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Engineered Interfaces in Fiber Reinforced Composites Part 13

Tham khảo tài liệu 'engineered interfaces in fiber reinforced composites part 13', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 344 Engineered interfaces in fiber reinforced composites and latter laminates. The tensile normal stress is harmful as it opens up the free edge leading to delamination. The presence of 15 and 45 layers in a laminate also changes drastically the magnitude and sign of the interlaminar normal stress ơz depending on the layer stacking sequence. Typical distributions of the interlaminar normal stress a. obtained near the free edge when subjected to an uniaxial tension are presented in Fig. 8.13 b Pagano and Pipes 1971 for the laminates with stacking sequences 15 45 s 15 45 -15 s and 45 15 s. It is clearly shown that the 15 45 s laminate has the highest tensile stress concentration in the mid-plane due to the largest difference in the stacking angle. From design considerations stacking sequence should be selected which can result in low tensile or compressive normal stresses under tension. The influence of material and stacking sequence on failure of boron fiber -epoxy matrix laminates was studied by Daniel et al. 1974 and is summarized in Table 8.3. It is noted that the ultimate tensile strength depends largely on the stress concentration and the volume fraction of 0 plies. Laminates with a high fraction of 0 plies but with sufficient number of 45 plies have the highest strength among those studied due to the low stress concentrations. Laminates without either 0 or 45 layers fail prematurely due to the delamination initiated from the free edges laminates without 45 plies give the lowest notch strength whereas those without 0 layers show the lowest unnotched strength Daniel et al. 1974 . The other parameter which influences the interlaminar stresses is the ply thickness. Thick plies tend to encourage higher interlaminar stresses thus causing premature delamination. It is shown that the critical strain for the onset of delamination decreases with increase in 90 ply thickness in the laminate in particular when placed in the midplane O Brien 1983 . Table 8.3 Effect of .