Cleavage Fracture Toughness Of Steels Research Articles

Development of New Formulation and Likelihood Function on Probabilistic Fracture Initiation Model

The authors propose a new model on cleavage fracture toughness of steels. Fracture toughness of steels is controlled by the weakest link mechanism, so it has intrinsic scatter. Beremin proposed Weibull stress through probabilistic fracture initiation model based on the weakest link theory, and it has been widely used for describing fracture toughness scatter. In his model, only propagation of micro crack is considered. However, micro crack nucleation should also be incorporated in order to estimate fracture toughness distribution more accurately. Bordet introduced micro crack initiation to the Beremin model, in which probability of micro crack nucleation is assumed proportional to plastic strain. Some previous studies have shown, however, that micro crack nucleation probability increases non-linearly with plastic strain. Then, we developed a new probabilistic model by introducing micro crack nucleation probability as a non-linear function of plastic strain.Furthermore, the authors developed a new method for obtaining Weibull parameters, in which not only distribution of fracture toughness values but also location of fracture initiation sites are considered through a newly developed likelihood function. We conducted fracture toughness tests with different specimen configurations and carried out convergence calculation for determining Weibull parameters by applying the likelihood function mentioned above. As a result of the calculation, the authors confirmed that the present model can obtain the Weibull parameters less sensitive to the specimen configurations and simulate distribution of fracture toughness more accurately than the previous models.

Mezzo-scopic Analysis of Fracture Toughness in Steels

The cleavage fracture toughness of steels was mezzo-scopically analyzed on the basis of the statistical local fracture criterion approach. The statistical stress criterion at the crack tip region suggests that the cleavage fracture toughness in steels can be described as a function of the yield stress, the cleavage fracture stress, and other mechanical properties of the materials. Formulation of the cleavage fracture toughness was first examined through an investigation on correlation between the cleavage toughness and the cleavage fracture stress obtained in notched round bar specimens in accordance with the theoretical prediction. Then, the scatter of the toughness, specimen thickness effect on the toughness, deterioration of the toughness due to cold working and irradiation, and improvement of the toughness caused by the Ni addition, were analyzed through the formulation of the toughness.

Quantitative prediction of embrittlement of low-carbon steels due to prestrain and effects of yield ratio

Summary The cleavage fracture toughness of steels is well-known to deteriorate under prestrain. Quantitative prediction of the fracture toughness after prestrain is examined in this paper. Based on the low fracture criterion approach, the cleavage fracture toughness of steels can be described in terms of the cleavage fracture strength and yield stress of the material. Since the experimental cleavage fracture strengths before and after prestrain are almost constant, loss of fracture toughness can be attributed to the work hardening behaviour of the base steel. The predicted fracture toughness values correspond well with the experimental results. The effect of the yield ratio on the loss of cleavage fracture toughness is also investigated. The experimental fracture toughness results show a heavier loss at a lower yield ratio as well as saturation under relatively high prestrain. These trends are qualitatively described by a numerical analysis based on the low fracture criterion approach.

予ひずみによる鋼材の破壊靭性低下量の定量的予測と降伏比の影響

It is well known that the cleavage fracture toughness of steels deteriorates by pre-straining. Quantitative prediction of fracture toughness after pre-strain was examined in this work. Based on the local fracture criterion approach, the cleavage fracture toughness of steels can be described in terms of the cleavage fracture strength and the yield strength of the materials. Since the experimental cleavage fracture strengths before and after pre-strain were almost constant, deterioration in fracture toughness can be attributed to the work hardening behavior of the base steel. The predicted fracture toughness successfully agreed with the experimental results. The effect of the yield ratio on the deterioration in cleavage fracture toughness was also investigated. The experimental results in fracture toughness showed larger deterioration in lower yield ratio steel and showed the saturation under the relatively high pre-strain. These tendencies could be qualitatively described with the numerical analysis based on the local fracture criterion approach.

高張力鋼溶接熱影響部の脆化に及ぼす島状マルテンサイトの影響の微視力学的検討

In the previous work, the state of the martensitic islands and its influence on the fracture toughness in low carbon steels simulated to the heat affected zone of multi-pass weld were investigated.It has been shown by the authors that cleavage fracture toughness of steels can be described in terms of the cleavage fracture stress and yield stress of materials.In the present work, experimental results in the previous work were analyzed along with the local criterion approach, and successful correlations were obtained. Through this correlation, the influence of the martensitic islands on the deterioration in toughness was investigated. It was suggested that the maximum size of martensitic islands was not an unique metallurgical factor which deteriorated the toughness, and mean free path and the statistical distribution of the martensitic islands had also a great influence on cleavage fracture toughness.