Yu Sekiguchi

Adhesion between side surface of an elastic beam and flat surface of a rigid body

A theory of adhesion between an elastic beam and a rigid body is proposed using linear beam theory. Normalized force between the elastic beam and the rigid body considering adhesion of the side surface of the elastic beam is investigated theoretically. Adhesion of an elastic beam is important to analyze gecko adhesion, and peeling mechanism of an adhered film. This adhesion is also important in design of grip-and-release devices. The force between an elastic beam and a rigid body is investigated by considering shear force and total energy, and is obtained as a function of the displacement of the elastic beam. The proposed theory is different from Kendall’s thin-film peeling theory in terms of the elastic energy. The proposed theory considers bending elastic energy, whereas Kendall’s theory considers extension elastic energy. Two different contacts, line contact and area contact, are taken into account to discuss the loading and unloading processes in terms of the relation between the force and the displacement. Non-dimensional parameter, which relates to the work of adhesion and the specifications of the elastic beam, is introduced to explain the normalized maximum tensile force.

Experimental study of the Mode I adhesive fracture energy in DCB specimens bonded with a polyurethane adhesive

ABSTRACT 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 displacement. To overcome the difficulty of measuring the crack length, a calculation method based on Winkler’s elastic foundation was introduced applying the compliance-based beam method (CBBM). Finally, the fracture energy of the polyurethane adhesive was discussed by comparing the calculation methods with and without measuring the crack length.

Experimental investigation of mode I fracture energy of adhesively bonded joints under impact loading conditions

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 GIC 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.

Novel Method to Measure the Creep Strength of Adhesively Bonded Butt Joints Subjected to Constant Loading Using a Hydro-Pneumatic Testing Machine

This article proposes a new method to measure the creep strength of adhesively bonded joints using a hydro-pneumatic testing machine and a specimen holder, on which multi-specimens can be mounted in one testing machine. Creep tests were conducted on stainless steel butt joints bonded with epoxy adhesives. A hydro-pneumatic loading system was introduced to avoid successive failures of multi-specimens as well as to achieve a stable and constant loading through the experiments. Even after a failure occurs in one of the joints and thus generates an impact, the loading system is capable of absorbing the shock so that the other remaining joints do not fail simultaneously. It was experimentally verified that choke valves, which were introduced in the hydraulic circuit of the system, worked as a damper when failure occurred. Additionally, it was established that automatic reloading to the remaining specimens after the failure was short enough compared with the creep rupture time. As this new method relates to the efficiency of creep testing, the utility of the proposed approach with the multi-specimen setup has been verified.

The Critical Energy Release Rate of Welded Joints Between Fiber-Reinforced Thermoplastics and Metals When Thermal Residual Stress is Considered

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 energy release rate of welded double cantilever beam (DCB) joints. The effect of the thermal stress on the critical energy release rate is discussed in terms of the experimental results of a welded DCB specimen composed of a FRTP and an aluminum alloy.

Adhesion mechanism of a gecko-inspired oblique structure with an adhesive tip for asymmetric detachment

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.

Mechanisms for Grip-and-Release Process of Adhesion Contact Using Material with Variable Elastic Modulus

Adhesion mechanisms with and without a defect for gripping and releasing objects are proposed, considering the adhesion between a semi-infinite elastic body and a rigid object with a sinusoidal surface roughness. These mechanisms are investigated theoretically. The total energy consists of the elastic term, the interfacial adhesion term, and the mechanical potential energy term due to the external pressure. The grip-and-release process is discussed in terms of the changes in the total energy and the contact width. The gripping stress can be expressed as a function of the elastic modulus. Using polymers with variable elasticity, the grip-and-release process is demonstrated experimentally and discussed in terms of the theory.

Expansion characteristics of thermally expandable microcapsules for dismantlable adhesive under hydrostatic pressure or in resin

ABSTRACT 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 was smaller than that of shear stress. It was observed that the expansion of the microcapsules was limited when there were many microcapsules in the vicinity of the interface. In other words, a complicated stress state occurred, inducing interfacial failure along the interface.

Experimental investigation of the effect of tip shape in gecko-inspired adhesive devices under asymmetric detachment

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.

Analytical determination of adhesive layer deformation for adhesively bonded double cantilever beam test considering elastic–plastic deformation

ABSTRACT 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 length and strain during the crack propagation are obtained, and the effect of the traction–separation profile on the DCB test results is investigated. The fracture energy is given by the area under the traction–separation curve and is not affected by the curve profile. However, the crack length of the DCB specimen is strongly affected by the adhesive deformation, leading to a calculation error in the fracture energy. Therefore, several crack length correction methods have been proposed. An analytical approach to describe the plastic zone at the crack front would help better understand DCB tests for adhesives. In this study, an analytical solution for a DCB test is discussed assuming that the adhesive layer undergoes an elastic–plastic deformation. The elastic zone of the specimen is replaced with a beam on an elastic foundation and the plastic zone with a beam having a uniformly distributed load. Influence of the plastic zone at the crack front in the DCB tests is analytically described by assuming an elastic–perfectly plastic material.