CompositesWorld
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JULY 2016 12 CompositesWorld DESIGN & TESTING ยป In addition to mechanical tests used to determine material properties of composite materials, tests to determine struc- tural properties are commonly performed on multidirectional composite laminates. A subset of these are bearing tests, commonly performed to determine the bearing stress at which failure occurs in a composite laminate, or the bearing strength, of mechanically fastened composite joints. Bearing stress is defned as the applied load divided by the bearing area of the hole (hole diameter multi- plied by the specimen thickness). Bearing failures typically consist of crushing of a localized region of the composite laminate that is in contact with the fastener (Fig. 1). Te bearing strength of a composite laminate depends on many factors, including the joint confguration (single or double shear), the type of fastener (pin or bolt), the number of fasteners (typically, one or two) and the specimen loading (tension or compression). Two ASTM standards are available for bearing testing of compos- ites. ASTM D 953 1 , originally developed for plastics, uses a double- shear confguration intended for use with pins. ASTM D 5961 2 , developed specifcally for polymer matrix composite laminates, includes four joint confgurations, referred to as Procedures A-D. Additionally, SACMA SRM 9 3 uses a pin-loaded, double-shear confguration. (Te fxtures for all six are pictured in Fig. 2). Sawicki 4 provides an excellent comparative evaluation of the various bearing test methods available for composites. Perhaps the most signifcant diference between the tests is whether the fastener(s) are loaded in single or double shear. Single-shear bearing tests are described in ASTM D 5379 Procedures B and C for two- and one-piece composite specimens, respectively. Although single-shear joint confgurations are more commonly used in application, they produce an asymmetrical loading that results in bending of the composite specimen and fastener during loading. As a result, the bearing stress varies through the specimen thick- ness and the resulting bearing strength is reduced. In contrast, double-shear bearing tests, described in ASTM D 953, ASTM D 5379 Procedures A and D, and SACMA SRM 9, use a one- piece composite specimen fastened between two steel components of the loading fxture. Double-shear confgurations provide symmet- rical loading of the fastener and a relatively uniform bearing stress distribution through the thickness of the composite laminate. Te choice between a single-shear and double-shear confgura- tion often is made based on the intended uses of the test results. For material qualifcation and acceptance or as part of a material char- acterization program, it is desirable to have a bearing test confgura- tion that identifes material-related efects under bearing load. For these uses, double-shear confgurations are preferred, due to the more uniform bearing stress distribution. For use in structural design and analysis, the bearing test confguration typically is chosen to best represent the application of interest; thus single-shear confgurations are commonly used. Regardless of the joint confguration chosen, however, the measured bearing strengths cannot be considered to Bearing testing of composites be a material property, nor can the bearing strengths obtained from diferent joint confgurations be used interchangeably. Another important consideration in bearing test selection is the type of fastener used. Te choice of pins vs. bolts can afect the resulting bearing strength due to the diference in out-of-plane restraint provided in the region of bearing failure. Te through- the-thickness compression loading produced by a bolt torque can signifcantly increase the measured bearing strength compared to that produced using a pin. Because preloaded bolts are commonly used in actual applications, they are typically used in bearing tests conducted for structural design and analysis. Te efects of fastener-specifc preloading are eliminated using pinned connec- tions, and therefore pinned bearing tests are favored for compari- sons between diferent materials or laminates. Tese compari- sons are particularly important because diferences in bearing strength do not necessarily correlate with other laminate stifness or strength properties. Te test procedures in ASTM D 5961 may be used with either pins or bolts, but ASTM D 953 and SACMA SRM 9 are for use with pins only. In addition to the type of fastener, the amount of clearance between the fastener and the machined Fig. 1 Examples of bearing failures The photos show bearing failures in composite laminates without and with the fastener installed. Source | Dan Adams Fig. 2 Bearing test method fixtures These fxtures are used with the six methods discussed: (from left) ASTM D 953, ASTM D 5961 procedures A, B, C and D, and SACMA SRM 9. Source | Dan Adams5