Papers by Keyword: Fracture Strain

Abstract: This paper assesses various newly developed ductile fracture criteria including modified Mohr-Coulomb (MMC), DF2012, DF2014, DF2016, Hu-Chen and Mu-Zang, which were all proposed in the last decade. The AA2024-T351 is used for the assessment by comparing the predicted fracture limits to the experimental results both in strain and stress spaces. Fracture loci are also constructed by these criteria to evaluate their characteristics. The evaluation demonstrates that the Lode parameter and stress triaxiality should be properly coupled for reasonable modeling of ductile fracture in wide loading conditions. This study also shows that the coupling of the Lode parameter can also be realized by introducing the effect of the largest shear stress in fracture criteria.

Abstract: This study observes the behaviors of selective laser melted (SLM) Ti-6Al-4V cylindrical samples when compressed along different directions. The effects of defect and microstructures upon compression behaviors are discussed. The disc-shape defects formed between the building layers does not affect the compression strength of SLM samples tested along different directions. The compression strength is up to 1330 MPa. The columnar structures formed along the building direction causes the plastic deformation to vary greatly with sample orientations. The samples compressed along the building Z direction sustain up to 24.4% deformation, while the samples compressed along the horizontal X and Y directions show a much lower fracture strain of 13.9%. The plastic deformation of columnar grains in crack propagation were observed greater than compressing with the columnar boundaries lying parallel to the compression surfaces. The disc-shape pores also tend to coalesce with each other in the X and Y samples causing a more brittle behaviors.

Abstract: This paper is concerned with the prediction of fracture strains for DP980 steel sheets using a modified Lou–Huh ductile fracture criterion. The usage of DP980 steel is significantly increasing in the automotive industry for weight reduction, enhancement of crashworthiness and safety of car body. The material behavior of AHSS show unpredictable and sudden fracture during sheet metal forming process. A modified Lou–Huh ductile fracture criterion is utilized to predict the formability of AHSS because the conventional FLD constructed based on necking is unable to evaluate the formability of AHSS. Fracture loci were extracted from 3D fracture envelopes by assuming the plane stress condition to evaluate equivalent plastic strain up to the point of fracture at a wide range of loading paths. Three different types of specimens such as pure shear, dog-bone and plane strain grooved specimens were used for tensile tests to construct 3D fracture envelopes of DP980. Fracture strain of each loading path was evaluated to show that there is little deviation between predicted fracture strains and experimentally acquired ones. From the comparison, it is concluded that the 3D fracture envelopes can accurately predict the onset of the fracture of DP980 steel sheets in complicated loading conditions including the pure shear condition.

Abstract: Under the influence of stresses and strains damage is progressively accumulated in the material leading to full damage viz. fracture corresponding to a critical damage parameter. The damage parameter varies in between zero and unity inclusive of both the values corresponding to non damaged and fully damaged condition. Also damage is a tensorial quantity with physical meaning. In order to represent this physical quantity, a damage-D plane is suggested. This is like a co-ordinate system to easy representation of damage as a function of fracture strain. The damage-D plane can be merged with engineering stress-strain curve beyond the UTS where the damage leads to fracture occurs in the material.

Abstract: Several possible routes are available for estimation of fracture behaviour from the results of small punch tests performed at constant rate of deflection. The routes include: (i) measurement of relevant dimensions directly on ruptured specimens, (ii) determination of critical deflections on the load vs. deflection curves and (iii) integration of these curves up to specific points. Equivalent fracture strain, fracture energy or fracture toughness are then evaluated from the obtained quantities. The mutual relations among the quantities are demonstrated by the results of small punch tests performed on a Fe-Al-based alloy in the temperature range extending from brittle up to ductile fracture appearance.

Abstract: This paper presents the calibration of three universal ductile fracture criteria with the stress triaxiality and the normalized third invariant of deviatoric stress dependence. Xue-Wierzbicki, Bai-Wierzbicki and Extended Mohr-Coulomb criteria are calibrated using butterfly specimen and newly designed notched tube specimen for an austenitic stainless steel. Different stress states necessary for successful calibration are generated by suitable combination of tension/compression-shear or tension/compression-torsion loading of both specimens, respectively. Suitability of the specimens for ductile fracture criteria calibration is evaluated comparing the range of reached stress states, the homogeneity of stress in process zone and appropriateness for the computational simulation of tests. Possible combination of above mentioned results with tensile testing of standard and notched cylindrical specimens is discussed, too.

Abstract: In heavy-duty cutting, large chips produced are wide, thick and of complex shapes. If the large chips are not controlled reasonably they will heavily influence the machining efficiency. This article researches on the generation process of large chips, and is combined with the research method of finite element simulation to establish curling model of large chips in heavy-duty cutting process. Through analyzing the fracture strain and cut strain suffered by the chips in the curling process of large chips and enlarging the large chips using electron microscopy, the generation and curling deformation law is revealed. And analyzing generation and curling mechanism of large chips can provide theoretical basis for the generation and control of ideal chips in heavy-duty cutting process.

Abstract: Based on previous results of both an increase of nearly 40% in static tensile strain by shortening fiber length from commercial 6.4 mm to 0.44 mm in an unsaturated polyester/styrene-butadiene GFRP-BMC composite containing 20 mass% short E-glass fibers and their acoustic emissions (AE), the fracture resistance mechanics of sub-mm length fiber dispersion reinforcement is proposed. Since the 40% strain increase acts to improve strength and toughness, the mechanics is useful. This paper aims to present the mechanism of strain-driven improvement where microcracks are prevented from propagating beyond the critical crack size (2a_c) for thermoset polymers, resulting in an increased and more dispersed total microcrack surface area as recorded by AE raising fracture strain.

Abstract: Damage of metals due to the influence of hydrogen is quite frequent and leads to dangerous failures. The characteristics of the hydrogen embrittlemnt of the 65Mn steel were evaluated with small punch test. With the increment of the amount of the hydrogen absorbed into the alloy at room temperature, the strength and the toughness of the material reduce. From the small punch experimental results, it is found the total impact energy, the fracture strain and the fracture stress decrease with the increment of the cathodic hydrogen charging time. The fracture surfaces change from the typical ductile fracture with big voids to the typical intergranular brittle fracture mode after hydrogen absorbed in the specimens with higher charging current density.

Abstract: The deformation behaviour of martensite and its effect on tensile properties of a lowcarbon dual-phase (DP) steel were investigated. DP steel samples with different martensite contents and morphologies were produced after intercritical annealing at different temperatures using low and high heating rates. Two distinct martensite morphologies were obtained for low and high heating rates. The investigated steel showed the unusual results that the true fracture stress and strain were found to increase with the martensite volume fraction. The plastic deformation of martensite was considered to be responsible for these results. Experimentally, it was observed that the martensite in DP steels with greater than 25-30% martensite can deform plastically during tensile straining. Finally, the effect of tempering on the martensite plasticity was also evaluated. It was found that the tempering process and an increase in the martensite content have a similar effect on promoting martensite plasticity.