International Journal of Adhesion & Adhesives 42 (2013) 6068
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International Journal of Adhesion & Adhesives
journal homepage: www.elsevier.com/locate/ijadhadh
Evaluation of kissing bond in composite adhesive lap joints using digitalimage correlation: Preliminary studies
R.L. Vijaya Kumar n, M.R. Bhat, C.R.L. Murthy
Department of Aerospace Engineering, Indian Institute of Science, Bangalore-560012, India
a r t i c l e i n f o
Accepted 20 December 2012The assembly of aerospace and automotive structures in recent years is increasingly carried out using
adhesives. Adhesive joints have advantages of uniform stress distribution and less stress concentration
Available online 16 January 2013
Digital image correlation
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esponding author. Tel.: 91 80 2293 3025;ail address: email@example.com (R.L. Vija
a b s t r a c t
in the bonded region. Nevertheless, they may suffer due to the presence of defects in bond line and at
the interface or due to improper curing process. While defects like voids, cracks and delaminations
present in the adhesive bond line may be detected using different NDE methods, interfacial defects in
the form of kissing bond may go undetected. Attempts using advanced ultrasonic methods like
nonlinear ultrasound and guided wave inspection to detect kissing bond have met with limited success
stressing the need for alternate methods. This paper concerns the preliminary studies carried out on
detectability of dry contact kissing bonds in adhesive joints using the Digital Image Correlation (DIC)
technique. In this attempt, adhesive joint samples containing varied area of kissing bond were prepared
using the glass fiber reinforced composite (GFRP) as substrates and epoxy resin as the adhesive layer
joining them. The samples were also subjected to conventional and high power ultrasonic inspection.
Further, these samples were loaded till failure to determine the bond strength during which digital
images were recorded and analyzed using the DIC method. This noncontact method could indicate the
existence of kissing bonds at less than 50% failure load. Finite element studies carried out showed a
similar trend. Results obtained from these preliminary studies are encouraging and further tests need
to be done on a larger set of samples to study experimental uncertainties and scatter associated with
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The need for stronger and lighter structures in aerospace andautomotive industry has driven the use of adhesive joints in theirassemblies. Adhesive bonding not only results in a uniform stressdistribution in joint structures but also allows the bonding ofdissimilar materials and assembly of composite structures. One ofthe major limiting factors of adhesive bonding is the nonavail-ability of a quantitative nondestructive technique that can detectand evaluate defects in the joint, particularly in the interfacialregion between substrate and adhesive layer. Also, degradation ofbonded joints in terms of loss of strength or other mechanicalproperties can be caused due to manufacturing errors or due toexposure of joints to moisture and temperature in service. Thisdegradation often results in change of adhesive elastic modulus.
Defects in adhesive joints can be classified into three broadcategoriesinternal gross defects, cohesive defects and adhesivedefects. Internal defects such as cracking, voids and porosity can
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be detected using conventional NDT inspection techniques suchas ultrasonics or X-ray . Poor adhesion between theadhesive and the substrate is often difficult to detect usingconventional ultrasonic techniques as this thin interfacial layercould be orders of magnitude less than the wavelength used .These defects are often termed kissing bonds or zero volumebonds, where the two surfaces are held together by a compressivestress with no molecular forces acting between them . Jiao andRose  have defined a kissing bond as a perfect contactbetween two surfaces which transmits no shear stress. Consider-able amount of literature is available on kissing bonds . Thecombination of severity and detection difficulty of this type ofdefects in an adhesively bonded structure makes it of significantpractical importance.
Attempts by some researchers using advanced ultrasonictechniques like nonlinear acoustics for detecting and characteriz-ing kissing bonds have met with some success . In caseof contacting surfaces the imperfect interface can introduce agreater degree of nonlinearity into the system. This nonlinearitycan be due to clapped surfaces with nonlinear stressstrainbehavior inherent in the contact leading to generation of harmo-nics of the interrogating ultrasonic frequency. These techniques
R.L. Vijaya Kumar et al. / International Journal of Adhesion & Adhesives 42 (2013) 6068 61
are largely confined to laboratory owing to difficulties involvedin generating and propagating such waves in complex structures.Another NDE method viz., pulsed thermography also has beentried by some researchers to detect kissing bond defects [19,20].Waugh et al.  have shown that phase pulsed thermographycan be a potential tool to detect kissing bond defects in adhesivelybonded joints.
However, owing to the limited success of conventional NDTmethods in detection of kissing bonds, search for alternativemethods is on. Digital Image Correlation (DIC) apparently hasgood potential in this regard. The aim of this paper is to explorethe ability of this technique to detect and evaluate kissing bondsin adhesive joints. Thus, a set of glass fiber reinforced compositeadhesive lap joints were prepared with planted kissing bonds ofdifferent sizes. Nondestructive inspection tools such as ultraso-nics and X-ray imaging were also utilized as supplementarytechniques. The joints were then loaded in a testing machine tillfailure to find the correlation between the size of kissing bondand the resulting bond strength. Digital images were recorded atdifferent stages of loading and analyzed using the DIC software.Kissing bonds could be detected at loads lesser than 50% of failureload. Details of the experiments conducted and results obtainedare presented in this paper.
2. Materials and samples
Glass fiber reinforced plastic (GFRP) composite laminates werefabricated with unidirectional (UD) E-glass fabric as reinforce-ment and thermoset epoxy resin LY556/HY 951 as the matrixwith a volume fraction of 65:35. The laminate was cured at roomtemperature with 14 layers of UD glass fabric all laid up in thesame direction. Single lap shear joint specimens (Fig. 1) wereprepared from strips cut out of unidirectional glass fiber rein-forced plastic (GFRP) laminates as substrates and a two partepoxy adhesive system (Araldite AV 138M and hardener HV 998)for the adhesive layer. Spacers were used to maintain a uniformthickness of 2.5 mm. The substrates were cut to the required sizeof 15050 mm2, the surfaces to be joined were prepared accord-ing to ASTM D 2093 standard for surface preparation of plastics.Adhesive and the hardener were mixed in the ratio of 100:40 asper manufacturers recommendation.
Fig. 1. GFRP adhesive single lap joint sample.
Fig. 2. Bonded region with planted kissing bo
The substrates were then bonded and cured at room tempera-ture for 24 h in a specially designed mold which helps inmaintaining a uniform bond line thickness of 0.2 mm whileensuring proper alignment of substrates under pressure (7 bar).An area of 5050 mm2 was bonded with kissing bonds intro-duced within this area in the form of ETFE (ethylene tetrafluor-oethylene) based, fluoro polymer release films having a thicknessof less than 20 mm (Fig. 2).
A total of four samples were prepared. One of the jointswithout any planted kissing bond was taken as a referencehealthy sample. The details of the samples prepared are providedin Table 1.
3. Experimental work
3.1. Ultrasonic inspection
Adhesive joint samples were subjected to normal incidencewater immersion ultrasonic scanning in both pulse echo andthrough transmission mode using a 5 MHz focused transducer.Fig. 3 shows the C-scan images obtained from ultrasonic pulseecho and through transmission methods. Though there was adecrease in received amplitude in through transmission inspec-tion scans which gave an indication of degradation qualitatively,this information was insufficient to prove the presence of kissingbonds as a similar trend can also be observed even in the presenceof cohesive defects like voids and pores or changes in the materialproperties like modulus or density . It was not possible tocharacterize or size the kissing bond defect looking at theC-scan images shown in Fig. 3. The pulse echo techniqueapproach also failed to provide a good measure of size of thekissing bond as it was difficult to identify the echo from theinterface in the noisy signal.
The ultrasonic lamb wave and nonlinear acoustic methods didnot yield any significant consistent outcome with regard todifferent sizes of induced k