A modified short beam shear test was proposed to measure interlaminar shear strength (ILSS) of fiber reinforced carbon-carbon composites (CCC). The direct loading in the short beam shear (SBS) test is replaced by an indirect loading through a rubber pad and an aluminum seat. This loading reduced flexure stresses to less than one fourth of the SBS test and eliminated transverse shear stress concentration under the load point. Both beam and contact finite element analyses were conducted to validate the proposed method. The finite element analysis showed that the ILSS calculated from beam equation overestimates the strength by 5% for shear flexible composites. This reduction was found to be the same for both polymeric and carbon matrix composites. The test data confirmed the interlaminar shear failure and very low data scatter in measured strength. The measured ILSS of T300 CCC was 2.68 ksi and it is one of the highest values reported in literature.
A B S T R A C TA total fatigue life model proposed previously by the same authors for laminated composite structures subjected to mode-I fracture loading is evaluated by two newer models. The new models differ from the original model by normalization factor and approximation of stable delamination growth rate equation. All three models include the delamination growth in subcritical (delamination initiation), linear (stable growth) and final (unstable growth) fracture domains. All three models were evaluated by comparing the total life predictions for two different block loadings. One is a normal operation of a rotor crafttype structure and the other is an aggressive loading. Predicted results of the three models were compared with each other and with the experimental data. The material considered was woven-roving E-glass fibre and vinyl ester matrix and the laminate was made by vacuum assisted resin transfer molding.Keywords delamination growth rate; fatigue; glass/vinyl ester; resistance curve.
N O M E N C L A T U R Ea = delamination length da/dN = delamination growth rate G C = critical energy release rate G IC = opening Mode I interlaminar fracture toughness G Imax = maximum cyclic mode I energy release rate G IR = opening Mode I interlaminar fracture toughness resistance G Ith = threshold value of mode I energy release rate P Imax = maximum mode I cyclic load R = ratio of minimum to maximum cyclic displacement δ Imin = minimum value of cyclic displacement δ Imax = maximum value of cyclic displacement = delamination length correction factor for modified beam theory
I N T R O D U C T I O NSusceptibility to delamination is a major weakness of composite laminates. Knowledge of material's resistance to interlaminar fracture and fatigue is essential to establish design-allowable and damage-tolerance guidelines for structures. Fracture mechanics-based delamination growth models are required to predict fatigue life and establish suitable inspection intervals so that a delamination can be found and repaired long before it becomes critical or when the applied stress exceeds the residual strength of the component. Fatigue delamination growth laws that Correspondence: Kunigal Shivakumar. cover the threshold, the stable growth and the unstable fracture domains are needed for total life estimation. Such growth laws were proposed in the past for metallic materials 1,2 and are now becoming accepted in damage-tolerant designs. Similar methodology is needed for composite laminates. Hypothetically, we can assume that the delamination growth rate has three domains, namely, subcritical (slow growth or delamination initiation), linear (stable growth) and unstable growth rate domains (see Fig. 1).The growth rate depends on microscope details of fibre architectures and resin properties in domain 1; on crack driving force (energy release rate G or G) in domain 2; and on the fracture characteristics in the unstable or the third domain. In composite laminates substantial research 3-13
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