The influence of local geometry on the strength of adhesive joints
Abstract
This paper describes a two-part investigation into the effect of local geometry changes at the edges of the overlap in single lap joints. In the first part, finite element analysis has been used to model the effects on the stress distribution of geometry changes which are small in relation to the dimensions of the local geometry, in order to provide an improved model for failure prediction. The model used is that of an adhesive around a rigid corner, and the effect of rounding the corner has been considered.
The second part deals with local geometry changes of the same order of magnitude as the dimensions of the geometry and their effect on joint strength. Three types of joint — one with a square-edged adhesive layer, one with a fillet of adhesive and one with an adhesive fillet plus a radiused adherend — were manufactured, tested and analysed. Improvements to the finite element models were also made following the results of part 1.
It is found that finite element analysis is capable of predicting the significant strength increases that may be achieved in single lap joints by filleting the adhesive at the edges of the overlap and rounding the ends of the adherends.
References (4)
- J.A. Harris et al.
Int J Adhesion and Adhesives
(1984) - J.R. Rice et al.
J Mech and Phys of Solids
(1968)
Cited by (245)
Prediction of the strength of adhesive lap joints. An investigative review
2024, International Journal of Adhesion and AdhesivesThe principal methods of predicting the strength of lap shear bonded joints have been introduced for metallic and composite adherends. The early theories of Volkersen and of Goland and Reissner provided indications of the shear and transverse [peel] stresses. These equations were not easy to use and were increasingly displaced by finite element analysis [FEA]. The importance or otherwise of the high stresses around the embedded corner of the unloaded adherend were examined and shown to be the most likely cause of initial cracking, but not necessarily of joint failure. By combining an extensive experimental programme of testing both adhesives and joints with FEA, knowledge was gained of the reasons why lap shear joints fail. Some simple equations were developed with failure criteria based on yield of the adherend and/or the adhesive which produced predictions in good agreement with experimental results. Finally, the bonding of composites was considered, and it was shown that the key factor is the transverse tensile strength of the composite.
Testing method to determine the strength and fracture toughness of adhesives in a single continuous test
2023, Theoretical and Applied Fracture MechanicsIn this work, a new testing method for adhesive joints under tensile loading is presented. This method comprised three pull-off tests and one peel test in one single loading step, which enables the determination of the strength and fracture toughness of the adhesive. The properties determined can be of the adhesive, if cohesive failure occurs, or of the interface, in the event of adhesive failure. To validate the testing method, the adhesive used was a commercially available silicone, for which strength and fracture toughness had already been determined using standard methods. It was concluded that this method accurately enables the determination of strength and toughness, using one single joint and loading step.
Assessment of failure mechanism of double-strap 3D-FML adhesively bonded joints under tensile and compressive loadings using cohesive zone modelling approach
2023, Composite StructuresDespite the widespread applications of Fiber Metal Laminates (FMLs), there is a clear lack of information regarding the performance of bonded joints mating 2D-FMLs and the recently-developed 3D-FMLs, particularly when considering their performances under compressive loading. Therefore, as a follow-up to the authors’ recent investigations, an extensive series of numerical analyses are conducted in the present study to simulate the tensile and compressive (i.e., buckling, and post-buckling) responses of double-strap adhesively bonded joints mating 3D-FMLs. The 3D-FMLs are joined using carbon fiber-reinforced plastic (CFRP) straps and structural epoxy resin. The damage initiation and evolution in the bonded regions are modelled by seven different FE models, which incorporated the mixed-mode trapezoidal Cohesive Zone Modelling (CZM) approach in conjunction with continuum elements. Firstly, the effects of different numbers of CZM layers and thicknesses on simulating the damage mechanism are investigated. Subsequently, the influence of CFRP straps’ thickness and length on the performance of the adhesive joints is parametrically studied. The distributions of peel and shear stresses along the length of the bonded regions are also systematically explored. Based on the results, the optimal joint configuration is established, and the most effective modelling framework is highlighted by comparing numerically predicted results against experimental results.
A novel profiling concept leading to a significant increase in the mechanical performance of metal to composite adhesive joints
2023, Composites Part B: EngineeringTraditional adhesive joints with straight edged adherends suffer from a significant stress concentration in the composite coincident with the edge of the metal adherend, which can lead to accelerated translaminar failure of the substrate. In this work, we developed a novel profiling concept which improves the mechanical performance of adhesive joints between metallic adherends and composite substrates. We conducted quasi-static four-point bending (4PB) tests which showed that profiling the edge of the metallic adherend could improve the peak load by at least 27%, and that the stability of failure was simultaneously improved. We investigated varying the profile parameters and were able to conclude that further significant mechanical performance gains could be achieved by increasing any of the profile: amplitude, frequency, or number of fractal length-scales. By analysing in-situ acoustic emission (AE) monitoring data we were able to observe that profiling of the metallic adherend results in failure initiation occurring at higher loads, which suggests that the concept is successful in providing better stress distributions and lowering peak stresses. By analysing the fracture surfaces, it is apparent that the profiling concept is successful in deflecting the translaminar fracture path; and additionally that a debonding mechanism occurs at the profile tips which is thought to be an important additional mechanism for creating damage tolerant joints.
Standard test methods and their need to evolve
2023, Advances in Structural Adhesive Bonding, Second EditionAdhesive bonding technologies are continually changing to support advances in joint design and to meet the needs of advanced manufacturing for an expanding number of applications. Adhesive strength and toughness are important for high-performing bonds, but traditionally strong adhesives have not been tough. Newer adhesives have increased toughness along with improvements in processing, robustness, and durability. This has opened up structural adhesive bonding to more demanding applications. Complex joint designs or renewable, reusable adhesives are more important to the aerospace, automotive, energy, and infrastructure sectors. Introducing novel materials, manufacturing methods, or installation methods into joint designs is a challenge as the risks from failure can be significant. Stakeholders require a common framework to trust the performance claims of any new adhesive or joining technology. Standard test methods serve as the gatekeepers of trust, so a recurring challenge is to maintain pace with technological advances. As engineers and material scientists, it is important to understand the limitations of current test methods and consider how standards may need to evolve with advancing changes in technology. The goals of this chapter are to provide a brief overview of the stakeholders and process involved in the curation of standard measurements and test methods. A series of case studies is provided to illustrate the interplay among standards, adhesive performance, and supporting new technologies. The reader should gain an appreciation for the evolving roles between the technical understanding of adhesive performance, provided in the following chapters, and the standards process.
Improving joint performance through graded materials and geometries
2023, Advances in Structural Adhesive Bonding, Second EditionThis chapter describes a set of practical techniques that can be used to reduce the stress concentration on the adhesive layer of bonded joints, via the localized control of adhesive and adherend properties. High stresses and sites for damage often occur at the edges of the bonded overlaps where abrupt geometrical changes are present. Gradients in geometry or of material properties have been shown to be viable ways to mitigate these effects. The chapter includes discussions of the different stresses acting on a commonly used bonded joint geometry, the presence of stress concentration, and the classical methodologies for reducing these stress concentrations. The concept of the mixed adhesive joint is then explained, demonstrating the improvements in mechanical and thermal performance that can be achieved using this configuration. This document then advances toward an explanation of the functionally graded joints, focusing first on the functional grading of the adhesive layer and the practical techniques that can be employed to achieve this. Functionally graded adherends are discussed next, with their basic concept and practical implementation techniques being described.