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This paper seeks to further investigate a recently proposed novel empirical-based Delamination Reduction Trend (DRT) for stitched composites. The DRT is capable of predicting the effective reduction in impact-induced delamination area due to the influence of stitching. DRT simply relates two parameters: normalised delamination area and stitch fibre volume fraction, to characterize the effectiveness of stitching in impact damage suppression. In this work, quasi-static indentation (QSI) test is performed on stitched composites with the aim to offer new observations, provide validations and understand limitations in DRT. Test data coupled with experimentally observed damage mechanisms reveal limitations of DRT, particularly across two regions: first, when damage is initiated and delamination growth has not been developed; second, when delamination propagation is influenced by boundary conditions and saturation of delamination damage has occurred. Validation of DRT is demonstrated in domain outside of these regions. The investigation is further analysed by statistical approach using a three-factorial design of experiments (DoE). The main influence and coupling interaction of stitch density, stitch thread thickness and indentation displacement on indent-induced delamination area are established and discussed. Graphical and statistical results provide further insights and validation to DRT. The novelty of this work lies in showing validation and understanding limitation of DRT for its effective use and accurate prediction of delamination damage in stitched composites., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations, 資料番号: PA1620008000

Many joint models available to predict secondary bending moments in the structure have a stiffness mismatch, while this type of structure widely used in aircraft. To determine how to represent a structure with a stiffness mismatch in a combined joint (bonded/riveted), a non-linear finite element analysis was performed. The detailed validation of this analysis identified the composite stiffened skin as the most suitable model in three dimensions. The use of this model for validating the secondary bending moment to calculate the behavior of the stiffener edge is straightforward and reliable. Experiments were performed to determine the distribution of the load in a combined joint under a tensile load that creates a secondary bending moment in a structure with a stiffness mismatch. The influence of related joint design considerations on the load transferred by the joint were examined through a finite element parameter analysis. The results are compared to determine best approach to predict the mechanical behavior at the edge of the stiffener. A close agreement between the finite element analysis and experimental results was obtained. Test observations using a C-scan compared well with the predictions of the onset of crack growth., 資料番号: PA1420016000

A joint combining riveting and bonding is considered in terms of structural performance if the composite structure has a mismatched stiffener. The transfer loading is correlated with high performance aerospace joints to increase delamination resistance in the out-of-plane direction. However, combined joints (rivet/bonded) will create a bearing area that induces another potential damage source aside from secondary bending moment on the edge of the stiffener. Another problem is that the structure is difficult to be inspected by using conventional methods because of limited accessibility. The use of embedded fiber Bragg grating (FBG) technology in the structure as a strain sensor can potentially solve the problem in structures that have a stiffness mismatch. The FBG can be used to detect and characterize delamination before it reaches a critical stage. The model used to represent this problem is a thin composite stiffened skin under two load cases: tension and three-point bending. Finite element modeling using a traction versus separation theory is performed to determine the critical area on the specimen for placement of the FBG before manufacturing and testing. Experiments were presented to determine the distribution of load in a combined joint under both loading cases using ideal loads to create a secondary bending moment and bearing loads in the stiffness-mismatched structure. In this research, the FBG successfully detected and characterized the delamination caused in both loading cases. In addition, FBG can predict the delamination growth quantitatively. A spectrum graph of the FBG results can be used to replace the conventional mechanical graph in composite structural health monitoring in real applications., 資料番号: PA1420017000

In this paper, the damage progression and failure characteristics of stitched composites under out-of-plane loading are experimentally investigated. Test specimens, stitched with various stitch densities and stitch thread thicknesses, are studied using quasi-static indentation test. Test specimens are loaded and unloaded in 0.5 mm incremental indentation displacement to examine for damage phenomena using non-destructive inspection techniques namely ultrasonic C-scan, X-ray radiography and X-ray micro-computed tomography to elucidate complex damage mechanisms and fracture behavior. Recorded test history of load–displacement curves indicate that damage progression can be characterized into three stages: damage initiation, damage propagation and final damage failure . Results show that damage initiation occurs at a lower load in stitched composites due to the presence of resin-rich regions which act as crack initiation sites. X-ray radiography convincingly shows that stitch-induced matrix crack lines are joined between stitch loops, being particularly evident in densely stitched composites. During damage propagation, stitching becomes highly effective in suppressing delamination growth, resulting in stitched laminates having much smaller delamination area compared to unstitched laminates, and the rate of delamination growth being inversely related to stitch density. It is found out that final failure is distinguished by a sharp load drop in the load–displacement curve. It is revealed that the final failure load increases with increasing stitch fiber volume fraction. The final failure mechanism in unstitched and moderately stitched composite is mainly delamination failure; while densely stitched composite failed by indenter penetration comprising of in-plane fiber fracture and matrix crushing. Energy absorption throughout the quasi-static indentation process is presented and discussed. This work provides novel insight to the damage progression and damage penetration of stitched composites.

宇宙構造・材料シンポジウム(第25回)(2009年12月4日、宇宙航空研究開発機構宇宙科学研究本部), 25th Symposium on Aerospace Structure and Materials (December 4, 2009. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)), Sagamihara, kanagawa Japan, 資料番号: AA0064732020

宇宙構造・材料シンポジウム(第24回)(2008年12月12日、宇宙航空研究開発機構宇宙科学研究本部), 24th Symposium on Aerospace Structure and Materials (December 12, 2008. Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency (JAXA)), Sagamihara, Kanagawa Japan, 資料番号: AA0064731017

資料番号: AA0063716012