The Compelling Case for Indentation as a Functional Exploratory and Characterization Tool

Abstract

The utility of indentation testing for characterizing a wide range of mechanical properties of brittle materials is highlighted in light of recent articles questioning its validity, specifically in relation to the measurement of toughness. Contrary to assertion by some critics, indentation fracture theory is fundamentally founded in Griffith-Irwin fracture mechanics, based on model crack systems evolving within inhomogeneous but well-documented elastic and elastic-plastic contact stress fields. Notwithstanding some numerical uncertainty in associated stress intensity factor relations, the technique remains an unrivalled quick, convenient and economical means for comparative, site-specific toughness evaluation. Most importantly, indentation patterns are unique fingerprints of mechanical behavior and thereby afford a powerful functional tool for exploring the richness of material diversity. At the same time, it is cautioned that unconditional usage without due attention to the conformation of the indentation patterns can lead to overstated toughness values. Limitations of an alternative, more engineering approach to fracture evaluation, that of propagating a precrack through a "standard" machined specimen, are also outlined. Misconceptions in the critical literature concerning the fundamental nature of crack equilibrium and stability within contact and other inhomogeneous stress fields are discussed.

Experimental and finite element study of residual thermal stresses in veneered Y-TZP structures

“…Different methodologies have been used to study the residual stress in bilayer specimens: analytical models [6,18], the hole-drilling method [19,20], optical birefringence technique [7,21], Vickers indentation method [8, 22–24] and finite element analysis [10,25]. Some of these methods can estimate the residual stresses only in simple bilayer geometries, e.g.…”

Nanoindentation of HMX and Idoxuridine to Determine Mechanical Similarity

“…Additionally, yield points during initial loading can be indicative of the onset of dislocation nucleation [20]. Finally, indentation-induced fracture [21][22][23][24][25][26] has been used by many researchers as a surrogate to quantify toughness in materials where meeting American Society for Testing and Materials (ASTM) standard specimen geometries would be challenging. Elastic modulus, hardness, and yield behavior are often measured using a Berkovich indenter probe, but more acute probes, such as the cube corner geometry, can be useful in initiating fracture.…”

XFEM investigation on Knoop indentation cracking: Fracture toughness and aspect-ratio of radial-median cracks

“…σ is the damage-initiating stress threshold, and 〈…〉 denotes the Macaulay brackets, thus ensuring that only tensile stresses can cause damage. 1 The resulting damage plane is oriented normal to traction σ. It is to be noted that damage initiation is mesh-dependent, and the exact time (i.e.…”

“…Indentation, on the other hand, has the compelling advantages that it is directly applicable to small structures and does not require pre-cracks, which makes the method practical and inexpensive. However, analysis requires a profound understanding of the indentation stress and deformation field and fracture mechanics [1].…”

“…Bilinear traction-separation (σ -δ) law constituting damage initiation and propagation 1. The Macaulay brackets are defined as follows: 〈x〉 = 0 for x b 0, and 〈x〉 = x for x ≥ 0.…”