Why is the stress behind an environmental stress cracking failure often missing from the design load case?
Environmental stress cracking (ESC) requires sustained tensile stress, and in a large share of environmental stress cracking failures that stress is not in the design load case at all: it was frozen into the part during molding, imposed when the part was assembled, or introduced by a fit or interference nobody accounted for. Because environmental stress cracking only needs the total to exceed a threshold, a part carrying substantial residual stress can crack under a service load that on paper is trivial. An environmental stress cracking analysis that stops at the service load will conclude, wrongly, that the stress leg was not satisfied.
What threshold has to be exceeded for environmental stress cracking to start, and how do the stress contributions combine?
Susceptibility to environmental stress cracking (ESC) is usefully expressed as a critical strain below which cracking does not initiate in a given resin and agent combination, and residual, assembly and service strains superpose toward that threshold. What matters is the total strain at the location where cracking started, not any single contribution to it. Because the contributions add, apportioning them is the real work of an environmental stress cracking investigation, and any one of them considered alone can look harmlessly small.
What is molded-in stress, and where does it concentrate in an injection-molded part?
Molded-in stress is residual stress that injection molding freezes into a plastic part as it cools under packing pressure in a constrained cavity, and it concentrates near gates, across weld lines, around cores and inserts, and wherever wall section changes abruptly. Cooling imbalance between the cavity and core sides adds a bending component that persists after ejection. None of this molded-in stress is visible, and none of it appears in a drawing, but it is often the largest single contributor to the total strain in an environmental stress cracking failure.
How can polarized light show residual stress in a plastic part?
In transparent and translucent polymers, examination between crossed polarizers renders the stress field directly as birefringence fringes, and the fringe density indicates where stress concentrates. Comparing a failed part against an unused part from the same lot, and against a part that has been annealed to relieve residual stress, separates what the molding process contributed from what the service condition added. Polarized-light examination is a fast, non-destructive observation that frequently reorients an environmental stress cracking investigation at the outset.
How can annealing be used to diagnose residual stress in a molded plastic part?
Annealing works as a diagnostic, rather than a fix, because heating a molded part above its relaxation range and cooling it slowly relieves residual stress and, in doing so, quantifies it: the dimensional change on annealing is a measure of what was locked in. A molded part that cracks readily as molded and resists cracking after annealing, with the same agent and applied stress, has demonstrated that residual stress was the governing contribution to its environmental stress cracking.
Which stress contribution is most often overlooked in an environmental stress cracking failure?
Assembly stress is the contribution most often overlooked in an environmental stress cracking (ESC) failure. Over-torqued fasteners, press fits, snap fits and interference with an adjacent component all impose sustained tensile stress that persists for the life of the assembly, and unlike a service load, assembly stress never relaxes and never cycles off.
Boss cracking around a screw is the archetypal case of assembly stress: the load is entirely from installation, and the crack appears at whatever moment an agent reaches it. Reconstructing assembly stress requires the assembly, its fasteners and the actual installation torque, not the specified one.
What does the location of a stress crack say about where the stress came from?
The location of an environmental stress crack points back at its own cause: cracking that follows a weld line implicates molding, cracking radiating from a boss or fastener implicates assembly, and cracking at a point of maximum bending under a known service load implicates the applied load. Crack location is evidence because cracks initiate where total tensile strain is highest. Where cracks appear consistently at one feature across many parts, the geometry is doing most of the explaining.
How do you tell whether a stress concentration comes from the part design or the molding process?
Comparing parts across production dates and cavities is what separates a stress concentration built into the part geometry from one that varies with how the tool was running. A sharp internal corner, an under-radiused boss or a rib junction that concentrates stress is a design characteristic present in every part from the tool. Inconsistent packing, an out-of-balance cooling circuit or a drifting process produces variation between parts and between lots.
Can chemical compatibility data show whether a plastic will resist environmental stress cracking?
Not by itself: published chemical compatibility ratings are typically generated on unstressed specimens and describe chemical resistance rather than stress-cracking resistance. A material listed as compatible with a fluid can still crack under that fluid when stressed. Assessing a material selection against environmental stress cracking (ESC) requires data generated at a realistic strain, which is what the bent-strip and constant-strain protocols exist to provide.
What evidence should be preserved after an environmental stress cracking failure?
After an environmental stress cracking (ESC) failure, preserve the complete assembly rather than the cracked part alone, with fasteners in place if possible and the installation torque recorded before anything is disturbed. Keep unused parts from the same lot for residual-stress comparison. Preserve the molding process record — cavity, gate, cycle and any tooling changes — and a sample of the fluid involved, since the stress analysis only matters once the agent leg is also established.
This article is general technical orientation, not a failure analysis, an engineering opinion, or advice on any specific matter. Determining the cause of a particular incident requires hands-on examination by a credentialed expert.