In order to reduce fuel consumption, lighter-weight materials such as aluminum alloys and carbon fiberreinforced plastics（ CFRP） have been widely used in aircraft and automobile structures. These materialsdemand composite bonded joints as structural elements. Adhesive bonding has several advantages fromviewpoint of joint strength when compared with other mechanical joints. Therefore, it is important to estimatethe strength of these joints. The strengths of adhesive bonds are usually determined by destructive testswhich require trial and error procedure. In order to reduce time and cost estimating these joint strengths, thepractical evaluation method for these joint strengths using FEM has been desired. Recently, cohesive zonemodel（ CZM） attracts special attention for prediction method of the strengths for bonding structures. In thepresent situation, the cohesive zone model has been installed in various FEM programs. Hence, CZM has beenwidely used for the strengths prediction of adhesive joints.In this review, the author overviewed an outline of cohesive zone model and its application for strengthprediction of adhesively bonded lap joints. In order to estimate the strength of the lap joints, the parametersof the CZM element under mode I and II conditions were necessary, which were determined from the DCB andENF tests in this case. Then, the strengths of the lap joints were estimated by using these parameters, whereinthe simulated fracture loads of the lap joints were well agreed with the experimental ones.In order to estimate the joint strengths with higher accuracy, it is necessary to clarify the fracture criterionsunder the mixed mode condition. Hence, in this review the several fracture toughness testing methods ofthe adhesive joints under mixed mode conditions were also described. The fracture loads of DCB and ENFtests were determined by crack growth simulation using cohesive element. It was found the relationship ofload-displacement curves showed good agreements with experimental results and analytical results. Thestrength of single-lap adhesive joints was estimated by FE analysis（FEA） using cohesive elements. Cohesiveparameters GIC and GIIC were obtained by DCB and ENF tests. The fracture load was calculated based on theconcept of a power law criterion as mixed-mode loading condition. The failure load predicted by FEA washigher than experimental result. The failure criterion must be identified by mixed-mode fracture toughnesstest. Finally, we conducted mixed-mode fracture toughness test of adhesive joints available for FEA usingcohesive element.

This paper shows the technique to implement constitutive equations for adhesions into the commercial finiteelement methods. This reports deals with the two types of implementations;（a） the existing conventionalmethod（ closest point projection, CPP） deal and（b） the new proposed method by using the complex stepderivative approximations（CSDA）. The calculated result by 3 point bending process by the propose methodwas also exhibited. It was found that the stable finite element calculation with the high accurate constitutiveequation by CSDA was realized. The calculation also showed that the present method with consideration ofthe equations enable us to provide more accurate finite element calculation than the conventional calculation.

CFRP is superior in specific strength and specific elastic modulus, which is suitable for weight reduction ofstructures. However, it is impossible to fabricate the structures with only CFRP components, CFRP-to-metalbonding is necessary. In the present situation, adhesive joints have several advantages form the viewpointof joint strength when compared with other mechanical joints. Hence, it is important to clarify the strengthcharacteristics of these joints.In this review, the fracture toughness and fatigue crack propagation characteristics of DCB/metal adhesivelybonded joints were explained form the viewpoint of stiffness asymmetry of CFRP and metal adherends andresidual stress caused in curing process of the adhesives.

The adhesive strength of structural joints has been intensively investigated for the past three decadesfor mainly epoxy adhesives.Recently, instead of the epoxy type, acrylic adhesives, which have much higherductility than epoxy adhesives, have been developed. This type of adhesive will have a wide variety ofindustrial applications.In this review, Tensile lap shear tests of aluminium single-lap joints were performed at various temperatureof 10-40 ℃ at several tensile speeds. Based on the experimental results, a new constitutive model oftemperature-dependent elasto-viscoplasticity of the adhesive was presented. Experiments to determineadhesive strength of butt-joint which was subjected to combined tension and torsion were performed . In thisexperiment, aluminum butt-specimens with an acrylic adhesive were used. A tubular butt-joint was subjectedto torsion under a tensile load. In order to examine the deformation speed and temperature on strength,several levels of torsional speeds and temperatures were chosen. The fracture strength strongly depends onthe torsional speed and the temperature, i.e., it becomes higher at lower temperature and at higher speed. Inorder to discuss the effect of the strain rate and temperature, numerical simulations of the combined-stresstest were conducted using a rate and temperature-dependent constitutive model. The calculated results are ingood agreement with the corresponding experimental data.

This review introduces how affect the residual stress in composites on its interlayer adhesive strength. Thesimple evaluation method of residual stress in resin products is also explained. The carbon fiber reinforcedthermo-plastics（ CFRTP） was fabricated by stacking and heat pressing the plain woven carbon cloth and thinsheetof PA6. For evaluation of interlayer adhesive strength, the simulated interlayer was prepared by insertingthe thin polyimide sheet before heat pressing. The needle with hemispherical tip was vibrated to axial directionby using ultrasonic transducer. The vibrated needle was stabbed to CFRTP toward thickness direction byusing CNC controlled XYZ table. By applying the tensile load to the simulated interlayer, the adhesive strengthwas evaluated. Test results showed that the interlayer adhesive strength of CFRTP was improved when needlewas stabbed with appropriate velocity. The improvement in adhesive strength was depending on the area ofsimulated interlayer. The hardness distribution measured by using model specimen revealed that the hardnessof model specimen around stabbed point was improved. The hardness measurement of resin under uni-axialstress revealed that the hardness of resin shows almost inverse linear relationship. These results suggestedthat the existence of compressive residual stress due to ultrasonic needle punching improves the interlayeradhesive strength of CFRTP.

This paper reviews the techniques for improving the interfacial adhesion strength of carbon fiber reinforcedplastics（ CFRPs）, especially the modification of resin matrix by adding the nano fibers is explained. Toimprove the interfacial adhesion strength between carbon fiber and matrix, many approaches have beenconducted, such as surface modification of carbon fiber, chemical treatment. In this paper, our recent studiesfor improving the interfacial adhesion strength of CFRPs by adding the cellulose nano fibers（ CNFs） to itsmatrix are introduced. Two types of CNFs in which only fiber length was different, were added to the epoxymatrix of CFRPs for investigating effect of the fiber length on its interfacial adhesion strength and mechanicalproperties. Test results showed that the interfacial adhesion strength between matrix and carbon fiber wasimproved by the modification. However, there is no significant difference in adhesion strength betweendifferent lengths of CNFs. In contrast, the bending properties were improved with increase of the length ofadded CNFs. Bridging of CNFs between carbon fibers was observed on the fractured surface after fatigue testswhen the longer CNFs were added. It was suggested that the bridging of CNFs might be effective to improvethe interfacial adhesion strength between carbon fiber and matrix.

In Japan, the innovative structural materials are developing through the collaboration among industry,government and academia. The synergistic effect based on the combination of materials is very important andthe optimum arrangement of the good properties of each material, such as steel, aluminum, and FRP, is desiredas“ Multi-materials”. One of the remarkable technologies for Multi-materials is joining/bonding for dissimilarmaterials with considering the decomposability for recycling. The adhesive bonding technology for metaland other materials with high joining strength and easy decomposition is needed. In this paper, the currentof the research and development for decomposable adhesives were reviewed, and the heating technologiesfor decomposition were described. Furthermore, the adhesive technology with thermosetting adhesives andceramics particles for a GFRP joint and decomposition due to the microwave irradiation were introduced. Theacceleration of the progress for the research and development in this field is expected by the collaboration ofthe related industry and academia.