V. P. Lawlor

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.

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).

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.

Measurement of load distribution in multibolt composite joints, in presence of varying clearance

Abstract An experimental study is presented on the effects of bolt-hole clearance on the load distribution in multibolt composite joints. Single shear, three bolt joints were studied. The specimens were manufactured from graphite-epoxy HTA/6376, with quasi-isotropic layups. Protruding head bolts of 8 mm diameter, torqued to finger tight conditions, were used. Different combinations of clearance were obtained by using four different reamers and a purpose designed, high precision drilling jig. Instrumented bolts with strain gauges attached for measurement of shear strain at the shear plane, were used to measure bolt load distribution. Prior to use in the multibolt joints, the bolts were calibrated in single bolt joints. For assembly of the multibolt joints, a special jig had to be designed to allow simultaneous aligning of the bolt along the axis of the joint, centring of each bolt in its hole, and torquing of the bolts to a prescribed level. The results of the study show that relatively small amounts of clearance can have substantial effects on load distribution. As an example, the middle bolt in a joint of this type is normally assumed to carry less load than the outer bolts, and is therefore considered not to be under threat of failure. But in the presence of clearance the load can shift to the middle bolt, potentially causing an unexpected failure mode.

Experimental study on effects of clearance on single bolt, single shear, composite bolted joints

Abstract An experimental study is presented on the effects of bolt-hole clearance in composite bolted joints, in single shear, single bolt configurations. Variable clearances were obtained using specially manufactured reamers, ranging from neat fit to 240 μm (which is somewhat larger than the largest allowed clearances in the aircraft industry). The specimens were manufactured in accordance with the appropriate ASTM standard from graphite-epoxy HTA/6376, with quasi-isotropic and zero dominated layups. Both protruding head and countersunk bolts were used, with two different applied torque levels. Purpose built jigs were designed for drilling high quality holes and centring the bolts within the holes (to remove bolt position as a variable). Some tests were performed to failure with several repeats of each clearance, and some to percentages of failure for later analysis with x-ray and microscopy. Joint stiffness, 2% bearing offset strength, ultimate bearing strength, and ultimate bearing strain were obtained. The main effects of clearance were on stiffness and ultimate strain with less effect on strength; a delay in load take-up also occurred with the higher clearance joints, which has implications for load distributions in multibolt joints.