Experimental investigation of mode I fracture energy of adhesively bonded joints under impact loading conditions
© The Author(s) 2017
Received: 10 September 2016
Accepted: 22 February 2017
Published: 28 February 2017
Double cantilever beam (DCB) tests under impact loading conditions were conducted using a falling-wedge impact test machine and a high-speed camera. The change in mode I fracture energy G IC was investigated in comparison with the results obtained under the quasi-static loading condition. Two types of adhesives with significantly different mechanical properties were used for the DCB tests, and the change in rate dependency of the adhesive types was observed. Adhesively bonded joints have been widely used in various engineering products, such as automobiles, ships and airplanes. The strength of the joints is important for product safety. To evaluate the mode I fracture energy of adhesively bonded joints, DCB tests have been standardized under the quasi-static loading condition. Additionally, several tests have been proposed to evaluate the impact resistance of the joints. However, impact loading makes it difficult to evaluate the fracture energy accurately because of the dynamic effects. Therefore, specialized evaluation methods for dynamic fracture must be considered, and a load-independent analysis of the fracture energy was used to avoid load measurement problems due to the dynamic effects in this study.
Recently, the automotive industry has paid attention to multi-material structures to reduce CO2 emission. Hence, the use of light-weight and high-strength materials such as Al–Mg alloys, high-tensile-strength steel and carbon-fiber-reinforced plastics (CFRP) is essential to lighten the vehicle weight . Joining methods for these dissimilar materials are the key to apply them into the structural part of the vehicles in combination. When the CFRP is jointed with other materials, welding, which is one of the most standard joining methods for vehicles, cannot be applied, and other methods must be considered. When dissimilar materials are jointed, thermal deformation at the joints often becomes a problem because of the difference in thermal coefficient of expansion, and reduction methods must be considered. Adhesive bonding can be applied to the joint among various types of materials to overcome these issues, as well as absorb vibration, prevent electrolytic corrosion, seal clearances, etc. Therefore, adhesive bonding can be one of the leading joining methods for the structural part of vehicles.
The fracture energy of adhesive joints can be experimentally measured using the double cantilever beam (DCB) test method for mode I fracture . However, this test is only standardized under the quasi-static condition. Conversely, the strength of the joint at high loading rates is an important issue for the safety of vehicles. Block impact tests  were conducted for the fracture strength in shear, whereas impact wedge-peel tests  and DCB tests with a hydraulic tensile test machine  were conducted for the mode I fracture energy in a wide range of loading speed. These methods have difficulty in measuring the load because the dynamic effects are significant. Additionally, an asymmetric fracture is observed when one side of the DCB specimen is pulled at a high speed . To measure the mode I fracture energy under the impact loading condition in another manner, a falling-wedge impact test machine using a DCB specimen was proposed by Xu et al. . An opening displacement and a crack length are measured with a high-speed camera. With a load-independent analysis, the fracture energy can be calculated without measuring the load. Therefore, the difficulty of the G IC measurement because of the dynamic effects can be avoided. Additionally, the specimen symmetrically fractures because of the symmetrical wedge shape. The change in G IC with the testing temperature has been investigated using three different epoxy adhesives in Ref. .
In this study, two types of adhesives are used to fabricate the DCB test specimens: a two-component type epoxy adhesive and a single-component type polyurethane adhesive. From the viscoelastic viewpoint, the glass transition temperature T g can be a key factor for the rate dependency of G IC of the adhesively bonded joints [6, 7]. Because T g of standard epoxy adhesives is much higher than the room temperature, the standard epoxy adhesives tend to be brittle. Hence, lower rate dependence is expected in the impact tests at room temperature. Although the polyurethane adhesives designed for structural usage can absorb much more energy in fracture than the epoxy adhesives , high rate dependence is expected because of the lower T g and ductile behavior. The effect of the loading rate on the G IC values for brittle and ductile adhesives is experimentally investigated with the falling-wedge impact testing machine.
Double cantilever beam test specimens
Mechanical properties of the epoxy and polyurethane adhesive
Failure strain (%)
Approximately 6 MPa
Double cantilever beam test
Results and discussion
Calculation of the fracture energy
We denote it as the load-independent method (LIM). Although the crack length correction cannot be used without measuring the load, it only depends on the elastic properties of the adherend . Therefore, the calculated value of |Δ| from the average value in the quasi-static tests was applied to calculate the impact test results.
Experimental results and discussion
Additionally, the calculation of G IC without measuring the load ensures a profitable discussion of the rate dependency of mode I fracture. Symmetrical fractures of the DCB specimens were obtained with the falling-wedge impact test machine at the opening speed of approximately 1.8 m/s. Although a taller machine is required for the tests with faster opening speeds, this test method has a high potential for impact DCB tests with various types of adhesives.
In this study, DCB tests under the impact loading condition were conducted to evaluate the fracture energy G IC in comparison with the results of the quasi-static condition. In the impact tests of the DCB specimens, a falling-wedge impact test machine was used to propagate the crack, and a high-speed camera was used to measure the opening displacement and crack length. The experimental results confirm that the fracture energy of the epoxy adhesive is independent from the strain rate. In contrast, the fracture energy of the polyurethane adhesive with impact loading is approximately 1.5 times larger than that with the quasi-static loading. Therefore, the experiment indicates that the fracture energy of the ductile adhesive is more likely affected by the loading condition than that of the brittle adhesive. In addition, the falling-wedge impact test machine can be used to evaluate the rate dependence in a wide range of fracture energy, including the structural adhesives.
YY performed the experiments and wrote the manuscript. XL performed the experiment for the polyurethane adhesive under impact loading. YS and CS helped to write the manuscript. All authors read and approved the final manuscript.
This paper is based on the results from a future pioneering program commissioned by the New Energy and Industrial Technology Development Organization (NEDO). The authors would like to express their sincere appreciation to the project members who have provided valuable information and participated in useful discussions. They also wish to thank Sunstar Engineering Inc. (Japan) for the material supply.
The authors declare that have no competing interests.
New Energy and Industrial Technology Development Organization.
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