Environmental stress cracking is involved with some 25 % of plastic part failures

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.