Showing papers on "Indentation published in 1980"

Elastic/Plastic Indentation Damage in Ceramics: The Median/Radial Crack System

Abstract: A theory for describing the evolution of the median/radial crack system in the far field of sharp-indenter contacts is developed. Analysis is based on a model in which the complex elastic/plastic field beneath the indenter is resolved into elastic and residual components. The elastic component, being reversible, assumes a secondary role in the fracture process: although it does enhance downward (median) extension during the loading half-cycle, it suppresses surface (radial) extension to the extent that significant growth continues during unloading. The residual component accordingly provides the primary driving force for the crack configuration in the final stages of evolution, where the crack tends to near-half-penny geometry. On the hypothesis that the origin of the irreversible field lies in the accommodation of an expanding plastic hardness impression by the surrounding elastic matrix, the ensuing fracture mechanics relations for equilibrium crack growth are found to involve the ratio hardness-to-modulus as well as toughness. Observations of crack evolution in soda-lime glass provide a suitable calibration of indentation coefficients in these relations. The calibrated equations are then demonstrated to be capable of predicting the widely variable median and radial growth characteristics observed in other ceramic materials. The theory is shown to have a vital bearing on important practical areas of ceramics evaluation, including toughness and strength.

Atomically sharp cracks in brittle solids: an electron microscopy study

Abstract: The issue of bond rupture versus microplasticity as an essential mechanism of crack propagation in brittle solids is addressed. A detailed survey of existing theoretical and experimental evidence relating to this issue highlights the need for direct observations of events within the crack-tip “process zone”, at a level approaching 10 nm. Transmission electron microscopy is accordingly used to study arrested cracks about sharp-contact (Vickers indentation and particle impact) sites in Si, Ge, SiC and Al2O3. The nature of the deformation which accommodates the irreversible contact impression is first investigated, in the light of Marsh's proposal of an “equivalence” between indentation and crack-tip zone processes. Interfacial and tip regions of the surrounding cracks are then examined for any trace of a plasticity-controlled fracture process. Dislocation-like images are indeed evident at the crack planes, but these are shown to be totally inconsistent with any conventional slip mechanism. The close connection between the dislocation patterns and moire fringe systems along the cracks points to “lattice mismatch” contrast in association with a partial closure and healing operation at the interface. Analysis of all other details in the crack patterns, e.g. the presence of a crack-front contrast band indicative of a residual strain field and the disposition of interfacial fracture steps relative to the dislocation/moire system, reinforces this interpretation. It is concluded that the concept of an atomically sharp crack provides a sound basis for the theory of fracture of brittle solids.

Fracture toughness of silicon

Abstract: The paper presents a study to determine the fracture toughness and to characterize fracture modes of silicon as a function of the orientation of single-crystal and polycrystalline material. It is shown that bar specimens cracked by Knoop microhardness indentation and tested to fracture under four-point bending at room temperature were used to determine the fracture toughness values. It is found that the lowest fracture toughness value of single crystal silicon was 0.82 MN/m to the 3/2 in the 111 plane type orientation, although the difference in values in the 111, 110, and 100 planes was small.

The Response of Solids to Elastic/ Plastic Indentation

Abstract: A new approach for analysing indentation plasticity and indentation fracture is presented, The analysis permits relations to be established between material properties (notably hardness, yield strength and elastic modulus) and the dimensions of the indentation and plastic zone. The predictions are demonstrated to be fully consistent with observations performed on a wide range of materials. The indentation stress fields can also be adapted to generate predictions of indentation fracture thresholds for the three dominant crack types: radial, median and lateral cracks. The predictions are generally consistent with experimental observations.

Plastic deformation produced during indentation phase of rotary forging

Abstract: The concept of rotary forging in its two basic forms is described and a technique is presented for simulating the indentation phase of the process. Annealed aluminium alloy work pieces of differing height/diameter ratios were indented and examined internally. The approximate shapes of the deformation zones were established by hardness measurements on sections from as-worked samples and by metallographic observations of sections from work pieces which were recrystallized after indenting. Large recrystallized grains define regions which were cold worked by critical amounts during indentation. The three-dimensional shapes of the plastically deformed zones are discussed and the implications are considered in relation to the rotary phase of forging. Mechanisms are suggested for forging faults such as mushrooming of the surface worked by the indenting tool.

Compressive Strength and Damage Mechanisms in Ceramic Materials. I. Temperature-Strain Rate Dependence of Compressive Strength and Damage Mechanism in Aluminum Oxide. II. Threshold Microfracture during Elastic-Plastic Indentation of Ceramics.

Abstract: : The results of compression tests of Al2O3 performed over a wide range in temperature and strain rate are interpreted in terms of dominant damage mechanisms. It is shown that compressive failure in Al2O3 is caused by one of three different mechanisms, each based on tensile (Mode I) growth of predominantly axial microcracks, and each characteristic of a specific temperature-strain rate regime. The concepts developed should be applicable to other strong ceramics. In addition, indentation experiments were carried out for a variety of ceramics. It is found that the threshold for microfracture during elastic-plastic indentation corresponds to radial, rather than subsurface median, crack formation. This is contrary to the fundamental assumption of existing models for threshold crack nucleation by sharp indenters or particles; the results indicate the need to modify the stress field calculations used in these models. (Author)

Plastic strain and dislocation structure of the KDP group crystals

Abstract: Plastic strain of the KDP, DKDP, RDP, ADP crystals has been studied by the methods of uniaxial compression, selective etching, and indentation. It has been found that at room temperature the crystals had little plasticity, while at high temperature their plasticity was large. Rows of dislocations and dislocation rosettes have been observed. Glide systems and activation parameters of the process of plastic glide have been determined. [Russian Text Ignored.]

Hertzian indentation of thorium dioxide, ThO2

Abstract: The Hertzian indentation technique was used to study the fracture properties of ThO2 and to measure the fracture surface energy, γ, of sintered ThO2 Optical microscopy and acoustic emission were employed to detect ring crack formation Perfect cracks were always formed and no indication of permanent plastic deformation was observed From the observed crack behaviour, a fracture surface energy, γ, of 25±02 J m−2 at room temperature and a fracture toughness, KIc, of 107 MN m−3/2 were deduced

Method and apparatus for the measurement of hardness testing indentations

Abstract: A method and apparatus are provided for measuring the magnitude of spherical hardness testing indentations in solid surfaces by scanning the indentations with a light beam. Scanning is achieved by tilting the beam in one or several planes perpendicular to the testpiece surface through a point which is the center of rotation for the beam tilting as well as the center of curvature of the indentation area in the plane of tilting. At least the part of the indenter making contact with the indented surface is spherically shaped, and the scanning light spot follows an arcuate path in the indentation being measured. The scanning light beam is sent through a transparent indenter, or scanning is carried out directly on the indentation after removal of the indenter. The intensity of the light reflected from the indentation in the direction of incidence is continuously measured, and the angle between the two principle intensity variations on each scan serves for determining the magnitude of the indentation and thus the Brinell hardness of the tested surface.

Use of Vickers indentation method for evaluation of fracture toughness of phase-separated glasses

Abstract: An attempt was made to use indentation approach to determine the fracture toughness of phase-separated glasses having particulate microstructure. The samples selected for study were lead borate glasses whose composition lies in the immiscible region of the PbOB 2 O 3 system. Indentation fracture experiments were performed on the polished surface of the specimens using a Vickers microhardness tester. The extent of surface traces of well-developed half-penny-like cracks growing from the corners of Vickers indents were measured. Fracture toughness values were then evaluated based upon the established indentation fracture mechanics. These values were compared with fracture toughness data obtained for the same glasses by the conventional three-point bending technique. It was found that Vickers indentation testing can be an efficient method for fracture toughness evaluation of opaque two-phase glasses.

Strength and fracture toughness of indented glass-nickel compacts

Abstract: Strength and fracture toughness of glass-nickel compacts have been measured in a four-point bend test. Knoop microhardness indentation technique was used to introduce controlled surface flaws in hot-pressed sodium borosilicate glass containing up to 10 vol% spherical nickel powders for the bend test and also to assess the strength of the compacts. It was found that as the number of voids increased, the load to induce indentation flaws, the subsequent fracture strength and toughness all increased.

The indentation of an elastic layer by a rigid stamp under condition of complete adhesion

Abstract: In this paper the indentation of an elastic layer by a rigid stamp is treated under conditions of complete adhesion beneath the stamp, where the ratio of the half-width of the contact region and the thickness of the layer is assumed to be small. The cases of a flat stamp and a polynomial one are considered successively and two applications are treated.

An experimentally-determined slip-line field for plane-strain wedge indentation of a strain-hardening material

Abstract: PRINTED grids are used to measure the plastic flow in very slow speed plane-strain wedge indentation experiments. Using a radial grid with its centre coinciding with the origin of the indentation it is shown that the necessary condition for the maintenance of geometric similarity during indentation, namely that all elements initially on a given radial line should describe the same trajectory in the unit diagram, is satisfied within experimental accuracy over a wide range of indentation depths. A slip-line field is then constructed using the directions of maximum shear strain increment calculated by making an incremental indentation of an initially square grid printed on a previously-indented specimen. In making a stress analysis of this field it is shown that consistent results can only be obtained if account is taken in the stress equilibrium equations of variations in flow stress with strain.

An inexpensive vertical-displacement indentation tester

Abstract: A displacement transducer is used to follow the vertical movements of an indenter tip, which is forced into the material specimen under a selected dead weight load. Measurement of indentation depth under load and vertical recovery following load removal allows both plastic and elastic characteristics of the material to be evaluated. The equipment gives consistent results, is readily portable, and is eminently suited to the testing of relatively soft materials such as compacted powders, plastics and foodstuffs.

Method and apparatus for automatic evaluation of indentations in hardness testing of materials

Abstract: Method and apparatus for automatically evaluating an indentation produced in a material according to the Vickers hardness testing method. A diaphragm apertured in two coordinate directions is guided over an image of the indentation, while a photoelectric receiver system produces an electrical signal representing the luminous flux passing through the diaphragm. The electrical signal is differentiated, and changes in the differentiated signal exceeding predetermined thresholds are used to indicate detection of edges of the indentation. The respective positions of the diaphragm when the indentation edges are detected, and a known dimension of the diaphragm, are used to calculate the length of a diagonal of the indentation.

Influence of impurity and indentation time on dislocation rosettes

Abstract: It has been shown that the impurity type and indentation time affect appreciable on dislocation rosettes of NaCl single crystals. It was established that the growth of indentation diagonal was not practically followed by the increase of the dislocation zone size. This phenomenon connected with the cross slip processes, which are perhaps more intensive for prolonged indentation time and high temperatures. The assumption is made that the two mechanisms of responsible for indentation formation the movement of dislocations and their multiplication by the cross slip. [Russian Text Ignored].

Preparation of semiconductor device

Abstract: PURPOSE:To obtain an insulation separating layer between elements, by using low temperature and less processing stages. CONSTITUTION:Using a resist mask 4, an Si substrate 1 is provided with an indentation 6 by being plasma-etched in a reacting tube using a mixture gas of CF4 and O2. By conducting plasma CVD, an SiO2 layer 7 is formed on the resist of the indentation 6. At the time, a surrounding wall of an opening 5 of a mask 4 is to be formed so that is becomes in such a position a being vertical or overhanging toward the substrate 1. After formation of the layer 7, it is converted (or switched) to only O2, and treated with ash to remove the resist film 4 together with an SiO2 film 4, the SiO2 layer 7 is left in the indentation. And then, the substrate is taken out from the reacting tube, and residual of the resist, etc. is removed. By using this process, the Si substrate is not required to be specially heated, an ordinary photo- resist film can be used, and therefore, an insulation separating layer can be obtained without formation and removal of special mask and with an extensive reduction of processing stages.

Analogy Between Indentation And Blasting Tests On Brittle Rocks

Abstract: Analogy Between Indentation and Blasting Tests on Brittle Rocks This paper presents a common mechanism of breaking quasi-brittle rocks either by drilling and blasting or by indentation. Only minor modifications, to account for the prevailing boundary conditions pertaining to either one of these cutting processes, are required. Rock breakage by drilling and blasting or by indentation is based on the formation of a heterogeneous stress field due to the application of concentrated loads either by the indentor or by the detonation of the explosives. Rock behavior varies according to the stress level generated into the material. In the high stress zone it is plastic or pseudoplastic. In low stress region, failure is of the brittle extension type. In indentation the formation of a confined plastic zone is essential before chipping. However, for blasting plasticity is not necessary to occur prior to chipping because the high pressure gases that are generated from the explosion are usually quite sufficient — under suitable conditions — to create failure by extension cracking. Energy consumed in these rock cutting processes is a function of the relative amount of fines produced and their size distribution (or degree of fineness). Further improvements in rock breakage efficiency can be achieved by the proper choice of the important parameters for the specific techniques. The above analogy can be extended to other well known rock testing and breakage techniques.