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Engineered Interfaces in Fiber Reinforced Composites Part 5

Tham khảo tài liệu 'engineered interfaces in fiber reinforced composites part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 104 Engineered interfaces in fiber reinforced composites Tj z -p 0 ợi a z . 4.29 The radial compressive stress qo is caused by the matrix shrinkage and differential thermal contraction of the constituents upon cooling from the processing temperature. It should be noted that 71 a z is compressive i.e. negative when the fiber has a lower Poisson ratio than the matrix vf vm as is the normal case for most fiber composites. It follows that 71 z acts in synergy with the compressive radial stress qo as opposed to the case of the fiber pull-out test where the two radial stresses counterbalance to be demonstrated in Section 4.3. Combining Eqs. 4.11 4.12 4 18 and 4.29 and for the boundary conditions at the debonded region L Q fm L o . 4.30 Solutions for the stress components are obtained as Of z U fT r l exp 2 z 4-31 4 z Ơ - ycu ỡ ơ 1 - exp 2 - z 4.32 Tm r z 2 - 0 1 g exp -2 - z 4.33 Tj a z Oị ỡ ơ exp -Ầ L-z . 4.34 Fig. 4.7 shows the approximate stress distributions in the constituents along the axial direction of the left half of the fiber for the carbon fiber-epoxy matrix composite with the relevant interface properties for the XA100 fibers given in Table 4.2. The FAS increases from the end towards the center while the MAS decreases in the same direction both of which are opposite to the axial stress distributions in fiber pull-out and fiber push-out to be shown in Sections 4.3 and 4.4. The IFSS increases from the debond crack tip towards the fiber ends in the debonded region. This response makes the debond propagation very difficult and is attributed to the radial compressive stress qi arising from the differential Poisson contraction between the fiber and matrix which should be added to the residual compressive stress qo. If the Poisson effects are completely neglected in the fiber fragmentation analysis the frictional shear stress would become constant over the whole debonded region. 4.23.2. Interface debond criterion The interface debond criterion used in this analysis