Stress Testing Molded Parts I’ve seen parts pass visual inspection and fail in the field. The customer returns the product, and suddenly everyone’s asking why. Stress testing catches failures before your customers do. Here’s how to use it.

Key Takeaways

| Aspect | Key Information |

Understanding Stress Testing

Purpose of Stress Testing PurposeApplicationVerify designProve part will performValidate materialConfirm material selectionFailure preventionFind failure modesSafety assuranceCritical applicationsCustomer requirementsContractual obligation

Types of Stress Stress TypeWhat It TestsTypical MethodTensileAxial strengthPull testFlexuralBending strength3-point bendImpactShock resistanceIzod/CharpyCreepLong-term loadingSustained loadFatigueCyclic loadingRepeated stressEnvironmentalReal-world conditionsEnvironmental chamber

Mechanical Testing Methods

Tensile Testing StandardASTM D638ISO 527Specimen typeType I-V1A, 1BTest speed0.2-20 in/min1-500 mm/minResultsStrength, elongation, modulusStrength, elongation, modulus

Flexural Testing StandardASTM D790ISO 178Configuration3-point, 4-point3-pointSpan-to-depth16:1 typical16:1 typicalResultsStrength, modulus, strainStrength, modulus, strain

Impact Testing TestStandardUseIzodASTM D256Notched impactCharpyASTM D6110Notched/unnotchedDrop weightASTM D3763High-rate impactDart dropASTM D5628Product impact

Fatigue Testing StandardASTM D3479ISO 13003TypeTension-tension fatigueFatigueFrequency1-30 Hz1-100 HzResultsS-N curveFatigue life

Testing Equipment

Universal Testing Machine SpecificationTypical RangeApplicationLoad capacity0.1-100 kNTensile, flexuralSpeed range0.001-20 in/minMultiple methodsAccuracy±0.5% of readingPrecision testingExtensometerRequiredStrain measurement

Impact Testers TypeCapacityApplicationIzod0.5-5 ft-lbSmall specimensCharpy1-50 ft-lbLarger specimensDrop tower0.5-100 ft-lbProduct testing

Environmental Chambers CapabilityRangeUseTemperature-40°C to +200°CThermal cyclingHumidity10-95% RHHumidity exposureCombinedFull controlEnvironmental testing

Sample Preparation

Specimen Requirements MethodSpecimen TypePreparationInjection moldedDirectly moldedPer ASTM/ISOCut from partPer standardAvoid damageConditioningPer standard24 hrs @ 23°C, 50% RH

Conditioning Requirements ConditionTemperatureHumidityTimeStandard73±4°F50±10% RH40 hours minimumElevatedPer specPer specPer specDry as molded23°C<0.1%Oven dryWet conditioned73°FImmersion48 hours

Number of Samples Test TypeMinimum SamplesRecommendedScreening3 specimens5 specimensQualification5 specimens10 specimensProduction verificationPer control planPer spec

Test Procedures

Tensile Test Procedure StepAction1Condition specimens2Measure dimensions (3 points, average)3Mount in grips (parallel alignment)4Attach extensometer5Zero load and displacement6Start test at specified speed7Record load vs. displacement8Note yield, break, modulus9Record failure mode

Impact Test Procedure StepAction1Condition specimens2Notch specimens (if required)3Measure notch dimensions4Position in striker5Verify pendulum height6Release pendulum7Record energy absorbed8Inspect fracture

Flexural Test Procedure StepAction1Condition specimens2Measure dimensions (3 points)3Position on supports (span per standard)4Zero load and deflection5Apply crosshead at specified rate6Record load vs. deflection7Calculate stress at 5% strain8Record failure mode

Acceptance Criteria

Typical Property Requirements PropertyTypical SpecificationNotesTensile strength>6,000 psi (ABS)Material dependentFlexural modulus>300,000 psi (ABS)Material dependentImpact strength>3 ft-lb/in (ABS)Material dependentElongation at break>5%Ductility indicator

Industry-Specific Requirements IndustryStandardKey RequirementsAutomotiveGM, Ford, Chrysler specsSpecific test methodsAerospaceASTM, SAE specsStringent requirementsMedicalISO 10993BiocompatibilityConsumerUL, CSASafety requirements

Test Report Requirements InformationDetailsTest date___Test operator___Material identificationGrade, lotSpecimen identificationPart, locationTest methodStandard usedEquipmentID, calibrationResultsAll measured valuesCalculated valuesStrength, modulusObservationsFailure modes

Design Validation Testing

Test Plan Development ElementContentsPurposeWhat is being validatedScopeWhat will be testedAcceptance criteriaPass/fail criteriaTest methodsSpecific proceduresSample requirementsQuantity, conditioningReportingResults documentation

Accelerated Testing MethodPurposeApplicationElevated temperatureSimulate long-termCreep, agingHumidity exposureMoisture effectsEnvironmental stabilityUV exposureWeatheringOutdoor applicationsThermal cyclingFatigueTemperature extremesSalt sprayCorrosionHarsh environments

Failure Analysis Analysis TypeUseVisual inspectionFracture originMicroscopyFailure initiationFractographyFailure mechanismChemical analysisMaterial verification

Common Test Failures

Tensile Test Failures SymptomLikely CauseActionLow strengthMaterial degradationCheck materialLow modulusVoids, porosityCheck processingBrittle fractureLow temperatureCondition properlyGrip failureSpecimen preparationImprove grip technique

Impact Test Failures SymptomLikely CauseActionLow energyNotch sensitivityCheck notch qualityVariable resultsSpecimen inconsistencyImprove consistencyBrittle fractureLow temperatureCondition properlyHigh scatterMaterial variationCheck material

Fatigue Failures SymptomLikely CauseActionEarly failureStress concentrationRedesignLow cyclesOverstressReduce loadScatterSurface finishImprove finishProgressive damageCumulativeRedesign

Documentation and Records

Test Report Template

 STRESS TEST REPORT Report #: __________ Date: __________ Part #: __________ Material: __________ Lot #: __________ PURPOSE _________ TEST CONDITIONS Test Date: __________ Operator: __________ Temperature: __________ Humidity: __________ TEST RESULTS Test 1: Strength: ______, Modulus: ______, Elongation: ______ Test 2: Strength: ______, Modulus: ______, Elongation: ______ Test 3: Strength: ______, Modulus: ______, Elongation: ______ Average: Strength: ______, Modulus: ______, Elongation: ______ Standard Deviation: ______ ACCEPTANCE CRITERIA Specification: __________ Result: [ ] PASS [ ] FAIL FAILURE MODE OBSERVATIONS _________ REMARKS _________ APPROVALS Tested By: __________ Date: __________ Reviewed By: __________ Date: __________

Checklist

Test Planning Test objectives defined Acceptance criteria established Test methods selected Sample requirements determined Test plan approved

Test Preparation Equipment calibrated Specimens prepared Conditioning complete Procedure reviewed Personnel trained

Test Execution Tests performed per procedure Data recorded completely Failures documented Equipment verified Results calculated

Reporting Test report complete Results compared to criteria Failure analysis complete Recommendations made Report approved

The Bottom Line Stress testing validates that your parts will perform. Without it, you’re guessing—and guessing gets expensive when parts fail in the field. Test the right things. Test them the right way. Document the results. That’s how you prove your parts will perform.