جستجوی مقالات مرتبط با کلیدواژه « ductile fracture » در نشریات گروه « مکانیک »

In this research, the load-bearing capacities of epoxy-based nanocomposite specimens containing rounded-tip V-shaped notches made of epoxy resin LR 630 and nanographene oxide were studied both experimentally and theoretically under pure opening mode conditions. In order to fabricate the studied specimens, first, the tensile properties and fracture toughness of pure epoxy resin and nanocomposite materials were determined by uniaxial monotonic tension and three-point bending tests. Rectangular plates containing a central rhombic hole with four blunt V-shaped corners with a notch angle of 60° and radii of 1, 2, and 4 mm were utilized as the samples for fracture tests. Then, the samples were subjected to uniaxial tensile loading, and their load-carrying capacities (LCC) were measured. For theoretical predictions, due to the ductile behavior of the studied specimens, a combination of the equivalent material concept (EMC) with the well-known brittle fracture criterion, maximum tangential stress (MTS), was employed. Then, experimental and theoretical results were compared. The results of the experiment showed that by adding nanoparticles to the epoxy resin, its strength improved by about 8%, and it was found that the maximum discrepancy between the theoretical and experimental results was related to the groove with a radius of 4 mm, approximately 9.2%. Finally, it was observed that the new criterion (EMC-MTS) could predict the experimental results well without performing any time-consuming and complex elastic-plastic analysis.

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 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.

Among the various models of ductile fracture, Gurson-Tevergard-Needleman (GTN) damage model has been widely used due to consider three steps; nucleation, growth, and the coalescence of voids during plastic deformation. Important issues in GTN model is the exact calculation of the model parameters which in experimental methods are very time consuming and costly. Therefore, the finite element simulation method is used for this purpose. The porous metals plasticity model in Abaqus software does not consider the coalescence of voids step and analyzes issues on the basis of two steps of nucleation and growth of voids. This causes an error in the results. In this study, the uniaxial tension test of aluminum alloy 5083-O is simulated with GTN damage model. Simulation is carried out by using finite element software Abaqus through writing code in the UMAT subroutine. The GTN damage model parameters of AA5083-O are evaluated by matching experimental engineering stress-strain curve and simulated curve. The results showed that the written code through UMAT subroutine addition to led to improve the simulation of GTN damage model in Abaqus software, provides an acceptable prediction of the GTN model parameters.

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.

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.