In this work, the state of stress between adhesively bonded single lap-joints was determined. The state of stress between adhesively bonded single lap-joints was determined by using a linear elastic analysis and experimental testing. In the analysis part of this work, the shear and normal stresses in an adhesively bonded single lap –joints were predicted by subjecting the joint to tension; shear force, and bending moments. Differential equations were used to model the problem and the joint was analyzed using a linear elastic analysis. The solution of the differential equations was used in predicting the shear and normal (peel) stress distributions over the single lap-joint. The boundary conditions used limit the analysis to two adherends having the same thicknesses, lengths, and material properties. In the experimental testing, twelve single lap-joints were produced. These joints were produced using four different overlap lengths(12.7mm,14mm,16mm,and 18mm),and then grouped into three with each group having four single lap-joint of 12.7mm,14mm,16mm,and 18mm overlap lengths. Each of the three groups were subjected to tension, shear force and bending moments by using a universal testing machine and dead weights. The shear and normal stress distributions over the joints were obtained by dividing the applied loads with the area of the overlap region of the joint. It became evident from this work that increasing the overlap length results in an increase in joint strength, and a decrease in both the shear and normal stresses due to an increase in the area of bond. It was also found from this work that maximum stresses occurs the ends of the overlap region of the joint and are hence the critical regions of the bonded assembly. It was also found from this work that the failure associated with single lap-joint are mainly as a result of the failure in the adhesive layer known as adhesive failure. The shear and normal stresses obtained from these tests were in agreement with those obtained from analysis in this work. These tests were used to check the analytical results and to establish confidence in theory. This work could be applied in the determination of the state of stress between adhesively bonded laminated composite door panels.
TABLE OF CONTENTS
Approval Page i
Abstract v Table of Content vi
List of Figures x
List of Tables xii
1.1 Aims and Objectives 1
1.2 Scope of the Project Research 2
1.3 Significance of Study 2
LITERATURE REVIEW 3
2.1 Volkersen’s Theory 3
2.2 Theory of Goland and Reissner 4
2.3 Plantema’s Modification 5
2.4 Modification of Kelsey and Benson 5
2.5. Review of Different Types of Joints 7
2.6 Adhesive Properties 8
2.7 Failure Modes of Laminate Composites 9
2.8 Environmental Factors Affecting Laminate Composites 10
2.9. Advantages and Disadvantages of Bonded Joints 11
2.10 Summary 11
3.1Tension Test 12
3.2 Shear Test 13
3.3 Bending Test 14
3.4 Analysis 16
3.5 Detail Design Considerations 23
EXPERIMENTAL RESULTS AND DATA ANALYSIS 24
4.1 Joint Calculations for Single Lap-Joint 1 27
4.2 Joint Calculations for Single Lap –Joint 2 29
4.3 Joint Calculations for Single Lap –Joint 3 31
4.4 Joint Calculations for Single Lap –Joint 4 33
4.5 Experiment Results 35
4.51 Tension Test Result 35
4.6. Shear Test Result 42
4.7. Bending Test Result 47
4.8. Comparison of Results of Experimental Testing and Analysis 61
DISCUSSIONS AND CONCLUSION 67
Adhesive bonding is a material joining process in which an adhesive, placed between the adherend surfaces, solidifies to produce an adhesive bond.
Adhesively bonded joints are increasing alternatives to mechanical joints in engineering applications and provide many advantages over conventional mechanical fasteners. Among these advantages are lower structural weight, lower fabrication cost, and improved damage tolerance. The application of these joints in structural components made of fibre-reinforced composites has increased significantly in recent years. The traditional fasteners usually result in cutting of fibres, and hence the introduction of stress concentration, both of which reduce structural integrity.
Bonded joints are more continuous and have potential advantages of strength-to-weight ratio, design flexibility, and ease of fabrication. To design structural joints in engineering structures, it is necessary to be able to analyze them. This means to determine stresses and strain under a given loading, and to predict probable points of failure. There are two basic mathematical approaches for the analyses of adhesively bonded joints: closed-form analysis (Analytical methods) and numerical methods (ie forward difference method and finite-element analyses).The literature dealing with joining composite structures with adhesives concentrates on investigating the bond strength.
There are several factors affecting the qualities of a bonded assemble like the effect of surface preparation and effect of joints configuration. The surface plays an important role in the bonding process and is perhaps, the most important process governing the quality of an adhesively bonded joint. Surface treatment prior to the application of adhesives is recommended to achieve maximum mechanical strength. Typical composite surface treatments include solvent cleaning techniques for thermo sets composites, where as thermoplastics composites require surface topographical changes to ensure strong and durable bond strength. For the effect of joints configuration, since joints represent one of the greatest challenges in the design of structures in general and in composite structures in particular since they entail discontinuities in the geometry of the structure. Unlike surface preparation, joint configuration is usually a product of design. The strength of a given type of joint depends, for a given type of load, on the stress distribution within the joint, which in turn depends on the joint geometry and the mechanical properties of the adhesive and adherend. Joints made with high strength adhesives more likely to fail prematurely in composite before failure in the adhesives occurs.
1.1 Aims and Objectives
- To determine the local stresses in the adherends (materials being bonded, or substrates) and adhesive, so that predictions can be made of critical locations in the bonded assembly.
- To determine the failure mechanism in single lap- joint (that is interfacial and cohesive failure as a result of normal or shear stresses exceeding the bond strength).
- To determine the effects of increasing overlap length on the shear and normal stress distributions.
1.2 Scope of the Work
This work will involve both experimental testing and analysis. Twelve single lap-joints will be produced. Twelve of the joints will be divided into three groups (four per group with the different overlap lengths). The first group with four single lap-joint with different overlap lengths will be subjected to tensile loading, while the second and third groups will be subjected to shear force and bending moments using a universal testing machine and dead weights. The shear and normal stresses will be obtained by dividing the applied loads with the area of the overlap region of the joint.
In the analysis part of this work, the joints will be analysed by subjecting them to tension, shear force and bending moments. Differential equations will be used to model the problem and the joint will be analysed by using a linear elastic analysis. The solution of the differential equations will be used in determining the shear and normal stresses over the joint.
1.3 Significance of Study
- This work will help in determining the stresses involved in opening and closing of door panels made of fibre reinforced composites.
- This work will help to identify the relationship between the increase in stresses at the interface of a bonded joint and the joint strength.
- This work will identify if it is the increase porosity (air voids) in the bond line that causes the joint to fail, or the increase in stresses observed along the interface of the bond line.
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