A I S I 4135 kl 127 5 136 0 16k 5 181 5 2 0 2 5 none PH 15-7 Mo 15-F 135 5 1I+8 5 193 5 215 0 — — expxosive peripheral PH 15-7 Mo 15-R 163 5 167 5 181+ 5 197 0 2 5 none I ( wyHMJt<R'i«i»mt»l'^llf*;*liQl'"**iB'l'l<w

-237- fracture Behavior By the notch tension t e s t used, m a t e r i a l s are evaluated as b r i t t l e when the notch s t r e n g t h / t e n s i l e s t r e n g t h equals one For s t e e l s , t h i s occurs when the t e n s i l e s t r e n g t h i s increased i n t o the range above 200 to 300,000 p s i For t i t a n i u m , the c r i t i c a l s t r e n g t h range i s from 140,000 to 170,000 p s i The bulge f r a c t u r e t e s t data m Table I I are i n e s s e n t i a l agreement w i t h these values At low s t r e n g t h l e v e l s , f r a c t u r e s propagat^ to unload the s e c t i o n and then stop At hi g h s t r e n g t h s , f r a c t u r e s propagate a t s u f f i c i e n t l y h igh v e l o c i t y to promote s h a t t e r i n g ACKNOWLEDGMENT The work r e p o r t e d m t h i s paper was completed f o r C u r t i s s - W r i g h t Corporation, Wright A e r o n a u t i c a l D i v i s i o n , a t Syracuse U n i v e r s i t y i REFERENCES 1 P Ludwik see A Nadai, "Theory o f Flow and Fracture o f S o l i d s , " Vol I , p 250 McGraw-Hill, New York, 1950 ' 2 R von Mises "Mechanik der f e s t e n Koerper im plastisch-deformablen Zustand," Nachr Ges Wiss Gottmgen, Math -physik Klasse, 1913 3 C Gazzara "The Modulus o f E l a s t i c i t y and Y i e l d S t r e n g t h o f Titanium Sheet," Thesis, Syracuse U n i v e r s i t y , June 1957

•238- SIMPLE LABORATORY BULGE TESTS FOR SHEET METAL FRACTURE TRANSITION STUDIES AND FOR WELDMENT PERFORMANCE EVALUATION P P Puzak and J gtoop M e t a l l u r g y D i v i s i o n Naval Research Laboratory, Washington, D C INTRODUCTION Considerable v o r k on the t e s t i n g o f h i g h - s t r e n g t h sheet m a t e r i a l s i n the presence o f notches has been conducted by v a r i o u s i n v e s t i g a t o r s During the past two years, these i n v e s t i g a t i o n s have been a c c e l e r a t e d because o f the f i n d i n g s and r e c o g n i t i o n t h a t such m a t e r i a l s are su b j e c t t o b r i t t l e f r a c t u r e when con s t r u c t e d as welded pressure vessels f o r r o c k e t casings^^^ Three general types o f t e s t s have gained r e c o g n i t i o n f o r the e v a l u a t i o n o f the f r a c t u r e r e s i s t a n c e c h a r a c t e r i s t i c s o f sheet metal These t e s t s can be described b r i e f l y as f o l l o w s A Large Notched Sheet T e n s i l e Test - I r w i n Test^^^ -Sheet t e n s i l e specimens, 12 x 3- i n , are c e n t r a l l y notched (mechanically sharpened a t the ends) and t e s t e d a t room temperature Test c o n d i t i o n s are adjusted t o permit the c a l c u l a t i o n o f a q u a n t i t y termed the "crack-extension-force " Fr a c t u r e s t r e n g t h i s d e f i n e d as the c h a c t e r i s t i c value, Gg o f the crack- e x t e n s i o n - f o r c e necessary f o r the "onset o f f a s t f r a c t u r e " B Edge Notch Sheet T e n s i l e Test - Brown Test^-*) -Sheet t e n s i l e t e s t s are conducted a t room temperature w i t h 1-in wide specimens having 60° in c l u d e d angle notches machined on both edges t o a depth o f 0 15-i n The notches are s y m e t r i c a l l y p o s i t i o n e d i n r e l a t i o n t o the c e n t e r - l i n e o f the specimen The c r i t e r i o n f o r notch s e n s i t i v i t y i s based upon the r a t i o o f nominal notch s t r e n g t h t o nominal t e n s i l e s t r e n g t h o f the given m a t e r i a l

•239- I Hofch s t r e n g t h r a t i o s o f 1 00 t o 1 05 I n d i c a t e e q u i v a l e n c e t o t h a t o f t h e base l i n e c a l c u l a t e d f o r I n f i n i t e l y wide sheets M a t e r i a l s v t i ^ c h d i s p l a y notch s t r e n g t h r a t i o s l e s s than 1 00 are considered notch s e n s i t i v e , and the smaller the value o f t h i s r a t i o , the more notch s e n s i t i v e the raateriaj. C Center-Cracked Sheet T e n s i l e Test - Srswley Test^^^ -Narrow (1 / 2 - l n wide) i n t e r n a l l y s l o t t e d , but s h a r p l y precracked t e n s i l e - t y p e specimens are used Net f r a c t u r e s t r e s s (nominal notch s t r e n g t h a t f r a c t u r e ) and percent shear f r a c t u r e appearance are determined f o r specimens t e s t e d over a range o f temperatures Both o f these c r i t e r i a e x h i b i t t r a n s i t i o n beha%'lor as a f u n c t i o n of t e s t temperature The c r i t e r i o n f o r behavior o f m a t e r i a l s I s based upon whether a r b i t r a r i l y d e f i n e d t r a n s i t i o n temperatures o f e i t h e r n e t f r a c t u r e s tress or f r a c t u r e appearance are above (pr o b a b l y unsafe f o r use) o r below (probably safe f o r use) the lowest o p e r a t i n g temperature o f the Intended a p p l i c a t i o n s I n d i s c u s s i n g the n o t c h - s t r e n g t h and f r a c t u r e t r a n s i t i o n f e a t u r e s o f h i s t e s t s , Srawley has concluded t h a t the sharp drop i n f r a c t u r e appearance w i t h temperature provides as meaningful a c r i t e r i o n o f p e r f o m a n c e as any o t h e r pro- posed Index^^^ Such a p o s i t i o n i s taken i n r e c o g n i t i o n o f t h e f a c t t h a t , t o be proven u l t i m a t e l y s i g n i f i c a n t , a l l t e s t s must be c o r r e l a t e d w i t h s e r v i c e experience, and f r a c t u r e appearance provides f o r such as r e a d i l y as any o t h e r Index The p o s s i b i l i t y o f using a f r a c t u r e t r a n s i t i o n index f o r the c r i t e r i o n of performance suggested the development o f a simple and inexpensive f r a c t u r e t e s t The d e s i r e d t e s t specimen should be easy t o prepare, and r e q u i r e l i t t l e ( i f any) machining The specimen should be l a r g e enough t o e l i m i n a t e problems concerning i n t e r p r e t a t i o n and r e l i a b i l i t y which may be encountered i n f r a c t u r e t e s t s o f small specimens Yet, the simple specimen should be s n a i l enough to be s u i t a b l e f o r both r o u t i n e acceptance t e s t i n g and f o r c o r r e l a t i o n w i t h s e r v i c e

-240- f a l l u r e m a t e r i a l s Since the t r a n s i t i o n aspects r e q u i r e t h a t t e s t s be conducted over a range o f temperatures, the t e s t procedures should a f f o r d easy c o n t r o l of the specimen temperature Also complicated g r i p p i n g devices t o prevent buckling o r e c c e n t r i c i t y of l o a d i n g should not be I n v o l v e d since they also preclude ra p i d consecutive t e s t s o f sever a l specimens I n a d d i t i o n , the a c c e p t a b i l i t y o f a simple t e s t would be enhanced i f a l a r g e number o f samples could be evaluated i n a s h o r t time by s k i l l e d or s e m i - s k i l l e d t e c h n i c i a n s a f t e r o n ly b r i e f i n s t r u c t i o n s Some o f the p o s s i b l e uses o f such a simple t e s t would be m a t e r i a l s q u a l i t y c o n t r o l check, heat treatment coupons t o Insure adequacy o f furnace t r e a t - ment used f o r the f a b r i c a t e d s t r u c t u r e , establishment o f heat t r e a t i n g procedures | f r e q u i r e d t o develop optimum combination o f s t r e n g t h and notch d u c t i l i t y i n a | gi v e n a l l o y , comparison o f f r a c t u r e c h a r a c t e r i s t i c s o f d i f f e r e n t a l l o y s as a f u n c t i o n o f s t r e n g t h l e v e l , i n v e s t i g a t i o n s o f m i l l processing or shop f a b r i c a t i o n v a r i a b l e s , e t c An urgent need a l s o exxsts f o r a simple t e s t capable o f e v a l u a t i n g weld c h a r a c t e r i s t i c s and weldment performance I t should be recognized t h a t from the thermal e f f e c t s o f welding a v a r i e t y o f notch toughness p r o p e r t i e s could be developed i n the v a r i o u s weld-zones (weld, f u s i o n l i n e , o r HAZ) which are unique t o welded s t r u c t u r e s I n any welded s t r u c t u r e , the propagation o f f r a c t u r e s may occur by any one o f the several paths as shown s c h e m a t i c a l l y i n F i g 1 The d e s c r i p t i o n s given f o r c e r t a i n r o c k e t casing f a i l u r e s ^ ^ ^ suggest degradation of prime p l a t e p r o p e r t i e s from w e l d i n g , s i n c e , i n some uses, the f r a c t u r e s propa- gated e x t e n s i v e l y and p r e f e r e n t i a l l y along HAZ areas A simple t e s t should, t h e r e f o r e , be capable o f I n d i c a t i n g the path o f l e a s t f r a c t u r e r e s i s t a n c e developed I n a p r a c t i c a l weldment Such a t e s t c o u l d be u s e f u l t o study the s u i t a b i l i t y o f e x i s t i n g or new weld metal compositions, welding procedures, w e l d i n g techniques, w e l d - r e p a i r s made subsequent to f i n a l heat treatment, I I

5 1 ^ Mi FRACTURE PATHS FUSION LINE WELD PLATE STRESS F i g . 1 - Schematic illufltration of the varioua potential paths of catastrophic failure which may be developed in practical veldosnts.

1 -242- heat treatment response check, screening t e s t , w e l d - q u a l i f I c a t l o n t e s t , etc The t e s t s which are t o be described h e r e i n were developed to meet the above-stated needs The same equipment and techniques used t o r a t e prime p l a t e m a t e r i a l on the basis o f f r a c t u r e appearance are r e a d i l y adaptable f o r weld zone performance t e s t s o f p r a c t i c a l weldments DESCRIPTION OF CRACK-STARTER BULGE TEST I t i s v e i l known t h a t the f r a c t u r e t r a n s i t i o n o f a given s t e e l i s not s e n s i t i v e t o notch a c u i t y Any t e s t procedure which uses a notch severe enough t o s t a r t a f r a c t u r e w i t h o u t gross pre-deformation, can be used to measure the in h e r e n t f r a c t u r e t r a n s i t i o n o f a given s t e e l A c c o r d i n g l y » . i n v e s t i g a t i o n s were conducted t o develop a simple l a b o r a t o r y bulge t e s t which | In c o r p o r a t e d the f e a t u r e s o f an a r b i t r a r y f l a w or c r a c k - s t a r t e r For materials r a n g i n g i n thickness from 0 040- t o 0 125-in , the d e s i r a b l e s i z e o f the bulge t e s t specimen was e s t a b l i s h e d e m p i r i c a l l y a t 6 x 6 inches i n dimension The specimens are sheared from as-received, h o t - r o l l e d and annealed p l a t e , and t numbered on diagonal corners so as t o correspond w i t h the p r i n c i p a l d i r e c t i o n o f r o l l i n g and f a c i l i t a t e o b s e r v a t i o n r e l a t e d t o transverse o r l o n g i t u d i n a l d i r e c t i o n s The specimens are then heat t r e a t e d t o the de s i r e d s t r e n g t h l e v e l i n n e u t r a l s a l t baths For the m a r t e n s i t i c type s t e e l s , the hard, b r i t t l e HAZ c o n d i t i o n n o r m a l l y developed upon welding was found t o pro v i d e a simple but e f f e c t i v e means o f i n t r o d u c i n g a c r a c k - s t a r t i n g d e f e c t F i g u r e 2 depi c t s s c h e m a t i c a l l y t h e c r a c k - s t a r t e r used f o r bulge specimens o f such s t e e l s The h e a t - t r e a t e d specimens are clamped t o a copper back-up bar and a 3/4-to 1-inch i n e r t arc f u s i o n weld ( w i t h o u t f i l l e r metal added) i s made a t the p l a t e c e n t e r l i n e The weld i s made w i t h argon gas s h i e l d i n g (30 CFH) and a 3/32" tungsten e l e c t r o d e Depending on the specimen t h i c k n e s s , 60 to 80 amps and 8 to 9 v o l t s .

-243- 6" \ FUSIO VHARD a B R I T T L E NOTCH WELD ' " f a l ? e ^ \ \ \ c ^ ~ : L ° ' c r a C - ^ t a r t e r employed for

-244- I ! however, are r e q u i r e d f o r specimens between 0 040 and 0 050-in t h i c k a t bath temperatures o f -100° t o -300°F and a t furnace temperatures o f 300° and 400OF Bulge t e s t i n g equipment i n v o l v i n g the use o f h y d r a u l i c and compressed axr p i s t o n systems were s t u d i e d but r e j e c t e d because o f d i f f i c u l t i e s i n i n s t r u - mentation, temperature c o n t r o l or time t o conduct each t e s t The simplest and most p r a c t i c a l technique devised f o r t h e b u l g e - t e s t i n g o f u l t r a - h x g h - s t r e n g t h sheet metals was t h a t which I n c o r p o r a t e d the p r i n c i p l e s employed i n forming processes which u t i l i z e the h y d r o s t a t i c compression o f f u l l y - c o n t a i n e d rubber t o form complicated sheet metal shapes An exploded-view o f the necessary t e s t apparatus i s shown i n F i g 3 Three sheets o f neoprene rubber (durometer hardneoa 45 - 55) are f u l l y contained i n t h e bottom o f t h e rubber r e t a i n i n g box w i t h i n s i d e dimensions o f 6 x 6% inches The rubber i s used t o form the bulge, the top piece i s considered expendable and i s replaced when necessary Three sides o f the r e t a i n i n g box are extended so as t o guide and prevent r o t a t i o n o f the d i e A clearance o f 1/16 i n c h between t h e o u t s i d e dimensions o f the d i e and i n s i d e dimensions o f t h e box i s s u f f i c i e n t f o r the purpose The b o l s t e r p l a t e , which i s b o l t e d t o the bottom o f the ram, i s s l o t t e d t o pr o v i d e a convenient means f o r interchange and attachment o f d i f f e r e n t dies t o the ram The s p r i n g holds the ram-die above t h e rubber i n the r e t a i n i n g box t o per m i t easy t r a n s - ference o f the specimen from the temperature b a t h t o the t e s t apparatus w i t h metal tongs as shown i n F i g 4 On the average, each bulge t e s t can be completed i n approximately 6 seconds ( i n c l u d i n g t r a n s f e r - t i m e t o p o s i t i o n t h e specimen) i f the energy t o bulge the specimen i s a p p l i e d by means o f a drop-weight The s t r i k e r p l a t e takes the impact blow o f the drop-weight and prevents damage t o the upper p a r t o f the ram Weights o f 300 to 500 pounds are dropped from an 8 f o o t h e i g h t t o

1/.- FRAME RAM SPRING RUBBER RETAINING BOX RUBBER STRIKER PLATE OPEN DIE BOLSTER PLATE KNURLED 4 NUT ^ ro I F i g . 3 - Exploded view of equipment r e q u i r e d t o conduct c r a c k - e t a r t e r bulge t e s t s .

•246- 1 J F i g A " Assembled equipment and p o s i t i o n i n g of specimen f o r c r a c k - s t a r t e r bulge t e s t

-247- DGSP, a t approximately 5 ipm t r a v e l speed are employed t o o b t a i n f u l l pene- t r a t i o n A small mechanical notch i s ground l o n g i t u d i n a l l y i n the f u s i o n weld t o a depth o f approximately 1/4 t o 1/2 o f the p l a t e thickness by means of a r o t a t i n g abrasive d i s c The c r a c k - s t a r t e r d e f e c t described above i s r e l a t i v e l y easy t o prepare and has been used s u c c e s s f u l l y f o r shear f r a c t u r e e v a l u a t i o n o f several m a r t e n s i t l c s t e e l s I t i s b e l i e v e d t h a t a sharp c h i s e l - c u t or s i m i l a r mechanical notch w i l l be adequate t o serve as a s u i t a b l e c r a c k - s t a r t e r i n bulge t e s t s o f the n o n m a r t e n s i t i c m a t e r i a l s The c r a c k - s t a r t e r bulge specimens a r e brought t o the d e s i r e d t e s t temperature by immersion f o r approximately 10 t o 15 minutes i n a l i q u i d bath Denatured a l c o h o l and dry i c e are used t o o b t a i n temperatures down t o isopentane (Mlsche Sludge Test Solvent) cooled w i t h l i q u i d n i t r o g e n i s used to o b t a i n temperatures below -100°F Water (heated by immersion heaters) may be used f o r t e s t temperatures up t o 200°? However, the evaporation o f l i q u i d from t h e su r f a c e o f specimens which are immersion heated i n water I n the range of 160° t o 212°F r e s u l t s i n a very r a p i d drop i n temperature a f t e r removal from the bath A specimen between 0 040 and 0 050 inches t h i c k , f o r example, w i l l drop about 30 degrees I n s i x seconds even when covered w i t h f i b e r g l a s s c l o t h (0 007 i n c h t h i c k ) t o r e t a r d the r a t e o f c o o l i n g Heating the specimen i n an a i r c i r c u l a t i n g furnace r e s u l t s i n a l e s s r a p i d drop i n the temperature when the specimen i s exposed t o the ambient temperature The corresponding drop i n temperature o f a furnace-heated specimen wrapped I n fiberglass c l o t h i s o n l y about 3 degrees i n 6 seconds a f t e r removal from t h e furnace For 0 062 t o 0 125-in t h i c k specimens, the bath and furnace temperatures are ad j u s t e d t o w i t h i n 5 t o 10 degrees o f the d e s i r e d t e s t i n g temperature Adjustments o f 15 t o 20 degrees.

I 00 I DROP-WEIGHT RUBBER DROP WEIGHT RUBBER P R E S S U R E Pig. 5 - Schematic illustration of crack-starter bulge test procedures. (See text for details.)

4 A •249- ^ Impact t h e die-ram The choice o f t h e proper weight i s dependent on the t h l c k - ^ oess and energy a b s o r p t i o n c h a r a c t e r i s t i c s o f the m a t e r i a l T e s t i n g o f any sample I s d i s c o n t i n u e d i f not completed w i t h i n 10 seconds W i t h i n t h i s time I n t e r v a l a l l p o s i t i o n s i n the c e n t r a l 5 - l n d i a r e g i o n o f 0 062 t o 0 125-ln t h i c k specimens were found t o remain w i t h i n 5** t o lO^F o f the bath temperature x\ I The sequence o f s i g n i f i c a n t steps are as f o l l o w s The a c t i o n o f a dropping weight. F i g 5 - L e f t , causes the d i e to contact the specimen and squeeze the rubber, thus f o r c i n g the specimen t o bulge i n t o the open c a v i t y o f the d i e . F i g 5 - R i g h r When the specimen begins to bulge, the b r i t t l e weld-notch r e g i o n cracks r e a d i l y , and thereby a u t o m a t i c a l l y introduces a sharp crack ( e q u i v a l e n t t o f a t i g u e crack, e tc ) The dynamic "rubber-forming" process continues t o propagate t h i s sharp crack i n t o the prime m a t e r i a l as (a) f u l l y b r i t t l e f r a c t u r e , o r (b) p a r t i a l l y b r i t t l e f r a c t u r e , o r (c ) f u l l shear (45°) f r a c t u r e _ The n a t u r e and e x t e n t o f the r e s u l t i n g f r a c t u r e i s , o f course, determined by the p r o p e r t i e s o f the s t e e l f o r t h e t e s t temperature I n v o l v e d RESULTS OF PRELIMINARY CRACK-STARTER BULGE TESTS The p r e l i m i n a r y c r a c k - s t a r t e r bulge t e s t s r e s u l t s are presented h e r e i n to e s t a b l i s h the p o t e n t i a l i t i e s o f the t e s t method and t o provide a comparison of f r a c t u r e appearance t r a n s i t i o n s o b t a i n e d w i t h o t h e r t e s t s For the most p a r t , the m a t e r i a l s employed. Table 1 , represent m a t e r i a l s p r e v i o u s l y s t u d i e d or p r e s e n t l y under i n v e s t i g a t i o n f o r f u t u r e r e p o r t i n g by Srawley During the e a r l y development o f the t e s t procedures, samples were t e s t e d p r i n c i p a l l y a t room temperature and o c c a s i o n a l l y a t 32°F ( i c e - w a t e r bath) F i g u r e 6 d e p i c t s

•250- 2«F ^ F i g , 6 - Fractijre c h a r a c t e r i s t i c s developed by four high-strength s t e e l s i n crack-starter bulge t e s t s conducted at 32° F. S t e e l samples developed 100̂ ^ Shear (No. 1 ) , Shear (No. 2 ) , 15% Shear (No. 3) and 1005?. B r i t t l e (No. U) Fractures respectively.

" 1 i •251- representative types isi itactures developed I n the cr a c k - s t a r t e r bulge t e s t s for several hlgh-stre^g^t sheet s t e e l s which were tested at 32°? As shown I n Table 1, these foi>- it«els were a l l heat treated to the same approximate hardness l e v e l (53-54 ' i ^ ^ , however, i t i s obvious from the v i s u a l appearance of samples alone that ccm^Ietely d i f f e r e n t crack r e s i s t a n c e properties were developed i n these s t e e l s . To e s t a b l i s h t i ^ shear fracture appearance t r a n s i t i o n curve for a given s t e e l , a minimum or s i x to eight c r a c k - s t a r t e r bulge t e s t specimens are required These are tested over an appropriate range of temperatures which v a r i e s according to the steel involved The i n i t i a l sample i s tested at room temperature, and the mea<:ured amount of shear fracture appearance determines the t e s t temperature which should be used for the second sample Upon the completion of each t e s t , shear fracture appearance measurements are made within the central 5-ln dia region of the specimen at any location 1/2-in or more from the fusion-weld crack-starter A minimum of three low magnification v i s u a l measurements are made across the specimen thickness of only the cleavage portion of the f r a c t u r e The average value of these measurements i s subtracted from the thickness dimension of the specimen and the r e s u l t represents that portion of the f r a c t u r e which occurred v i a shear Excluding the heat treatment schedule, shear f r a c t u r e appearance ratings ( i e , specimen preparation, t e s t i n g and measurements) can be established for a given s t e e l i n l e s s than one hour In addition, when numerous s t e e l s are involved, so that t e s t s of several s t e e l s could be conducted at the same preset bath-temperature, i t i s estimated that two technicians could t e s t and measure more than 100 specimens i n an 8-hour day One of the f i r s t s t e e l s that was a v a i l a b l e i n s u f f i c i e n t quantity to e s t a b l i s h shear f r a c t u r e appearance ratings was a 0 063-in thick consumable

LUji imvnti*^ CM TABLE 1 DATA FOR STEELS USED IN CRACK-STARTER BULGE TEST INVESTIGATIONS Alloy Heat Treatment Hardness •If Composition - Per Cent No Type /us t e n l t l z e d Tempered Rc ( Mn Si Cr Ni Mo V 1 SAE 4340 15750F, 1 hr/0 Q 4250F, 2+2 hr 53 0 40 0 70 0 28 0 80 1 80 0 25 2 Low Alloy 1575°F, 1 hr/0 Q 600°F, 2+2 hr 54 0 43 0 80 1 60 0 85 1 85 0 38 0 10 3 Hot Work Tool 1900OF, 1/2 hr/A C IOOQOF, 3+3+3 hr 54 0 40 0 30 0 90 5 00 — 1 30 0 4 5 4 Hot Work Tool 1950OF, 1/2 hr/A C 1020°F, 2+2 hr 54 / 1 0 40 0 55 1 00 3 25 — 2 80 0 35 5A AMS 6434 1575°F, 1/2 hr/0 Q 400OF, 2 hr i 52 0 31 0 67 0 40 0 99 1 84 0 31 0 17 5 8 AMS 6434 1575°F, 1/2 hr/0 Q 500OF, 2 hr 49 0 31 0 67 0 40 0 99 1 84 0 31 0 17 5C AMS 6434 1575°F, 1/2 hr/0 Q 725°F, 2 hr 45 0 31 0 67 0 40 0 99 1 84 0 31 0 17 6 Low Alloy 1700OF, 1600OF, 1/2 1/4 hr/A hr/0 C Q 600°F, 2+2 hr 52 0 42 0 87 1 63 1 02 1 81 0 39 0 09 7 AISI 410 1800OF, 1/2 hr/A C 600OF, 3 hr 40 0 13 0 35 0 47 12 4 0 2 2 0 10 0 01 8 Low Alloy 1700°F, 1/4 hr/A c 700°F, 1/2 hr 54 0 38 0 91 1 50 2 12 — 0 53 0 06 9 Experimental 1 5 7 5 0 F , 3/4 hr/0 Q 5 7 5 O F , 2 hr 54 0 42 0 91 1 40 0 83 — — 0 02 10 Ti-4Al-3Mo-lV 1650OF, 1/4 hr/W Q 9 2 5 0 F , 12 hr.,A C, 45 0 023 c, 4 2 Al, 2 : I Mo , 1 1 V, 0 12 Fe * Nominal Analyses given for Steel Nos 1 to 4

•253- electrode, vacuum-remelt AMS 6434 a l l o y ( S t e e l No 5) For one condition of heat treatment (No 5C i n Table 1 ) , the v i s u a l changes i n fracture character- i s t i c s CiChibited by the samples as a function of t e s t temperatures are shown in F i g 7 Representative fracture sections used to e s t a b l i s h the fracture appearance t r a n s i t i o n curve for Steel No 5C are shovm i n F i g 8 As can be seen i n F i g 8, a precipitous change i n fracture appearance i s developed over a r e l a t i v e l y narrow range of te s t i n g temperatures This precipitous change from e s s e n t i a l l y f u l l shear to r e l a t i v e l y low values (15 to 307) of shear fr a c t u r e appearance i s r e a d i l y detectable (without measurements) by v i s u a l observation of the amount of cleavage i n fractured specimens which are tested i n increments of 20°F At lower t e s t i n g temperatures, however, a t a i l - l i k e region i s developed i n the shear f r a c t u r e appearance curve U n t i l more ex- perience IS gained, the r e l a t i v e notch toughness qu a l i t y of a given s t e e l w i l l be discussed i n terms of the bulge f r a c t u r e t r a n s i t i o n (BFT) which i s defined, a t t h i s time, as the approximate temperature at which the precipitous change i n shear f r a c t u r e appearance i s developed i n c r a c k - s t a r t e r bulge t e s t s of a given s t e e l (BFT for Steel No 5C i s approximately -100°F) A q u a l i t a t i v e comparison of d i f f e r e n t materials, or of the same material i n d i f f e r e n t conditions of heat treatment, i s based upon th e i r r e l a t i v e BFT temperature positions Figure 9 indicates the cra c k - s t a r t e r bulge t e s t f r a c t u r e appearance t r a n s i t i o n s for St e e l No 5 i n three conditions of heat treatment ( a u s t e n i t i z e d 1575°F and tempered re s p e c t i v e l y at 400°, 500° and 725°F) The BFT temperatures are indicated to be approximately -210°F for the 400OF temper, -120OF for the 500°F temper and -100°F for the 7250F temper The embrittling propensities shown for Steel No 5 as the tempering temperature i s r a i s e d from 400°F to 725°F are not s t a r t l i n g Such b r i t t l e c h a r a c t e r i s t i c s

• 254- N0.5C N0.5C atSSfce- 160 *F N0.5C Na5CF !90°F F i g , 7 - Fractiire c h a r a c t e r i s t i c s exhibited by Ste e l No. 5C iln crack-starter bulge t e s t e (See F i g . 8 for shear fracture appearance data.)

100 I m CM I DC < X CO H 8 0 6 0 li j 4 0 o UJ 2 0 STEEL NO 5C FRACTURE APPEARANcfe CRACK-STARTER BULGE TEST HEAT TREATMENT I 5 7 5 ° F 1/2 HR OQ 725 F 2 HR AC/ 0 - 3 0 0 m - 2 0 0 HOO 0 TESTING TEMPERATURE (<^F) ^ " S«''S^ci2tSS^bS .̂"^°^ ^ representative fracture Bectians taken i r o n cracit-Btarter bul^o t e s t samples of S t e e l No 5C. u.

100 01 8 0 r CO 6 0 UJ 40 Q / / 5A / i UJ 0 - 3 0 0 J I \—L STEEL NO 5 F R A C T U R E APPEARANCE C R A C K - S T A R T E R B U L G E T E S T HEAT TREATMENT A L L S P E C I 5 7 5 » F | H R O Q A' '5A- X 4 0 0 » F 2HR A C (52 Rc) 5B- 0 5 0 0 ' F 2HR A C (49 Ro) 5C-A 725 " F 2HR A C (45 Rc) I I ' l l I I I I - 2 0 0 -100 0 TESTING TEMPERATURE ("F) i I M F i g 9 - Crack-eterter bulge t e s t fracture appearance t r a n s i t i o n curves developed for three conditions of heat treatment for S t e e l No 5

i -257- I ere developed at Intermediate tempering temperatures i n many of the martensitic alloy s t e e l s Of p a r t i c u l a r s i g n i f i c a n c e with respect to the performance I n other t e s t s I s the f a c t that the BFT temperatures for a l l heat treatment con- ditions studied are shown to be s i g n i f i c a n t l y below the ambient temperatures Thus, at room temperature, the notch d u c t i l i t y behavior I n the various t e n s i l e - type t e s t s (edge-notched or center-cracked) are expected to be highly uniform f o r t h i s material i n a l l three temper conditions and i n d i c a t i v e of high r e - sis t a n c e to fracture concomitant with 100% shear ( d u c t i l e ) behavior A comparison of r e s u l t s obtained with several s t e e l s indicates that good agreement can be expected between the fracture appearance t r a n s i t i o n curves determined i n either the cr a c k - s t a r t e r bulge t e s t or the center-cracked t e n s i l e t e s t For the 0 083-ln thick martensitic low-alloy s t e e l * for which complete f r a c t u r e appearance t r a n s i t i o n curves were established by both t e s t procedures, excellent agreement was obtained as shown I n F i g 10 Accordingly, the simple and rapid evaluations made i n the cra c k - s t a r t e r bulge t e s t are expected to predict the t r a n s i t i o n temperature range over which the various tensile-type t e s t s are l i k e l y to Indicate high "numbers" (top shel f , shear) or low "numbers" (bottom of t r a n s i t i o n range, b r i t t l e ) Figure I I depicts r e s u l t s of fracture appearance t r a n s i t i o n determi- nations for a 0 039-ln t h i c k A I S I type 410 s t a i n l e s s s t e e l The fracture appearance t r a n s i t i o n was not firmly established I n the center-cracked t e n s i l e t e s t but was deflned(^) as occurring i n the temperature range between 0° and -IZO^F I n the cr a c k - s t a r t e r bulge t e s t , good agreement I n fracture appearance data are Indicated with the center-cracked specimen data for temperatures of -150°F and lower However, 1007 shear fractures were obtained I n crack-starter NOTE * Reported as Steel C by Srawley^^)

100 < UJ V) us o q: UJ a . 80 60 40 E A SRAWLEY T E S T X CRACK-STARTER BULGE T E S T E A S T E E L NO 6 .X A A 20 o ' ' ' HEAT TREATMENT I 7 0 0 « F jHR A C I 6 0 0 « F ^ H R O Q 6 0 0 « F 2 + 2 H R A C ROOM TEMP PROP T S 2 7 0 , 0 0 0 P S I Y S 2 2 0 , 0 0 0 P S I NJ 00 I J I I L j l J L J I I I 0 100 TESTING TEMPERATURE (*F) 200 F i g 10 - Comparison of crack-starter bulge and center-cracked t e n s i l e t e s t fracture appearance t r a n s i t i o n cirrve data for S t e e l No 6

-259- 250 41 U $ n «) •p o H h (D C Is ^ P 0) 6 ^ 0 L° u c V! H o +> g s ^ § n I (0 CD UJ c r H 200 150 100 < LU c n L J O q: U J C L 80 60 4 0 20 U L T I M A T E T E N S I L E S T R E N G T H N E T F R A C T U R E S T R E S S ^ L £ ^ T R E N G T H (SRAWLEY TEST DATA) • X ¥r FRACTURE A P P F A R A N C E A S R A W L E Y T E S T X C R A C K - S T A R T E R B U L G E T E S T / / S T E E L NO 7 / NO DATA / I I / / r H E A T T R E A T M E N T / / ISOO'F 2-HR AC ' 600»F 3HR AC R O O M T E M P P R O P TS 195,000 PSI YS 140,000 PSI 1 -300 - 2 0 0 -100 0 TESTING TEMPERATURE (°F) 100 F i g 11 - Coraparison of c r a c k - s t a r t e r bulge and center-cracked t e n s i l e t e s t data for S t e e l No 7

-260- bulge t e s t s of t h i s s t e e l at test temperatures of -90° and -120°F, and the per cent shear fracture dropped precipitously to approximately 237 shear when the s t e e l was tested at -150°F Although t h i s s t e e l i s recognizably too low in strength to be considered suitable for advanced rocket casing applications, i the r e s u l t s of c r a c k - s t a r t e r bulge t e s t s are interpreted to imply satisfactory | fr a c t u r e performance to temperatures at l e a s t as low as -100°F In terms of ^ the center-cracked specimen net f r a c t u r e - s t r e s s t r a n s i t i o n temperature concept*, s a t i s f a c t o r y service at -lOOOp i s also implied by the data ( F i g 11, Top) which show net f r a c t u r e - s t r e s s values at -120°? approximately equivalent to the nomi- nal t e n s i l e strength for t h i s s t e e l Another s t e e l for which additional data are to be obtained to establish the fracture appearance t r a n s i t i o n firmly i n the center-cracked t e n s i l e test^''^ I s the 0 075-in thick low a l l o y , a i r hardenable proprietary s t e e l (No 8) designed to obtain minimum y i e l d strength l e v e l s of 230,000 p s i As shown in F i g 12, center-cracked t e n s i l e t e s t s were not conducted between 80° and 300°F For c r a c k - s t a r t e r bulge t e s t s conducted at 40OF and lower, excellent agreement i n shear fracture appearance data was obtained with that of the center-cracked t e n s i l e t e s t s At 90° and 120°F, however, the fracture appearance ratings developed i n the c r a c k - s t a r t e r bulge t e s t s were 857 and 1007 shear, respectively The BFT temperature of t h i s s t e e l i s defined as approximately 9 0 0 F As can be seen i n F i g 12, the c r a c k - s t a r t e r bulge test fracture appearance t r a n s i t i o n curve for t h i s s t e e l i s r e l a t i v e l y steep at room tem- perature Because of possible v a r i a t i o n s i n the amount of shear developed at NOTE The net f r a c t u r e - s t r e s s t r a n s i t i o n temperature i s a r b i t r a r i l y defined as the temperature at which the net fracture s t r e s s i s equal to the y i e l d strength for the p a r t i c u l a r s i z e of specimen used

CM I 100 < llJ I - 2 4 0 o a: Lu 2 0 CL FRACTURE APPEARANCE A SRAWLEY T E S T 0 CRACK-STARTER BULGE T E S T NO DATA 1 /'STEEL NO 8 / , ^ H E A T TREATMENT / / I700°F j-HR AC .4^ 7 0 0 ' F iHR AC ROOM TEMP PROP TS 276,000 PSI YS 234 ,000 PSI 0 -100 0 100 200 TESTING TEMPERATURE ^ F ) F i g . 12 - Comparison of crack-etarter Imlge and center-cracked t e n s i l e t e s t f r a c t u r e appearance t r a n s i t i o n curve data for S t e e l So. B, A L 300 {4

-262- "room temperature," the tensile-type t e s t determination conceivably could exhibit s i g n i f i c a n t s c a t t e r with small changes I n room temperature For material from t h i s same heat and with s i m i l a r heat treatments, Hodge(^) has reported considerable s c a t t e r for the c h a r a c t e r i s t i c crack-extension force v a l u e s , , obtained for t r i p l i c a t e samples tested at "room temperature " i t i s believed that the t r a n s i t i o n features exhibited by t h i s s t e e l ( F i g 12) could provide an explanation of the sc a t t e r obtained i n the "room temperature" wide sheet notched t e n s i l e t e s t s DESCRIPTION OF WELD-ZONE PERFORMANCE TEST Because of the complexities involved i n welding, i t i s not possible to evaluate f u l l y a l l of the weld-zone regions by conventional crack propa- gation t e s t techniques However, bulge t e s t s i n which the deformation i s adequate to develop fractures are uniquely suited to observe p r e f e r e n t i a l , weak or b r i t t l e paths of f a i l u r e i f such e x i s t i n p r a c t i c a l weldments For example, i f a p o t e n t i a l l y b r i t t l e weld path of f a i l u r e e x i s t s i n a given weldment. I t w i l l be evident i n fra c t u r e s by b r i t t l e n e s s of the weld per se Figure 13 depicts such a b r i t t l e weld path of f a i l u r e i n a low a l l o y high strength s t e e l (No 9) weldment which was tested at room temperature i n the ad-welded condition s o l e l y for i l l u s t r a t i v e purposes i n t h i s report The weld zone performance t e s t i s not Intended to evaluate the fracture c h a r a c t e r i s t i c s of the prime plate The temperatures at which d u c t i l e f r acture performance (1007 shear) can be expected i n prime p l a t e are established more r e a d i l y i n the c r a c k - s t a r t e r bulge t e s t previously described The weld zone performance t e s t s are aimed at es t a b l i s h i n g the welding pro- cedures, techniques and materials which are required to Insure that trouble- free s e r v i c e i n otherwise tough a l l o y s w i l l not be jeopardized by the development of embrittled weld-zone paths of potential f a i l u r e • fHUM miij^f

-263- NO. 9 41 rtWtHhi I I t i l I 75^F 'le. 13 - B r i t t l e veld f a i l u r e developed at room temperature i n weld-zone bulge t e s t (open d i e ) . Ductile plate performance i s indicated by resistance to propagation of the numerous transverse weld cracks. ^ y ^

i -264- The same te s t apparatus and general procedures described for crack* s t a r t e r bulge t e s t s are also applicable for weld-zone performance studies The weldments (6 x 6-in ) are prepared for bulge te s t i n g by grinding of the weld crown for 1-1/4-in from each end i n order to insure f l a t contact with the die A r t i f i c i a l crack-starters are not employed because the weld-zone performance t e s t i s expected to indicate which of the various weld-regions are inherently most susceptible to the s t a r t i n g of cracks Simulated service weldments can be investigated or samples may be prepared under controlled laboratory conditions to investigate the ef f e c t s of s p e c i f i c welding variables I n the absence of embrittled weld-zone regions, i t should be recog- nized that d u c t i l e plate metal tears may be forced to i n i t i a t e from weld defects Figure 14 i l l u s t r a t e s such a f a i l u r e developed i n a room tempera- ture open-die bulge t e s t of a weldment made with Steel SB ( d u c t i l e plate performance at room temperature previously predicted by crack-starter bulge t e s t BFT temperature of -120°F, see F i g 9) I n the open-die bulge t e s t , the extent of d u c t i l e tearing i s a function not only of the fracture c h a r a c t e r i s t i c s of the plate but also of the -drop-weight energy applied Thus, to be f u l l y s i g n i f i c a n t , the weld-zone performance t e s t must be conducted under conditions which provide information on the following (a) Origin of f a i l u r e , (b) l e v e l of prefracture deformation at which f a i l u r e i n i t i a t e s , ( c ) the presence or absence of p r e f e r e n t i a l paths of f a i l u r e , and (d) extent and type of r e s u l t i n g f a i l u r e The forced fractures obtained i n open-die bulge t e s t s of p r a c t i c a l weldments are examined to screen those welding procedures or materials which develop obviously b r i t t l e or p r e f e r e n t i a l weld-zone paths of f a i l u r e To

-265- N0.5B Ig. H. - I l l u s t r a t i o n of forced ductile plate f a i l u r e originating from weld defect i n room temperature weld-zone bulge t e s t (open d i e ) .

•zee- establish weldment fr a c t u r e performance under known conditions of el a s t i c or p l a s t i c s t r a i n s . I t I s necessary to l i m i t the allowable bulge deformation I t i s planned to control the amount of bulge deformation by the use of inter- changeable, s o l i d die-cups of increasing bulge depths I n comparison to the open-die. Fig 15 i l l u s t r a t e s s o l i d die-cups w i t h 0 1, 0 2 and 0 3-ln bulge depths, respectively Preliminary studies are under way wi t h these die-cups to establish the r e l a t i v e f r a c t u r e performance i n various high strength steel weldments prepared under conditions which simulate those used i n service For f i l l u s t r a t i v e purposes i n t h i s report, a HAZ f a i l u r e obtained i n a high strength st e e l weldment bulge tested w i t h the 0 2-in die-cup, and a predominantly weld f a i l u r e obtained i n another weldment tested w i t h a 0 3-in die-cup are shown i n Fig 16 CONTEMPLATED FUTURE PROGRAM For the most par t , the work completed to date was concentrated upon the crack-starter bulge t e s t The procedures, as described herein, represent a simple, Inexpensive and rapid means of obtaining q u a l i t a t i v e comparisons of , d i f f e r e n t materials based upon a fr a c t u r e appearance t r a n s i t i o n c r i t e r i o n I t i i s planned to continue w i t h add i t i o n a l steels the comparisons of crack-starter bulge t e s t data w i t h those obtained i n other tests These tests w i l l also be extended to Include nonmartensitlc materials f o r which more suitable notching methods must be developed To demonstrate the p o t e n t i a l i t i e s of mechanical notches. Figure 17 I l l u s t r a t e s the f r a c t u r e obtained i n a T l high strength sheet a l l o y (No 10) which was notched with crossed chisel cuts p r i o r to heat treatment to a hardness l e v e l of R̂ 44 - 45 The weld-zone performance t e s t investigations have only recently begun The fractured weldment samples which have been discussed herein were

3^ . .-.^...A>*«»- •̂•̂ v-^'i.:';_v;'.-. . ;J f . : . • I A- rr"i«rtlfc.. I I F i g . 15 - I l l u s t r a t i o n of open-die (upper l e f t ) and s o l i d die-cups of increasing depths used to control the amount of bulge deformation which can be developed i n weld-zone performance t e s t .

t 00 vO CM I SESrft • tV: F i g , 16 - Appearance of weld zone performance t e s t samples of two d i f f e r e n t high strength s t e e l weldments. HAZ failiore ( l e f t ) was developed with the 0.2-in. die-cup, and the predominantly weld f a i l u r e ( r i g h t ) with the 0.3-in. die-cup.

-269- v4 crack-Starting purposes,

-270- prepared to I l l u s t r a t e the p o t e n t i a l i t i e s of t h i s t e s t procedure to demonstrate the path of least f r a c t u r e resistance i n a given weld assembly Because of the l i m i t e d experience w i t h the weld-zone performance t e s t , i t i s not possible at t h i s time to define s p e c i f i c c r i t e r i a r e l a t e d to expected weld-zone behavior Preliminary explorations of the type already conducted for the crack-starter bulge t e s t are required f o r p r a c t i c a l weldments These are under way Upon completion of the preliminary studies w i t h open-die and s o l i d die-cups, ex- tensive weld-zone performance te s t s are contemplated f o r future investigations These w i l l Include studies i n v o l v i n g s i m i l a r and d i s s i m i l a r weld- and plate- metal combinations and w i l l be aimed at establishing the welding procedures and materials required f o r the development of optimum properties f o r specific service applications A l l of the various c r i t e r i a proposed f o r the p r e d i c t i o n of service behavior must be considered as q u a l i t a t i v e u n t i l they have been validated by correlations w i t h service f a i l u r e steels, vrtilch, of course, represent the ultimate i n " t e s t " r e s u l t s The new test procedures described herein are r e a d i l y amenable f o r c o r r e l a t i o n purposes not only fo r prime plate but also f o r weld-zone service f a i l u r e s Such correlations are planned and samples are being s o l i c i t e d ACKNOWLEDGEMENT The authors acknowledge the o r i g i n a l suggestions of Mr W S P e l l l n l f o r a drop weight loaded, rubber formed bulge t e s t f o r f r a c t u r e and weld evaluation which prompted t h i s I n v e s t i g a t i o n

•271- REFERENCES 1 Kles, J A , "Casing Failures", Materials Advisory Board Meeting of Committee on Materials f o r Large Solid Propellant Rocket Motors, February 1959 ( t o be published by NRL) 2 I r w i n , G R , "Fracture Strergth", Report of NRL Progress, November 1957, pp 10-19 3 Espey, G B , Jones, M H , and Brown, W F , Jr , "Preliminary Report on Sharp Notch and Smooth Tensile Characteristics f o r a Number of High Strength Sheet Steel Alloys", to be presented at Annual Meeting of ASTM, A t l a n t i c C i t y , June 21 and 22, 1959 4 Srawley, J E , and Beachem, C D , "Crack Propagation Tests of High-Strength Sheet Steels Using Small Specimens", NRL Report 5127, A p r i l 9, 1958 5 Srawley, J E , and Beachem, C D , "Crack Propagation Tests of Some High- Strength Sheet Steels", NRL Report 5263, January 10, 1959 6 Puzak, P P , Schuster, M E , and P e l l l n l , W A , "Crack-Starter Tests of Ship Fracture and Project Steels", WELDING JOURNAL RESEARCH SUPPLEMENT, p 481-s, October 1954, and Ship Structure Committee Report, SSC-77 7 Srawley, J E , "Current Position of Crack Propagation Testing of High Strength Sheet Steels i n the Physical Metallurgy Branch at N R L ", Materials Advisory Board Meeting of Committee on Materials f o r Large Solid Propellant Rocket Motors, February 1959 8 Hodge, J M , "Summary of U S Steel's Experience w i t h Crack Propagation Testing", Materials Advisory Board Meeting of Committee on Materials f o r Large Solid Propellant Rocket Motors, February 1959