The current revision of the standard recommends use of two specimen designs from E1820 that provides provision for the load-line displacement measurement. Load-line displacement measurement is the preferred method for J calculation in E1921. In addition to these designs, the E399 design is also allowed. In the E399 design, the razor blades for extensometer are located at the front face which is 0.25W distance from the load-line position. To accommodate the front face displacement measurement, Section 7.1 prescribes that “the load-line displacement can be inferred by multiplying the measured values by the constant 0.73.” The correction multiplier comes from 1980 work published by J.D. Landes (22). In this work (22), the coefficient 0.73 was derived using simple geometrical correction.
However, during the November 2017 task group meeting, it was recognized that while the load-line measurement method would remain the preferred method for J calculation, there are some conditions when use of load-line displacement is impractical. Examples for such conditions include testing in extreme environment as well as use of small size compact specimens. In addition, better analytical solutions have been developed to accommodate inference of the load-line displacement when measurements are performed at locations that are different from the load-line. One of these solutions is described in the book of K. Wallin, “Fracture Toughness of Engineering Materials - Estimation and Application”. Based on the discussion during the 2017 task meeting it was suggested to prepare this revision of the standard.
This ballot adds a fourth compact specimen design and allows the use of displacement measurements at any position up to 0.35W in front of the load-line. In section 7.1, a new equation describes how load-line compliance shall be inferred from compliance at measurement locations that are different from the load-line. The reference (22) is updated to Wallin’s book. Finally, section 9.1.4 describes how the load-line plastic area (AP) shall be inferred from the plastic area measured at locations other than the load-line (APx). Please note that the proposed Equation 15a takes into account an erratum in reference (22).