Topic Abrasion Made by Specialist

A portable field instrument was designed and built to measure changes in the abrasion resistance hardness of geologic materials, especially those changes resulting from weathering. Weathering is spatially and temporally heterogeneous and outcrops can become case-hardened or coresoftened, i.e. a differential hardness between the exterior surface and the underlying material can develop as a result of non-uniform chemical changes during weathering in which the exterior surface appears to become harder. These differential weathering effects can affect the rock morphology and lead to the development of cavernous weathering [1, 2]. In discussing rock hardness in its relation to weathering, the term hardness is usually applied to the rock as a whole and not to its constituent mineral grains. Rock hardness can be thought of as the friability, the resistance to erosion, the abrasion resistance, the durability, the cohesion, or other descriptions and combination of descriptions of physical properties that apply to rock material. Because of the variety of properties involved in rock hardness, it is difficult to choose and perform a suitable test for comparing hardness among different rocks, especially in the field. Recently Day and Goudie [3] have been successful in using a Schmidt Test Hammer to measure rock hardness and its relation to landform development. The Schmidt Hammer measures the coefficient of restitution (the fractional rebound distance of a mass upon impact) which depends on the cohesion of the surface and subsurface material. The Schmidt Hammer is not applicable to the study of case hardening or core softening because (1) most exterior crusts are quite thin (mm to cm), (2) the hammer will be influenced by the underlying softer material and (3) the measurement will physically break the exterior crust. Gillespie [4] used acoustic wave * Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A. t Present address: 15171 Auburn St., Detroit, MI 48223, U.S.A. 141 speeds to measure the degree of weathering of glacial erratics in Sierra Nevada moraines and discovered that these measurements were affected by the development of case-hardened crusts. However, acoustic wave speeds measure a bulk property and, in practice, it is difficult to use them to investigate small areas within individual rocks. In materials science, extensive hardness testing has been conducted on homogeneous materials such as metals and hard rubber. These tests consist of Vickers and Brinell indentation tests. The tests are inapplicable to heterogeneous materials such as rocks because the durability of rock in the face of erosive agents may depend only slightly on the indentation properties of the individual grains. A useful test of rock hardness would measure a property or set of properties that influences the erosion behaviour of a rock. Resistance to abrasion is such a set of properties which determines the behaviour of the rock in response to the shear stresses and gouging action of abrasion. The abrasion resistance hardness tester (ARHT), a portable field instrument, was developed with the above ideas in mind (Fig. 1). ARHT abrades the material by means of a diamond coring bit, which is turned slowly by a constant-speed motor and which is driven into the rock by a constant normal force or load. The measured time required to abrade to a fixed depth is related to the material's abrasion resistance. During the operation of ARHT, the operator has control of the load, L, and the depth of penetration, D. Convenient units obtained from the instrument, that discriminate hardness among different materials, are the abrasion resistance hardness value (/4,) and the coefficient of relative hardening for an exterior surface (C), defined as: Ha = ~-~ × 10 -5, (1) /4. (exterior) C = H. (interior)' (2) where /4, is the abrasion resistance hardness value (105N see/m), C is the coefficient of relative hardening (dimensionless), L is the load, or normal force

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