During the quadrennial term 1979–1982, major advances have been made in our knowledge of the rheology of the oceanic lithosphere by the skillful combination of experimental and theoretical rock mechanics, seismology and marine geophysics in increasingly sophisticated models for the flexure of the oceanic lithosphere at seamounts and island chains, along transform faults, and at subduction zones. The relative simplicity of plate bending geometry, thermal history, and mineralogical and chemical compositions of the oceanic plates in these settings make the geophysical observations very powerful constraints on the in situ rheology of the oceanic lithosphere. In the first part of this paper, I review the laboratory work on materials appropriate to the oceanic lithosphere with emphasis on contributions during the quadrennial period and the need for future work. The important results of flexure models incorporating realistic material properties are then summarized.

Rheology of the lithosphere

Microcracks in rocks: a review

Influence of shape and locations of initial 3-D cracks on their growth in uniaxial compression

Fracture of rocks containing a multitude of pre-existing cracks is considered from both theoretical and experimental points of view, paying attention mainly to the underlying mechanisms The competition between a number of mechanisms in producing tear or shear type fractures is discussed in relation to the properties of the rock and the system of pre-existing cracks on the one hand and the type of loading on the other hand First, 2-D theoretical models and experimental results aimed at the explanation and description of brittle fracture under compression are considered Their insufficiency and the necessity to address the 3-D peculiarities of crack growth in rock are shown on the basis of new experimental results on 3-D crack propagation in transparent rock-like brittle materials under uniaxial compression The results show that in contrast to the 2-D case, a single 3-D crack cannot propagate any appreciable distance and the loading results in dynamic, burst-like failure of the sample Possible mechanisms of the routinely observed extensive fracture propagation in rock samples (splitting), as well as the possibility of shear (oblique) fracture in uniaxial compression, are discussed in connection with these experiments

Mechanisms of brittle fracture of rock with pre-existing cracks in compression

Coalescence of two parallel pre-existing surface cracks in granite

Crack extension from flaws in a brittle material subjected to compression

There is a need to know accurately the strength of brittle materials in compression, both to distinguish between competing statistical, micromechanical theories of strength and to provide data necessary for engineering designs for the optimum use of these materials as load bearing elements. The scope of application of high density, high-strength brittle materials as structural elements is increasingly rapidly. Materials, such as alumina, silicon carbide and silicon nitride, are being used in application where their corrosion resistance, strength at high temperature, light weight and very high compressive strength can be used to advantage. Potential design applications include biaxially prestressed hypersonic airfoils [1,2], tiled domed structures beneath the sea [3], jet engine bearings [4], and gas turbine components [5].

Marc Adams

Papers

Crack extension from flaws in a brittle material subjected to compression

Compression testing of ceramics