3-D Experimental and FEA Investigations of Thick Single-lap Bolted Joints

Stresses in single-lap bolted joints of thick plates are complex and difficult to analyze. The few previous studies of stress through the thickness of bolted joints have been limited to finite element simulations and have been implemented only for the joining of relatively thin plates. Experimental and numerical analyses were conducted to analyze the stress distribution inside thick bolted plates along the bearing plane normal to the plate surface for both composite and isotropic materials. Experimental analysis was conducted via embedded-polariscope photoelasticity, embedded resistance strain gages, and embedded fiber-optic strain sensors. The FEM analysis was performed with commercial codes using material properties and other data obtained experimentally as input. Experimental and numerical results agreed reasonably well, and are believed to depict the behavior of the joint under load well enough to facilitate improvement of joint design. Some alternative designs were analyzed with the objective of decreasing the value of the maximum stress. The first plate presented a steel bushing around the hole, and the maximum stress in that case was decreased by 50%. The second design had the edges of the hole chamfered at a 45-degree angle. This design did not exhibit a decrease in the maximum stress, but it did show an advantageous change in the position of the maximum stress. INTRODUCTION Bolt fastening is the most widely spread method of joining because it is a non-permanent joint, it is easy to set up in a variety of conditions, it facilitates disassembly and repair, and, most of the time, it does not require manufacturing of special parts, which makes it less expensive. The problem of fastening composite materials using bolted joints is a very troublesome, because stress concentrations develop around the holes, severely reducing the strength and fatigue life of the structure. To utilize the full potential of Fiber Reinforced Polymers (FRP) in structural elements, appropriate methods for stress analysis must be developed. Camanho and Matthews (1997) [1] summarized the previous work in predicting the failure of bolted joint composites in their review paper. The conclusion was that by taking the composite to the ultimate load, very little information could be extracted. Most of the studies on mechanically fastened joints have focused on failure tests of the structures [1, 2, 3, 4, 5, 6, 7]. Relatively few studies deal with 3-D stress analysis of bolted joints. Among the first finite element solutions created for studying contact stresses were the ones of Chen, Lee and Yeh in 1995 [8]. They were based on an incremental variational principle and the transformation matrix derived from three-dimensional contact kinematics conditions. The model took into account the effects of friction, clearance, bolt elasticity, stacking sequence and contact tractions around the bolted joint. Another finite element analysis for laminate composite joints was developed by Barbero, Luciano, and Sacco [9]. In their paper, a contact/friction iso-parametric finite element for the analysis of connections between laminated composite plates is presented. A three-dimensional finite element model of bolted composite joints was developed by I reman [1 0] to determine the non-uniform stress through the thickness of the composite laminates in Report Documentation Page Form Approved OMB No. 0704-0188 Public reporting burden for the collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, to Washington Headquarters Services, Directorate for Information Operations and Reports, 1215 Jefferson Davis Highway, Suite 1204, Arlington VA 22202-4302. Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to a penalty for failing to comply with a collection of information if it does not display a currently valid OMB control number. 1. REPORT DATE 14 OCT 2004 2. REPORT TYPE Journal Article 3. DATES COVERED 14-10-2004 to 14-10-2004 4. TITLE AND SUBTITLE 3-D EXPERIMENTAL AND FEA INVESTIGATIONS OF THICK SINGLE-LAP BOLTED JOINTS 5a. CONTRACT NUMBER