Structural Engineering & Statics
where the load goes
Concept

Crack growth — where it appears

The extension of an existing crack a little at each cycle of stress, at a rate set by the stress intensity at its tip. It is what a fatigue life is, which is why a welded detail — whose cracks are present from the day it was made — has no initiation phase and no material dependence.

Named by 2 essays across 2 fields — each of them below, with the objects they name alongside it.

Also named here as detail category, endurance limit, inspection, stress range — the same set of essays touches all of them, so they are one junction rather than several.

Three details, and no material anywhere on the plot. Stress range against cycles to failure for three detail categorys — 160, 90, 36 N/mm² at two million cycles. The lines are parallel because they share a slope of three, and the spread between them is a factor of 4.4 in stress and therefore 88 in life. Nothing on this plot depends on the strength of the steel: the same detail in a grade twice as strong lies on the same line. At a stress range of 70 N/mm² the lives are 160: 6.8e+7, 90: 4.3e+6, 36: 2.7e+5 cycles. The knee in each line is the constant-amplitude limit, past which the slope becomes five.

The load that never came near failing anything

A detail survives sixty-eight million cycles at a stress range that another detail in the same steel survives two hundred and seventy thousand of. The two lie a factor of two hundred and fifty apart, and the material is not on the plot anywhere.

materials · fatigue Four details, and no material anywhere on the plot. Stress range against cycles to failure for four detail categorys — 160, 112, 71, 36 N/mm² at two million cycles. The lines are parallel because they share a slope of three, and the spread between them is a factor of 4.4 in stress and therefore 88 in life. Nothing on this plot depends on the strength of the steel: the same detail in a grade twice as strong lies on the same line. At a stress range of 62 N/mm² the lives are 160: unlimited, 112: 2.1e+7, 71: 3.0e+6, 36: 3.9e+5 cycles. The knee in each line is the constant-amplitude limit, past which the slope becomes five.

The detail decides and the steel does not

A fatigue check contains no material strength anywhere. The same detail in a steel twice as strong lies on exactly the same line, because a fatigue life is decided by the geometry of a weld and by the stress range it sees — and two per cent of the traffic does most of the damage, because life goes as the inverse cube of the range.

connections · detail category

Named alongside it

The objects these essays reach for when they reach for this one.

Detail categoryEndurance limitFatigueInspectionResidual stressStress concentrationStress rangeEquivalent loadFractureGeometryLoad spectrumMiner sum

All concepts