Fatigue stress based approach

Many factors, in addition to stress, strain, amphtude, etc., influence the fatigue life of materials. Here, the effects of (a) stress-based and (b) strain-based evaluations of fatigue life will be considered for low- and high-cycle cases. In the past and even now, the stress-based approach is quite common. 

The engineering approach recognizes that many plastic products have been in use at least since the 1940s and have been exposed to long-term static and/or dynamic loads based on varying environmental conditions such as temperature. Designers thus consider creep, fatigue, stress, temperature, time, and other data (see Chapters 3-5). 

The study of the fracture and fatigue properties of UHMWPE materials has received considerable attention over the last 2 decades. The fatigue resistance of UHMWPE is now widely accepted as critical to the structural performance of orthopedic implant bearings, and such data is now commonplace in regulatory approval submissions. With regard to fatigue properties, both the total life (stress analysis based) and defect tolerant (fracture mechanics based) approaches... 

A more common method for medical devices is to run the life test until failure occurs. Then an exponential model can be used to calculate the percentage survivability. Using a chi-square distribution, limits of confidence on this calculation can be established. These calculations assume that a failure is equally likely to occur at any time. If this assumption is unreasonable (e.g., if there are a number of early failures), it may be necessary to use a Weibull model to calculate the mean time to failure. This statistical model requires the determination of two parameters and is much more difficult to apply to a test that some devices survived. In the heart-valve industry, lifetime prediction based on S-N (stress versus number of cycles) or damage-tolerant approaches is required. These methods require fatigue testing and ability to predict crack growth. " ... 

Rates of CF crack propagation are uniquely defined by the linear elastic fracture mechanics stress intensity factor range that combines the effects of applied load, crack size, and geometry 17,40. The similitude principle states that fatigue and CF cracks grow at equal rates when subjected to equal values of AK [6-S]. The dal N versus AK relationship may be complex however, an effective approach is based on a power (or Paris) relationship of the form [4/]... 

The criterion used here is the approach used in the ANSI B31.3 Petroleum Refinery Piping, that is, to limit the maximum calculated principal stress to the allowable stress range rather than using the shear theory of failure. The value 1.25(.S a + provides a safety margin against the possibility of fatigue due to localized stresses and other stress conditions (see Fig. 2.1). Obviously, stresses must be computed both with and without thermal expansion, since allowable stresses are much smaller for conditions without thermal expansion. As an additional safety precaution, the computed stresses are usually based on the modulus of elasticity E at room temperature [ 8 ]. 

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