Torque Limit for Composites Joined with Mechanical Fasteners

Current design guidelines for determining torque ranges for composites are based on tests and analysis from isotropic materials. Properties of composites are not taken into account. No design criteria based upon a systematic analytical and test analyses is available. This paper is to study the maximum torque load a composite component could carry prior to any failure. Specifically, the torque-tension tests are conducted. NDT techniques including acoustic emission, thermography and photomicroscopy are also utilized to characterize the damage modes. I. Introduction erospace structures utilize innovative, lightweight composite materials for exploration activities in low earth orbit, for the space shuttle and space station and will be used for future space exploration beyond low earth orbit to the moon, Mars and other destination as well as in future crew exploration vehicles. Due to size limitations, manufacturing facilities, contractual obligations or particular design requirements, the joining of composite components will be required. The common methodologies for joining composite components that have analytical precedence and practical applications are the adhesively bonded and mechanically fastened joints. In some applications both methods are simultaneously incorporated into the design. Guidelines and recommendations for establishing design criteria, analyzing and testing composites are readily available for engineers to adapt for their particular applications. However, guidelines and recommendations based on analysis and testing are not available for specifying a fastener torque range used in joining composite components. The purpose of this investigation is to develop an initial process for recommending a torque range to apply to metallic mechanical fasteners used to join composite components and select an acceptable non-destructive failure detection methodology to determine composite specimen failure. Recommended torque values to apply to fasteners vary from industrial specifications, fastener type or specific applications. At NASA’s Marshall Space Flight Center (MSFC) an in-house standard, MSFC-STD-486, “Standard Threaded Fasteners, Torque Limits for,” is the guideline used to specify torque values and ranges. The torque values specified in MSFC-STD-486 are based on fastener tests and are dependant upon the fastener material and strength. The torque value is directly related to the tension in the fastener. When the fastener is used to join components and is tightened, the bolt elongates producing a tension or pre-load in the fastener which then results in a compressive load on the components being mated. Isotropic materials used to join components have documented material properties for design and analysis purposes. An-isotropic materials, such as laminated composites, require material properties in the through-the-thickness direction that may not be available and have, generally, much lower strength than that in the plane of isotropic materials. One method for developing recommendations for torquing mechanical fasteners to join composite components involves testing composite specimens with bolts. To establish a testing methodology and acceptance/failure criteria the Torque versus Tension machine located at NASA’s Marshall Space Flight Center is used to perform these tests. In addition, an Acoustic Emissions transducer is placed on each specimen, to capture the transient elastic waves generated during the tightening sequence for later analytical evaluation, to “listen” for any cracking of the matrix material or fiber breakage resulting from the compressive force during the tightening process. After test completion