Load Paths: Why Structures Fail Where They Do

Follow the load

Every applied load must reach a reaction. The sequence of members carrying it there is the load path, and tracing it deliberately is the single most useful habit in structural design.

In a wing, aerodynamic pressure is distributed over the skin, gathered by stringers and ribs, carried in bending by the spar caps, resisted in shear by the spar webs, and finally delivered into the fuselage through the wing attachment fittings. Each transition is a place where the structure changes its mind about how the load is carried, and each is a candidate for failure.

If you cannot trace a continuous path from every load to a reaction, the structure has a gap — and the load will find its own way there, usually through something not designed to carry it.

Load carried is proportional to stiffness

This is the principle that surprises people, and it explains a large fraction of unexpected failures.

When several members share a load in parallel, they do not share it equally or by strength. They share it in proportion to stiffness. A stiff member attracts load away from a flexible one adjacent to it, regardless of which is stronger.

The practical consequences are direct. Adding a stiff bracket to reinforce an area draws load into the bracket and its attachments, which may now be the weakest element. Replacing a component with a stiffer one changes the load distribution throughout the assembly. A repair doubler that is much stiffer than the parent structure concentrates load at its edges.

Stiffening a structure locally does not automatically strengthen it. It redistributes the problem.

Stress concentration

Nominal stress — load divided by area — is a fiction anywhere the geometry changes. At holes, fillets, notches and section changes, the actual peak stress is higher by the stress concentration factor K_t.

For a circular hole in a wide plate under uniform tension, K_t = 3. The stress at the hole edge is three times the nominal value, regardless of hole size. Sharp re-entrant corners are far worse, and a genuinely sharp corner is a crack by another name.

This is why fillet radii are specified rather than left to the machinist, why holes are deburred, and why cutouts in loaded structure need reinforcement. Under static loading a ductile material may yield locally and redistribute. Under cyclic loading it will not forgive you.

Fatigue: the failure that ignores your margin

A structure can be entirely adequate statically and still fail from fatigue after enough cycles at a fraction of its static capability.

Fatigue cracks initiate at stress concentrations, propagate slowly and stably under cyclic loading, and then fail suddenly when the remaining section can no longer carry the peak load. The great majority of fatigue life is consumed in initiation and slow growth, which is what makes inspection viable as a strategy.

Two consequences shape aerospace practice. Surface condition dominates initiation — machining marks, corrosion pits and handling damage are where cracks start, so surface finish in fatigue-critical areas is a structural requirement, not a cosmetic one. And damage tolerance assumes a crack exists and demonstrates it can be found before it becomes critical, rather than assuming a flawless part.

Buckling: failure without exceeding a stress

Slender members in compression fail by instability, not by exceeding an allowable stress. The Euler critical load,

P_cr = π² · E · I / (K · L)²

depends on the elastic modulus, the second moment of area, and the effective length set by end restraint. Material strength does not appear.

This has an important implication: for a buckling-critical member, choosing a stronger alloy achieves nothing. Only stiffness, geometry, or end fixity help. Thin-walled aerospace structures are frequently buckling-critical, and panels may be permitted to buckle in service while the stiffeners continue to carry load — a deliberate design state, not a failure.

Practical discipline

• Trace every load to a reaction, and treat any path you cannot trace as a finding.
• Ask what your stiffness changes do to load distribution, especially after a repair.
• Radius every re-entrant corner; specify the radius.
• Check buckling separately from strength for anything slender in compression.
• Report margins against a stated allowable, and state which failure mode governs.