Influence of Fabrication Residual Thermal Stresses on Rubber-toughened Adhesive Tubular Single Lap Steel-Steel Joints under Tensile Load

The tensile load bearing capability of adhesively-bonded tubular single lap joints which is calculated under the assumption of linear mechanical adhesive properties is usually much less than the experimentally-determined because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives. Also, as the adhesive thickness increases, the calculated tensile load bearing capability with the linear mechanical adhesive properties increases, while, on the contrary, the experimentally-determined tensile load bearing capability decreases.

In this paper, the stress analysis of adhesively-bonded tubular single lap steel-steel joints under tensile load was performed taking into account the nonlinear mechanical properties and fabrication residual thermal stresses of the adhesive. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive.

Using the results of stress analysis, the failure criterion for the adhesively-bonded tubular single lap steel-steel joints under tensile load was developed, which can be used to predict the load-bearing capability of the joint. From the failure criterion, it was found that the fracture of the adhesively-bonded joint was much influenced by the fabrication residual thermal stresses.     

Analysis of the Tubular Single Lap Joint with Nonlinear Adhesive Properties

The majority of the load transfer of an adhesively-bonded joint is accomplished by the nonlinear behavior of the adhesive. In this paper, the torque transmission capability and shear stress distribution of the tubular single lap joint were calculated by incorporating the nonlinear shear properties of the adhesive. The nonlinear shear properties were represented by three different mathematical models such as two-parameter exponential, linear perfectly-plastic and multilinear strain-softening approximations.

From the analyses and experiments, it was found that all the analyses with nonlinear approximations predicted the torque transmission capabilities accurately, but the two-parameter exponential approximation gave the best predictions with the simplest form for use in numerical calculation.     

An Iterative Solution for the Torque Transmission Capability of Adhesively-Bonded Tubular Single Lap Joints with Nonlinear Shear Properties

The adhesively-bonded tubular single lap joint shows large nonlinear behavior in the load-displacement relationship, because structural adhesives for the joint are usually rubber-toughened, which endows adhesives with nonlinear shear properties. Since the majority of load transfer of the adhesively-bonded tubular single lap joint is accomplished by the nonlinear behavior of the adhesive, its torque transmission capability should be calculated using nonlinear shear properties. However, both the analytic and numerical analyses become complicated if the nonlinear shear properties of the adhesive are included during the calculation of torque transmission capabilities.

In this paper, in order to obtain the torque transmission capabilities easily, an iterative solution which includes the nonlinear shear properties of the adhesive was derived using the analytic solution with the linear shear properties of the adhesive. Since the iterative solution can be obtained very quickly due to its simplicity, it has been found that it can be used in the design of the adhesively-bonded tubular single lap joint.     

The Torque Transmission Capabilities of the Adhesively-Bonded Tubular Single Lap Joint and the Double Lap Joint

With the wide application of fiber-reinforced composite materials in aircraft, space structures and robot arms, the design and manufacture of composite joints have become a very important research area because they are often the weakest areas in composite structures.

In this paper, the stress and torque transmission capabilities of the adhesively-bonded tubular single lap joint and the double lap joint were experimentally tested. In order to compare the experimental results with the calculated results, the stress and torque transmission capabilities were analyzed by the 3-dimensional finite element method taking into consideration the nonlinear properties of the adhesive.

From the experiments it was found that the torque transmission capabilities of the adhesively-bonded double lap joint was 2.7 times as large as that of the single lap joint. Also, it was found that the fatigue limit of the double lap joint was 16 times as large as that of the single lap joint.     

Nonlinear Analysis of theTorque Transmission Capability of Adhesively Bonded Tubular Lap Joints

Calculated torque transmission capability of adhesively bonded tubular lap joints using linear elastic material properties is usually much less than the experimentally-determined one because the majority of the load transfer of the adhesively bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives.

Although the adhesively bonded tubular double lap joint has better torque transmission capability and reliability than the single lap joint, the nonlinear analytic or numerical analysis for the adhesively bonded tubular double lap joint has not been performed because of numerical complications.

An iterative solution that includes the nonlinear shear behavior of the adhesive was derived using the analytic solution. Since the iterative solution can be obtained very quickly due to the simplicity of the algorithm, it is an attractive method of designing adhesively bonded tubular single and double lap joints.     

Optimal Design of the Adhesively-Bonded Tubular Single Lap Joint

In this paper, a method for the optimal design of the adhesively-bonded tubular single lap joint was proposed based on the failure model of the adhesively-bonded tubular single lap joint. The failure model incorporated the nonlinear mechanical behavior of the adhesive as well as the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermal stresses induced by fabrication.

The effects of the design parameters for the adhesively-bonded tubular single lap joint, such as the thicknesses of adhesive layer and adherends, the bonding length, and the scarfs of adherends, on the torque transmission capability and the efficiency of the adhesive joint were investigated.     

Torque Transmission Capabilities of Bonded Polygonal Lap Joints for Carbon Fiber Epoxy Composites

Although carbon fiber epoxy composite materials have excellent properties for structures, the joint in composite materials often reduces the efficiency of the composite structure because the joint is often the weakest area in the composite structure.

In this paper, the effects of the adhesive thickness and the adherend surface roughness on the static and fatigue strengths of adhesively-bonded tubular polygonal lap joints have been investigated by experimental methods. The dependencies of the static and fatigue strengths on the stacking sequences of the composite adherends were observed.

From the experimental investigations, it was found that the fatigue strength of the circular adhesively-bounded joints was quite dependent on the surface roughness of the adherends and that polygonal adhesively-bonded joints had better fatigue strength characteristics than circular adhesively-bonded joints.     

Development of a Failure Model for the Adhesively Bonded Tubular Single Lap Joint

The accurate calculation of the stresses and torque capacities of adhesively bonded joints is not possible without understanding the failure phenomena of the adhesive joints and the nonlinear behavior of the adhesive.

In this paper, an adhesive failure model of the adhesively bonded tubular single lap joint with steel-steel adherends was proposed to predict the torque capacity accurately.

The model incorporated the nonlinear behavior of the adhesive and the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermally-induced stresses from fabrication.     

A Closed-form Solution for the Torque Transmission Capability of the Adhesively Bonded Tubular Double Lap Joint

The adhesively bonded tubular double lap joint has better torque transmission capability and reliability in bonding than the single lap joint.

In this paper, an analytic solution for the torque transmission capability and stress distribution of the adhesively bonded tubular double lap joint was derived assuming linear properties of the adhesive.

From the analytic solution, it was found that the torque transmission capability of the double lap joint was more than 40% larger than that of the single lap joint.     

Optimal Design of the Adhesively-Bonded Tubular Single Lap Joint

In this paper, a method for the optimal design of the adhesively-bonded tubular single lap joint was proposed based on the failure model of the adhesively-bonded tubular single lap joint. The failure model incorporated the nonlinear mechanical behavior of the adhesive as well as the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermal stresses induced by fabrication.

The effects of the design parameters for the adhesively-bonded tubular single lap joint, such as the thicknesses of adhesive layer and adherends, the bonding length, and the scarfs of adherends, on the torque transmission capability and the efficiency of the adhesive joint were investigated.     

Development of a Fatigue Failure Model for the Adhesively Bonded Tubular Single Lap Joint under Dynamic Torsional Loading

The adhesively bonded tubular single lap joint shows nonlinear torque transmission capability and deformation characteristics under static torsional loading because of nonlinear properties of the adhesive. However, the dynamic or fatigue torque transmission capability can be calculated with linear, analysis because the stress-strain relation under torsional fatigue loading is linear, due to the small dynamic transmission capability compared with the static torque transmission capability.

In this paper, a failure model for the adhesively bonded tubular single lap joint under torsional fatigue loading was developed with respect to the adhesive thickness, which is the critical factor for the static torque transmission capability. Also, a design method for the adhesively bonded tubular single lap joint under torsional fatigue loading was proposed.     

Tensile Strength of Single Lap Joint and Scarf Joint between CFRP and Carbon Steel

The strength of single lap joints and scarf joints between carbon cloth laminated plastics (CFRP) and carbon steel bonded with epoxy resin was investigated both analytically and experimentally. The stress and strain distributions under tensile loads of the joints were analyzed by applying the elastic finite element method.

The strength of the joints was predicted by applying the strength law of CFRP, metal, adhesive layer and their interfaces to the calculated stress distributions. The predicted strength was compared with the experimental strength of the joints. The critical positions of the joints and the effects of the overlapped length on the joints were examined.     

Estimation of Fatigue Strength for Adhesively-Bonded Lap Joints Based on Fatigue Failure Criterion Under Multiaxial Stress Conditions

A method for estimation of endurance limits for adhesively-bonded single and single-step double-lap joints was proposed which considers the stress multiaxiality in the adhesive layer. At first, fatigue tests and finite element analysis were conducted for these lap joints. Then, endurance limits of these joints were estimated using their stress distributions and critical regression equations which were obtained from the S-N data of adhesively-bonded scarf- and butterfly-type joints. The results indicate that the endurance limits of these lap joints can be estimated from the regression equation based on the maximum principal stress.     

A Method of Predicting the Stresses in Adhesive Joints after Cyclic Moisture Conditioning

The durability of adhesive joints is of special concern in structural applications and moisture has been identified as one of the major factors affecting joint durability. This is especially important in applications where joints are exposed to varying environmental conditions throughout their life. This paper presents a methodology to predict the stresses in adhesive joints under cyclic moisture conditioning. The single lap joints were manufactured from aluminium alloy 2024 T3 and the FM73®-BR127® adhesive-primer system. Experimental determination of the mechanical properties of the adhesive was carried out to measure the effect of moisture uptake on the strength of the adhesive. The experimental results revealed that the tensile strength of the adhesive decreased with increasing moisture content. The failure strength of the single lap joints also progressively degraded with time when conditioned at 50°C, immersed in water; however, most of the joint strength recovered after drying the joints. A novel finite element based methodology, which incorporated moisture history effects, was adopted to determine the stresses in the single lap joints after curing, conditioning, and tensile testing. A significant amount of thermal residual stress was present in the adhesive layer after curing the joints; however, hygroscopic expansion after the absorption of moisture provided some relief from the curing stresses. The finite element model used moisture history dependent mechanical properties to predict the stresses after application of tensile load on the joints. The maximum stresses were observed in the fillet areas in both the conditioned and the dried joints. Study of the stresses revealed that degradation in the strength of the adhesive was the major contributor in the strength loss of the adhesive joints and adhesive strength recovery also resulted in recovered joint strength. The presented methodology is generic in nature and may be used for various joint configurations as well as for other polymers and polymer matrix composites.     

Tensile Strength of Joints Bonded With a Nano-particle-Reinforced Adhesive

In order to improve the tensile lap shear strength of adhesively bonded joints, nano-particles were dispersed in the adhesive using a 3-roll mill. The dispersion states of nano-particles in the epoxy adhesive were observed with TEM (Transmission Electron Microscopy) with respect to the mixing conditions, and the effect of nano-particles on the mechanical properties of the adhesive was measured with respect to dispersion state and weight content of nano-particles. Also the static tensile load capability of the adhesively bonded double lap joints composed of uni-directional glass/epoxy composite and nano-particle-reinforced epoxy adhesive was investigated to assess the effect of nano-particles on the lap shear strength of the joint. From the experimental and FE analysis results, it was found that the nano-particles in the adhesive improved the mechanical properties of the adhesive. Also the increased failure strain and the reduced CTE (coefficient of thermal expansion) of the nano-particle-reinforced adhesive improved the lap shear strength of adhesively bonded joints.     

Strength of Adhesively-Bonded Butt Joints of Tubes Subjected to Combined High-Rate Loads

The dynamic strength of adhesively-bonded joints was investigated experimentally. The strength of the bonded joints under combined high rate loading was measured using the clamped Hopkinson bar method. Tubular butt joints bonded by epoxy resin were used for the experiment. Combined stress waves of tension and torsion were applied to the specimens. The strength of the adhesively-bonded joint was determined by measuring the stress waves propagated in the load output tube of the specimen. It was found that the dynamic strength of the adhesive joints was greater than the static strength under tensile and shear load.     

Application of Adhesive Joining Technology for Manufacturing of the Composite Flexspline for a Harmonic Drive

A new manufacturing method for the cup-type composite flexspline for a harmonic drive was developed using adhesive joining technology to obviate the manufacturing difficulty of the conventional one-piece cup-type steel flexspline and to improve the dynamic characteristics of the flexspline.

In this method, the boss, tube and tooth sections of the flexspline were designed and manufactured separately, and adhesively bonded. The tube section was manufactured with high strength carbon fiber epoxy composite material and its dynamic properties were compared with those of the conventional steel flexspline.

The torque transmission capability of the adhesively-bonded joint was numerically calculated using the nonlinear shear stress-strain relationship which was represented by an exponential form.

From the test results of the manufactured composite flexspline and the conventional steel flexspline, it was found that the manufactured composite flexspline had better torque transmission characteristics. Also, it was found that the damping capacity of the composite flexspline was considerably improved.     

Adhesive Joining Technology for Manufacturing of the Composite Flexspline for a Harmonic Drive

A new manufacturing method for the cup-type composite flexsplinc drive was developed using adhesive joining technology to obviate the manufacturing difficulty of the conventional one-piece cup-type steel flexspline and to improve the dynamic characteristics of the flexspline.

In this method, the boss, tube and tooth sections of the flexspline were designed and manufactured separately, and adhesively bonded. The tube section was manufactured with high strength carbon fiber epoxy composite material and its dynamic properties were compared with those of the conventional steel flexspline.

The torque transmission capability of the adhesively-bonded joint was numerically calculated using the nonlinear shear stress-strain relationship which was represented by an exponential form.

From the test results of the manufactured composite flexspline and the conventional flexspline, it was found that the manufactured composite flexspline had better torque transmission characteristics. Also, it was found that the damping capacity of the composite flexspline was considerable improved.     

An efficient analysis model for functionally graded adhesive single lap joints

Employing a functionally graded adhesive the efficiency of adhesively bonded lap joints can be improved significantly. However, up to now, analysis approaches for planar functionally graded adhesive joints are still not addressed well. With this work, an efficient model for the stress analysis of functionally graded adhesive single lap joints which considers peel as well as shear stresses in the adhesive is proposed. Two differential equations of the displacements are derived for the case of an axially loaded adhesive single lap joint. The differential equations are solved using a power series approach. The model incorporates the nonlinear geometric characteristics of a single lap joint under tensile loading and allows for the analysis of various adhesive Young׳s modulus variations. The obtained stress distributions are compared to results of detailed Finite Element analyses and show a good agreement for several single lap joint configurations. In addition, different adhesive Young׳s modulus distributions and their impact on the peel and shear stresses as well as the influence of the adhesive thickness are studied and discussed in detail.     

Torque Transmission Capabilities of Bonded Polygonal Lap Joints for Carbon Fiber Epoxy Composites

Although carbon fiber epoxy composite materials have excellent properties for structures, the joint in composite materials often reduces the efficiency of the composite structure because the joint is often the weakest area in the composite structure.

In this paper, the effects of the adhesive thickness and the adherend surface roughness on the static and fatigue strengths of adhesively-bonded tubular polygonal lap joints have been investigated by experimental methods. The dependencies of the static and fatigue strengths on the stacking sequences of the composite adherends were observed.

From the experimental investigations, it was found that the fatigue strength of the circular adhesively-bounded joints was quite dependent on the surface roughness of the adherends and that polygonal adhesively-bonded joints had better fatigue strength characteristics than circular adhesively-bonded joints.