Stress cracks in plastic

Hansen CM, Just L (2001) Prediction of environmental stress cracking in plastics with Hansen solubility parameters. Ind Eng Chem Res 40( 1) 21—25... 

Hansen, C.M. and Just, L. (2001) Prediction of Environmental Stress Cracking in Plastics with Hansen Solubility Parameters, Ind. Eng. Chem. Res. 40,21-25. 

The effect of liquid surfactants can powerfully accelerate stress crack formation. Nevertheless, stress crack formation in plastics must be distinguished from stress crack corrosion as known in particular in metallic materials. Corrosion is understood as the erosion of atoms from the material by chemical processes and in metals particularly by electro-chemical reactions. Additional influence by stresses leads to crack formation and brittle fracture which often resembles of the failure of stress cracks in plastics. Stress crack formation in thermoplastics is, however, a purely physical process. No chemical changes take place in the material even under the influence of surfactants. The terminology is nevertheless not completely uniform. The accelerating effect of liquids on stress crack formation in plastics is occasionally described as stress crack corrosion although no real corrosion process is connected with it. 

Andrews, E.H. (September 1966) Fracture mechanics approach to corrosion stress cracking in plastics in Proceedings of the Conference on the Physical Basis of Yield and Fracture, Oxford, (ed. A.C. Strickland), Institute of Physics The Physical Society London, p. 127. 

Some agglomerates of different materials have been observed to fail because of internal flaws driven by a number of stresses (e.g., internal tensile stress cracks in the surface plastic flow at the surface between the agglomerate and platen and shear stress within the sphere). For brittle particle agglomerates with significant internal flaws, the tensile strength is small compared to the compressive and shear strength, and failure is likely initiated by the internal tensile stress. In any case, a careful microscopic examination of failed pieces can provide much information on the dominant failure mode (Bika et al., 2001). 

Stress rupture tests on test pieces are very important under conditions where, in addition to the stress, the atmosphere is chosen to accelerate failure. The best known t> pe of test is a test of the so-called environmental. stress cracking of plastics, where the aggressis e atmosphere is a chemical that causes cracking when the material is in a strained state. These tests are usually considered as a form of chemical resistance test and are cosered in Chapter 14. Ozone cracking of rubber, also an environmental resi.stance test, is another example. 

Environmental stress cracking is considered to be the most fi-equent mode of failure of plastic materials.It was estimated in 1996 that 20% of plastics failed because of environmental stress cracking. By environmental stress cracking, we understand a failure in which materials in contact with a fluid are subjected to stress. " It is quite difficult to simulate environmental stress cracking in the labo-... 

A. Lustiger, Understanding Environmental Stress Cracking in Polyethylene, in Medical Plastics Degradation, Resistance Failure Analysis ed. R.C. Portnoy (SPE, Plastic Design Library, Rice RC, Tritsch DE, 1998), pp. 66-71... 

This relatively new family of adhesives does not yet comprise enough individual formulations to cope with all the potential industrial applications where they could make a major contribution. Gap-filling remains a problem with many formulations and some induce stress cracking in susceptible plastics. 

A. Lustiger, Understanding environmental stress cracking in polyethylene. Medical Plastics and Biomaterials Magazine, published July, 1996 accessed from www.devicehnk.com. 

These plastics exhibit low dielectric losses and stable dielectric constants over a broad range of temperatures and frequencies. The chemical resistance of the materials is their weakest point. Although resistant to most common solvents, acids and alkalis, they exhibit stress cracking in the presence of organic ketones, esters and chlorinated hydrocarbons. Polysulphones are used in the electronics field for connectors, chip carriers and capacitor dielectrics, and they are the first of the high temperature thermoplastics to be used for printed wiring board fabrication. 

The determination of stress loads in plastic parts is based on creep rupture tests using different exposure media (see Section 2.5.5.2.1.1). Figure 2.28 shows the time to crack formation and the time to fracture for specimens under constant stress in a test medium. If a part made from the same material is exposed to ethanol for 1000 s and does not exhibit crack formation, it can be assumed that the internal stress level is below 3 MPa. 

In common with many organic liquids, liquid anaerobics can embrittle and stress-crack some plastics and, in general, testing needs to be done before using them on plastics. Speeding up curing with primers will often eliminate this problem. 

For plastics, we do have the D-3929, Practice for Evaluating the Stress Cracking of Plastics by Adhesives Using the Bent-Beam Method. It recognizes that some adhesives may interact with plastic adherends in such a way as to induce areas of weakness leading to stress cracking. 

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