8.1 INTRODUCTION

Dimensional stability with time in the absence of applied stresses was discussed in Chapter 7. When applied stresses are added to significant times, we become concerned with creep. The causes of temporal instability must be kept in mind when the applied stress is low (microcreep) for then the deformations due to creep may be on the same order of magnitude as those due to relief of residual stresses from fabrication, post-curing, thermal, physical or chemical aging, phase transformations, metallurgical processes such as annealing, settling due to gravity, or slow microcracking. Techniques to differentiate these mechanisms include weight-change measurements, glass transition temperature tests, matrix sensitive mechanical tests such as interlaminar shear, microscopy and surface roughness measurements [1].

Creep has been defined as a time-dependent permanent strain at a constant level of applied stress [2]. Cases of variable applied stress might be included, such as creep during fatigue conditions. The term creepocity has been used to denote the percentage increase in strain during some time period [3]. Engineers make the distinction between regular creep and microcreep. The former implies the largest deformation rates, up to 1%/min [4], with damage accumulation eventually leading to stress rupture. Microcreep is conveniently related to another precision engineering property-the microyield strength (MYS) (see Section 3.12). Microcreep may be defined as creep that occurs at MYS stress levels (and hence, strains below about 10 ^?5). The MYS test is inherently imprecise, since creep may occur during the short-term loadings and will thus modify...