Nondestructive adhesive joints

Adhesive Disadvantages. There are some limitations in using adhesives to form assemblies. The major limitation is that the adhesive joint is formed by means of surface attachment and is, therefore, sensitive to the substrate surface condition. Another limitation of adhesive bonding is the lack of a nondestructive quality control procedure. Finally, adhesive joining is still somewhat limited because most designers of assemblies are simply not familiar with the engineering characteristics of adhesives. 

The interfaces in bonded structures influence the mechanical behavior of components significantly. Therefore, an important task in nondestructive testing (NOT) is the investigation of the interaction forces in adhesive joints and the development of techniques to evaluate the bond quality. The load capacity of such joints is often limited by regions of weak bonding. As in all materials, the... 

For Class K (space-grade), the additional tests of nondestructive wire bond pull, FIND, and radiography are imposed. Screen tests assure the reliability of the entire electronic product of which the adhesive joints are a part. 

Nondestructive Testing of Adhesive Joints. Many commercially available adhesives are based on polymers. NDT of adhesive joints (180) poses challenges because of (2) the small size of possibly deleterious defects (eg, porosity), (2) the thinness of the adhesive layers, and (3) sometimes, the size, shape, and material of the adherends. Acousto-ultrasonics (181) and various types of ultrasonics (182,183) are applicable to adhesive joints. Ultrasonics can be combined with noncontact optical methods (184) and pulsed thermography (185). However, the assessment of the quality or of the quantitative strength of the adhesive bond based on the NDT data often proves difficult. 

Arnold, W., Nondestructive determination of the strength of adhesive joints. In Trends in NDE Science and Technology. Proc. 14th World Conf. on NDT, 1, 1997, pp. 93-98. 

Filimonov, S.A., Resonance method as a means for inspecting the quality of adhesive joints. Sov. J. Nondestruct. Test., 26, 853-860(1991). 

Lavrentyev, A.l. and Rokhlin, S.I., Models for ultrasonic characterization of environmental interfacial degradation in adhesive joints. Rev. Prog. Quant. Nondestruct. Evai, 13B, 1531-1538 (1994). 

Nagy, P.B. and Adler, L., Nondestructive evaluation of adhesive joints by guided waves. J. Appl. Phys., 66,4658-4663 (1989). 

Mustafa, V., Chahbaz, A., Hay, D.R., Brassard, M., Dubois, S., Imaging of disbond in adhesive joints with Lamb waves. In Nondestructive Evaluation of Materials and Composites. SPIE 2944, Scottsdale, AZ, 3-5 December, 1996. 

During the production phase, and also in service with critical structures, it is essential to use nondestructive tests to assess the quality and fitness for purpose of the product. The nondestructive test will not measure strength directly but will measure a parameter which can be correlated to strength. It is, therefore, essential that a suitable nondestructive test is chosen and that its results are correctly interpreted. Typical defects found in adhesive joints are described and an indication given of their significance. The limits and likely success of current physical nondestructive tests will be described, and future trends outlined. 

Ultrasonic spectroscopy or the measurement of the through-thickness vibration characteristics of an adhesive joint can be used for the nondestructive testing of the cohesive properties of the adhesive layer (Cawley and Hodson 1988). The modulus of the adhesive can be determined from measurement of the through-thickness natural frequencies if the thickness of the adhesive layer is known. The Fokker Bond Test Mk II (Schliekelmann 1975), which uses a spectroscopic measurement is the only commercially available instrument which claims to indicate the... 

Cawley P, Hudson MJ (1988) The NDTof adhesive joints using ultrasonic spectrscopy. In Thompson DO, Chimenti DE (eds) Review of progress in QNDE 8B. Plenum Press, New York, pp 1377-1384 Curtis GJ (1982) Nondestructive testing of adhesively-bonded structures with acoustic methods, ultrasonic testing - non conventional testing techniques. Wiley, Chichester... 

Bond Inspection. After the adhesive or sealant is cured, the joint area can be inspected to detect gross flaws or defects. This inspection procedure can be either destructive or nondestructive. The nondestructive type of tests can be visual or use advanced analytical tests. These types of bond inspections are described below. 

Hagemaier, D. J. End-Product Nondestructive Evaluation of Adhesive Bonded Metal Joint, in Adhesives and Sealants, vol. 3, Engineered Materials Handbook, ASM International, Materials Park, OH, 1990. 

There are several ways that actual joints can be tested. For quality control purposes, a proof test is commonly used, as described above. This test imposes a stress on the specimen but limits it to a point well below where any destruction of the joint can occur. This type of test only looks for serious flaws in the bonding processes such as interface contamination, air entrapment in the joint, or undercured adhesive. The prototype joints can also be tested to destruction by using similar test methods described above for standardized testing. This, however, is generally not done at great frequency because of the cost. Generally, most actual joint tests are performed to ascertain the cause of failure (i.e., forensic analysis of the failed joint) or for nondestructive determination of the adequacy of the bond. 

Each individual process step is of decisive importance for the total quality of the bonded product. In this context, it may be permitted to define an adhesive as a process material. This demand derives decisively from the fact that nondestructive testing for bonded joints is only possible to a limited extent and/or their realization involves a great metrological effort. 

S. Yang, L. Gu, R.F. Gibson, Nondestructive detection of weak joints in adhesively bonded composite structures. Compos. Struct. 51, 63-71 (2001)... 

Nondestructive testing (NDT) is far more economical, and every assembly can be tested if desired. However, there is no single nondestructive test or technique that will provide the user with a quantitative estimate of bond strength. There are several ultrasonic test methods that provide qualitative values. However, a trained eye can detect a surprising number of faulty joints by close inspection of the adhesive around the bonded area. Table 7.16 lists the characteristics of faulty joints that can be detected visually. The most difficult defects to be found by any method are those related to improper curing and surface treatments. Therefore, great care and control must be given to surface-preparation procedures and shop cleanliness. 

Figure U.l Ultrasonic inspection techniques, (a) Contact pulse echo with a search unit combining a transmitter and receivers, (b) Contact through transmission. Transmitting search unit on top and receiving search unit on the bottom, (c) Immersion pulse echo with search unit (transmitter and receiver) and part inspected under water, (d) Immersion through transmission with both search units (transmitter and receiver) and part under water, (e) Immersion reflector plate. Same as (c) but each unit requires a reflector plate below the part being inspected. (Ref Hagemaier, D.J., End Product Nondestructive Evaluation of Adhesive Bonded Metal Joints , Adhesives and Sealants, vol. 3, Engineered Materials Handbook, ASM International, Materials Park, OH, 1990)...

One problem, always present in the use of structural adhesives, is how to determine the reliability of the bonded structure. This may be at the time of manufacture, to ensure that a good joint has been produced, or during service, to monitor behavior under operating conditions. It is often impractical, or impossible, to use the types of test methods employed to determine mechanical properties and interest has centered on nondestructive test methods."" " ... 

E. Segal and J. L. Rose, Nondestructive testing of adhesive bond joints, in Research Techniques in Nondestructive Testing, Vol. IV (R. S. Sharpe, ed.), Chapter 8, Academic Press, London (1980). 

R. J. Schliekelmann, Nondestructive testing of adhesive bonded joints, Fokker-VFW Technical Report, NTIS (N81-28190), 38 pp. (April, 1982). 

Hagemaier, D.J., Adhesive-bonded joints. In Nondestructive Evaluation and Quality Control. ASM Handbook, Vol. 17, 1992, pp. 610-640. 

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