Showing papers on "Indentation published in 1975"

Indentation fracture: principles and applications

Abstract: The basic principles and practical applications of indentation fracture are reviewed.

Equilibrium penny-like cracks in indentation fracture

Abstract: A study is made of the mechanics of two basic types of indentation fracture, cone cracks (“blunt” indenters) and median cracks (“sharp” indenters). The common feature which forms the central theme in this work is that both crack types, in their well-developed stages of growth, may be regarded as essentially “penny-like”. On this basis a universal similarity relation is derived for equilibrium crack dimension as a function of indentation load. Experimental measurements confirm the general form of this relation. A more detailed fracture mechanics analysis is then given, to account for additional, contact variables evident in the data. Notwithstanding certain analytical limitations, the study serves as a useful basis for investigating a wide range of contact-related problems, both fundamental and applied, in brittle solids.

Electron microscopy of microcracking about indentations in aluminium oxide and silicon carbide

Abstract: Transmission electron microscopy is used to examine the nature of microcracking about small-scale indentations in two highly brittle solids, sapphire and carborundum. The observed crack geometry is discussed in terms of an earlier model of indentation fracture beneath a point force, in which both loading and unloading half-cycles contribute to the crack growth. The residual interfaces are generally found to exhibit moire fringe contrast, and occasionally to contain dislocation networks. These observations are discussed in relation to spontaneous closure and healing mechanisms, and the associated “lattice mismatch” is estimated at about one part in a thousand. It is suggested that cleavage steps comprise the main source of obstruction to lattice restoration across the interfaces. Mechanical and thermal treatments of the indented specimens are found to influence the extent of the residual cracking. Some practical implications concerning the strength degradation of brittle solids are discussed.

Strength Degradation of Brittle Surfaces: Blunt Indenters

Abstract: Indentation fracture mechanics is used to develop a theoretical basis for predetermining the strength properties of brittle surfaces in prospective contact situations. Indenters are classified as blunt or sharp; only the first is considered in the present work. The classical Hertzian cone crack conveniently models the fracture damage incurred by the surface in this class of indentation event. Significant degradation is predicted at a critical contact load; when the load is increased beyond this critical level, further degradation occurs at a relatively slight rate. Bend tests on abraded glass slabs confirm the essential features of the theoretical predictions. The controlling variables in the degradation process, notably starting flaw size and in-denter radius, are investigated systematically. An indication is also given as to optimization of material parameters.

Investigation of the deformation mechanism of MgO crystals affected by concentrated load

Abstract: The mechanism of deformation under the indentation of MgO crystals has been studied by transmission and scanning electron microscopy. The dislocation structure and Burgers vectors of rosette dislocations are determined. The nature and geometry of cathodoluminescence around indentation have been also investigated. The luminescence radiation is shown to be caused by interstitial point defects built up under indentation. [Russian text ignoreed].

Young's modulus of lignin from a continuous indentation test

Abstract: The deformation of lignin in a continuous ball indentation test was almost entirely elastic up to a stress of 2.2 × 108 Pa (22 kg mm−2). The load versus depth of indentation curve of the lignin followed closely the classical Hertz equation thus enabling the Young's modulus of lignin to be calculated. From these results a stress-strain curve for lignin was drawn.

Development of Corrugations on Surfaces in Rolling Contact

Abstract: The self-generation of corrugations on metal surfaces in rolling contact has been studied experimentally in a rolling contact disc machine and theoretically using a computer simulation. The system vibrates in the ‘contact resonance’ mode excited by surface irregularities. If the damping is low and the load high, the vibration may be severe enough to cause plastic indentation of the surface in one revolution which then amplifies the vibration in the next revolution. A stability map has been produced which indicates the ranges of load, damping and surface roughness for which corrugations would be expected to develop.

Impact Damage Tolerance of Graphite/Epoxy Sandwich Panels.

Abstract: The resistance of graphite/epoxy sandwich panels to low energy, foreign object impact damage was studied. Falling weight impact tests were performed on 104 rectangular sandwich panel specimens. The effect of several material, geometry, and loading parameters on damage susceptibility was explored. Damage observed visually was related to residual strength of specimens taken from the impact region, and in some cases, to static indentation tests carried out on companion specimens. Sandwich test specimens were fabricated with face sheets of graphite or S-glass/epoxy with a 1-in. depth nomex honeycomb core. Fiber type, core density, and laminate orientation were fabrication variables. Drop weight tests were accomplished using a 2-in.-diameter steel ball. Initial and residual static shear strength were measured on notched, four-point bend specimens. Impact tests showed that graphite sandwich panels are much more susceptible to foreign object damage than S-glass panels. The impact energy required to sustain the same relative damage level was an order of magnitude greater for the S-glass than for the graphite panels. The failure characteristics observed suggest that this is due primarily to the low strain to failure of graphite composites. Local core crushing occurred in all tests, and all but the S-glass panels suffered fiber fracture and permanent indentation at low energy levels. Core stiffness had an observable effect on impact resistance, but other parameters studied did not appear to significantly affect damage tolerance. An analysis was carried out in which the sandwich panel was represented as an orthotropic sheet on an elastic foundation, and a classical, double Fourier series approach was taken. The load condition was approximated by an influence function technique to effect a constant radius of bending curvature, simulating the region around the drop weight ball. The capability exists to vary the material constants of the face sheet and core to investigate the nature of indentation failure, and to identify factors contributing to impact resistance. Numerical results were obtained for some of the material parameters employed in the impact tests.

Experimental Study of Edge Loading of Ice Plates

Abstract: The interaction of an ice cover and a fixed narrow structure was modelled by edge loading large rectangular (0.4 m × 0.5 m) plates of columnar-grained ice. The plates were set on one of the long edges and the load was applied to the opposite edge by means of indentors of widths varying between 12 and 150 mm. Ice plates 25 to 100 mm thick were used. Observations were made of the dependence of the indentation load on indentor width and ice thickness. All loading was within the range of ductile behavior of the ice. The study indicated that the indentation pressure was a function of the width of the indentor, but not of the thickness of the ice plate. The mode of deformation was observed to be primarily two dimensional. The results of this study are in contrast to observations under field conditions, from which it is concluded that the indentation pressure depends on the ratio of the width of the indentor to the thickness of the ice cover. This suggests the presence of an effect which must be given attention ...

Luminous marking in an indentation of an object

Abstract: A luminous mark on an object is provided by a phosphor and a source of nuclear radiation in an indentation of the object, the materials being held in place by a transparent cover closing said indentation.

Germanium microhardness and the correlation of indentation creep with dislocation velocity

Abstract: Temperature- and time-dependent measurements of the Vickers microhardness of germanium have been made. The measured hardness can be seen to be the result of two separate mechanisms. When an indentor is placed upon a germanium surface a time-independent, relatively temperature-independent mechanism is responsible for the formation of an initial impression. The indentation enlarges by a strongly temperature- and time-dependent glide process. The first process may be the result of either the semiconductor to metal phase transformation with subsequent deformation of the metallic phase or the athermal motion of dislocations over the Peierls barrier. At the present time there is insufficient evidence to distinguish between these two mechanisms. On the other hand, the second process can be shown to be a direct result of plastic flow by the thermally-activated motion of dislocations in germanium. A simple model is developed through which the time and temperature dependence of germanium microhardness can ...

Transient disturbances in a half-space during the first stage of frictionless indentation of a smooth rigid die of arbitrary shape

Abstract: A solution is obtained by the method of self-similar potentials for the time-spatial distribution of the contact stress between a homogeneous, isotropic, linearly elastic half-space and a smooth rigid die having an arbitrary indenting velocity and shape. The solution holds as long as the outward speed of the contact zone does not fall below the speed of the dilatational wave in the elastic medium. A proof is given that the instantaneous value of the force required to indent the die during this stage of contact is directly proportional to the product of the area of contact and the velocity of indentation at that instant. Introduction. In this paper the method of self-similar solutions [1-7] is used to solve problems in which rigid dies of arbitrary shape are pressed into a linearly elastic, homogeneous isotropic half-space at a rate sufficient to cause the contact to be superseismic. For these problems, no disturbance propagates along the surface of the halfspace more rapidly than the boundary of the region of contact. Consequently, there will be no deformation of the surface at points beyond the region of contact. Moreover, the deformation of the surface within the contact zone will be completely defined by the portion of the rigid die which has crossed the original position of the surface of the halfspace. In [5-7] it was shown that the rate of penetration must exceed a certain value for the contact between a half-space and a die with a cusp at the point of initial contact to be superseismic. However, the situation is quite different if the die is smooth in the region of contact. For problems of this type, the contact must always be superseismic for a finite interval of time. Moreover, the length of this interval of time can easily be computed from the shape and indentation velocity of the die and the velocity of the dilatational wave in the half-space. The problem in the title belongs to that class of elastodynamic problems the boundary conditions of which may be expressed in terms of functions which are homogeneous functions of space and time. For such problems, solutions may readily be obtained by either the self-similar potential approach or by the more familiar transform methods [8]. The virtual equivalence at these methods when applied to self-similar problems has most recently been demonstrated by Norwood [9]. * Received October 14, 1973; revised version received February 21, 1974. 216 A. R. ROBINSON AND J. C. THOMPSON Two-dimensional problems. For plane strain contact problems, it is convenient to denote the vertical displacement of points on the surface of the y = 0 half space by U(x, t). (We use the notation of [5-7] in this paper.) If the contact is frictionless, the boundary conditions defined below are sufficient completely to determine the stress and displacement fields at every point in the half-space: uv(xf 0 ft) — U(Xj f) j

The Varied Role of Plasticity in the Fracture of Inductile Ceramics

Abstract: Even in the most brittle solids there is usually a measure of local microplasticity which influences the temperature dependence of the fracture stress. Deviations in behavior from the usual response of solids which exhibit increasing ductility with temperature have been examined for a range of nominally inductile ceramics including a LAS glass ceramic, Si3N4, SiC, and Al2O3. The temperature dependence of the fracture behavior has been evaluated using specimens in which cracks of controlled size were introduced by a hardness indentation technique. The effects on the fracture behavior of varying crack lengths by using different indentation loads, of blunting the implaced cracks, and of varying temperature at constant crack size were determined. The results are consistent with the anticipated influence of plasticity on fracture behavior in metals, polymers and more ductile non-metallics.

Electron Microscopic Observations of Microcracking about Indentations in Aluminum Oxide and Silicon Carbide.

Abstract: : Transmission electron microscopy was used to examine the nature of microcracking about small-scale indentations in two highly brittle solids, sapphire and carborundum The observed crack geometry is discussed in terms of an earlier model of indentation fracture beneath a point force, in which both loading and unloading half-cycles contribute to the crack growth The residual interfaces are characterized mainly by moire patterns, sometimes by dislocation networks These observations are discussed in relation to spontaneous closure and healing mechanisms, and the 'lattice mismatch' necessary for their production estimated at about one part in a thousand It is shown that cleavage steps comprise the main source of obstruction to lattice restoration across the interfaces Mechanical and thermal treatments of the indented surfaces are found to influence the extent of the residual cracking Some practical implications of the observations are discussed

Regularities of plastic deformation in indentation on the (111) plane of NaCl single crystals

Abstract: Dislocation structures arising in indentation on the (111) face of NaCl single crystals are studied by etch-pit technique. The deformation temperature varied in the interval from 77 to 520 °K. The effect of radiation defects introduced by γ-irradiation on the shape of dislocation rosette was investigated. Three types of dislocation rosettes depending on the experimental conditions were observed. A possible explanation of this phenomenon is proposed. It is shown that the polarity of plastic deformation observed under indentation is an universal phenomenon which is revealed in the temperature range from 77 to 470 °K. The phenomenon of dislocation rosette inversion was first observed. [Russian Text Ignored]

Impact Response Characteristics of Polymeric Materials

Abstract: : Stress relaxation and creep behavior of PC and PMMA were studied under conditions of constant velocity loading followed by halts. This information was used to deduce relaxation-corrected, force-incursion depth behavior. Measurements of force versus depth were also made on sinusoidally programmed indentation. The evaluation of the 'densified' zone forming under the contact area was followed cinematographically. Models were developed including one that leads to the time and velocity dependence of the stress relaxation. (Author)

The indentation of a newtonian fluid in a finite cylindrical container by a right circular cylinder

Abstract: The indentation of the free surface of a Newtonian fluid in a finite cylindrical container by a right circular cylinder is considered. It is assumed that weight and inertia effects are negligible compared to viscous effects.

Fractured Structure around Vickers Hardness Indentation in TiC-Mo-Ni Cermets

Abstract: A peculiar structure was found to develop around Vickers hardness indentation in TiC base cermets, whereas it has never developed in WC base cemented carbides. Therefore, in this report, the above structure was investigated at room temperature using various cermets with various Ni, C and Mo contents and carbide grain size.As the result, it was made clear that the structure was mainly associated with trans- and intergranular cracks of carbide under the heavy stress generated by Vickers impression, so that it was considerably influenced by various factors as shown above. The fractured structure was considered to be due to lower strength and plasticity of titanium carbide solid solution than those of WC. These results would be a help for understanding the mechanical properties of cermets.