Abstract
The scratch test is used for quality control mostly in phenomenological ways, and whether fracture toughness can be obtained from this test is still a matter of debate requiring further elucidation. In this paper, values of the fracture toughness of copper obtained by different scratch-based approaches are compared in order to examine the applicability of scratch-based methodologies to characterize the fracture toughness of soft metals. The scratch response of copper to a Rockwell C diamond indenter is studied under a constant normal load condition. The variations of penetration depth, residual depth, and residual scratch width with applied normal load are quantified from spherical to sphero-conical contact regimes by piecewise functions. A newly proposed size effect law is found to be the most suitable for scratch-based approaches to characterizing the fracture toughness of soft metallic materials with significant plasticity. A simple expression relating the nominal stress to the penetration depth is proposed for the spherical contact regime and gives almost the same value of fracture toughness. The residual scratch width provides useful information on pile-up of material and on the spherical tip radius of the indenter. It is found that the values of the fracture toughness obtained from the microscratch test are influenced by the data range for analysis.
Highlights
The scratch test is a convenient means of investigating material properties1–5 such as scratch hardness,6 coefficient of friction,7,8 wear response,9–12 fracture,13 elastic recovery,14 scratch resistance,15,16 and tribological8,17,18 and stick-slip19 behavior of, for example, polymers,20–25 metals,26 ceramics,27,28 and interfaces.29 The use of this test is fundamental to the understanding of contact-induced surface deformation30,31 and scratch-induced flaw/defect/damage/cracking37–44
The contact condition becomes stable with fully developed pile-up of material after the initially transient stage, and the measured variables fluctuate around average values. The recorded variables such as penetration and residual depths together with the tangential force keep fluctuating during the scratch test, in spite of the constant normal load applied, which can be attributed to dynamic effects, material pile-up, fracture, the stick–slip phenomenon,76,137 and effects due to randomness
Using data from microscratch tests, both linear elastic fracture mechanics and the energetic size effect laws proposed by Akono and Hubler and by Ulm underestimate the fracture toughness of metallic materials
Summary
The scratch test is a convenient means of investigating material properties such as scratch hardness, coefficient of friction, wear response, fracture, elastic recovery, scratch resistance, and tribological and stick-slip behavior of, for example, polymers, metals, ceramics, and interfaces. The use of this test is fundamental to the understanding of contact-induced surface deformation (e.g., reciprocating sliding contact, sliding contact, and single-asperity contact36) and scratch-induced flaw/defect/damage/cracking (e.g., mechanical glass frosting, machining/manufacturing, cutting, material removal, microabrasion, wear, stick–climb–slipinduced damage, polishing and grinding, and even phase transformation). Fracture toughness obtained via a microscratch test based on the energetic size effect law was found to be in excellent agreement with measurements from conventional fracture tests (e.g., the compact tension test and three-point bending test on notched specimens) for homogeneous material.120. By using a rectangular cutter with a strategy devised by Dagrain et al.,122 they found that side wall friction had a significant effect on the scratch test, which can be explained by noting the increase in specific energy with the side wall friction caused by dilation of the material ahead of the cutter.123 They argued that fracture toughness cannot be measured by a wide shallow groove cut,124 since the mechanism of the failure mode varies with cutting depth, and the simple size effect law requires a contact condition of geometrical similarity. A simple expression relating nominal contact pressure in the scratch direction with penetration depth is proposed, and it is found that fracture toughness can be obtained under the spherical contact regime
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