Showing papers on "Indentation published in 1971"

Plastic Deformation of Aluminum Oxide by Indentation and Abrasion

Abstract: Transmission electron microscopy provided direct evidence that plastic deformation occurs during the room-temperature indentation and abrasion of Al2O3. Examination of single-crystal and polycrystalline specimens showed that high densities of dislocations are produced within the near-surface regions by mechanical polishing with a fine diamond compound (0.25 μm) and that plastic deformation by both slip and mechanical twinning occurs during the placement of Vickers microhardness indentations. The occurrence of plastic deformation in this normally brittle material is considered to be a consequence of the nature and magnitude of the local stresses developed under pointed indenters and irregularly shaped abrasive particles. Preliminary results on the effect of annealing on the retained substructure are also presented. Annealing at 900°C and higher resulted in the reduction of residual stresses through the motion of dislocations and their rearrangement into lower-energy configurations.

Anisotropy in the hardness of single crystals

Abstract: The results of this work, and those published by other researchers who have used Knoop indentation measurements, confirm that the nature of anisotropy in hardness is essentially determined by the crystal structure and the primary slip systems which accommodate dislocation motion during indentation. Materials belonging to the same class of crystal structure and having common slip systems possess similar anisotropic properties. The varying extent of work-hardening or fracture, associated with indentations, does not appear to influence the anisotropy— although twinning on the basal planes of hexagonal closepacked metals may have a significant effect. An analysis of the indentation process is presented which establishes a clear relationship between the ‘effective resolved shear stress’ (t 0 9), in the bulk of the crystal beneath the indenter, and the observed hardness. Directions which correspond to the minimum values of t9 e , on specific crystallographic surfaces, are those of maximum hardness and conversely. The analysis is shown to be equally applicable to a wide range of crystalline solids including nonmetallic materials, of various crystal structure, and typical f.c.c., b.c.c. and c.p.h. metals. Finally, anisotropy in hardness can be used to identify active slip systems in those crystals where it is possible for dislocations to move on more than one system.

The Plane Strain Indentation of Anisotropic Aluminium Using a Frictionless Flat Rectangular Punch

Abstract: In Part 1, the slip-line field solutions and the associated load requirements necessary for the indentation of anisotropic solids are presented. The analysis is based on Hill's approach to the analysis of anisotropic material. All results are recorded in terms of a lumped anisotropic parameter, c.In Part 2, the results of an investigation to determine the anisotropic parameters of a commercially available aluminium are reported. Specimens machined from the aluminium at selected orientations to the anisotropic axes were indented using a nominally frictionless flat rectangular punch. A comparison between the theoretical and experimental indentation loads is given.

Indentation lifetime of swollen poly (β-hydroxyethyl methacrylate) networks

Abstract: An original indentation lifetime method was used in the investigation of the ultimate properties of crosslinked poly(β-hydroxyethyl methacrylate) gels swollen with water to equilibrium. The method consists in the determination of the time (indentation lifetime,τ) which is needed to produce a failure in a polymer sample by means of a thin cylindrical indentor under constant load. The curves obtained by plotting the indentation lifetimex in dependence on stressσ at various temperatures can be superimposed to yield a universal dependence; the temperature dependence of the shift factorsa T obeys theWLF equation. It was found that thea T values corresponded to the shift factors of the viscoelastic statical measurements at much shorter times. From the temperature dependence of the indentation lifetime, the valuesT s from theWLF equation were determined for samples swollen to equilibrium and prepared at various concentrations of the crosslinking agent or diluent at polymerization. The difference found between the dependence of theT s values on the concentration of the crosslinking agent and diluent and the same dependence determined in dry samples by measuring the creep compliance was elucidated. The non-monotonous dependence of the indentation lifetime values on stress was explained in terms of the specific deformation and stress geometry of the indentation test.

A numerical method for treating indentation problems

Abstract: Abel transforms are used to simplify the equations describing the deformation of an elastic medium by a rigid indentor with a circular cross-section. The equations derived constitute a convenient starting point for the numerical solution of such problems. As a particular example the case of a parabolic rough punch is treated.

Nature of the hardness anisotropy on the (001) faces of PbS and CaF2 single crystals

Abstract: Indentation and scratching methods were used to study the anisotropy of the mechanical properties of the (001) faces of PbS and CaF2 single crystals. The surface relief near the indentations and scratches was studied. A study was made of the dislocation rosettes around the indentations and of the dislocation zones around the scratches, in the directions of maximal and minimal hardness. A scheme is proposed for the plastic deformation which occurs in PbS and CaF2 crystals scratched along these directions. A qualitative explanation is advanced for the experimental hardness anisotropy.

Mechanical Properties of Plastic Foams. II. The Modulus of Elasticity and Indentation of Bead Type Foams

Abstract: Part one ( 1 ) of this series was concerned with geometry, modulus of elasticity, and yield stress of closed cell and open cell plastic foams. It is intended in this paper to investigate the modulus of elasticity of bead type foams through an analysis of compression behavior of two foam beads. Since the indentation of soft material involves finite deformation, a contact theory for finite deformation is developed, first, for smooth beads, then the discussion of indentation is given for two beads of rough surfaces. Lastly, the theoretical aspect of the modulus of molded bead foam is presented. The mechanical properties of foam play an important role in impact cushioning design such as the foam-padded automobile dashboard for forehead safety. Figure 1 shows the stress-strain curve of molded bead foam. This experimental result is presented here as an aid to the derivation of the theory. Figure 2 shows our preliminary data of Young’s modulus vs. the density of molded bead foam.