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