The application of indentation techniques to the evaluation of fracture toughness is examined critically, in two parts. In this first part, attention is focused on an approach which involves direct measurement of Vickers-produced radial cracks as a function of indentation load. A theoretical basis for the method is first established, in terms of elastic/plastic indentation fracture mechanics. It is thereby asserted that the key to the radial crack response lies in the residual component of the contact field. This residual term has important implications concerning the crack evolution, including the possibility of post indentation slow growth under environment-sensitive conditions. Fractographic observations of cracks in selected “reference” materials are used to determine the magnitude of this effect and to investigate other potential complications associated with departures from ideal indentation fracture behavior. The data from these observations provide a convenient calibration of the Indentation toughness equations for general application to other well-behaved ceramics. The technique is uniquely simple in procedure and economic in its use of material.

A Critical Evaluation of Indentation Techniques for Measuring Fracture Toughness: I, Direct Crack Measurements

A Critical evaluation of indentation techniques for measuring fracture toughness

Evaluation ofKIc of brittle solids by the indentation method with low crack-to-indent ratios

Evaluating Fractured Reservoirs Reservoir Management Detecting And Predicting Fracture Occurrence and Intensity Analysis Of Anisotropic Reservoirs Analysis Procedures in Fractured Reservoirs Appendix A: List of Documented Fractured Reservoirs Appendix B: Procedures Checklist Appendix C: Averaging Techniques

Geologic Analysis of Naturally Fractured Reservoirs

A review of the observations of indentation-induced fracture suggests that there is no simple generalization which may be made concerning crack initiation sequences. Here, we investigate the material dependence of the initiation sequence of indentation cracks (cone, radial, median, half-penny, and lateral) using an inverted tester allowing simultaneous viewing of the fracture process and measurement of the indeter load and displacement during contact. Two normal glasses, two anomalous glasses, and seven crystalline materials are examined. Key results include (i) direct evidence that the surface traces of cracks observed at indentation contacts are those of radial cracks, rather than median-nucleated half-penny cracks (at least for peak contact loads <40 N) and (ii) that, in crystalline materials, radial cracks form almost immediately on loading of the indenter, in anomalous glasses at somewhat greater loads, but in normal glasses during unloading. A detailed consideration of the stress fields arising during indentation contact predicts material-dependent initiation sequences, in agreement with observations, particularly those of radial crack formation on loading for materials with large modulus-to-hardness ratios. In addition, a new, unexplored crack system is demonstrated, the shallow lateral cracks, which appear to be responsible for material removal at sharp contacts.

Direct Observation and Analysis of Indentation Cracking in Glasses and Ceramics

The median and lateral cracks around Vickers and small-diameter spherical indentations in soda-lime glass have been studied. The indentations were sectioned to alloy a study of the subsurface deformation by making the indentations on and near the tip of a pre-existing crack. These observations are important to the application of indentation technique in determining fracture mechanics parameters as well as to the abrasion, erosion and wear processes in brittle materials.

The origin of median and lateral cracks around plastic indents in brittle materials

A systematic study of spherical indentations on various glasses has shown that for small indenters (<or approximately=1 mm) plastic flow as well as Hertzian cone cracking occurs. What is of more interest however, is that on unloading, a series of radial cracks forms about the area of contact. With continued unloading, these cracks grow and just before complete unloading, an entirely independent system of lateral cracks, which are responsible for material removal processes, occurs. The indentation process was followed using an inverted microscope and indentation stress-strain curves were produced. These curves were compared with recent indentation correlation expressions and found to fit reasonably well. The radial cracking about the contact site was analysed in terms of a stress field which was a composite of the normal Hertzian loading situation coupled with the expansion of a cavity adjacent to a free surface. This model agrees reasonably well with experiment and provides a qualitative treatment of lateral cracking phenomena.

Indentation plasticity and the ensuing fracture of glass

Some observations of overlapping interacting cracks

Determination of the surface residual stresses in tempered glasses by indentation fracture mechanics

Intense Rayleigh waves produced by the impact of high-velocity liquid jets on brittle solids were arranged to interact with well-defined surface flaws of dimensions 50 to 200 μm. The extent of crack growth was monitored as a function of distance from the impact site. It was found that considerable crack growth as well as crack branching occurred for cracks parallel to the incident wavefront and little or no growth for orthogonal cracks. The form of the surface wave was monitored using piezoelectric crystals attached to the surface. The results are discussed in terms of recent fracture mechanics analysis of stress-wave interaction with cracks. The significance of this study to strength degradation of brittle bodies subjected to rain-drop impact is pointed out.

J. T. Hagan

Papers

The origin of median and lateral cracks around plastic indents in brittle materials

Indentation plasticity and the ensuing fracture of glass

Some observations of overlapping interacting cracks

Determination of the surface residual stresses in tempered glasses by indentation fracture mechanics

Rayleigh wave interaction with, and the extension of, microcracks