Sheet Metal Fatigue near Nuts Welded to Sheet Structures and Bolted to a Rigid Attachment

Journal Article
05-15-04-0022
ISSN: 1946-3979, e-ISSN: 1946-3987
Published May 10, 2022 by SAE International in United States
Sheet Metal Fatigue near Nuts Welded to Sheet Structures and Bolted
                    to a Rigid Attachment
Sector:
Citation: Noll, S., "Sheet Metal Fatigue near Nuts Welded to Sheet Structures and Bolted to a Rigid Attachment," SAE Int. J. Mater. Manf. 15(4):2022.
Language: English

Abstract:

Stress-based sheet metal fatigue near nuts welded to thin sheets is one of the necessary design processes for car bodies. In this investigation, the influence of the attachment contact on the localized fatigue mechanism is examined through finite element (FE) models and controlled fatigue experiments. First, a fatigue experimental setup, which includes a thin-sheet closed-hat section with a weld nut bolted to a thick attachment piece, is designed to minimize the uncertainty of the influence of the fixtures on the experimental results. The experiments are carried out on 0.9- and 1.0-mm thick hat sections with a square weld nut under force control conditions with complete reversed loading. Due to the contact, the test specimen performs as a bilinear spring that has a lower stiffness in the upstroke direction when compared to the downstroke direction where full contact of the attachment occurs with the hat section. The results of the experiments are Fa-N curves for the combinations of the load levels and sheet thickness. Numerical analyses are then carried out to transform the curves into Wöhler (σa -N) curves and fit them with a straight line. Due to the thick attachment contact interaction, the outer fiber of the hat section near the weld nut undergoes two tensile cycles through a single loading cycle of the experimental setup, with the upstroke stress level dominating the damage per cycle. When normalized with respect to stress, the bounds of the 95% confidence interval fall within a factor of 3 in fatigue life for all of the experimental results, regardless of sheet thickness.