Springback Behavior Prediction of Benchmark

Springback Behavior Prediction of Benchmark Problem II Numisheet 2008 Model Under Smooth Drawbead 

Waluyo A. Siswantoa, Badrul Omar , Agus D. Anggonob, Annevella A. Mathewa b

a Advanced Dynamics and Vehicle Safety, UTHM Universitas Muhammadiyah Surakarta, PhD Student under AdVeS – AMMC , UTHM Faculty of Mechanical and Manufacturing Engineering Universiti Tun Hussein Onn Malaysia, 86400 Parit Raja - Batu Pahat Johor Darul Ta’zim, Malaysia Phone: +60 7 4536709. Fax: +60 7 4536080 E-mail: [email protected]

Abstract An accurate modelling of the sheet metal deformations including the springback prediction is one of the key factors in the efficient utilisation of Finite Element Method (FEM) process simulation in industrial application. The accurate simulation of the drawing products will not be useful since springback will occur after the tools are removed off the drawing die set. The springback prediction should be further performed to see the final formed shape after the elastic recovery. The elastic behaviour of the metal will be contributing to other geometrical forming defects, such as thinning and wrinkling. This paper presents an evaluation of a standard benchmark model defined as Benchmark II of Numisheet 2008. This is an S channel model with various drawbeads. For benchmark evaluation purpose, only smooth drawbead is chosen and two finite element application packages based on implicit codes i.e DYNAFORM and AUTOFORM are utilised for simulating the forming process as well as the calculation of the springback distortion. Results are presented with comparison of the two packages. The simulation strategy involved are discussed to provide a general approach that may influence the results. The results are also revealing the influence of the smooth drawbeads when this type of bead is introduced to the model. The simulation results show that the application of implicit finite element code in predicting springback distortion is sufficiently effective. The two simulation results from different commercial packages are in a good agreement. One thing that should be taken into consideration for further research is accommodating springback to obtain a better geometrical accuracy. Keywords: Smooth Drawbead, DYNAFORM, AUTOFORM, Springback

1. INTRODUCTION Springback describes the change in shape of a formed sheet upon removal from the constraint or tooling. It can be considered as a dimensional change which happens during unloading, due to the occurrence of primarily elastic recovery of the material. In recent years, the high strength steels and aluminum alloys are increasingly used for sheet metal parts in the automotive industry to reduce mass. However, because of these materials higher ratios of yield strength to elastic modulus, precise prediction and

control of essential.

springback

become

Since 1993, four benchmarks (Ubending, S-rail, AUDI door and unconstrained bending) are setup for springback prediction in Numisheet. The benchmark of Numisheet 2008 has included an additional problem which is related to springback. There are mainly two types of finite element approaches for springback simulation, i.e. static implicit method and dynamic explicit method [1,2]. Numisheet 2008 Benchmark 2 is provided by Daimler AG. The objective of this benchmark is to

evaluate the capability of the simulation software to model different drawbead scenarios with respect to the prediction of drawing and springback and the investigation of the predictability of the springback for deep drawn parts where the stress state is influenced by drawbeads [4]. The benchmark model requires two approaches regarding the forming process steps. The first is standard approach with the processes of closing, drawing, trimming and springback. The second is advanced approach with the processes of closing, drawing, springback, clamping in fixation device, trimming and springback [4]. The forming limit of formability in sheet metal processing is an important problem and the Forming Limit Curve (FLC) or the Forming Limit Diagram (FLD) is a useful concept [3-5]. They are also used in workshops to analyze actual and potential problems of sheet forming or to compare the formability of different materials. To make the sheet forming analysis accurate and precise, full material data including the FLC is required. However, experimental determination of FLC or FLD is very time consuming, and it’s difficult to obtain consistent results due to the different experimental methods and subjective measurement approaches adopted. As a result, many researchers turn to develop analytical models as an alternative to fill up the deficiency of experiment. By integrating numerical simulation, probability optimization, DFSS (design for six sigma) and robust design together, the procedure can be used not only to improve the reliability, but also to decrease the sensitivity of design quality to the uncertainties [6]. The proposed method can reach the desired blank shape within a few iterations and it can be further applied to optimum blank design of other practical sheet metal-forming problems [6-7].

2. PROBLEM DEFINITION Springback is a common phenomenon in sheet metal forming since the elastic recovery of the internal stresses in induced after removal of the tools. Springback is a consequence of the unbalanced stresses through the thickness of the sheet undergoing bending. This paper considers the prediction of springback behavior a model of benchmark problem 2 Numisheet 2008 under smooth drawbead. Drawbeads which can influence the springback need to be investigated to see the performance in way of the minimisation of the springback. The investigations approach includes the following steps: - closing - drawing - trimming and - springback The trimming of the part will be done in the closed drawing tools. This is a simplification compared to the result of DYNAFORM software, which was done outside of the drawing tools. 3. SPRINGBACK SIMULATIONS Many researchers [4]-[7] have studied the material flow controlled by the restraining force of the drawbeads. Figure 1. shows the drawbead apply to the assembly of deep drawing process.

Figure 1. Deep drawing process with drawbead

Drawbeads will control the material flow by using the penetrations or restraining force to the sheet metal.

Accuracy of the springback prediction for different restraining forces and therefore different states of stress in the sheet.

Dies with smooth drawbead and punch are designed to form the sheet metal and also to assist shaping and improve formability.

The types of drawbead are smooth and lock, smooth bead is rounded model and lock bead is angular model. Smooth drawbead model will be used in this research.

Binder face is used to hold blanks to avoid wrinkle before feeding into drawing cavities. Generally, its shape is made by offset and extending profile of stamping part surface. Draw bead design is used to control blanks to be evenly fed into drawing cavities to elude any defects through changing its locations, lengths and cross sections.

The drawing and dimensions of S-rail Benchmark 2 model Numisheet 2008 could be seen in Figure 2. It shows the location of holes for fixing in measurement device. This dimensions become a reference model which use to measure the springback.

S tart

S urface Data

Define tools elem ent (Die, punch, binder, blank)

S et m aterial property of blank

P rocess defined (Moving, binder force, trimm ing, springback)

Figure 2. Dimensions of S-Rail (mm)

Assembly design includes dies, blank, binder and punch as shown in3.

S olve on A utoform NO

E valuation

OK

Minimum springback

Figure 4. The flowchart process of form ability

Figure 3. Assembly of forming process

In the layout design stage, we perform the formability analysis on the 3D drawing die face using AUTOFORM. In order to ensure the success of the formability analysis, users must be able to not only feed the system with models and parameters that can faithfully reflect the actual forming

process, but also accurately interpret the result and provide recommendations. The result of the analysis will be used to modify the process design included the blank holder force to get an optimal die face. When performing formability analysis, we treat dies or cavities, punches, and binder as rigid bodies. The blanks are represented by their neutral planes. The flow of analysis is presented in Figure 4. Blank material is HX260LAD. Forming parameters and material properties need to be specified, which include sheet thickness (1 mm), the friction coefficient between blanks and dies (0.14), Poisson’s ratio (0.3), Young’s ratio (210 MPa), yield strength (176 MPa) strain hardening curve and yield surface, as shown in Table 1

Strain analyses indicated that the highest strain established in the same region between the process before and after trimming, also got the same value, as shown in Figure 6.

(a)

Table 1. Material for Benchmark 2 Material type Thickness Rolling direction Frictional coef.

HX260LAD 1.0 mm Parallel to global x 0.13

4. RESULT AND DISCUSSION According to the wrinkling preliminary analyses indicated that the risk of wrinkling is highest in “O” region by applied of 400 kN blank holder force (BHF), as shown in Figure 5.

Figure 5. Highest risk of wrinkling region in 400 kN BHF

(b) Figure 6. Plastic strain distribution, (a) Before trimming and (b) After trimming

The evaluation of springback results will be presented in the following. The simulation results will be compared for each participant as plots on the reference section “a”, “b” and “c”, as shown in Figure 7.

Figure 7. Springback observations sectioning.

The comparisons between springback and the reference geometry of these simulations with isotropic material and isotropic hardening are described in Figure 8. The highest springback is 25.9 % in section A, then 23.2 % in section C, and the lowest is 15.6 % in the section B. The higher springback is located in the outer section. It shown that In order to accurately compare numerical based on AUTOFORM and DYNAFORM results. DYNAFORM values were obtained based on previous work by the authors conducted using similar conditions. A quick look to the section diagrams for punch force and displacement shows that on the whole the numerical analyses simulations delivered results which qualitatively follow the DYNAFORM, and are able to represent the sensitivities correctly, as shown in Figure 9. After draw in results are verified, springback is computed by releasing drawing stresses. The half profiles of section “a”, “b” and “c” are shown in Figure 10, respectively, the height of product drawing before trimming and after trimming. The highest deviation is about 1.75 mm at the tip point of section “a” by applying BHF of 400 kN. Application of larger BHF on the process such as 650 and 850 kN provide a lower springback at the tip point of section “a”, but thinning and wrinkling are more probably during sheet metal forming.

Reference geometry (inside)

(a)

Reference geometry (inside)

(b)

Reference geometry (inside)

(c)

Figure 8. Comparison sheet metal after spring back with reference geometry, (a) Section A, (b) Section B, (b) and (c) Section C.

Figure 9. Comparisons of AUTOFORM and DYNAFORM punch force vs displacement at FBH 400 kN.

5. CONCLUSIONS The results show that springback

Springback

behave in a relatively different manner at different sections and blank holder force levels. In terms of the capability of two packages, i.e. AUTOFORM and DYNAFORM, both show a good agreement.

Smooth bead (before trimming)

(a)

Before Springback

The springback do not behave in a predictable manner until it is analysed involving holder force and the drawbeads. This indicates that springback can be somehow minimized but cannot be eliminated. The mode of the springback depends on the material flow shown in the part geometry complexity. In the specific benchmark model No.2 with smooth drawbeads, the springback deviations fall about 0.5 – 1 mm.

(b)

The results indicate an important elastic recovery phenomenon that cannot be avoided. Minimising it by introducing the combination of drawbeads and blankholder forces could not eliminate. This supports a necessity to optimise the geometry of the tooling dies to get accurate result as expected. The springback should be accommodated.

6. ACKNOWLEDGEMENT

(c) Figure 10. Height of drawing before trimming and after springback at each section a, b and c. (mm)

Thanks to AMMC that allow the authors to use AUTOFORM. This work has been conducted under Advance Dynamic and Vehicle Safety (AdVeS) research group. Part of the work using DYNAFORM has been supported by Research Centre and Innovation, University Tun Hussein Onn Malaysia (UTHM), Vot. No.0506 Shortgrant.

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