Study of fatigue loads in spot weld joints for a side impact beam

Abstract: Gestamp HardTech develops and produces press-hardened safety components in
boron steel for the passenger car industry.

During door slam testing performed by a car manufacturer, the front door
is opened and closed 100 000 times. If the spot welds joining the Side
impact beam with the inner panel receive too high stress levels during this
test, fatigue fracture will occur.
Today, one or two additional weld tabs are often applied to the rear
joining position to handle the fatigue loads. The drawback with this design
is that the shear tabs are driving the blank size. There are also
manufacturing difficulties associated to the shear tab design.

The purpose of this thesis is to, by simulation loops, find a design of the
rear end of the Side impact beam, including positions- and number of welds,
that provides equal or better fatigue resistance as a “shear tab” design.

The fatigue life is evaluated through FE-simulations of the door slam where
the fatigue parameter called Equivalent structural stress is determined for
each spot weld. This parameter is analytically calculated from the spot
weld forces during Post-processing.

The thesis has also included fatigue testing of spot welded specimens for
determination of material data used in the formation of Equivalent
structural stress.

The results show that by changing the placements of the welds and by minor
changes of the Side impact beam, the Equivalent structural stress in the
most critical weld is reduced to the same level as when a shear tab is
present.