ABSTRACT

The aim of fracture mechanics theories is to describe the influence of cracks on the mechanical performance of structures. It was not realized until the catastrophies with the Liberty Ships in the 1940s that efficiently loaded conventional materials (metals) suffer from crack formation during service. These cracks generally develop and grow under varying amplitude loading until the structure finally fails catastrophically. With the aid of fracture mechanics it should be possible now to determine a critical crack length which leads to catastrophic failure under the expected load spectrum. Knowing this crack length, it is now the task of the inspection and maintenance engineers to ensure that (existing) cracks do not grow to the critical length in the period up to the next inspection. This procedure assumes that the state of damage of the structure can be determined during inspection with adequate nondestructive testing equipment. Especially for aircraft maintenance, this procedure is proved to be economically efficient and guarantees a satisfying degree of safety. This must also be the final aim for structures produced from fiber-reinforced materials (in this case fiber-reinforced metals). Our experience with this class of materials is, however, still too small to make efficient use of them. Metal matrix composites (MMCs) have been produced and investigated for about 30 years. The fibers (or whiskers, or particles) used are mainly those which are strong and stiff but consequently brittle (e.g., boron, SiC, Al20 3 fibers). The negative aspect of the brittleness can be partly reduced by embedding the fibers in a ductile matrix. Metal matrix composites thus consist of fibers (whiskers or particles), matrix, and the range between fiber and matrix, the interface (or interphase), where the matrix is coupled to the fiber. Thus it is clear that the mechanical behavior (e.g., the fracture toughness) of this

material is influenced by these components. For this reason cracks can form in any of these components. This leads to the phenomenon that, in contrast to conventional metals, failure mostly takes place not as a result of one or a few cracks but as a result of statistically distributed damage in one or more components which finally leads to catastrophic failure with or without the formation of one or more macrocracks. This makes fracture mechanical concepts difficult, especially if one likes to use these concepts to guarantee safe inspection and maintenance procedures and thus to make efficient use of MMC structures.