Cooling stress cracking

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

General rules for the occurrence of stress corrosion cannot be given, since the phenomenon is very specific for certain combinations of polymer and environment, and is also dependent on the processing conditions in manufacturing the article (such as the occurrence of cooling stresses and orientations). When plastics are used in articles which are subjected to mechanical stress, such as pipes, crates, bottles, screw-caps, etc., the risk of stress corrosion in the presence of fat, soap or organic liquids, should always be taken into account. Proper choice of material and dimensions can minimize the occurrence of crack formation. 

Discoidal stress cracks Disc-shaped cracks caused by stress in a material such as amber, when it is heated and cooled. Sometimes called sun spangles or lily pads . 

Chapter 6 dealt with residual stresses that occur when products are cooled rapidly from both the sides. There are biaxial compressive stresses in the surface layers and biaxial tensile stresses in the interior. If a hole is drilled through such a product, it cuts through the tensile stress region, and acts as a stress-concentrating feature with a q value of 2. If there is ingress of a stress cracking fluid, radial cracks may form from the bore of the hole, perpendicular to the residual circumferential stresses. These cracks will be at the mid-thickness of the product. 

Early thermal contraction cracking, also known as thermal stress cracking or curling, is caused by excess temperature gradients (deferential) or moisture content between the top and bottom of a slab, attributed to rapid cooling when ambient temperature drops and, in the case of thick (mass) concrete elements, to excess heat generation during hydration of the cement. 

Isotactic polybutene crystallizes into three different forms. When it cools from the melt, it originally crystallizes into a metastable crystalline one. After several days, however, it transforms into a different form. Noticeable changes in melting point, density, flexural modulus, yield, and hardness accompany this transformation. The third crystalline form results from crystallization from solution. The polymer exhibits good impact and tear resistance. It is also resistant to environmental stress-cracking. 

Analysis of extruded and injection molded iPP shows that a skin-core structure is formed due to high shear and thermal gradients upon cooling. These frozen-in tensile stresses in the skin can act as the stress component in the environmental stress cracking process [4]. In injection moldings, molecular weight has a secondary effect. This effect is that residual orientation in the direction of flow increases with higher... 

The non-electrochemical techniques include direct immersion of materials samples in the test fluid in either the laboratory or plant. These s lmples sometimes have an artificial crevice generated with a serrated washer. They may be welded to determine the effects of welds and weld heat affected zones. Real-time information can be obtained using electrical resistance probes. Heat transfer effects can be evaluated by having a test sample that is exposed to the corrodent on one side and the other side heated or cooled. Stressed samples are used to evaluate stress corrosion cracking tendencies [33]. 

The quality of cooling water and steam must he considered when selecting materials of construction. The chosen materials may be resistant on the process side, but fail on tbe utility side. For example, 316 stainless steel may be applicable for vertical heat exchanger tubing that handles a weak organic acid on the process side, but it may fail from chloride stress cracking on the waterside. A duplex stainless steel may be a better choice. 

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