Environmental stress cracking is involved with some 25% of plastic part failures.Jeffrey A. JansenStork TechnimetNew Berlin, WisconsinEnvironmental stress cracking (ESC) is aphenomenon in which a plastic resin is de-graded by a chemical agent while understress, and it is the leading cause of plasticcomponent failure. It is a solvent-induced failuremode, in which the synergistic effects of the chem-ical agent and mechanical stresses result in cracking.A recent study showed that 25% of plastic part fail-ures are related to ESC.To adequately understand the ESC failure mech-anism, some background on analogous cracking inair is required. In the absence of chemical interac-tion, cracking is associated with prolonged staticstress through a creep mechanism. Creep, some-times called static fatigue, is a brittle fracture modein which continuous stress results in molecular dis-entanglement within the polymer chains. The creep failure mechanism involves a series ofdistinct steps. The first step is craze initiation, thesecond is craze growth that leads to crack initiation,then crack extension, and finally catastrophic frac-ture. Creep failure is common within plastic mate-rials at room temperature, but rare in metals. It isa result of the viscoelastic properties of polymericmaterials.This article details the steps involved with ESC,describes the characteristics of such failures, anddiscusses the three factors involved with failure.Two case histories illustrating ESC failures are alsopresented. Steps in environmental stress crackingESC is a phenomenon in which a particularplastic resin is cracked through contact with a spe-cific chemical agent while under stress. The syner-gistic effects of the chemical agent and mechanicalstresses result in cracking. The chemical agent does not cause direct chem-ical attack or molecular degradation. Instead, thechemical penetrates into the molecular structureand interferes with the intermolecular forcesbinding the polymer chains, leading to acceleratedmolecular disentanglement. The mechanism steps involved in ESC failure aresimilar to those responsible for creep failure, andinclude fluid absorption, plasticization, craze initi-ation, crack growth, and finally fracture. Becausethe ESC process depends on the diffusion of thechemical into the polymer structure, the rate of fluidabsorption is a critical parameter in the rate of bothcraze initiation and crack extension. The more rap-idly that the chemical agent is absorbed, the fasterthe polymer will be subjected to crazing and sub-sequent failure. Recent comparisons have illustrated creep as aspecial condition of ESC. Under this model, creep issimply ESC with air as the chemical agent, the prin-cipal difference being that the presence of an activechemical agent accelerates the disentanglementprocess. This acceleration results in a significant re-duction in the time to crack initiation, and sub-stantially increases the speed of the extending crack,thus shortening the time to failure. Alternatively,ESC cracking develops at reduced stress or strainlevels relative to creep failure in air. It has been theorized that “Highly localized fluid absorption is probably themechanism for acceleration. The fluid is preferentiallyabsorbed at sites under high dilatational stress suchas a stress concentrating defect, a craze, or the tip ofa crack. The absorbed fluid locally plasticizes the ma-terial, reducing its yield strength. Critical strains andstresses for craze initiation with the most active fluidscan be as low as 0.1% and a few megapascals. Stressesand strains due to processing and/or assembly canoften exceed the critical condition.” (Rapra Tech-nology) Characteristics of ESCEnvironmental stress crack failures share severaltypical characteristics: • Brittle fracture: ESC failures are caused by brittlefracture, even in materials that would normally beexpected to produce a ductile yielding mechanism.The crack initiation sites for ESC failures are alwayson the surface. They normally correspond to local-ized areas of high stress, such as microscopic de-fects or points of stress concentration. The initia-tion location is generally related to direct contactwith an active chemical agent, either liquid or gas. • Multiple cracks: Multiple individual cracks areinitiated, and these subsequently coalesce into aunified fracture. Numerous crack origins and thecorresponding unions are illustrative of an ESCfailure mechanism. •Smooth morphology: The crack origin areas usu-ally exhibit a relatively smooth morphology, in-dicative of slow crack growth. However, aggres-Environmental Stress Cracking – THEPLASTICKILLER50 ADVANCED MATERIALS & PROCESSES/JUNE 2004environ.qxd 5/13/04 9:12 PM Page 2A high number of latch handles on an enclosure sud-denly began to fail after a rel-atively short time. Standardservice included periodic actu-ation of the handles at normalexterior temperatures. Thehandles were molded froma commercial grade of a poly-carbonate / polyacrylonitrile:butadiene:styrene (PC/ABS)resin blend. The handle as-sembly is held together in abase unit with a metallic roll pin and a spring. A reviewof molding and assemblyprocesses revealed no varia-tions to account for the sudden change in performance.A visual inspection of the failed parts showed signifi-cant cracking, consistent across all of the failed parts. Thecracks were present within the molded boss that securedthe roll pin, and had a shape that was irregular but con-tinuous. Upon disassembly of the units, additional non-catastrophic cracks were apparent within similar locationsaround the boss hole. The fracture surface displayed fea-tures characteristic of brittle fracture, with many crackorigins adjacent to the inner diameter surface. A typicalcrack is shown in Fig. 3. An oily residue was readily ap-parent on and adjacent to the fracture surface.Typical fracture surfaces were further examined viascanning electron microscopy (SEM). The SEM inspec-tion of the fracture surface confirmed the presence ofmultiple apparent crack origins along the inner diam-eter of the molded boss in an area that had been in di-rect contact with the roll pin. Locations within the crackorigins showed evidence of craze remnants, as shown inFig. 4. This suggested the formation of micro-crazes aspart of the crack initiation. Locations adjacent to the ap-parent crack origins revealed features indicative of