Showing papers on "Indentation published in 1978"
TL;DR: In this article, the median and lateral cracks around Vickers and small-diameter spherical indentations in soda-lime glass have been studied and sectioned to alloy a study of the subsurface deformation by making the indentations on and near the tip of a pre-existing crack, which is 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.
Abstract: 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.
186 citations
TL;DR: In this article, the authors made an analysis of the relationship between load and crack extension in the Hertzian indentation of a solid surface with a spherical indentor and proposed that the minimum load for ring-crack formation becomes independent of the flaws in the solid if these have a size above a certain threshold value.
137 citations
TL;DR: Brookes et al. as discussed by the authors suggested that crack-free hardness indentations may also be made at low loads in single crystals of germanium and silicon (C. A. Brookes and his unpublished data) and suggested that this may be due at least in part, to a different mechanism.
Abstract: A HARD indenter pressed into the surface of a ductile solid such as a metal will produce a permanent indentation. The mean pressure over the indentation is known as the hardness of the solid and is a measure of its yield or flow stress1. With many relatively brittle materials such as glass and rock salt, it is possible to form plastic indentations at low loads. This is because the indentation process involves a very large component of hydrostatic stress which inhibits brittle failure: the material then yields by flow and the hardness is again a measure of the (ductile) yield stress of the solid2–4. At high loads the indentation is usually accompanied by appreciable cracking. Crack-free hardness indentations may also be made at low loads in single crystals of germanium and silicon (C. A. Brookes and our unpublished data) and it is suggested here that this may be due at least in part, to a different mechanism.
98 citations
77 citations
TL;DR: In this paper, a sharp crack is introduced into bend-test specimens using a wedge indenter and residual stresses are relieved by removing the surface layer containing the indentation, and it is found that annealing is not a satisfactory way of removing residual stresses from specimens precracked by indentation since it produces effects that give too high a value for fracture toughness.
Abstract: A technique has been devised for precracking fracture-toughness specimens of hardmetals whereby a sharp crack is introduced into bend-test specimens using a wedge indenter and residual stresses are relieved by removing the surface layer containing the indentation. The technique is reproducible, simple to use, and does not suffer from the inaccuracies and difficulties of interpretation inherent in using the spark-machining and pyramid-indentation techniques. It is found that annealing is not a satisfactory way of removing residual stresses from specimens precracked by indentation since it produces effects that give too high a value for fracture toughness. Spark-machined specimens do not give reliable fracture-toughness data because there are practical problems in introducing a notch sufficiently narrow to simulate the effect of a thin crack and because thermal damage caused by the spark discharge probably alters the properties of the material at the notch root.
36 citations
TL;DR: In this article, the formation of stable radial cracks at indentations in PMMA is considered, and the distribution of tensile stress in the surface adjacent to an impression made with a spherical indenter is taken to be that round an expanded hole in an infinite plate of plastic-elastic material.
Abstract: The formation of stable radial cracks at indentations in PMMA is considered. The distribution of tensile stress in the surface adjacent to an impression made with a spherical indenter is taken to be that round an expanded hole in an infinite plate of plastic-elastic material, and the strain energy release rate G at a crack propagating in this stress field is calculated. As in the case of Hertzian fracture there is a size effect: G is proportional to the product of the radius of the plastic-elastic boundary (and hence to the radius of the impression) and the energy density of the stress field, so that according to this or any qualitatively similar model the radius (or depth) of the impression which first initiates fracture is a measure of the fracture properties. Indentation may therefore be used to assess ductile-brittle transitions in polymers in a variety of circumstances, such as increasing molecular orientation, decreasing temperature, or decreasing molecular weight. An experiment with irradiated PMMA which confirms the existence of the effect is described to illustrate the latter situation.
36 citations
TL;DR: In this article, an earlier theory of contact-induced strength degradation of brittle materials is extended to include plates in residual surface compression, with the modifying effect of the residual field incorporated into both indentation and strength equations.
Abstract: An earlier theory of contact-induced strength degradation of brittle materials is extended to include plates in residual surface compression. The scale of the strength-controlling flaw is predicted by indentation fracture mechanics, with the modifying effect of the residual field incorporated into both indentation and strength equations. Experimental verification of the predictions is obtained from diamond-pyramid indentation tests on thermally tempered glass plates. As with untempered plates, the theory accounts for the load dependence of the strength loss; it also explains the insensitivity of the degradation characteristics to initial flaw distribution and identifies toughness as the controlling material parameter. Most significant, however, is the demonstration that surface strengthening can produce dramatic improvements in degradation resistance. The possibility of obtaining all parameters necessary for a complete degradation analysis of a given tempered inaterial entirely by routine indentation/strength testing is discussed.
35 citations
01 Jan 1978
TL;DR: In this article, the basic principles of indentation fracture were reviewed and applied to practical contact problems, with special emphasis on the problem of contact-induced strength loss, and how these principles may be applied to real-world contact problems.
Abstract: Brittle ceramic components may suffer severe degradation as a result of localised cracking in contact (notably impact) situations. Such situations, although complicated by a variety of factors, e.g. nature and extent of damage modes operating during the contact, prior state of the exposed surface and the environment, indenter geometry, are most conveniently analysed in terms of “indentation fracture mechanics”. This paper accordingly sets out to review the basic principles of indentation fracture and to indicate how these principles may be applied to practical contact problems, with special emphasis on the problem of contact-induced strength loss.
35 citations
TL;DR: In this paper, the physical meaning of the empirical surface toughness parameter derived from the Palmqvist test of cemented carbides was analyzed and the theoretical results were in close agreement with experimental data for composites based on tungsten carbide and on titanium carbide.
32 citations
TL;DR: In this article, Caron et al. investigated the effect of 7' and 7' compounds on the stability of the 7' precipitates on prolonged aging in a cupronickel alloys.
Abstract: which a re otherwise (meta)s table a re r ende red uns table in the p re sence of ft. In s u m m a r y , the fo rmat ion of 7 ' and 7\" compounds in a cupronickel ma t r ix s e e m s to be cont ro l led by the e / a ra t io of these compounds, as suggested by previous investigators in nickel and iron base alloys, s'n Also the formation of Y\" and its subsequent transformation to the equilibrium fi'Ni3(Nb) phase affects the stability of the 7' precipitates on prolonged aging. The author would like to thank Dr. Ronald N. Caron for many helpful discussions and review of the manuscript; and the Siemens Corporation, Karlsruhe, West Germany, for conducting the microanalysis in a Siemens ST 100F microscope. Ms. Sheila Pressman prepared the manuscript. This work was supported by the Olin Brass Group.
21 citations
TL;DR: In this paper, a class of aluminum-filled epoxy composites were subjected to indentation tests over a wide temperature range and the effect of aluminum content, applied load, and adhesion efficiency between matrix and aluminum particles on the indentation behavior was studied.
Abstract: A class of aluminum-filled epoxy composites were subjected to indentation tests over a wide temperature range The tests were carried out under constant load and continuously varying temperature The effect of aluminum content, applied load, and adhesion efficiency between matrix and aluminum particles on the indentation behavior was studied Measured indentation values were found to lie within limits predicted theoretically
TL;DR: In this article, the authors measured the contact time between pendulum indentor tip and rubber surface, in addition to the rebound resilience, allowing calculation of deformation, speed and acceleration of the damped sinusoidal motion.
Abstract: When rubber is deformed an energy input is involved which is released in part when the rubber returns to its original shape. The part which is not returned as mechanical energy is dissipated as heat. The “rebound resilience” is defined as the ratio of the energy returned to the energy applied for deformation by an indentation due to a single impact. In this experiment, a pendulum mass and a test piece, as long as they are in contact, may be considered an oscillating system having one degree of freedom in which the rubber is a Voigt model. The correlation between rebound resilience and loss angle of the rubber may thus be derived. The measurement of the contact time between pendulum indentor tip and rubber surface, in addition to the rebound resilience, allows calculation of deformation, speed and acceleration of the damped sinusoidal motion. The penetration of the spherical indentor under a constant force depends on Young's modulus. Scott found an empirical relationship among force applied, depth of indentation, radius of the indentor tip and the modulus. This relationship has been recently improved by Stiehler and coworkers, The Stiehler formula has been used here for evaluating the storage modulus of rubber vulcanizates by using the maximum force transmitted by the impacting mass and the indentation depth of the test piece at the same time.
TL;DR: The indentation test provided an accurate estimate of Young's modulus or the shear modulus for trapped lobes, and the results suggest that it is a valid method for estimating these constants in nontrapped lobes.
Abstract: Isolated dog lobes were maximally trapped with air, and their parenchymal elastic properties were measured at the trapped volume. Indentation tests were performed on the surface of the lobes, followed by uniaxial and torsion tests on excised pieces of parenchyma. Similar values for Young's modulus were obtained from the indentation and uniaxial tests. The values for the shear modulus from the torison tests also were consistent with Young's modulus measured by the other procedures. The indentation test provided an accurate estimate of Young's modulus or the shear modulus for trapped lobes, and the results suggest that it is a valid method for estimating these constants in nontrapped lobes.
TL;DR: In this article, the authors determined the temperature dependence of the critical resolved shear stress in oxygen-free silicon in the temperature range 800-1050°C, using the method of indentation dislocation rosettes.
Abstract: The temperature dependence of the critical resolved shear stress in oxygen‐free silicon was determined in the temperature range 800–1050 °C, using the method of indentation dislocation rosettes. We found the stress to vary only about 30% in the temperature range studied, that is, from about 4.3×107 to about 3.2×107 dyn/cm2. The implications on the phenomenon of thermal slip in silicon‐wafer processing are discussed.
TL;DR: In this paper, the effect of time of indentation and time of recovery on the Rockwell hardness of a series of glassy and crystalline polymers has been determined, based on conventional viscoelastic approaches.
Abstract: The effect of time of indentation and time of recovery on the Rockwell hardness, of a series of glassy and crystalline polymers has been determined. An analytical technique, based on conventional viscoelastic approaches, has been developed. This technique permits the resolution of the hardness number into its components of permanent deformation, retarded elastic compression, and retarded elastic recovery. Subtraction of components due to retarded elastic response results in hardness numbers dependent only the time-dependent permanent deformation. Extrapolation of this data to zero time results in an absolute or time-independent value of Rockwell hardness, designated Ro.
TL;DR: In this article, the influence of indentation size in relation to the associated fracture in semi-brittle materials is discussed, with particular reference to oriented glassy polymers, and it is concluded that a strong correlation should exist between the critical indentation sizes for fracture and the degree of molecular orientation, and this has been observed in hot drawn PMMA.
Abstract: The influence of indentation size in relation to the associated fracture in semi-brittle materials is discussed, with particular reference to oriented glassy polymers. It is concluded that a strong correlation should exist between the critical indentation size for fracture and the degree of molecular orientation, and this has been observed in hot-drawn PMMA.
Patent•
17 Nov 1978
TL;DR: In this paper, the oblique light is directed so as to be specularly reflected from a facet of the indentation vertically with respect to the general specimen surface, to provide enhanced brightness differentation between a multifaceted indentation and other surface features.
Abstract: Hardness testing apparatus, for use in assessing dimensions, from which a hardness value can be determined, of an indentation formed under controlled conditions in a surface of a specimen, having a photosensitive detector for sensing brightness over an image in which the indentation differs in brightness from the rest of the image. The detector produces electrical signals representative of sensed brightness. Discrimination circuitry discriminates signals representative of brightness within the indentation from signals representative of brightness outside the indentation. Correlation of discrimination results provides an assessment of indentation dimensions. Preferably oblique lighting is used to provide enhanced brightness differentation between a multifaceted indentation and other specimen surface features. The oblique light is directed so as to be specularly reflected from a facet of the indentation vertically with respect to the general specimen surface.
Patent•
08 Mar 1978
TL;DR: In this paper, the Vicker's hardness test stably in a high precision by photoelectrically measuring the diagonal length of a indentation with the use of an image sensor.
Abstract: PURPOSE:To accomplish the Vicker's hardness test stably in a high precision by photoelectrically measuring the diagonal length of a indentation with the use of an image sensor
TL;DR: In this paper, a microscopic investigation of the initial part of the incubation period of flow-induced cavitation erosion on austenitic stainless steel was performed, and it was found that the number of indentations is proportional approximately to the 3.5th power of the hydrostatic collapse pressure.
Abstract: A microscopic investigation of the initial part of the incubation period is performed of flow-induced cavitation erosion on austenitic stainless steel. Slip lines are readily seen all over the exposed area and up to three different slip systems are observed to operate simultaneously in a single grain. The amount of work hardening (as measured by microhardness tests) due to a single collapse is found to be negligible, although a change of surface topography as well as an increase in the surface layer disorder is found. It is argued that the energy of a single indentation can be calculated from 'static' material properties and a method of calculation is presented. The energy absorbed in a single indentation is found to be comparable to the energy of a cavity which initially has a diameter of 300 mu m. It is found that the number of indentations is proportional approximately to the 3.5th power of the hydrostatic collapse pressure.
TL;DR: In this article, the authors analyzed the frictionless indentation of an elastic half-plane by a relatively blunt, symmetric elastic punch at an ar: bitrary speed by treating the more general problem of frictionless Hertzian contact between elastic solids.
Abstract: Frictionless indentation of an elastic half-plane by a relatively blunt, symmetric elastic punch at an ar: bitrary speed is analyzed by treating the more general problem of frictionless Hertzian contact between elastic solids. As in the quasi-static problem, the analysis assumes that the solid surface contours are approximately flat. In addition, the contact strip expands at a constant rate and the imposed rigid body motions and surface contours are represented by polynomial curves. Homogeneous function techniques allow analytic solutions to the basic mathematical problem. As an example, the general results are then applied to the uniformly accelerating parabolic punch on a half-plane.
01 Jan 1978
TL;DR: 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.
Abstract: SummaryIn 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.ZusammenfassungDie vorliegende Arbeit befaßt sich mit dem Eindringen eines starren Stempels in eine elastische Schicht für den Fall, daß die Unterseite des Stempels auf der Schicht befestigt ist.Dabei wird angenommen, daß die halbe Breite des Berührungsgebietes klein ist gegenüber der Dicke der Schicht.Zwei Sonderfälle werden näher untersucht: der flache Stempel und der polynomförmige Stempel. Zwei Anwendungen werden behandelt.
TL;DR: In this paper, the role of environmentally assisted crack growth in the contact-induced strength degradation of brittle surfaces was studied, and Indentation fracture mechanics, incorporating a standard crack-velocity function, were used to predict remaining strength as a function of contact load and duration.
Abstract: The role of environmentally assisted crack growth in the contact-induced strength degradation of brittle surfaces was studied. Indentation fracture mechanics, incorporating a standard crack-velocity function, are used to predict remaining strength as a function of contact load and duration. Strength tests on annealed and tempered glass disks, indented with a diamond pyramid or tungsten carbide sphere in a water environment, are in accord with the predicted degradation characteristics. The results indicate that fatigue effects are likely to be of only secondary importance in designing for maximum resistance to in-service contact damage.
01 Jan 1978
TL;DR: In this article, a research program was carried out to assess the mechanisms of damage in a strong ceramic subject to compressive loading, and to determine, if possible, the relevancy of such mechanisms to indentation damage.
Abstract: During the impact of ceramics by small hard particles, indentation damage is caused, which can lead to erosion and/or strength degradation of the ceramic. It is suspected that certain types of indentation-induced flaws may be generated by the compressive fields known to be associated with such loading. Accordingly, a research program was carried out to assess the mechanisms of damage in a strong ceramic subject to compressive loading, and to determine, if possible, the relevancy of such mechanisms to indentation damage.
TL;DR: In this paper, the authors considered a rigid smooth half-wedge which simultaneously translates tangentially and normally at constant speeds with respect to the surface of an elastic half-space, and they found that if no tangential speed is imposed, the halfwedge must approach a flat punch configuration and a maximum normal force must be applied.
TL;DR: In this paper, a class of plasticized epoxy polymers were subjected to indentation tests over a wide range of temperature and plasticizer percentage, and the materials exhibited creep behavior according to the temperature and the amount of plasticizer.
Abstract: A class of plasticized epoxy polymers were subjected to indentation tests over a wide range of temperature and plasticizer percentage. Constant or continuously varying temperature tests were carried out under constant load, and the materials exhibited creep behavior according to the temperature and the amount of plasticizer. Thermal expansion properties were also studied, and glass transition temperature as well as thermal expansion coefficients a1 and a2 were determined for each individual material. Theoretical predictions were found valid under the present conditions.
20 Apr 1978
TL;DR: In this paper, the authors used a 2-degree-of-freedom dynamic model to accurately simulate low velocity, hard object impacts on sandwich structures for impact velocities up to 6.1 m/s.
Abstract: : The resistance of composite aircraft structures to handling and impact loadings is an important consideration in assessing their suitability for long term operational service. The specific threat addressed in this study is that of low speed (0 to 30m/s), hard object, transverse normal impact (dropped tool, runway stones, etc.). The objective is to characterize the damage which occurs to composite faced sandwich structures under realistic impact conditions and determine the effect of this damage on residual compression fatigue properties. The sandwich construction selected for study consisted of AS/3501-6 graphite/epoxy face sheets of (0/+ OR - 45/0) SUBS CONSTRUCTION BONDED TO HRP-3/16-5.5 honeycomb core. This construction is representative of sandwich construction generally used in flaps, spoilers, and access doors. Static indentation tests were conducted on sandwich panels supported on a rigid foundation, and dropped weight impact tests were performed on simply supported sandwich beams. Comparison of the results of these tests indicates that static tests in conjunction with a simple 2-degree-of-freedom dynamic model can be used to accurately simulate low velocity, hard object impacts on sandwich structures for impact velocities up to 6.1 m/s. Test results of indentation versus static load is used in the dynamic model to predict the indentation which would occur for any particular impact case.
TL;DR: In this paper, a new experimental method is presented for determining trajectories of flow for non-steady-state processes which exhibit geometric similarity (quasisteady state processes (1)∗).
Abstract: A new experimental method is presented for determining trajectories of flow for non-steady-state processes which exhibit geometric similarity (quasi-steady-state processes (1)∗). One process that is normally regarded as quasi-steady-state (plane strain wedge indentation) is investigated using this method. It is shown that this process satisfies the requirements of geometric similarity within experimental accuracy. Results are presented for wedge angles of 30°, 60° and 90° and a range of indentation depths up to approximately 3 mm.Finally, it is suggested how this method can be combined with well-known orthogonal-grid techniques to provide information for the calculation of effective strains throughout the deforming region.
31 May 1978
TL;DR: In this article, the authors used selected area electron channeling to characterize the extent of subsurface damage produced in ceramics during grinding and polishing operations and found that polishing damage depths can be correlated using a simple model based on a sliding elastic-plastic indenter.
Abstract: : The threshold loads and crack sizes for indentation cracking were investigated for a number of ceramics using scanning electron microscopy and acoustic emission. It was found that the cracking behavior could be predicted, using a fracture mechanics/dimensional analysis approach modified to take into account the stress field at an elastic-plastic indentation. Selected area electron channeling was used to characterize the extent of subsurface damage produced in ceramics during grinding and polishing operations. It was found that polishing damage depths can be correlated using a simple model based on a sliding elastic-plastic indenter. Finally, the effect of temperature and strain rate upon material removal and cracking threshold was analyzed, using recent experimental compressive strength and crack growth measurements as inputs to current predictive models. The results are shown to have surprising implications with regard to dynamic, high temperature indentation (particle impact). (Author)
TL;DR: In this paper, the authors considered the indentation by a smooth rigid punch of a half-space composed of linear elastic material of hexagonal symmetry whose plane boundary is parallel to the basal planes.
Abstract: The problem is considered of the indentation by a smooth rigid punch of a half-space composed of linear elastic material of hexagonal symmetry whose plane boundary is parallel to the basal planes. The case is considered in which the area of contact between the punch and the half-space is circular, the end of the punch with is in contact with the half-space having an arbitrary profile. An integral equation is formulated and solved for the boundary value of the normal displacement in the half-space, and an expression is derived for the distribution of pressure under the punch.