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Recent research programs have indicated that the frequency of cyclic loading has a significant effect on the fatigue life of certain composite materials and lay-ups. In some cases, reduction in loading frequency has resulted in a proportional reduction in fatigue life. In these cases, data from high-frequency fatigue tests do not provide accurate assessment of life for service loadings which occur at much lower rates. In many of these test programs, materials, lay-ups, and loading frequencies were selected to examine the effect of load frequency on matrix degradation and fatigue life. These materials and loading frequency selections are not representative of those used in aircraft structures.
The current program was performed to examine the effect of cyclic loading frequency on the fatigue life of lay-ups which represent those used in aircraft structure. Fatigue tests on three AS/3501-6 graphite/epoxy laminates were performed at frequencies ranging from 0.1 to 10 Hz. Tests of a (±45)2s lay-up gave results similar to those of Sun and Chan for the same lay-up in T300-5208 graphite/epoxy. At low frequencies, the AS/3501-6 material appeared to follow the data from T300-5208. But at 10 Hz the life for AS/3501-6 was one tenth that of the T300-5208 material.
Lay-ups having 0° plies were found to have a lesser, but still significant, effect of frequency on fatigue life. Life for these specimens increased by factors of 3 to 4 when load frequencies were increased by a factor of 100. Preliminary analyses indicate that the sensitivity of fatigue life to load frequency appears related to the matrix stress state. Fatigue life sensitivity was correlated with the ratio of matrix shear stress to applied axial stress.
composite materials, laminates, fatigue tests, frequency dependence, temperature effect, composites
Technical specialist, Structural Research, McDonnell Aircraft Co., McDonnell Douglas Corp., St. Louis, MO