Abstract
Joining by forming is a commonly applied technique in the automotive industry to assemble parts of thin metal sheets to meet the demands of lightweight design. The joining operation induces changes in material behaviour due to cold forming, that can be observed in increased hardness in the area close to the joint neck compared to the base material. Complex geometrical features of clinched joints on a small scale and the lack of non-destructive methods to track local stresses and strains require a combined approach utilizing numerical and experimental techniques. Numerical process and loading simulation are performed utilizing commercial finite element software LS-Dyna®. Hardness measurements in the joint are carried out to assess the impact of forming operation. Cyclic material properties are derived from Vickers hardness to estimate fatigue life with the Local Strain Approach using the damage parameter PSWT. Fatigue life estimation with failure criterion crack initiation obtained from simulation results is compared to those from experiments. The results obtained indicate that the Local Strain Approach is suitable for fatigue life estimations of clinched joints under constant amplitude loading as long as the influence of the forming process is considered.
Highlights
Mechanical clinching is an efficient joining technique used in automotive industry and manufacturing of white goods
The present study focuses on the fatigue life estimation with the Local Strain Approach (LSA) taking into account the forming history including changes of cyclic material properties due to cold working
With ∆f = 0.1 Hz, the number of cycles up to crack initiation is close to those corresponding to the failure criterion fracture
Summary
Mechanical clinching is an efficient joining technique used in automotive industry and manufacturing of white goods. Two or more sheets are joined through localized cold forming without additional fasteners Different variants exist, such as clinching with fixed grooved dies, extensible dies, shear clinching and flat clinching, resulting in joints with distinctive joint properties [1]. Extensive research, both through experimental work and numerical simulation, has been conducted so far to better understand the material flow in the contact region during forming operation [2,3,4]. The cold forming of the material is assumed to introduce significant changes in cyclic material properties, resulting in experimentally obtained higher fatigue life compared to estimation based on numerical estimation [9].
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