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Experimental Methods to Determine a Tearing Resistance Curve


A tearing resistance curve (R-curve) characterises a metal’s behaviour to absorb ductile energy at the plastic zone at the tip of a crack propagating by slow stable ductile tearing. The R-curve is the power-law (or offset power-law) curve fit through experimental data plotted as fracture toughness (CTOD or J) against stable crack extension, Δa. 

Experimentally, a fracture mechanics specimen, i.e. prepared with a crack-sharp notch, is incrementally loaded and the value of fracture toughness in terms of CTOD or J is determined, along with a corresponding value of the crack length at the point of measurement. The values of J and CTOD will be based on formulae given in established fracture toughness testing standards, and may be also corrected for the crack extension during the test. Data is generated for a range of tearing from less than half a millimetre, up to where the specimen has exceeded maximum load during test.

The conventional method to determine an R-curve is to perform a series of identical tests (at least six), and take each specimen up to different load, typically three which are unloaded before reaching maximum load in the test, and three which are taken to different levels beyond maximum load. The value of fracture toughness at the unloading point (NOT the maximum load point) is determined, and the amount of stable ductile tearing is measured from the fracture face after test (often heat tinting is used to make this clearer). These six results are used to plot a ‘multiple-specimen’ R-curve. The number of separate tests required can make this approach less desirable, and there are other methods to estimate the crack length as it grows throughout a single test. One such method is known as the ‘unloading compliance’ method, where, as the specimen goes through its full loading during the test, is repeatedly partially elastically unloaded. The slope of the unloading is related to the specimen stiffness (compliance), and hence crack length at that point. Individual values of J and CTOD can also be determined at each unloading point. In this way one specimen may be used to generate several dozen J and Δa data points to fit the R-curve. The specimens are usually instrumented using clip gauges at the notch mouth to determine the compliance based on crack mouth opening displacement (CMOD) values. Machining side grooves on the test specimen can promote straighter-fronted tearing. Typically triplicate specimens are used, in a similar manner to single point fracture toughness tests. As an alternative to unloading compliance, sometimes the crack length can be monitored using direct current potential drop (dcpd) throughout the continuously rising load test, and used to define values of Δa for plotting an R-curve.

There are three main standards which describe the fracture toughness testing of ductile alloys by generating tearing resistance curves: BS 7448-4 (1997), ISO 12135 (2016) and ASTM E1820 (2018). Each of these standards differs in significant details relating to the generation and analysis of the test data. Find out more about the techniques and methods given in these standards for the determination of tearing resistance curves (R-curves) in single-edge notched bend (SENB) fracture toughness test specimens.

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