M.A. 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.

Numerical investigation of a crash test of a composite helicopter subfloor structure

Abstract Composite energy-absorbing aircraft structures are being studied within a European Commission research programme (CRASURV – Design for Crash Survivability). One of the aims of the project is to evaluate the current capabilities of crashworthiness simulation codes for modelling future composite primary structures. In this paper, a detailed analysis is presented of a generic module of a composite helicopter subfloor structure, subjected to crash loading. The analysis is performed with the explicit finite element code PAM-CRASH and is compared with the results of a drop test. It has been found that pre-test simulations with only coupon data as input are capable of providing a reasonable overall representation, but to closely match the behaviour of the test, a significant amount of post-test work is required. The calibration of the post-failure material properties proved to be more crucial than the behaviour up to initial failure. The representation of fabric materials was found to be inadequate and a new fabric material model is under development as a result. The importance of modelling frictional effects was highlighted, and a mesh density study showed the model to be robust over a range of mesh densities.

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.

Modelling of Bird Strike on an Aircraft Wing Leading Edge Made from Fibre Metal Laminates – Part 1: Material Modelling

Fibre Metal Laminates with layers of aluminium alloy and high strength glass fibre composite have been reported to possess excellent impact properties and be suitable for aircraft parts likely to be subjected to impacts from objects such as runway debris or birds. In a collaborative research project, aircraft wing leading edge structures with a glass-based FML skin have been designed, built, and subjected to bird strike tests that have been modelled with finite element analysis. In this first part of a two-part paper, a material model developed for FML suitable for use in impact modelling with explicit finite element analysis is presented. The material model is based on a recent implementation in the commercial finite element code PAM-CRASH/SHOCK of a Continuum Damage Mechanics (CDM) model for composites, incorporating anisotropic strain rate effects. Results from the model are compared with experimental results on FML at variable strain rates and the model is shown to be capable of capturing most of the complex strain rate dependent behaviour exhibited by these materials.

An Experimental Study of Bolt-Hole Clearance Effects in Single-lap, Multibolt Composite Joints

An experimental study on the effects of variable bolt-hole clearance in single-lap, multibolt composite joints is presented. To the authors’ knowledge, this is the first time that clearance effects in multibolt joints have been quantified experimentally. Joints with different clearances in each hole have been tested and the effects on load distribution, quasi-static strength, fatigue life, and failure modes are reported. Instrumented bolts have been used for measuring load distribution and specialized jigs have been used for positioning the bolts in the holes and drilling the joints. The clearances examined ranged from neat-fit to clearances slightly larger than those allowed in the aircraft industry. Earlier finite element studies with linear elastic material properties have indicated a significant effect on load distribution, and have postulated significant effects on strength due to reduced contact areas between bolts and holes, with correspondingly increased bearing stresses. The present experimental results confirm that clearance has major effects on the load distribution; the measured load distribution effects agree well with a three-dimensional finite element analysis with a nonlinear contact analysis. However, no significant effect on the ultimate quasi-static strength has been found, which is in line with earlier experimental studies on single-bolt joints. On the other hand, clearance has been found to have quite significant effects on fatigue life, which represents the first published data on this issue to the authors’ knowledge. For both quasi-static and fatigue loadings, clearance had a stronger influence on failure initiation loads than on ultimate failure loads, indicating that clearance is of concern when designing for no damage (e.g., limit load design of aircraft).

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.

Analytic integration of kernel shape function product integrals in the boundary element method

Abstract In this paper, analytic integration procedures are presented for non-singular, nearly singular and nearly hyper-singular boundary element integrals in two-dimensional (2-D) elastostatics. Both curved and straight boundaries are considered for this purpose. In the former case, a series approximation is adopted and in the latter case the integrals are evaluated exactly. In the analytical results the geometry is kept in symbolic form. Integral result for a particular element can be obtained after giving appropriate numerical values to the analytical results. Convergence aspects of these analytical results are studied in detail. The analytical integral results are then used for analysis of 2-D structures in elasticity.

Nonlocal elasticity based magnetic field affected vibration response of double single-walled carbon nanotube systems

The behaviour of carbon nanotubes in a magnetic field has attracted considerable attention in the scientific community. This paper reports the effects of a longitudinal magnetic field on the vibration of a magnetically sensitive double single-walled carbon nanotube system (DSWNTS). The two nanotubes of the DSWNTS are coupled by an elastic medium. The dynamical equations of the DSWNTS are derived using nonlocal elasticity theory. The two nanotubes are defined as an equivalent nonlocal double-Euler-Bernoulli beam system. Governing equations for nonlocal bending-vibration of the DSWNTS under a longitudinal magnetic field are derived considering the Lorentz magnetic force obtained from Maxwell’s relation. An analytical method is proposed to obtain nonlocal natural frequencies of the DSWNTS. The influence of (i) nanoscale effects and (ii) strength of longitudinal magnetic field on the synchronous and asynchronous vibration phase of the DSWNTS is examined. Nonlocal effects with and without the effect of magnetic field are illustrated. Results reveal the difference (quantitatively) by which the longitudinal magnetic field affects the nonlocal frequency in the synchronous and asynchronous vibration modes of a DSWNTS. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4720084]

Characterisation of damage development in single shear bolted composite joints

Abstract Experiments have been performed to study the force-deflection and damage development characteristics of bolted joints in carbon-epoxy composite materials, in the presence of variable bolt-hole clearance. Single lap, single bolt joint configurations, sized to induce bearing failure, were used. An initial set of tests involved loading joints up to ultimate failure. The primary failure mode was bearing failure. The secondary failure mode was bolt failure for the lower clearance joints, while the larger clearance joints exhibited large displacements without bolt failure. A further series of tests were then performed up to a load level corresponding to the first significant change of slope in the load-deflection curve of the larger clearance joints. These specimens were examined using optical microscopy and SEM to compare the damage in specimens with different levels of clearance. The joints with the largest clearance were found to exhibit the most damage.

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.