To determine the fracture toughness from an SENB specimen, the rising load is recorded during the test, and the corresponding specimen displacement is monitored. This displacement can either be the load line displacement (LLD, q) or the crack mouth opening displacement (CMOD, V). In SENB specimens the load line displacement is the vertical movement of the middle of the specimen as it rotates and bends under three point loading. Practically, experimental measurements of LLD using transducers or comparator bars are less accurate than estimates of LLD based on measurements of CMOD using clip gauges. To determine compliance (e.g. for unloading compliance R-curves) or CTOD then the value of CMOD is necessary. Therefore, it is common to instrument SENB specimens with either one clip gauge at the crack mouth, or a pair of clip gauges at knife edges at two distances above the crack mouth (known as double clip).
However, it is possible to determine fracture toughness in terms of J without using clip gauges, for example when testing in conditions challenging for clip gauge performance, such as in aggressive corrosive environments, or high temperature. The load line displacement can be determined from the crosshead ram displacement of the testing machine. The errors in the raw ram displacement data can be corrected for by testing a dummy un‐notched specimen through an equivalent load cycle in the same test machine, and measuring the ram displacement in this case. The extraneous displacement is then subtracted from the ram displacement measured at each point during the test. This approach is sometimes known as the ‘roller indent’ method as it accounts for the indentation of the roller into the bar of material for SENB specimens. In a comparison of different methods to determine J, using LLD from the roller indent method gave a similar, but slightly conservative value of J compared to when clip gauges were used (Moore & Pargeter, 2018).
There are three main standards which describe the fracture toughness testing: BS 7448-1 (1991), 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. If testing without clip gauges, the value of J can be used to estimate a value of CTOD, based on formulae given in ASTM E1820, since CTOD cannot be determined using rigid-rotation assumptions without CMOD. The ram-correction method could however, also permit multiple-specimen R-curves to be determined without clip gauges. However, it is not recommended to use the unloading compliance method to generate R-curves without clip gauges, since the LLD-based compliance equations are not as accurate as CMOD-based versions.
Moore P & Pargeter A, 2018 ‘Comparison of using the crack mouth displacement (CMOD) and load line displacement (LLD) methods in the determination of critical J integral in SENB specimens’. Fatigue Fract Eng Mater Struct (FFEMS). 2018;41:1997–2009. https://doi.org/10.1111/ffe.12837
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.