Non-destructive testing of adhesively bonded

Adams, R.D. and B.W. Drinkwater, Non-destructive testing of adhesively-bonded joints. International Journal of Materials and Product Technology, 14(5/6), 1999. 

Fig. 1. Comparison of destructive shear strength tests on laminates with the predicted strengths using a Fokker bond tester. A series of laminates is represented where the adherend thickness t varies as follows ( ), t = 0.6 mm (O), t = 0.8 mm (x), f = 1.0 mm (-h), f = 1.2 mm (A), t = 1.5 mm. To construct this standard plot, 120 specimens of Dural 2024-T3 were used 95% of all results are within the range 0.36 kg mm . (From R J Schliekelmann, Non-destructive testing of adhesively bonded joints, in Adhesion, Fundamentals and Practice, McClaren, London, 1966)...

Many widely used Tests of adhesion can be applied to the rubber to metal bonds Blister test. Fracture-mechanics test specimens. Non-destructive testing of adhesively-bonded structures. Peel tests. Shear tests. Tensile tests and Wedge test. This particular article is concerned with those aspects that are of practical concern in the rubber-processing industry. 

Non-destructive testing of adhesively-bonded structures G J CURTIS Acoustic wave techniques, resonance and pulse-echo testers... 

Schliekelmann RJ. Non-destructive testing of adhesive bonded metal-to-metal joints. Nondestructive Test 1972 5(2) 79—86. 

Tests to measure the bond which can be obtained with the concrete of the structure to be strengthened are best carried out on the structure itself. A possibility is to utilise a pull-off test as developed for the non-destructive testing of concrete(21). A circular steel probe is bonded to the concrete surface and specially designed portable apparatus is then used to pull off the probe, along with a bonded mass of concrete, by applying a direct tensile force. Any defects in bond would be revealed by the occurrence of failures at the adhesive-concrete interface. 

Figure 1.16 In the British Aerospace 146 one of the largest components is the wing-skin assembly which is manufactured using aluminium alloy. The stiffening stringers are bonded onto the skin using a modified phenolic adhesive. The two gentlemen are conducting non-destructive tests on the bonded joints.

The basic aim of any non-destructive test of an adhesive joint must be the direct correlation of some parameter measurable by the test with the failure property to be measured or predicted, without impairing the effectiveness of the bonded part for its intended application [145]. 

The cohesive strength of the adhesive is really the only parameter which can be estimated with any degree of confidence and it is this which features in most non-destructive tests of bonded joints. 

Also emphasised in this book are the various means of destructive and non-destructive testing of bonded joints, with special mention of structures fabricated by adhesive bonding. 

The best approach to achieving good performance in bonded joints is to select the proper adhesive, design the joint properly for the specific application, and maintain rigid process control. One disadvantage in the use of adhesives is the absence of good non-destructive tests to determine the strength of adhesive-bonded joints in a wide variety of ord applications... 

On that basis, the book intends to bridge current issues, aspects and interests from fundamental research to technical apphcations. In seven chapters, the reader will find an arrangement of latest results on fundamental aspects of adhesion, on adhesion in biology, on chemistry for adhesive formulation, on surface chemistry and pretreatment of adherends, on mechanical issues, non-destructive testing and durability of adhesive joints, and on advanced technical applications of adhesive joints. Prominent scientists review the current state of knowledge about the role of chemical bonds in adhesion, about new resins and nanocomposites for adhesives, and about the role of macromolecular architecture for the properties of hot melt and pressure sensitive adhesives. Thus, insight into detailed results and broader overviews as well can be gained from the book. 

ADHESION ASSOCIATES Raymond F. Wegman, 34 Mt. Arlington Road, Ledgewood NJ 07852, (201) 584-5232. Consultant in areas of adhesion, adhesives, surface treatments for adhesive bonding and non-destructive testing. 

Appropriate test methods for the control of fabrication procedures and non-destructive testing (NDT) are basic requirements for the formation of structural adhesive joints. Such methods should be based upon relatively easily measurable parameters that have a close identity with the properties of the bonded assembly that need to be controlled. However the quality of bonded joints depends upon many factors, requiring a range of very different procedures. 

After the adhesive is cured, the joint area can be inspected to detect gross flaws or defects. This inspection procedure can be either destructive or non-destructive, as discussed in Section 12.1. Destructive testing generally involves placing samples of the production run in simulated or accelerated service and determining if it has properties similar to a specimen that is known to have a good bond and adequate service performance. The causes and remedies for a number of faults revealed by such mechanical tests are described in Table 12.1. Most of the destructive (mechanical) tests that can be carried out on adhesive bonds are listed in Section 12.2.1.1 (ASTM methods for mechanical properties). Non-destructive tests are far more economical, and every assembly can be tested, if desirable. ... 

Particular attention needs to be paid to establishing an effective system of quality assurance for adhesive connections. Non-destructive test methods based on ultrasound, X-rays or the measurement of electrical or thermal conductivity are available but only of limited value in practice. Specific data on adhesion cannot always be obtained by these methods. This section examines the issue of quality assurance from a practical point of view. Many years of experience have shown that only a quality assurance system that takes into account the specific features of adhesive bonding technology can produce consistently satisfactory results over an extended production period. The information outlined here should be viewed as a general checklist to be adapted to the specific requirements of each manufacturing environment. 

The most commonly used methods for evaluation of adhesive bonds are mechanical tests such as tensile shear and peel tests that determine the weakest link in a bonded assembly. Although these tests are useful in the development and quality control of adhesives, they are destructive and cannot offer failure prediction for in-service components. Ultrasonic inspection is the most commonly used non-destructive test method and can accurately assess de bonding in single adhesive bonds, providing that the sensor is perpendicular to the defect plane. However, ultrasound has some limitations in... 

Figure 6.33 An example of holographic non-destructive testing. The test specimen was an adhesively bonded carbon fibre laminate honeycomb structure and a vacuum was applied to cause increased displacement of the face sheet when a defect was present. Debonds were present at the interface between the face sheet and honeycomb core (below the disturbance in the regular pattern that can be seen second from top and bottom) and crushed honeycomb core was also present (top and third from top) [149].

By far the majority of non-destructive testing (NDT) techniques associated with adhesive bonds take place after the joint has been made, and can usually be carried out at the manufacturing point or during service. Most techniques are void detectors and, although it is claimed that the cohesive strength of the adhesive is being assessed, this is unlikely to be the case. 

Adhesion Society. He is active in organising conferences and publishing and was a founder member of EURADH and WCARP, the main European and World series on Adhesion. He has been Joint Editor-in-Chief of the International Journal of Adhesion and Adhesives since 1999. His main research area is on adhesively bonded joints and he pioneered the application of finite element analysis for determining the stresses, strains, and strength of such joints. In addition to his adhesives research, he has also worked extensively on vibration properties in composites and developed low-velocity impact tests for non-destructively testing composites and sandwich structures. 

Non-destructive testing can be as simple as a visual inspection as a trained eye will detect a surprising number of faulty joints, including close inspection of the adhesive bond line at the joint periphery, misaligned parts or uneven adhesive bond lines. 

There are also a number of more sophisticated non-destructive techniques (NDT) which measure the frequency response of the bonded component part and can thus identify the presence (or absence) of adhesive in the joint. These NDT methods use ultrasonic testing, whereby high-frequency, highly directional sound waves analyse the adherends and find hidden internal flaws [2]. 

Adhesion test, non-destructive A test that can be performed to establish the presence of a specified amount of adhesion without destroying the film. Examples Tape test of a mirror surface pull-to-limit wire-bond test. 

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