Experimental study of effects of prior overload on fracture toughness of A533B steel

Fatigue Fract Eng Mat Struct

et al. 2004

A B S T R A C T Quantification of the enhancement in cleavage fracture toughness of ferritic steels following warm pre-stressing has received great interest in light of its significance in the integrity assessment of such structures as pressure vessels. A Beremin type probability distribution model, i.e., a local stress-based approach to cleavage fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre-stressing, WPS) in two ferritic steels with different geometry configurations. Firstly, the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels are identified. These parameters are then used in two-and three-dimensional finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures (WPS) to determine the probability of failure. Using both isotropic hardening and kinematic hardening material models, the effect of hardening response on the predictions obtained from the suggested approach has been examined. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the importance of the crack tip residual stresses. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. Predictions are compared with the available experimental data. The paper finally discusses the results in the context of the non-uniqueness of the Weibull parameters and investigates the sensitivity of predictions to the Weibull exponent, m, and the relevance of m to the stress triaxiality factor as suggested in the literature. a = crack length (mm) W = ligament (mm) a/W = crack/ligament ratio B = specimen thickness (mm) B 0 = reference thickness (mm) B/B 0 = thickness correction ratio i = order number (ascending) of a specific specimen tested in a group (i = 1, . . . , N ) β = 'shape parameter' exponent for toughness-based distribution m = Weibull exponent N = total number of specimens tested under the same conditions (sample size) K 0f = reference fracture toughness (MPa √ m) K f = fracture toughness after WPS (MPa √ m) K min f = minimum (threshold) fracture toughness (MPa √ m)

Section: Introductionmentioning

confidence: 99%

A local approach to cleavage fracture in ferritic steels following warm pre‐stressing

Fatigue Fract Eng Mat Struct

et al. 2004

“…Test results from two pressure vessel ferritic steels, A533B and A508 are summarised here. Both steels were used in earlier test programmes by Smith and Garwood [6], Fowler [5], and Swankie [13] For the A533B steel, tests were carried out by Smith and Garwood [6] using single edge notch bend (SE(B)) containing fatigue pre-cracks with nominal a/W=0.5. Fourteen specimens in the as received conditions were fractured tested at -170°C with another fourteen preloaded and unloaded at room temperature and then cooled and fractured at -170°C (the LUCF loading cycle).…”

Section: Summary Of Experimental Resultsmentioning

confidence: 99%

“…Studies by Reed and Knott [3,4] and Fowler [5] suggest that crack tip residual stresses are the dominating factor. Experimental observations indicate considerable uncertainty (wide scatter) in the cleavage fracture toughness of ferritic pressure vessel steels before and after WPS [6,7]. Statistical models mostly based on Weibull three and/or four parameter distributions are used to describe the scatter in the test results.…”

Section: Introductionmentioning

confidence: 99%

Predicting How Crack Tip Residual Stresses Influence Brittle Fracture

Computational Weld Mechanics, Constraint, and Weld Fracture

et al. 2002

A probability distribution model, based on the local approach to fracture, has been developed and used for estimating cleavage fracture following prior loading (or warm pre-stressing) in two ferritic steels. Although there are many experimental studies it is not clear from these studies whether the generation of local residual stress and/or crack tip blunting as a result of prior loading contribute to the enhancement in toughness. We first identify the Weibull parameters required to match the experimental scatter in lower shelf toughness of the candidate steels. Second we use these parameters in finite element simulations of prior loading on the upper shelf followed by unloading and cooling to lower shelf temperatures to determine the probability of failure. The predictions are consistent with experimental scatter in toughness following WPS and provide a means of determining the relative importance of the crack tip residual stresses and crack tip blunting. We demonstrate that for our steels the crack tip residual stress is the pivotal feature in improving the fracture toughness following WPS. The paper finally discusses these results in the context of the non-uniqueness and the sensitivity of the Weibull parameters.

“…Earlier experimental results for twenty eight A533B specimens were obtained by Smith and Garwood [4] using single edge notch bend SEN(B) specimens containing fatigue pre-cracks with nominal a/W=0.5 and W=2B=100 mm. Fourteen specimens in the as-received condition were loaded to fracture at -170°C with another fourteen in the warm pre-stressed condition at same temperature.…”

Section: Methodsmentioning

confidence: 99%

“…The experimental data of pre-cracked specimens carried out by Garwood-Smith [4] were available for SEN(B), B=50 mm. These data can be converted for use with the C(T), B=25 mm model based on the toughness dependence with specimen thickness as follows: where K B1 and K B0 are the fracture toughness corresponding to B 1 and B 0 (the reference thickness).…”

Section: Probability Analysis 41 Calibration Of the Weibull Parametersmentioning

confidence: 99%

The Role of Constraint and Warm Pre-Stress on Brittle Fracture in Ferritic Steels

et al. 2004

KEM

This paper presents the results of an experimental and numerical study carried out to investigate the effect of warm pre-stressing on cleavage fracture in ferritic steels using cracked and notched specimens. It is shown that the local approach based on Weibull theory predicts the increase in toughness following warm pre-stressing in highly constrained geometries. The observed effect of pre-loading in low constraint specimens such as round notched bars is less. The local approach could not predict the differences and it is suggested that the variation of triaxiality factor, the ratio of hydrostatic stress to Von Mises, in the plastic zone, is a contributing factor.