Strain Stress cracking

When environmental stress cracking tests are carried out to detect the time to onset of crazing, results are generally obtained as a function of applied strain, and possibly also of temperature. There is then an apparent threshold strain below which crazing does not occur for a very long time. A discussion of how this data can be treated has been given by Wright [18]. 

When a plastic exposed to air is subjected to a stress or a strain below its yield point, cracking can occur after a very long duration. The simultaneous exposure to a chemical environment under the same stress or strain can lead to a spectacular reduction of the failure time. The accelerated cracking in this way corresponds to environmental stress cracking (ESC). 

Abstract When subjected to a mechanical loading, the solid phase of a saturated porous medium undergoes a dissolution due to strain-stress concentration effects along the fluid-solid interface. Through a micromechanical analysis, the mechanical affinity is shown to be the driving force of the local dissolution. For cracked porous media, the elastic free energy is a dominant component of this driving force. This allows to predict dissolution-induced creep in such materials. 

Stress and strain Nature of environment Temperature Molecular weight Molecular architecture Crystallinity Orientation > < Dissolution Softening Stress-cracking Embrittlement Chemical degradation Photochemical degradation k Biological degradation... 

Kambour et al. performed extensive studies on the mechanisms of plasticization [18-25]. The correlation observed between the critical strain to craze and the extent of the glass-transition temperature (Tg) depression speaks strongly in favor of a mechanism of easier chain motion and hence easier void formation. In various studies on polycarbonate [19,24], polyphenylene oxide [20], polysulfone [21], polystyrene [22], and polyetherimide [25], Kambour and coauthors showed that the absorption of solvent and accompanying reduction in the polymer s glass-transition temperature could be correlated with a propensity for stress cracking. The experiments, performed over a wide range of polymer-solvent systems, allowed Kambour to observe that the critical strain to craze or crack was least in those systems where the polymer and the solvent had similar solubility values. The Hildebrand solubility parameter S [26] is defined as... 

A constant tensile-stress method is outlined in ISO 6252, in which a test specimen is exposed to a constant tensile force while immersed in a stress cracking agent so as to determine the time to rupture under a specified stress. This uniaxial test leads to the determination of the lifetime of the specimen with accuracy, but it is time consuming and requires complex equipment. Variations of this test include a tensile creep test that monitors the strain and a monotonic creep test that uses a constant stress rate instead of a fixed stress [1]. 

Another bent-strip method for evaluating the ESCR is presented in ISO 4599. In this test, strips of a plastic are positioned in a fixed flexural strain state and exposed to a stress cracking agent for a predetermined period. The test is uniaxial and simple to perform, and the deformation is constant. Because of the molecular chain relaxations, the stress state is well defined only at the beginning of the test. After exposure to the medium, the strips are removed from the straining rig, examined visually for changes in appearance, and then tested for some indicative property such as tensile strength. 

Turnbull A, Maxwell AS, Pillai S (2000) Comparative assessment of slow strain rate, 4-pt bend and constant load test methods for measuring environment stress cracking of polymers. Polym Test 19(2) 117—129... 

Al-Saidi LF, Mortensen K, Almdal K (2003) Environmental stress cracking resistance behaviour of polycarbonate in different chemicals by determination of the time-dependence of stress at constant strains. Polym Degradat Stabil 82(3) 451—461... 

The critical stresses and strains for cracking, the toughness of a layer,... 

Environmental Stress Cracking - Cracking or crazing that occurs in a thermoplastic material subjected to stress or strain in the presence of particular chemicals or weather conditions or as a result of aging. Also called ESC. 

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