Meaning
Experimental evaluation of material yield strength and flow stress across logarithmic increments of deformation speed quantifies rate-dependent mechanical behavior in solid polymers. Conducting strain rate sensitivity testing provides critical constitutive data for simulating high-velocity impact events, drop tests and crashworthiness in plastic assemblies. This testing methodology spans quasistatic to high-strain-rate regimes, losing applicability above speeds where wave propagation effects dominate specimen stress uniformity.
Rate Dependence
Polymers exhibit visco-elastic response, shifting from ductile to brittle behavior as deformation velocity increases. Through strain rate sensitivity testing, material analysts measure the increase in yield stress occurring when loading speed jumps from quasi-static to dynamic rates. Amorphous polymers like polycarbonate demonstrate pronounced strain rate sensitivity, whereas highly filled compounds show localized matrix cracking before yield stresses rise.
Datasheets listing single-speed tensile test values fail to predict part performance during high-speed impact or drop loading.
Test Instrumentation
Specialized testing equipment must capture rapid load and displacement responses without introducing inertia artifacts into the data. Servo-hydraulic systems and split-Hopkinson pressure bars serve strain rate sensitivity testing across strain rates ranging from one per second to several thousand per second. High-speed optical extensometry measures localized strain field evolution without physical contact, bypassing mechanical grip slippage at elevated testing speeds.
Thermal imaging during testing reveals localized adiabatic heating, which softens the specimen and counteracts strain-rate hardening.
Moulding Effect
Processing conditions dictate molecular orientation and crystallinity, directly altering material response across different strain rates. High shear during injection molding produces anisotropic orientation that magnifies the outcomes of strain rate sensitivity testing depending on whether specimens are loaded parallel or perpendicular to flow lines. Incorporating regrind material reduces molecular weight, lowering the strain rate threshold at which brittle fracture replaces ductile yielding.
Moulders must align test specimen orientation with anticipated part load vectors to gain valid impact predictions.
