18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS PROGRESSIVE DAMAGE IN STITCHED COMPOSITES UNDER IMPACT LOADING K.T. Tan 1 *, N. Watanabe 1 , A. Yoshimura 2 , Y. Iwahori 2 , T. Ishikawa 2 1 Department of Aerospace Engineering, Tokyo Metropolitan University, Tokyo, Japan, 2 Advanced Composite Technology Centre, Japan Aerospace Exploration Agency, Tokyo, Japan * Corresponding author (tan-kwektze@sd.tmu.ac.jp) Keywords : stitching, impact, quasi-static indentation, damage progression, delamination 1 Introduction [+45/90/-45/0/0/+45/90/90/-45/0] S . The linear Damage in carbon fibre reinforced plastics (CFRP) density of Vectran stitch threads used in this study is due to impact loading is an extremely complex 200denier, with a stitch space and pitch of 3mm x phenomenon that comprises of multiple failure 3mm (densely stitched) or 6mm x 6mm (moderately mechanisms like intra-laminar matrix cracks, inter- stitched). Vectran is selected as the stitch fibre laminar delamination, fibre pull-out and fibre because, besides having comparable properties with fracture. In stitched composites, impact damage Kevlar, it is more superior due to its very low behavior is further complicated by the presence of propensity to absorb moisture and performs better in through-thickness stitching [1, 2], which not only interlaminar strengthening of stitched composites favorably increases mode I/II interlaminar strength [7]. The type of stitch used is the Modified Lock [3, 4], but also inevitably creates geometrical defects stitch. After the stitching process (if any), resin like weak resin-rich pockets around stitch threads transfer moulding (RTM) technique, using resin and misalignment of in-plane fibres. Computational XNR/H6813, was adopted to consolidate the modeling has been used to simulate progressive composite. Specimens of 100mm width and 150mm damage effectively [5]. However, the complexity of length were then cut out from a mother plate using a impact damage progression in stitched composites diamond wheel cutter. The averaged plate thickness would need to be first understood and appreciated by of the 20-ply specimen is 4.1mm. All specimens are physical experimental observations. physically examined for any poor-resin regions and In this study, quasi-static indentation (QSI) test is ultrasonic C-scanned for any internal delamination performed for the first time on stitched composites. to ensure that they are free from any manufacturing QSI offers a good validation and comparison with related defects. low-velocity impact (LVI) test [6], and provides 2.2 Experimental Methodology good understanding on damage progression in Quasi-static indentation (QSI) test was performed composite structures under impact loading. Damage initiation, propagation and ultimate failure are using Instron 8852 test machine (100kN load cell) with a displacement rate of 0.5mm/min. The semi- investigated due to the effect of stitching, particularly the influence of stitch density. Non- spherical indenter has a diameter of 15.9mm and the specimen was placed on a support frame similar to destructive evaluation (NDE) techniques namely the one used in the low-velocity impact (LVI) test ultrasonic c-scan analysis, x-ray radiography and x- ray micro computed tomography are employed to [1, 2]. After each indentation step of 0.5mm, the specimen was unloaded and observed for damages, elucidate various damage mechanisms in stitched by conducting ultrasonic inspections using a 5 MHz composites. probe and X-ray inspections using ZnI 2 penetrant. The specimen was subjected to both x-ray 2 Experimental Details radiography and x-ray micro-computed tomography examination to observe in-plane and cross-sectional 2.1 Test Specimens damages respectively. More details on these non- The specimens were made using T800SC-24K destructive evaluation techniques can be referred to (Toray Industries) carbon fibre fabric of 20-ply [8]. After damage inspection, the specimen was
again subjected to indentation at higher loading at delamination propagation. This is due to the fact that the next 0.5mm step. By repeating these steps, when stitches are closely spaced, stitches act as damage progression of stitched composite was effective crack arrestors for delamination damage, investigated. The test was terminated once final thus preventing sudden delamination widespread. failure was reached. This test was similarly repeated (a) Densely Stitched for all unstitched and stitched composites. 3x3 Final Failure P failure 16 0.5mm 3 Results and Discussion 1.0mm 14 1.5mm 2.0mm 3.1 Load-Displacement Curves 12 Damage 2.5mm Force (kN) Propagation 3.0mm 10 Typical Load-Displacement curves obtained from 3.5mm 4.0mm 8 the quasi-static indentation tests are presented in Fig. 4.5mm Damage 1. It is observed that damage progression can be 5.0mm 6 Initiation 5.5mm categorized into three stages: damage initiation , 6.0mm 4 P initiation 6.5mm damage propagation and final failure . 2 7.0mm 7.5mm Damage initiation is characterized by the first 0 observable slope change in the load-displacement 0 1 2 3 4 5 6 7 8 graph, which often occurs between 1.0mm to 1.5mm Indentation Displacement (mm) indentation displacement in this study. Final failure (b) Moderately Stitched is identified by the maximum force in the load- 16 0.5mm 6x6 1.0mm displacement curve, which is accompanied by an 14 1.5mm 2.0mm abrupt load drop. Final failure occurs during the 2.5mm 12 3.0mm 7.5mm step for densely stitched composites (Fig. 1a), Force (kN) 3.5mm 10 and the 8.5mm step for moderately and unstitched 4.0mm 4.5mm 8 specimens (Fig. 1b, c). The force values of damage 5.0mm 5.5mm 6 initiation and final failure for all specimens are 6.0mm 6.5mm extracted from all curves and the average values are 4 7.0mm 7.5mm given in Fig. 2. It is revealed that final failure load 2 8.0mm 8.5mm increases with increasing stitch density, as stitches 0 are effective in suppressing delamination growth and 0 1 2 3 4 5 6 7 8 9 eventually raising ultimate strength [1]. However, Indentation Displacement (mm) results also showed that damage initiation load (c) Unstitched decreases with increasing stitch density. This means 16 0.5mm 1.0mm that stitched composites are more susceptible to 14 1.5mm 2.0mm damage initiation at lower loads and this is attributed 12 2.5mm to the presence of weak resin-rich region around Force (kN) 3.0mm 3.5mm 10 stitch loops, acting as crack initiation sites [2]. 4.0mm 4.5mm Detailed discussion will be made in later sections. 8 5.0mm 5.5mm 6 6.0mm It is further observed that during the damage 6.5mm 4 propagation stage (after damage initiation and 7.0mm 7.5mm before final damage failure), densely stitched 2 8.0mm 8.5mm composite exhibits very smooth curve line, 0 indicating that composite damage is gradual and 0 1 2 3 4 5 6 7 8 9 delamination spread is smooth. However, in Indentation Displacement (mm) moderately stitched and unstitched composites, the Fig.1. Load-Displacement Curves of (a) Densely curves exhibit some irregular curve pattern with Stitched; (b) Moderately Stitched; (c) Unstitched small abrupt load drop, especially at higher Composites. displacement, implying sudden damage growth and
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