What Is Delamination and Adhesive Failure? Mechanisms and Detection
What Is Delamination?
Delamination is the separation of bonded layers within a laminated composite, coated substrate, or adhesively bonded assembly — occurring along the interface between plies, between coating and substrate, or within the adhesive bond line. It is one of the most prevalent and damaging failure modes in composite structures, coated products, and bonded assemblies across the aerospace, automotive, electronics, and glass industries.
What Is Adhesive Failure?
Adhesive failure is a specific type of bond failure in which separation occurs at the interface between the adhesive and one of the adherend surfaces — rather than within the adhesive bulk (cohesive failure) or within the adherend material (substrate failure). Adhesive failure at the adhesive-adherend interface indicates insufficient wetting, inadequate surface preparation, or fundamental incompatibility between the adhesive and substrate chemistry.
Delamination in Composite Materials
How Delamination Develops
In fibre-reinforced composite laminates, delamination initiates and propagates along the resin-rich layer between adjacent plies when:
- Interlaminar tensile stress exceeds the through-thickness tensile strength (from bending loads at free edges, holes, or ply drops)
- Interlaminar shear stress exceeds the interlaminar shear strength (ILSS) of the resin-fibre interface
- Impact damage causes internal delamination invisible from the surface (barely visible impact damage, BVID) — a primary structural concern in carbon fibre aerospace structures
- Fatigue cycling progressively grows existing delaminations under cyclic interlaminar loading
Detection of Composite Delamination
Scanning Acoustic Microscopy (SAM) — described in Blog 65 — is the primary non-destructive method for imaging delaminations in composite panels. Ultrasonic C-scan imaging maps delamination area and depth. Thermographic inspection (active thermography, lock-in thermography) detects delaminations from anomalies in heat diffusion through the laminate. For in-service inspection, phased array UT and pulse-echo UT detect delaminations from their normal incidence back-wall echo reduction.
Adhesive Failure in Glass and Coated Products
Delamination of Glass Laminates
Laminated safety glass consists of two or more glass sheets bonded with PVB (polyvinyl butyral) or EVA interlayers. Delamination between the glass and interlayer occurs from:
- Moisture ingress: Water penetrates the edge seal and plasticises the PVB, reducing its adhesion to the glass
- Thermal cycling: CTE mismatch stresses accumulate at the glass-PVB interface over repeated temperature cycles
- Incompatible surface chemistry: Silane coupling agent deficiency on the glass surface reduces PVB adhesion
- Impact damage: Severe impact separates the glass from the interlayer at the impact zone
Edge delamination in laminated glass is a significant aesthetic and structural defect — detected by visual inspection and characterised by microscopy and FTIR analysis of the delaminated interface.
Coating Delamination
Coating delamination from glass, metal, or polymer substrates results from:
- Hydrophobic substrate contamination (silicone, oil) — as described in Blog 62 (paint craters)
- Osmotic pressure from trapped moisture at the coating-substrate interface
- CTE mismatch stress from thermal cycling
- UV degradation of the adhesion-promoting primer layer
Testing and Characterisation of Delamination and Adhesive Failure
Peel Testing (ASTM D903, ASTM D1876)
Quantifies the force per unit width to separate bonded layers — providing adhesion strength data for coating and adhesive bond quality assurance.
Cross-Cut Adhesion Test (ASTM D3359)
Rapid qualitative screening for coating adhesion to substrate — used for quality control of painted and coated surfaces.
Pull-Off Adhesion Test (ASTM D4541)
Measures the tensile strength of the adhesive bond to the substrate — applicable to coatings on glass, metal, and concrete.
Scanning Acoustic Microscopy (SAM)
Non-destructive internal imaging of delaminations in composites, laminates, and bonded assemblies — described in Blog 65.
FTIR-ATR Analysis of Delaminated Surfaces
Chemical identification of delamination interface chemistry — detecting contamination, degradation products, or adhesion primer deficiency at the failure interface.
Why Choose Infinita Lab for Delamination and Adhesive Failure Analysis?
Infinita Lab provides delamination detection (SAM, UT, thermography), adhesion testing (peel, pull-off, cross-cut), and failure interface chemistry analysis (FTIR, SEM-EDS) through our nationwide accredited materials testing and failure analysis laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is the difference between delamination and disbond in composite structures? Delamination refers to separation between plies within the composite laminate itself — at the resin-rich interlaminar layer. Disbond refers to separation between the composite face sheet and a bonded core (honeycomb, foam) or between co-bonded substructures. Both involve interface failure but at different locations within the sandwich or bonded assembly.
What is BVID (Barely Visible Impact Damage) and why is it critical for composite aircraft structures? BVID is impact damage that causes internal delamination in CFRP structures without visible surface marks — the damage is invisible to visual inspection but significantly reduces the residual compression-after-impact (CAI) strength of the laminate. Aircraft structures are designed to maintain structural integrity despite the presence of BVID — making internal delamination detection by SAM or UT critical for airworthiness maintenance.
Why does moisture cause delamination in laminated glass? Moisture ingress from the glass edge plasticises the PVB interlayer — reducing its glass transition temperature and adhesion to the glass surface. The plasticised region loses cohesive strength and can separate from the glass surface under thermal stress. Proper edge sealing and selection of PVB with moisture resistance additives prevents this degradation mechanism.
How does adhesive failure differ from cohesive failure in practical terms for quality assessment? Adhesive failure — separation at the adhesive-substrate interface — leaves one substrate clean and the other substrate surface coated with adhesive. It indicates a bonding problem (contamination, improper surface preparation, or adhesive-substrate incompatibility) and is considered a bond quality deficiency. Cohesive failure splits the adhesive layer — both surfaces show adhesive residue — confirming that the adhesive-substrate bond exceeds the adhesive's internal strength. Cohesive failure is preferred for most bonding applications.
What NDE method is most sensitive for detecting delamination in CFRP aircraft skin panels? Phased array ultrasonic testing (PAUT) combined with automated scanning provides the best combination of sensitivity, coverage speed, and defect sizing capability for production inspection of CFRP aircraft skin panels. For thin skins (<3 mm), high-frequency PAUT (5–10 MHz) detects delaminations as small as 5 mm diameter. Scanning acoustic microscopy at 15–50 MHz provides higher resolution for smaller panels and bonded assemblies.
