Cracking, crazing and stress whitening in plastic parts

When a part splits in service, the first useful question is not which material to switch to — it is what the break is telling you. Most plastic part cracking causes fall into a small number of families, and each family leaves a different signature. Read the signature before you change the grade, because the wrong substitution often reproduces the same failure in a more expensive polymer.

This article walks through the four signatures we see most often in machined stock shapes, and what each one points to.

Reading a fracture surface

Start at the origin, not the end. A fracture has an initiation region, a propagation region and often a final fast break. A part that failed slowly shows a dull, matte propagation zone and a small final zone; a part that failed in one event shows almost no propagation before it lets go. If the origin sits at a sharp internal corner or a thread root, the material was probably overloaded at that point. If it sits in a smooth field with no feature, look for a chemical or thermal cause instead. Colour helps: a yellowed or bleached zone near the origin usually points to heat, while a clean break points to mechanical overload.

It helps to know what the stock was meant to do. Our materials guide sets out the service window of each polymer, and comparing that window with the failed part’s duty is often enough to rule a cause in or out.

Crazing: what it looks like and what starts it

Crazing shows as a fine, silvery web of micro-cracks that reflects light. It is not the same as a true crack: the crazes still carry load across thin fibrils, so the part looks intact and then fails with little warning. Crazing begins where local stress meets a hostile environment — a solvent wipe, a cleaning agent, or simply high locked-in stress from machining. Clear grades make it visible; opaque grades hide it until the surface whitens or the part snaps.

Environmental stress cracking and the media list

Environmental stress cracking is the combination of a modest mechanical stress and a chemical that the material tolerates poorly under that stress. Neither factor alone would damage the part. The media that catch people out are ordinary ones: cutting fluids, thread-locking compounds, mould release sprays, chlorine-based cleaners and some hydraulic oils. Many polycarbonate grades are especially sensitive to alkaline cleaners and to some amines. Amorphous materials such as PC and PEI craze more readily than semi-crystalline ones such as acetal (POM), which tend to swell or soften instead of crazing.

The practical test is simple. Wipe an offcut with the suspect fluid, stress it slightly and leave it overnight. If it crazes, the fluid is a candidate in the real part too, and it moves to the top of the plastic part cracking causes worth checking first.

Stress whitening at assembly points

Stress whitening is the pale halo that appears around a screw boss, a press-fit bore or a staked joint. It is the material yielding locally, and it is a warning rather than a failure. On a clear part it is obvious; on a black part you will only see it under grazing light. Whitening at an assembly point almost always means the joint is over-stressed for the material and for the wall thickness behind it.

Treat whitening as a stop signal. If it appears on the first few parts of a batch, the assembly process is outside the material’s comfort range, and the rest of the batch will fail in the field on the same route.

Over-torqued fasteners and press-fit damage

A fastener driven to a steel torque value will strip or split a plastic boss. Plastic relaxes under load, so a joint that feels tight on the bench can lose preload and then be re-tightened until it cracks. Press fits behave the same way: an interference that is fine in aluminium can raise hoop stress in acetal or nylon past yield, and moisture uptake in nylon adds to it over the first weeks. That slow build is why press fits and over-torqued joints sit among the plastic part cracking causes that appear only after a batch has already passed inspection. Use a torque figure derived for the polymer, a wider boss wall, or a metal insert where the load is high.

Questions buyers ask about plastic part cracking

How do I tell crazing from a true crack?

Crazing is a web of fine, reflective micro-cracks that still carry some load; a true crack is a single sharp separation. Under a low-power microscope the crazed zone looks silvery and diffuse, while a crack has crisp edges. A crazed part can survive a while, but it will fail without much warning.

Which plastics are most prone to stress cracking?

Amorphous, glassy polymers such as polycarbonate are the most sensitive, because they craze rather than yield. Semi-crystalline grades such as POM and the nylons resist crazing but can swell in aggressive media. The service medium and the locked-in stress matter more than the polymer name alone.

Can annealing prevent cracking?

Annealing relieves locked-in stress from machining and can remove one of the two ingredients in environmental stress cracking. It does not make a material chemically compatible with a fluid it already dislikes. Use it as one measure among several, not as a substitute for the right grade.

What should I check first when a batch cracks in assembly?

Check the fit and the torque before the material. Measure the actual interference or drive torque, compare it with the figure the part was designed for, and look for whitening on early parts. If the joint is inside its design values and cracks still appear, then review the fluid contact and the grade. Our FAQ page covers the common compatibility questions, and you can send us the failed part for a second view.