The big idea: A structure fails because of overloading, material choice, size, or shape, and usually more than one at once.
Why the cause matters: Identifying which of the four is the real cause decides the fix: a bigger section, a different material, a different geometry, or a lower rated load.
Finite element analysis colours the stress. Read the red, then ask which of the four causes put it there.
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| Cause | What it looks like | The fix |
|---|---|---|
| Overloading | A load larger, more sudden or more off-centre than the one designed for — somebody standing on the top step, or two people on a one-person swing | A larger safety factor, a clearer rating, or a design that physically prevents the misuse |
| Material choice | The material was too weak, too flexible, too brittle for an impact, or degraded by its environment — a polymer that went brittle in UV, a steel that corroded | A different material, or a finish that protects the one chosen |
| Size | The section was too small or too slender: a thin shelf that sags, a long thin leg that buckles | More depth, a shorter span, a shorter effective length — geometry, not grade |
| Shape | A stress concentration at a sharp internal corner, a hole in the wrong place, or a section that is the wrong shape for the load | A radius instead of a sharp corner, material moved away from the neutral axis, a hole moved to where the stress is low |
The one students miss: sharp corners: Stress concentrates at a sharp internal corner, and the sharper it is the higher the local stress climbs.
A crack therefore starts at a corner, a hole or a tool mark, and runs from there — which is why a fillet radius is a structural feature and not a cosmetic one.
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What an FEA plot is telling you
The colours are stress, not strength
Red is the highest stress in THIS model, not necessarily a failure. Compare the number on the scale with the material's yield strength before concluding anything.
Red at a corner is a warning
It marks a stress concentration. Adding a radius there usually drops the peak stress substantially for no extra material.
Large blue regions are wasted material
Material carrying almost nothing can be removed — which is what topology optimisation does, and why printed brackets look organic.
The model is only as good as its inputs
The loads, the constraints and the material data were all assumed by somebody. An FEA plot of the wrong load case is confidently, colourfully wrong.
Using FEA data in an answer: Quote the peak stress, compare it with the yield strength, and say what the ratio is.
"The peak is 240 MPa against a yield of 250 MPa, so the part is at 96% of yield with no margin" is a finding. "It is red at the corner" is a description.
How this is tested — identifying why a structure has failed, including interpreting FEA data. It comes up two ways:
Paper 1 — multiple choice
- Identify the cause of a described failure.
- Choose the change that removes a stress concentration.
Paper 2 — analysing a product
- Explain why a named product failed, using the data given.
- Interpret an FEA result and recommend a change.
The trap: Answering "the material was not strong enough" to everything. Size and shape cause more failures than material does, and they are far cheaper to fix.
A moulded polymer bracket cracks after eight months outdoors. FEA shows a peak stress of 18 MPa at a square internal corner, against a material yield of 45 MPa. Explain the failure.
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