EFFECT OF THE FREQUENCY OF THE ALTERNATING LOAD ON FATIGUE DELAMINATION OF LAMINATES

When a composite laminate is loaded by a tensile or compressive force, and the laminate has free edges, high interlaminar stresses would be built near these edges, depending on the specific lay-up. Finite eleme nt technique was used to calculate the interlaminar stresses. It is sh own that, for graphite fibers in an epoxy matrix quasi-isotropic lamin ate, these interlaminar stresses are quite high relative to the in-pla ne stresses. For an alternating load, the stress will alternate too. A nalysis of these effects has been done. The matrix material was modele d by viscoelastic behavior to determine the heat generation from these alternating stresses and the heat dissipation was also considered. Th e analysis shows that heat build-up will occur when the heat conductio n is slower than the heat generation. Such a situation exists for most of the composites with polymeric matrices. When the frequency of the alternating load changes, the viscoelastic complex modulus will also b e changed, but very slowly, and, therefore, the heat generated per cyc le load would not change much. On the other hand, the heat dissipation per cycle load would be much smaller for a higher load frequency, and therefore heat would build-up and the strength, at these particular r egions, would be degraded. An experimental program on [0-degrees, +/-4 5-degrees, 90-degrees]2s graphite/epoxy laminate confirmed the analysi s. Fatigue alternate loading of this laminate with frequencies of 1, 2 .8, and 10 Hz showed degradation of the fatigue strength with frequenc y. The failure process always starts with delamination at the free edg es, followed by crack propagation and finally, by buckling of the rema ining thin laminae.