Showing papers on "Indentation published in 1979"

Hardness, Toughness, and Brittleness: An Indentation Analysis

Abstract: The ratio H/Kc, wjere His hardness (resistance to deformation) and Kc. is toughness (resistance to fracture), is proposed as an index of brittleness. Indentation mechanics provides the scientific basis for this proposal. The analysis, developed in terms of a model contact system, indicates that all materials are more susceptible to deformation in small-scale loading events and to fracture in large-scale events. By normalizing the characteristic dimensions of the two competing processes and the contact load in terms of appropriate functions of H and Kc a universal deformation/fracture diagram can be constructed. From this diagram the mechanical response of any material of known hardness and toughness may be predicted for any prospective in-service contact loading conditions. The concept offers a simple approach to materials classification for design purposes.

The crack-initiation threshold in ceramic materials subject to elastic/plastic indentation

Abstract: The threshold for indentation cracking is established for a range of ceramic materials, using the techniques of scanning electron microscopy and acoustic emission. It is found that by taking into account indentation plasticity, currant theories may be successfully combined to predict threshold indentation loads and crack sizes. Threshold cracking is seen to relate to radial rather than median cracking.

Size effects in abrasion of brittle materials

Abstract: Small indentations and scratches can be made in highly brittle materials such as glass or silicon without any associated fracture. It is proposed that this effect, which is usually explained in terms of flaw statistics, is in fact governed by a strain energy criterion which can be defined quantitatively if the tensile field which initiates fracture is known. Using a model of the field of residual stress around an indentation proposed by Swain and Hagan (1976), it is shown that the critical size of indentation for fracture to occur should be about 12(ET/Y2), where E is Young's modulus, Gamma fracture surface energy, and Y the yield stress for plastic flow in uniaxial compression. The critical size parameter is evaluated for silicon.

Indentation plasticity and microfracture in silicon carbide

Abstract: The extent of the plastically deformed region associated with indentation in silicon carbide is determined by means of selected-area electron channelling. It is found that the extent of the plastic zone beneath an indent is quite large, i.e. equal to about five times the impression radius. Microcrack formation is studied in the SEM, and the combined results are discussed in terms of current elastic-plastic indentation fracture models. The first cracks to form are radial microcracks; their morphology, and the observed indentation plastic zone dimensions, support the elastic-plastic model of Perrott for indentation cracking inα-SiC.

Microhardness Indentations on Artificial White Spot Lesions

Abstract: In this study, Knoop hardness experiments on artificial carious enamel (pH 5.0, 4.5 and 4.0) are presented with the load perpendicular to the subsurface lesion. The results show that for human and bovine enamel the indentation length is proportional to the square root of the load. The proportionality constant is numerically estimated for all pH values for human and bovine enamel. The indentation measurements reflect the decalcification despite the presence of surface layer covering the lesion.

The role of distributed viscoelastic coupling in sensory adaptation in an insect mechanoreceptor

Abstract: 1. Time-dependent mechanical coupling in proprioceptive campaniform sensilla of the cockroach leg was investigated by measuring the compliance of the sensillar cap with punctate, sinusoidal mechanical stimuli at forcing frequencies ranging 3 mHz (0.003 Hz) to 100 Hz. Coupling was described in terms of linear transfer functions for cap compliance and for sensory discharge sensitivity to both cap indentation and applied force. 2. Most sensilla stiffened viscoelastically as a power of frequency, with median power coefficient −0.058 for compliance, and with indentation lagging force by a constant phase angle, median −8.4°. This behavior indicates broadly distributed viscoelastic rate constants in individual sensilla. The extent of viscoelasticity also varied among sensilla; about a third were purely elastic, with constant compliance and zero phase angle. 3. Discharge sensitivity increased with forcing frequency, usually as a power function, and the sinusoidally modulated discharge peaked sooner than both indentation and force by essentially constant phase angles. Median power coefficients were 0.51 for indentation sensitivity and 0.38 for force sensitivity; corresponding phase angles were somewhat smaller than for a power law, respective medians 39° and 30°. Purely elastic caps gave identical values for indentation and force. 4. This means all sensilla adapted, and all were rate sensitive; but in viscoelastic ones these effects were greater for indentation than for force. 5. Discharge sensitivity for a few sensilla deviated systematically from a power law in that power coefficients and phase angles increased somewhat with forcing frequency. 6. Computed responses for viscoelastic caps showed creep under constant force, and stress relaxation under constant indentation, with faster and greater sensory adaptation for indentation than for force. 7. We conclude cap compliance exerts a mild filtering action throughout the entire time course of adaptation in viscoelastic sensilla, but has no role in the adaptation of purely elastic ones.

Dynamic hardness testing of metallic materials

Abstract: A method of hardness testing is described based on the principle of energy measurement. The construction and operation of an apparatus operating on this principle is given and a comparison is made with static indentation hardness tests.

Selected properties of dental composites.

Abstract: The traditional parameters used to evaluate dental composites in the laboratory have been working time, polymerization contraction, compressive, tensile, shear, and bond strength, resistance to indentation, water sorption and solubility, leakage, color stability by the 24-hour ultraviolet lamp test, and thermal coefficient of expansion. Other less well documented properties of composites are their modulus of elasticity, abrasive wear, wetting by water or saliva, and staining. It is the purpose of this paper to review the present state of knowledge of this latter set of properties and to attempt to interpret them with respect to the application of dental composite filling materials.

Portable hardness tester

Abstract: A hardness tester includes a frame with a penetrator slidably carried by said frame for movement along a penetration axis, and thereby adapted to form an indentation in a specimen upon application of a load thereto. The resulting depth of the indentation is determinative of the hardness number of the specimen. The means by which a load is applied to the penetrator is rotatably actuated. The rotational displacement of the loading means bears a predetermined relationship to the linear dimension of the indentation depth formed during hardness testing of a specimen. The hardness tester further includes electro-optical mechanical encoding means, and further adapted to emit a coded electrical signal indicative of the sensed angular displacement. Furthermore, an electronic counting circuit decodes the output signals of the encoding means in the form of a count. The count, determinative of the resulting depth of penetration, is indicative of the hardness number of the tested specimen. Optical numerical display means are also provided for visually displaying the specimen hardness number.

Contact‐Damage Resistance of Partially Leached Glasses

Abstract: Strength degradation characteristics of partially leached glass rods damaged in sharp contact are investigated Residual compressive stresses in the outer layer (clad) of the treated rods inhibit the growth of surface cracks toward the inner region (core) The strength of rods indented quasi-statically with a Vickers diamond pyramid shows functional dependences on contact load and clad stress in good agreement with predictions based on indentation fracture theory The analysis identifies the degree of residual compression as the key parameter in the strengthening effect; however, in designing for high-performance applications it may be necessary to optimize material properties (eg toughness, elastic moduli) and dimensions of the clad/core composite Partial leaching, with its potential for developing high, uniform stresses over the clad thickness, may be advocated as a most useful means of obtaining high resistance to adverse contact conditions

Indentation-debonding of an Adhered Surface Layer

Abstract: In many technological applications (e.g. circuitboards), relatively thin, soft polymer layers are bonded to a harder (metal) surface. A new mechanical test for the adhesive bond strength between these layers using sharp, needle-like indenters, has been developed. This test and its experimental-analytical features are described in this paper. In the central indented region of the bonded polymer, compressive deformations of the top layer and subsurface take place. At the rim of the central indented area, however, a tensile stress is induced in the interfacial bond between the top layer and the substrate. Debonding occurs when this stress exceeds the bond strength. A debonded polymer surface layer displays Newton's rings because it diffracts light. Observations indicate that the debonded segment of the surface layer behaves as an elastic plate between the rim of indentation and the unfailed bonded region. The “peeling moment” capacity of adhesive at the bonded edge limits the bond resistance. Considering the...

Reverse Plastic Flow Associated With Plastic Indentation

Abstract: When an indenter is pressed into a surface as in a hardness test, the material beneath the indenter undergoes plastic flow. When the load is removed from the indenter, a second plastic flow occurs in the opposite direction. This paper presents experimental evidence for this reverse plastic flow and mentions a number of practical implications of this behavior.

Method for forming shaped buried layers in semiconductor devices utilizing etching, epitaxial deposition and oxide formation

Abstract: A method for forming a shaped buried layer in a semiconductor structure includes the steps of removing a portion of semiconductor material from adjacent the surface of the semiconductor substrate to form an indentation, introducing a dopant into the surface of the indentation to form regions of impurity in the semiconductor substrate, forming a region of epitaxial material on the surface of the indentation, and forming regions of insulating material to surround the epitaxial material.

Effect of Workpiece Properties on Grinding Forces in Polycrystalline Ceramics.

Abstract: : Grinding forces have been measured on a number of polycrystalline ceramics and correlations between the forces and workpiece properties have been attempted in an effort to gain insight into mechanisms of chip formation. The closest correlations were with a parameter related to the indentation fracture characteristics of each workpiece. This suggest that a major mechanism of chip formation during multipoint abrasive machining of ceramics is fracture resulting from the plastic indentation of the workpiece by the abrasive grains. Grinding forces during up and down grinding were different but both correlated with the indentation fracture parameter when the comminution characteristics of the chips during down grinding were accounted for. (Author)

Contact‐Induced Failure of Prestressed Glass Plates

Abstract: An indentation fracture technique was used to determine critical contact conditions under which prestressed brittle surfaces are subject to catastrophic failure. A theoretical model based on the growth of a well-developed, contact-induced half-penny crack leads to a simple inverse-cube power relation between indentation load and tensile prestress. The analysis is developed in terms of fracture parameters which are readily calibrated in routine indentation/strength tests. Experiments on glass disks loaded simultaneously in biaxial flexure and Vickers indentation confirm the essential failure predictions of the theory; toughness is the key material parameter controlling resistance to failure. The results emphasize the danger of spurious tensile stresses in ceramic systems exposed to severe contact events.

Indentation processes in poly (methyl methacrylate).

Abstract: Indentation processes in PMMA are examined in order to test the theory developed by Puttick, Smith and Miller (1977). Ball, cone and wedge indenters of various configurations are considered and the effect of loading rate is also considered. For ball and cone indenters the effect of friction is found to be most important and corrections are made for this. By use of fractography and other methods values are assigned to the c/a ratio for various conditions. The values are found to be between 1.8 and 2.0 for ball indenters and 1.5 and 1.6 for cone indenters. Using these c/a ratios the theoretical predictions of indentation pressures are shown to be in close agreement with the experimental results for the test conditions used. For wedge indenters the model is of limited use due to changes in deformation mode with wedge angle.

Evaluation of effective sueface energy by a microindentation method

Abstract: 1. The Indentation made by a Vickers pyramid, like the elastic indentation of a spherical indenter, produces a tapered crack in brittle materials under indentation. The size of visible radial cracks surrounding the pyramid indentation is practically the same as that of a tapered crack. 2. A method is proposed for the evaluation of the effective surface energy from the results of microhardness tests. Some features connected with allowances for plastic strain under the indentation are noted.