Effect of data reduction and fiber-bridging on Mode I delamination characterization of unidirectional composites

Abstract

Reliable delamination characterization data for laminated composites are needed for input in analytical models of structures to predict delamination onset and growth. The double-cantilevered beam (DCB) specimen is used to measure fracture toughness, G Ic , and strain energy release rate, G Imax , for delamination onset and growth in laminated composites under mode I loading. The current study was conducted as part of an ASTM Round Robin activity to evaluate a proposed testing standard for Mode I fatigue delamination propagation. Static and fatigue tests were conducted on specimens of IM7/977-3 and G40-800/5276-1 graphite/epoxies, and S2/5216 glass/epoxy DCB specimens to evaluate the draft standard "Standard Test Method for Mode I Fatigue Delamination Propagation of Unidirectional FiberReinforced Polymer Matrix Composites." Static results were used to generate a delamination resistance curve, G IR , for each material, which was used to determine the effects of fiber-bridging on the delamination growth data. All three materials were tested in fatigue at a cyclic G Imax level equal to 90% of the fracture toughness, G Ic , to determine the delamination growth rate. Two different data reduction methods, a 2-point and a 7-point fit, were used and the resulting Paris Law equations were compared. Growth rate results were normalized by the delamination resistance curve for each material and compared to the nonnormalized results. Paris Law exponents were found to decrease by 5.4% to 46.2% due to normalizing the growth data. Additional specimens of the IM7/977-3 material were tested at 3 lower cyclic G Imax levels to compare the effect of loading level on delamination growth rates. The IM7/977-3 tests were also used to determine the delamination threshold curve for that material. The results show that tests at a range of loading levels are necessary to describe the complete delamination behavior of this material.

Delamination growth in polymer-matrix fibre composites and the use of fracture mechanics data for material characterisation and life prediction

“…[47,48] were the first to note that, for a given R-ratio, the da/dN versus G max curve determined from DCB tests subjected to fatigue loading was not unique but depended on the starting value of G max . This dependency was subsequently confirmed in [4,42]. As a result it has been argued [47,48] that for real aircraft structures (from a design and from a sustainment/maintenance perspective) it is important to use the da/dN versus G max curve which has the fastest growth rate and a means for determining this curve was proposed.…”

“…Here it should be recalled that, when originally formulating the equations for crack growth, Paris argued that, since Irwin [20,21] had shown that the stress-intensity factor, K, uniquely characterises the near tip stress field, then the rate of FCG should be a function of ΔK and K max [22,23]. In [24] [2,[41][42][43][44][45][46][47][48][49][50][51][52][53][54][55]. (Where ∆√G is given by (�G max -�G min ); and G min is the minimum value of the energy release-rate in a fatigue cycle.…”

“…The apparent increase in fracture toughness due to fibre-bridging, measured using the Mode I DCB test subjected to quasi-static loading, has led several authors [4,13,14,42] to express the apparent toughness, G CR , as a function the length, a, of the advancing delamination by the amount by which the delamination exceeds its initial length, a 0 . The proposed expressions are of the form shown below, and it should be noted that these present discussions are all concerned with Mode I failure, and hence, the subscript 'I' has been omitted for clarity:…”

“…Murri [4,42] was the first to present experimental data that revealed the large scatter associated with da/dN versus G max curves from DCB fatigue tests. To illustrate the extent of the scatter, and hence the difficulty in determining the 'average' growth curve that is required in JSSG2006 [1], Figure 9 presents the da/dN versus G max curves presented in [4,31,39,42,50] for tests using an IM7/977-3 CFRP composite laminate.…”

“…The use of Mode I DCB tests is the most common approach employed to characterise delamination growth as a function of the energy release-rate, G [2,4,5,19,26,[38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][76][77][78]. However, a major problem with this method is the large retardation that can develop due to fibres bridging the delamination [4,5,13,14,36,42,49]. (A detailed review of the effects of fibre-bridging and the current test standards is given in [78].…”