Composite and adhesively bonded structures fail in ways the surface does not show: a skin lets go of its honeycomb core, plies separate, or an adhesive layer never bonded in the first place. Bond testing finds those disbonds, delaminations and voids, whether you are verifying a production process or inspecting structure periodically in service.
- Finds
- Disbonds, delamination, voids in adhesive layers, water ingress in honeycomb
- Typical use
- Composite and honeycomb aircraft structure, bonded panels, wind blade assemblies
What bond testing finds
- Disbonds between skins and honeycomb core, or between bonded metal parts
- Delaminations between plies of a composite laminate
- Voids and porosity in adhesive layers
- Water ingress and core damage in honeycomb
- Impact damage that leaves little or no mark on the surface
The techniques
No single technique covers every structure, so the procedure chooses the one that fits the construction and the flaw:
- Tap testing: the simplest check. A disbonded or delaminated area sounds different when it is tapped, manually or with an instrumented tapper.
- Resonance: a probe coupled to the surface detects the change in resonant response over a disbond or delamination, useful on laminates and metal-to-metal bonds.
- Pitch-catch: separate sending and receiving elements on one probe detect how sound travels through the near skin, suited to honeycomb and composite skins without couplant.
- Mechanical impedance analysis (MIA): a probe measures the stiffness of the structure under it; a disbond makes the area less stiff.
Where the structure needs a volumetric examination, conventional ultrasonic and phased array techniques are used as well.
Where it’s used
- Aerospace composites: control surfaces, fairings, panels and honeycomb structure
- Defense aircraft and equipment
- Bonded metal panels and assemblies
- Wind blade assemblies and other layered structure
- Production verification that a bonding process is producing sound work
Codes and standards
- Aircraft manufacturers’ NDT manuals and structural repair manuals, which specify the technique, reference standards and acceptance for each area
- ASTM E2533, the guide to nondestructive testing of polymer matrix composites used in aerospace applications
- Customer, prime-contractor and production specifications
Bond testing is compared against reference standards made to match the structure being examined, so the drawings or the manual reference for the part are needed when the job is planned.
Who does the work
Every Midgard technician who performs bond testing holds current certification in that method, issued under a written practice. For most industrial work the written practice follows ASNT’s SNT-TC-1A; for aerospace and defense work it follows NAS410. A Responsible Level III oversees the program: approving procedures and technique sheets, examining and certifying technicians, and answering the technical questions that come up on the job.
If your contract calls for a different scheme, such as ANSI/ASNT CP-189 or NAVSEA requirements, tell us when you ask for a quote.
Training in bond testing
Midgard also trains in bond testing, along with all the other major methods. Instruction is given at Level I, II and III by certified Level III instructors, to SNT-TC-1A, CP-189, NAS410, NAVSEA or whichever code you certify to, with general, specific and practical examinations at your facility or ours. It suits new hires working toward first certification as well as technicians due for re-examination. See training and certification, or ask about a class.
Common questions
What is the difference between a disbond and a delamination?
A disbond is a failure of the adhesive joint between two parts, such as a skin separating from its honeycomb core. A delamination is a separation between plies inside a composite laminate. Both weaken the structure and neither may show on the surface.
Is tap testing enough?
Sometimes. Tap testing is quick and finds larger disbonds near the surface of thin skins, and many manuals allow it for those areas. Smaller or deeper flaws, thicker laminates and critical areas need an instrumented technique such as resonance, pitch-catch or mechanical impedance analysis.
Does bond testing need couplant?
It depends on the technique. Resonance testing uses a couplant, while pitch-catch, mechanical impedance analysis and tap testing are dry, which suits honeycomb and parts that must not get wet.
Why are reference standards needed?
Bond test instruments compare the structure against a known good area and a known flaw of the same construction. The reference standard sets the instrument up so that a real disbond or delamination in your part is recognized.
Tell us the structure and its construction, the manual or specification, and whether reference standards are available. Call 1-844-MIDGARD, email contact@midgardscientific.com, or use the contact page.