جستجوی مقالات مرتبط با کلیدواژه "شکست نرم" در نشریات گروه "مکانیک"

The size effect in microscale occurs due to the presence of a limited number of grains in deformation area and affects the mechanical behavior of the material. Based on this, describing the material behavior in the micro scale requires new relations and theories. In this article, the fracture behavior of 304 stainless steel thin sheet was tested by performing a uniaxial tensile test on the specimens with different thicknesses and grain sizes in the micro scale. The results showed that the fracture strain of different specimens in the micro scale decreases with decreasing thickness or increasing the grain size of the sheet. In order to consider the interactive effect of sheet thickness and grain size, the aspect ratio parameter was defined as the ratio of thickness to the average grain size of the sheet. Based on this, Ayada phenomenological ductile fracture criterion was modified by considering the size effect in the micro scale. Predicting the fracture locus using the modified criterion is in accordance with the fracture strain of different specimens in the micro scale, and this criterion can predict the fracture strain of different specimens with a maximum error of 4.4%. Based on the obtained results, it was found that the proposed fracture criterion can be used to predict fracture in the micro scale processes.

In this study, the phenomenon of ductile fracture in single-stage roll forming processes was investigated. In this regard, the fracture behavior of the 6061-T6 aluminum alloy was examined using the Modified Mohr-Coulomb fracture criterion. Subsequently, the calibration of the fracture criterion was carried out using a combined experimental-simulation approach. The roll forming process was modeled using finite element analysis, and the fracture criterion was implemented within the appropriate subroutine of the finite element simulation. According to the results, the Modified Mohr-Coulomb criterion with a 4.47% error capability could predict the onset of fracture in the single-stage roll forming process. Considering the sensitivity of the damage distribution accuracy to the stress state during the process, the effects of process parameters including thickness, bending angle, and corner radius on the triaxial stress parameter were investigated. The results indicate that these parameters do not have a significant impact on the triaxial stress parameter, and its average value remains within the range of 0.057 throughout all cases. Therefore, the fracture prediction results obtained with this criterion can be extended to other roll forming scenarios. Based on this, the influence of process parameters on damage and fracture during the roll forming process was examined. According to the results, an increase in sheet thickness and bending angle leads to an increase in damage, while an increase in the corner radius results in a reduction in damage area and a delayed occurrence of fracture during the process

The theory of continuum damage mechanics with a phenomenological approach is used in predicting the phenomenon of material failure, and since each material has its own behavior, it is especially important to find out the damage behavior of common used metals. In this study, the constants of two damage models, Ganjiani and Bonora, for analyzing the elastoplastic damage behavior of several metals by writing VUMAT subroutine in Abaqus software, simulation and comparison with experimental data in literature, were determined and the ability and efficiency of models in the analysis of metals damage behavior was ensured. In finding the constants of the models, stress-strain, damage-strain and failures train-stress triaxiality diagrams were used and after the simulation, with the help of force-displacement diagram, which had a good agreement with the experimental diagram for each metal, the accuracy of the performance was confirmed. In general, the constants during a trial and error process were determined to be the optimal fit. The studied metals were steel 1045, aluminum 2024-T351and HY130 steel. For the first two metals, only simple tension test is simulated, and for HY130 steel a more general test, three point bending test, is simulated and the obtained force-displacement diagram was compared with the experimental diagram. Also, because the strains were in the range of large strains, the relationships related to large strains have been used.

The present study is devoted to experimental and numerical investigation of in-situ tensile tests to recognize the mechanisms of ductile fracture under different stress states. The GTN model, which is a micromechanical based damage model, has used for numerical simulations. The parameters related to this model for St12 steel were identified by response surface method (RSM) through minimizing the difference between numerical and experimental results of the tensile test on a standard specimen. The void related parameters of GTN model were determined 0.00107, 0.00716, 0.01, and 0.15 for ff, fc, fN, f0, respectively. After calibrating the damage model for the studied material, the tensile tests were carried out on the in-situ specimens with different geometries. The fractographic analysis was performed to identify the ductile fracture under a wide range of stress states and two failure mechanisms were observed. The calibrated damage model was applied to FE simulations of in-situ tensile specimens for numerical study of the experimentally observed fracture phenomenon. The extracted numerical results showed a good agreement with experimental observations comparing load-displacement plots with a margin of error within 5%. The location of fracture initiation, crack growth orientation, and the displacement at fracture zone in numerical studies also showed close correspondence with experiments.

In this paper, a coupled plasticity-phase field model for ductile fracture is proposed. The Drucker-Prager plasticity model, which have been applied to metals, concrete, polymers, foams, and other pressure-dependent materials, is coupled with the phase field method. The governing equations are determined by a minimization principle that results in balance laws for the coupled displacement-fracture phase field problem. Furthermore, the finite element implementation, discretization and integration algorithms for the proposed model are presented for three-dimensional, plane strain and plane stress states. In addition, to control the influence of the plastic work and its effect on the crack propagation process, a threshold variable is introduced. Using a numerical example, it is demonstrated that a specific length scale and a certain minimum element size is necessary such that the regularized crack surface converges to the sharp crack. The accuracy of the proposed model and integration algorithm is verified by comparing the obtained results with existing experimental data. In addition, the Arcan sample, by means of a special test setup, allows to load a sample at different direction, and thus performing mixed mode fracture investigation using the model.

A Forming Limit Diagram (FLD) is a graph which depicts the major strains versus values of the minor strains at the onset of localized necking. Experimental determination of a FLD is usually very time consuming and requires special equipment. Many analytical and numerical models have been developed to overcome these difficulties. The Gurson- Tvergaard- Needlemann (GTN) damage model is a micromechanical model for ductile fracture. This model describes the damage evolution in the microstructure with physical equations، so that crack initiation due to mechanical loading can be predicted. In this work by using the GTN damage model، a failure criterion based on void evolution was examined. The aim is to derive constitutive equations from Gurson''s plastic potential function in order to predict the plastic deformation and failure of sheet metals. These equations have been solved by analytical approach. The Forming Limit Diagrams of some alloys which studied in the literatures have been predicted using MATLAB software. The results of analytical approach have been compared with experimental and numerical results of some other researchers and showed good agreement. The effects of GTN model parameters including 〖 f〗_0 〖،f〗_C 〖،f〗_N،f_f، as well as anisotropy coefficient and strain hardening exponent on the FLD and the growth procedure of void volume fraction have been investigated analytically.

Assessment of fracture toughness in pipe weldment is important for pipeline engineers and designers as welded zone is the most critical site for crack initiation and propagation. This research reports the determination of fracture toughness in spiral seam weld of API X65 pipe steel. This steel pipe has vast usage in Iran’s gas transportation projects. To measure the material fracture toughness، single edge notch bend (SENB) specimens were machined from the pipe. For evaluating the fracture toughness in test material، two techniques were used (based on multi-specimen tests in ASTM E 1820 standard). These techniques were based on determination of crack tip opening displacement (CTOD)، and JIC. A value of 0. 23 mm was obtained for CTOD، and its equivalent KIC was 265 MPa√m from the first method. The second test method gave a value of 396 KJ/m2 for JIC، whose equivalent KIC was 332 MPa√m. A discussion on the reasons for KIC difference from the two test techniques، and comparison between the obtained results and similar data from the literature conclude the paper.

Abstract- Damage of metals is a progressive physical process which finally leads to the failure of them. In this study, first, a coupled elastic- plastic- Lemaitre's ductile damage model combined with large deformations theory is developed and implemented as a subroutine into ABAQUS/EXPLICIT code. Then, by performing standard tensile and Vickers micro-hardness tests, mechanical and damage properties for St14 steel are determined. For validation of the damage model and also identified properties, cutting and fine cutting processes are simulated by the model in two cases of large and small deformation theories. Comparison of the numerical simulation results and experimental reports show that Lemaitre's ductile damage model combined with large deformation theory can accurately predict damage evolution, crack initiation, propagation, and ductile fracture in the metal forming processes with large deformations. Keywords: Lemaitre's Ductile Damage Model, Large Deformations Theory, Cutting and Fine Cutting Processes, Ductile Fracture.