Unstitched Laminates Research Articles

Influence of Stitch Angle on the Flexural Response of Composite Structures

Utilizing lightweight composites in aerospace vehicles plays a pivotal role in sustainability by improving fuel efficiency and mitigating emissions. However, due to their low interlaminar strength, delaminations and microcracking degrade their strength and stiffness properties. Through-thickness stitching improves the interlaminar strength, thus increasing the delamination tolerance of composite structures. Therefore, in this study, the influence of repeated periodic stitching patterns, stitched at three different angles (0, 45, and 90°) w.r.t. the transverse direction, on the flexural properties of stitched composites is investigated. The strain distributions of these specimens were obtained using a single continuous optical fiber bonded to the tensile surface of the specimens. The results show that stitching at a 45° angle produced lower strains and higher flexural strength and bending modulus when compared to the stitched and unstitched laminates. An increase of 32.3 and 47.4% in the flexural strength and bending modulus were obtained, respectively, with reference to the unstitched laminates. The failure morphology of specimens was also analyzed using optical microscopy and revealed that the stitch seams arrested delaminations. However, due to the 45° angle, the damage progressed along a longer distance, thus resulting in an increased ultimate failure load.

Effect of Stitching, Stitch Density, Stacking Sequences on Low-Velocity Edge Impact and Compression after Edge Impact (CAEI) Behavior of Stitched CFRP Laminates.

Low-velocity edge impact and compression after edge impact (CAEI) behavior of stitched carbon-fiber-reinforced plastic (CFRP) laminates were experimentally investigated in the paper. Five groups, including three stacking sequences (P1, P2, P3) and two stitch densities (stitch space × stitch pitch is 10 mm × 10 mm and 15 mm × 15 mm) of stitched/unstitched CFRP laminates, were prepared by the VARTM technique and subjected to low-velocity edge impact and compression after edge impact experiments. The damage of CFRP laminates was detected by optical observation and micro-CT. The effects of stitching, stitch density, stacking sequences and impact energy on properties of edge impact and CAEI were discussed. The results show that the damage of edge impact of stitched laminates is smaller than that of unstitched laminates. The main failure mode of CAEI of the unstitched laminates is delamination and that of the stitched laminates is global buckling. The addition of stitches can effectively improve the edge impact resistance and damage tolerance of CFRP laminates. Compared with the unstitched laminates with the same stacking sequence, the peak impact force of the laminates with stitch density 15 mm × 15 mm increases by 5.61-12.43%, and the increase in residual compression strength is up to 5-20.9%. The peak impact force of the laminates with stitch density 10 mm × 10 mm increases by 8.1-31.4%, and the increase in residual compression strength is up to 24.2-27%. Compared with the other two stacking sequences (P1 and P2), the stacking sequence P3 has excellent resistance of edge impact and CAEI properties.

Effect of Stitch Orientation on Tensile and Flexural Bending Mechanical Properties and Damage Mechanisms of Glass/Epoxy Composites Laminate

This study aims to investigate the effect of stitch row directions on tensile and flexural bending mechanical properties of composite laminates reinforced by glass-fabric. For the fabrication of stitched laminates, a polyester thread was used to stitch the dry fabric glass in fourth cases (longitudinal stitch 0°, transversal stitch 90°, multi-stitching 0°/90°, and 45°/-45° stitch) with 4 mm stitch spacing. The responses of stitched laminates specimens subject to the tests mentioned were compared to the unstitched laminates and discussed. According to our results, the effect of stitching plays opposite roles on the tensile property by decreasing of about 45% and 36% in tensile strength and longitudinal modulus compared to unstitched specimens, respectively. However, an increasing approximately 11.69% to 20.25% in flexural strength found in samples stitched along 0° and multi-stitching 0°/90°, due to the stitch lines through-thickness by arrests temporally of cracks propagation and the delamination progressively propagate between layers.

Tensile Properties of a Novel Graphene Pattern Stitched Carbon/Epoxy 3D Composite

The focus of this study is on the tensile properties of a novel graphene pattern stitched carbon/epoxy 3D composite which could be considered in aerospace industry applications. Tensile modulus and strength are reduced by18-25% due to filament breakage during stitching; this reduction varies with stitching density. Fabrication induced voids and resin rich regions also contribute. However, extensive delamination observed in the tension loaded unstitched laminate is constrained by the graphene pattern stitching and the proposed 3D reinforced composite is expected to be more damage resistant under lateral impact loading.

Electromagnetic interference shielding of stitched carbon fiber composites

This study reports on the improvements in the electromagnetic interference shielding effectiveness of unidirectional carbon fiber-epoxy composites as a results of stitching with conductive and nonconductive threads. Unidirectional carbon fiber laminates stitched with copper, titanium, Kevlar metallic, Kevlar 10 and Dyneema T90 threads were manufactured using vacuum assisted resin infusion process. Representative samples from each laminate were tested in directions normal and axial to the fiber direction using rectangular waveguide. Experimental results showed that composites with improved shielding effectiveness in both directions were produced. For example, stitching with copper doubled the shielding effectiveness in the axial direction when compared to the unstitched laminate. In addition, composite stitched with Dyneema exhibited the most balanced increase in the shielding effectiveness in both axial and normal directions (91.5% and 17.3%, respectively) compared to the unstitched laminate. The shielding mechanism of the composites was mainly reflection.

Effect of stitching on the flexure after impact behavior of thin laminated composites

The paper investigates the effect of stitching on the post-impact flexural performance of two classes of thin carbon/epoxy laminates. Unstitched and stitched laminated samples were first impacted with energies ranging between 1 J and 8 J by an instrumented drop-weight testing machine. The residual flexural properties of impacted samples were then assessed by three-point bending tests. The damage induced by impact and by post-impact bending was characterized in detail by visual inspection and penetrant-enhanced X-radiography. The results of the experimental analyses are illustrated and discussed to highlight the correlation between the residual flexural properties and the key fracture modes occurring in the laminates. The study shows that even though stitching is capable of improving the delamination resistance of the laminates, it may also drastically reduce their flexural strength. It is also seen that the residual post-impact flexural properties, which are controlled by the peculiar impact damage mechanisms of stitched and unstitched laminates, may be greatly degraded by impacts with energy above a critical threshold value.

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 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.

Effects of Stitching Parameters on Tensile Strength of FRPs under Hygrothermal Conditions

An investigation based on experimental and analytical approaches was conducted to evaluate the behavior of the stitched laminates under hygrothermal conditions. Tensile strength of laminates under different environmental conditions was predicted using FEM model. Effects of stitching parameters upon strength performance of laminates were studied using FEM model and validity of results was checked by comparing with the experimental results. Test results have shown that hygrothermal environment condition has no significant effect on the tensile strength of unstitched laminates, but improved the strength of stitched laminates significantly. Stitching decreased the tensile strength of laminates under 20°C dry environment; however, it improved the tensile strength under 20°C wet environment. It was also found from analytical results that, the failure strength of stitched laminates is higher for smaller thread radius (R<0.25) and relatively greater (5~6mm) stitching distance (5~6mm).

Effect of Stitching on Plain and Open-hole Strength of CFRP Laminates

Experimental and analytical investigation is conducted to explore the effects of stitching on plain (without hole) and open-hole compressive and tensile strength of uniweave T300/QY9512 laminates under different environmental conditions (20 °C/dry and wet, 150 °C/dry and wet). Strength performance of stitched composite laminates is also studied using finite element analysis (FEA) model and compared with the experimental results to validate the model. It is found that under similar environmental conditions, the open-hole compressive strength of stitched laminate is decreased and open-hole tensile strength increased as compared to the unstitched laminates. Predicted tensile and compressive strengths are found to be in a good agreement with the test results and the relative error in all cases is less than 15%.

Open hole fatigue characteristics and damage growth of stitched plain weave carbon/epoxy laminates

Fatigue response of stitched plain weave carbon/epoxy laminates containing circular holes is experimentally investigated. Two carbon/epoxy laminates of cross-ply [(0/90)]20 and quasi-isotropic [(±45)(0/90)2(±45)2(0/90)2(±45)2(0/90)]s are reinforced using Kevlar-29® yarns in through-thickness direction. The laminates are drilled to produce a circular hole with diameter of 5.7mm. Stitch configuration for cross-ply laminates is round stitch and parallel stitch, while that for quasi-isotropic laminates is parallel stitch only. For round stitch configuration, the hole is surrounded by circular stitch line of 7-mm diameter. For parallel stitch, the distance between two stitch lines (spacing) is 15mm. In all, three independent cases are presented in this paper: Case 1 (cross-ply laminates, round stitch, tension–tension fatigue); Case 2 (cross-ply laminates, parallel stitch, tension–tension fatigue); Case 3 (quasi-isotropic laminates, parallel stitch, compression–compression fatigue). In each case, comparison with unstitched laminates is made. Case 1 shows that round stitch reduces tension fatigue curve of carbon/epoxy laminates. Round stitch seems to aggravate the damage, which is emanating from the hole rim of laminates. It gradually diverts the damage towards the edge of the specimen and causes premature fatigue failure. Case 2 shows that although parallel stitch generally does not influence the fatigue life of laminates, the damage growth due to parallel stitch is apparently unstable after 8 million cycles. As a result, laminates with parallel stitch eventually fail before reaching 10 million cycles. In contrast, unstitched laminates are able to sustain fatigue load for more than 10 million cycles. Case 3 shows that under compression fatigue load, fatigue limit of stitched plain weave laminates is better than that of the unstitched ones due to damage redistribution along the stitch lines.

Mode-I Fracture and Impact Analysis on Stitched and Unstitched Glass/Epoxy Composite Laminate

A number of approaches have been used to improve the impact damage resistance and tolerance of composite materials. These include control of fibre/matrix interfacial adhesion, laminate design and through the thickness reinforcement. One of the techniques of arresting delamination is providing through-the-thickness reinforcement like stitching. The objective of this work is to understand the influence of fibre orientation and stitching on the mechanical response of the laminate. In this paper, the drop weight impact properties and mode I interlaminar fracture toughness of stitched and unstitched glass fibre epoxy laminate are were studied experimentally. The ultrasonic C-Scan image of the damaged impact specimen is used to find the damage area. A detailed comparison between the stitched and unstitched laminates experimentally confirmed that the stitching effectively suppressed the out of plane impact damage and increased the interlaminar fracture toughness.

Experimental And Finite Element Analysis Of Mechanical Response On Unidirectional Glass Fibre Epoxy Laminate

Research in low velocity impact loading of composites is aimed at reducing the degree of damage in order to improve the damage tolerance. A number of approaches have been used to improve the impact damage resistance and tolerance of composite materials. These include control of fiber/matrix interfacial adhesion, laminate design and through the thickness reinforcement. One of the techniques of arresting delamination is providing through-the-thickness reinforcement like stitching. The objective of this work is to understand the influence of stitching on the impact of the laminate. In this paper, the drop weight impact properties of stitched and unstitched glass fibre epoxy laminate are compared experimentally and the damaged specimen are assessed using back lighting photographic technique. A detailed comparison between the stitched and unstitched laminates experimentally confirmed that the stitching effectively suppressed the out of plane impact damage.

Buckling Analysis of Stitched and Unstitched Glass Epoxy Composites

In this paper, the effects of delamination on critical buckling load failure of unidirectional E-glass/epoxy composite laminated plate consisting of delamination at the middle of the mid plane were studied. Experimental work was carried out to determine the buckling load for rectangular composite plates consisting of delamination for various aspect ratios ranging from 0.5 to 3. Stitched and unstitched laminates were prepared by introducing inplane delamination at the mid of the mid plane using Teflon film. These specimens were made with unidirectional fiber of orientation of (0o/-45o/45o/90o)s with single inplane delamination at the mid of the mid plane of through width and various lengths of 130 mm and 50 mm. The Plates were made to buckle experimentally by simply supporting the top and bottom edges, and sides were kept free. Buckling load has been found from the graph. The critical buckling load is found to be increased in the stitched laminates when compared with unstitched laminates and is decreased in both stitched and unstitched laminates as the aspect ratio increases. The experimental results were validated by using commercial finite element software of ANSYS.

The effect of stitch incline angle on mode I fracture toughness – Experimental and modelling

Abstract In this study, double cantilever beam (DCB) experiments were conducted to determine the effect of stitch incline angle on fracture toughness. The laminates were stitched with a square wave pattern having a pitch and spacing of 4 mm and three different angles being 0°, 22.5° and 45°. A semi-analytical model for DCB specimens was developed based on the Timoshenko beam theory and validated against the experiment results using a previously developed traction law. The results were also compared against a finite element analysis (FEA). The 0° stitched laminates had a mode I fracture toughness 2.35 times higher than the unstitched laminates at maximum load. The testing showed that increasing the incline angle from 0° to 22.5° and 45° reduced fracture toughness at peak load by 12.2% and 20.2%, respectively. As the crack propagated further, the effect of angle reduced due to the traction laws of higher incline angle having less load but greater displacement. The semi-analytical model predicted loads and displacements within 15% of the experimental results. The accuracy is influenced by the traction law and was more accurate for lower incline angles. This is due to more progressive failure as the angle increases rather than a clean break as the model assumed. The 2D FEA was conducted using the load and traction law determined from the semi-analytical solution. The displacements and crack lengths were within 5% and 8%, respectively, of the semi-analytical results.

Improvement of Out-of-Plane Impact Damage Resistance of CFRP Due to Through-the-Thickness Stitching

The present study investigated, both experimentally and numerically, the improvement of low-velocity impact damage resistance of carbon fiber reinforced plastic (CFRP) laminates due to through-the-thickness stitching. First, we conducted drop-weight impact tests for stitched and unstitched laminates. The results of damage inspection confirmed that stitching did improve the impact damage resistance, and revealed that the improvement effect became greater as the impact energy increased. Moreover, the stitching affected the through-the-thickness damage distribution. Next, we performed FEM analysis and calculated the energy release rate of the delamination crack using the virtual crack closure technique (VCCT). The numerical results revealed that the stitching affected the through-the-thickness damage distribution because the stitch threads had a marked effect on decreasing both the modes I and II energy release rate around the bottom of the laminate. Comparison of the results for models that contained delaminations of various sizes revealed that the energy release rate became lower as delamination size increased; therefore the stitching improved the impact resistance more effectively when the impact energy was higher.

板厚方向の縫合によるＣＦＲＰ積層板の面外衝撃損傷抑制効果

The present study investigated the improvement of low-velocity impact damage resistance of CFRP laminates due to the through-the-thickness stitching experimentally and numerically. First, we conducted drop-weight impact tests for stitched and unstitched laminates. The results of damage inspection confirmed that stitching did improve the impact damage resistance, and revealed that the improvement effect became greater as the impact energy increased. Moreover, the stitching affected the through-the-thickness damage distribution. Then we performed FEM analysis and calculated the energy release rate of the delamination by using Virtual Crack Closure Technique (VCCT). The numerical results revealed that the stitching affected the through-the-thickness damage distribution because the stitch threads particularly decreased the mode I energy release rate around the bottom of the laminate. Comparison of the results of models which contain delamination of various size revealed that the energy release rate became smaller as delamination size increased, therefore the stitching improved the impact resistance more effectively when the impact energy was larger.

Characterization of fracture modes in stitched and unstitched cross-ply laminates subjected to low-velocity impact and compression after impact loading

The insertion of transverse reinforcing threads by stitching is a very promising technique to restrict impact damage growth and to improve post-impact residual strength of laminates. In order to develop general models capable of addressing the issues of impact resistance and damage tolerance of stitched laminates, detailed understanding of the nature and extent of damage, identification of the dominant fracture modes and assessment of the effect of stitches on the damage development are essential. In this study, both instrumented drop-weight tests and compression-after-impact tests were carried out to examine and compare the damage responses of stitched and unstitched graphite/epoxy laminates subjected to low-velocity impact. The progression of damage and its effect on post-impact performance was investigated in detail in two classes of cross ply laminates ([0 3/90 3] s and [0/90] 3s) by means of an extensive series of damage observations, conducted with various complementary techniques (X-radiography, ultrasonics, optical microscopy, deply). The results of the analyses carried out during the study to characterize the key fracture modes and to clarify their relationship with the structural performance of both stitched and unstitched laminates are reported and discussed in the paper.

Effect of stitching on the low-velocity impact response of [03/903]s graphite/epoxy laminates

Abstract This study examines the effect of stitching on the impact performance of a class of graphite/epoxy cross-ply laminates with the aim of investigating the ability of through-thickness reinforcement to improve the delamination resistance of laminates. Unstitched and stitched rectangular specimens (65 mm × 87.5 mm) were simply supported by a steel plate having a rectangular opening 45 mm × 67.5 mm in size and impacted at the center with energies ranging between 1 and 13 J. Stitched and unstitched laminates revealed similar structural performances in terms of force versus displacement response, energy absorption and residual indentation depth. It was also observed that whereas stitching does not appear capable of preventing the initiation and spread of delaminations, it induces a clear reduction of damage area when stitches bridge delaminations sufficiently developed in length.

Hygrothermal Properties of Stitched and Unstitched Uniweave T300/QY9512 Laminates (With a Hole)

Experimental investigation was conducted in order to understand the hygrothermal properties of stitched and unstitched uniweave T300/QY9512 laminates (with a hole) in this paper. The tests were conducted in a hygrothermal environment and a normal environment, respectively. Experimental results for compression, transverse impact and compression after impact of stitched and unstitched laminates were presented in this paper. The results showed that a hygrothermal environment degrades the compression properties of stitched and unstitched laminates (with a hole), and hygrothermal environment had a beneficial effect on low-velocity impact property.