Plastics : Stress cracking management in the plastic industry

Stress cracking is the formation of cracks in plastic materials due to the simultaneous presence of mechanical stress and exposure to certain chemicals or environmental conditions. 

It results in unexpected failures of plastic parts even when the applied stress is well below the material's normal breaking point.

Mechanism:

• Occurs when localized stresses in a plastic part allow chemicals to penetrate the polymer structure
• Creates microscopic cracks that gradually propagate through the material
• Results in sudden, brittle failure without significant deformation
• Often begins at stress concentration points like sharp corners, notches, molded-in stresses which are internal tensions trapped within plastic parts during manufacturing

Types of stress cracking:

• Environmental Stress Cracking (ESC): most common form, caused by exposure to chemicals that don't dissolve the polymer but facilitate crack growth
• Chemical Stress Cracking: direct chemical attack that weakens molecular bonds
• Thermal Stress Cracking: temperature changes create internal stresses that exceed material strength
• Ultraviolet (UV) Stress Cracking: UV radiation degrades polymer chains, making them susceptible to cracking under stress

Common causes:

• Residual molding stresses: uneven cooling during manufacturing creates internal tensions
• Assembly stresses: overtightened screws, press-fits, or snap fits
• Chemical exposure: detergents, oils, solvents, even seemingly harmless substances
• Cyclic loading: repeated stress application, even at low levels
• Environmental factors: temperature fluctuations, humidity changes
• Design issues: sharp corners, thin sections, abrupt thickness transitions

Most vulnerable materials:

• Polyethylene (PE): particularly susceptible to ESC in detergents and surfactants
• Polycarbonate (PC): sensitive to chemicals like alcohols and certain oils
• Acrylics (PMMA): prone to cracking from solvents and alcohols
• Polystyrene (PS): highly vulnerable to many solvents and oils
• ABS (Acrylonitrile Butadiene Styrene): susceptible to automotive fluids and some cleaning agents

ESC (Environmental Stress Cracking): failure mechanism where plastic cracks under mechanical stress when exposed to specific chemicals, even though these chemicals wouldn't normally damage the plastic alone.
Surfactants: chemical compounds that reduce surface tension between liquids or between a liquid and a solid, commonly found in detergents, cleaning products, and personal care items.
ABS (Acrylonitrile Butadiene Styrene): common thermoplastic polymer known for its impact resistance and toughness, widely used in automotive parts, electronic housings, and consumer goods.

Industry impact:

• Unexpected product failures in the field
• Safety concerns, especially in pressure-containing or structural applications
• Warranty claims and product recalls
• Damaged brand reputation
• Design limitations for certain material or application 

Prevention strategies:

Material selection:

• Choosing stress-crack resistant grades of polymers
• Using copolymers or blends with better resistance
• Selecting materials with chemical compatibility for the application environment

Design considerations:

• Eliminating sharp corners and notches (using generous radii)
• Uniform wall thickness to minimize stress concentrations
• Avoiding press-fit or snap-fit assemblies where critical
• Designing to minimize residual stress

Radii: rounded corners or edges in plastic parts that distribute stress more evenly, reducing stress concentrations that could initiate cracking.
Uniform wall thickness: consistent material thickness throughout a plastic part that prevents localized stress buildup by ensuring even cooling and stress distribution.
Press-fit assemblies: joining method where one part is forced into another with an interference fit, creating a secure connection through friction and material deformation without additional fasteners.
Snap-fit assemblies: Connection system where a flexible projection (hook, bead, or tab) on one part deflects temporarily during assembly then returns to its original position to lock into a mating feature on another part.

Residual stress: internal forces that remain within a plastic part after processing, even when no external loads are applied, they result from uneven cooling or molding pressure during manufacturing and can contribute to premature part failure.

Processing improvements:

• Optimized molding parameters to reduce residual stresses
• Annealing parts to relieve internal stresses
• Controlling cooling rates during manufacturing
• Proper gate location and design in injection molding

Annealing parts: controlled heating process for plastic components that relieves internal stresses by heating the material below its melting point, allowing polymer chains to relax and reorganize, then cooling it slowly to reduce stress cracking potential.

Testing methods:

• Bent strip immersion tests (ASTM D1693): standardized test where bent plastic strips are partially immersed in a chemical solution while under stress, measuring time until cracking occurs to evaluate environmental stress crack resistance.
• Constant tensile load tests (ASTM D2552): method where plastic specimens are subjected to a fixed tensile load while exposed to chemicals or environmental conditions, measuring time to failure to determine stress cracking susceptibility.
• Ball or pin impression tests with chemical exposure: test where a ball or pin creates localized stress on a plastic surface while chemicals are applied, observing if and when cracks form around the impression point.
• Accelerated environmental cycling tests: procedures that expose plastic materials to rapidly changing environmental conditions (temperature, humidity, UV, chemicals) while under stress to quickly evaluate long-term stress cracking performance.

Stress cracking remains one of the most challenging failure modes to predict and prevent in plastic products, often appearing suddenly after weeks or months of seemingly successful use.