Ductile failure static load

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

The test for stress-rupture of water pipes is illustrated schematically in Figure 12.1. Below a critical load the relation between static load, expressed as stress or pressure, and time to failure follows a power law relation as described in Chapter 7. Above the critical load ductile... 

SEM examination showed ductile features (Fig. 5.16) on the center of the bolt and intergranular features on the outer circumference (Fig. 5.17). Both sides of cracking were caused by static overload failure with the ductile features at the center present throughout. The intergranular appearance at the edge is suggestive of embrittlement, leading to premature failure at loads below those expected. 

FIG. II-5. Conversion of a triangular dynamic pressure into an equivalent static load. peak of the dynamic triangular puke (Pa) P static pressure (Pa) t duration of the triangular pulse (ms) T first excited period of the one-degree-of-freedom structure (ms) iax- tnaximum displacement at failure (m) Ay maximum elastic dkplacement (m) p maximum ductility. 

Unlike ductile metals, composite laminates containing fiber-reinforced thermosetting polymers do not exhibit gross ductile yielding. However, they do not behave as classic brittle materials, either. Under a static tensile load, many of these laminates show nonlinear characteristics attributed to sequential ply failures. One of the difficulties, then, in designing with laminar composites is to determine whether the failure of the first ply constitutes material failure, termed first-ply failure (FPF), or if ultimate failure of the composite constitutes failure. In many laminar composites, ultimate failure occurs soon after first ply failure, so that an FPF design approach is justified, as illustrated for two common laminar composites in Table 8.9 (see Section 5.4.3 for information on the notations used for laminar composites). In fact, the FPF approach is used for many aerospace and aircraft applications. 

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