Stitch Thread Research Articles

Analysis on fracture behaviour of stitched foam sandwich composites using interlaminar tension test

Sandwich composites are susceptible to interfacial delamination, owing to the mismatches in the material properties between the face sheets and core. Previous studies have shown that stitching can improve the performance of sandwich composites. In this study, an analysis approach was developed to investigate the fracture behaviour of stitched foam sandwich composites. The stitched foam sandwich composites were manufactured by a vacuum-assisted resin transfer moulding process. Interlaminar tension tests revealed the effects of the linear thread density on the failure mechanisms of the stitched foam sandwich composites. Asymmetric double cantilever beam tests were performed to investigate the influences of the stitch thread reinforcement on the fracture behaviour. An analytical approach combining extended finite element method and nonlinear spring elements was proposed to predict the failure behaviour of the stitched sandwich composites. Experiment and simulation approaches were employed to investigate the influences of the stitch parameters (stitch pitch and linear thread density) on the ultimate load and energy absorption. The results show that stitched method can significantly enhance the mechanical properties of sandwich composites. The energy absorption and ultimate load values of the specimens tend to increase with an increase in the linear thread density or a decrease in the stitch pitch.

Interlaminar shear behaviour and meso damage suppression mechanism of stitched composite under short beam shear using X-ray CT

The stitching pattern and space play important roles in the mechanical behaviour of stitched composites, while their effects on interlaminar shear behaviour and corresponding damage suppression mechanism have not been studied. Here, the short beam shear (SBS) test and X-ray computed tomography (XCT) were systematically performed to explore the influences of different stitching patterns and spaces on the interlaminar shear behaviour and damage suppression mechanism of stitched composites. The SBS test results reveal that a reduction in the stitching space improves the interlaminar shear performance. Especially, when the stitching patterns enable the stitch thread to bind the load-bearing yarns, the final SBS load is increased after reaching the elastic limit. Furthermore, the XCT results identify the suppression mechanism. That is, the enclosed loops formed by the stitch thread separate the fabrics into several cells. Damage propagation is interrupted within the cells in the cross-sections containing the stitch thread. Damage propagation is also suppressed in the cross-sections parallel or perpendicular to the stitch thread. Stitched composites exhibit a lower damage extension and damaged volume than unstitched composites. In particular, when the stitching pattern is perpendicular to the damage propagation direction, the damage extension is significantly reduced and depends on the stitching space. The results obtained in this study can contribute to the mechanical performance design of stitched composites in future engineering applications through purposeful modulation of the stitching pattern and space.

Forming characteristics and surface damages of stitched multi-layered para-aramid fabrics with various stitching parameters for soft body armour design

In this study, effects of different stitching parameters on formability behaviour of multi-layerd para-armide panels were investigated. Different stitching pattern namely, Uni-axial longitudinal, Bi-axial straight, Uni-axial-diagonal, Bi-axial-diagonal, Edge stitch and Unstitched were used for binding different layers. Stitch length and gap parameters were also considered while formability using different values. Diamond quilted stitched with less stitch gap and short stitch length shows severe surface wrinkling and stitch thread breakage, whereas unstitched layers show a better deformability without wrinkling and stitch thread breakage. Similarly, diamond quilted stitched shows a very less drawing-in values both in warp and weft directions. As stitch density increases, the drawing-in measurement value reduced due the formation of more rigid layer to resist the deformation. Unstitch and diamond quilted preform possess the highest and least deformation recovery percentage respectivly. Moreover, deformation recovery percentage of the layers become very less as the stich gap and stitch length reduced.

Finite element analysis of mode II delamination suppression in stitched composites using cohesive zone model

To provide an accurate prediction of mode II delamination properties of stitched composites, a single stitched laminate under pure shear load (interlaminar shear) is investigated. An interlaminar shear test (IST) is conducted, using a specially constructed fixture. A finite element model of the IST is developed, using a spring connector to represent the stitch thread. Furthermore, an end notch flexural test is simulated based on the IST model and with a cohesive zone element used to simulate crack propagation. The effects of the cohesive zone modelling parameters (interfacial strength, stiffness and viscosity coefficient) are also investigated. Close agreement is obtained between the simulated and experimental results. Finally, the verified model is extended to study the effect of the number of stitch rows in the crack region. The results confirm that the crack propagation starting point (peak load) depends on the number of stitch rows in the crack region.

Further investigation of Delamination Reduction Trend for stitched composites

Abstract 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.

Finite element model for compression after impact behaviour of stitched composites

A finite element (FE) model using coupling continuum shell elements and cohesive elements is proposed to simulate the compression after impact (CAI) behaviour and predict the CAI strength of stitched composites. Continuum shell elements with Hashin failure criterion exhibit the composite laminate damage behaviour; whilst cohesive elements using traction-separation law characterise the laminate interfaces. Impact-induced delamination is explicitly modelled by reducing material properties of damaged cohesive elements. Computational results have demonstrated the trend of increasing CAI strength with decreasing impact-induced delamination area. Spring elements are introduced into the model to represent through-thickness stitch thread in the composite laminates. Results in this study validate experimental finding that CAI strength is improved when stitching is incorporated into the composite structure. The proposed FE model reveals good CAI strength predictions and indicates good agreement with experimental results, making it a valuable tool for CAI strength prediction of stitched composites.

Damage characteristics in 3D stitched composites with various stitch parameters under in-plane tension

Three-dimensional (3D) reinforcement by stitching is effective in improving the impact resistance of composites. Stitching, however, adversely affects the composite’s in-plane mechanical responses, and alters its damage mechanisms due to stitch-induced irregularities. We experimentally investigate the effect of two important stitch parameters, stitch density and thread diameter, on the damage characteristics of 3D stitched multidirectional composites under in-plane tension using X-ray radiography, X-ray micro-computed tomography and digital image correlation (DIC). Our study shows that composites stitched with thicker thread exhibit improved tensile strength due to effective hindrance of edge-delamination. We also found that stitch thread affects damage behaviors. A higher number of transverse cracks develops in the middle portion of thin 90° fiber tows; the inter-crack distance is reduced by dense stitching. DIC is able to identify the cracks that appear in resin-rich channels and distinguish strain fields due to different stitch densities.

Effects of stitch density and stitch thread thickness on mode II delamination properties of Vectran stitched composites

Mode II delamination properties of Vectran stitched composites were investigated, and tabbed end notch flexural specimen testing was used to prevent premature failure. The effects of stitch density and stitch thread thickness were explored, and fibre compaction due to the stitching process was also verified. The results show that, in moderately stitched laminates (low stitch density), the improvement in GIIC was negligible. Crack bridging by the stitch threads at the crack zone were mostly compensated for the effect of fibre compaction, which reduced the GIIC values. Conversely, in densely stitched laminates (high stitch density), GIIC values were improved significantly (2·4 times higher than those of unstitched laminates). The effects of stitch thread thickness appeared to be negligible in moderately stitched laminates. For densely stitched laminates, thicker stitch thread (500 denier) possessed GIIC values that were 45·7% higher than thinner stitch thread (200 denier).

Effect of stitch density on fatigue characteristics and damage mechanisms of stitched carbon/epoxy composites

The effect of stitch density (SD) on fatigue life, stiffness degradation and fatigue damage mechanisms in carbon/epoxy (T800SC/XNRH6813) stitched using Vectran thread is presented in this paper. Moderately stitched composite (SD=0.028/mm2; ‘stitched 6×6’) and densely stitched composite (SD=0.111/mm2; ‘stitched 3×3’) are tested and compared with composite without stitch thread (SD=0.0; ‘unstitched’). The experiments show that the fatigue life of stitched 3×3 is moderately better than that of unstitched and stitched 6×6. Stitched 3×3 pattern is also able to postpone the stiffness degradation onset. The improvement of fatigue properties and postponement of stiffness degradation onset in stitched 3×3 is primarily due to an effective impediment of edge-delamination. Quantification of damage at various cycles and stress levels shows that stitch density primarily affects the growth rate of delamination.

PS17 4点曲げ試験による縫合複合材曲面板の力学特性の評価

Experimental evaluations of interlaminar tensile strength for stitched CFRP curved panels were carried out. Vectran^[○!R] fiber with 200 denier was used as stitch thread. L-shape specimens were loaded by four point bending test, and interlaminar tensile strength was measured. Interlaminar tensile strength of stitched specimens show significant increment compared to those of unstitched one. The fracture behavior of stitched specimens were examined by cross sectional observation. Almost all of the stitch threads were not broken. Stitching process sufficiently improve the interlaminar tensile strength.

Effect of stitch density and stitch thread thickness on damage progression and failure characteristics of stitched composites under out-of-plane loading

Abstract 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.

Effect of stitch density on tensile properties and damage mechanisms of stitched carbon/epoxy composites

Experimental investigation is performed to study tensile properties, damage initiation and development in stitched carbon/epoxy composites subjected to tensile loading. T800SC-24kf dry preforms with tow orientation of [+45/90/−45/02/+45/902/−45/0]s are stitched using 200 denier Vectran® thread. Modified-lock stitch pattern is adopted, and stitch density is varied, viz. moderate density (stitched 6×6: stitch density=2.8cm−2) and high density (stitched 3×3: stitch density=11.1cm−2). The stitched preforms are then infiltrated by epoxy XNR/H6813 using resin transfer molding process. Tensile test is conducted to obtain in-plane mechanical properties (tensile strength, failure strain, tensile modulus and Poisson’s ratio). Effect of stitch density on the mechanical properties is assessed, and it is found that stitched 3×3 modestly improves the tensile strength by 10.4%, while stitched 6×6 reduces the strength by only 1.4%. In stitched 3×3 cases, the strength increase is mainly due to an effective impediment of edge-delamination. Tensile stiffness and Poisson’s ratio of carbon/epoxy are slightly reduced by stitching. Fiber misalignment in in-plane and out-of-plane directions is responsible for stiffness reduction, whilst reduction of Poisson’s ratio is probably caused by the orthogonal binding effect of modified-lock stitch architecture. Damage mechanisms in stitched and unstitched composites are studied using acoustic emission testing and interrupted test coupled with X-ray radiography and optical microscopy. The detailed damage observation reveals that stitch thread promotes early formation of transverse and oblique cracks. These cracks rapidly develop, and higher density of cracks ensues in stitched composites. Although this behavior triggers early formation of delamination, stitched 3×3 effectively impedes the growth the delamination. In contrast, stitched 6×6 is ineffective in suppressing the delamination yet the cracks are vast in this specimen. One of the plausible reasons of the rapid development of cracks in stitched composites is fiber compaction effect whereby fibers are compacted and the gap among fibers is reduced. The verification of compaction effect is done experimentally by performing burn-off test to measure the local fiber volume fraction. It is confirmed that fiber compaction indeed occurs as indicated by higher local fiber volume fraction between stitch lines.

Tensile Failure Strength Analysis and Experimental Confirmation of Stitch Reinforced Composite of T-stiffened Structure

Based on the progressive failure analysis method, taking account of 7 kinds of failure modes: stitch thread failure mode, resin extend (or compress) failure mode, fibre extend (or compress) failure mode, extend (or compress) delamination failure mode, a new failure analysis FE model was proposed for predicting the tensile strength of stitch-reinforced composite T-stiffened structures. The 3D solid element and truss element were used to simulate laminate and stitch thread individually in the FE model. The Hashin failure criterion and stiffness degradation technology are used to analyze damage failure of the stitch T-stiffened structure. Comparing the initial and final damage failure strength with those of experiments on stitch composite T-stiffened structures validated the Python secondary development program for the FE model presented in the paper. There was good agreement between the failure strength from the damage failure analysis and that of the experiment. This indicated that the model was correct, and that it could be a reference for stitch-reinforced composite design and analysis.

Effect of stitch density and stitch thread thickness on compression after impact strength and response of stitched composites

In this paper, the damage failure and behaviour of stitched composites under compression after impact (CAI) loading are experimentally investigated. This study focuses on the effect of stitch density and stitch thread thickness on the CAI strength and response of laminated composites reinforced by through-thickness stitching. Experimental findings show that stitched composites have higher CAI failure load and displacement, which corresponds to higher energy absorption during CAI damage, mainly attributed to greater energy consumption by stitch fibre rupture. The coupling relationships between CAI strength, impact energy, stitch density and stitch thread thickness are also revealed. It is understood that the effectiveness of stitching has high dependency on the applied impact energy. At low impact energy range, CAI strength is found to be solely dependent on stitch density, showing no influence of stitch thread thickness. It is however observed that stitch fibre bridging is rendered ineffective in moderately stitched laminates during compressive failure, as local buckling occurs between stitch threads, resulting in unstitched and moderately stitched laminates have similar CAI strength. The CAI strength of densely stitched laminates is much higher due to effective stitch fibre bridging and numerous stitch thread breakages. At high impact energy level, CAI strength is discovered to be intimately related to both stitch density and stitch thread thickness. Since CAI failure initiates from impact-induced delamination area, stitch fibre bridging is considerable for all specimens due to the relatively large delamination area present. Stitch threads effectively bridge the delaminated area, inhibit local buckling and suppress delamination propagation, thus leading to increased CAI strength for laminates stitched with higher stitch density and larger stitch thread thickness. Fracture mechanisms and crack bridging phenomenon, elucidated by X-ray radiography are also presented and discussed. This study reveals novel understanding on the effectiveness of stitch parameters for improving impact tolerance of stitched composites.

Understanding effectiveness of stitching in suppression of impact damage: An empirical delamination reduction trend for stitched composites

In this paper, a novel empirical trend has been identified to understand the delamination reduction in stitched composites subjected to impact loading. This empirical-based Delamination Reduction Trend (DRT) is developed based on an extensive series of low-velocity impact tests using specimens of different laminate thicknesses, stitch densities and stitch thread linear (mass) densities, subjected over a range of impact energy levels. The DRT simply relates two parameters: normalized delamination area (DelamNorm) and stitch fiber volume fraction (Vft), to characterize the effectiveness of stitching in impact damage suppression. DRT evidently shows a bi-linear behavior: first, an initial linearly decreasing relationship of DelamNorm with Vft; and second, a plateau which indicates maximum delamination area reduction limit of 40% by stitching. Experimentally observed mechanisms are presented and discussed with the aim to justify and explain the bi-linear behavior of DRT. The DRT is further validated with numerous published literature results and has demonstrated excellent agreement.

X-ray radiography and micro-computed tomography examination of damage characteristics in stitched composites subjected to impact loading

In this study, X-ray radiography and X-ray micro-computed tomography (μCT) are employed to characterise impact damage in stitched composites, paying particular attention to the influence of stitch density and stitch thread thickness. Laminated composites, stitched with varying stitch densities and stitch thread thicknesses, are subjected to low-velocity impact loading at various energy levels. X-ray radiography is used to observe in-plane matrix cracks and overall delamination propagation; whilst μCT is exploited to examine cross-sectional views showing detailed through-thickness matrix cracks distribution and 3D delamination damage pattern. It is revealed that stitches act as crack initiation sites, due to the presence of weak resin-rich pockets around stitch threads, thus inevitably resulting in densely stitched composites having more stitch-induced matrix cracks upon impact loading. It is also concluded that specimens with higher stitch density and thread thickness are more capable of impeding delamination growth by effectively bridging delamination cracks and arresting crack propagation. μCT evidently shows that 3D delamination pattern in densely stitched composite is more cylindrical in shape, whereas that of a moderately stitched or unstitched composite is more conical.

Impact Damage Resistance, Response, and Mechanisms of Laminated Composites Reinforced by Through-Thickness Stitching

In this article, the study of impact damage of laminated composites reinforced by through-thickness stitching is investigated and presented in threefold. Specimens stitched with varying stitch density and stitch thread thickness are subjected to low-velocity impact via a drop-weight machine. Impact damage resistance is first studied by examining the extent of delamination area in damaged specimens using ultrasonic C-scan analysis. It is revealed that higher stitch density is more capable of impeding delamination growth by arresting cracks at closer interval and suppressing crack propagation. The use of thicker stitch thread offers slight improvement to damage resistance by marginal reduction in delamination propagation, and is more pertinent at high impact energy levels. Impact damage response is then analyzed from the impact history response curves of impacted laminates. The impact response of load–time graphs demonstrates that the onset of delamination is not influenced by stitch density and stitch thread thickness, but the maximum residual impact force is related to the delamination size of the laminates, which is sequentially related to stitch parameters. Finally, impact damage mechanisms are elucidated by employing X-ray radiography and micro-Computed Tomography to reveal subsurface damages, primarily dominated by intralaminar matrix cracks, interlaminar delamination, and stitch fiber/matrix debonding. It is revealed that stitches act as crack initiation sites, due to the presence of weak resin-rich pockets around stitch threads, thus inadvertently resulting in densely stitched composites having more stitch-induced matrix cracks upon impact loading. Contrarily, specimens with higher stitch density and thread thickness are more capable of impeding delamination growth by effectively bridging delamination cracks and arresting crack propagation. Principal mechanisms responsible for impact resistance performance of stitching namely crack arresting and crack bridging are presented and discussed.

Effect of stitch density and stitch thread thickness on low-velocity impact damage of stitched composites

Effect of stitch density and stitch thread thickness on low-velocity impact damage of stitched composites is investigated experimentally. Physical examination on damage surfaces shows that stitches perform as crack initiators, as well as crack arrestors. Longer matrix cracks are observed in densely-stitched composites, in contrast to isolated matrix cracks found in moderately-stitched composites. Ultrasonic C-scan evidently compares the delamination areas and concludes that specimens with higher stitch density and thread thickness are more capable of impeding delamination growth by effectively bridging delamination cracks. Load–time curves reveal that the onset of delamination is not influenced by stitch density and thread thickness. Energy consumption for the impact event is evaluated and discussed with the conclusion that, although absorbed energy is independent of stitch density and thread thickness, the proportion of energy consumption for damage mechanisms like delamination, matrix cracks and stitch debonding are different for laminated composites stitched with different stitch parameters.

Stitch fiber comparison for improvement of interlaminar fracture toughness in stitched composites

In this study, strain energy release rates are measured and compared for laminated composites reinforced by through-thickness stitching using different stitch fiber materials — Carbon, Kevlar, and Vectran. Strain energy release rates are evaluated experimentally using the DCB test and validated computationally using FEA. The FE model of the stitched composite incorporates a novel four-step stitch fracture process, namely: interfacial debonding, slack absorption, fiber breakage, and pull-out friction. The FE predictions of GIC show good agreement with the experimental results. It is revealed that the relationship between G IC and stitch density, or stitch thread volume fraction for all stitch materials follow a linear law. It is concluded that Vectran provides the toughest interlaminar reinforcement and is most suitable for stitch fiber application. The characterization and difference in fracture behavior between Vectran and Kevlar stitch fibers, by using a novel Interlaminar Tension Test (ITT), are presented and discussed.

Interlaminar Fracture Toughness of Vectran-stitched Composites - Experimental and Computational Analysis

In this article, mode I interlaminar fracture toughness (GIC) of Vectran-stitched laminated composite is determined experimentally and computa- tionally. Critical strain energy release rates are measured by performing double can- tilever beam test on composites stitched with Vectran as stitch fiber, and are found to increase with increasing stitch thread thickness and stitch density. It is also revealed that the relationship between GIC and stitch density or stitch thread volume fraction appears to be linear. Interlaminar tension test is conducted to identify important fracture behavior of a single Vectran stitch fiber thread. The finite-element (FE) model of the stitched composite incorporates the novel four-step stitch fracture pro- cess, namely, interfacial debonding, slack absorption, fiber breakage, and pullout friction. The FE predictions of load-displacement curves and critical mode I strain energy release rates show good agreement with the experimental results. The differ- ences in interdependent stitch mechanisms between moderately stitched and densely stitched composites are discussed.