PSI - Issue 38

Boris Spak et al. / Procedia Structural Integrity 38 (2022) 572–580 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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1. Introduction With the prevailing demands to deliver lightweight products and reduce the CO 2 footprint during operation and manufacturing especially in the automotive and aerospace industries, driven either by legal framework conditions or social self-commitment of the manufacturers, considerable research is undertaken on the use of lightweight materials and design. Aluminum alloys are of great interest due to their high strength, low density, corrosion resistance and processability. Considering the field of joining operations, the mechanical clinching technology offers several advantages over other methods of joining like self-piercing riveting (SPR) and resistance spot welding (RSW). Although showing lower static strength compared to SPR and RSW, clinched joints exhibit remarkable fatigue properties especially in the high cycle fatigue as has been shown by Mori et al. (2012). Further advantages support the use of clinched joints as a joining method, e. g. low energy consumption and fumeless emissions. With mechanical clinching, two or more sheets are joined through localized cold forming in the contact region between a punch and a die, resulting in an interlock. The joint is formed without any other materials, such as fasteners, although glue can be used to alter the properties of the clinched joint. Literature review indicates that a lot of effort has been undertaken to understand the lock forming mechanisms by Mucha (2011) and to assess the characteristics of the joint quality through geometric properties, like the neck thickness, bottom thickness, interlock and undercut with an enclosed optimization for a specific set of tools as exemplified by Wang et al. (2018). With the great variety and possibility of combinations of available tool geometries and materials to be joined, new ways are needed to virtually explore the properties of clinched joints. Ewenz et al. (2021) point out that as of today no model exists that could capture all essentials spanning from creating a joint without cracks during forming operation up to evaluation of fatigue properties under cyclic loading. Su et al. (2015) investigated the applicability of an analytical model, derived from the assessment of spot welds, to clinched joints. The results yield a non-conservative fatigue life estimation for the material and joint geometry in consideration. Furthermore, a strong assumption has to be made to justify the application of that analytical model, namely that cold working in the neck region results in material properties similar to those of a rigid body.

Nomenclature E

Young’s modulus

Cyclic hardening coefficient

K’ N f

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

P SWT

Nominal stress

S b c

Fatigue strength exponent Cyclic ductility exponent Cyclic hardening exponent Local strain amplitude Cyclic ductility coefficient

n’ ε a ε f ’ σ a σ f σ f ' σ m σ 1

Local engineering stress amplitude

Yield stress

Fatigue strength coefficient

Mean local stress

Local first principal stress

Degree of forming

φ

The present study investigates the possibilities to estimate fatigue life of clinched joints with the Local Strain Approach (LSA). Thus, the failure criterion is crack initiation. Firstly, the material behavior is described by identification of the constitutive parameters from quasi-static tension tests. The stress-strain response of the material is processed in a 2D process simulation with the commercial LS-Dyna® finite element code to create two different rotationally symmetric clinched joints with bottom thickness of 1.0 mm and 1.4 mm (hereafter referred to as variant 1.0 and variant 1.4, respectively). The simulated geometry of the joint is mapped to 3D and loaded with constant