Fracture toughness of adherends bonded with two-part acrylic-based adhesive: double cantilever beam tests under static loading
© The Author(s) 2016
Received: 9 May 2016
Accepted: 22 June 2016
Published: 29 June 2016
Adhesives are used in various industries to bond materials. The failure criteria of adhesive joints are based on the strength (peel) and fracture mechanisms of the materials. It is important to investigate these criteria in relation to the propagation and separation of Mode I, II, and III cracks. The purpose of this study is to use double cantilever beam (DCB) tests to measure fracture toughness in aluminum alloy (5052-H34), glass fiber-reinforced polypropylene matrix composite, and carbon fiber-reinforced epoxy matrix composite adherends bonded with a two-part acrylic-based adhesive. The fracture behaviors of the specimens are also discussed. DCB tests are carried out to measure fracture toughness under Mode I loading of adhesively bonded joints with different types of adherends. The fracture toughnesses of the aluminum alloy, glass-fiber-reinforced polypropylene matrix composite (GF/PP), and carbon fiber-reinforced epoxy matrix composite (CF/EP) specimens are 1071, 1438, and 1652 Jm−2, respectively. The fracture surfaces of the aluminum alloy, GF/PP, and CF/EP specimens are observed to be of the interfacial, adherend, and cohesive types, respectively.
Adhesives are used in various industries to bond materials. The drawbacks of using adhesives include relatively low resistance to high temperatures and problems with quality assurance . However, the advantages include a continuous bond, lower process temperatures, and the ability to join several components together in a single operation .
Adhesively bonded lap joints are commonly studied to measure the shear strength of adhesive joints [3–5]. These joints produce tensile stresses (peel) and shear at their ends . The failure criteria of adhesive joints are based on the strength (peel) and fracture mechanisms of the materials . It is important to investigate these criteria in relation to the propagation and separation of Mode I, II, and III cracks. In terms of the fracture mechanisms, double cantilever beam (DCB) tests are commonly used in Mode I to measure the fracture toughness of adhesive joints [8, 9]. The advantages of using this test include its relatively simple method and the facility to measure the fracture toughness by using beam theory [10, 11]. DCB tests of steel and fiber-reinforced plastic (FRP) adherends bonded with adhesives (e.g., polyurethane, epoxy, silicone, phenol–formaldehyde, and methacrylate) have been conducted to measure fracture toughness [12–15].
A few detailed reports have been made about the fracture behavior of adherends bonded with acrylic-based adhesive under Mode I loading [16–18]. These describe the effects of surface treatments on Mode I fracture energy using FRP and aluminum alloy adherends bonded with a toughened acrylic adhesive. The results show that fracture toughness is enhanced by applying surface treatments such as grit blast and γ-methacryloxypropyltrimethoxysilane (γ-MPS). Adherends also need to be considered not only to determine the designs of structures but also to deal with various materials such as metal/metal, thermoset, and thermoplastic matrices.
The purpose of this study is to measure fracture toughness by means of DCB tests on aluminum alloy, glass/polypropylene, and carbon/epoxy adherends bonded with an acrylic-based adhesive. The fracture behaviors of the specimens are also discussed.
An aluminum alloy (5052-H34, average thickness = 3.0 mm), a GF/PP matrix composite (Tepex dynalite 104-RG600(6)/47 %, average thickness = 3.0 mm, produced by Bond Laminates, Germany), and a CF/EP matrix composite (F6343B-05P (0/90)14, average thickness = 3.3 mm, produced by Toray Industries Inc., Japan) were used for the adherends. A two-part acrylic-based adhesive (3 M™ Scotch-Weld™ Structural Plastic Adhesive DP8005, Japan) was used to bond the adherends. The advantages of this adhesive include its ability to bond adherends and polyolefins and its good resistance to water, humidity, and chemicals; moreover, it does not require pretreatment of the adherends and is a solvent-free adhesive system.
Bond-line thicknesses measured on the DCB specimens
Standard deviation (SD) (mm)
An edge surface of the specimen was coated with white spray to highlight the crack tip. Perpendicular lines were drawn at 10 mm intervals on the edge of the DCB specimens.
The DCB tests were conducted based on ASTM D3433  using a tensile test machine (Compact Table-Top Universal/Tensile Tester, EZ-S, produced by the Shimadzu Corporation, Japan) at a crosshead speed of 0.5 mm/min. The crack length was measured through a microscope (VH-ZST Swing-head Zoom Lens, VHX-5000 Digital Microscope, produced by the Keyence Corporation, Japan). To introduce an artificial crack, the DCB specimens were loaded until the crack length reached approximately 70 mm, at which point the load was removed. This initial load–displacement curve was not used to measure the crack extension resistance. The numbers of specimens that were tested were three, four, and three for aluminum alloy, GF/PP, and CF/EP, respectively.
Macroscopic and microscopic methods of fracture-surface observation
A digital camera (IXY Digital 70, produced by Canon Inc., Japan) was used for macroscopic analysis of the entire area of each fracture surface. A microscope (see “DCB tests” section) was used as a means of microscopic analysis of the fracture surfaces (enlargement scale: 20× to 1000×).
Results and discussion
Load–displacement curves and maximum load versus crack length
Results of average initiation, rest state, and G m −G i fracture toughness and crack propagation length for the DCB tests
Fracture toughness (Jm−2)
Initiation fracture toughness (Jm−2)
Rest-state fracture toughness (Jm−2)
Difference between maximum and initiation fracture toughness (Jm−2)
Crack propagation length (mm)
G max = G IC
G m −G i
a m −a i
a r −a m
a r −a i
Fracture surface observation
The aluminum alloy specimens had the lowest fracture toughness because of a lack of bonding between the aluminum alloy and the adhesive. The highest fracture toughness was obtained from the CF/EP specimens. This is because the constituent materials (i.e., carbon fiber, epoxy, and adhesive) were bonded together well, as compared to the GF/PP specimens in which poorer bonding was observed between the glass fibers and PP. The results of the DCB tests under Mode I static loading showed a correlation between the fracture toughness and fracture morphology of the specimens, i.e., the values of fracture toughness (fracture morphology) can be ranked as aluminum alloy (interfacial) <GF/PP (adherends) <CF/EP (cohesive).
DCB tests were conducted to measure fracture toughness in different types of adherends bonded with a two-part acrylic-based adhesive, i.e., aluminum alloy, GF/PP, and CF/EP. The values of fracture toughness for the aluminum alloy, GF/PP, and CF/EP specimens were 1071, 1438, and 1652 Jm−2, respectively. It was confirmed by investigating the fracture surfaces that the fractures of the aluminum alloy specimens occurred mainly between the aluminum alloy and the adhesive. Relatively poor bonding between the GFs and PP caused delamination of the adherends in the GF/PP specimens. The fracture surfaces of the CF/EP specimens were cohesive failures. It can be concluded that the fracture toughness of the specimens is closely related to the fracture morphology of the fracture surfaces. Future work includes an investigation of proper surface treatments and modification of the adhesive to improve the fracture toughness.
HK, KN, and HO made the specimens. KN prepared the experimental setup. HK conducted the experiments. KN and HO supported for HK to write the paper. All authors read and approved the final manuscript.
This paper is based on results obtained from a future pioneering project commissioned by the New Energy and Industrial Technology Development Organization (NEDO).
The authors declare that they have no competing interests.
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