Ship and offshore structures are full of holes: lightening holes, access manholes, pipe and cable penetrations, pin-jig openings. Each one is necessary — and each one is a stress raiser. Where the load has to flow around an opening, it crowds together at the edge, and the local stress can rise well above the average in the surrounding plate.
Why holes concentrate stress
Think of force as a stream of parallel lines flowing through the plate. A cut-out forces those lines to divert around it, bunching together at the sides of the hole. The tighter they bunch, the higher the peak stress — the stress concentration. For a circular hole in a wide plate under tension, the peak at the edge is famously about three times the far-field stress.
Where σmax appears
The peak (σmax) sits at the edge of the opening, on the axis perpendicular to the load — exactly where fatigue cracks like to start. Sharp corners make it worse: a square cut-out with tight radii can concentrate stress far more severely than a generous, rounded one. The first design move is almost always simply to increase the corner radius.
Designing the reinforcement
Where geometry alone is not enough, the edge is strengthened — with a thicker insert plate, a doubler or a welded edge stiffener that carries the diverted load smoothly past the opening. The aim is not to eliminate the concentration but to keep the peak stress within the fatigue and yield limits the class society requires.
Verified by FEM
Hand formulas cover the classic cases, but real cut-outs sit in real structures with nearby welds, brackets and combined loads. Finite element analysis makes the true peak visible, so reinforcement is added precisely where it is needed and nowhere it is not — lighter, stronger and certifiable, in the spirit of iki (粋).
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Frequently asked questions
How much does a hole increase stress in a plate?
For a small circular hole in a wide plate under uniform tension, the peak stress at the hole edge is about three times the nominal (far-field) stress. Sharper openings can be considerably higher.
How are cut-outs reinforced?
By increasing corner radii and, where needed, adding thicker insert plates, doublers or edge stiffeners so the diverted load passes the opening within allowable stress and fatigue limits.
