Simulation & Stress Analysis in Sheet Metal Restrike Process to Optimize the Required Shape & Reduce Cost of Trials in NPD
#1Mr.Aviraj Newase, #2Dr.Jagtap
2ur guide email id
#1,2M.E(Design Engineering), Department of Mechanical Engineering, Savitribai Phule Pune University, Pune.
Abstract— The objective of restrike processes is primarily to produce a desired shape by plastic deformation of sheet metal. The final product quality is conditional on both the sheet substantial features and progression variables such as stress & strain. These changeable are subjective by the tool design, blank geometry, properties of the lubricant used (such as coefficient of friction and heat capacity) and restrike process speed. A deviating product shape can result if incorrect recipes of these development parameters are used. A deviating shape is usually caused by pliable spring back of the job after restrike process and retracting the tool.
During restrike & Knotching, the material characteristics of the sheet being formed change and affect the process parameters during processing. The blank material properties improved during and at the end of each of these Restrike process processes. It is specifically because of this that the design of restrike process quiet be contingent on the acquaintance and ability of the die designer, wherein the choice of ideals for the various process constraints is based on test and error methods.
Index Terms— Sheet Metal, Restrike & Knotching process, Stress Analysis, Simulation & Optimization.
In sheet metal Restrike process operations, the amount of useful deformation is limited by the existence of unsteady distortion which mostly precedes the form of contained wrinkling. Localized necking occurs when the stress state leads to an increase in the surface area of the sheet at the expense of a reduction in the thickness. After the localized neck initiates, further deformation of the material concentrates in this localized region, and homogeneous deformation away from neck region vanishes completely. The localized neck is therefore a very important phenomenon in determining the amount of useful deformation that can be imposed on a work piece. The mechanism for initiation of localized band involves a number of factors including material properties and punch profile. The phenomenon is attributed to the softening effect, including geometric softening (the decrease with strain of the cross-section area which bears the Restrike process load, the generation of voids), and material softening (flow stress decreases with the increase of the effective strain) .
Restrike process typically faces challenges associated with Burrs, wrinkling, and optimization of shapes, all of which are expensive as they lead to wastage of material and loss of production time. Simulation can predict such defects during product development and often prevent their occurrence during production with the attendant saving time and material, by identifying necessary and often times simple changes in design. It is of importance therefore, for sheet metal fabricating companies to adopt product development simulation techniques in order to reduce or eliminate the aforementioned problems and thus remain competitive.
The main benefits offered by full process simulation in sheet metal Restrike process is the identification and correction of flaws in the production line prior to manufacture in order to cheaply and efficiently produce the desired product. The cost of die production is also reduced because the simulation substitutes the exclusive out-dated probationary and error procedure of constructing dies.
II. OBJECTIVES OF THE STUDY
Primary goal of this research is to perfectly simulate and optimize the required shape in the sheet metal Restrike process with the help of deep restrike process. This is expected to enable with the help of software simulation process design and to then enable redesign where necessary in order to meet the requirements of producing desired shapes(crack free sheet metal shapes) using these coupled processes.
The following are the objectives of the study:
1. Simulate restrike processes for SS304/SS316/SS410 sheet metal restrike process.
2. Simulate knotching processes for SS304/SS410 after sheet metal restrike process.
3. Couple the simulated processes in order to develop a full process simulation.
III. LITERATURE REVIEW
Mane, T., Goel, V., Kore, S. D.  High velocity sheet metal Restrike methods, such as electromagnetic Restrike process and electro-hydraulic Restrike process (EHF), are based on high voltage electrical energy. This paper gives theoretical details about high strain rate restrike process in the Electro-hydraulic restrike process. Following the experimental results in the literature, a simulation of the high strain rate restrike process was prepared in ABAQUS-CAE wherein a dynamic loading on a sheet blank was applied and was allowed to plastically deform following the Johnson-Cook material model. The results of the simulation were validated from the experimental values obtained from existing literature.
Hagenah, H., Geiger, M., Merklein, M. 
The paper will present developments towards the improvement of the robustness of sheet metal Restrike process chains. The possibilities and challenges will be demonstrated using two examples. Firstly, the effect of heat treatment to precipitation hardened aluminium sheet metals in order to enlarge the process window for deep restrike process operations will be discussed. Numerical simulation can be used to predict the effects of the heat treatment. Using more sophisticated approaches the heat treatment layout and its effects on the restrike process can be computed and predicted. Thus the process window can be enlarged and in consequence the process chain will receive an increased robustness. Secondly the integrated tube and sheet hydro-restrike process that also includes joining operation will be used as an example of the positive impact numerical simulation can have on complex process chains. This highly integrated process chains needs carefully designed tools that ensure robust production. Furthermore, several effects occurring during the Restrike process operations are difficult to understand since the process cannot be stopped at arbitrary moments or easily investigated in situ. The needed understanding and there for the mandatory input for successful and robust process design has to be derived from advanced numerical models and the according simulation runs.
An especially interesting field is simulation optimization where optimization methods are used to generate a design concept early in the design process.
Fig.1. Analysis flow chart & Methodology
V. RESTRIKE & KNOTCHING CALCULATIONS
a) Type of operation:
h / d ≤ 0.5 – shallow restrike
h / d > 0.5 – deep restrike
Where, h = shell height, d = shell diameter
b) Estimation of blank Diameter (Theoretical):
D = √ (d2 + 4dh) …………..……………. (1)
Where, D – Blank diameter in mm,
d – Shell outer diameter in mm
h – Shell Height in mm,
r – Corner radius of punch
c) Considering Trim allowance:
Trim allowance = 0.05mm for every 10 mm diameter of drawn cup
Where, D1= Initial diameter of blank (D1)
= D (Theoretical diameter.) +Trim allowance … (2)
d) t / D Consideration:
Table 4.1: t / D decides the severity of wrinkling
t / D Percentage Severity of wrinkling
Up to 0.5 Wrinkling is a severe and compressive load must be reduced
Blank holder must be used, so a double action press is preferable
Above 0.5 up to 1.5 Wrinkling is moderate and low blank holding forces are permitted
Above 1.5 up to 2.5 Wrinkling is very light so, single action press is enough
Over 2.5 No wrinkling so blank holder is unnecessary even with high compressive load
e) Estimation of restrike process pressure:
Restrike process pressure,
p = π x d x t x S x ((D / d) – C) …………………... (3)
P = Restrike process force in ‘kg-f’
d = Shell outer diameter
D = Blank diameter
t = thickness of sheet in ‘mm’
S = Ultimate tensile strength in N/mm2
C = constant to cover friction and bending (0.6 to 0.7 for ductile material)
f) Blank holding pressure:
Blank holding pressure = 1/3rd of restrike process pressure
g) Press capacity:
Press capacity = (Restrike process pressure + Blank holding pressure) x 1.3 ……………………….. (4)
h) Restrike process speed:
Table 4.4: Restrike process speed
Material Single action restrike process Double action restrike process
Ft/min m/sec Ft/min m/sec
Steel 60 0.3048 30-55 0.1778-0.27
Stainless Steel - - 20-30 0.1016-0.27
Aluminum 180 0.144 100 0.1016-0.1524
Aluminum alloys - - 30-40 0.508
Copper 150 0.762 85 0.1524-0.2032
Brass 200 1.016 100 0.4318
i) Restrike process die clearances:
Table 4.5: Restrike process dies clearances
Blank thickness(t) 1st draw 2nd draw Sizing draw
Up to 0.38 1.07t-1.09t 1.08t-1.1t 1.04t-1.05t
0.4-1.27 1.08t-1.1t 1.1t-1.12t 1.05t-1.06t
1.28-3.18 1.1t-1.12t 1.12t-1.14t `.07t-1.09t
3.5 & above 1.12t-1.14t 1.15t-1.2t 1.08t-1.1t
‘t’ is the thickness of the original blank
j) Punch radius:
Punch radius = 4t to 10t (or) Radius on product restrike process. Where, t = sheet thickness
m) Draw radius (or) die radius:
R = 6t to 8t ……………… (5)
Tolerance = ± 0.005” (or) ± 0.127 ……………… (6)
VI. STRESS ANALYSIS OF DESIGN DATA 
The FEA based commercial software consists of three steps: Pre-processing, Processing (or Solution) and Post-processing. A complete finite element analysis is a logical interaction of these three steps.
Pre-processing is the primary step on FEM. This step consumes most time out of the three steps. It includes geometry creation, meshing and applying boundary conditions. The software’s like ANSYS includes tools to create geometry in itself, but it is preferred to create 3D model in CAD software’s. Then model is to be imported in the analysis software to create meshing. Meshing is the process of discretization of the geometry into elements and nodes. After meshing the suitable boundary conditions are to be applied like Pressure, force, displacement, fixed support etc. In general we can say that pre-processing is the step of creating a deck for the analysis.
2) Processing or Solution
In this step user has to give only (or execute) solve command. This step is fully computer or software based. In this step software solves the simultaneous equations based on type of analysis and boundary conditions.
Post-processing is the step of viewing results, verifications, conclusions and thinking about what steps could be taken to improve the design. In this step we check vonMises stresses and deformation of the component.
VII. SIMULATION  
It is the first and basic step of each and every analysis as well as design process. As the objective of the project is to simulate the sheet metal Restrike process; so we need to model both component and other components of the restrike process (like die, blank holder and punch).
Fig.2. Blanking tool – Section view
Fig.3. Blanking tool bottom die – open view
Fig.4. Blanking tool top die – open view
Fig.5. Isometric view of Restrike tool
Fig.6. Open view of Knotching tool
Fig.7. Isometric view of Knotching tool
Meshing is the process of discretization of the geometry into elements and node. There are two types of meshing available one is Free (or auto) meshing and other is Mapped meshing. Free meshing is usually employed on complex shaped areas and volumes. It generally incorporates quad shape elements for area and tetrahedron elements for volumes. No regular arrangement of nodes and elements observed in free meshing. Mapped meshing employed a regular pattern of nodes and elements in the model. It usually incorporates Quad elements for area and Tetrahedron or Hex elements for volumes.
Analysis of component using Simulation software
Solid Edge ST (104.00.00.082) Femap (10.2.1), Solver used NX Nastran (7.1) and Hyper-form
Fig. 8 Restrike operation Simulation with BHF 2.986 Kg-f, Stress Result on component: Von Mises
Result or Observation: Crack observed at bottom section of component (red colour)
Table 4.6: Stress Results:
Result component: Von Mises
Extent Value X Y Z
Minimum 0.00379 Mpa 10.478mm -29.142 mm -1.202 mm
Maximum 11.87 Mpa 2.867 mm 20.892mm -0.0879mm
Fig. 9 Double restrike simulation Material thickness 1.50mm with BHF 3.120 Kg-f, Stress Result: Von Mises
Result or Observation: Optimum Thinning (Safe or acceptable component) observed
Table 4.7: Stress Results
Result component: Von Mises
Extent Value X Y Z
Minimum 0.00981Mpa -3.725 mm -30.721 mm -1.512 mm
Maximum 7.4189 Mpa -18.641 mm -10.510 mm -0.0989 mm
It is usually done for regular shape of areas and volumes. Mapped meshing requires more time and user interface as compared to free meshing. In this project work free meshing is used with Sizing mesh modification method.
3) Material selection
Selection of material is one of the important tasks in analysis. If the material is not assigned to the model then analysis process or solver will not run. The type of analysis like linear or nonlinear can be decided by material properties. The ANSYS Workbench data base consist of standard materials available globally or commonly used. The user has to select the one which he/she required.
For our project material used is SS304/SS316/SS410 with properties as below:
Young’s Modulus of Elasticity, E = 2.05*10^5 Mpa,
Poisson’s Ratio, μ = 0.28
Density, ρ = 7900 Kg/m3,
Yield strength, Syt =342 Mpa
4) Boundary Conditions
The boundary conditions are mainly of two types one is load and other support or constraints. Applying specific boundary conditions is important as it will affect the results obtained. The load can be given in terms of force, pressure, moment, etc. while support can be fixed, simply support, cylindrical support, etc. Support restricts the degrees of freedom of the component.
In this step we have to select type of analysis like static, dynamic, model, etc. Then we can select type of solver; ANSYS Workbench provides solvers like Mechanical APDL, Auto-DANYA, etc. for structural analysis and CFD solver for analysis of Fluid Dynamics. We can also change the analysis settings like step control, solver control, restart control, nonlinear control etc.
After this we execute Solve command and then software solves the simultaneous equations in the form of matrix and gives output.
The software gives results in terms of graph with range of values from minimum to maximum. The user has to decide whether results obtained are proper or not. We checked Equivalent vonMises and total deformation or displacement of the component (roller).
VIII. VALIDATION & EXPERIMENTATION
Experiments are to be conducted on a hydraulic press of a suitable capacity. The die would be mounted on the bolster plate of the press and the speed of the ram would be set based on the historical data as well as the input received from the analysis data. Restrike process problems can be predicted before tool fabrication through the use of software that can be integrated into production routes which rely increasingly on computer technology. The prediction of Restrike process difficulties at the component design stage ensures that the chosen geometry is compatible with the draw ability of steel. Restrike process has become a highly technical process, and the development of a steel Restrike process route no longer involves simple trial and error methods. The parameters influencing the draw operation and evident during the trials are:
i) Type of material
ii) Thickness of the component
iii) Mechanical properties
iv) Use of lubricant
v) Blank size and development
vi) Blank holding pressure
vii) Speed of the operation
Fig.10. Validation & experimental setup 
Fig.11: Experimentation dies of forming or draw or flaring process
Fig.12: Restrike dies open view from top side
IX. CONCLUSION AND PROPOSED FUTURE SCOPE
i) To overcome the entire problem related to thinning & crack in sheet metal forming simulation.
ii) Use simulation reduces time as well as design & development & development (trial & error) cost in sheet metal.
Proposed future scope:
1) To improve and to do simulation of the sheet metal component over cad model with crack detection in simulation phase.
2) To optimize material cost, time of NPD & eliminate trial & error method in restrike & Knotching process.
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 Sponsorship of this project is given by AESSEAL INDIA PVT LTD, Pune. Problem definition is defined by this sponsored company to resolve the sheet metal thinning & crack issue with the help of modelling, simulation, new die design & development with reference to the simulation in forming operation.
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