PSI - Issue 38

574 Boris Spak et al. / Procedia Structural Integrity 38 (2022) 572–580 Author name / Structural Integrity Procedia 00 (2021) 000 – 000 3 amplitudes with stress ratio = 0.1 to replicate the experimental set up of lap shear specimens. The aim of the fatigue tests is to detect crack initiation. To obtain the material parameters for fatigue life estimation with LSA and damage parameter P SWT by Smith et al. (1970) as well as fatigue life estimation based on local strain amplitudes, cyclic tests with constant strain amplitudes are performed. Experimental results and fatigue life estimations are compared for both variants of clinched joints. 2. Process simulation of clinched joint The material investigated is a heat treatable aluminum wrought alloy EN AW-6060 T66. The chemical composition of supplied sheets with a thickness of 2 mm as a result of optical emission spectrometry is summarized in Table 1.

Table 1. Chemical composition of investigated aluminum alloy [wt%].

Material

Si

Fe

Cu

Mn

Mg

Cr

Zn

Ti

EN AW-6060 T66

0.45

0.18

0.02

0.04

0.44

0.01

0.01

0.02

The flow curve is obtained from quasi-static tension tests in compliance with the standard DIN EN ISO 6892-1 (2020) with dog bone specimen manufactured according to German standard DIN 50125 (2016). A speckle pattern was applied on each of the nine tested specimens. Full field measurement of displacements on the specimen surface was conducted with digital image correlation system GOM Aramis. The mechanical properties were determined with yield strength R p02 = 233 MPa, ultimate tensile strength R m = 247 MPa, Young’s modulus E = 65.165 MPa and Poison ratio ν = 0.332. From displacement measurements, strains are computed up to onset of necking. For higher strains that are present during forming operation of clinched joints, typically up to a degree of deformation φ ≈ 2…3, an extrapolation according to Voce (1948) is utilized for the yield stress σ f .   16.48 ) 233 (295.93 295.93 −  − − = MPa e MPa MPa f (1)

Fig. 1 a) 2D model of process simulation. b) Comparison of force displacement curve from experiment and simulation. c) Simulation result of cross section (red line) of clinched joint variant with bottom thickness of 1.4 mm. d) Hardness distribution in clinched joint variant 1.4

The process simulation is carried out utilizing LS-Dyna R11.1.0 in an axisymmetric 2D model with implicit solver. The elastic-plastic material behavior is put in as a piecewise linear curve. Isotropic hardening is assumed with a von Mises yield surface and r -adaptive mesh refinement with an element size of 0.125 mm is implemented. Punch, die and holder are modelled as rigid bodies. Discrete elements are introduced and parametrized to model the stiffness of the real test rig, see fig. 1 a). The static and dynamic coefficient of friction is assumed at 0.5 and 0.25, respectively.