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Experimental fatigue and durability assessment plays a prominent role in light-weight design. There is broad evidence, however, that the fatigue strength of materials and structures determined under constant amplitude loading may be not relevant—quantitatively or even qualitatively—for variable amplitude or spectrum loading. Various phenomena both on a microstructural and a macrostructural level contribute to this finding as discussed in the paper including consideration of the fatigue limit, the role of crack growth mechanisms and fracture surface topography, residual stresses, scatter, relative proportions of crack initiation and crack propagation phases, detection of fatigue-critical locations, and ranking of materials and design details. Some general aspects regarding the application of variable amplitude testing for design and clearance purposes such as the required experimental efforts are also considered.

Within a continuing program on high tensile graphite/epoxy composite, stress-strain, axial fatigue, and compliance behavior of unnotched, notched (3-mm diameter hole), and jointed specimens made of [O2/±45/O2/±45/90]s T300/914C laminates (177°C curing temperature) have been studied. In addition, the behavior of unnotched specimens cut from (1) the same laminate but with the longitudinal specimen axis now perpendicular to the zero-degree fiber direction, and (2) the high modulus fiber laminate with the same build-up was investigated. Stress-strain curves, S-N curves, and increase-in-compliance versus percentage-of-total life curves were determined for all specimen types for stress ratios, R, ranging from R = +5.0 (compression-compression cycling, C-C) to R = +0.1 (tension-tension cycling, T-T). An overall comparison of results from specimens with different stress raisers shows that the stress raisers diminish fatigue strength in the low-cycle range, but in the high-cycle range their influence has vanished. Effective stress concentrations were found to be different for compression and tension. During T-C cycling, increase of compliance was lowest for the fastener-filled no-load transfer joint and largest for the single-shear load transfer joint. The large compliance changes of the load-transfer specimens were attributed to increased bearing damage. In general, the scatter in static and fatigue strength was found to be comparable with that for similar features in metals. When the plain material was loaded transversely instead of longitudinally, static and fatigue strength were lower by a factor of about 3.

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Bei praktisch allen Abschatzungen der Schwingfestigkeit eines Bauteils miissen Kompromisse bei der Auswahl der Bemessungs-Diagramme eingegangen werden. Die fur das Bauteil nicht zutreffenden Parameter der Bemessungsunterlagen miissen recht pauschal durch Abschlagfaktoren beriicksichtigt werden. Zusammen mit den Ungenauigkeiten der Miner-Regel konnen sich daraus recht erhebliche Fehleinschatzungen der Betriebslebensdauer eines Bauteiles ergeben. Wenn das Bauteil zur Verfiigung steht, konnen diese Unsicherheiten durch einen Versuch mit betriebsahnlichem Kollektiv, verbunden mit einer Relativ-Miner Rechnung zum grogten Teil vermieden werden. Aus dem gleichen Grunde sollten alle Kenntnisse uber das Schwingfestigkeitsverhalten des Bauteils im Betrieb bei der Berechnung der Lebensdauer beriicksichtigt werden.