Stiffness Degradation and the Fatigue Behavior of Carbon Fiber Sheet Molding Compounds on the Component Level
Sprache des Vortragstitels:
ICFC 8 - the Eighth International Conference on Fatigue of Composites
Sprache des Tagungstitel:
Carbon Fiber Sheet Molding Compounds (CF-SMC) are discontinuous fiber reinforced composites with high fiber fractions of 40 vol% or more. The stochastic meso-scale morphology and typical fiber lengths between 25 and 50 mm cause large scatter in fatigue tests of standardized specimens. This presents a major obstacle in the estimation of practical endurance limits for CF-SMC applications. In a previous publication of the authors, significant stiffness degradation was observed during uniaxial cyclic loading on the coupon level and the fatigue strength was defined by the weakest link. In this study the discussion of the fatigue behavior of CF-SMC is extended to the component level. Sub-components of a hat profile geometry were compression molded in a low-flow setup. Glass fiber tracers were used to investigate the fiber orientation via X-ray computed tomography. The component tests were performed under monotonic and cyclic 4-point bending loading conditions (R=0.1). Digital image correlation and in-situ passive thermography were used to monitor failure initiation and damage progression. One hat profile was further equipped with five strain gauges to record the local strain evolution. The component level tests showed considerably less scatter than the standardized fatigue tests. All tested parts revealed a significant initial stiffness drop of about 20 % in the early stages of the fatigue life. This is followed by a continuous but slighter stiffness degradation for most of the fatigue life until a sharper drop occurs before final failure. Weak links in the meso-scale morphology resulting in localized damage sites could be identified by temperature peaks in the thermographic images. A visible local increase in temperature and strain concentrations before rupture indicated the final failure sites. The findings show the importance of component level investigations and the need for stiffness degradation to be considered in the fatigue life prediction of CF-SMC structures.