A bonded assembly fails in the field or during validation testing. The epoxy failed — but why? Was the material defective, the cure process incomplete, the surface preparation inadequate, the joint design flawed, or the in-service environment more severe than predicted? Without systematic troubleshooting, failures appear random and inexplicable. Root-cause analysis requires combining fractography (microscopic examination of failure surfaces), property testing, material analysis, and process review to identify the precise failure mechanism. This diagnosis is critical because the corrective action depends entirely on the root cause — treating surface contamination as a material defect leads to wasted expense and repeated failures.

Pre-Failure Failure Analysis: Non-Destructive Examination

Before opening a failed assembly, examine it carefully for clues:

Visual inspection under magnification (10–50×):
– Interfacial failure (adhesive pulled cleanly from substrate): Suggests weak bonding (surface contamination, inadequate wetting, or wrong primer application)
– Cohesive failure (adhesive tore, leaving some material on both adherends): Indicates good adhesive strength but design or environmental stress exceeded material capability
– Fracture surface characteristics: Smooth/glossy surfaces indicate old failure (oxidation or environmental degradation); rough/granular surfaces indicate recent, brittle failure (poor cure or high stress)
– Crack initiation site: Failure typically initiates at stress concentrations — sharp corners, voids, surface defects, or edge regions with thin bondlines

Non-destructive testing:
– Thermography: Thermal imaging can reveal internal delamination (regions of poor contact show different heat transfer and appear as hot spots)
– Ultrasonic inspection: A-scan or C-scan ultrasonic testing reveals internal voids, delamination, or areas of poor bonding by measuring sound reflection
– Tap testing: Tapping a bondline with a hammer reveals delamination — bonded regions ring; delaminated regions sound dull
– X-ray or computed tomography (CT): Reveals internal porosity, void size/distribution, and internal crack extent

Destructive Analysis: Fractography and Material Testing

Once non-destructive examination is complete, dissect the failed assembly to perform materials analysis:

Macro-fractography (visual examination of fracture surface):

Parts are carefully broken along the failure plane and examined at 10–50× magnification. The fracture surface reveals:

• River marks: Patterns radiating from the failure origin, pointing toward the initiation site
• Mirror region: Smooth, reflective area around the initiation site (indicates slow initial crack growth under low stress)
• Hackle marks: Rough, jagged regions where crack accelerated after initiation (indicates higher stress or brittle failure)
• Voids or inclusions: Visible defects that concentrated stress and initiated failure
• Adhesive thickness and uniformity: Visual assessment of bondline consistency; uneven thickness suggests poor clamping or excess squeeze-out during bonding

Micro-fractography (scanning electron microscopy, SEM):

High-magnification (100–5,000×) imaging reveals micro-scale failure mechanisms:

• Interfacial fracture (failure at substrate-adhesive boundary): Indicates weak adhesive-substrate bonding (surface contamination, inadequate wetting, or no mechanical interlocking)
• Adhesive fracture (failure within the epoxy polymer): Indicates adequate surface adhesion but inadequate epoxy strength or toughness
• Cohesive fracture within the substrate: Rare, but indicates adhesive was so strong it pulled material out of the substrate itself
• Presence of voids, bubbles, or inclusions: Reveals manufacturing defects (air entrapment, moisture, or contamination)
• Evidence of plastic deformation vs. brittle fracture: Plastic deformation (ductile fracture) indicates adequate toughness and moderate stress; brittle fracture indicates low toughness or very high stress

Elemental analysis (X-ray fluorescence, X-ray photoelectron spectroscopy):

Surface analysis of fracture faces reveals:

• Presence of substrate elements (Si, Al, Fe): If high substrate elemental concentration is found on the epoxy fracture surface, this indicates interfacial failure — the adhesive was weak at the interface
• Oxidation products (O₂, CO₂): Evidence of oxidative degradation or moisture absorption
• Contamination elements (Cl, Na): Evidence of salt contamination (marine exposure or inadequate surface cleaning)

Chemical Analysis of the Adhesive

Retrieve adhesive samples (from the failed assembly, archive production batches, and uncured material) for chemical analysis:

Gel-permeation chromatography (GPC):
Measures molecular weight distribution of the cured epoxy polymer. Changes in molecular weight indicate:
– Under-cure: Low molecular weight indicates incomplete cross-linking
– Over-cure: Extremely high molecular weight can indicate cross-linking so dense that the polymer becomes brittle
– Environmental degradation: Molecular weight reduction from oxidation or hydrolysis indicates environmental attack

Thermogravimetric analysis (TGA):
Measures material loss as temperature increases, revealing:
– Moisture content: Peak weight loss at 100–150°C is typically water; high moisture indicates absorption or inadequate drying during cure
– Volatile content: Loss >2% volatiles indicates inadequate cure or trapped solvents
– Decomposition temperature: Onset of polymer decomposition (typically 300–400°C for epoxy) indicates Tg and degradation resistance

Differential scanning calorimetry (DSC):
Measures thermal transitions:
– Glass transition temperature (Tg): Direct measurement compared to specification baseline; deviation suggests cure problems or environmental degradation
– Residual cure enthalpy: Remaining exothermic heat indicates incomplete cure — should be near-zero for properly cured samples
– Moisture absorption: Endothermic transition around 100–150°C indicates absorbed water

Mechanical Property Testing of Failed Components

Compare properties of failed samples to baseline (new, properly cured material):

Lap shear strength testing (ASTM D1002):
– Failed assembly: If coupons bonded to the same adherend material show >70% of baseline shear strength, the adhesive itself is adequate; failure suggests design, stress, or environmental factors
– If shear strength <50% of baseline, investigate adhesive defects (under-cure, contamination, environmental degradation)

Peel strength testing (ASTM D1876):
– Indicates adhesive toughness and interfacial strength
– <70% of baseline suggests brittle failure or interfacial weakness

Tensile testing of the adhesive itself:
– Remove adhesive samples from the failed assembly (carefully, without contaminating the sample)
– Test tensile properties (tensile strength, elongation-to-break, modulus)
– Compare to baseline
– Lower tensile strength + lower elongation suggests brittleness (under-cure or oxidative degradation)
– Lower strength with maintained elongation suggests plasticization (moisture absorption or thermal softening)

Property Retention Testing for Environmental Degradation

If environmental degradation is suspected:

Thermal aging: Expose cure samples to elevated temperature (80–90% of Tg, typically 300–350°F) in air for 500–1,000 hours, then test properties

Moisture conditioning: Expose samples to 95% relative humidity at 140°F for 7 days per ASTM D1141, then test properties

Combined environmental exposure: Thermal cycling (ASTM D1141) combined with humidity and salt spray (ASTM B117) for comprehensive environmental screening

Property loss correlation:
– 10–20% property loss from baseline: Environmental degradation is occurring but not severe; component may have exceeded its service life
– 30–50% loss: Environmental degradation is significant; material selection or design needs revision
– >50% loss: Severe environmental attack; material is unsuitable for the application environment or service life

Common Root-Cause Findings and Corrective Actions

1. Interfacial failure (adhesive-substrate interface):

Root cause indicators:
– Fractography shows clean substrate surface with little or no adhesive residue
– SEM reveals no mechanical interlocking of adhesive into substrate asperities
– Elemental analysis shows high substrate elements (Si, Al, Fe) on the epoxy fracture surface

Possible causes:
– Surface contamination (oils, oxides, dust) preventing wetting and adhesion
– Inadequate surface preparation (smooth surface, poor roughness)
– Inadequate adhesion promoter (silane primer) application
– Substrate stored too long after preparation (oxidation reform

ed)

Corrective actions:
– Implement grit-blasting to 40–60 micron roughness with immediate silane primer application
– Reduce time between surface preparation and bonding (<4 hours)
– Validate surface quality with contact angle measurement or adhesive tape test

2. Cohesive failure with low strength (weak adhesive):

Root cause indicators:
– Fractography shows rough, granular failure surfaces on both adherends
– DSC shows residual cure enthalpy (incomplete cross-linking)
– Thermal aging and moisture conditioning show high property loss

Possible causes:
– Incorrect mixing ratio (wrong resin-to-hardener ratio)
– Insufficient cure temperature or dwell time
– Uncured or partially cured epoxy in the bondline
– Under-cure due to insufficient oven temperature (oven setpoint reached but part interior didn’t reach target)

Corrective actions:
– Verify mixing by weight on a calibrated scale, document for each batch
– Implement oven temperature monitoring with thermocouples on representative parts
– Verify cure schedule is adequate for bondline thickness (thicker bondlines require longer dwell times)
– Establish cure process validation with property testing on samples

3. Brittle failure (low toughness):

Root cause indicators:
– Fractography shows very rough, jagged fracture surface with no plastic deformation
– SEM reveals no ductile failure characteristics
– Tensile test shows high strength but very low elongation-to-break (<1%)

Possible causes:
– Over-cured adhesive (excessive cure temperature or dwell time) creating cross-link density so high the polymer becomes brittle
– Environmental oxidative degradation at elevated temperature
– Moisture absorption followed by drying, leaving the epoxy in a brittle state
– Wrong material selection (brittle formulation used instead of toughened formulation)

Corrective actions:
– Review cure schedule; ensure temperature and dwell time match material specification (not exceeded)
– Switch to toughened epoxy formulation with higher elongation-to-break and fracture toughness
– Implement environmental protection (coatings, encapsulation) to limit oxidation and moisture absorption
– Increase design margin — design for stress <50% of adhesive failure stress to accommodate property variation

4. Void-initiated failure:

Root cause indicators:
– Fractography shows visible voids at fracture initiation site
– SEM shows void size and density
– Radiography or CT scan shows internal porosity distribution

Possible causes:
– Air entrainment during mixing (high-speed mixing, vibration)
– Volatiles (solvents, water) boiling during cure, especially if heating ramp is too fast
– Inadequate de-gassing of mixed adhesive before application
– Improper clamp pressure allowing air to be trapped in the bondline

Corrective actions:
– Slow mixing speed (50–100 rpm vs. 500+ rpm) to minimize air introduction
– Implement slow heating ramps (2–5°C/minute) to allow volatiles to escape before gelling
– Allow mixed adhesive to sit for 5–10 minutes post-mixing to allow large bubbles to rise and pop
– Verify clamp pressure is adequate (50–150 psi typically) to squeeze out excess adhesive and air

Timeline and Documentation

Complete root-cause analysis typically requires:

• Visual/fractographic examination: 1–2 days
• Non-destructive testing: 2–5 days depending on equipment availability
• Mechanical property testing: 1–2 weeks (testing, data reduction, analysis)
• Chemical analysis and thermal testing: 2–4 weeks depending on lab capacity
• Report writing and corrective action planning: 1 week

Total timeline: 3–6 weeks for comprehensive analysis

Documentation: Maintain photographs, fractography images, test data, and a detailed report with root-cause determination and recommended corrective actions. This documentation supports design changes, material qualification modifications, or process improvements to prevent recurrence.

Learning from Field Failures

Each failed assembly is an opportunity to improve design, material selection, or manufacturing processes. Systematic root-cause analysis prevents the same failure mode from occurring in future products or programs.

Email Us to perform fractography, root-cause analysis, and materials investigation for failed bonded assemblies, including thermographic inspection, elemental analysis, and corrective action planning.

Visit www.incurelab.com for more information.