On Springback Prediction With Special Reference To Constitutive Modeling

The springback phenomenon that occur in thin metal sheets after forming is mainly a stress driven problem, and the magnitude is roughly proportional to the ratio between the magnitude of the residual stresses after forming and Young's modulus. An accurate prediction of residual stresses puts, however, high demands on the material modeling. A phenomenological plasticity model is made up of several ingredients, such as a yield criterion, a plastic hardening curve, a hardening law, and a model for the degradation of elastic stiffness due to plastic straining. The authors have recently, Ref. [1], showed the importance of a correct modeling of a cyclic stress-strain behavior via a phenomenological hardening law, in order to obtain an accurate stress prediction. The main purposes of the present study are to study the influence of two other constitutive ingredients: The yield criterion and the material behavior during unloading. The material behavior during unloading is evaluated by loading/unloading/reloading tension tests, where the material is unloaded/reloaded at specific plastic strain levels. The slope of the unloading curve is measured and a relation between the "unloading modulus" and the plastic strains is established. In the current study, results for four different materials are accounted for. The springback of a simple U-bend is calculated for all the materials in the rolling-, transverse- and diagonal directions. From the results of these simulations, some conclusions regarding constitutive modeling for springback simulations are drawn

A comprehenisve analysis of benchmark 4: Pre-strain effect on springback of 2D draw bending

In order to be able to form high strength steels with low ductility, multi-step forming processes are becoming more common. Benchmark 4 of the NUMISHEET 2011 conference is an attempt to imitate such a process. A DP780 steel sheet with 1.4 mm thickness is considered. In order to understand the pre-strain effect on subsequent forming and springback, a 2D draw-bending is considered. Two cases are studied: one without pre-strain and one with 8% pre-stretching. The draw-bending model is identical to the "U-bend" problem of the NUMISHEET'93 conference. The purpose of the benchmark problem is to evaluate the capability of modern FE-methods to simulate the forming and springback of these kinds of problems. The authors of this article have previously made exhaustive studies on material modeling in applications to sheet metal forming and springback problems, [1],[2],[3]. Models for kinematic hardening, anisotropic yield conditions, and elastic stiffness reduction have been investigated. Also procedures for material characterization have been studied. The material model that mainly has been used in the current study is based on the Banabic BBC2005 yield criterion, and a modified version of the Yoshida-Uemori model for cyclic hardening. This model, like a number of other models, has been implemented as User Subroutines in LS-DYNA. The effects of various aspects of material modeling will be demonstrated in connection to the current benchmark problems. The provided material data for the current benchmark problem are not complete in all respects. In order to be able to perform the current simulations, the authors have been forced to introduce a few additional assumptions. The effects of these assumptions will also be discussed

Urethral pressure response patterns induced by squeeze in continent and incontinent women.

Our aim was to compare the urethral pressure response pattern to pelvic floor muscle contractions in 20-27 years old, nulliparous continent women (n=31) to that of continent (n=28) and formerly untreated incontinent (n=59) (53-63 years old) women. These women underwent urethral pressure measurements during rest and repeated pelvic muscle contractions. The response to the contractions was graded 0-4. The young continent women showed a mean urethral pressure response of 2.8, the middle-aged continent women 2.2 (NS vs young continent), and the incontinent women 1.5 (p < 0.05 vs middle-aged continent, p < 0.001 vs young continent). Urethral pressures during rest were significantly higher in the younger women than in both groups of middle-aged women. The decreased ability to increase urethral pressure on demand seen in middle-aged incontinent women compared to continent women of the same age as well as young women seems to be a consequence of a neuromuscular disorder rather than of age

Abnormal urethral motor function is common in female stress, mixed, and urge incontinence.

Aim: To investigate the urethral motor function in incontinent women. Materials and Methods: The intraurethral pressure was measured continuously in the high-pressure zone of the urethra at rest and during repeated short squeezes around the microtip transducer catheter in a group of 205 women with clinically manifest urinary incontinence (severe), and compared with the findings of investigations in 87 middle-aged women (53-63 years) with treatment naive incontinence (mild-to-mode rate) and healthy controls. Results: Women with established incontinence significantly (P < 0.001) more often (66%) had a pressure fall during or immediately following squeeze than women with treatment naive incontinence (35%) or asymptomatic women (25%). The acceleration of urinary flow and the maximal flow rate were significantly (P < 0.01) increased in patients with incontinence: acceleration was 13 +/- 2.2 (17.8), 20 +/- 2.8 (18.9), and 32 +/- 4.9 (24.9) degrees (mean +/- SEM;SD) for incontinence, naive incontinence and no incontinence, respectively; maximum urinary flow rate was 23, 22, and 16 ml/sec. No statistical differences in any of these measures were seen when stress and urge incontinence were compared. Conclusion: Women with stress, urge, and mixed urinary incontinence seem to have a primary neuromuscular disorder in the urethra, which presents itself as an overactive opening mechanism with a urethral pressure fall instead of a pressure increase on provocation during the filling phase of the bladder, and during bladder emptying a more efficient opening of the bladder outlet than in normal women. We suggest that one and the same pathophysiological mechanism participates in female stress, urge, and mixed incontinence

On constitutive modeling for springback analysis

The springback phenomenon that occurs in thin metal sheets after forming is mainly a stress driven problem, and the magnitude is roughly proportional to the ratio between residual stresses and Young\u27s modulus. An accurate prediction of residual stresses puts, in turn, high demands on the material modeling during the forming simulation. A phenomenological plasticity model is made up of several ingredients, such as a yield condition, a plastic hardening curve, a hardening law, and a model for the degradation of elastic stiffness due to plastic straining. The authors of this paper have recently, [1], showed the importance of a correct modeling of a cyclic stress-strain behavior via a phenomenological hardening law, in order to obtain an accurate stress prediction. The main purposes of the present study are to study the influence of two other constitutive ingredients: the yield criterion and the material behavior during unloading. Three different yield criteria of different complexity are evaluated in the present investigation: the Hill\u2748 criterion, the Barlat-Lian Y1d89 criterion, and the 8-parameter criterion by Banabic/Aretz/Barlat. The material behavior during unloading is evaluated by loading/unloading tension tests, where the material is unloaded/reloaded at specified plastic strain levels. The slope of the unloading curve is measured and a relation between the "unloading modulus" and the plastic stain is established. In the current study, results for four different materials are accounted for. The springback of a simple U-bend is calculated for all the materials in the rolling-, transverse- and diagonal directions. From the results of these simulations, some conclusions regarding constitutive modeling for springback simulations are drawn. (C) 2010 Elsevier Ltd. All rights reserved