Environmental stress cracking (ESC) is a phenomenon in which a plastic resin is degraded by a chemical agent while under stress, and it is the leading cause of plastic component failure. It is a solvent-induced failure mode, in which the synergistic effects of the chemical agent and mechanical stresses result in cracking. A recent study showed that 25% of plastic part failures are related to ESC.

To adequately understand the ESC failure mechanism, some background on analogous cracking in air is required. In the absence of chemical interaction, cracking is associated with prolonged static stress through a creep mechanism. Creep, sometimes called static fatigue, is a brittle fracture mode in which continuous stress results in molecular disentanglement within the polymer chains.

The creep failure mechanism involves a series of distinct steps. The first step is craze initiation, the second is craze growth that leads to crack initiation, then crack extension, and finally catastrophic fracture. Creep failure is common within plastic materials at room temperature, but rare in metals. It is a result of the viscoelastic properties of polymeric materials.

This article details the steps involved with ESC, describes the characteristics of such failures, and discusses the three factors involved with failure. Two case histories illustrating ESC failures are also presented.

Steps in environmental stress cracking

ESC is a phenomenon in which a particular plastic resin is cracked through contact with a specific chemical agent while under stress. The synergistic effects of the chemical agent and mechanical stresses result in cracking.

The chemical agent does not cause direct chemical attack or molecular degradation. Instead, the chemical penetrates into the molecular structure and interferes with the intermolecular forces binding the polymer chains, leading to accelerated molecular disentanglement.

The mechanism steps involved in ESC failure are similar to those responsible for creep failure, and include fluid absorption, plasticization, craze initiation, crack growth, and finally fracture. Because the ESC process depends on the diffusion of the chemical into the polymer structure, the rate of fluid absorption is a critical parameter in the rate of both craze initiation and crack extension. The more rapidly that the chemical agent is absorbed, the faster the polymer will be subjected to crazing and subsequent failure.

Recent comparisons have illustrated creep as a special condition of ESC. Under this model, creep is simply ESC with air as the chemical agent, the principal difference being that the presence of an active chemical agent accelerates the disentanglement process. This acceleration results in a significant reduction in the time to crack initiation, and substantially increases the speed of the extending crack, thus shortening the time to failure. Alternatively, ESC cracking develops at reduced stress or strain levels relative to creep failure in air.

It has been theorized that

"Highly localized fluid absorption is probably the mechanism for acceleration. The fluid is preferentially absorbed at sites under high dilatational stress such as a stress concentrating defect, a craze, or the tip of a crack. The absorbed fluid locally plasticizes the material, reducing its...