Papers by Keyword: Cleavage Fracture

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Authors: Yoon Suk Chang, T.R. Lee, Jae Boong Choi, Young Jin Kim, Bong Sang Lee
Abstract: The scatter of measured fracture toughness data and transferability problems for specimens with different crack configurations and loading conditions are major obstacles for integrity assessment of ferritic steels in ductile-brittle transition region. To address these issues, recently, concerns for local approach adopting micro-mechanical damage models are being increased again in connection with a progress of computational technology. In this paper, cleavage fracture evaluation based on Weibull statistics was carried out for SA508 carbon steel. A series of three dimensional finite element analyses as well as corresponding fracture toughness tests were performed for 1T-CT and PCVN specimens at -60°C. Also, failure probability analyses for different configurations and sensitivity analyses for Weibull parameters were conducted. Thereby, promising results have been derived through comparison between measured and estimated fracture toughness data, which can be utilized to make the basis for demonstrating real safety margins of components containing defect.
Authors: Andrzej Neimitz, Ihor Dzioba, Urszula Janus
Abstract: In the paper the fracture mechanisms in ultra-high-strength steel are examined. However, the emphasis is on cleavage fracture, which was observed in the whole temperature range tested. The extent of cleavage depends on the specimen thickness and temperature. The experimental program consisted of tensile and fracture tests and was followed by scanning microscope observations of the fracture surfaces. Then, a full 3D elastic-plastic finite element analysis was carried out assuming finite strains. The numerical results support the postulate that the onset of cleavage jump is observed when the maximum opening stresses become higher than the critical value (the material property at a given temperature) over a distance greater than the critical value (material property independent of temperature). A discussion of so-called 3D stress parameters is presented.
Authors: M.J. Balart, Claire L. Davis, Martin Strangwood, J.F. Knott
Abstract: The effects of matrix microstructure and features of non-metallic inclusion (morphology, type, volume fraction and size) on cleavage initiation in medium-carbon Ti-V-N and V-N microalloyed resulphurized forging steels have been determined by examining fracture surfaces produced in room temperature Charpy impact tests. The steels were generally Al-deoxidised but one V-N steel was Si-deoxidised. It has been found that, in the Ti-treated steel, having a ferrite-pearlite microstructure, brittle fracture initiation occurred at cracked coarse (Ti,V)(C,N) single phase or [(Ti,V)(C,N)/Al2O3/MnS] multi-phase inclusions. In the Ti-free steels, cleavage initiation was dependent on matrix microstructure and non-metallic inclusions. In the low strength Ti-free steels, with a ferrite-pearlite microstructure, the absence of a continuous grain boundary ferrite layer led to initiation from interfacing pearlite colonies. For the bainitic microstructure, cleavage initiated close to the notch, but the microstructural feature responsible could not be identified. For the ferritepearlite microstructure in the Si-deoxidised V-N steel, cleavage initiated at cracked Mn-Al-(Ca) silicate inclusions. The higher matrix strength and more continuous nature of grain boundary allotriomorphic ferrite in the V-N steel deoxidised with Al was associated with cleavage initiation from V-rich (V,Ti)(C,N)-containing inclusions. These were generally of smaller size than those in the Ti-treated steels.
Authors: Saeid Hadidi-Moud, A. Mirzaee-Sisan, Christopher E. Truman, David John Smith
Abstract: Potentially both global and local approaches may be used to predicting the effect of loading history on cleavage fracture toughness distribution of ferritic steels. In this paper the dramatic increase in the apparent lower shelf fracture toughness of A533B steel following warm pre-stressing (WPS) has been predicted using these approaches. Extensive experimental evidence suggesting significant enhancement in fracture toughness of ferritic steels within the lower shelf temperatures following WPS are used to verify and compare the applicability and the extent of validity of the models. The global approach is based on the distribution of toughness data described by Wallin statistical model in conjunction with the Chell model for WPS effect. The local approach on the other hand is a Beremin type model that uses the Weibull stress to predict the WPS effect. Weibull stresses would essentially reflect the WPS effect on redistribution of stress-state around the crack tip. Predictions for apparent toughness using the two approaches are discussed in the light of the suggestion that residual stresses are the main cause of the enhancement, at least for the material and geometry used in this study.
Authors: Han Ok Ko, Sun Jung Kang, Yoon Suk Chang, Jae Boong Choi, Young Jin Kim, Min Chul Kim, Bong Sang Lee
Abstract: Fracture toughness data from the cleavage resistance test of structural steels often show a large scatter. Geometry dependency as well as the scatter makes it difficult to evaluate appropriate fracture integrity of cracked components. To address these restrictions, several stochastic models have been proposed by Beremin group, Mudry and other researchers while each of them employs specific estimation scheme and micro-mechanical parameters. The purpose of this paper is to investigate applicability of the Weibull stress model in transition temperature regime and to quantify constraint effect among different-sized CT and PCVN specimens. The constituting parameters m and σu are determined at three temperatures by maximum likelihood estimate (MLE) technique in use of FE analysis results and experimental data of PCVN specimens. Also, failure probabilities of PCVN and CT specimens are calculated from the Weibull parameters, which are used for derivation of a prototype of toughness scale diagram. The diagram provides a technical basis to resolve transferability issue in the same material under different temperatures and constraint conditions.
Authors: Huan Ju He, Ling Feng Zhang, Shao Zhe Li
Abstract: The dynamic compression test was carried out for the AZ91 Magnesium alloy of as-cast and aging state with a split Hopkinson pressure bar, and the dynamic behavior has been investigated. Finally the fracture surface of samples at the different strain rates was analyzed by scanning electron microscope (SEM). The results show that compared with the as-cast AZ91, it is more impressible for the solid solution and age-treatment AZ91 at the similar strong strain rates, while the stress-strain curve has the duality of positive and negative effects as the variation of strain rate.
Authors: A. Ray, Debalay Chakrabarti
Abstract: Charpy impact testing (over the transition temperature rage) on different samples of a Ti-microalloyed steel, having the same average-TiN particle size but different average-ferrite grain sizes, showed that in spite of the presence of large TiN cuboides, ferrite grain refinement can significantly improve the impact toughness, provided the meso-texture (i.e. the intensity of low-angle boundaries) and matrix strength can be restricted to low values.
Authors: Yan Guo Liu, Jin Ma, Xian Ming Sun
Abstract: A research on the effect of strip defect in the notched specimen of low alloy hot rolled steel 16MnR at -196°C is carried out in this paper. 4-point positive and negative bending experiments of notched specimen with different preloads are carried out to introduce strip defects of different dimensions in the front of notch. And then the residual stress and work hardening is eliminated through high temperature tempering. Bending fracture experiment is carried out at -196°C. Through microscopic observation and the measurement and analysis of mechanical parameters, it is discovered that: when the preload ratio P0/Pgy0.861, the Pf /Pgy rapidly decreases as the P0/Pgy increases. That’s because the increase of local high stress-strain region caused by the strip defect at the front notch end makes the distribution of crack-shaped nucleus active region of cleavage fracture increase, causes the cleavage initiation and leads to discrete numeric values of material notch toughness Pf and W.
Authors: Qing Song Wu, Hong Chuan Zhu, Jiang Tao Zhao, Fang Yi Sun, Kuan Hei Hu
Abstract: The relationship between texture and secondary work embrittlement of interstitial-free steels was studied by X-ray analysis technique and scanning electron microscopy analysis technique. The results indicated that the stronger the γ fiber texture was, the higher the plastic strain ratio (r-value) was, and Secondary work embrittlement temperature rose as r-value increased or γ fiber texture intensified. The mode of secondary work embrittlement fracture was a blend of cleavage fracture and intergranular fracture, and the fraction of cleavage fractures decreased gradually as secondary work embrittlement temperature rose.
Authors: H. Nakumura, H. Kobayashi, Takamitsu Shimizu, S. Kanno, M. Saito, M. Asano, Han Ki Yoon
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