Measurement and prediction of CTOD in austenitic stainless steel

Abstract

A B S T R A C T Variation of Crack Tip Opening Displacement (CTOD) test values can have a significant effect on the Engineering Critical Assessment of a structure. This paper examines the development of CTOD with increasing load in an austenitic stainless steel. The silicone replication method giving variation of CTOD across the specimen thickness, and Digital Image Correlation (DIC) are compared to each other, and in turn to clip gauge measurements from tests. Results from Finite Element models are also presented. Estimations of CTOD from BS 7448-1, ISO 12135 and ASTM E1820, and a proposed modification from JWES are compared to the experimental data from the crack cast in silicone compound -assumed to be the actual CTOD. The DIC measurement showed consistency with crack replicas, and a formula is given to estimate CTOD using DIC. For high strain hardening austenitic stainless steel, both the JWES and ASTM E1820 estimations provide adequate accuracy for CTOD. N O M E N C L A T U R EA p = plastic area under P versus V p a 0 = initial crack length B = specimen thickness B 0 = remaining ligament, W À a 0 b = position on section as a ratio of B / 2 E = modulus of elasticity J = strain energy around the crack K = stress intensity factor K I = stress intensity factor in mode I loading m = plane strain function used in JWES m ASTM = function relating J to CTOD n = strain hardening exponent P = load r p = rotational factor for plastic hinge assumption V g = clip gauge opening displacement V p = plastic component of clip gauge opening displacement W = specimen width z = knife edge height δ = crack tip opening displacement (CTOD) δ 5 = direct CTOD measurement from two points at the specimen surface 5 mm apart, placed directly at the crack tip δ 5 DIC = δ 5 measured using the DIC technique δ SRC = CTOD measured on the silicone replicas δ FE = CTOD obtained from the FE model v = Poisson's ratio σ ys =0.2% proof strength at test temperature σ uts = ultimate tensile strength at test temperature σ y = flow stress at test temperature, (σ ys + σ uts ) / 2 ε = strain ɳ = geometrical based calibration function for J Correspondence: C. J. Brown.

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

“…Green's observation on photo‐elastic images of notched bend specimens showed that the yielding pattern at the crack tip region is similar to that predicted in a slip‐line field . Digital image correlation observation on a SEN(B) specimen showed that the strain distribution near the crack tip is similar to that described in the slip line field theory by Green …”

“…A total of 50 quarter Bx2B single‐edge notched bend (SENB) specimen models were generated to investigate CTOD for a range of a 0 / W and σ ys / σ uts using ABAQUS v6.14. A blunted crack tip of 0.03‐mm radius was used in the models, which allows better plastic deformation at the crack tip region . The modelling technique used in this paper was shown to give good prediction of CTOD, further described in previous published works .…”

“…A blunted crack tip of 0.03‐mm radius was used in the models, which allows better plastic deformation at the crack tip region . The modelling technique used in this paper was shown to give good prediction of CTOD, further described in previous published works . The crack tip was designed to blunt continuously as the crack open without stable ductile tearing and thus leads to lower values of CTOD …”

“…The concept of rigid rotational point has been successfully implemented for the determination of CTOD with various degree of accuracy . To investigate the rotational factor, r p for SEN(B) specimens, an opened crack was investigated geometrically based on the similar triangles concept (Figure ).…”

Comparison of methods to determine CTOD for SENB specimens in different strain hardening steels

“…11,23 However, where there is curvature of the crack (for example, as a consequence of fatigue pre-cracking), the opposite may be true. This is not necessarily the case, 10,11,14 but Hutchison and Pisarski found that the curvature of the crack front greatly influences the resultant fracture toughness across the crack tip in the thickness direction.…”