Plastic Instability and Forming Limits in Anisotropic Sheet Metal

1997 ◽  
pp. 373-378 ◽  
Author(s):  
J. Chakrabarty
Keyword(s):  
Sheet Metal ◽  
Plastic Instability ◽  
Forming Limits

On the Theories of Sheet Metal Necking and Forming Limits

1978 ◽  
Vol 100 (3) ◽  
pp. 303-309 ◽  
Author(s):  
A. S. Korhonen
Keyword(s):  
Sheet Metal ◽  
Present State ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Strain Path ◽  
Plastic Instability ◽  
Stretch Forming ◽  
Theoretical Limit ◽  
Forming Limits ◽  
Localized Necking

The history and the present state of the theory of sheet metal forming limits are reviewed. The theory of necking and plastic instability (Swift-Hill and Marciniak-Kuczyn´ski models) is discussed and theoretical limit strains are calculated. The influence of the strain path on the theoretical limit strains is discussed with computational examples. At the present no theory can fully explain the localized necking in stretch forming.

Sheet metal forming limits as classification problem

2017 ◽  
Author(s):  
Christian Jaremenko ◽  
Xiaolin Huang ◽  
Emanuela Affronti ◽  
Marion Merklein ◽  
Andreas Maier
Keyword(s):  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Classification Problem ◽  
Forming Limits

Experimental and Numerical Analysis of Forming Limits in CNC Incremental Sheet Forming

2007 ◽  
Vol 344 ◽  
pp. 511-518 ◽  
Author(s):  
Markus Bambach ◽  
M. Todorova ◽  
Gerhard Hirt
Keyword(s):  
Sheet Metal ◽  
Sheet Forming ◽  
Incremental Sheet Forming ◽  
Negative Slope ◽  
Evaluation Procedure ◽  
Forming Limit ◽  
Limit Curve ◽  
Forming Limit Curve ◽  
Forming Limits ◽  
Straight Line

Asymmetric incremental sheet forming (AISF) is a relatively new manufacturing process for the production of low volumes of sheet metal parts. Forming is accomplished by the CNC controlled movements of a simple ball-headed tool that follows a 3D trajectory to gradually shape the sheet metal blank. Due to the local plastic deformation under the tool, there is almost no draw-in from the flange region to avoid thinning in the forming zone. As a consequence, sheet thinning limits the amount of bearable deformation, and thus the range of possible applications. Much attention has been given to the maximum strains that can be attained in AISF. Several authors have found that the forming limits are considerably higher than those obtained using a Nakazima test and that the forming limit curve is approximately a straight line (mostly having a slope of -1) in the stretching region of the FLD. Based on these findings they conclude that the “conventional” forming limit curves cannot be used for AISF and propose dedicated tests to record forming limit diagrams for AISF. Up to now, there is no standardised test and no evaluation procedure for the determination of FLCs for AISF. In the present paper, we start with an analysis of the range of strain states and strain paths that are covered by the various tests that can be found in the literature. This is accomplished by means of on-line deformation measurements using a stereovision system. From these measurements, necking and fracture limits are derived. It is found that the fracture limits can be described consistently by a straight line with negative slope. The necking limits seem to be highly dependent on the test shapes and forming parameters. It is concluded that standardisation in both testing conditions and the evaluation procedures is necessary, and that a forming limit curve does not seem to be an appropriate tool to predict the feasibility of a given part design.

scholarly journals Recent Approaches for the Determination of Forming Limits by Necking and Fracture in Sheet Metal Forming

2015 ◽  
Vol 132 ◽  
pp. 342-349 ◽  
Author(s):  
M.B. Silva ◽  
A.J. Martínez-Donaire ◽  
G. Centeno ◽  
D. Morales-Palma ◽  
C. Vallellano ◽  
...  
Keyword(s):  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Forming Limits

APPLICATION OF FORMING LIMIT CRITERIA BASED ON PLASTIC INSTABILITY CONDITION TO METAL FORMING PROCESS

2008 ◽  
Vol 22 (31n32) ◽  
pp. 5680-5685
Author(s):  
SEONG-CHAN HEO ◽  
TAE-WAN KU ◽  
JEONG KIM ◽  
BEOM-SOO KANG ◽  
WOO-JIN SONG
Keyword(s):  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Thin Sheet ◽  
Plastic Instability ◽  
Theoretical Background ◽  
Forming Limit ◽  
Preliminary Evaluation ◽  
Forming Process ◽  
Element Analysis

Metal forming processes such as hydroforming and sheet metal forming using tubular material and thin sheet metal have been widely used in lots of industrial fields for manufacturing of various parts that could be equipped with mechanical products. However, it is not easy to design sequential processes properly because there are various design variables that affect formability of the parts. Therefore preliminary evaluation of formability for the given process should be carried out to minimize time consumption and development cost. With the advances in finite element analysis technique over the decades, the formability evaluation using numerical simulation has been conducted in view of strain distribution and final shape. In this paper, the application of forming limit criteria is carried out for the tube hydroforming and sheet metal forming processes using theoretical background based on plastic instability conditions. Consequently, it is confirmed that the local necking and diffuse necking criteria of sheet are suitable for formability evaluation of both hydroforming and sheet metal forming processes.

Bifurcation Analysis of Forming Limits for an Orthotropic Sheet Metal

2014 ◽  
Vol 136 (5) ◽  
Author(s):  
Shuhui Li ◽  
Ji He ◽  
Z. Cedric Xia ◽  
Danielle Zeng ◽  
Bo Hou
Keyword(s):  
Sheet Metal ◽  
Bifurcation Analysis ◽  
Yield Criterion ◽  
Forming Limit Diagram ◽  
Experimental Result ◽  
Forming Limit ◽  
Forming Limits ◽  
R Value ◽  
Value Effect ◽  
The Hill

A bifurcation analysis of forming limits for an orthotropic sheet metal is presented in this paper. The approach extends Stören and Rice's (S–R) bifurcation analysis for isotropic materials, with materials following a vertex theory of plasticity at the onset of localized necking. The sheet orthotropy is represented by the Hill’48 yield criterion with three r-values in the rolling (r0), the transverse (r90) and the diagonal direction (r45). The emphasis of the study is on the examination of r-value effect on the sheet metal forming limit, expressed as a combination of the average r-value raverage and the planar anisotropy (Δr). Forming limits under both zero extension assumption and minimum extension assumption as well as necking band orientation evolution are investigated in detail. The comparison between the experimental result and predicted forming limit diagram (FLD) is presented to validate the extended bifurcation analysis. The r-value effect is observed under uniaxial and equal-biaxial loadings. However, no difference is found under plane strain condition in strain-based FLD which is consistent with Hill's theory. The force maximum criterion is also used to analyze FLD for verification.

Forming Limits of a Sheet Metal After Continuous-Bending-Under-Tension Loading

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
Ji He ◽  
Z. Cedric Xia ◽  
Danielle Zeng ◽  
Shuhui Li
Keyword(s):  
Finite Element ◽  
Sheet Metal ◽  
Sheet Metal Forming ◽  
Metal Forming ◽  
Loading Condition ◽  
Hybrid Approach ◽  
Forming Limit ◽  
Forming Limits ◽  
Continuous Bending ◽  
Bending Under Tension

Forming limit diagrams (FLD) have been widely used as a powerful tool for predicting sheet metal forming failure in the industry. The common assumption for forming limits is that the deformation is limited to in-plane loading and through-thickness bending effects are negligible. In practical sheet metal applications, however, a sheet metal blank normally undergoes a combination of stretching, bending, and unbending, so the deformation is invariably three-dimensional. To understand the localized necking phenomenon under this condition, a new extended Marciniak–Kuczynski (M–K) model is proposed in this paper, which combines the FLD theoretical model with finite element analysis to predict the forming limits after a sheet metal undergoes under continuous-bending-under-tension (CBT) loading. In this hybrid approach, a finite element model is constructed to simulate the CBT process. The deformation variables after the sheet metal reaches steady state are then extracted from the simulation. They are carried over as the initial condition of the extended M–K analysis for forming limit predictions. The obtained results from proposed model are compared with experimental data from Yoshida et al. (2005, “Fracture Limits of Sheet Metals Under Stretch Bending,” Int. J. Mech. Sci., 47(12), pp. 1885–1986) under plane strain deformation mode and the Hutchinson and Neale's (1978(a), “Sheet Necking—II: Time-Independent Behavior,” Mech. Sheet Metal Forming, pp. 127–150) M–K model under in-plane deformation assumption. Several cases are studied, and the results under the CBT loading condition show that the forming limits of post-die-entry material largely depends on the strain, stress, and hardening distributions through the thickness direction. Reduced forming limits are observed for small die radius case. Furthermore, the proposed M–K analysis provides a new understanding of the FLD after this complex bending-unbending-stretching loading condition, which also can be used to evaluate the real process design of sheet metal stamping, especially when the ratio of die entry radii to the metal thickness becomes small.

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