A bonded assembly fails in the field or during validation testing. The epoxy failed — but why? Was the material defective, the cure incomplete, the surface preparation inadequate, the joint design flawed, or the in-service environment more severe than predicted? Root-cause analysis combines fractography, property testing, material analysis, and process review to identify the precise mechanism, because the corrective action depends entirely on the root cause — treating surface contamination as a material defect leads to wasted expense and repeated failures.
Non-Destructive Examination Before Opening the Assembly
Visual inspection under 10–50× magnification is the first step. Interfacial failure — the adhesive pulling cleanly from the substrate — suggests weak bonding from contamination, inadequate wetting, or wrong primer application. Cohesive failure — tearing with material left on both adherends — indicates adequate adhesive strength but stress that exceeded material capability. Smooth, glossy surfaces point to older failures from oxidation; rough, granular surfaces point to recent, brittle failure. Cracks typically initiate at stress concentrations — sharp corners, voids, surface defects, or thin edge bondlines.
Non-destructive testing adds detail without destroying evidence: thermography reveals delamination as hot spots; ultrasonic A-scan or C-scan testing reveals internal voids by sound reflection; a tap test distinguishes bonded regions (which ring) from delaminated ones (which sound dull); and X-ray or CT reveals internal porosity and crack extent.
Fractography and Material Testing
Once non-destructive examination is complete, dissect the assembly. Macro-fractography at 10–50× reveals river marks radiating from the failure origin, a smooth mirror region indicating slow initial crack growth, rough hackle marks where the crack accelerated, and bondline thickness irregularities suggesting poor clamping or excess squeeze-out.
Scanning electron microscopy at 100–5,000× resolves the failure mode at the micro scale: interfacial fracture indicates weak bonding from contamination or inadequate wetting; fracture within the epoxy itself indicates adequate adhesion but inadequate polymer toughness; and the presence or absence of plastic deformation distinguishes ductile failure (moderate stress) from brittle failure (low toughness or very high stress). Elemental analysis (XRF or XPS) on the fracture face adds a final layer: high substrate-element signal (Si, Al, Fe) on the epoxy side confirms interfacial failure, oxidation products indicate environmental attack, and chloride or sodium indicates salt contamination from marine exposure or inadequate cleaning.
Chemical and Thermal Analysis
Gel-permeation chromatography measures the cured polymer’s molecular weight distribution: low molecular weight indicates under-cure, unusually high molecular weight indicates over-cure and brittleness, and a reduction relative to baseline indicates oxidative or hydrolytic environmental attack. Thermogravimetric analysis tracks mass loss with temperature — a peak near 100–150°C typically indicates absorbed moisture, volatile loss above 2% indicates inadequate cure or trapped solvent, and the onset of decomposition (typically 300–400°C for epoxy) benchmarks degradation resistance. Differential scanning calorimetry measures Tg directly against the specification baseline and detects residual cure enthalpy, which should be near zero in a properly cured sample.
Mechanical Property Testing of Failed Components
Comparing failed-sample properties to baseline, properly cured material isolates the failure mode. Lap shear strength per ASTM D1002 above 70% of baseline points away from the adhesive itself and toward design, stress, or environmental factors; below 50% points to an adhesive defect such as under-cure or contamination. Peel strength per ASTM D1876 below 70% of baseline suggests brittle or interfacial weakness. Tensile testing of adhesive removed from the failed joint — checking strength, elongation-to-break, and modulus against baseline — separates brittleness (lower strength and elongation, from under-cure or oxidation) from plasticization (lower strength with maintained elongation, from moisture absorption or thermal softening).
Environmental Degradation Testing
When environmental attack is suspected, thermal aging (500–1,000 hours at 80–90% of Tg) and humidity conditioning (95% RH at 140°F for 7 days, per ASTM D2247) followed by property retesting quantify the damage; combined thermal cycling with salt-spray exposure per ASTM B117 screens for the worst-case field environment. A 10–20% property loss from baseline suggests degradation within normal service-life expectations; 30–50% loss calls for a material or design revision; and losses above 50% indicate the material is unsuitable for the application environment.
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Common Root-Cause Findings and Corrective Actions
Interfacial failure shows a clean substrate with little adhesive residue and high substrate-element signal on the epoxy fracture face. Usual causes: surface contamination, inadequate roughness, insufficient primer, or a substrate that oxidized again after sitting too long between prep and bonding. Fix: grit-blast to 40–60 micron roughness with immediate silane primer, keep the prep-to-bond gap under four hours, and validate with contact-angle measurement or a tape adhesion check.
Cohesive failure with low strength shows rough, granular surfaces on both adherends and residual cure enthalpy on DSC. Usual causes: an incorrect resin-to-hardener ratio, insufficient cure temperature or dwell time, or an oven that reached setpoint at the surface before the part interior did. Fix: verify mixing by weight per batch, monitor oven temperature with thermocouples on representative parts, and confirm the cure schedule accounts for bondline thickness.
Brittle failure shows a rough, jagged fracture with no plastic deformation and under 1% elongation-to-break despite high strength. Causes include over-cure, oxidative degradation, moisture absorption followed by drying, or simply the wrong (rigid rather than toughened) formulation. Fix: confirm cure temperature and dwell time against spec rather than exceeding it, switch to a toughened formulation, add environmental protection against oxidation and moisture, and design for stress below 50% of failure stress.
Void-initiated failure shows visible voids at the fracture origin and internal porosity on CT. Typical causes: air entrainment during high-speed mixing, volatiles boiling out under too fast a heating ramp, inadequate de-gassing, or clamp pressure too low to squeeze out trapped air. Fix: mix at 50–100 rpm rather than 500+, ramp at 2–5°C/minute, let mixed adhesive stand 5–10 minutes before application, and confirm clamp pressure (typically 50–150 psi).
A complete investigation — fractographic examination, non-destructive testing, mechanical property comparison, and chemical/thermal analysis — typically runs three to six weeks depending on lab capacity. Documenting photographs, fractography images, and test data alongside the root-cause conclusion supports design changes, requalification, or process improvements that prevent recurrence. The same diagnostic sequence applies whether the joint in question uses ultra-high-temperature inorganic adhesive systems or a conventional structural formulation, such as those covered in what is the strongest structural epoxy for industrial repairs; and where the corroded surfaces point to galvanic attack rather than a cure or contamination defect, see electrochemical corrosion in bonded joints.
Each failed assembly is an opportunity to improve design, material selection, or manufacturing process control. Systematic root-cause analysis is what prevents the same failure mode from recurring in the next build or program.
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