Showing papers on "Indentation published in 1989"

Vickers indentation fracture toughness test Part 1 Review of literature and formulation of standardised indentation toughness equations

Abstract: There is considerable interest in determining the fracture toughness of brittle materials by measuring the extent of cracking associated with a Vickers indentation because of the ease of specimen preparation and the simplicity of the test. However, confusion has been engendered by the multitude of models and equations in the literature relating the degree of cracking to the fracture toughness. In Part 1 of this work, nineteen of these equations are reviewed and then modified in a standard manner for both experimental convenience and direct comparison.MST/1050a

An explanation of the indentation size effect in ceramics

Abstract: A quantitative model is proposed to explain the indentation size effect (ISE) often observed in the hardness response of hard brittle materials, namely that hardness is observed to increase with decreasing indentation size. The model is based on a mixed elastic/plastic materials deformation response whereby plastic deformation occurs in a discrete manner progressively to relieve stresses created by elastic flexure of the surface at the edges of the indentation. During unloading of the indenter, recovery of the elastic increment of deformation, which precedes each new band of plastic deformation, results in the indentation appearing smaller than expected, particularly as the indentation sizes decrease to approach the scale of the plastic deformation band spacing. The model fits observed experimental data well and analysis of hardness/size data in this way is shown to allow both a bulk hardness value and a characteristic deformation band scale to be calculated for a given sample.

Vickers indentation fracture toughness test Part 2 Application and critical evaluation of standardised indentation toughness equations

Abstract: The standardised indentation fracture toughness equations formulated in Part 1 have been applied to a range of brittle materials: namely, glass ceramics, aluminas, zirconias, and WC–Co cermets. Ana...

Indentation Fracture Toughness of Sintered Silicon Carbide in the Palmqvist Crack Regime

Abstract: The fracture toughness of a sintered dense alpha-SiC was estimated by the Vickers indentation microfracture method in the low-load Palmqvist crack regime. It was observed that the use of simultaneously obtained Vickers hardnesses does not yield reliable fracture toughness values, nor does application of the median-crack-derived equations. It is necessary to utilize a load-independent, crack-free hardness value with this toughness estimation method. Although several of the curve-fitting equations yield similar toughnesses, it is concluded for the Palmqvist crack system in this alpha-SiC that the Niihara-Morena-Hasselman (1982) equation is the only one which yields fracture toughness values in agreement with conventional measurement techniques.

Grinding mechanisms and strength degradation for ceramics

Abstract: This paper presents a critical review and evaluation of knowledge of the grinding mechanisms for ceramic materials and their influence on the finished surface and mechanical properties. Two main research approaches are identified: a machining approach and an indentation fracture mechanics approach. The machining approach has typically involved measurement of the grinding forces and specific energy coupled with microscopic observations of the surface morphology and grinding detritus. Any proposed mechanisms of abrasive-workpiece interaction must be consistent with the magnitude of the specific energy and its dependence on the grinding conditions. The indentation fracture mechanics approach assumes that the damage produced by grinding can be modeled by the idealized flow system produced by a sharp indentor. Indentation of a ceramic body is considered to involve elastic/plastic deformation with two principal crack systems propagating from the indentation site; lateral cracks which lead to material removal and radial/median cracks which cause strength degradation. Each of these approaches provides insight into grinding behavior and strength degradation, but each has its shortcomings.

Field indentation microprobe for structural integrity evaluation

Abstract: Apparatus and methods for the in-field measurement of mechanical and physical properties of metallic structures. The apparatus is a Field Indentation Microprobe (FIM) using an indenter that is caused to contact and indent the structure using cyclically applied and released successive increasing loads at the same location. The load and penetration depth data during both cyclic loading and unloading are used to determine the flow properties and fracture toughness of the structure. An X-Y driven testing head supports a load cell which is connected to an indenter holder and the indenter. A displacement transducer is carried by the load cell to measure the depth of penetration of the indenter into the structure, and one or more ultrasonic transducers determine the physical phenomena such as crack size, material pile-up around indentation, and residual stress presence and orientation. A polishing tool is provided to prepare the surface of the structure prior to indentation. Also, a video camera is provided to view the surface, and to measure the final diameter of indentation. A data acquisition system and a computer appropriately programmed for automated testing and for acquiring and processing test data are used to provide the desired mechanical and physical property information. Accordingly, changes in local mechanical and physical properties of a structure, due to either normal service or damage, can be determined nondestructively in the field, and hence structural integrity can be evaluated.

Nanoindentation of silver-relations between hardness and dislocation structure

Abstract: The depth dependence of hardness in a well-annealed single crystal of silver has been characterized in nanoindentation experiments. The work is based on similar experiments performed by Chen and Hendrickson, but extends their results to indent depths on the nanometer scale. The hardness is generally found to increase with decreasing depth, with a rather sharp increase observed at depths of less than 50 nm. Using etch pitting to reveal the surface dislocation structure after indentation, the sharp rise in hardness is found to be associated with the disappearance of dislocation rosette patterns and any signs of near-surface dislocation activity, thereby suggesting that very small scale indentation plasticity may take place by nondislocation mechanisms. However, order of magnitude calculations show that possible alternatives, specifically, diffusional mechanisms, are too slow to make significant contributions. It is suggested that for very small indents, either the surface dislocation debris is quickly annealed out before it can be observed or indentation plasticity is accommodated entirely by subsurface dislocation activity.

Indentation and rotation in the western Alpine arc

Abstract: Summary The overall structural aspects of the external western Alpine arc and foreland may be consequences of NW-SE compression ahead of crustal thrusting on the northern front and of mainly sinistral wrenching along its southwestern lateral border. However such a model assumes a simple straight-line displacement which, while possibly valid on the continental scale, cannot be considered for the external arc since a model of linear translation and rotation around a distant pole is not compatible with the structural organization of the western arc of the Alps. The constant tangential stretching parallel to the curved front, the fan-ring shortening and shear directions on décollement levels, the complex nappe transport trajectories and the widespread dextral wrench-faulting around the arc, can be better explained by local rotation of a block belonging to the European margin, dragged along the western sidewall of a large indenter moving northwestwards. This laterally displaced block produces an anticlockwise ring shear about a pole of rotation within it. Consequently, the Alpine arc cannot be understood in terms of plane-strain models along local ‘transport’ directions. The deformation process corresponds instead to lateral rotation-expulsion of a deformed unit belonging to the European Plate margin, jammed against its stable foreland.

Improved techniques for measuring the indentation and thickness of articular cartilage.

Abstract: A review of the techniques previously employed in the indentation and measurement of the thickness of articular cartilage has led to new and improved techniques for performing both measurements. By utilizing high-speed, microcomputer-controlled data logging techniques, simultaneous monitoring of signals from a dynamic load cell and a displacement transducer could be made throughout an indentation test. The position of the indenter as it touched the articular surface could thus be determined automatically by identifying the moment at which a positive change in the load signal occurred. Less accurate and more time consuming techniques previously required for determining the position of the cartilage surface were hence avoided. The apparatus also included a critically damped dashpot which prevented any transient loads being applied to the cartilage. Depths of indentation could be measured to an accuracy of 0.005 mm with a measurement repeatability of 2.14 per cent.By replacing the indenter with a sharp needl...

Measurement of adhesion of thin polymer coatings by indentation

Abstract: A microindentation technique was developed to measure the adhesive shear strength of thin polymer coatings on glass substrates. Indentation‐induced debonding of the coating was observed to occur under three different conditions: Type I was with the deformations underneath the indenter being essentially elastic; Type II was with the deformations underneath the indenter being plastic; and Type III was after the indenter penetrated the substrate. Stress analyses to calculate the interfacial shear stress for the indentation‐induced debonding of thin coatings are presented for the three types of debonding. All three stress analyses are based upon the linear, elastic analysis of the contact stresses arising from indentation of a soft coating on a rigid substrate. These analyses provide a basis for using controlled indentation debonding as a quantitative measure of the interfacial shear strength of thin coatings to rigid substrates.

Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

Abstract: The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R-curve behavior, both of which are independent of temperature from 20° to 1400°C.

SURFACE DEFORMATION BEHAVIOUR OF TiC AND TiN COATED STEELS: 1 INDENTATION RESPONSE

Abstract: Indentation hardness testing using loads of 25–1000 gf on a Vickers profile diamond indenter has been used to study the deformation and fracture behaviour of CVD TiC and CVD and PVD TiN coatings on a range of steel substrates. The coatings were prepared by proprietary processes and were 2–5 /lm thick. The variation of hardness with indentation size and depth has been compared with a number of models for the composite hardness behaviour of coated systems. Good fits were found both to a law of mixtures model incorporating indentation size effect behaviour, and to a simple empirical polynomial. In both cases, parameters can be calculated from which the hardness of the coating itself can be assessed for purposes of quality control etc. For the first time, the variation of hardness with temperature for coated systems has been investigated. For the coating/substrate composites studied here, hardness fell rapidly with temperature, and ‘displayed marked hysteresis of both hardness andfracture behaviour. T...

Cyclic Fatigue Behavior of an Alumina Ceramic with Crack-Resistance Characteristics

Abstract: The behavior under cyclic tension—tension loading of an alumina ceramic with pronounced crack-bridging (R-curve) characteristics is studied. Tests on disk specimens with indentation cracks reveal no failures below the static fatigue limit. Theoretical predictions of the stress-lifetime response, based on the premise that environmentally assisted slow crack growth is the sole factor determining lifetime, are consistent (within experimental scatter) with the data. The results indicate that there is no significant cyclic degradation from potential damage to the bridges, at least in the short-crack region pertinent to strength properties.

Dependence of the Vickers indentation fracture toughness on the surface crack length

Abstract: On etudie les variations de K c en fonction de longueurs croissantes de fissures superficielles pour plusieurs materiaux fragiles (ceramiques, vitroceramiques et cermets).

The Effects of Thin Compressive Films on Indentation Fracture Toughness Measurements

Abstract: Thin compressive films have been shown to decrease the lengths of radial cracks produced by a Vickers indentation in a variety of non-metallic materials. The intrinsic stress of submicrometre thick films deposited by reactive ion beam sputtering was measured by a cantilever technique. The change in the apparent indentation fracture toughness of the coated material was correlated with film thickness and stress, indentation load, and the nature of the substrate.

Structural integrity evaluation based on an innovative field indentation microprobe

Abstract: A brief description is given for a field indentation microprobe (FIM) apparatus which was developed to evaluate, nondestructively in situ, the integrity of metallic structures. The FIM consists of an automated ball indentation (ABI) unit for determining the mechanical properties (yield strength, flow properties, fracture toughness, etc.) and a nondestructive evaluation (NDE) unit (consisting of ultrasonic transducers and a video camera) for determining the physical properties such as crack size, material pile-up around indentation, and residual stress presence and orientation. The laboratory version used here performs only ABI testing. Results of the laboratory version of the FIM tests on unirradiated and irradiated A212 grade B pressure vessel steel are discussed in this paper. Excellent agreement was obtained between yield strength and flow properties (true-stress/true-plastic-strain curve) measured by the ABI tests and those of uniaxial tensile tests. 14 refs., 9 figs., 1 tab.

Dynamic indentation system

Abstract: The dynamic indentation system cyclically indents a test material for the purpose of determining mechanical properties that cannot be measured directly, such as in-plane wall stress in intact hearts. A probe or an end surface thereof cyclically indents the test material while the indentation stress acting on the face of the probe and the position of the probe are measured. The indentations can be servo-controlled for added stability. The transverse stiffness of the material is calculated as the slope of the relation between the indentation stress and indentation strain during each indentation cycle. The transverse stiffness is then used as an estimate of wall stress, and the relation between the transverse stiffness and in-plane strain can be used as a direct estimate of material properties. The dynamic indentation system is suitable for estimating in-plane wall stress and material properties from different parts of a test material. Since the vibrations are high in frequency, any gross movement of the test material or of the probe during the period of one oscillation is very small and does not influence the transverse stiffness. The dynamic indentation system allows, for example, estimation of wall stress throughout the course of a single cardiac contraction.

The effect of fiber coatings on interfacial shear strength and the mechanical behavior of ceramic composites

Abstract: Thin coatings deposited on ceramic fibers prior to densification employing chemical vapor infiltration techniques have been used to limit fiber-matrix bonding This has resulted in improvements in strength and toughness at room and elevated temperatures in Nicalon® fiber-reinforced/SiC matrix composites The properties of the fiber-matrix interface in fiber-reinforced ceramic composites have been examined utilizing an indentation method in which a standard microhardness indentor is used to push on fibers embedded in the ceramic matrix Compositions and microstructures at the interface have been characterized employing analytical electton microscopy Correlations between interfacial phenomena and observed mechanical behavior have been made

Modification of the mechanical properties of ceramics by ion implantation

Abstract: The effects of ion implantation on the mechanical properties are reviewed for alumina, silicon carbide and fully-stabilized zirconia. It is demonstrated that by ion implantation, (a) hardness of the near surface layer is varied over a wide range, depending on ion dose and implantation temperature, (b) significant increases in the flexural strength are observed, (c) the resistivity against the crack initiation and propagation associated with Vickers indentation is increased, (d) the resistivity against lateral and radial crackings associated with scratching by a loaded diamond stylus is increased, (e) a reduction in the friction coefficient from 0.6 to 0.15 is observed for polycrystalline silicon carbide in sliding contact with a steel. These results are discussed in terms of microstructural changes, surface compression stress and amorphized surface layer of an increased plasticity.

Indentation size effect and strain-hardening

Abstract: Analyse theorique de la relation de Meyer entre la charge de l'indenteur spherique (de Brinell) L et d le diametre d de l'indentation: L=ad n

Mechanical properties (hardness and adhesion) of a‐C:H thin films produced by dual ion beam sputtering

Abstract: Hydrogenated amorphous carbon films (a‐C:H) are very hard, inert to organic solvents and inorganic acids, and relatively transparent. This combination of mechanical and optical properties makes such material an ideal candidate for optical coatings. The dual ion beam sputtering technique has been used for a‐C:H thin‐film deposition. By the ultramicrohardness Knoop indentation and scratch test technique, hardness and adhesion of samples have been measured. The correlation between hardness value at an indentation size of 10 μm and ion beam energy of the second gun is reported.