C.T. McCarthy

Bolt-hole clearance effects and strength criteria in single-bolt, single-lap, composite bolted joints

Abstract Effects of bolt-hole clearance on the stiffness and strength of composite bolted joints were investigated. The configuration studied was single-lap, single-bolt. Four different clearances were obtained using variable size reamers, ranging from neat-fit to 240 μm. The specimens were manufactured in accordance with ASTM standard D5961/D5961 M-96, from graphite/epoxy HTA/6376, with quasi-isotropic and zero-dominated lay-ups. Both protruding head and countersunk bolts were used, with two different applied torque levels. Specimen dimensions were chosen to obtain bearing as the primary mode of failure, with ultimate failure being mostly through bolt failure. Joint stiffness, 2% offset bearing strength, ultimate bearing strength and ultimate bearing strain were obtained according to the Standard. In addition, an alternative definition of strength was derived, which has some advantages over the offset method, and the results were evaluated according to this definition. Increasing clearance was found to result in reduced joint stiffness and increased ultimate strain in all tested configurations. Finger-tight joints with protruding head bolts showed a link between clearance and strength, but countersunk and torqued joints did not. A delay in load take-up also occurred with the higher clearance joints, which has implications for load distributions in multi-bolt joints.

Experiences with Modeling Friction in Composite Bolted Joints

Finite element analyses of composite bolted joints are common in the literature. However, the important issue of friction is often given superficial treatment. Friction introduces added difficulties to an already complex contact problem in terms of numerical convergence, and there can be a temptation to accept any method that will give a convergent solution. However, friction can significantly alter the stress distribution in the laminate at the bolt-hole interface, and carries a major proportion of the load in torqued joints; hence is important to model correctly. In the present study, experiences with modeling friction in composite bolted joints using commercial code MSC.Marc are presented. Unlike previous studies, both physical friction parameters and nonphysical convergence parameters within the available models are examined in detail and the findings should be helpful to other researchers analyzing similar problems. Two available models within the code are examined for their ability to model load transfer by friction in torqued joints, and the stress distribution at the bolt-hole interface in a pinned joint. The torqued joints include a large clearance so that both static and kinetic friction effects occur as the joint begins to slide and clearance is taken up. Results from the torqued joint models are compared with the experimental results. The stress distribution at the bolt-hole interface of the pinned joint is compared with a solution from an analytical method. It has been found that only one of the two models available in the code is capable of producing satisfactory results, and even with that model significant modification to the default friction parameters was required. It has also been found that using friction coefficients measured under ideal (clean) conditions in the model of the torqued joints did not give very good agreement with the joint experiments, which involved routine handling of the specimens. Finally, the developed friction model is used in a case study of a multibolt joint with variable degrees of bolt torque and bolt-hole clearances, and it is shown that such models can provide useful information for the design of composite bolted joints.

A three-scale finite element investigation into the effects of tissue mineralisation and lamellar organisation in human cortical and trabecular bone

Bone is an exceptional material that is lightweight for efficient movement but also exhibits excellent strength and stiffness imparted by a composite material of organic proteins and mineral crystals that are intricately organised on many scales. Experimental and computational studies have sought to understand the role of bone composition and organisation in regulating the biomechanical behaviour of bone. However, due to the complex hierarchical arrangement of the constituent materials, the reported experimental values for the elastic modulus of trabecular and cortical tissue have conflicted greatly. Furthermore, finite element studies of bone have largely made the simplifying assumption that material behaviour was homogeneous or that tissue variability only occurred at the microscale, based on grey values from micro-CT scans. Thus, it remains that the precise role of nanoscale tissue constituents and microscale tissue organisation is not fully understood and more importantly that these have never been incorporated together to predict bone fracture or implant outcome in a multiscale finite element framework. In this paper, a three-scale finite element homogenisation scheme is presented which enables the prediction of homogenised effective properties of tissue level bone from its fundamental nanoscale constituents of hydroxyapatite mineral crystals and organic collagen proteins. Two independent homogenisation steps are performed on representative volume elements which describe the local morphological arrangement of both the nanostructural and microstructural levels. This three-scale homogenisation scheme predicts differences in the tissue level properties of bone as a function of mineral volume fraction, mineral aspect ratio and lamellar orientation. These parameters were chosen to lie within normal tissue ranges derived from experimental studies, and it was found that the predicted stiffness properties at the lamellar level correlate well with experimental nanoindentation results from cortical and trabecular bone. Furthermore, these studies show variations in mineral volume fraction, mineral crystal size and lamellar orientation could be responsible for previous discrepancies in experimental reports of tissue moduli. We propose that this novel multiscale modelling approach can provide a more accurate description of bone tissue properties in continuum/organ level finite element models by incorporating information regarding tissue structure and composition from advanced imaging techniques. This approach could thereby provide a preclinical tool to predict bone mechanics following prosthetic implantation or bone fracture during disease.

Finite element analysis of effects of clearance on single shear composite bolted joints

Abstract A three-dimensional finite element analysis is presented on the effects of bolt-hole clearance in composite bolted joints. Both single bolt and multibolt single shear joints were modelled and the results are compared with those from a parallel experimental programme. The specimens studied were made from graphite-epoxy HTA/6376, with quasi-isotropic layups. Protruding head bolts of 8 mm diameter, torqued to finger tight conditions, were used. The models showed excellent ability to quantify the effects of increasing clearance, which included reduced contact area and overall stiffness in the single bolt case, and substantially changed load distribution in the multibolt case.

Modelling Bird Impacts on an Aircraft Wing - Part 2: Modelling the Impact With an SPH Bird Model

Abstract In a collaborative research project, aircraft wing leading edge structures with a glass-based Fibre Metal Laminate (FML) skin have been designed, built, and subjected to bird strike tests that have been modelled with finite element analysis. In this second part of a two-part paper, a finite element model is developed for simulating the bird strike tests, using Smooth Particle Hydrodynamics (SPH) for modelling the bird and the material model developed in Part 1 of the paper for modelling the leading edge skin. The bird parameters are obtained from a system identification analysis of strikes on flat plates. Pre-test simulations correctly predicted that the bird did no penetrate the leading edge skin, and correctly forecast that one FML lay-up would deform more than the other. The SPH bird model showed no signs of instability and correctly modelled the break-up of the bird into particles. The rivets connecting the skin to the ribs were found to have a profound effect on the performance of the structure.

BOLJAT: a tool for designing composite bolted joints using three-dimensional finite element analysis

This paper discusses the development of a software tool for design of composite bolted joints, using three-dimensional finite element analysis. The tool allows the user to create the joint geometry through a menu-driven interface and then generate a customised mesh according to the users needs. Contact parameters are defined automatically, which shields the user from the most difficult part of the process. Boundary conditions, bolt pre-loads, and material properties can also be set. Only a few manual steps are necessary to complete the finite element code generation process. By automating the time-consuming model creation process, the tool facilitates the increased use of three-dimensional finite element analysis in the design of composite bolted joints. A case study is shown to demonstrate the usefulness of the tool.

Numerical micromechanical investigation of interfacial strength parameters in a carbon fibre composite material

A finite element micromechanical model of a high strength composite material is subjected to a range of loading conditions to demonstrate its ability to predict failure. An investigation into the relative magnitude and distribution of the normal and shear stresses within the interface region of a single fibre embedded in a matrix region is compared to that of a multi-fibre representative volume element, where the fibre placement is statistically equivalent to that of a real material. A study is subsequently undertaken in which the relative magnitudes of the shear and normal strengths of the fibre–matrix interface are varied under transverse tension and shear. The results are interpreted with relation to the yield strength of the matrix. The predicted performance of the composite is shown to compare well with published experimental data, under transverse tension and in-plane shear. It is concluded that a single set of interface strength parameters can be used to represent the behaviour of the composite material. The results also show that interfacial shear strengths are expected to be equal, and higher than the interfacial normal strength.

COMM Toolbox: A MATLAB toolbox for micromechanical analysis of composite materials

The COmposite MicroMechanics (COMM) Toolbox is a design tool, developed in MATLAB, which provides efficient pre- and post-processing capabilities for micromechanical analyses of composite materials with finite element analysis. The COMM Toolbox has automated all manual tasks associated with micromechanical analyses of composite materials, providing a simple, convenient environment for creating, submitting, monitoring, and evaluating results from micromechanical analyses. The interactive pre-processing capability currently enables a variety of fiber distributions to be generated, allowing either nonuniform or regular fiber arrangements for both low and high fiber volume fractions to be analyzed under mechanical and/or thermal loading. The functionality of the above features has been demonstrated by carrying out a case study examining the effect of fiber volume fraction on material behavior. Importantly, the COMM Toolbox means that advanced multiscale modeling concepts, which determine material behavior based on the physical interactions of the constituent phases, are likely to gain more widespread exposure from potential academic or industry-based interests. The COMM Toolbox is freely available to the community and can be obtained by contacting the corresponding author.

Formation of reworkable nanocomposite adhesives by dielectric heating of epoxy resin embedded Fe3O4 hollow spheres

Epoxy resin (ER) thermosetting adhesives provide highly cross-linked 3-dimensional structures leading to highly stable and strong mechanical/physical performance in a wide range of bonding applications. However, such excellent physical attributes pose a significant challenge with respect to disassembly of the bonded adherends and previous disassembly methods have resulted in damage to the adherends. Hence, this paper presents a specifically engineered re-workable nanocomposite adhesive, created by embedding dielectric sensitive Fe3O4 hollow nanospheres (HNSs) in epoxy resin. This nanocomposite adhesive can be completely degraded by dielectric heating, resulting in facile disassembly of bonded adherends. FESEM and 3D Micro-CT characterisation demonstrates good dispersibility of the HNSs in cured ER, while the dielectric degradation performance and hardness/modulus were investigated by FESEM and nanoindentation. Results show that the Fe3O4 HNSs can effectively convert the microwave energy into thermal energy to significantly degrade the mechanical properties of the adhesive modulus and hardness by 83.4% and 90%, respectively. FESEM and HRTEM imaging attributes the reduction in nanocomposite adhesive properties to the formation of spatial voids nucleating from the embedded nanomaterials. Prior to dielectric heating, tensile loaded single lap-shear bonded joint tests indicated that the nanocomposite adhesive was 19.3% stronger than its neat ER adhesive counterpart due a nano-reinforcement toughening mechanism. However, after 3 minutes of dielectric heating exposure, the nanocomposite adhesive joint strength was reduced by 96.3% compared to just 18.7% for the neat ER adhesive, demonstrating the excellent re-workable performance of our new nanocomposite adhesive.

Micromechanical failure analysis of advanced composite materials

Abstract This chapter introduces the reader to the important area of micromechanical modelling for the analysis of failure in advanced composite materials. The chapter gives a general introduction to the micromechanical failure in composite laminates and describes finite element (FE) modelling of unidirectional composites to model such failure. We introduce the concept of representative volume elements and describe statistical methods to characterise composite microstructures in detail. A novel approach to re-create statically equivalent microstructure geometries, termed the nearest neighbour algorithm, is then developed and used to create advanced two- and three-dimensional FE models. The important concepts of periodicity and homogenisation are introduced, and then used to extract macroscale elastic moduli. Matrix plasticity and fibre–matrix interface failure models are then introduced and used to predict macroscale strength properties, where extensive material parameter, thermal and cyclic loading studies are carried out. We then develop applications towards modelling macroscale materials and structures by creating macroscopic failure surfaces and extending the micromechanical models to model composite–adhesive joints, where adhesive and composite failure coexist in one microscale model. Again, extensive parameter studies are carried out. A number of experimental techniques to both calibrate and validate micromechanical models are then described, with some applications. It is shown that whereas the micromechanical models require a solid understanding of composites, geometric topology, FE analysis and non-linear material modelling, once they are created they can be used for extensive and cost-efficient parameter studies for both composite material design and application to structural features such as bonded joints.