Impact strength of joints bonded with high-strength pressure-sensitive adhesive

https://doi.org/10.1016/j.ijadhadh.2014.09.005Get rights and content

Abstract

The impact strength of joints bonded with a double-coated high-strength pressure-sensitive adhesive (PSA) was experimentally investigated. PSA has recently been used to join parts of mobile devices such as smart-phones, which are often subjected to drop impacts. Consequently, the impact strength of PSA bonded joints has become important.

Two types of specimens, butt joint specimens and double cantilever beam (DCB) specimens bonded with adhesives were utilized for the experiments. Quasi-static tests and impact tests of the specimens were carried out using a mechanical testing machine and an impact testing machine. The PSA layers in the specimens were observed using a high-speed digital camera. The deformation and strain distribution in the adherends of the DCB specimens were also measured using a novel high-speed digital camera with photoelastic imaging capability.

Though the strength of the butt joints increased as the loading rate increased, the critical fracture energy of the DCB specimens decreased at high loading rates. This may be attributed to the transition to the brittle nature of the PSA in the loading range in which no cavitation occurred. To verify the critical fracture energy obtained with the DCB tests, finite element analyses (FEA) based on the cohesive zone model (CZM) were carried out, and the load–displacement curves of the DCB tests were simulated. The predicted results showed good agreements with the experimental results.

Introduction

The use of adhesively bonded joints has recently been expanding in many applications. For instance, instead of mechanical fastening, adhesives are used for the assembly of electronic devices such as mobile phones owing to cost and space constraints. However, an ordinary adhesive needs curing time, which leads to poor production efficiency. Pressure-sensitive adhesives (PSAs) have been used for certain applications instead of chemically curing adhesives because they do not need curing time. PSA tapes have other advantages, i.e., it is easy to keep the thickness of the adhesive layer in a joint even, and they can be provided as die-cut shapes for easier application.

One of the important requirements for mobile equipment is sufficient strength for the impact loads caused by items being dropped to the floor. However, the impact strength of joints bonded with PSAs has not yet been investigated well. Only the peel strength of PSA tapes in static or intermediate strain-rate ranges has been measured [1], [2], [3], [4]. In addition, it has been reported that the delaminating process of PSAs is complicated, and cavitation and fibrillation in the PSA layers occurred in probe-tack tests during a quasi-static loading process [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. However, the dynamic characteristics of PSAs under high strain-rate conditions are still not clear, and methods for predicting the impact strength of PSA joints have not been established well thus far. Consequently, an empirical approach based on the impact tests of actual mobile devices is widely used for the actual design, though computer aided engineering (CAE) is essential for accelerating the design process.

In this research, the strength of PSA joints is investigated changing the strain-rate of applied loads from quasi-static to impacts. Using butt joint specimens and double cantilever beam (DCB) specimens bonded with PSA, the strength and critical fracture energies are measured, and their dependency on the strain-rate is clarified. Based on the experimental results, the traction and separation rules of the PSAs were determined and applied to finite element analyses (FEA) based on the cohesive zone model (CZM), by which the load displacements curves of the DCB tests were simulated in order to verify the critical fracture energies obtained experimentally.

Section snippets

Strengths of butt joints bonded with a PSA

In this research, butt joint specimens bonded with a type of PSA were tested to measure the tensile strength of the joints under quasi-static, intermediate, and impact conditions. The delamination process of the joints was observed using a high-speed digital camera. For impact tests of adhesively bonded joints, four types of experimental set-up have mainly been used such as the split Hopkinson bar [16], [17], [18], [19], [20], drop-weight testing machines [21], [22], [23], [24], [25], [26], [27]

Results of butt joint tests

Images on the PSA layer in a butt joint specimen subjected to quasi-static load are shown in Fig. 7. Voids were generated at an early stage of loading, and the diameter increased with increasing load. This phenomenon is called “cavitation”. It is thought that the voids are generated as a result of negative hydrostatic stress in the PSA layer. If the Poisson’s shrinkage of the PSA layer is suppressed by the constraint of the adherends, negative hydrostatic stress occurs. When the negative

Analytical simulation of strength of DCB specimens

Prediction of load–displacement curves for the DCB specimens was carried out using FEA. Recently, the cohesive zone model (CZM) has frequently been utilized for the strength prediction of joints bonded with ductile adhesives [38], [39], [40], [41], [42], [43], [44], [45], [46]. In this research, ABAQUS ver.6.8 (Dassault Systems Simulia Corp.), in which a type of CZM element was implemented, was used for two-dimensional static analysis of the DCB specimen having PSA layers of 50 or 100 μm

Discussion

As shown in Fig. 18, the critical fracture energy of DCB specimens having a 100 μm thick PSA layer exhibits an interesting phenomenon in impact conditions, namely, that the specimens can be categorized into two groups of high and low critical fracture energy. The low critical fracture energy makes sense because the PSA became brittle due to the strain rate increase, but the increase of critical fracture energy is unusual.

Fig. 26 shows the images of the DCB specimens identified as specimens 1 and

Conclusion

In this research, the strength and critical fracture energy of joints bonded with a double-side pressure-sensitive adhesive were experimentally investigated. Through butt joint tests, it was confirmed that the strength of the joints depended on the strain rate. The strength increased with increasing loading velocity. In contrast, the mode I critical fracture energy measured by DCB tests had a peak around intermediate loading velocity and decreased in the range of impact loading because the PSA

Acknowledgments

We gratefully acknowledge tesa tape K.K. and tesa SE for providing us with materials and information.

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