Can photographs show whether a cracked plastic part failed by environmental stress cracking or chemical attack?
No: photographs cannot separate environmental stress cracking (ESC) from chemical attack in a cracked plastic part that was in contact with a fluid. The fluid may have chemically degraded the polymer, or the fluid may have acted as a stress-cracking agent without altering the polymer’s chemistry at all. The two explanations point at different parties: chemical attack points at the material and the chemical that attacked it, and environmental stress cracking points at whatever put the part under sustained tensile stress. The laboratory separates environmental stress cracking from chemical attack reliably, and it does so by asking a single question: did the polymer change?
What is the difference between chemical attack and environmental stress cracking?
Chemical attack degrades the polymer directly, while environmental stress cracking (ESC) is a physical process that leaves the polymer chemically essentially intact, even though the two mechanisms share one appearance. In chemical attack, bonds in the polymer backbone are cleaved, or additives are extracted, and the material is measurably different afterward than it was before. Environmental stress cracking does no such thing: a surface-active agent lowers the energy required for a craze to nucleate and grow, allowing fracture at a stress far below the material’s rated strength.
How does molecular weight testing distinguish chemical attack from environmental stress cracking?
Molecular weight is the primary discriminator: chain scission from chemical attack shows up as a measurable reduction in molecular weight, typically with a broadened distribution, while an environmental stress cracking (ESC) failure characteristically shows a molecular weight distribution indistinguishable from unexposed reference material. Gel permeation chromatography measures the molecular weight distribution of the polymer, and it is the single most direct available test for separating environmental stress cracking from chemical attack.
Gel permeation chromatography depends entirely on having a reference, meaning unexposed material of the same grade and ideally the same lot, because absolute molecular weight varies between grades and production runs.
What does an infrared spectrum add when separating environmental stress cracking from chemical attack?
An infrared spectrum detects new chemical species: in an environmental stress cracking (ESC) failure, the spectrum of the fracture region should look like the spectrum of the bulk and like the reference material. Oxidation introduces carbonyl absorption that was not there before, and hydrolysis of an ester or amide backbone produces its own characteristic changes. An infrared spectrum showing new functional groups concentrated at the fracture surface is pointing toward chemical involvement rather than a purely physical mechanism.
How does the fracture surface separate environmental stress cracking and chemical attack from an overloaded plastic part?
The fracture surface separates both environmental stress cracking (ESC) and chemical attack from overload, since the fracture morphologies are separable under magnification: environmental stress cracking produces a characteristic morphology of crazed regions where fibrils have drawn and ruptured, generally with a slow-growth zone and a distinct transition to final fast fracture, whereas ductile overload shows gross deformation, drawing and shear lips, and brittle mechanical fracture shows a flat surface with radial features pointing back to an origin.
These fracture morphologies establish that a failure was environmental stress cracking rather than simply an overloaded part in the presence of a fluid, which is a third possibility, beside environmental stress cracking and chemical attack, that gets less attention than it deserves.
What three conditions does environmental stress cracking need?
Environmental stress cracking (ESC) needs a susceptible resin, a sustained tensile stress and a surface-active agent, all present together, and the absence of any one of the three stops it. That structure is diagnostically useful: if any leg can be excluded, environmental stress cracking can be excluded as the mechanism. It also means an environmental stress cracking finding always implicates at least two contributions, which is why matters involving environmental stress cracking rarely resolve to a single responsible party without further work on which leg was the abnormal one.
What kinds of fluids act as environmental stress cracking agents?
The agents behind environmental stress cracking (ESC) are usually unremarkable, and aggressive chemicals are not required: detergents, lubricants, oils, alcohols, adhesives and some cleaning products are documented stress-cracking agents for susceptible resins. A fluid that is entirely benign against an unstressed specimen can crack the same material readily once stress is present. This is why compatibility data generated on unstressed coupons routinely misleads, and why the stress condition has to be part of any compatibility assessment that is going to mean anything.
Does identifying environmental stress cracking show where the stress came from?
No: establishing environmental stress cracking (ESC) identifies the mechanism but not the source of the stress leg, and where the stress came from is a separate inquiry. The stress may be residual stress frozen in during molding, an interference or clamping stress imposed at assembly, or an applied service load. Polarized-light examination visualizes residual and assembly stress directly in transparent and translucent materials, and comparing a failed part against an unassembled exemplar often shows how much of the total stress the assembly itself contributed.
How can an environmental stress cracking finding be confirmed?
Reproduction is the strongest confirmation of environmental stress cracking (ESC): where exemplar parts and the suspect fluid are both available, reproducing the failure under controlled stress and exposure moves the finding from inference to demonstration. The bent-strip approach in ASTM D1693 is the common reference for that kind of reproduction. A reproduction that generates the same fracture morphology at a comparable stress is considerably more persuasive than a chain of analytical results alone.
What should be preserved when a cracked plastic part may have failed by environmental stress cracking or chemical attack?
Four things should be preserved when a cracked plastic part may have failed by environmental stress cracking (ESC) or chemical attack: the cracked part without cleaning, a sample of every fluid the part contacted, unused parts from the same lot, and the assembly context. The cracked part should not be cleaned, since cleaning removes the agent that is half the evidence. Each fluid sample should come with concentration and exposure conditions if known. Unused parts from the same lot serve both for reference measurement and for reproduction testing. The assembly context, meaning torque values, fits, and how the part was constrained, matters because the stress leg cannot be reconstructed from a loose part.
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.