Practical Methods for Estimating Formability of Sheet Materials

Transcription

1 Practical Methods for Estimating Formability of Sheet Materials By David Diaz Infante, Berk Aykas, Ali Fallahiarezoodar and Taylan Altan Formability is the ability of a sheet material of a given composition and thickness to deform without failure or fracture. Information on formability significantly help process and tool design engineers predict failure during analysis of sheet metal forming processes for tool design. In addition, formability information is also used to evaluate manufacturability of the designed part and for material selection based on complexity of part features and strength requirements. In sheet metal industry, the initial estimation of formability is usually based on total elongation, measured from the tensile test. It is assumed or considered that the larger is the value of total elongation, the more formable is the material. This inexpensive and simple way of evaluating formability, however, is not very reliable because while the tensile test is conducted under uniaxial conditions of stress and strain, the forming conditions in practical stamping operations are biaxial. Thus, most stamping engineers who use process simulation, use Forming Limit Diagram (FLD). The FLD s represent the limits of fracture for a variety of stress and strain states, i.e. under uniaxial or biaxial deformation conditions. However, it takes a number of experiments and considerable time and expense to create a single FLD for a specific material with a specific thickness. Therefore, at the Center for Precision Forming (CPF) of The Ohio State University ( and it is proposed to estimate formability, approximately, using data from the uniaxial tensile test, biaxial bulge test and the FLD. Tensile Test A typical engineering stress vs. engineering stress curve, obtained from a tensile test, is seen in Figure 1. Details of this test were discussed in previous Stamping Journal articles (May/June 2011, page 12 and Sept/Oct. 2015, page 16). The most commonly used tensile data include Yield Stress (YS), Ultimate Tensile Strength (UTS), total elongation and uniform elongation, Figure 1. 1

2 While total elongation values give an indication about the formability of the tested material, uniform elongation is a more useful value since it does not include necking strain that occurs after the UTS stress, given in the engineering stress/strain curve. Both, uniform and total elongation values are used in the present study, as indications of formability. The tensile data, Figure 1, can be used to determine the true stress/true strain curve that is often expressed, after curve fitting to the original stress-strain data, as σ = K ε n (σ = true stress, ε = true strain, K = strength factor, n = strain hardening cofficient). It can be shown that n is equal to the value of uniform strain, as seen in Figure 1. Thus, often n assumed to be constant with varying strain, is also used as an indication of formability. However, in Advanced High Strength Steels (AHSS) such as Dual Phase steels, the value of n may vary with increasing strain. Thus, the value of n may not be a good indicator of formability. Bulge Test The bulge test emulates the deformation of sheet material under biaxial deformation conditions and it is illustrated in Figure 2. In this test, it is possible to measure the pressure and the bulge height during the test, while the round blank is held at the flange with a lock bead to prevent metal flow towards the bulging material. The bulge height at fracture is also an indication of formability, i.e. formability of the tested material increases with the increasing bulge height, measured at fracture. Forming Limit Diagram Most engineers who conduct process simulation, using a commercial code, such as PAM-STAMP, Autoform or DYNA, also use the FLD to predict fracture in forming a part, Figure 3. In conducting a simulation, the strains can be calculated at any location in the part, during deformation. The strains above the FLD curve, Figure 3, indicate potential fracture. While FLD has been very useful in die and process design for low carbon steels, it is not always reliable when using AHSS because FLD values (a) vary with blank thickness, used in the tests and (b) are obtained for one batch of material while 2

3 material properties and formability may vary, for the same nominal material, from batch to batch. Furthermore, obtaining an FLD for a given material and thickness, requires considerable time and cost. Thus, FLD can also be used as an indication of formability but it is not always reliable, especially when applied to AHSS. Therefore, in the industrial practice often the percentage thinning is used as an indication of potential fracture. The lowest point in the FLD, Figure 3, corresponds approximately to the plane strain condition, i.e. where the minor strain is close to zero. Thus, the point A in Figure 3, can also be used as an indicator of formability. Considering the cost and time needed to obtain the FLD, it is reasonable to use the FLD data from literature, or given by a material supplier, for the same type of material, with similar thickness and UTS. Approximate Evaluation of Formability The objective of the present study is to estimate formability of a given material/thickness with as little effort and cost as possible. For this purpose, it is suggested to use: a) Tensile data (i.e. uniform and total elongation). This data is available for all sheet materials, from material suppliers b) Bulge Test data that can be obtained, at a relatively low cost. For many materials, Center for Precision Forming (CPF) has conducted bulge tests c) FLD data, which may be available from material suppliers, or in the open literature At CPF, formability data, as discussed above, has been collected for a variety of AHSS, namely for DP980 (1.4 mm), CP800 (1.4 mm), DP980 (1.2 mm), DP780-HY (1 mm), DP780 (1 mm), TRIP1180 (1.2 mm), CR-DP780 (1 mm), TRIP780 (1 mm), DP600 (1 mm), DP590 (1.4 mm), TWIP900 (1.1 mm), and TWIP980 (1.3mm). As examples, several AHSS are considered in this article, as seen in Figure 4. It is observed that the formality (a) increases with decreasing UTS, (b) new TWIP and TRIP steels, considering their UTS, have relatively good formability. 3

4 From this brief study it can be concluded that, considering the inherent errors in measurements and for most AHSS, all the formability indicators show similar trends. While FLD is not always reliable, it still appears to be the best indicator of formability. FLD is not easily available for each batch of sheet material. Therefore, the tensile data (uniform and total sheet elongation) and bulge test data (bulge height at fracture) provide useful indications of formability. In the industrial practice, potential fracture is related to local thinning (as percentage). This local thinning prediction is still a very good method for predicting fracture through simulation (although simulations may have some inaccuracies). It is desirable to evaluate, for a given material batch, the thinning at fracture with some of the formability indicators as far as they are available. In practice, comparing thinning at fracture with uniform elongation and total elongation, obtained from tensile test, may be the easiest way to evaluate formability. References [1] Altan, T., et al., Sheet Metal Forming Fundamentals, Volume 1, Chapter 4, ASM International. [2] POSCO Automotive Steel Data Book,

5 Figure 1: Typical Engineering Stress - Strain curve obtained from tensile test Figure 2: Schematic of the bulge test that is used to determine formability as well as true stress/true strain curve 5

6 Figure 3: Schematic of a Forming Limit Diagram (FLD) and the Location of the Plane Strain value (Minor strain=0) 6

7 Figure 4: Uniform Elongation, Total Elongation, Bulge Height at Fracture and Major Strain in FLD, as indicators of formability for various AHSS (FLD data from POSCO Automotive Steel Data Book 2016, for DP980 1 mm, DP590 1 mm and TWIP mm. Elongation Values are average of data in rolling, transverse and 45 directions). David Diaz Infante (diazinfantehernandez.1@.osu.edu), Berk Aykas (aykas.2@osu.edu) and Ali Fallahiarezoodar (fallahiarezoodar.1@buckey .osu.edu) are Graduate Research Associates at the Center for Precision Forming (CPF) at The Ohio State University, 1971 Neil Ave., Room 339 Baker Systems Engineering Building, Columbus, OH Taylan Altan (altan.1@osu.edu) is a Professor Emeritus and Director of CPF, and 7