Fatigue dynamic

Fatigue data is usually published in the form of Wohler curves where stress or strain amplitude is plotted against the number of cycles to failure on a logarithmic scale. 

Dynamic fatigue is a complex issue. However, the following common observations can be made  

Fatigue strength is sensitive to stress concentration such as that caused by notches or sharp corners. It depends on the stress frequency - its effect at low frequencies is much more severe, and therefore failure can occur earlier than that predicted using high-frequency tests. The number of cycles to failure decreases with increasing temperature. 

Reproduced with permission from I.M. Ward and J. Sweeney, An Introduction to the Mechanical Properties of Solid Polymers, Wiley, Chichester, UK, 2004. Copyright Wiley, 2004. 

Reproduced with permission from D. Tripathi, Practical Guide to Polypropylene, Rapra Technology, Shrewsbury, UK, 2002. Copyright Rapra Technology, 2002 

There is a large literature concerning the dynamic fatigue of polymeric 

When designing dynamic fatigue tests the experimentalist has a wide range of variables to consider which contribute in different ways to the fatigue process. The following list is an extended version of that compiled by Andrews [4], namely  

A periodically varying load or stress system with a characteristic stress amplitude, cTa[o-a = l/2(o-max- O min)]  

A characteristic wave-form (sinusoidal, square, etc.) for both the stress and strain  

The ambient and internal temperature of the specimen which in general will not be the same  

---

Fatigue or dynamic fatigue can be defined as the decrease in load bearing capacity with time under cyclic or intermittent load, the term static fatigue being sometimes used to describe creep-rupture (see Sections 4.9.2 and 6.10). 

The dynamic fatigue can be fair or good for certain grades if care is taken to limit the strains by restricting the stresses to values in keeping with the low modulus. 

Figure 4.121. Polyimides examples of creep modulus (GPa) versus time (h) at 100°C and 300°C 4.26.5 Ageing Dynamic fatigue...

---