Theory 052/Engineering & Materials /Joining
Weld Life Is Residual Relaxation
Residual stress is treated as a static debit. It is a variable that decays.
Fatigue life of a welded joint is set by how fast residual stress relaxes rather than by its initial magnitude, so a joint with high but stable residual stress can outlast one with lower stress that relaxes quickly.
01The evidence
Post-weld treatments improve fatigue life by modifying residual stress, and the improvement varies with service temperature and loading.
02The tension
Design rules apply a fixed penalty for residual stress. That assumes it persists, which is untrue at service temperature.
03The mechanism
Welding leaves tensile residual stress at the toe, which drives crack initiation. At service temperature the stress relaxes by creep, and the rate depends on temperature and on the local microstructure.
That makes relaxation rate the controlling variable, and predicts that joints which relax quickly should recover their fatigue life over time while stable ones do not.
04What this is not
Penalties assume the stress is permanent. This treats it as decaying, which predicts life recovery with service time.
05Predictions
- P1Fatigue life should correlate with relaxation rate.
- P2Joints should improve with service time.
- P3Temperature should shift the rate.
06Refutation
Life tracking initial residual stress regardless of relaxation would restore the static penalty.
07What it changes
It would change post-weld treatment decisions.