Failure under Sustained Loads

The terms A and B are material constants dependent on the environment and the rate of loading. The curve also indicates a threshold value of AGi(AGith), defined as that value of AGj below which crack growth is too small to be detected, even after 10 cycles have been applied. For a given bonded system it is found that Gjth iscc ic- 

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Delayed Failure see under Sustained Load Failure. 

The fatigue strength under sustained loads and cyclic loads is an important material characteristic. In all structures, the long-term stress should be maintained below the fatigue strength and only for infrequent and exceptional accidental situations may higher stress be admitted without serious risk of a failure. Also, in many structures, variable loads and their range between the maximum and minimum values determine their behaviour for example. 

When polymers are applied for any practical purpose it is necessary to know, among other things, what maximum loads they can sustain without failing. Failure under load is the subject of the present chapter. 

Filaments do not fail in a catastrophic manner due to the nested basal plane structure and under-go a pull-out or sword-in-sheath failure. Under a tensile load, a surface initiated crack is arrested after progressing through several layers into the fiber but, may well crack rapidly around the circumference. The stress concentration within the fiber increases and the process is repeated at an internal flaw elsewhere along the length until eventually, the diameter left cannot sustain the applied load and a pull-out failure occurs (Figure 8.23). The brittle failures of a PAN and a pitch based carbon fiber are compared in Figure 8.24 with a pull-out failure of a VGCF treated at 2200°C [51]. 

Stress distribution shear lag solution D A DILLARD Stresses in shear joints Creep A D CROCOMBE Occurrence protection against creep failure Durability creep rupture D A DILLARD Creep under sustained stress Durability fatigue D A DILLARD Effect of cyclic loads... 

Creep strength n. The initial stress at which failure occurs after a measured time under load. Thus, creep strength (at any temperature) must be labeled with the time to failure. Like creep modulus, creep strength is useful to designers for applications in which plastic articles or members will carry sustained loads. 

They tested at 1800 cycles per minute, 10 cycles per hour, and 0.8 cycles per hour the highest test frequency was least severe and the lowest frequency the most severe, in terms of damage per cycle, as indicated by the number of cycles to failure shown in Figure 25. In terms of hours to failure there is a less clear-cut distinction, but, at low stress levels, the lowest frequency would probably produce the earliest failures. Specimens maintained under a sustained load were also tested and no failures were reported during the time required to fail the specimens subjected to cyclic loading. 

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