In-situ compression of high performance polymeric and related fibers within the Scanning Electron Microscope

Abstract

A large portion of the interest currently given high performance polymer fibers stems from their potential use in composite construction. Although many of these fibers display good tensile properties, compressive strength is a major concern. There are many studies which have observed the compressive failure morphology of polymeric fibers. These studies usually involve the postmortem observation of samples which have failed, either partly or completely, following compressive strain. Observations have included such events as kink band formation, brittle failure and other modes as well. But to our knowledge these failure modes have not been recorded, in process, using the scanning electron microscope (SEM). In this article we introduce a technique which allows observation of compressive failure within the SEM, or using light microscopy (LM), in process. We also present some results unique to this procedure. This technique allows us to follow this "process" of compressive failure including both it's initiation and the more advanced stages of failure. Since the compressive failure of polymeric fibers is a process rather than an instantaneous event we have been able to record this process of failure, in it's various stages, in relatively high resolution SEM micrographs. Our methodology involves the use of relatively low accelerating voltages to minimize charging artifacts associated with damaged areas of the fiber. This induced damage can disrupt the 3 - 5nm sputter coating used to prevent such artifact resulting in uncoated fiber being exposed to the electron beam. This technique also lends itself readily to video recording. We have observed compressive failure processes for Kevlar 49®, Spectra 1000®, Hercules AS-4® carbon and various experimental fibers such as Polybenzobisoxazole (PBO). Our observations have included such modes of failure as kink band formation, delamination, buckling and brittle failure. It is noted that some of these processes occur simultaneously in any given fiber, and with the exception of brittle failure, these processes can be recorded at every stage of development visible to the SEM.