Search

Optical components are often adjusted to maintain their alignment along the optical axis. They are fixed at their optimal positions particularly using ultraviolet (UV) ray-curable adhesives. To ach...

The plastic zone at the crack front of an adhesively bonded double cantilever beam (DCB) specimen is analytically expressed considering the deformation of the adhesive layer. The plastic zone lengt...

Although adhesive-adherend interfaces play an important role in improving adhesive joint strength, their observation is a challenging task. If the adhesive and adherend can be clearly separ...

A method to prevent substrate damage in a double cantilever beam (DCB) test was experimentally investigated by changing bond-line width. Highly toughened adhesives are developed due to an increase in the demand for structural adhesives. Furthermore, structural materials that are lightweight and excellent in mechanical properties but have limitations in thickness are developed. In contrast, plastic deformation of the substrates is expected when the DCB test is performed with conventional DCB specimens using the aforementioned adhesives and materials. The reduction in the maximum stress on the substrate surface by narrowing a bond-line width is a practical method to prevent plastic deformation when a substrate possesses limited thickness. In this study, the influence of the bond-line width on an adhesive fracture energy in mode I was experimentally investigated by manufacturing DCB specimens in which the bond-line width is narrower than the substrate width. Additionally, the maximum bond-line width ...

A method for fabricating adhesively bonded joints with graded properties along the bond-line was proposed. By utilizing two types of acrylic adhesives, one hard and the other soft, the properties of the mixture were changed by controlling the mixing ratio. An applicator for the adhesives was developed and used for the experiments, in which property-graded adhesive layers were actually produced. The properties, i.e., stress-strain curves at various points of the property-graded adhesive layer, were experimentally measured, and the validity of the proposed method was confirmed. The Young’s moduli at multiple points of the adhesive layer changed from 34.7 MPa to 634 MPa by changing the mixing ratio of the adhesives.

Understanding the changes in the adhesive properties with the temperature and loading rate is important, particularly for designing adhesively bonded joints in structural applications. The viscoelastic behavior of adhesives is widely known to be based on the time–temperature superposition principle. In the case of the elastoplastic behavior, the stress–strain relationship is significantly affected by the external conditions, particularly for ductile adhesives. Similarly, the fracture behavior is expected to exhibit a temperature–loading rate dependence; however, few studies have focused on them, especially for structural acrylic adhesives.In this study, the impact of the temperature and test speed on the fracture energy of structural acrylic adhesives was investigated. Bulk tensile tests and double cantilever beam (DCB) tests were performed at different temperatures and test speeds. Bulk behavior showed an increase in the maximum stress and a decrease in the elongation at lower temperatures and higher loading rates. Conversely, the fracture toughness increased with a combination of higher temperatures and loading rates when the crack stably propagated. The mode I fracture energies of the test speed of 500 mm/min at 60°C increased by approximately 1.5–2 times that of 0.05 mm/min at room temperature. In addition, stick-slip crack propagation was observed at lower temperatures or higher loading rates.

The expansion characteristics of thermally expandable microcapsules (TEMs) under hydrostatic pressure or in resin were experimentally investigated. For the experiments, the expansion of the microcapsules was observed in high-pressure nitrogen at high temperature utilizing optical microscopy with a digital camera installed. The TEMs used for the experiments were expanded by heating under hydrostatic pressure up to 3 MPa, but the expansion degree decreased with increasing pressure. A cured bulk specimen of epoxy resin containing the microcapsules was made, and the expansion of the microcapsules was again observed with the microscope. It was found that the expansion of the microcapsules in the resin was saturated at a certain temperature. The stress distribution in the resin produced by the expansion of the microcapsules was calculated by the finite-element method. It was found that normal stress occurred, but it was mainly compressive. Tensile stress was also generated, although the maximum value wa...

The separation characteristics of an electrically dismantlable adhesive, ElectRelease, were experimentally investigated. The adhesive has a strength that decreases by applying voltage. The residual strengths of cylindrical butt joints bonded with the adhesive were measured after voltage application. Various experiments were carried out in order to investigate the influences of the bonding area and the type of adherend materials on the residual strength by changing the dimensions and material of the adherends. The influence of constant voltage and current applications was also examined. As a result, it was confirmed that the residual strength of a joint does not depend significantly on the bonding area but on the type of adherend material. The residual strength was able to be determined and predicted by using an areal density of the total charge passing through the bonding area. Higher applied voltage was able to accelerate the decay of the residual strength and shorten the time for a given decreas...

Adhesively bonded double cantilever beam (DCB) test specimens with residual stress were manufactured. Deforming the adherends during the curing process caused the DCB specimens to curve due to the residual stress and shear load arose at the adhesive layer. Therefore, mixed mode fractures were generated by applying mode I loading to the specimens. The relation between the specimen curvature and the strain energy due to the residual stress was theoretically investigated and the mode II energy release rate for each specimen was calculated using measured specimen curvature. Because the mode I energy release rate for each specimen can be calculated with the DCB test result, the fracture toughness in mixed mode conditions was discussed. By manufacturing the specimens with different residual stresses, mixed mode fracture tests with various mixed mode ratios were conducted and the effect of the mixed mode ratio on the energy release rates was investigated.

With an increasing demand for adhesives, the durability of joints has become highly important. The fatigue resistance of adhesives has been investigated mainly for epoxies, but in recent years many other resins have been adopted for structural adhesives. Therefore, understanding the fatigue characteristics of these resins is also important. In this study, the cyclic fatigue behavior of a two-part acrylic-based adhesive used for structural bonding was investigated using a fracture-mechanics approach. Fatigue tests for mode I loading were conducted under displacement control using double cantilever beam specimens with varying bond-line thicknesses. When the fatigue crack growth rate per cycle, da/dN, reached 10−5 mm/cycle, the fatigue toughness reduced to 1/10 of the critical fracture energy. In addition, significant changes in the characteristics of fatigue crack growth were observed varying the bond-line thickness and loading conditions. However, the predominance of the adhesive thickness on the fatigue crack growth resistance was confirmed regardless of the initial loading conditions. The thicker the adhesive bond line, the greater the fatigue toughness.

Designing joints requires a clear understanding of the appropriate thickness of the adhesive used. Structural acrylic adhesives have rarely been studied in terms of their thickness effect on the joint performance. To this end, the fracture resistance of a second-generation acrylic (SGA) adhesive was experimentally investigated by conducting a double cantilever beam (DCB) test. Because the SGA adhesive whitened when plastically deformed, the change of the plastic region with the crack growth was visualized. Therefore, the relationship between the fracture energy and the adhesive thickness was explained in terms of the plastically deformed area. With a thinner adhesive layer, the entire layer was whitened, and a linear relationship was obtained. In this region, the fracture energy increased from approximately 2 kJ/m2 with an adhesive thickness of 0.2 mm to approximately 4 kJ/m2 with 0.6 mm thickness. With more increase in the thickness, the fracture energy increased over 8 kJ/m2. Increasing the thickness, however, resulted in partial whitening, yielding a non-linear relationship. Moreover, increasing the loading rate changed fracture behavior. At the opening speed of 2.5 m/s, stick-slip crack propagation was observed in any adhesive thickness, and the critical fracture energy dramatically decreased. In contrast, the arrest fracture energy under unstable crack propagation was independent of the loading rate and the adhesive thickness as approximately 1 kJ/m2.

Gecko has a special micro hair on its foot to attach to a surface and climb walls. Mimicking its hair, easily detachable adhesive devices have been developed creating micro-structures on the surfaces. Adhesion strength of these devices changes with peeling direction. Therefore, this dependency enables a compatibility of strong adhesion and easy detachment. In this study, the effect of peeling speed on the adhesive strength of the devices is experimentally investigated.

The influence of loading rates and the combined stress states of tension and shearing on the strength, strain, and absorbed energy of an adhesively bonded joint was experimentally investigated. Cylindrical butt joint specimens were prepared and strength tests were performed on the specimens with a servo-controlled hydraulic testing machine that combined tension and torsion loading. Two types of epoxy adhesives, ductile and brittle, were applied to the specimens. The tests were performed under a quasi-static condition of 6.67×10 −2 MPa/s and a high-rate loading condition of 1.00×10 3 MPa/s. The results of the combined loading tests showed that the states of the fractured surfaces were not affected by the loading rates. As for the ratio of tensile and shear loading, adhesive failure tended to partially occur when the ratio of shear loading was very high. The strength points for the specimens bonded with each adhesive were distributed in a stress plane of tension and shearing and could be fitted with a curve that was described by an equation with exponential parameters that were not influenced by the strain rate; however, other parameters that described the intercepts were influenced. The failure strains and absorbed energies for the brittle adhesive were slightly dependent on the strain rate, but this dependency was unclear for the ductile adhesive.

Fiber reinforced plastics (FRPs) are in high demand as a means to reduce the weight of vehicle structures. When utilizing FRPs with other materials, the joining technology between them is a key technical issue because the joining methods are related to the safety of the vehicle and also restrict the vehicle design. Polypropylene (PP) is a widely utilized thermoplastic material in the automotive industry because of its reasonable price and high resistance to water absorption. Therefore, PP has a potential to be applied to a matrix resin for FRPs. When PP is utilized as a matrix resin for FRPs, welding is a joining method that has a low cost and high productivity between fiber reinforced polypropylene (FRPP) and metals. However, the welding strength between a FRPP and a metal does not have sufficient strength to be used in the structures. Therefore, in this paper, a chemical etching treatment and sandblasting treatment were applied to the surfaces of metal adherends that were used to increase the welding strength to verify the effect of mechanical interlocking, and the critical energy release rates of the welded joints were measured on welded double cantilever beam (DCB) specimens. The experimental results indicate that the chemical etching treatment is a very effective surface treatment method, with the resulting metal surface exhibiting a drastic increase of welding strength because the treatment made the surface morphology complex enough to enhance the effect of mechanical interlocking and obtain a higher bonding strength compared to the morphology produced via sandblasting. In addition, the employed chemical etching methods increase the bonding strength between GFRPP and metal to a level that exceeds the interlaminar strength of GFRPP. Therefore, the chemical etching depth did not affect the welding strength, and the maximum bonding strength only depends on the interlaminar strength of the GFRPP.

Thermal effects on welded joints between fiber-reinforced thermoplastics (FRTPs) and metals have been investigated theoretically and experimentally. Because FRTPs use thermoplastics as the matrix resin, they have advantages over fiber-reinforced thermoset plastics (FRPs or FRSPs), including the ability to be welded. When dissimilar materials are welded together, however, thermal stress occurs due to the different thermal expansions of the materials and affects the energy release rate of the joint. Therefore, a method for evaluating the true energy release rate, including the effect of thermal stress, is necessary for strength evaluation tests. Although several theories that compensate for the thermal stress and evaluate the true energy release rate have already been proposed, they require parameters that are difficult to measure. Therefore, it is difficult to apply them in experimental investigations. In this article, a theoretical method with easily measurable parameters is proposed to calculate the ener...

The Mode I fracture energy of a polyurethane adhesive with low Young’s modulus was investigated. Metal adherends in standardized double cantilever beam (DCB) tests are typically too stiff for soft adhesives, making it difficult to measure the fracture energy accurately. However, soft adhesives, such as a single-component polyurethane adhesive tested in this paper, are in high demand in the automobile industry. Thus, accurate measurement techniques must be established. Flexible substrates composed of spring steel were used for the DCB tests to accommodate the deformation of the adhesive layer. First, the applicability of the flexible substrates was discussed using specimens bonded with an epoxy adhesive. For soft adhesives, however, the deformation of the adhesive layer must be considered in the calculation methods of the fracture energy. Although the deformation effect on the DCB tests has been discussed with Winkler’s elastic foundation, the crack length must be measured along with the load and d...

Geckos and some insects can easily adhere to and detach from surfaces with micro/nanoscale hair structures on their foot, called setae and spatulas. Here, a model is developed to describe the detachment of the seta. In this model, the seta is assumed to be a beam whose tip adheres to a surface. When normal and tangential forces are applied to the root of the beam, a moment is generated at the contact tip and detachment occurs. The detachment conditions depend heavily on flexibility of the hair. The effects of Young's modulus and aspect ratio of length versus thickness of the beam on the detachment condition are theoretically investigated. The Poisson effect on the detachment conditions was also examined with the experimental results using fabricated silicone rubber beam arrays.

In this study, high-frequency (HF) welding of glass–fibre-reinforced polypropylene (GF/PP) with thermoplastic adhesive layers consisting of zinc oxide (ZnO), anatase-type titanium oxide or silicon carbide was investigated. Effects of the ceramic type and content on the dielectric and temperature characteristics of these adhesive layers were evaluated experimentally, and the ratio of the dielectric loss tangent to the relative dielectric permittivity (tan δ / e ′), which was the index of the HF heating efficiency, exhibited different tendencies by these parameters. This value increased rapidly with increasing temperature even at 10 vol% ZnO, suggesting that the heating of the adhesive layer may be accelerated by combining temperature rise with a small amount of ZnO. During the HF welding process, the ZnO-containing adhesive layer bound to GF/PP in the shortest time (18 s) with high bond strength (~14 MPa). The effects of temperature (50 °C) and moisture (80% relative humidity) on the mechanical strength of the HF-welded specimens after a long-term exposure were also examined.

Stress concentration in adhesively bonded joints, which is considered a major factor affecting their strength, can be avoided by tailoring the material properties of the adhesives using a functionally graded adhesive (FGA). The material properties of second-generation acrylic (SGA) adhesives can be simply changed by changing the mixing ratio of the agents. However, the superiority of FGA joints using SGA adhesives has not been experimentally clarified yet. Therefore, in this study, a shear strength test and a constant load low-cycle shear test were conducted on single lap joint (SLJ) specimens tailoring the adhesive layer stepwise. The FGA specimen was compared with the non-tailored specimens using stiff or flexible adhesives. The SLJ test results showed 16% improvement in the joint strength by stepwise tailoring of the adhesive layer. Additionally, the difference in the strain distributions among the different adhesive layers was investigated via a digital image correlation (DIC) method, and the shear strain at the edge of the FGA specimen was more than 40% decreased compared to the non-tailored specimen using the flexible adhesive. The low-cycle test results also showed the superiority of the FGA specimen to the other specimens. The FGA specimen held up more than 4 times the number of cycle of other specimens with an applied load of 18 kN or more. This was attributed to the suppressed plastic deformation at the edges of the adhesive layer owing to the introduction of FGAs.

Gecko’s foot hairs exhibit significant frictional anisotropy that enables a strong foot grip in a specific direction and an easy detachment in the opposite direction. In this study, we fabricate adhesive devices with frictional anisotropy mimicking gecko’s foot hair based on oblique micro-beam arrays. The devices adhesion force is strongly anisotropic along the beam tilting direction and depends on the stress distribution at the contact area which, in turn, is affected by the geometry of the beam tips. This dependence is investigated by fabricating and testing micro beam arrays with various tip shapes.

An adhesion model of an oblique structure with an adhesive tip is proposed by considering a limiting stress for adhesion to describe the detachment mechanism of gecko foot hairs. When a force is applied to the root of the oblique structure, normal and shear stresses are generated at contact and the adhesive tip is detached from the surface when reaching the limiting stress. An adhesion criterion that considers both the normal and shear stresses is introduced, and the asymmetric detachment of the oblique structure is theoretically investigated. In addition, oblique beam array structures are manufactured, and an inclination effect of the structure on the asymmetric detachment is experimentally verified.