PSI - Issue 57

Boris Spak et al. / Procedia Structural Integrity 57 (2024) 445–451 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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cladding. A step up in strength can be achieved through different temper conditions, while at the same time potentially reducing the formability. While higher strength, that is commonly associated with better performance under cyclic loading, is desirable, a reduced formability negatively affects the prospects of a crack free component manufactured via forming operations such as roll forming. A more flexible manufacturing approach is being in development using automated, robot-assisted roll forming. Instead of moving the metallic sheet along several roll pairs arranged in a specifically order one after another as described in Halmos (2006), the sheet is kept in place and the rolls are moved on a predefined tool path along the sheet to obtain the desired resultant geometry. Abvabi et al. (2015) have studied the influence of residual stresses on the forming process finish of a dual phase steel and observed the introduction of tensile residual stress on the sheet surface. Tensile residual stresses might result in a reduced fatigue life, see Schijve (2009). It is therefore necessary to consider resulting residual stresses in the design of critical structure components, next to changes in surface finish and local changes in material properties due to cold working, to achieve a more accurate fatigue life estimation. The study at hand investigates the impact of roll forming on the fatigue life using the Local Strain Approach (LSA) with the damage parameter P SWT . Firstly, the process simulation is presented along with the constitutive relationship of the aluminum alloy obtained from quasi-static tension tests. Three different tool paths are considered and simulated. A brief summary of the LSA is presented together with the experimentally determined cyclic materialproperties. The loading simulation is described for different loading conditions. The results of the fatigue life estimation is discussed for the three different tool paths.

Nomenclature E

Young’s modulus Tangent modulus

E TAN F max

Peak force in cyclic loading Cyclic hardening coefficient

K’ N f

Number of cycles to failure (crack initiation) Damage parameter according to Smith-Watson-Topper

P SWT R, R ε R p02

Stress ratio, strain ratio

Yield stress

b c Fatigue strength exponent Cyclic ductility exponent Cyclic hardening exponent Local strain amplitude ε f ’ Cyclic ductility coefficient ν Poisson ratio σ a σ f Yield stress n’ ε a

Local engineering stress amplitude

Equivalent stress (von Mises) Fatigue strength coefficient

σ eqv

σ f ' σ m

Mean stress

Principal stresses

σ 1,2,3

2. Process simulation of robot-assisted roll forming The material used is the aluminum alloy EN AW-7475 in the stable temper condition T761. The chemical composition, provided by the supplier, in the as received sheet with a thickness of t = 4 mm is given in table 1. Table 1. Chemical composition of investigated aluminum alloy EN AW-7475 T761 [%]. Material Si Fe Cu Mn Mg Cr Zn Ti EN AW-7475 T761 0.038 0.074 1.637 0.000 2.309 0.199 5.695 0.035