Incremental forming process strategy variation analysis through applied strains
© Lora and Schaeffer; licensee Springer. 2014
Received: 11 November 2013
Accepted: 5 January 2014
Published: 22 January 2014
Nowadays, different studies on Incremental Sheet Forming have been taken due to the increasing demand for flexible manufacturing processes. This process is applicable in different areas, such as prototyping and the production of small batches of parts, always searching for lower manufacturing costs. This work analyzes how the variation of the process strategies (punch diameter and the vertical step down) influences the workpieces strain ends. These experimental studies were compared to the numerical simulation. The results found show concordance between simulated and experimental results.
KeywordsIncremental forming Forming strategy Numerical simulation
Over the last decades, the forming area has improved its knowledge, both in terms of the materials used as well as in the flexibility and process cost reduction. The Incremental Sheet Forming (ISF) is basically focused on the production of small batches of parts, fast prototyping with process flexibility and reduced operational cost (Jeswiet 2005 and Arruda 2010). The cost reduction is focused on the need to manufacture a low cost material die and also be partially applied to the process, according to Kwiatkowski et al. (2010), Hirt et al. (2004) and Bambach (2010). The ISF is characterized by through the punctual application of load over the metal sheet performing repeated strains. Kopac and Kampus (2005) and Lora et al. (2013) comments that this type of strain applied to the workpiece, makes great strains occur in the material, far greater than the strains of the conventional processes.
As a failure criterion for sheet metal forming process, commonly the Forming Limit Diagram (FLD) in combination with a forming limit curve (FLC) is used. Among the tests that can define an FLC, there is the Nakajima methodology, which presents good reliability in the results. A conventional FLC cannot be directly applied to ISF process as a failure criterion due to strains in ISF process be highly superior and have no linear strain path. Silva et al. (2010) and Kuzman et al. (2010) find maximal strains superior to FLC when applied the ISF process in the metal sheet forming.
The need to search for a new strain limit for the materials applied to ISF process has occurred, and studies have been developed in the last decade defining parameters that influence the material drawability, according to Jeswiet et al. (2005). This work evaluates the strategies of the ISF process through variation of the vertical step down and punch diameter variations, obtaining information about the true strains applied to the material.
An important point to be emphasized is the need/capacity of the blank holder to move in vertical direction. As in the TPF case, the blank holder has to follow the punch movement towards the increment; this way strains will happen only in the punch contact region.
The tests were conducted on a simple straight line geometry, in which varied forming strategies were applied. The sheets were engraved with circles separated by 2mm (from the centers) through an electrochemical corrosion and an electric current. This corrosion applied to the material does not damage the same.
All experiments were conducted on a DC04 sheet blank (thickness = 1mm) with a work space of 360 × 70mm2 (length x width). Table 1 summarizes all applied strategies, went always a feed rate of 2000m/min was used.
Parameters analyzed in the experiments
Punch diameter (PØ) [mm]
Vertical step down (p) [mm]
The blankholder, punch and die were assumed rigid. A Coulomb model was considered for frictional (μ = 0.1) actions. The Gauss integral was adopted at thickness direction with 3 points integration. The mesh was adopted 2x2mm quadratic. Three integral dots and four nodes linear Belytschko-Tsay shell element were used. The material model of Barlat 36 with anisotropic materials under plane stress conditions was adopted. The exponent “m” in Barlat’s yield surface is set as 6.0, and the plastic yield expressed with the Hollomon formula, has its properties presented in Table 2.
DC04 material data
kf = 564 · φ0,2
Yield Stress (kf 0)
kf 0 = 186MPa
Results and discussions
Final depth in the experiments
The simulation shows an approach with the major strain achieved by the workpiece. The experimental strain peaks found in the graphs come from regions where ruptures in the tested workpiece occurred. The simulation did not anticipate these strain peaks; however, it was in major concordance with the analyses along the workpiece, such as in approximately 0.85 and 0.6 for strain in tests #1 and #2, respectively. The main strain peaks were 1.19 and 0.75 in the tests #1 and #2, respectively.
In test #3, for example, it may be considered the same observations of test #2, where there is a good concordance between the simulation and the experimental results, but it does not anticipate the strain peaks achieved by the sheet in the rupture region. In this test, the main strain in the “X” coordinate center of the workpiece and in the simulation are 0.45 in the experiments; however, the main strain is approximately 0.58 for both cases in the ends of the workpieces. Test #4 did not show homogeneity in the main strain profile, due to the major severity of the strategy. The strains achieved 0.35 in the simulation and 0.47 in the experiments.
The main strain peaks were measured in the workpiece longitudinal ends, near to the rupture region. As the simulation, which has no failure criterion, does not anticipate these peak values, it is understood that these peaks provoke material rupture.
The strains achieved in test #1 are higher than the remaining tests and the final depth is larger than in the other tests. A punch with a larger diameter can distribute better the stress over the contact area and make the sheet less severe. Also as a softer increment, it can apply higher strains and larger depth.
In test #4, it was observed a positive parabola for the strain profile along the test specimen, due to the more severe conditions imposed in the process, consequently, major possibility of springback.
The maximal strain that can be imposed to the sheets in the incremental process vary according to the strategy adopted in the process and cannot apply the strain conventional limits.
Finite Element Method
Forming Limit Curve
Forming Limit Diagram
Incremental Sheet Forming
Simple Point Forming
Two Point Forming.
The authors gratefully acknowledge LdTM/UFRGS, SENAI CIMATEC and IBF/RWTH-Aachen for the support in the development of these work and CAPES for the financial support of scholarship.
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