Indent edge cracks

Figure 3.1. Cracked Vickers indent on a (001) plane of the perovskite Nao.vsWOj showing slip lines parallel to the indent edges. After McColm and Wilson. ...

In equation (5.92), v is Poisson s ratio, P is the indenter load, E is Young s modulus, c is the crack depth, z is the depth of the indentation zone where j no tensile stress component is present, and ( is a term required to convert ) a straight edged crack to a semielliptical geometry. Q is defined by the ) expression... 

For most substances used in the experiments an indentation force greater than 0.1 N leads to cracks or even lift-offs at the edges or the ends of the diagonals of the indentation produced by the Vickers pyramid. Therefore the values of the measured hardnesses have a large spread and usually are not reproducible. Most of the literature data were produced at a time where ultra-micro-hardness devices were not available, so that forces larger 5-10 N have been used. 

Gragert and Meyer (Fig. 6.2.1) and Boyarskaya (Fig. 6.2.2) by observation of surface deformations induced by indentation with a tungsten carbide ball and by scratch. The observations were carried out using secondary electron beam and in cathodoluminescence. They demonstrated on MgO and LiF crystals the occurrence of cracks around the impression of the ball similar to those induced by a Vickers indenter, and also the occurrence of a concentration of screw and edge dislocations in the area of the cracks. 

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

Due to its simplicity, its nondestructive nature, and the fact that minimal machining is required to prepare the sample, the use of the Vickers hardness indentations to measure Ki. has become quite popular. In this method, a diamond indenter is applied to the surface of the specimen to be tested. Upon removal, the sizes of the cracks that emanate (sometimes) from the edges of the indent are measured, and the Vickers hardness H in GPa of the material is calculated. A number of empirical and semiempirical relationships have been proposed relating Ki., c, Y, and //, and in general the expressions take the form... 

The three-dimensional properties of a laminate given by Eqns (6.11), (6.12), and (6.32) are needed in situations where out-of-plane stresses develop. Besides the obvious case of out-of-plane loading such as the local indentation and the associated solution of contact stresses in an impact problem, out-of-plane stresses typically arise near free edges of laminates, in the immediate vicinity of plydrops and near matrix cracks or delaminations. Typical examples are shown in Figure 6.4. The red lines indicate regions in the vicinity of which out-of-plane stresses [Pg.132]

The standard deviaticm for tensile tests can be relatively high due to microscopic defects, and the number of tested specimens should be stated. Care must be taken so that the samples are not broken at the grips (Fig. 6.6d), which would lead to irreproducible results. This can be prevented by either the use of mbber-coated grips or, if rubber-coated grips are not available, by protecting the clamped part with tape. Cracks can grow from indentations at the edge of the sample and to prevent premature failure, it is recommended to cut the sample with a sharp blade. 

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