environ qxd 5 13 04 9 12 PM Page 2 Environmental Stress Cracking THE PLASTIC KILLER Environmental stress cracking is involved with some 25 of plastic part failures Jeffrey A Jansen Stork Technimet New Berlin Wisconsin E nvironmental 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 50 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 yield strength Critical strains and stresses for craze initiation with the most active fluids can be as low as 0 1 and a few megapascals Stresses and strains due to processing and or assembly can often exceed the critical condition Rapra Technology Characteristics of ESC Environmental stress crack failures share several typical characteristics Brittle fracture ESC failures are caused by brittle fracture even in materials that would normally be expected to produce a ductile yielding mechanism The crack initiation sites for ESC failures are always on the surface They normally correspond to localized areas of high stress such as microscopic defects or points of stress concentration The initiation location is generally related to direct contact with an active chemical agent either liquid or gas Multiple cracks Multiple individual cracks are initiated and these subsequently coalesce into a unified fracture Numerous crack origins and the corresponding unions are illustrative of an ESC failure mechanism Smooth morphology The crack origin areas usually exhibit a relatively smooth morphology indicative of slow crack growth However aggres ADVANCED MATERIALS PROCESSES JUNE 2004 environ qxd 5 13 04 9 13 PM Page 3 Latch handle failure A high number of latch handles on an enclosure suddenly began to fail after a relatively short time Standard service included periodic actuation of the handles at normal exterior temperatures The handles were molded from a commercial grade of a polycarbonate polyacrylonitrile butadiene styrene PC ABS resin blend The handle assembly is held together in a base unit with a metallic roll pin and a spring A review of molding and assembly processes revealed no variations to account for the sudden change in performance A visual inspection of the failed parts showed significant cracking consistent across all of the failed parts The cracks were present within the molded boss that secured the roll pin and had a shape that was irregular but continuous Upon disassembly of the units additional noncatastrophic cracks were apparent within similar locations around the boss hole The fracture surface displayed features characteristic of brittle fracture with many crack origins adjacent to the inner diameter surface A typical crack is shown in Fig 3 An oily residue was readily apparent on and adjacent to the fracture surface Typical fracture surfaces were further examined via scanning electron microscopy SEM The SEM inspection of the fracture surface confirmed the presence of multiple apparent crack origins along the inner diameter of the molded boss in an area that had been in direct contact with the roll pin Locations within the crack origins showed evidence of craze remnants as shown in Fig 4 This suggested the formation of micro crazes as part of the crack initiation Locations adjacent to the apparent crack origins revealed features indicative of brittle fracture and secondary cracking was also apparent Examination of the final fracture zone showed continued