CompositesWorld
Issue link: https://cw.epubxp.com/i/620463
JANUARY 2016 16 CompositesWorld DESIGN & TESTING » Unlike most mechanical tests that measure stifness and strength properties, fracture mechanics testing addresses the growth of delaminations in composite laminates. Te property measured is the material's critical energy release rate, G c , or fracture toughness. Tis experimentally measured value of G is compared to the available energy release rate, obtained from engineering analysis, to determine whether a composite delamination will propagate under a particular loading condition. Just as diferent test methods are used to measure stifness and strength properties under tension, compression and shear loading, there are diferent fracture toughness test methods and properties associated with three modes of delamination loading (Fig. 1). Mode I corresponds to an interlaminar tension loading and is the most commonly performed fracture mechanics test. Mode II corre- sponds to interlaminar shear loading, and is also commonly used when predicting delamination growth. Mode III corresponds to less commonly tested interlaminar "scissoring" shear loading. In actual composite structures, delaminations are subjected to a combination of loading modes and, thus, the fracture toughness corresponding to mixed-mode delamination growth is required. Here, therefore, we will focus on Mode I, Mode II and Mixed Mode (Mode I and II) testing, for which standardized tests exist. Tese three tests may be performed using specimens cut from identical 3-5-mm thick test panels in which a thin (13 µm) strip of non-adhering PTFE flm is inserted at the laminate midplane during panel layup to produce the required implanted delamination. Unidi- rectional laminates, with fbers oriented in the direction of delamina- tion propagation, are used to prevent delamination from migrating through the thickness to neighboring plies, invaliding test results. Mode I testing is performed using the Double Cantilever Beam (DCB) test specifed in ASTM D 5528 1 . Te specimen is 125 mm long, 25 mm wide, with a 63-mm long implanted delamination at one end. No specialized test fxturing is required, but loading hinges are bonded to the top and bottom surfaces for tensile loading the speci- men's delaminated end (Fig. 2a, p. 17). During the test, the displace- ment rate is a constant, and the applied load, crosshead displacement and delamination length are recorded as stable delamination growth occurs. Typically, specimen edges have been coated with white paint to aid in locating the delamination tip. Although the DCB test is relatively simple to perform, calculating the Mode I fracture toughness from test results can be confusing. For starters, G IC is calculated using beam theory equations, assuming that the portions of the specimen above and below the delamination act as dual cantilever beams. But because the supported end of the "beams" are not truly cantilevered, the equations must be corrected, using a calibration procedure that uses measured specimen compliances obtained for several delamination lengths. ASTM D 5528 describes three methods for performing this calibration and provides correc- tions to the beam theory equations that adjust either the delamina- tion length or the specimen compliance. Typically, all three (modifed beam theory, compliance calibration, and modifed compliance cali- bration) produce similar results. Modifed beam theory is the recom- mended method, but all three are listed on the recommended data reporting sheet. Additionally, three methods are provided for deter- mining the load at which delamination growth occurs, an input to G IC calculations. Te options currently provided in ASTM D 5528 produce a total of nine values for G IC , but eforts are underway to reduce the number of values and provide more specifc recommendations. Although G IC can be calculated for any delamination length produced during testing, values often increase as the delamination propagates due to fber bridging. Tat is, fbers remain intact across the delamination plane and resist delamination growth. Terefore, two initiation values of G IC typically are calculated, corresponding to the onset of growth from the implanted PTFE flm as well as a short, natural precrack. Although G IC for growth from the flm (non- precracked) typically is higher because of the blunt crack tip, it may be of use for predicting growth of some delaminations produced in composite structures during manufacturing. Te precracked G IC value is typically lower, and considered more representative for delamina- tions produced from impacts and for progressive damage modeling. For Mode II testing, the three-point End Notched Flexure (ENF) test was standardized as ASTM D 7905 2 in 2014. Te 160-mm long ENF specimen is loaded using a conventional three-point fexure fxture, with the 45-mm implanted delamination extending to the midpoint between the outer and central loading points (Fig. 2b, p. 17). Because delamination growth is unstable, compliance data for calibration purposes are obtained at relatively low load levels, shifting the specimen in the fxture to produce three delamination lengths. Similar to the DCB test, G IIC values are obtained for the onset of growth from the implanted insert and a natural precrack. Unlike the DCB test, ASTM D 7905 specifes a single calibration method (compli- ance calibration) and recommends a single method for load determi- nation (maximum load point), resulting in single G IIC values for the non-precracked and precracked conditions. Fracture mechanics testing of composites FIG. 1 Fracture mechanics loading modes. Source (all images) | Dan Adams Mode I Mode II Mode III