interface adhesion strength of adhesive-bonded materials ...this reflection coefficient can be...

Journal of Multidisciplinary Approaches in Science 11, Issue 1 (2019) 8-17

8

Journal of Multidisciplinary Approaches

in Science (JMAS)

Journal homepage: https://jmas.biz/index.php

ISSN: 2652-144X

Interface Adhesion Strength of Adhesive-Bonded Materials Using Ultrasonic Technique

Mukarram Ali1, Muhammad Shahid1,*, Haseeb Ahmed Khan Lodhi1, Danyal Naseer1, Imran Sadiq1 1 School of Chemical and Materials Engineering, National University of Sciences and Technology, H-12, Islamabad, Pakistan

ARTICLE INFO ABSTRACT

Article history: Received 12 September 2019 Received in revised form 27 October 2019 Accepted 7 November 2019 Available online 8 November 2019

One of the potential applications of ultrasonic testing (UT) technique is non-destructive assessment and analysis of an interface between two adhesively bonded substrates. The ultrasonic wave when reflects after interacting with the adhesively bonded interface, can be evaluated to estimate the shear strength of the adhesively bonded imperfect bond between two dissimilar materials. A correlational analysis can be employed using relationship of sound waves to shear and regression analysis to assess the reflected signal from the interface of two dissimilar adhesively bonded materials. Two adhesively bonded dissimilar substrates (1) A 4340 Steel plate and (2) Silica fabric reinforced Phenolic resin composite plate, adhesively bonded using polyurethane adhesive, were investigated. The reflection signal from the bonded material was analyzed and compared with the reference signal acquired from the unbonded part using ultrasonic testing technique, in order to obtain an interfacial stiffness (Kn), reflection coefficient (RL) and return loss (R). The three variables were evaluated and correlated with the shear strength of the joint as determined by tensile testing. Analysis of the experimental data showed that the shear-strength was cumulatively linear with the rise in ‘Gain’ and ‘Reflection Coefficient’, approximately with upto 98% accuracy. The correlation was validated between the shear strength and the reflection coefficient data of two dissimilar adhesively bonded materials, using ultrasonic non-destructive testing (NDT).

Keywords: Interface, adhesion, shear strength, ultrasonic testing Copyright © 2019 JMAS - All rights reserved

1. Introduction

Increasing use of complex structures demanding high performance joint properties e.g. lightweight, quick and cost-effective production, corrosion resistance along with foremost requirement of reasonably good shear strength. Adhesive joints offer the better solution compared with other joining techniques such as welding/brazing, riveting etc. [1]. Uniform stress distribution and reduction of joint weight, offer an added advantage of improved fatigue resistance [2]. The quality of the engineering structure is also dependent on joint’s shear strength of the adherends. One of the biggest limitations in using adhesive bond is the lack of procedures to quantify adhesive bonding quality through non-destructive testing [3] [4].

*Corresponding author. E-mail address: Muhammad Shahid ([email protected])

Open Access

Journal of Multidisciplinary Approaches in Science Volume 11, Issue 1 (2019) 8-17

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Therefore, there was a need to develop a NDT procedure in order to predict the bond quality [5] Various NDT Techniques such as ultrasonic testing, acoustic emission testing etc., are available and can be used for the testing of the adhesively joined materials [6]; (UT is considered one of the attractive technique for adhesive bond strength estimation.

Failures in adhesively bonded joints have been found generally due to improper material

processing and contamination in surface preparation of the adhesive bonding process. Adhesive bond defects fall into three broad categories [7] [8]: (1) Gross defects; (2) Cohesive defects; (3) Adhesive defect.

Gross defects include peeling, cavities, cracking and porosity, which can be detected using

standard UT. Poor adhesion of the adhesive may lead to degradation of adhesive bond strength resulting from various reasons, such as ecological effects or curing abnormalities. [5] The adhesion defect may reduce the bond strength between the adhesive and the adherends, however, it is difficult to be detected using conventional methods.

Estimation of bond strength using UT NDT without damaging the joint’s components, serving

twofold purposes; the percentage of uncertainty in the results is reduced and the joint assembly is saved from damage [7].

Ultrasonic reflections and transmission measurements provide one of the most accurate

approaches to finding interfacial hardness, which is further used to assess the binding strength [9]. The reflection coefficient of the interface depends on the frequency [10]. By measuring the frequency using the spring boundary conditions, the interfacial hardness can be estimated leading towards determination of the binding strength [11] [12]. Another method was also used wherein the intensity of the sound was noticed during ultrasonic waves passing through the interface; this factor was associated with rigidity of the connection and the binding strength [13-14]. This paper analyzes the binding force determined by tensile testing, correlated with interfacial stiffness and followed bya comparison between the two, by applying multiple patterns [15] [16].

2. Theoretical Background

Considering two imperfectly bonded dissimilar materials, if the size and spacing between the imperfections is much smaller than the wavelength, the ultrasonic wave interaction with the interface can be understood using spring boundary conditions, as schematically illustrated in Figure 1 [17].


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When sound waves enter into the material a specific pressure ‘P’ is exerted on the interface

which can be represented in terms of velocity ‘v’ [18].

Therefore,

𝑃 = 𝑣𝜔𝜄𝜌 (1)

𝑣 =

𝑃𝜔𝑖𝜌

(2)

where P = Pressure, 𝑣 = velocity of the wave, 𝜔 = frequency of the wave. The incident wave pressure as Pinc and reflected wave as Pref, can be defined by Eq.3 and 4. .

So,

𝑃)*+ = 𝑒)-(/0)*1234561) (3)

𝑃89: = 𝑅𝑒)-(/0)*1<34561)

(4)

where θ is the angle of incidence of the wave and "k" is wave number which is made by the normal to the wave front and the z-axis. The total field in the upper-medium can be determined by adding Eq.3 and 4. [19]:

𝑃=5=>? = 𝑒)-/0)*1@𝑒2)-34561 + 𝑅𝑒)-34561B (5) The reflection coefficient as “R” which indicates the intensity of a wave reflected by an

impedance discontinuity in the transmission medium and can be determined using Eq.6, whereas the transmission coefficient ‘T’ can be determined by Eq.7 using the spring boundary conditions and the law of impedances [20]:.

𝑅 =

(𝑍D + 𝑍E)(𝑍E − 𝑍G)𝑒2)-H + (𝑍D − 𝑍E)(𝑍E + 𝑍G)𝑒)-H

(𝑍D + 𝑍E)(𝑍E − 𝑍G)𝑒2)-H + (𝑍D − 𝑍E)(𝑍E + 𝑍G)𝑒)-H

(6)

Figure 1 Imperfect interface between two adhesively bonded solids [17]

Adhesive

Steel 4340

Composite

Z1, θ, ρ1

Z2, θ1,

Z3, ρ3

Incident Wave

Reflected Wave

Z = 0

Z = d


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𝑇 =

4𝑍D𝑍E(𝑍D − 𝑍E)(𝑍E − 𝑍G)𝑒)-H + (𝑍D + 𝑍E)(𝑍E + 𝑍G)𝑒2)-H

(7)

where Z1, Z2, and Z3 are the impedances of three different mediums. Shearing of the material ‘Ƞ’ at the interface during the propagation of sound waves an be determined by Eq.8 [15].

Ƞ =1𝐾*

(8)

This Ƞ is related to compression coefficient and shear strength as indicated by Eq.9. Ƞ =

𝜇𝜏

(9)

where 𝜇 is the compression coefficient. Substituting 𝑒)-H and 𝑒2)-H b by 𝑖𝑤ȠQ in equation 6,

𝑅 =RST2SU2)

VWXSTSUYRSU<SZ2)

VWXSUSZY<RST2SZ2)

VWXSUSZYRST2SU2)

VWXSTSUY

RST<SU2)VWXSTSUYRSU<SZ2)

VWXSTSZY

(10)

The value of Transmission coefficient evaluated by putting the value of 𝑒)-H and 𝑒2)-H in eq 7: [21]

𝑇 = 2𝑍D R𝑍E + 𝑍G − 𝑖

𝑤𝐾*𝑍E𝑍GY + 2𝑍E R𝑍D + 𝑍E − 𝑖

𝑤𝐾*𝑍D𝑍EY

R𝑍D + 𝑍E − 𝑖𝑤𝐾*𝑍D𝑍EY R𝑍E + 𝑍G − 𝑖

𝑤𝐾*𝑍D𝑍GY

(11)

Now because of infinite reflections in layered mediums, the total reflection coefficient will be a sum of all the reflections and Reflection Coefficient [7]. The sum of all internal reflections can be determined by Eq.12:

𝑅9 =

𝑇DE𝑇ED𝑒E)-\

1 − 𝑅DEE𝑒E)-\𝑅ED +

𝑇EG𝑇GE𝑒E)-\

1 − 𝑅EDE𝑒E)-\𝑅GE

(12)

The Total Reflection will be calculated by adding Eqs. 10 and 12 [22] as given in Eq.13:

𝑅] = 𝑅 + 𝑅9

(13)

This reflection coefficient can be evaluated in terms of interfacial stiffness and can be compared with shear strength [23] [24] [25].

3. Design of Experiments and Results

A plate of Steel-4340 of specific dimensions (density: 7800 kg/m3) was bonded to a silica fabric reinforced Phenolic resin composite of the similar dimensions as of steel plate (density 1.65 g/cm3) [22] [26], using a hand layup layer of polyurethane adhesive (density of 0.8 g/cm3) [5] [27]. The ratio


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of the two-parts polyurethane resin was kept as 1:1 (adhesive: hardener) [5] [28]. The sound velocity of steel was 5850 m/s whereas the composite had sound velocity of 2700 m/s [6] [29].

Multiple samples were tested under varying conditions since the curing process had significant

impact on mechanical properties of the bond. These conditions were established during curing of the specimens under various loading conditions. The adhesive joint was made by following ASTM standard D1002 [30, 31]. Pre-set parameters for shear testing are given in Table 1.

Initial samples were cured at ambient conditions without applying any pressure. The other two

samples were cured under a weight of 5 kg by holding the sample between a vice, etc. The ultrasonic experiments were conducted using EPOCH LT 910-258 UT machine. A UT probe of 1 inch diameter and frequency of 5.0 MHz was placed above the test specimen. The velocity of incoming wave was set as 5849 m/s, the damping resistance was set to 50 ohm, Gain was set to 30dB and rejection was set 0%. The thickness of the specimen was determined using a Vernier Callipers. The thickness of all the samples tested was 12mm. The probe was focused through the top of the steel plate onto the adhesive-bonded region between the steel plate and the composite plate. The longitudinal ultrasonic waves were transmitted through the transducer [32]. The shear testing of samples was performed using 20 kN UTM Shimadzu machine based on ASTM D1002 standard [30] [31]. The strain rate used to all the experiment was kept as 127mm/min.

The reflection coefficient was calculated from the mathematical model based on experimental results. Then a correlational analysis of reflection coefficient and gain values were determined by shear strength using MINITAB 16 STATISTICLE SOFTWARE. Table 3 lists the results based on the mathematical model.

Table 2. Shear strength values determined using the

mathematical model Sample dB Shear Strength (MPa) Return Loss RL

1 -3 7.1 -1.5 1.5

2 -3 8.2 -1.5 1.5

3 -4 9.4 -2 2

4 -4 9.35 -2 2

Table-1. Pre-set parameters for shear testing

Parameters Set Value Machine Type UTM SHIMAZDU

Standard followed ASTM D1002 [30] Length of the steel and

composite samples 180mm

Width 25.4mm Thickness 6mm each plate

Strain Rate of the shear test

1.27mm/min


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5 -5 10.9 -2.5 2.5

6 -5 10.9 -2.5 2.5

7 -6 12.2 -3 3

8 -6 11.6 -3 3

9 -6 12.5 -3 3

10 -6 11.5 -3 3

11 -6 11.5 -3 3

Return Loss was determined using the relationship given in Eq.14 [33].

Return Loss(RL) = 10 log ^)^8

(14)

4. DISCUSSION

The correlational analysis was validated using regression analysis [34]. The correlation of shear strength calculated using RL (Table 2) was analysed to be 98% while shear strength calculated using dB was related 96.9%. The residual plots for dB are shown in Figs. 2-3 and shear strength are shown Fig. 4.

Fig.2 shows a normal probability plot displaying showing deviation of dB residual from the ideal

line.. It was evident that percentage residue was significantly away from the base line (ideal residue = 0) whereas RL results were relatively closer to the base line as shown in Fig.4(b). It could be deduced that the shear strength results based upon dB model were inconsistent as compared to RL based model. This difference between the results of dB and RL was the result of direct and indirect relationship with shear strength. Shear strength of an interface directly depends on Reflection coefficient using the relation formulated in the theoretical model. However, in case of dB, another parameter known as interfacial stiffness was incorporated which was not appropriate to calculate employing the equipment used for calculating shear strength. In case of reflection coefficient this interfacial stiffness was incorporated in the modified mathematical model developed in this study removing the errors appeared in case of dB results.

Figure 3(a) and 4(a) show histograms displaying frequency of consistent values in the data, for

dB and RL, respectively. Although value of RL should be ideally zero, however, it could not be brought to zero in model based calculations. One of the objectives of this analysis was to find type of the residual occurring the most. Smaller positive residual was acceptable as it was easier to exclude, however, negative residual would be unsuitable for substitution and calculations. In case of dB model based results maximum of the residual lies in the negative region (Fig-3(b)) mainly because of non-incorporation of interfacial stiffness in calculation but in case of RL based calculations the residual is positive and lower which makes RL based model more effective and dependable to use. Figures 3(b) and 4(b) display the observation order plotted X-axis and residual on Y-axis which exhibit deviation of the values with order of reflection.


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Figure 2: Normal Probability plot showing deviation of dB residual from the ideal line.

Figure 3(a): (dB) Histogram exhibiting frequency of the results in positive and negative ranges.

Figure 3(b): (dB) Variation in the fitted values to the observative results.


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Figure 4(a): (RL) Histogram showing the consistent values of residual using reflection coefficient in negative and positive ranges; majority of the results are in the positive range showing the credibility of the results.

Figure 4(b): (RL) The plot displays the variation in the fitted values to the observative results.

Figure-5. Effect of Reflection coefficient on shear strength

In light of all the analysis done based upon dB and RL model calculations, it was deduced that the RL model calculations were more efficient, reliable lesser error (approx.2%). Hence, a relation between shear strength and reflection coefficient was obtained from the model as shown in Fig. 5 displaying a linear approximation.

S = 2.8RL + α (14)

where S is the shear strength (MPa), RL = Reflection coefficient and α = Strength Constant.

02468

10121416

0 1 2 3 4 5 6

Shea

r Str

engt

h

Reflection Coefficient

Reflection Coefficient vs Shear Strength

SS


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Although, many methods are in use to obtain the value of strength constant, however, in this case, it was determined from the graph by extending the line till it intercepted the Y-axis; the point of Y-intercept gave the value of the strength constant (3.53 in this particular case).

5. Conclusions

A relationship was derived between reflection coefficient and bond strength of two adhesively-bonded dissimilar materials using spring boundary conditions. After physical shear strength testing, the ultrasonic testing technique was applied in order to estimate shear bond strength by applying two theoretical models: (1) Reflection coefficient method and (2) Decibel loss method. The obtained results were then statistically analysed by applying Regression analysis and Correlation analysis. The Reflection coefficient was observed to be more accurate and viable then Decibel loss method. Moreover, experimental results proved that the adhesive shear bond strength between dissimilar materials increased linearly with an increase in interfacial stiffness. The relationship “S = 2.8RL + α” proved to be valid for determining the shear strength of adhesively-bonded dissimilar materials.

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interface adhesion strength of adhesive-bonded materials ...this reflection coefficient can be...

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