The Effect of Material Strain Hardening on the Determination of CTOD R-Curves Using SENB Specimens

Abstract

In ductile materials the fracture toughness is usually characterised by a tearing resistance curve, or R-curve, plotting the fracture toughness in terms of J or CTOD against crack extension. Recent research has evaluated the methods to determine CTOD in engineering alloys with a wide range of yield to tensile (Y/T) ratios for single point CTOD. This work develops the investigation into R-curves, and reviews the assumptions about SENB specimens deforming under rigid rotation, the evaluation of CTOD from J for R-curves, and the nature of tearing initiation in low Y/T ratio stainless steel, from comparisons against a series of silicone replicas cast from the SENB specimen notch during fracture toughness tests. For CTOD R-curves, the methods based on CTOD from J in ISO 12135 and ASTM E1820 gave lower and less accurate R-curves than the rigid rotation methods in BS 7448-4 and WES 1108. However, the accuracy of the BS 7448-4 formula varied for the different strain hardening materials, and overestimated the R-curves in the low tensile ratio stainless steel. Investigations into the effect of the assumption about rigid rotation in different strain hardening materials led to a rotational factor function of tensile ratio, rp sh, to be developed from numerical modelling. When this function was substituted into standard equations in place of the fixed value of rp an improvement in the accuracy of BS 7448-4 R-curves compared to replica measurements was seen for the range of strain hardening investigated, but it did not significantly improve the accuracy of the WES 1108 formula, which accounts for strain hardening in other parameters. A combination of the elastic CTOD part of the WES 1108 formula, with the plastic CTOD incorporating the modified rotational factor, was concluded to offer the optimum method to determine CTOD R-curves for a range of strain hardening materials.

Finite element analysis on grey cast iron assets: a case study on penstocks in the wastewater system

“…It should be noted that FEA modelling (Han et al, 2014, Khor et al, 2018, Benamara et al, 2016, Wang et al, 2000, Moore and Khor, 2018 and imaging techniques (Budzik andJensen, 2016, Wang andShuai, 2012), such as digital image correlation (Quino et al, 2020, Yates et al, 2010, Hart and Bruck, 2021, have been successfully applied on the analysis of cracks. However, these approaches would require detailed understanding of the material properties, loading condition and boundary conditions.…”

A CTOD equation based on the rigid rotational factor with the consideration of crack tip blunting due to strain hardening for SEN(B)

“…For the range of 0.44 ≤ σ ys / σ uts ≤ 0.98 and 0.3 ≤ a 0 / W ≤ 0.7, CTOD between 0.2 and 1.0 mm was considered for the investigation. CTOD below 0.2 mm is considered “critical” in standards, where the elastic‐based stress intensity factor, K , is more relevant; CTOD beyond 1.0 mm is generally large, where some significant stable crack propagation would be expected in real specimens …”