Fumiyoshi Minami

Estimation procedure for the Weibull parameters used in the local approach

The local approach was recently proposed by Beremin and Mudry for evaluating the statistical behaviour of toughness results of materials. This approach introduces a stress parameter σw, termed the Weibull stress, as a measure of the fracture resistance of materials instead of the conventional toughness parameters such as Kc, δc and Jcl (critical stress intensity factor, CTOD and J-integral, respectively). The Weibull stress σw obeys the Weibull distribution with the two parameters m and σu (the shape and the scale parameter, respectively). The first parameter m is normally estimated to be 22 irrespective of the kind of material. In this paper a procedure for the determination of the Weibull parameters m and σu is developed. This procedure consists of the determination of the plastic zone ahead of the crack tip, from which cleavage fracture originates, and of the maximum likelihood estimation of the parameters m and σu based on the stress distribution in the plastic zone. Calculations using this procedure confirm that the distribution of the Weibull stress σw is a material property independent of specimen thickness, and in particular that the shape parameter m depends on the material, e.g. m≃12 for a German reactor pressure vessel steel (20 Mn Mo Ni 5 5). Using these parameters for the distribution of the Weibull stress the size effect in fracture toughness values is predicted and an improved agreement between theory and experiments is obtained compared to the Weakest Link model.

Evaluation of Prestraining and Dynamic Loading Effects on the Fracture Toughness of Structural Steels by the Local Approach

On the occasion of recent great earthquakes, great concern is focused on the prevention of unstable fracture of steel structures against the seismic loading. This paper employs the local approach for the evaluation of prestraining and dynamic loading effects, experienced during an earthquake, on the fracture toughness of structural steels. The prestraining and dynamic loading lead to a similar result: increasing the yield stress and tensile strength and decreasing the fracture toughness. It is shown, however, that the combined effects of prestraining and dynamic loading is not equivalent to the sum of each individual effect. The analysis using the local approach demonstrates that the critical Weibull stress at brittle fracture initiation is independent of prestraining and dynamic loading. Based on the Weibull stress fracture criterion, the prestraining and dynamic loading effects on the fracture toughness can be predicted from static toughness results of the virgin material. As an engineering application, a simplified method is proposed for the estimation of fracture toughness under the seismic condition. This method uses a reference temperature concept: the dynamic fracture toughness at the service temperature T with prestrain is displaced by the static toughness of the virgin material at a lower temperature T -ΔT PD , where ΔT PD is a temperature shift of the fracture toughness caused by prestraining and dynamic loading. The temperature shift ΔT PD is provided as a function of the flow stress elevation in the seismic condition.

Damage Model for Predicting the Effect of Steel Properties on Ductile Crack Growth Resistance

This study pays attention to reveal material properties that control a resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. Crack growth tests using 3-point bend specimens with a fatigue pre-crack are conducted for two steels that have different ductile crack growth resistance, whereas both steels have the same mechanical properties in terms of strength and work hardening. Observation of crack growth behaviors provides that different mechanisms between ductile crack initiation from fatigue pre-crack and subsequent growth process can be applied. It is shown that two types of ductile properties of steel associated with ductile damage can mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from a surface of notch root, and the other one is a stress triaxiality dependent ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve is proposed taking the above two ductile properties into account, where the ductile crack initiation from crack-tip is in accordance with local strain criterion, and the subsequent crack growth triaxiality dependent damage criterion. The proposed model accurately predicts the measured different R-curves between two steels used that have the same ‘strength properties’, and also the stress triaxiality dependence of R-curve.

A Statistical Approach for Fracture of Brittle Materials Based on the Chain-of-Bundles Model

An analytical method for the assessment of failure probability of brittle materials exhibiting progressive cracking prior to cleavage fracture is presented. The underlying fracture mechanism is based on the assumption that instability of a critical flaw no longer leads to failure and causes redistribution of the local stresses. The fracture process progresses by consecutive unstable propagation of the surviving flaws up to total failure. A limiting distribution for the fracture stress, which is identical with the first asymptotic distribution of smallest values, is derived on the basis of a chain-of-bundles probability model. Numerical procedures for calculating the parameters of the limiting distribution are also described. Due to the nature of the resulting distribution, the method employs the maximum likelihood estimation of parameters combined with a finite element solution to the crack-tip fields. An application of the present model to analyze the effect of notch depth on fracture toughness values obtained from single-edge notch bend (SENB) specimens is also included.

Fracture mechanics analysis of Charpy test results based on the weibull stress criterion

Abstract The material fracture toughness is often estimated from the Charpy impact energy using empirical correlations. However, each correlation has a limitation of application, although some are implemented in fabrication standards. This study employs the Local Approach to interpret Charpy test results. Instrumented Charpy tests and fracture toughness tests are performed in the lower-transition range for structural steels of 490 and 780 MPa strength class. Stress fields are addressed by 3D-FEM considering the strain rate effect and temperature rise during dynamic loading. It is shown that the critical Weibull stress at brittle fracture initiation is almost independent of the loading rate. This enables the Charpy results to be transferred to the material fracture toughness. As an alternative to the instrumented test, a simplified procedure is proposed: The evaluation of fracture initiation at 0.6 KV to 0.8 KV leads to a good estimation of brittle fracture toughness in the lower-transition range, where KV is a total impact energy.

Local Approach to Dynamic Fracture Toughness Evaluation

Fracture toughness properties of structural steels of 490-MPa strength class and weld metals under dynamic loading are studied in the light of the near crack tip stress fields. The loading rate in toughness tests range from 0.1 to 500 mm/s. The near-tip stress fields are analyzed by 3D-FEM considering the strain rate effect on the flow properties of materials. Temperature rise caused by high-speed plastic deformation is also included in the FE analysis. The critical CTOD at brittle fracture initiation decreases when increasing the loading rate. This is due to the elevation of a local stress near the crack tip. The local approach is applied to the dynamic fracture toughness evaluation. It is shown that the brittle fracture resistance evaluated in terms of the Weibull stress, an integrated stress over a highly stressed region near the crack tip, is a material property independent of the loading rate. Based on the Weibull stress fracture criterion, the dynamic fracture toughness can be predicted from static toughness results. An engineering procedure to estimate the loading rate effect on the fracture toughness is also presented.

Fe calculations of stress fields from cracks located at the fusion line of weldments

Abstract This paper investigates the stress fields for a crack located at the fusion line of a weldment. The strength mis-matching and the size of the HAZ were varied, and the corresponding distribution of the maximum principal stress was examined. The weld metal strength was globally overmatched with respect to the base material, but locally over- and undermatched with respect to the heat affected zone. Three cases of mis-match were compared, and it was found that reducing the strength of the HAZ lowered the maximum principal stresses.

Constraint Correction of Fracture Toughness CTOD for Fracture Performance Evaluation of Structural Components

A correction of CTOD for constraint loss in large-scale yielding conditions is made on the basis of the Weibull stress fracture criterion that eliminates an excessive conservatism in the conventional fracture assessment and material fracture toughness requirement. A CTOD ratio β = δ3P / δWP (

A probabilistic analysis on thickness effect in fracture toughness

Abstract Fracture toughness values are often influenced by specimen thickness and they indicate generally decreasing toughness with increasing thickness. In the present paper, a probabilistic analysis has been carried out by using various kinds of toughness data in order to clarify the applicability of the weakest link model to thickness effect in fracture toughness. Moreover, a new statistical method is proposed for determining fracture toughness distribution, which is necessary for the above analysis, with taking the temperature dependency of fracture toughness into account. Thickness effect in fracture toughness is brought about by its statistical nature and the weakest link model can be applied to evaluate the thickness effect for both steel plate and its welds with heterogeneity in toughness. This thickness effect is considerably affected by Weibull shape parameter and the probability of cleavage fracture for the material. The statistical method proposed newly in this paper is sufficiently applicable and superior to the conventional method. By using this new method, Weibull parameters at a temperature of interest can be determined with much the same reliability as in the conventional method, and also Weibull parameters at lower and higher temperatures can be obtained with a certain confidence depending on the number of specimens tested.

Fracture Toughness Evaluation of Laser Beam-Welded Joints of 780 MPa-Strength Class Steel

For fracture assessment of steel structures with laser beam-welded joints, it is significantly important to evaluate the fracture toughness of the weld metal. However, the fracture toughness of weld metal is often impossible to measure by standard Charpy impact testing in the brittle-to-ductile transition or upper-shelf temperature region, because of fracture path deviation (FPD) from the weld metal due to narrow weld bead and a high degree of overmatching in strength. In this work, evaluated fracture toughness of laser beam-welded joints of 780 MPa-strength class (HT780) steel is examined by Charpy impact testing and CTOD fracture toughness testing. The improved Charpy impact tests, using the specimen with three parallel welds or the side-groove specimen, are conducted in order to prevent FPD. The applicability and significance of the improved impact test methods to the laser beam-welded joints of high-strength steel are investigated by means of Weibull stress analysis. The Weibull stress analysis indicates that the 3-weld method could not necessarily prevent FPD in laser beam-welded joints of HT780 steel, since side beads are not as hard as main weld beads, due to reheating at the main weld. On the other hand, a side-groove specimen is effective for avoiding FPD, but intrinsic VE of the weld metal could not necessarily be obtained by using this specimen.