Unstitched Composites Research Articles

Drop-weight impact behaviour of stitched composites: Influence of stitching pattern and stitching space

The stitching pattern and space play important roles in the mechanical behaviour of stitched composites in the non-perforation and perforation impact loading. This paper presents the systematically drop-weight impact experiments to explore the influence of stitching pattern and space on the impact behaviour of stitched composites under a wide range of impact energies. The stitching effectively improves the peak load and the force plateau in the impact process, indicating the impact resistance of stitched composites is enhanced. Especially, the diagonal stitching pattern with the smaller stitching space provides the superior impact resistance. Besides, the fiber breakage and the delamination area of stitched composites are reduced compared with that of the unstitched composite. Whereas, under the non-perforation impact loading, with the increase of impact energy, the reduction in delamination area of stitched composites with the large stitching space and the longitudinal pattern is not obvious.

Experimental study on flexural strength of stitched foam composites

Sandwich composites and laminate composites are considered as major classes of lightweight structures in a wide range of engineering applications. Lightweight potential of fabric has demonstrated the specific mechanical properties of sandwich composites with or without core material. Many studies have shown that tension, compression, flexure, stress is improved or degraded up to 20% by a stitched process. The present paper focuses on the study of structural improvements of stitched foam composite and also to compare it with unstitched composite of the same glass fiber material. The use of vacuum bagging process to make the stitched foam composite structure and a simple hand lay-up technique for unstitched composite. These composites are made by using E Glass fiber 180GSM, aradur hy 951 hardener, Araldite ly 556 epoxy resin and glass fiber yarn. For stitched foam composite, laminates are stitched in transverse direction i.e., through thickness direction with high strength yarn. 3-point bending test is done to find flexural strength of both composites and compare their results stitched sample showed higher value of flexural strength and flexural rigidity after three-point bending test the strength of stitched composite is 40% more than unstitched composite material.

Effect of stitching on improving the tensile strength of sisal fabric/epoxy composites for internal bone plate applications

Permanent metallic implants have been used traditionally for orthopaedic bone fractures. Though metallic bone plates are medically acceptable, they are also linked with numerous weaknesses like a modulus disparity, corrosion, and delay in fracture healing. Hence, preparing biodegradable implants using bio-composites is the current trend in research. Natural materials may be a good alternative due to their properties closer to the mechanical properties of bone and easily biodegradable too. Sisal is one of the degradable and reprocessing natural fibers, which can be used for implant applications. Nevertheless, low stiffness and strength properties are a common issue in the natural-based composite. However, the high moisture absorption behaviour of natural fibers leads to a loss of mechanical properties. These issues in the natural composite structures can be resolved by stitching methods. Hence, in this connection, the strengthening of composite in the thickness direction was done through the stitching process on improving the tensile properties of non-treated sisal fabrics/biodegradable epoxy resin composite. The results showed that the stitching aided in avoiding the complete withdrawal of fibers by bonding in the thickness direction. The results also showed that an improvement of tensile strength in the stitched composite was around 90% higher than the non-stitched specimen. An in-vitro degradation test was done to estimate the reduction of mechanical strength during biodegradation. The study also proved that stitched sisal composite surges less moisture compared to unstitched composite. Though there was a slight decrease in tensile properties of the composite due to moisture, yet stitched composite could be employed as a bone plate due to their enough tensile strength. Thus, the decomposable stitched sisal composites are a capable substitute bone plate for patients, as they offer provisional mechanical support at the fracture site.

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.

Multifunctional Carbon Fiber Composite with Improved Electromagnetic Interference Shielding and Interlaminar Fracture Toughness Characteristics

In this study, multifunctional carbon fiber-epoxy composites with improved electromagnetic interference shielding and interlaminar fracture toughness were manufactured. The interest in manufacturing multifunctional composites has been driven by the need for materials that simultaneously perform structural and non-structural functions. Unidirectional carbon fiber-epoxy composite laminates stitched with Dyneema13, Copper, and Kevlar13 were prepared using resin infusion process. Representative specimens from the unstitched and the stitched composites were tested using rectangular waveguide and Mode I interlaminar fracture tests. The electromagnetic shielding effectiveness experimental results showed that stitched composites exhibited improved shielding effectiveness compared to unstitched composites. For example, composite stitched with copper showed the highest increase of 102.2% in shielding effectiveness whereas composite stitched with Dyneema13 showed the smallest increase of 75.4% in shielding effectiveness compared to the unstitched. Results revealed that the dominant effect of shielding in these composites is absorption. The Mode I experimental results showed that composite stitched with Kevlar13 exhibited the highest crack initiation interlaminar fracture toughness (GIC-initiation), whereas composite stitched with Dyneema13 exhibited the highest maximum crack propagation interlaminar fracture toughness (GIC-maximum). The experimental results comply with the goal of the study to produce multifunctional composite with improvement in the mechanical and electromagnetic properties.

Effect of Stitching Patterns on Tensile Strength of Kenaf Woven Fabric Composites

In the past years, the natural fibre had attracted much attention by many researchers as an alternative fibre to replace synthetic fibre composite materials today. An experimental study was conducted to explore the effect of stitching on woven kenaf fabric. The hand lay-up and vacuum bagging technique were used. The composites were made of woven kenaf fabric as a reinforcement and epoxy resin as a matrix. The composites were made in different patterns of stitches that split into two categories which were basic pattern that was stitched by the single cross which includes Vertical, Horizontal, Tilt 30 and Tilt 60 . The other one was complex pattern, stitched by the double cross which includes Box, Tilt 45 /90 , Tilt 30 /30 and Tilt 60 /60 . It was found that the increasing specific strength for single stitch composite of the V, T60, and T30 patterns were 14.51 %, 1.49 % and 0.64 % respectively, compared to the unstitched composite. The specific strength produced by V stitch which was about 9.53 MPa/g,was higher than other stitch patterns. The results also showed that the specific strength for all the double stitch patterns were gradually increased compared to the unstitched ones. T60/60 exhibited the highest specific strength which increased about 53.17 % compared to the unstitched composite. The evidence from this study suggested that the double stitch composite gave better performance in specific strength, while stitching patterns and stitching angle gave significant effect to the performance of woven stitch kenaf composite compared to the unstitched ones. Implications of the results and future research direction were also presented.

A novel quartz fibre reinforced dense polyimide matrix composite: effect of preform stitching

ABSTRACTA dense polyimide (PI) composite reinforced by quartz fibres (QFs), was prepared by vacuum assisted polymer infiltration and hot-pressing in this work. The microstructures and the mechanical properties of the as-fabricated composites were investigated. The results show a high flexural strength (≈686 MPa) of the composite, due to its dense nature. Moreover, there was no significant change in the bending strength of the stitched composites. The laminates mainly suffered delaminations and fracture of the QFs. The intralaminar bonding of the composite was enhanced by a novel stitching technique (all 28 layers stitched together by untwisted quartz filament tows in the plain stitch). As a result, the interlaminar fracture energy with 1 stitches cm−2 stitch density was increased by 87.1%, compared to that of the unstitched composite. In the case of stitched specimens, stitching plays an important role in improving the Gc and suppresses the influence of in-plane fibre orientation.

The Effect of Stitching with Conductive and Nonconductive Materials on the Mode I Interlaminar Fracture Toughness of Carbon Fiber Composites

This study presents the effects of the stitching with seven threads, having a variety of mechanical properties from brittle to highly ductile behavior, on the delamination resistance/interlaminar fracture toughness of stitched carbon fiber‐epoxy laminated composites. Conductive Copper, Titanium, and Kevlar metallic, as well as non‐conductive Dyneema‐13 Kg, Dyneema‐T90, Kevlar‐10 Kg, and Kevlar‐13 Kg threads were tested for their mechanical properties. Next, laminates of unidirectional carbon fiber‐epoxy composites stitched with these fibers were fabricated using a vacuum assisted resin transfer molding process. Double cantilever beam tests were conducted on samples from the stitched and unstitched composites for comparison. Subsequent fractographic analysis using scanning electron microscope and optical microscope were conducted to reveal the relation between the stitch type and the energy release rate. Experimental results showed that during the crack initiation stage, composites stitched with Kevlar metallic exhibited an average 393% increase in Mode I critical energy release rate, GIC, the largest increase of the seven thread material types studied. At the fully developed crack propagation stage, composites stitched with Dyneema exhibited a 984% increase in GIC. However, composite stitched with copper and titanium exhibited 375% and 497% increase in GIC at the propagation stage, respectively. SEM and optical microscopic analysis revealed a strong relation between the inherent features of the thread materials and the delamination resistance. POLYM. COMPOS., 40:E1252–E1262, 2019. © 2018 Society of Plastics Engineers

Interlaminar fracture toughness and electromagnetic interference shielding of hybrid-stitched carbon fiber composites

In the current study, the production of multifunctional hybrid-stitched composites with improved interlaminar fracture toughness and electromagnetic interference shielding effectiveness is reported. Unidirectional carbon fiber-epoxy composite laminates stitched with Kevlar, nylon, hybrid stitched with both Kevlar and nylon and unstitched were prepared using resin infusion process. Representative specimens from unstitched and stitched composites were tested using rectangular waveguide and Mode I double cantilever beam tests. The Mode I experimental results showed that composite stitched with Kevlar exhibited the highest crack initiation interlaminar fracture toughness (GIC-initiation), whereas composite stitched with nylon exhibited the highest maximum crack propagation interlaminar fracture toughness (GIC-maximum). The four-hybrid stitching patterns exhibited higher GIC-initiation than the unstitched and stitched with nylon composites and lower than stitched with Kevlar composite, whereas they had higher GIC-maximum than the unstitched and stitched with Kevlar composites, although lower than stitched with nylon composite. The electromagnetic shielding effectiveness experimental results showed that stitched composites exhibited improved shielding effectiveness compared to unstitched composites. For example, composite stitched with nylon had highest shielding effectiveness value of 52.17 dB compared by the composite stitched with Kevlar which had 40.6 dB. The four hybrid-stitched composites exhibited similar shielding effectiveness with an average value of 32.75 dB compared to the unstitched composite shielding effectiveness of 22.84 dB. The experimental results comply with the initial goal of this study to manufacture multifunctional hybrid stitching composites with combined properties between Kevlar and nylon-stitched composites.

Impact Performance of Stitched and Unstitched Composites in Extreme Low Temperature Arctic Conditions

The impact performance of T650-35 stitched polyimide laminates and unstitched CFRP woven laminates is investigated at 23 and − 70 °C. Three impact energy levels of 6.7, 13.4, and 18.4 J/mm are selected to analyze the resulting damage modes. The results from the force–displacement and the absorbed energy curves evidently show that at − 70 °C, the specimens have noticeably higher initiation damage force for both material systems. Moreover, more significant load drops are observed at − 70 °C curves compared to the room temperature curves, which imply that multiple damage modes exist in greater extent at low temperature. The results further show that the T650-35 polyimide composites require higher impact load to initiate the matrix damage, indicating that the stitches efficiently suppressed the crack propagation on the samples. Furthermore, the T650-35 stitched polyimide composites can carry higher damage loads in both temperatures compared to the unstitched CFRP woven composites. X-ray micro-computed tomography technique is employed to reveal multiple complex impact damage modes. Results show that for unstitched composites impacted at 6.7 J/mm, − 70 °C samples experience multiple damage modes that include significant delamination, fiber breakage. However, for stitched composites impacted at 13.4 J/mm, similar complex damage modes are observed at 23 and − 70 °C.

Compression after low-velocity impact (CAI) properties of multistitched composites

ABSTRACTThe objective of this work was to investigate compression after low-velocity impact (CAI) properties of a multistitched composite. It was found that the CAI strength of the multistitched composite was high compared to the unstitched composites. In addition, stitching density, stitching type, stitching direction, and stitching fibers were found to be important structural parameters. The CAI load on the multistitched composites resulted in lateral damages near the impacted indentation region. The structures showed large damaged areas, but the out-of-plane layer delamination was confined to relatively small areas. Thus, the multistitching enhanced the CAI properties of the composite.

Characterization of multi‐stitched woven nano composites under compression after low velocity impact (CALVI) load

The purpose of this research was to determine the compression after low velocity impact (CALVI) properties of multi‐stitched nano composite. It was obtained that the CALVI strength of the multi‐stitched nano composite was higher than that of the unstitched composites with or without the nanomaterial. In addition, stitching types (hand stitch or machine stitch) and the particle size of nanomaterial or micro material types were found to be important structural parameters for the multi‐stitched nano composites. The CALVI load applied on the multi‐stitched nano composite resulted in lateral damages close to the impacted indentation region. The failure modes of the damaged zone were local matrix‐fiber debonding and delamination, kinking, and local multiple matrix‐fiber breakages in the in‐plane and out‐of‐plane directions of the composite. Multi‐stitched nano composite showed large damaged areas but the out‐of‐plane layer delamination was confined to relatively small areas. Thus, multi‐stitching and nano particle addition enhanced the CALVI properties of the composite structures. POLYM. COMPOS., 39:3750–3764, 2018. © 2017 Society of Plastics Engineers

In-plane shear properties of multistitched nano composites by bias tensile test

In this study, multistitched woven nano composites were developed and their in-plane shear properties were characterized. The in-plane shear strength of unstitched structure was low compared to that of the unstitched/nano structures. However, the in-plane shear strength of unstitched structure was high compared to the machine stitched structures because of stitching and stitching yarn-matrix interfacial region. Additionally, the in-plane shear strength of machine stitched/nano composite structure was slightly high compared to the unstitched structure. The multistitched and multistitched/nano structures had limited delamination in their cross-sections. Their delamination regions were confined at a narrow area due to multistitching. This was considered that the developed multistitched/nano composites has better demage tolerance compared to unstitched composites.

Compression properties and damage mechanisms of stitched carbon/epoxy composites

Abstract The effect of stitching on compression properties and damage mechanisms of carbon/epoxy composites is experimentally investigated. The waviness angle of load-bearing tows is also measured in both in-plane and out-of-plane directions, and a correlation is made between compression strength and the waviness angle. It is found that stitching generally reduces the compression strength of carbon/epoxy composites of up to 16%. The average compression strength has a stronger correlation with the in-plane waviness angle rather than the out-of-plane one. A comparative damage assessment, which studies the initiation, progression and final failure in stitched and unstitched composites under axial compression, reveals that the early cracking in resin-rich region is responsible for the lower compression strength experienced by the stitched composites. The interaction between resin cracking and fiber splitting near the flank of wavy fibers accelerates the development of fiber kinking, and causes an early failure in 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.

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.

Thermal degradation of the mode I interlaminar fracture properties of stitched glass fibre/vinyl ester composites

Changes to the Mode I interlaminar fracture toughness, GIc, and fracture mechanisms of stitched and unstitched fibreglass/vinyl ester composites were investigated after exposure to elevated temperatures. The fibreglass was stitched through the thickness with Kevlar®-49 thread in two orientations with two stitch densities, and then resin transfer moulded with a cold-curing vinyl ester resin. After curing at room temperature (∼20°C) for several weeks, the composites were heated to between 100 and 300°C for 1 h or at 175°C for times ranging from 0.25–100 h. The GIc values, which were measured using the double cantilever beam method, of stitched composites in the cold-cured condition were between 1.5 and 2.3 times higher than the unstitched composite. It was observed with scanning electron microscopy that this toughening occurred by deflection of the crack tip at the stitches, by the ability of the stitches to remain intact for a short distance (7–15 mm) behind the crack front, and by partial pull-out of broken stitches. The interlaminar fracture toughness of the unstitched composite increased slightly following heating, despite a possible breakdown of the chemical structure of the vinyl ester between 150 and 300°C. In contrast, the interlaminar toughness of the stitched composites was degraded significantly by heating, and this was probably caused by thermal deterioration of the Kevlar® stitches. This study reveals that the elevated-temperature post-curing of stitched composites will reduce the effectiveness of Kevlar® stitching in raising the Mode I interlaminar fracture toughness. © 1998 Chapman & Hall

Creep response of woven-fibre composites and the effect of stitching

This paper presents an investigation of the tensile creep behaviour of woven-fibre composites stitched with cotton and carbon threads along or perpendicular to the loading direction. Creep tests were conducted at ambient temperature with low to medium stress levels. It has been found that through-thickness stitching can significantly improve the creep resistance of composites, provided the stitches are aligned in the direction of loading. At a given time, the stress level required to induce the same amount creep strain in stitched composites was about twice that for unstitched composites. The creep data were analysed by using the Findley equation and the time-hardening exponent was found to remain approximately constant, while the amplitude of transient creep strain varied with both stress levels and stitching methods.

Erosion of impact-notched holes in GFRP composites

The sand-water erosion behaviour of notched holes in glass/epoxy composites was investigated. Using through-the-thickness stitching, the amount of hole damage caused by impact testing prior to wear tests was varied. Erosion resistance was evaluated based on the amount of mass loss and the percentage increase in hole diameter. Results showed that the holes in unstitched laminate which incurred greatest impact damage, eroded the least. The holes in stitched laminates, however, were more susceptible to erosive wear even though the internal damage appeared less severe. Scanning electron micrographs showed that the lower half of the holes in the unstitched laminates remained intact while those in the stitched laminates were eroded substantially. Cut sections away from the hole were observed with an optical microscope. A comparison of cut sections showed that heavy stitching results in severe fibre impalement damage, thereby reducing the reinforcing capability of the fibres around the hole. For the unstitched composite, in spite of more widespread impact-induced delamination damage, the inherent presence of protruding fibres in the backplies provided additional resistance against erosion.