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Dynamic shear strength of adhesive joints made of metallic and composite adherents
Materials and Design 31 (2010) 2102–2109
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Dynamic shear strength of adhesive joints made of metallic and composite adherents Sohan Lal Raykhere a, Prashant Kumar a, R.K. Singh b, Venkitanarayanan Parameswaran a,* a b
Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208 016, India Defense Materials Stores Research and Development Establishment, Kanpur 208 016, India
a r t i c l e
i n f o
Article history: Received 20 July 2009 Accepted 21 October 2009 Available online 25 October 2009
a b s t r a c t The use of adhesive joints has gained good acceptance in the automotive and aerospace industries in recent years, particularly for joining glass fiber reinforced plastics (GFRP) to metals. Such joints will be subjected to short duration dynamic loads in service. The present study focuses on the evaluation of the shear strength of adhesive joints prepared using four different commercial adhesives at loading rates in the range of 0.6–1.2 MPa/ls. The adhesives used were Araldite 2014, Araldite 2011, Epibond 1590 and A/B Loctite 324. Joints were prepared with two different adherent combinations; aluminum–aluminum and aluminum–GFRP. The results of the study indicated that, depending on the adhesive and adherent combination, the dynamic strength was 2–4 times the static strength. Among the four adhesives, Epibond 1590 exhibited the highest rate sensitivity whereas Loctite 324 exhibited the least. Further it was also observed that the dynamic strength was not sensitive to the adherent combination whenever the failure was predominantly within the adhesive layer. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Adhesive joints are extensively used in automotive and aerospace industry in recent years. Particularly with the wide spread use of glass fiber reinforced plastics (GFRP) in these sectors, joining of GFRP to metals is inevitable. Conventional techniques of joining like bolting and riveting has the disadvantage of creating localized damages in GFRP. Adhesives offer the best alternative to conventional joining methods in this context. Invariably such joints are subjected to dynamic loads during service either by design or by accident. Estimating the load carrying capacity of such joints under dynamic loading is therefore important to their successful use. Adhesive manufactures provide information only on the static lap strength of the adhesive for a given adherent combination. The strength of adhesive lap joints subjected to dynamic loading has been investigated by several researchers using different techniques. The study by Harris and Adams [1], on the impact strength of adhesive lap joints, determined using the instrumented impact technique, indicated that the joint strength is not significantly affected by loading rate for the adhesives studied. On the contrary, Beevers and Ellis [2] reported that lap joints made of steel adherents and an epoxy adhesive can withstand higher loads under impact conditions. Recently there has been several investigations on the use of the split Hopkinson bar to evaluate the strength of adhesive joints. The dynamic strength of a lap joints made of steel * Corresponding author. Tel.: +91 512 2597528; fax: +91 512 2597408. E-mail address: [email protected] (V. Parameswaran). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.10.043
adherents and a general purpose epoxy adhesive was determined at various loading rates by Srivastava et al. [3] using the split Hopkinson pressure bar (SHPB) technique. A pin and collar type specimen was used in SHPB by Yokoyama and Shimizu [4] to determine the dynamic shear strength of adhesive lap joints. Sato and Ikegami [5] studied the strength of adhesively bonded butt joints under combined tension and torsion using the SHPB. Their results indicated that the failure of the butt joint under combined loading can be fitted with the von-Mises criteria. Adamvalli and Parameswaran [6] investigated the dynamic strength of adhesive lap joints at high temperatures using the SHPB technique. Wu et al. [7] evaluated the post impact strength of printed circuit boards (PCBs) in which the components were mounted using an isotropic conductive adhesive. The PCBs were first subjected to high strain rate bending using a SHPB at different strain rates. They reported that the post impact strength reduced with increasing bending strain rate. Goglio and Rossetto [8] have recently studied the behavior of lap joints subjected to impact loading using an instrumented Charpy pendulum. They have reported that the strength of the joint is higher under impact loading. They also observed that joints having thinner adhesive layer had higher strength. Apart from those listed above, there are also investigations addressing the response of adhesive joints subjected to impact loading. Vaidya et al. [9] reported an experimental cum numerical investigation on the transverse impact behavior of adhesively bonded lap joints. The lap joints in their study were subjected to transverse impact using a drop weight tester and a detailed stress analysis of the joint
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was carried out through finite element analysis. Sato [10] has outlined a theoretical approach to obtain the time variation of the stress distribution near the edge of the adhesive layer in a lap joint subjected to impact loading. The effect of modifying an epoxy adhesive with another biodegradable polymer on the impact strength was investigated by Jin and Park [11]. They evaluated the impact strength using an Izod impact tester. Their study indicated that addition of the biodegradable polymer improves the impact strength. In most of the studies reported above on evaluation of the adhesive joint strength [3–6], the adherents used were metals. In the present study, four different commercial adhesives were used to bond aluminum and GFRP and the dynamic strength of these joints were determined using a torsional split Hopkinson bar (TSHB). For comparison, the static strength was also determined. The joint geometry and preparation, testing method and the results are presented in the following sections.
the bond area, which was not desirable. The aluminum half of the joint was made of an alloy equivalent to 6101. The GFRP half was prepared in-house by stacking the required number of unidirectional prepregs in a [0/90]s configuration and then laminating them in a hot press, further details of which can be found in [12]. The static strength was also evaluated for the sake of comparison by testing single lap joints of geometry shown in Fig. 1b under compressive loading. 2.2. Adhesives used As mentioned earlier, the four different adhesives used in this study were Araldite 2011 [13], Araldite 2014 [14], Epibond 1590 A/B [15] and Loctite 324 [16]. The first three adhesives mentioned are two part adhesives manufactured by Huntsman Ltd., whereas Loctite 324, a fast curing adhesive, was manufactured by Henkel Technologies. The proportions of the two parts, physical properties and the curing details of these adhesives are provided in Table 1. Araldite 2011 is a general purpose adhesive whereas, Araldite 2014 and Epibond 1590 are recommended for aerospace applications. Loctite 324 is a specially formulated adhesive for toughness and impact strength. According to the manufacture’s catalogue these adhesives can be used for bonding similar or dissimilar adherents. Further details on each of the adhesives are available in the manufacturer’s catalogue [13–16].
2. Details of specimen, adhesives and bonding procedure 2.1. Specimen geometry The geometry of the adhesive butt joint used in this study to evaluate the shear strength is shown in Fig. 1. The joint had two parts. The aluminum half of the joint was in the form of a cylinder with a flange at one end as shown in Fig. 1a. The TSHB used in the study had bars of 25.5 mm diameter; hence the flange also had an outer diameter of 25.5 mm. In the case of aluminum–aluminum (henceforth referred to as AA) joint, the second half was an annulus with an inner diameter of 14 mm and an outer diameter of 25.5 mm. The bond area on which the adhesive was applied (shaded region in Fig. 1a) was 100 mm2. In the case of aluminum–GFRP (henceforth referred to as AG) joint, the GFRP half was a full disc of outer diameter 25.5 mm and 5 mm thickness. A full disc was used instead of an annulus for the AG joints as drilling an inner hole of 14 mm would induce damage in the GFRP within
2.3. Bonding procedure The surface preparation of the adherents was carried out as follows. The aluminum half of the joint was first degreased using acetone. Subsequently, the surface was roughened with a 200 grit paper. Then the roughened surface was cleaned with methanol. In the case of the GFRP half, the surfaces were roughened with 200 grit paper and then cleaned with methanol. The adherent halves were weighed before bonding using an electronic weighing
25.5
18
25.5
(a)
3
14
3 14
4
(b) 5
Adhesive 3
(c)
18 All dimensions in mm
(d)
Fig. 1. Geometry of (a) dynamic test specimens, (b) static test specimens and photographs of the assembled joint, (c) aluminum–aluminum joint and (d) aluminum–GFRP joint.
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Table 1 Properties, mix ratio and cure cycle for adhesives tested.a
a
Adhesive
Mixing ratio (A:B by wt)
Specific gravity
Mix-viscosity at 25 °C (Pa s)
Curing cycle
Araldite 2011 A/B Araldite 2014 A/B Epibond 1590 A/B Loctite 324
100:80
1.05
30–45
100:50
1.60
90
100:55
1.10
146
–
1.06
10–24
30 h at 25 °C 30 h at 25 °C 4 h at 60 °C 2 h at 25 °C
ness of 0.07 mm in the finally assembled joint. Since there was no specific recommendation on the adhesive thickness for Epibond 1590 A/B and Loctite 324, the weight for these two adhesives were also calculated corresponding to an adhesive layer thickness of 0.07 mm. An optical micrograph of an AA lap joint bonded with Araldite 2014 is shown in Fig. 2. It can be observed that the final adhesive layer thickness is about 100 lm. 3. Experimental details 3.1. TSHB set up
From manufactures data sheet.
Fig. 2. Optical micrograph of adhesive layer in an AA lap joint bonded with Araldite 2014 (each small division in scale shown in the micrograph is 100 lm).
scale of 1 mg resolution. Subsequently, the two parts of the adhesive (except Loctite 324), were mixed as suggested by the manufacturer in the appropriate ratio (see Table 1). A known weight of the mixture was then applied to the adherents and the joint was assembled and placed in a fixture to maintain alignment of the two halves. After the joint was cured, the extra adhesive was cleaned and the joint was again weighed to determine the weight of adhesive (w) actually present in the joint. For Araldite 2011 and Araldite 2014 the manufacturer recommended an adhesive layer thickness of 0.05–0.1 mm for maximum strength, whereas there was no such recommendation for the other two adhesives. Hence the weight of the adhesive that should be finally retained in the joint (w) was calculated targeting a nominal adhesive layer thick-
The schematic of the TSHB set up used in this study is shown in Fig. 3. The incident and transmitter bars had a diameter of 25.5 mm and were respectively 2500 mm and 1600 mm long. The bars were made of high strength aluminum alloy. A pair of ±45° strain rosettes were installed diametrically opposite to each other on each bar such that the sensing direction of the gages were at ±45° to the bar axis. The gages were connected in a full bridge configuration using an Ectron 530 strain amplifier and the strain signals were recorded using a NI PCI 6115 data acquisition card at 1 MHz sampling rate. The two faces of the specimen were bonded respectively to the incident and transmitter bar. At one of its ends (A in Fig. 3), the incident bar had a pulley with a dead-weight loading arrangement. About 440 mm away from the pulley (point B in Fig. 3), the incident bar was clamped using a locking device (shown in Fig. 3). A static torque of the required magnitude was stored in this portion (AB in Fig. 3) of the incident bar by torquing the pulley using dead-weights. This torque is released suddenly by snapping the bolt used to clamp the incident bar at location B. This way a torsional pulse was produced in the incident bar. This pulse, called the incident pulse, travels towards the specimen and on reaching the specimen, a part of it is reflected back into the incident bar and the rest is transmitted through the specimen into the transmitted bar. The incident, reflected and transmitted pulses are recorded through the full bridge gage arrangement for further analysis. 3.2. Data analysis Using one-dimensional theory of wave propagation, the torque history at the two ends of the specimen T1(t) and T2(t) can be obtained from the recoded shear strains as follows:
T 1 ðtÞ ¼ 2GJ fcI ðtÞ þ cR ðtÞg D T 2 ðtÞ ¼ 2GJ fcT ðtÞg D
where cI ðtÞ; cR ðtÞ and cT ðtÞ are respectively the incident, reflected and transmitted engineering shear strains recorded using the strain
Locking device Incident bar
Pulley
Specimen
Transmitter bar
B A C
Weights
ð1Þ
Stain rosette Back pressure device
Fig. 3. Schematic of the TSHB used for determining the dynamic strength of adhesives.
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rosettes and D, G and J are respectively the diameter, modulus of rigidity and polar moment of inertia of the bar. It is important to note that, for an accurate evaluation of the strength of the specimen, the condition of dynamic equilibrium (T1 = T2) should be reached before failure. Once this has been attained, the shear strength of the joint is calculated from the maximum transmitted torque, T2MAX, as follows:
ss ¼
16Do T 2MAX
ð2Þ
pðD4o D4i Þ
where Do and Di are the outer and inner diameter of the bond area shown in Fig. 1. 3.3. Testing procedure The specimen to be tested was attached to the incident and transmitter bars by bonding the specimen faces to the bars using either Loctite 324 or Araldite 2011. The area of this bond was three times larger than the area of the bond in the joint. By this way it was ensured that the failure always occurs in the joint and not at the interface with the bars. After curing, the joint was bonded to the incident and transmitter bars and a light clamping pressure was applied through the back pressure devise shown in Fig. 3. Once the bond between the specimen and the bar has cured (2 h for Loctite 324 and 30 h for Araldite 2011), the incident bar was clamped using the notched bolt (see Fig. 3). The portion AB of the incident
bar was then torqued to the required level by adding dead-weights. The notched bolt was then tightened until it breaks. The strain data was recorded and analyzed as mentioned earlier. The static strength was determined by subjecting single lap joints (see Fig. 1b) to compressive loading in a universal testing machine. The test was conducted at a temperature of 26 °C and a cross head speed of 1 mm/min. It should be noted here that the shear stress in the adhesive layer in the static test is induced by subjecting the joint to a compressive loading whereas in the dynamic loading the shear stress is induced by pure torsion. It is in general understood that, in the case of single lap joints, the state of stress in the adhesive layer may be neither pure shear nor uniform in nature. However, when the adherents are thick as in the present case, the state of stress will be dominantly uniform shear and the lap strength is also a good estimate of the adhesive strength [17,18]. Hence even though there is a change of geometry and loading (compression versus torsion) between the static and dynamic tests, in both cases the state of stress in the adhesive layer will be dominantly shear and therefore the results should be comparable. Further discussion on this will be given in the next section. 4. Results and discussion Typical incident, reflected and transmitted signals for two cases, an AA joint bonded with Araldite 2011 and an AG joint bonded with Epibond 1590 A/B, are shown in Fig. 4. It can be seen that
35
1.2 0.9
Incident
25 Torque (Nm)
Signal (V)
0.6
30
Transmitted
0.3 0.0 -0.3
20
T1
15 10
-0.6
T2 5
-0.9
Reflected 0
-1.2 0
200
400
600
800
1000
0
1200
50
100
150
200
100 150 Time (µ s)
200
Time (µs)
Time (µ s)
(a) Aluminum-Aluminum joint bonded with Araldite 2011 1.2
35
0.9
Incident
30
Transmitted
T1
25
0.3
Torque (Nm)
Signav (V)
0.6
0.0 -0.3 -0.6
20
T2 15 10
Reflected
-0.9
5
-1.2
0
0
200
400
600 800 Time (µ s)
1000
1200
0
50
(b) Aluminum-GFRP joint bonded with Epibond 1590 A/B Fig. 4. Recorded signals and torque histories for AA and AG joint.
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the incident pulse has a duration of about 330 ls. Also shown in Fig. 4 are the time histories of the torques, T1 and T2 at the two ends of the joint calculated using Eq. (1). As shown in Fig. 4, T1 and T2 are nearly equal throughout the loading, and reaches maximum values in about 50–60 ls. This confirms that the joint attains dynamic equilibrium before failure. The shear strength of the adhesive layer was calculated from the maximum transmitted torque using Eq. (2) and the loading rate was determined by dividing the maximum shear stress by the time to reach T2MAX. For each adherent combination (AA and AG) and adhesive, at least five tests were conducted and the average strength along with the 95% confidence level was
Table 2 Static and Dynamic shear strength of adhesives.
A
Adhesive
Joint type
Static strength (MPa)
Dynamic strength (MPa)
Loading rate (MPa/l s)
Sensitivity
Araldite 2011
AA AG
15.6 ± 2.7 13.2 ± 1.6
44.0 ± 2.8 46.2 ± 2.6
1.1 ± 0.1 1.1 ± 0.0
2.8 3.5
Araldite 2014
AA
51.1 ± 3.9 (50.5)a 39.7 ± 2.1
1.2 ± 0.2 (1.33)a 0.8 ± 0.1
2.4
AG
21.6 ± 1.1 (22.0)a 21.3 ± 1.0
Epibond 1590 A/B
AA AG
13.1 ± 1.1 11.9 ± 2.4
38.9 ± 1.6 43.6 ± 2.5
1.0 ± 0.2 0.8 ± 0.1
3.0 3.7
Loctite 324
AA AG
18.6 ± 1.1 17.0 ± 1.8
32.9 ± 2.6 31.9 ± 1.9
0.6 ± 0.2 0.7 ± 0.2
1.8 1.9
1.7
Values in parenthesis are that from Ref. [6] for titanium lap joints.
obtained for each case. The condition of attaining equilibrium was also verified in all cases. The results of the study are presented in Table 2. The static strength values are also provided in Table 2 for comparison. The loading rate in the case of static testing was 1 MPa/s. As pointed out earlier in Section 3.3, there is a geometry difference as well as a difference in the way the shear stress is induced in the adhesive between the static and dynamic tests. In order to establish that the geometry and type of loading does not affect the strength obtained, a comparison is made between the results shown in Table 2 and that reported in our earlier investigation [6] (also shown in parenthesis in Table 2) for the adhesive Araldite 2014. In Ref. [6], the static strength of the lap joint with titanium adherents is reported as 22 MPa and the reported failure is cohesive in nature. In the present study, the static strength obtained with aluminum adherents for the same adhesive is 21.3 MPa which is in good agreement with that reported in [6] and this should be expected as the failure in the both cases is cohesive in nature. Further the dynamic strength reported in [6], obtained using the lap joint specimen in a compression SHPB, for Araldite 2014 is 50.5 MPa at a loading rate of 1.33 MPa/ls, whereas that given in Table 2, obtained using the butt joint geometry subjected to torsion in a TSHB, is 51.2 at a loading rate of 1.2 MPa/ls. This good agreement confirms that geometry and loading configuration does not bring in any additional effects. It can be observed from Table 2 that the dynamic strength is higher than the static strength uniformly for all adhesives and adherent combinations. The adherent combination (AA or AG) does not seem to have a significant effect on the static strength of the
Fig. 5. Photographs of specimen halves after dynamic test for Araldite 2014.
S.L. Raykhere et al. / Materials and Design 31 (2010) 2102–2109
adhesive. However the same is not true for the dynamic strength. In the case of Araldite 2014, the dynamic strength of an AG joint is 25% lower than that for an AA joint. Whereas, for Epibond 1590 A/B the strength of AG joint is about 12% higher than that of an AA joint. The adherent combination does not seem to have an influence on the dynamic strength for Araldite 2011 and Loctite 324. The ratio of the dynamic strength to static strength is a measure of the rate sensitivity. It can be observed from Table 2 that Epibond 1590 A/B has the highest rate sensitivity whereas Loctite 324 has the lowest. In order to understand the reduction in dynamic strength for AG joints when compared to AA joints, the specimen halves were inspected using a Leica S8 APO stereo microscope fitted with a 2 Mega pixel monochrome camera. Fig. 5 shows the photograph of the specimen surfaces without any magnification. The glossy portion of the aluminum half was masked with black non glossy paper to prevent reflections and improve clarity. It can be observed from Fig. 5, that the adhesive is uniformly present in both halves in the case of AA joint, except for a few locations where the adhesive did not wet either halves. In the case of the AG joint, shown in Fig. 5, the appearance of the aluminum half is similar to that in the AA joint. The GFRP half also shows a layer of adhesive, however, the adhesive layer in the GFRP half looks thinner and less uniformly
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spread over the GFRP. The photographs of the AA and AG specimens at a higher magnification are shown in Fig. 6. It can be observed from Fig. 6 that, in the case of AA joint, the adhesive layer present on both halves appears to be uniform and equally rich on both halves. There are few small regions (less than 100 lm) in both halves of the specimen, where the adhesive has not wetted the adherents. Despite these spots, it is clear that the failure is primarily within the adhesive layer or cohesive in nature. In the case of AG joint, shown in Fig. 6, the adhesive layer on the aluminum half is similar to that for the AA joint. However, the adhesive layer on the GFRP half is less rich than that in the aluminum half. Further, at several locations, the GFRP adherent is visible indicating that at these locations, the adhesive layer has separated from the GFRP. The relatively brighter locations indicate regions of adhesive failure. Thus the failure is mixed in nature and we believe this to be the reason for the reduction in the dynamic strength. To further reinforce this observation, the photographs of specimen halves of an AA and an AG lap joint after static test are shown in Fig. 7. In the case of AA joint the adhesive layer on the two halves appears uniform and identical in texture. Few small regions where the adhesive peeled off from one side are also visible in Fig. 7 for the AA joints. The adhesive layer on the aluminum and GFRP halves of the AG joint shown in Fig. 7, look similar to each other. Further
Fig. 6. Optical micrographs of specimen halves after dynamic test for Araldite 2014.
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Fig. 7. Optical micrographs of specimen halves after static test for Araldite 2014.
the appearance is also similar to that in the AA joint, indicating failure entirely within the adhesive layer. As indicated in Table 2, the static strength for AA and AG joints was the same in the case of Araldite 2014. For the other three adhesives considered in this study, failed specimens revealed substantial presence of adhesive on both parts of the joint indicating that the failure was predominantly cohesive in nature. In general the results indicate that the dynamic strength is considerably higher than the static strength for all adhesives and the adherent material does not influence the joint strength as long as the failure is cohesive failure of the adhesive. 5. Conclusions Adhesive joints with aluminum and GFRP as the adherents were prepared using four commercial adhesives namely Araldite 2014, Araldite 2011, Epibond 1590 and A/B Loctite 324. These joints were subjected to a short duration torque using a topsional split Hopkinson bar and their dynamic strength was determined. Both aluminum–aluminum and aluminum–GFRP joints were tested. For comparison the static strength was also determined. The results of the study indicated the following: – The dynamic strength was always higher than the static strength for all adhesives; however the amount of increase was different for different adhesives. – Among the adhesives considered, Epibond 1590 A/B exhibited the highest rate sensitivity whereas Loctite 324 had the lowest rate sensitivity.
– Whenever the failure was cohesive in nature, the combination of the adherents did not show any significant influence on the strength. – The findings of the study indicate that joints designed based on static strength will have a higher margin of safety against short duration dynamic loads.
Acknowledgements The authors acknowledge the support of Aeronautical Research and Development Board (Structures Panel) in funding this study through Project No. DARO/08/1051424/M/I. References [1] Harris JA, Adams RD. An assessment of the impact performance of bonded joints for use in high energy absorbing structures. Proc Inst Mech Eng 1985;199(C2):121–31. [2] Beevers A, Ellis MD. Impact behavior of bonded lap joints. Int J Adhes Adhes 1984;4(1):13–6. [3] Srivastava V, Shukla A, Parameswaran V. Experimental evaluation of dynamic shear strength of adhesively bonded lap joints. J Test Eval 2000;28(6):438–42. [4] Yokoyama T, Shimizu H. Evaluation of impact shear strength of adhesive joints with the split Hopkinson bar. JSME Int J, Ser A 2000;41(4):503–9. [5] Sato C, Ikegami K. Strength of adhesively-binded butt joints of tubes subjected to combined high-rate loads. J Adhes 1999;70(1):57–73. [6] Adamvalli M, Parameswaran V. Dynamic strength of adhesive lap joints at high temperature. Int J Adhes Adhes 2008;28:321–7. [7] Wu CML, Li RKL, Yeung NH. Impact resistance of SM joints formed with ICA. J Electron Packag 2002;124:374–8. [8] Goglio L, Rossetto M. Impact rupture of structural adhesive joints under different stress combinations. Int J Impact Eng 2008;35:635.
S.L. Raykhere et al. / Materials and Design 31 (2010) 2102–2109 [9] Vaidyaa UK, Gautam ARS, Hosur M, Dutta P. Experimental–numerical studies of transverse impact response of adhesively bonded lap joints in composite structures. Int J Adhes Adhes 2006;26:184–98. [10] Sato C. Dynamic stress responses at the edges of adhesive layers in lap strap joints of half-infinite length subjected to impact loads. Int J Adhes Adhes 2009;29:670–7. [11] Jin FL, Park SJ. Impact-strength improvement of epoxy resins reinforced with a biodegradable polymer. Mater Sci Eng 2008;A478:402–5. [12] Kumar P, Tiwari S, Singh RK. Characterization of toughened bonded interface against fracture and impact loads. Int J Adhes Adhes 2005;25:527–33. [13].
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[14]. [15] . [16] . [17] Maheri MR, Adams RD. Determination of dynamic shear modulus of structural adhesives in thick adherend shear test specimens. Int J Adhes Adhes 2002;22:119–27. [18] Da Silva LFM, Adams RD. Measurement of the mechanical properties of structural adhesives in tension and shear over a wide range of temperatures. J Adhes Sci Technol 2005;19(2):109–41.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Dynamic shear strength of adhesive joints made of metallic and composite adherents Sohan Lal Raykhere a, Prashant Kumar a, R.K. Singh b, Venkitanarayanan Parameswaran a,* a b
Department of Mechanical Engineering, Indian Institute of Technology, Kanpur 208 016, India Defense Materials Stores Research and Development Establishment, Kanpur 208 016, India
a r t i c l e
i n f o
Article history: Received 20 July 2009 Accepted 21 October 2009 Available online 25 October 2009
a b s t r a c t The use of adhesive joints has gained good acceptance in the automotive and aerospace industries in recent years, particularly for joining glass fiber reinforced plastics (GFRP) to metals. Such joints will be subjected to short duration dynamic loads in service. The present study focuses on the evaluation of the shear strength of adhesive joints prepared using four different commercial adhesives at loading rates in the range of 0.6–1.2 MPa/ls. The adhesives used were Araldite 2014, Araldite 2011, Epibond 1590 and A/B Loctite 324. Joints were prepared with two different adherent combinations; aluminum–aluminum and aluminum–GFRP. The results of the study indicated that, depending on the adhesive and adherent combination, the dynamic strength was 2–4 times the static strength. Among the four adhesives, Epibond 1590 exhibited the highest rate sensitivity whereas Loctite 324 exhibited the least. Further it was also observed that the dynamic strength was not sensitive to the adherent combination whenever the failure was predominantly within the adhesive layer. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Adhesive joints are extensively used in automotive and aerospace industry in recent years. Particularly with the wide spread use of glass fiber reinforced plastics (GFRP) in these sectors, joining of GFRP to metals is inevitable. Conventional techniques of joining like bolting and riveting has the disadvantage of creating localized damages in GFRP. Adhesives offer the best alternative to conventional joining methods in this context. Invariably such joints are subjected to dynamic loads during service either by design or by accident. Estimating the load carrying capacity of such joints under dynamic loading is therefore important to their successful use. Adhesive manufactures provide information only on the static lap strength of the adhesive for a given adherent combination. The strength of adhesive lap joints subjected to dynamic loading has been investigated by several researchers using different techniques. The study by Harris and Adams [1], on the impact strength of adhesive lap joints, determined using the instrumented impact technique, indicated that the joint strength is not significantly affected by loading rate for the adhesives studied. On the contrary, Beevers and Ellis [2] reported that lap joints made of steel adherents and an epoxy adhesive can withstand higher loads under impact conditions. Recently there has been several investigations on the use of the split Hopkinson bar to evaluate the strength of adhesive joints. The dynamic strength of a lap joints made of steel * Corresponding author. Tel.: +91 512 2597528; fax: +91 512 2597408. E-mail address: [email protected] (V. Parameswaran). 0261-3069/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2009.10.043
adherents and a general purpose epoxy adhesive was determined at various loading rates by Srivastava et al. [3] using the split Hopkinson pressure bar (SHPB) technique. A pin and collar type specimen was used in SHPB by Yokoyama and Shimizu [4] to determine the dynamic shear strength of adhesive lap joints. Sato and Ikegami [5] studied the strength of adhesively bonded butt joints under combined tension and torsion using the SHPB. Their results indicated that the failure of the butt joint under combined loading can be fitted with the von-Mises criteria. Adamvalli and Parameswaran [6] investigated the dynamic strength of adhesive lap joints at high temperatures using the SHPB technique. Wu et al. [7] evaluated the post impact strength of printed circuit boards (PCBs) in which the components were mounted using an isotropic conductive adhesive. The PCBs were first subjected to high strain rate bending using a SHPB at different strain rates. They reported that the post impact strength reduced with increasing bending strain rate. Goglio and Rossetto [8] have recently studied the behavior of lap joints subjected to impact loading using an instrumented Charpy pendulum. They have reported that the strength of the joint is higher under impact loading. They also observed that joints having thinner adhesive layer had higher strength. Apart from those listed above, there are also investigations addressing the response of adhesive joints subjected to impact loading. Vaidya et al. [9] reported an experimental cum numerical investigation on the transverse impact behavior of adhesively bonded lap joints. The lap joints in their study were subjected to transverse impact using a drop weight tester and a detailed stress analysis of the joint
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was carried out through finite element analysis. Sato [10] has outlined a theoretical approach to obtain the time variation of the stress distribution near the edge of the adhesive layer in a lap joint subjected to impact loading. The effect of modifying an epoxy adhesive with another biodegradable polymer on the impact strength was investigated by Jin and Park [11]. They evaluated the impact strength using an Izod impact tester. Their study indicated that addition of the biodegradable polymer improves the impact strength. In most of the studies reported above on evaluation of the adhesive joint strength [3–6], the adherents used were metals. In the present study, four different commercial adhesives were used to bond aluminum and GFRP and the dynamic strength of these joints were determined using a torsional split Hopkinson bar (TSHB). For comparison, the static strength was also determined. The joint geometry and preparation, testing method and the results are presented in the following sections.
the bond area, which was not desirable. The aluminum half of the joint was made of an alloy equivalent to 6101. The GFRP half was prepared in-house by stacking the required number of unidirectional prepregs in a [0/90]s configuration and then laminating them in a hot press, further details of which can be found in [12]. The static strength was also evaluated for the sake of comparison by testing single lap joints of geometry shown in Fig. 1b under compressive loading. 2.2. Adhesives used As mentioned earlier, the four different adhesives used in this study were Araldite 2011 [13], Araldite 2014 [14], Epibond 1590 A/B [15] and Loctite 324 [16]. The first three adhesives mentioned are two part adhesives manufactured by Huntsman Ltd., whereas Loctite 324, a fast curing adhesive, was manufactured by Henkel Technologies. The proportions of the two parts, physical properties and the curing details of these adhesives are provided in Table 1. Araldite 2011 is a general purpose adhesive whereas, Araldite 2014 and Epibond 1590 are recommended for aerospace applications. Loctite 324 is a specially formulated adhesive for toughness and impact strength. According to the manufacture’s catalogue these adhesives can be used for bonding similar or dissimilar adherents. Further details on each of the adhesives are available in the manufacturer’s catalogue [13–16].
2. Details of specimen, adhesives and bonding procedure 2.1. Specimen geometry The geometry of the adhesive butt joint used in this study to evaluate the shear strength is shown in Fig. 1. The joint had two parts. The aluminum half of the joint was in the form of a cylinder with a flange at one end as shown in Fig. 1a. The TSHB used in the study had bars of 25.5 mm diameter; hence the flange also had an outer diameter of 25.5 mm. In the case of aluminum–aluminum (henceforth referred to as AA) joint, the second half was an annulus with an inner diameter of 14 mm and an outer diameter of 25.5 mm. The bond area on which the adhesive was applied (shaded region in Fig. 1a) was 100 mm2. In the case of aluminum–GFRP (henceforth referred to as AG) joint, the GFRP half was a full disc of outer diameter 25.5 mm and 5 mm thickness. A full disc was used instead of an annulus for the AG joints as drilling an inner hole of 14 mm would induce damage in the GFRP within
2.3. Bonding procedure The surface preparation of the adherents was carried out as follows. The aluminum half of the joint was first degreased using acetone. Subsequently, the surface was roughened with a 200 grit paper. Then the roughened surface was cleaned with methanol. In the case of the GFRP half, the surfaces were roughened with 200 grit paper and then cleaned with methanol. The adherent halves were weighed before bonding using an electronic weighing
25.5
18
25.5
(a)
3
14
3 14
4
(b) 5
Adhesive 3
(c)
18 All dimensions in mm
(d)
Fig. 1. Geometry of (a) dynamic test specimens, (b) static test specimens and photographs of the assembled joint, (c) aluminum–aluminum joint and (d) aluminum–GFRP joint.
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Table 1 Properties, mix ratio and cure cycle for adhesives tested.a
a
Adhesive
Mixing ratio (A:B by wt)
Specific gravity
Mix-viscosity at 25 °C (Pa s)
Curing cycle
Araldite 2011 A/B Araldite 2014 A/B Epibond 1590 A/B Loctite 324
100:80
1.05
30–45
100:50
1.60
90
100:55
1.10
146
–
1.06
10–24
30 h at 25 °C 30 h at 25 °C 4 h at 60 °C 2 h at 25 °C
ness of 0.07 mm in the finally assembled joint. Since there was no specific recommendation on the adhesive thickness for Epibond 1590 A/B and Loctite 324, the weight for these two adhesives were also calculated corresponding to an adhesive layer thickness of 0.07 mm. An optical micrograph of an AA lap joint bonded with Araldite 2014 is shown in Fig. 2. It can be observed that the final adhesive layer thickness is about 100 lm. 3. Experimental details 3.1. TSHB set up
From manufactures data sheet.
Fig. 2. Optical micrograph of adhesive layer in an AA lap joint bonded with Araldite 2014 (each small division in scale shown in the micrograph is 100 lm).
scale of 1 mg resolution. Subsequently, the two parts of the adhesive (except Loctite 324), were mixed as suggested by the manufacturer in the appropriate ratio (see Table 1). A known weight of the mixture was then applied to the adherents and the joint was assembled and placed in a fixture to maintain alignment of the two halves. After the joint was cured, the extra adhesive was cleaned and the joint was again weighed to determine the weight of adhesive (w) actually present in the joint. For Araldite 2011 and Araldite 2014 the manufacturer recommended an adhesive layer thickness of 0.05–0.1 mm for maximum strength, whereas there was no such recommendation for the other two adhesives. Hence the weight of the adhesive that should be finally retained in the joint (w) was calculated targeting a nominal adhesive layer thick-
The schematic of the TSHB set up used in this study is shown in Fig. 3. The incident and transmitter bars had a diameter of 25.5 mm and were respectively 2500 mm and 1600 mm long. The bars were made of high strength aluminum alloy. A pair of ±45° strain rosettes were installed diametrically opposite to each other on each bar such that the sensing direction of the gages were at ±45° to the bar axis. The gages were connected in a full bridge configuration using an Ectron 530 strain amplifier and the strain signals were recorded using a NI PCI 6115 data acquisition card at 1 MHz sampling rate. The two faces of the specimen were bonded respectively to the incident and transmitter bar. At one of its ends (A in Fig. 3), the incident bar had a pulley with a dead-weight loading arrangement. About 440 mm away from the pulley (point B in Fig. 3), the incident bar was clamped using a locking device (shown in Fig. 3). A static torque of the required magnitude was stored in this portion (AB in Fig. 3) of the incident bar by torquing the pulley using dead-weights. This torque is released suddenly by snapping the bolt used to clamp the incident bar at location B. This way a torsional pulse was produced in the incident bar. This pulse, called the incident pulse, travels towards the specimen and on reaching the specimen, a part of it is reflected back into the incident bar and the rest is transmitted through the specimen into the transmitted bar. The incident, reflected and transmitted pulses are recorded through the full bridge gage arrangement for further analysis. 3.2. Data analysis Using one-dimensional theory of wave propagation, the torque history at the two ends of the specimen T1(t) and T2(t) can be obtained from the recoded shear strains as follows:
T 1 ðtÞ ¼ 2GJ fcI ðtÞ þ cR ðtÞg D T 2 ðtÞ ¼ 2GJ fcT ðtÞg D
where cI ðtÞ; cR ðtÞ and cT ðtÞ are respectively the incident, reflected and transmitted engineering shear strains recorded using the strain
Locking device Incident bar
Pulley
Specimen
Transmitter bar
B A C
Weights
ð1Þ
Stain rosette Back pressure device
Fig. 3. Schematic of the TSHB used for determining the dynamic strength of adhesives.
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rosettes and D, G and J are respectively the diameter, modulus of rigidity and polar moment of inertia of the bar. It is important to note that, for an accurate evaluation of the strength of the specimen, the condition of dynamic equilibrium (T1 = T2) should be reached before failure. Once this has been attained, the shear strength of the joint is calculated from the maximum transmitted torque, T2MAX, as follows:
ss ¼
16Do T 2MAX
ð2Þ
pðD4o D4i Þ
where Do and Di are the outer and inner diameter of the bond area shown in Fig. 1. 3.3. Testing procedure The specimen to be tested was attached to the incident and transmitter bars by bonding the specimen faces to the bars using either Loctite 324 or Araldite 2011. The area of this bond was three times larger than the area of the bond in the joint. By this way it was ensured that the failure always occurs in the joint and not at the interface with the bars. After curing, the joint was bonded to the incident and transmitter bars and a light clamping pressure was applied through the back pressure devise shown in Fig. 3. Once the bond between the specimen and the bar has cured (2 h for Loctite 324 and 30 h for Araldite 2011), the incident bar was clamped using the notched bolt (see Fig. 3). The portion AB of the incident
bar was then torqued to the required level by adding dead-weights. The notched bolt was then tightened until it breaks. The strain data was recorded and analyzed as mentioned earlier. The static strength was determined by subjecting single lap joints (see Fig. 1b) to compressive loading in a universal testing machine. The test was conducted at a temperature of 26 °C and a cross head speed of 1 mm/min. It should be noted here that the shear stress in the adhesive layer in the static test is induced by subjecting the joint to a compressive loading whereas in the dynamic loading the shear stress is induced by pure torsion. It is in general understood that, in the case of single lap joints, the state of stress in the adhesive layer may be neither pure shear nor uniform in nature. However, when the adherents are thick as in the present case, the state of stress will be dominantly uniform shear and the lap strength is also a good estimate of the adhesive strength [17,18]. Hence even though there is a change of geometry and loading (compression versus torsion) between the static and dynamic tests, in both cases the state of stress in the adhesive layer will be dominantly shear and therefore the results should be comparable. Further discussion on this will be given in the next section. 4. Results and discussion Typical incident, reflected and transmitted signals for two cases, an AA joint bonded with Araldite 2011 and an AG joint bonded with Epibond 1590 A/B, are shown in Fig. 4. It can be seen that
35
1.2 0.9
Incident
25 Torque (Nm)
Signal (V)
0.6
30
Transmitted
0.3 0.0 -0.3
20
T1
15 10
-0.6
T2 5
-0.9
Reflected 0
-1.2 0
200
400
600
800
1000
0
1200
50
100
150
200
100 150 Time (µ s)
200
Time (µs)
Time (µ s)
(a) Aluminum-Aluminum joint bonded with Araldite 2011 1.2
35
0.9
Incident
30
Transmitted
T1
25
0.3
Torque (Nm)
Signav (V)
0.6
0.0 -0.3 -0.6
20
T2 15 10
Reflected
-0.9
5
-1.2
0
0
200
400
600 800 Time (µ s)
1000
1200
0
50
(b) Aluminum-GFRP joint bonded with Epibond 1590 A/B Fig. 4. Recorded signals and torque histories for AA and AG joint.
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the incident pulse has a duration of about 330 ls. Also shown in Fig. 4 are the time histories of the torques, T1 and T2 at the two ends of the joint calculated using Eq. (1). As shown in Fig. 4, T1 and T2 are nearly equal throughout the loading, and reaches maximum values in about 50–60 ls. This confirms that the joint attains dynamic equilibrium before failure. The shear strength of the adhesive layer was calculated from the maximum transmitted torque using Eq. (2) and the loading rate was determined by dividing the maximum shear stress by the time to reach T2MAX. For each adherent combination (AA and AG) and adhesive, at least five tests were conducted and the average strength along with the 95% confidence level was
Table 2 Static and Dynamic shear strength of adhesives.
A
Adhesive
Joint type
Static strength (MPa)
Dynamic strength (MPa)
Loading rate (MPa/l s)
Sensitivity
Araldite 2011
AA AG
15.6 ± 2.7 13.2 ± 1.6
44.0 ± 2.8 46.2 ± 2.6
1.1 ± 0.1 1.1 ± 0.0
2.8 3.5
Araldite 2014
AA
51.1 ± 3.9 (50.5)a 39.7 ± 2.1
1.2 ± 0.2 (1.33)a 0.8 ± 0.1
2.4
AG
21.6 ± 1.1 (22.0)a 21.3 ± 1.0
Epibond 1590 A/B
AA AG
13.1 ± 1.1 11.9 ± 2.4
38.9 ± 1.6 43.6 ± 2.5
1.0 ± 0.2 0.8 ± 0.1
3.0 3.7
Loctite 324
AA AG
18.6 ± 1.1 17.0 ± 1.8
32.9 ± 2.6 31.9 ± 1.9
0.6 ± 0.2 0.7 ± 0.2
1.8 1.9
1.7
Values in parenthesis are that from Ref. [6] for titanium lap joints.
obtained for each case. The condition of attaining equilibrium was also verified in all cases. The results of the study are presented in Table 2. The static strength values are also provided in Table 2 for comparison. The loading rate in the case of static testing was 1 MPa/s. As pointed out earlier in Section 3.3, there is a geometry difference as well as a difference in the way the shear stress is induced in the adhesive between the static and dynamic tests. In order to establish that the geometry and type of loading does not affect the strength obtained, a comparison is made between the results shown in Table 2 and that reported in our earlier investigation [6] (also shown in parenthesis in Table 2) for the adhesive Araldite 2014. In Ref. [6], the static strength of the lap joint with titanium adherents is reported as 22 MPa and the reported failure is cohesive in nature. In the present study, the static strength obtained with aluminum adherents for the same adhesive is 21.3 MPa which is in good agreement with that reported in [6] and this should be expected as the failure in the both cases is cohesive in nature. Further the dynamic strength reported in [6], obtained using the lap joint specimen in a compression SHPB, for Araldite 2014 is 50.5 MPa at a loading rate of 1.33 MPa/ls, whereas that given in Table 2, obtained using the butt joint geometry subjected to torsion in a TSHB, is 51.2 at a loading rate of 1.2 MPa/ls. This good agreement confirms that geometry and loading configuration does not bring in any additional effects. It can be observed from Table 2 that the dynamic strength is higher than the static strength uniformly for all adhesives and adherent combinations. The adherent combination (AA or AG) does not seem to have a significant effect on the static strength of the
Fig. 5. Photographs of specimen halves after dynamic test for Araldite 2014.
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adhesive. However the same is not true for the dynamic strength. In the case of Araldite 2014, the dynamic strength of an AG joint is 25% lower than that for an AA joint. Whereas, for Epibond 1590 A/B the strength of AG joint is about 12% higher than that of an AA joint. The adherent combination does not seem to have an influence on the dynamic strength for Araldite 2011 and Loctite 324. The ratio of the dynamic strength to static strength is a measure of the rate sensitivity. It can be observed from Table 2 that Epibond 1590 A/B has the highest rate sensitivity whereas Loctite 324 has the lowest. In order to understand the reduction in dynamic strength for AG joints when compared to AA joints, the specimen halves were inspected using a Leica S8 APO stereo microscope fitted with a 2 Mega pixel monochrome camera. Fig. 5 shows the photograph of the specimen surfaces without any magnification. The glossy portion of the aluminum half was masked with black non glossy paper to prevent reflections and improve clarity. It can be observed from Fig. 5, that the adhesive is uniformly present in both halves in the case of AA joint, except for a few locations where the adhesive did not wet either halves. In the case of the AG joint, shown in Fig. 5, the appearance of the aluminum half is similar to that in the AA joint. The GFRP half also shows a layer of adhesive, however, the adhesive layer in the GFRP half looks thinner and less uniformly
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spread over the GFRP. The photographs of the AA and AG specimens at a higher magnification are shown in Fig. 6. It can be observed from Fig. 6 that, in the case of AA joint, the adhesive layer present on both halves appears to be uniform and equally rich on both halves. There are few small regions (less than 100 lm) in both halves of the specimen, where the adhesive has not wetted the adherents. Despite these spots, it is clear that the failure is primarily within the adhesive layer or cohesive in nature. In the case of AG joint, shown in Fig. 6, the adhesive layer on the aluminum half is similar to that for the AA joint. However, the adhesive layer on the GFRP half is less rich than that in the aluminum half. Further, at several locations, the GFRP adherent is visible indicating that at these locations, the adhesive layer has separated from the GFRP. The relatively brighter locations indicate regions of adhesive failure. Thus the failure is mixed in nature and we believe this to be the reason for the reduction in the dynamic strength. To further reinforce this observation, the photographs of specimen halves of an AA and an AG lap joint after static test are shown in Fig. 7. In the case of AA joint the adhesive layer on the two halves appears uniform and identical in texture. Few small regions where the adhesive peeled off from one side are also visible in Fig. 7 for the AA joints. The adhesive layer on the aluminum and GFRP halves of the AG joint shown in Fig. 7, look similar to each other. Further
Fig. 6. Optical micrographs of specimen halves after dynamic test for Araldite 2014.
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Fig. 7. Optical micrographs of specimen halves after static test for Araldite 2014.
the appearance is also similar to that in the AA joint, indicating failure entirely within the adhesive layer. As indicated in Table 2, the static strength for AA and AG joints was the same in the case of Araldite 2014. For the other three adhesives considered in this study, failed specimens revealed substantial presence of adhesive on both parts of the joint indicating that the failure was predominantly cohesive in nature. In general the results indicate that the dynamic strength is considerably higher than the static strength for all adhesives and the adherent material does not influence the joint strength as long as the failure is cohesive failure of the adhesive. 5. Conclusions Adhesive joints with aluminum and GFRP as the adherents were prepared using four commercial adhesives namely Araldite 2014, Araldite 2011, Epibond 1590 and A/B Loctite 324. These joints were subjected to a short duration torque using a topsional split Hopkinson bar and their dynamic strength was determined. Both aluminum–aluminum and aluminum–GFRP joints were tested. For comparison the static strength was also determined. The results of the study indicated the following: – The dynamic strength was always higher than the static strength for all adhesives; however the amount of increase was different for different adhesives. – Among the adhesives considered, Epibond 1590 A/B exhibited the highest rate sensitivity whereas Loctite 324 had the lowest rate sensitivity.
– Whenever the failure was cohesive in nature, the combination of the adherents did not show any significant influence on the strength. – The findings of the study indicate that joints designed based on static strength will have a higher margin of safety against short duration dynamic loads.
Acknowledgements The authors acknowledge the support of Aeronautical Research and Development Board (Structures Panel) in funding this study through Project No. DARO/08/1051424/M/I. References [1] Harris JA, Adams RD. An assessment of the impact performance of bonded joints for use in high energy absorbing structures. Proc Inst Mech Eng 1985;199(C2):121–31. [2] Beevers A, Ellis MD. Impact behavior of bonded lap joints. Int J Adhes Adhes 1984;4(1):13–6. [3] Srivastava V, Shukla A, Parameswaran V. Experimental evaluation of dynamic shear strength of adhesively bonded lap joints. J Test Eval 2000;28(6):438–42. [4] Yokoyama T, Shimizu H. Evaluation of impact shear strength of adhesive joints with the split Hopkinson bar. JSME Int J, Ser A 2000;41(4):503–9. [5] Sato C, Ikegami K. Strength of adhesively-binded butt joints of tubes subjected to combined high-rate loads. J Adhes 1999;70(1):57–73. [6] Adamvalli M, Parameswaran V. Dynamic strength of adhesive lap joints at high temperature. Int J Adhes Adhes 2008;28:321–7. [7] Wu CML, Li RKL, Yeung NH. Impact resistance of SM joints formed with ICA. J Electron Packag 2002;124:374–8. [8] Goglio L, Rossetto M. Impact rupture of structural adhesive joints under different stress combinations. Int J Impact Eng 2008;35:635.
S.L. Raykhere et al. / Materials and Design 31 (2010) 2102–2109 [9] Vaidyaa UK, Gautam ARS, Hosur M, Dutta P. Experimental–numerical studies of transverse impact response of adhesively bonded lap joints in composite structures. Int J Adhes Adhes 2006;26:184–98. [10] Sato C. Dynamic stress responses at the edges of adhesive layers in lap strap joints of half-infinite length subjected to impact loads. Int J Adhes Adhes 2009;29:670–7. [11] Jin FL, Park SJ. Impact-strength improvement of epoxy resins reinforced with a biodegradable polymer. Mater Sci Eng 2008;A478:402–5. [12] Kumar P, Tiwari S, Singh RK. Characterization of toughened bonded interface against fracture and impact loads. Int J Adhes Adhes 2005;25:527–33. [13]
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[14]