Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion,resulting in enhanced room temperature formability.The exact mechanism of the formability improvement is...Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion,resulting in enhanced room temperature formability.The exact mechanism of the formability improvement is not yet established.The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile(UT)tests.However,the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects.In the present work,an attempt is made to decouple the friction effect on formability improvement numerically.The improved formability is studied using a hole expansion test(HET).The limit of forming during hole expansion is modeled using the Hosford–Coulomb(HC)damage criteria,which is implemented as a user subroutine in a commercial explicit finite element(FE)software.Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading.Therefore,the difference between simulation and experimental hole expansion ratio(HER)can be used to decouple the friction effect from the overall formability improvement during hole expansion.The improvement in HER due to stress relaxation and friction effect is different.The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions.Compared to stress relaxation,friction effect played a major role during interrupted HET.展开更多
To clarify the effects of the vertical angle of a conical punch on stretch flangeability, hole expansion forming tests were conducted. Test results showed that the hole expansion ratio becomes larger as the vertical a...To clarify the effects of the vertical angle of a conical punch on stretch flangeability, hole expansion forming tests were conducted. Test results showed that the hole expansion ratio becomes larger as the vertical angle decreases.Results also showed that the fracture strain at the fracture location on the hole edge was constant and independent of the vertical angle. This is because the hole expansion ratio was controlled not only by the fracture strain, which is independent of the vertical angle, but also by deformation uniformity along the hole edge. From the result of numerical analyses, it was determined that deformation uniformity depends on the gradient of circumferential stress along the radius direction. When the vertical angle is sharp, the circumferential stress showed a steep decline and the deformation localization was suppressed. Consequently, the hole edge deformed more uniformly and the hole expansion ratio became larger. It is concluded that in order to improve stretch flangeability of high strength steel, it is important to uniformly deform the hole edge by applying a conical punch with a sharp vertical angle.展开更多
文摘Servo press forming machines are advanced forming systems that are capable of imparting interrupted punch motion,resulting in enhanced room temperature formability.The exact mechanism of the formability improvement is not yet established.The contribution of interrupted motion in the ductility improvement has been studied through stress relaxation phenomena in uniaxial tensile(UT)tests.However,the reason for improved formability observed when employing servo press is complicated due to the additional contribution from frictional effects.In the present work,an attempt is made to decouple the friction effect on formability improvement numerically.The improved formability is studied using a hole expansion test(HET).The limit of forming during hole expansion is modeled using the Hosford–Coulomb(HC)damage criteria,which is implemented as a user subroutine in a commercial explicit finite element(FE)software.Only the contribution of stress relaxation is accounted for in the evolution of the damage variable during interrupted loading.Therefore,the difference between simulation and experimental hole expansion ratio(HER)can be used to decouple the friction effect from the overall formability improvement during hole expansion.The improvement in HER due to stress relaxation and friction effect is different.The study showed that the model effectively captures the hole expansion deformation process in both monotonic and interrupted loading conditions.Compared to stress relaxation,friction effect played a major role during interrupted HET.
文摘To clarify the effects of the vertical angle of a conical punch on stretch flangeability, hole expansion forming tests were conducted. Test results showed that the hole expansion ratio becomes larger as the vertical angle decreases.Results also showed that the fracture strain at the fracture location on the hole edge was constant and independent of the vertical angle. This is because the hole expansion ratio was controlled not only by the fracture strain, which is independent of the vertical angle, but also by deformation uniformity along the hole edge. From the result of numerical analyses, it was determined that deformation uniformity depends on the gradient of circumferential stress along the radius direction. When the vertical angle is sharp, the circumferential stress showed a steep decline and the deformation localization was suppressed. Consequently, the hole edge deformed more uniformly and the hole expansion ratio became larger. It is concluded that in order to improve stretch flangeability of high strength steel, it is important to uniformly deform the hole edge by applying a conical punch with a sharp vertical angle.
