Indentation

About: Indentation is a research topic. Over the lifetime, 13002 publications have been published within this topic receiving 340476 citations.

Effects of anisotropy on elastic moduli measured by nanoindentation in human tibial cortical bone

Abstract: Many biological materials are known to be anisotropic. In particular, microstructural components of biological materials may grow in a preferred direction, giving rise to anisotropy in the microstructure. Nanoindentation has been shown to be an effective technique for determining the mechanical properties of microstructures as small as a few microns. However, the effects of anisotropy on the properties measured by nanoindentation have not been fully addressed. This study presents a method to account for the effects of anisotropy on elastic properties measured by nanoindentation. This method is used to correlate elastic properties determined from earlier nanoindentation experiments and from earlier ultrasonic velocity measurements in human tibial cortical bone. Also presented is a procedure to determine anisotropic elastic moduli from indentation measurements in multiple directions.

The Determination of Static and Dynamic Yield Stresses Using a Steel Ball

Abstract: The static and dynamical yield stress of the material of a thick steel plate may be estimated by pressing and by dropping a hard steel ball on a plane surface of the plate which has been ground and then polished. Under these conditions, the first appearance of an indentation on the polished surface can be detected with good accuracy, either by an optical interference method, or by an optical reflexion method. The statical experiment consists in finding the least force which must be applied to the steel ball to produce a permanent indentation, whilst the dynamical experiment consists in finding the least normal velocity of impact which gives similarly a permanent indentation. Using either the Guest-Mohr principal-stress difference or the von Mises shear strain energy hypotheses as criteria of failure, combined with an analysis of the stresses in the plate, it is shown how the appropriate yield stress can be calculated from the experimental data. Tests were made on a specimen of mild steel, two specimens of homogeneous armour plate and a very hard nickel-chrome steel of the type used for ball and roller bearings. The ratio of the dynamic value of the yield stress to the static value was found to increase as the hardness number decreases; the ratio was practically unity for the nickel-chrome steel, about 1⋅1 for the armour plate and about 2 for the mild steel. The values of the static yield stress found by the ball method and by an ordinary tensile or compression test are different; this is probably due partly to the inaccuracy of the criteria of plastic flow, partly to the difference in work-hardening in the two experiments, and partly to changes in the structure of the surface due to polishing. This discrepancy is without effect on the ratio of the dynamic to static yield stress as determined by the ball method, since the stress distributions in the static and dynamic ball experiments are identical.

Ultra-microhardness testing procedure with Vickers indenter

Abstract: Depth-sensing indentation equipment is widely used for evaluation of the hardness and Young's modulus of materials. The depth resolution of this technique allows the use of ultra-low loads. However, aspects related to the determination of the contact area under indentation should be cautiously considered when using this equipment. These are related to the geometrical imperfections of the tip, the diamond pyramidal punch and the formation of pileup or the presence of sink-in, which alter the shape and size of the indent. These and other aspects, such as the thermal drift of the equipment and the scattering at the zero indentation depth position related to surface finishing, are discussed in this work. A study concerning the hardness and the Young's modulus results determined by Vickers indentation on different materials was performed. Samples of fused silica, BK7 glass, aluminium, copper and mild steel (for which the values of Young's modulus were previously known) were tested using indentation loads in the range 10–1000 mN. Moreover, two methods are proposed for performing the indentation geometrical calibration of the contact area; these are compared with a former method proposed by Oliver and Pharr (OP). The present methods are based on: (i) analysis of the punch profile using atomic force microscopy (AFM); and (ii) a linear penetration-depth function correction (LM), based on knowledge of the values of the Young's modulus of several materials. By applying these methods to the indentation load/indentation depth results, it was possible to draw some conclusions about the benefit of the AFM and LM methods now under proposal.