Determination of Elastic Moduli of Fiber-Resin Composites Using an Impulse Excitation Technique

ABSTRACTThe elastic moduli of graphite/epoxy and graphite/cyanate ester composite specimens with variouslaminate lay-ups was determined using an impulse excitation/acoustic resonance technique andcompared to those determined using traditional strain gauge and extensometer techniques. Thestiffness results were also compared to those predicted from laminate theory using uniaxialproperties. The specimen stiffnesses interrogated ranged from 12 to 30 Msi. The impulseexcitation technique was found to be a relatively quick and accurate method for determining elasticmoduli with minimal specimen preparation and no requirement for mechanical loading frames.The results of this investigation showed good correlation between the elastic modulus determinedusing the impulse excitation technique, strain gauge and extensometer techniques, and moduluspredicted from laminate theory. The flexural stiffness determined using the impulse excitation wasin good agreement with that predicted from laminate theory. The impulse excitation/acousticresonance interrogation technique has potential as a quality control test.INTRODUCTIONThe objective of this test program was to determine if an acoustic resonance technique couldaccurately measure the elastic moduli of graphite epoxy materials and thus be used to supplementdata obtained by traditional mechanical test techniques. The advantage of the acoustic resonancetechnique explored for this memorandum is the reduced cost and time required to determinelaminate stiffness.The determination of elastic properties in composite materials typically involves the use of strain-gauged specimens subjected to mechanical loading. All aspects of both specimen preparation andtesting of these specimens are costly, time consuming, and are performed on specimens preparedindependently from the actual service components. The traditional elastic modulus measurementsbegin with the preparation of a sheet of material nominally 9 to 12 inches square. The sheet istrimmed and has fiberglass tabbing material bonded to the specimen in four places (top andbottom, front and back); the bonding of the tabs typically requires two bonding operations. Thecost is further increased by the use of tapered tabs. After the tabs are applied the tensile specimensare sliced from the sheet. Prior to testing, strain gauges are applied to the specimen surface, andthe lead wires are soldered to the strain gauge. The tensile specimen is loaded into a testingmachine and the load versus strain is recorded. After testing, the data is reduced to provide amodulus which is taken over a strain range that varies among testing specifications[I-4].This description of the overall procedure used to generate modulus data is provided to illustrateboth the time and the expense involved in measuring the mechanical properties of compositematerials. The transformation of composite materials into tensile specimens typically costs $100per specimen and takes a week or more to accomplish. By comparison, the impulseexcitation/acoustic resonance technique can be used on specimens that require substantially lesspreparation. The specimen used for the acoustic resonance technique need be only rectangular andof an appropriate size to have a frequency detectable by the acoustic detector. Both in-plane and