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Drawing No. EH–CS–007 // Civil & Structural Engineering

Fracture Mechanics & Crack Growth Simulator

Reviewed August 2026

A cracked component can fracture before its nominal stress reaches yield when the crack-tip stress intensity reaches the applicable toughness. Grow the crack, change the stress and the material, and watch K approach KIC; then explore Paris-law growth, crack-tip plasticity and a deliberately simplified leak-before-break geometry screen.

Educational model. Linear elastic fracture mechanics with constant geometry factors, Irwin plastic-zone estimates and a single-region Paris law. The leak-before-break tab is only a geometric LEFM screen; real assessment also requires crack-shape solutions, elastic-plastic tearing, leak detectability, load combinations, residual stress and code-specific acceptance criteria.
Stress intensity against toughness
the crack size that ends the component
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Parameters

Live readout

What to watch for

Critical size falls with the square of stress

ac goes as 1/σ². Doubling the working stress quarters the flaw you can tolerate, which is why highly stressed designs need far finer inspection than lightly stressed ones made of the same steel.

Paris-law growth accelerates as the crack grows

On the fatigue tab, watch where the cycles go. In a single-region Paris model, growth rate rises strongly with ΔK, so much of the integrated interval is often spent near the smaller-crack end. The exact split depends on the initial flaw, critical size, stress range and material constants.

Toughness beats strength for flaw tolerance

Swap between the materials. A high-strength steel with low toughness has a far smaller critical crack than a softer, tougher one at the same stress. Strength decides when it yields; toughness decides how big a flaw it survives.

Thickness changes crack-tip constraint

The plastic-zone tab shows the plane-stress zone at three times the plane-strain one. That is why published KIC is the plane-strain value — the lower bound — and why the thickness validity check matters when interpreting a test.

Leak-before-break is a geometry argument

This page uses only a geometry screen: it compares the ideal central-crack critical half-length ccrit with wall thickness t, equivalently comparing full lengths 2ccrit and 2t. Real leak-before-break qualification also needs crack-shape, tearing and leak-rate analysis.

The exponent is where the risk lives

Change Δσ by 25 % with m = 3 and the life changes by about a factor of two. Fatigue predictions are therefore only as good as the load spectrum they are fed.

Sources and technical basis

Linear elastic fracture mechanics with Irwin plastic-zone estimates and a single-region Paris growth law; material values are representative teaching anchors, not design allowables.

USAF AFGROW Damage Tolerance Design Handbook — stress intensityLinear-elastic fracture-mechanics basis for K and the dependence on applied stress, crack size and geometry.ASTM E399-24 — plane-strain fracture toughnessCurrent K_IC test method and the linear-elastic, high-constraint validity basis behind plane-strain toughness.USAF AFGROW — fatigue crack growth and ΔKDamage-tolerance treatment of cyclic stress, stress range and stress-intensity range for fatigue-crack-growth analysis.

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