IBOLT: A Composite Bolted Joint Static Strength Prediction Tool

Abstract

The objective of this paper is to describe the bolted joint analysis method developed and used by LM Aeronautics (named IBOLT) including the theoretical basis, required input, and coupon-level strength-prediction validation. IBOLT performs a fracture-mechanics-based static strength prediction for a rectangular composite joint element subjected to any combination of biaxial membrane loads, shear loads, and an off- axis bolt load. Out-of-plane bending moments are also treated. Joint configuration effects (single- or double-shear) are modeled by a beam-on-elastic-foundation analysis to account for the thickness and stiffness of the joint members as well as the bolt bending and shear stiffness. Empirical equations are included to account for fastener head geometry, effects of filled versus open holes, fastener/hole clearance, and fastener torque. Required input, in addition to the usual geometry, loads, fastener, and substrate elastic properties, includes a variety of special laminate-level joint properties. Uni-axial notched, un-notched, and bolted-joint coupon tests are conducted to develop these properties. In addition to these uniaxial coupon tests, bearing/bypass tension and bearing/bypass compression tests have been performed to validate the accuracy of IBOLT strength predictions. An overview of these validation results is provided, along with a brief summary of IBOLT's strengths and weaknesses as a practical bolted joint stress analysis tool.

Progressive Failure Analysis of Composites

“…Finally, examples of progressive failure analysis applied to bolted joints or pin-loaded laminates are given by Camanho [21,110], Xiao and Ishikawa [59], Tserpes [111], Qing [24], Chang [112], Riccio and Marciano [113], Kweon [67], Goswami [63], and Eisenmann and Rousseau [114]. McCarthy et al [61] studied the effect of bolt clearance and load bypass in progressive failure of multi-bolted double lap joints.…”

Residual strength of composite laminates containing scarfed and straight-sided holes

“…The early work by Whitney and Nuismer [1] and Pipes et al [2] yielded empirical models whose fitting parameters must be obtained by experiments for given stacking sequences and notch geometries. These types of semi-empirical methods require separate calibrations for specific configurations, such as the ratio of fastener load to bypass load, biaxial loading, stacking sequence of composite laminates, tapering geometry of countersunk fasteners, and out-of-plane bending [3].…”

“…Based on the concept of inherent flaws, Eisenmann and Rousseau [3] proposed an analytical method to incorporate the influence of laminate lay-up on the notch strength of composites. After accounting for other factors such as biaxial loading, fastener and hole configurations, this approach, named IBOLT, serves as the basis for designing bolted joints at Lockheed Martin Aeronautics (LM Aero) [3].…”

“…Based on the concept of inherent flaws, Eisenmann and Rousseau [3] proposed an analytical method to incorporate the influence of laminate lay-up on the notch strength of composites. After accounting for other factors such as biaxial loading, fastener and hole configurations, this approach, named IBOLT, serves as the basis for designing bolted joints at Lockheed Martin Aeronautics (LM Aero) [3]. Recently Camanho et al [10] reported that to accurately predict the notched strength of laminates the inherent-flaw based fracture mechanics approach would require the calculation of the length of inherent flaws for different notch geometries.…”

“…The results are compared to predictions of the ultimate strengths using a cohesive zone model, which is developed in this work for scarfed-hole laminates. The analysis is compared to the IBOLT inherent-flaw fracture mechanics approach developed at LM Aero [3], as an example of industry practice, and the continuum damage mechanics model implemented in Abaqus 6.7-1 [8,9], as an example of commercial FE codes. A focus of the analysis is to assess these approaches from the point of view of calibration of material properties.…”