Hair Breakage 254 As described in the previous chapter, in mechanical testing it is generally useful to consider the magnitude of the stress (i.e. force per unit area), rather than the force itself. Therefore, prior to testing, the dimensions of all fibers are measured using the laser micrometer attachment on a conventional Diastron 675 Mini Tensile Tester (MTT). Throughout this chapter, fatigue testing results will be compared to findings from conventional stress-strain experiments, which were performed using either a Diastron model 675, or 600 MTT. All instruments are small enough to be housed in benchtop environmental chambers which allow precise control of the relative humidity. Note on setting an upper limit on cycles-to-fail: As mentioned already, fatigue experiments under relatively low stress conditions can result in a great number of cycles before failure. Therefore, to aid with the time required to perform such experiments, it is useful to set an upper limit (300,000 cycles) above which the fiber is considered to have survived, and the equipment moves on to the next sample. As will be described, it is still possible for these survivors to be included in analyses by invoking the concept of censored data. Figure 1. Diastron CYC 800 Fatigue Tester (picture courtesy of Diastron Limited, Andover, UK)