mohammadmehdi kasaei

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.

The deformation behavior of the material in micro-forming processes is different from macro-scale due to the size effect. The size effect in micro-scale appears due to few grains in the deformation region and causes the material behavior to be affected by the thickness and grain size of the sheet. Because of this, conventional constitutive models are not suitable for predicting the material behavior in micro-forming processes. In this paper, a new constitutive model based on the Swift equation and considering size effect in micro-scale is presented to describe the strain-hardening behavior of the stainless steel 304 foil. Comparison of flow stress curves of specimens with different grain sizes showed that the prediction of material flow stress with the new constitutive model is improved compared to the existing model, especially at high strains, so that the average and maximum error of the new model is less than one-third and less than half of the conventional model error, respectively. Finite element simulation of the micro-tensile test was performed using the new constitutive model to investigate the size effect on the deformation behavior of the specimens. The new constitutive model was verified by comparing the results of experimental tests and finite element simulation of sheets with different grain sizes. Also, the results revealed that the estimation of the forming force using the new constitutive model is done with higher accuracy than the conventional and existing model for sheets with different grain sizes and high strain ranges.

In this research, the thermal analysis of additive fabrication by DMLS method has been investigated. In the DMLS method, the metal powder is melted by a laser heat source and finally a solid three-dimensional piece is formed. This analysis was performed by finite element method in Abaqus software. Laser heat distribution is considered Gaussian. The mechanical and thermal properties of the powder are considered as a function of melting temperature. Finally, the results obtained by the finite element method are compared with previous researches. The effects of laser speed and power on temperature distribution have also been investigated.

In this research, the twist and the longitudinal bow defects were studied by finite element analysis in the asymmetrical channel sections. Effects of the geometrical properties including the strip thickness, the web width, and the flange width were investigated on the above effects. Some experiments were performed on an industrial roll-forming machine to verify the accuracy of the finite element model. The results showed that the twist angle decreases with the increase of both strip thickness and web width. However, the twist angle increases with the increase of flanges width. Also the variations of the longitudinal bow height are similar to those of the average residual longitudinal strain of both flange edges.

In this paper, a new sealing method has been applied to eliminate the friction force between the tube and the die. The new sealing method was experimentally examined in the free bulge test of AA6063 aluminum tubes and necking limit strains, thickness and bulge height of specimens were measured. Results showed that the new sealing method compared to the conventional sealing method improves the material flow to the deformation zone and, therefore, increases the bulge height of aluminum tubes. Finite element simulations of the free bulge test were performed in ABAQUS software under the different process parameters. In these simulations, forming limit curve obtained from the experimental tests was utilized to predict of the necking. Results showed that an increase of friction coefficient, axial feed and initial length tub increases the bulge height of aluminum tubs in the new sealing method compared to the conventional sealing method.

In this paper, cold roll forming process of a high strength steel pipe using four types of flower pattern including circular, edge, double radius and reverse bending is simulated with finite element method in MSC Marc Mentat software. Due to importance of quality of final pipe and in order to achieve the desired geometry in high strength steel pipes, selecting the appropriate flower pattern to design the pipe roll forming production line is considered. Using finite element simulation results, deformation of sheet in this process is studied and effect of flower pattern type on geometry of final product, which includes curvature distribution, spring back and thickness distribution of pipe, is investigated. Results show that implementing reverse bending flower pattern, leads to reduction in deviation from mean curvature at edge of the sheet up to about 65 percent. Thickness distribution analysis shows that circular and edge flower patterns cause upsetting and thinning of edge of the sheet up to 0.2 millimeters, respectively. But, use of double radius and reverse bending patterns cause average thickness of edge to be well adjusted to reach 2.8 millimeters. Also, circular flower pattern has the lowest value of spring back in terms of variation of mean relative curvature of 0.69 percent and edge deviation of 0.15 millimeters. To validate the finite element simulation, experimental tests were designed and conducted using one forming stand. By comparing resultant data of experimental tests with simulation results, validity of finite element simulation confirmed.

Flexible roll forming is a modern process to produce products with variable cross section that can be used in construction and automotive industries. The portion which connects the slim section to the wide one is called transition zone. In this paper, effect of different bend curves on the web warping has been studied in the transition zone. Considering the geometrical boundary conditions of the transition zone, five different curves including circular, quadratic, cubic, quartic, and quintic curves were studied numerically and experimentally. According to the finite element results, longitudinal strains on profile edge which are less than the desirable strain increase the web warping defect. Product with circular bend curve showed the maximum amount of web warping while the quartic bend curve led to the lowest web warping. In order to verify the finite element analysis, the longitudinal edge strain and the web warping were measured in products with Circular bend curve experimentally. A good agreement between the experimental and finite element results confirmed the accuracy of the finite element model.

Flexible roll forming is a modern process for producing profiles with changing cross section. One of the important defects in this process is the web warping of product that causes failure to obtain dimensional and geometrical tolerances. In this paper، mechanism of web warping occurrence was investigated by finite element simulation in ABAQUS/CAE software. Results of simulation indicated that inadequate longitudinal strain in the edge of profile’s flange in transition zone is the reason of profile’s web warping. Furthermore، the effect of geometric parameters of product such as flange length، bend angle، radius of transition zone and thickness on the web warping were determined. Analysis of variance showed flange length and bend angle are recognized as the most effective factors on warping of profiles with specific thickness. An equation for prediction of warping was proposed in terms of geometrical parameters of product. In order to verify the finite element model، the longitudinal strain of deformed strip edge was obtained from simulations and compared with the experimental results of other researchers. A good agreement between them confirmed the accuracy of the finite element model.

Cage roll forming is one of the new methods of cold roll forming process which is widely used for producing ERW pipes. In addition to less production cost and time, using cage roll forming provides smooth deformation on the strip. Few studies can be found about cage roll forming because of its complexity, and the available knowledge is more experience-based and not science-based. In this paper, deformation of pipes with small thickness/diameter ratio is investigated by 3D finite element simulation in Marc-Mentat software. Edge buckling defect in roll forming of pipes with small thickness/diameter ratio is very important. Due to direct effect of longitudinal strain on the edge buckling phenomenon, longitudinal strains at the edge and center line of the deformed strip are investigated and the high risk stands are introduced. Deformation flower pattern and curvature of the deformed strip are predicted using the simulation results and the effects of each cage forming stage on the deformed strip profile are specified. In order to verify the simulation results, circumferential length, opening distance of the two edges and forming power in different forming stages are obtained from the simulations and are compared that of the experimental data which were measured from the production line. A good agreement between the experimental and simulated results is observed.