| Popis: |
An important result of our investigations of the fatigue response of composite materials has been the identification and characterization of the effect of frequency of cycled loads (or strain). Frequency has been found tosignificantly influence the fatigue response of boron/epoxy and boron/aluminum plate specimens with a center hole. A large data base for strain-controlled testing of boron/epoxy and boron/aluminum at different frequencies has been reported earlier. More recently, we have extended the data base to include load-controlled test results for boron/epoxy and boron/aluminum using stiffness change as the primary damage parameter, and allowable stiffness change as a failure criterion. In general, the data show that low-frequency fatigue loading produces more concentrated, local, hole-related damage with corresponding greater specimen stiffness reduction. In contrast, high-frequency loading produces more dispersed damage with more axial micro-structure-related damage and less stiffness reduction. Since these results are true for two widely different-materials and different testing conditions, the authors believe that these effects are representative of the mechanics of the phenomenon, and do not represent isolated effects. It appears that frequency effects can be expected in many fiberous composites and that they are large enough to be of significant consequence in many engineering applications. |
