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.
Understanding Stress Testing
Purpose of Stress Testing
| Purpose | Application | Verify design |
|---|---|---|
| Prove part will perform | Validate material | Confirm material selection |
| Failure prevention | Find failure modes | Safety assurance |
| Critical applications | Customer requirements | Contractual obligation |
Types of Stress Stress
| Type | What It Tests |
|---|---|
| Typical Method | Tensile |
| Axial strength | Pull test |
| Flexural | Bending strength |
| 3-point bend | Impact |
| Shock resistance | Izod/ |
| Charpy | Creep |
| Long-term loading | Sustained load |
| Fatigue | Cyclic loading |
| Repeated stress | Environmental |
| Real-world conditions | Environmental chamber |
Mechanical Testing Methods
Tensile Testing
| StandardASTM D638ISO 527 | Specimen type | Type I-V1A, 1BTest speed | 0.2-20 in/min |
|---|---|---|---|
| 1-500 mm/min | Results | Strength, elongation, modulus | Strength, elongation, modulus |
Flexural Testing
| StandardASTM D790ISO 178 | Configuration |
|---|---|
| 3-point, 4-point | 3-point |
| Span-to-depth | 16:1 typical |
| 16:1 typical | Results |
| Strength, modulus, strain | Strength, modulus, strain |
Impact Testing
| Test | Standard | Use |
|---|---|---|
| IzodASTM D256 | Notched impact | CharpyASTM D6110 |
| Notched/unnotched | Drop weightASTM D3763 | High-rate impact |
Fatigue Testing
| StandardASTM D3479ISO 13003 | Type | Tension-tension fatigue |
|---|---|---|
| Fatigue | Frequency | 1-30 Hz |
| 1-100 Hz | ResultsS-N curve | Fatigue life |
Testing Equipment
Universal Testing Machine
| Specification | Typical Range | Application |
|---|---|---|
| Load capacity | 0.1-100 kNTensile, flexural | Speed range |
| 0.001-20 in/min | Multiple methods | Accuracy±0.5% of reading |
| Precision testing | Extensometer | Required |
Impact Testers
| Type | Capacity | Application |
|---|---|---|
| Izod | 0.5-5 ft-lb | Small specimens |
| Charpy | 1-50 ft-lb | Larger specimens |
| Drop tower | 0.5-100 ft-lb | Product testing |
Environmental Chambers
| Capability | Range | Use |
|---|---|---|
| Temperature-40°C to + | 200°CThermal cycling | Humidity |
| 10-95% RH | Humidity exposure | Combined |
Sample Preparation
Specimen Requirements
| Method | Specimen Type | Preparation |
|---|---|---|
| Injection molded | Directly molded | Per ASTM/ISO |
| Cut from part | Per standard | Avoid damage |
| Conditioning | Per standard | 24 hrs @ 23°C, 50% RH |
Conditioning Requirements
| Condition | Temperature | Humidity | Time |
|---|---|---|---|
| Standard | 73±4°F50±10% RH40 hours minimum | Elevated | Per spec |
| Per spec | Per spec | Dry as molded | 23°C<0.1% |
| Oven dry | Wet conditioned | 73°FImmersion | 48 hours |
Number of Samples Test
| Type | Minimum Samples | Recommended |
|---|---|---|
| Screening | 3 specimens | 5 specimens |
| Qualification | 5 specimens | 10 specimens |
| Production verification | Per control plan | Per spec |
Test Procedures
Tensile Test Procedure
| Step | Action | 1 | Condition specimens |
|---|---|---|---|
| 2 | Measure dimensions (3 points, average) | 3 | Mount in grips (parallel alignment) |
| 4 | Attach extensometer | 5 | Zero load and displacement |
| 6 | Start test at specified speed | 7 | Record load vs. displacement |
| 8 | Note yield, break, modulus | 9 | Record failure mode |
Impact Test Procedure
| Step | Action | 1 |
|---|---|---|
| Condition specimens | 2 | Notch specimens (if required) |
| 3 | Measure notch dimensions | 4 |
| Position in striker | 5 | Verify pendulum height |
| 6 | Release pendulum | 7 |
| Record energy absorbed | 8 | Inspect fracture |
Flexural Test Procedure
| Step | Action | 1 |
|---|---|---|
| Condition specimens | 2 | Measure dimensions (3 points) |
| 3 | Position on supports (span per standard) | 4 |
| Zero load and deflection | 5 | Apply crosshead at specified rate |
| 6 | Record load vs. deflection | 7 |
| Calculate stress at 5% strain | 8 | Record failure mode |
Acceptance Criteria
Typical Property Requirements
| Property | Typical Specification | Notes |
|---|---|---|
| Tensile strength>6,000 psi (ABS) | Material dependent | Flexural modulus>300,000 psi (ABS) |
| Material dependent | Impact strength>3 ft-lb/in (ABS) | Material dependent |
Industry-Specific Requirements
| Industry | Standard | Key Requirements |
|---|---|---|
| AutomotiveGM, Ford, Chrysler specs | Specific test methods | AerospaceASTM, SAE specs |
| Stringent requirements | MedicalISO 10993 | Biocompatibility |
Test Report Requirements
| Information | Details | Test date___Test operator___Material identification |
|---|---|---|
| Grade, lot | Specimen identification | Part, location |
| Test method | Standard used | EquipmentID, calibration |
| Results | All measured values | Calculated values |
| Strength, modulus | Observations | Failure modes |
Design Validation Testing
Test Plan Development
| Element | Contents |
|---|---|
| Purpose | What is being validated |
| Scope | What will be tested |
| Acceptance criteria | Pass/fail criteria |
| Test methods | Specific procedures |
| Sample requirements | Quantity, conditioning |
| Reporting | Results documentation |
Accelerated Testing
| Method | Purpose | Application |
|---|---|---|
| Elevated temperature | Simulate long-term | Creep, aging |
| Humidity exposure | Moisture effects | Environmental stabilityUV exposure |
| Weathering | Outdoor applications | Thermal cycling |
| Fatigue | Temperature extremes | Salt spray |
Failure Analysis Analysis
| Type | Use |
|---|---|
| Visual inspection | Fracture origin |
| Microscopy | Failure initiation |
| Fractography | Failure mechanism |
| Chemical analysis | Material verification |
Common Test Failures
Tensile Test Failures
| Symptom | Likely Cause | Action |
|---|---|---|
| Low strength | Material degradation | Check material |
| Low modulus | Voids, porosity | Check processing |
| Brittle fracture | Low temperature | Condition properly |
| Grip failure | Specimen preparation | Improve grip technique |
Impact Test Failures
| Symptom | Likely Cause | Action |
|---|---|---|
| Low energy | Notch sensitivity | Check notch quality |
| Variable results | Specimen inconsistency | Improve consistency |
| Brittle fracture | Low temperature | Condition properly |
| High scatter | Material variation | Check material |
Fatigue Failures
| Symptom | Likely Cause | Action |
|---|---|---|
| Early failure | Stress concentration | Redesign |
| Low cycles | Overstress | Reduce load |
| Scatter | Surface finish | Improve finish |
| Progressive damage | Cumulative | Redesign |
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.