Predicting notched tensile strength of full-scale composite structures from small coupons using fracture mechanics

Abstract

The initial fracture propagation within a full-scale stiffened quasi-isotropic composite panel and coupons with stringer feet under tensile loads was investigated. The specimens were made from Non-Crimp Fabric through Vacuum assisted Resin Transfer Moulding. The failure loads of all configurations were successfully related using the same value of trans-laminar fracture energy. The method involved independent tests of scaled-down Over-height Compact Tension specimens and the Virtual Crack Closure Technique. It was found to be crucial to include the fully developed damage process zone in the crack length and to interpret the results carefully in order to identify the failure loads consistently.

In situ characterization of trans-laminar fracture toughness using X-ray Computed Tomography

“…In this paper, the ASTM E1922 standard [8] has been used and both its validity and limitations considered. The ASTM E1922 standard is capable of generating a partial Rcurve from in situ CT scans, which agrees well with the approximate value of 85 kJ/m 2 reported in [10] at about 3 mm effective crack increment in the 0° plies based on the previous interrupted OCT tests and conventional CT scans. Furthermore, the observed Rcurve behaviour is also consistent with the previously observed stable fracture propagation after fracture initiation in the full-size stiffened panel reported in [10], which was postulated to be due to the existence of an R-curve [10].…”

“…The in situ CT scan images are compared to the previous CT scan images from interrupted tests in Figure 5 [10]. Different from the conclusions in Hong et al [7] which compared in situ synchrotron X-ray against ex situ CT scan with dye penetrant, the quality of the current in situ CT scans was found to be as good as the conventional CT scans of the previous interrupted specimens with dye penetrant.…”

Deducing the R-curve for trans-laminar fracture from a virtual Over-height Compact Tension (OCT) test

“…On the one hand, R-curves have been reported for Mode I trans-laminar fracture e.g. in coupon tests [2][3][4][5][6][7] and observed in full-scale composite structural tests [8]. On the other hand, many existing approaches for modelling Mode I trans-laminar fracture [9][10][11][12] do not directly address R-curves and assume a constant Mode I trans-laminar fracture toughness value during fracture propagation.…”

“…In the current study, this crack length is referred to as the effective crack length aeff. This is a common procedure for the determination of Mode I trans-laminar fracture toughness for composites [8,27]. At the end of the paper an alternative definition of crack length, by distinguishing between the FPZ development and fracture propagation [26], is discussed.…”

Thickness effect on Mode I trans-laminar fracture toughness of quasi-isotropic carbon/epoxy laminates

“…is the half notch-to-width ratio, C is the initial notch length, W is the specimen width and ) sec( ) (   = f is a geometrical parameter to account for the effect of finite width and is equal to 1.025 for this case [11]. This equation was used for convenience, and is fine for comparison purposes, although it has been shown that the size of the damage process zone should really be accounted for in determining the true trans-laminar fracture toughness [12].…”