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Fatigue strain

A smaller plate – and yet a lower stress level!

Sometimes a smaller plate gives a better stress distribution. In this example the trapezoidal bracket carries a stress of 446.9 MPa in the corner. Smoothing the geometry of the bracket – making it more flexible, in effect – keeps the stress from piling up in one point and spreads it along the whole length of the bracket. In fatigue calculations this matters a great deal.

FEA model of two bracket variants with vertical and horizontal load, stress scale up to 446.9 MPa
Two variants of the same bracket under the same load.
Triangular bracket in the FEA model – stress of 446.9 MPa concentrated at the corner
A sharp corner piles the stress into one point – 446.9 MPa.
Bracket with a rounded outline – stress drops to 261.5 MPa and spreads along the edge
Rounding spreads it along the edge – 261.5 MPa with a smaller plate.
Fatigue crack at the gusset plate of a steel connection, indicated by a yellow arrow
The crack starts where the geometry changes most sharply.