Whitening, stress

In the direct design procedure the assumption is made that no abrupt changes occur in cross-section, discontinuities in the surface, or holes through the member. This is not the case in most structural parts. The stresses produced at these discontinuities are different in magnitude 

There are conditions of loading a product that is subjected to a combination of tensile, compressive, and/or shear stresses. For example, a shaft that is simultaneously bent and twisted is subjected to combined stresses, namely, longitudinal tension and compression, and torsional shear. For the purposes of analysis it is convenient to reduce such systems of combined stresses to a basic system of stress coordinates known as principal stresses. These stresses act on axes that differ in general from the axes along which the applied stresses are acting and represent the maximum and minimum values of the normal stresses for the particular point considered. There are different theories that relate to these stresses. They include Mohr s Circle, Rankine s, Saint Venant, Guest, Hencky-Von Mises, and Strain-Energy. 

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Polypropylene polymers are typically modified with ethylene to obtain desirable properties for specific applications. Specifically, ethylene—propylene mbbers are introduced as a discrete phase in heterophasic copolymers to improve toughness and low temperature impact resistance (see Elastomers, ETHYLENE-PROPYLENE rubber). This is done by sequential polymerisation of homopolymer polypropylene and ethylene—propylene mbber in a multistage reactor process or by the extmsion compounding of ethylene—propylene mbber with a homopolymer. Addition of high density polyethylene, by polymerisation or compounding, is sometimes used to reduce stress whitening. In all cases, a superior balance of properties is obtained when the sise of the discrete mbber phase is approximately one micrometer. Examples of these polymers and their properties are shown in Table 2. Mineral fillers, such as talc or calcium carbonate, can be added to polypropylene to increase stiffness and high temperature properties, as shown in Table 3. 

ABS materials are widely used as impact modifiers but cause opacity and have only moderate aging characteristics. Many grades show severe stress-whitening, generally a disadvantage, but a phenomenon positively employed in labelling tapes such as Dymotape. 

MBS materials vary considerably in their tendency to cause stress-whitening in PVC and in their effect on impact strength. They are generally considered to lead to better aging than ABS additives but are marginally more expensive. 

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

Overall behavior. It is the time-dependent strain at which crazing, stress whitening, and rupture decreases with a... 

Good stress-crack and craze resistance, brittle Good processibility, no stress whitening Higher-heat styrenic, brittle... 

Fig. 15 Craze-like defect formed in the stress-whitened zone in front of the crack in a POM compact tension specimen loaded at 100 °C (from [65])...

Internal cavitation was proved by comparison of the initial particle diameter with the diameter measured on a fracture surface (Huang et al., 1993b). An increase of about 20-70% of the initial volume was found, depending on the temperature. This voiding process participates in the energy consumption and is the cause of the stress whitening effect observed on deformed samples. 

The presence of particles can modify the fracture surface from a mirror-like surface (for a brittle material), to a rough stress-whitened surface. The roughness can act as a multiplication factor for the absorbed energy. 

A ductile behavior accounting for intense shear yielding. It is characterized by stable crack propagation, entire stress-whitening of the fracture surface and is accompanied by more or less pronounced shear lips. 

Failure Mechanisms. BPF polycarbonate develops crazes at ascending stresses and fractures in a pseudo-brittle manner similar to polystyrene or PMMA. At room temperature the block polymers develop few separate crazes. As the yield is approached, shear bands grow from the edges. Fracture initiates at an edge from a point where the two shear bands initiated. When a neck forms, the plastic strain in the neck is ca. 80% however fracture occurs shortly after the neck is formed so that the ultimate elongation of the specimen is only 10 or 12%. The shear bands and necks show some stress whitening (Figure 9). 

Thus, reinforcement of polypropylene with as little as 3 vol % of nylon fibers increases the elongation at which necking and stress whitening occur from 10% for the unreinforced matrix to 20% for the composite. We call this phenomenon "retarded necking. ... 

SBC are generally attacked by many hydrocarbon solutions. Degreasing of parts in many solvents can result in stress whitening, cracking or premature failure of SBC parts. Some bodily fluids contain high levels of lipids and are considered SBC stress cracking agents. Care should be taken any time SBC parts are exposed to any stress crack medium to ensure suitability for use. 

Table 3. Instability Phenomena in Amorphous Polymers Comparison between the Extension Ratio, X,", at the Initiation of Stress-Whitening and the Maximum Extension Ratio, X, of Chains between Entanglement Points...

Figure 25 shows that the tensile behavior of PMMA and PS, stretched some 10-30 °C below Tg, is very similar to that of PC. Like PC both PMMA and PS exhibit extensive stress-whitening at high extension ratios, k", where the peak or inflection of the stress-strain curve is observed. In Table 3, the value of k" measured for PMMA is compared to the maximum extension ratio, k , of chains between entanglement points. The fact that k" = k provides further evidence that the cavitation in PMMA must be attributed to the same deformation mechanism as discussed in Section 4.2 for PC. The value of k" = kikj" = 6.8 measured for pre-oriented PS is higher than the theoretical value of k = 4.1 included in Table 3. This result is not... 

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