1

Fatigue patterns in materials

Lars Damkilde Section for Structural MechanicsDepartment of Civil Engineering, Aalborg University

Fatigue – start and spreads

Beachmarks

2

Fatigue – combined failure

Top and bottom is cracked due to fatigue.

Middle part fails due to plastic failure

Part of a cranehook

3

Roller shaft failure

Photograph of fracture surface showing initiation site, beachmarks from fracture propagation, and small area of final fracture.

4

Composite failures

Metal fatigue failure is characterised by slow crack propagation; cracks initiate and propagate in a series of notch blunting and crack propagation events. By contrast, composite fatigue failure is a diffuse accumulation of damage, including multiple damage modes and complex failure mechanisms.

5

Welding may initiate cracks

6

Tubular joints – welded details

After many years in service, aluminum trichord sign structures often develop significant cracks and event complete failures in their welded connections, as shown in this photo of a failing joint. Redundant connections in most cases prevent pieces of the structure from falling onto the highway, but signs with failing joints need to be repaired or replaced.

7