Effects of Mode Ratio, Ply Orientation and Precracking on the Delamination Toughness of a Laminated Composite

Abstract

Results are presented from an investigation of the effects of mode ratio, interfacial ply orientation and precracking on the delamination toughness of a graphite/ epoxy composite. Delamination toughness tests were performed on specimens that exhibited growth at 00/00, 15°/15°, 15°/- 150, 30°/30° and 30°/-30° interfaces. Three mode ratios were considered. The double cantilever beam test was utilized for pure mode I, the end-notched flexure test was used for pure mode II, and the symmetrically delaminated single leg bending test was used for a mixed-mode condition, G,,IG, equal to approximately 0.4. All five interfaces were tested in both precracked and non-precracked conditions at each mode ratio. Also, for the 30°/30° and 30°/- 30° interfaces, two different stacking sequences were used to assess the effect of remote ply orientation on the perceived mode I and mode II toughness. For any mode ratio and interface angle, it was found that precracked toughness values were always lower than the corresponding nonprecracked result. No other consistent trends were observed in all of the data. For the mode I tests, the 30/30 interface exhibited the lowest toughness, for the mixed-mode tests, the 0/0 interface produced a minimum toughness, and for mode II no significant effect of interface angle on toughness was observed. These results are interpreted with the aid of recent three dimensional finite element analyses for energy release rate distributions along the delamination front for the different stacking sequences and test geometries. The accuracy of various data reduction techniques are also evaluated, and suggestions are made for future testing.

Mixed mode I + II interlaminar fracture of carbon/epoxy laminates

“…However, actual applications involve mixed mode loadings. Therefore, recent focus has been on mixed mode I + II fracture [2][3][4][5][6][7][8][9][10][11][12][13]. The mixed mode bending (MMB) test is considered the most appropriate for characterising interlaminar fracture resistance of unidirectional (UD) specimens over a wide range of mode combinations [2][3][4].…”

“…However, the MMB test is not suitable for experimental compliance calibration and inconsistencies in specimen compliance and toughness values have been reported when using beam models [3]. On the other hand, few studies have been conducted on multidirectional (MD) specimens [2][3][4][5][6][7][8][9][10][11][12][13], which are more representative of actual applications. Moreover, complicated problems exist in testing of MD specimens i.e intraply damage [5][6][7][8][9] and ambiguity in mode partitioning for delamination between differently oriented plies [13][14][15].…”

“…On the other hand, few studies have been conducted on multidirectional (MD) specimens [2][3][4][5][6][7][8][9][10][11][12][13], which are more representative of actual applications. Moreover, complicated problems exist in testing of MD specimens i.e intraply damage [5][6][7][8][9] and ambiguity in mode partitioning for delamination between differently oriented plies [13][14][15]. The approaches proposed to avoid this problem consist in adopting a characteristic length of crack extension [14] or using empirical mode partitioning equations that make compatible fracture test results from various types of specimens [15] This paper reports a study of the mixed mode I + II interlaminar fracture of carbon/epoxy specimens [13].…”

Mode I interlaminar fracture of carbon/epoxy multidirectional laminates

“…ANALYSIS OF MULTIDIRECTIONAL DCB SPECIMENS MD laminates usually present elastic couplings which can be the source of significant errors in G Ic measurements [5,10]. For example, bending-bending couplings may generate highly curved thumbnail shaped delamination fronts, while membrane-bending couplings can induce significant contributions of thermal residual stresses to the measured G Ic .…”

A single leg bending test for interfacial fracture toughness determination

“…Equation (10) is likely to be more accurate, as there is less influence of uncertainties in material properties on Go. A preferable, and more accurate method of obtaining Gc is by the compliance calibration method of data reduction [26]. In this approach, the specimen's compliance as a function of crack length is first determined experimentally by appropriate placement of the specimen in the loading fixture.…”