Design for fatigue with metal foams

In structural applications for metallic foams, such as in sandwich panels, it is necessary to take into account the degradation of strength with cyclic loading. A major cause of this degradation is the nucleation and growth of cracks within the foams. In a closed-cell foam, the cell faces are subject to membrane stresses while the cell edges bend predominantly. Consequently, crack initiation and growth occurs first in the cell faces and then progresses into the cell edges for a closed-cell foam. There is accumulating evidence that an additional fatigue mechanism operates in the cyclic deformation of foams: cyclic creep, also known as ratcheting, under a non-zero mean stress. When a metallic alloy is subjected to cyclic loading with an accompanying non-zero mean stress, the material progressively lengthens under a tensile mean stress, and progressively shortens under a compressive mean stress. Consequently, for a metallic foam, the cell walls progressively bend under a compressive mean stress and progressively straighten under a tensile mean stress. This leads to a high macroscopic ductility in compression, and to brittle fracture in tension.

We shall show later in this chapter that a characteristic feature of metallic foams is their high damage tolerance: the degradation in strength due to the presence of a hole or crack in a foam is usually minor, and there is no need to adopt a fracture mechanics approach. Instead, a design based on net section stress usually suffices. A word of caution, however. It is expected that there should be a critical crack size at which a transition from ductile to brittle behavior occurs for tensile loading and tension-tension fatigue of a notched panel. The precise value of the notch size for which the behavior switches has not yet been determined, but is expected to be large.

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