This paper examines the effect of plain stitching by untwisted fibre rovings in the in-plane mechanical properties and Mode I interlaminar fracture toughness of glass/polyester composites. The traditional FRP laminated composites with two dimensional fibre architecture have poor interlaminar fracture resistance and suffer extensive damage by delamination cracking when subjected to out of plane loading. Through-the-thickness reinforcement is done by stitching with twisted fibre yarns in sewing machine to increase the delamination strength, but, it degrades the in-plane mechanical properties. In this work glass woven roving mats (WRM) are stitched in the thickness direction with untwisted kevlar, glass and carbon fibre rovings of various Tex by the plain stitch. It is observed that in plain stitch, no thread cross is formed and thus no resin rich pockets are seen. Uniform distribution of fibres in the stitch roving, absence of resin rich region and lesser amount of fibre damage result in increased in-plane tensile, lap shear, flexural, transverse shear and impact strengths. Effect of stitching on Mode I delamination toughness (GIc) of glass/polyester laminates has been investigated by performing double cantilever beam (DCB) test. It is observed that stitching increases the Mode I delamination toughness up to 20 times higher than that of unstitched specimen. © 2006 Elsevier Ltd. All rights reserved.

R. Velmurugan

Department of Aerospace Engineering

Cracking (chemical)

Delamination

Fracture toughness

Mechanical properties

Polyesters

Toughness

Polymer-matrix composites

STITCHING

Glass fiber reinforced plastics

Carbon fiber

COMPOSITE PROPERTY

FIBER REINFORCED COMPOSITE

Fracture

GLASS FIBER

laminate

mechanical property

Polyester

polymer

ROVING

STITCH

SYNTHETIC RESIN

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Composites Science and Technology

For crashworthiness applications, hollow composite tubes have been proposed and studied as impact-bearing members in earlier studies to replace metal tubes and beams which resisted impact by bending. This study focusses on the effect of the addition of hollow glass microballoons (GMB) in composite tubes for compressive loading. Epoxy containing 0 to 0.3 volume fraction of GMB is used with woven glass fabrics to fabricate the specimens. The mean load and energy absorbed while progressively crushing the tubes are experimentally obtained. The variation of these properties with the addition of GMB is analyzed. It is found that the compression properties improve with the addition of hollow glass particles. To explain the different modes of energy dissipation, the basic mechanical properties of the GMB-filled composites are obtained by various mechanical tests. These results are also used in an energy-based model to predict the properties of the tubes under compression. © 2019

Thin-Walled Structures

Improvements in the crushing behaviour of glass fibre-epoxy composite tubes by the addition of hollow glass particles

This paper presents the effect of addition of nano-Alumina particles on the fracture properties of glass fiber reinforced plastic (GFRP) composite laminates. Epoxy resin is the most commonly used polymer matrix for advanced composite materials in view of its ability to adhere to a wide variety of fillers; on curing, they provide excellent stiffness and dimensional stability. However the highly cross linked epoxy often behaves undesirably brittle, because, plastic deformation is constrained, leading to poor resistance to crack initiation and propagation. Hence it is necessary to improve the toughness without sacrificing the other important mechanical and thermal properties. In this work, glass-fiber-reinforced composite with nano-Alumina modified epoxy matrix was successfully produced with a hand lay- up process and characterized by EDAX and XRD technique for its composition. The experimental results show that the composites exhibited improvements in inter-laminar toughness values (GIC and GIIC) along with improvements in other mechanical properties, especially in toughness related properties. The Mode-I interlaminar fracture toughness for 2 phr (per hundred gram resin) nano-Alumina was 2.5 times higher than that of unfilled epoxy and the Mode-II inter-laminar fracture toughness improved by 37 %. The significant increase in Mode-I fracture toughness and improvement in Mode II inter-laminar fracture toughness resulting from the nano-particle modification, indicates a pronounced increase in matrix toughness. Impact tests suggest that the energy absorption capability of the GFRP considerably improved with the addition of equi-Axed nano-Alumina particles with epoxy resin. The laminate and fracture surface morphology analysis was done to understand the fracture and toughening mechanisms behind these property changes. The bending characteristic such as ILSS and Flexural properties recorded the maximum improvements of 14 % and 17 % respectively for the laminate with nano-Alumina modified epoxy. A significant improvement in flexural modulus of over 37 % was noticed with respect to unmodified epoxy. The experimental results show that the tensile modulus exhibited 15% improvement compared to laminate without nano-Alumina, while, a modest change was observed in the tensile strength. Copyright © 2012 by ASME.

ASME International Mechanical Engineering Congress and Exposition, Proceedings (IMECE)

The inter-laminar fracture and mechanical behavior of nano-Alumina modified glass fiber/ epoxy composite

The influence of in-plane fibre orientation on the mode I interlaminar fracture toughness, GIc of unstitched and stitched glass/polyester composites is investigated in this paper. The GIc of planar specimens depends on the fibre orientation, θ in the layers adjacent to the fracture plane, in addition to the property of matrix material. The mode I fracture toughness and fracture behavior of unstitched and stitched 0/0, 30/-30, 45/-45, 60/-60, 90/90 and 0/90 interfaces of unidirectional fibre mats (UD) and 30/-30, 45/-45 and 90/90 interfaces of woven roving mats (WRM) are studied. WRM layer orientation is represented by the direction of warp fibres. Stitching is done by untwisted Kevlar fibre roving of Tex 175 g/km at the stitch densities (number of stitches per unit area) of 10.24 and 20.48 stitches/inch2. The specimens having same stitch density, but different stitch distributions are prepared, and the influence of stitch distribution on GIc is studied. Double cantilever beam (DCB) tests are carried out and the GIc is determined using modified beam theory. The GIc of both unstitched and stitched specimens increases with increase in orientation angle, θ upto 45° above which it decreases. The GIc values of unstitched 45/-45 delamination interface is around 2.4 times that of the unstitched 0/0 interfaces. The influence of fibre orientation on GIc is clearly observed in unstitched specimens, whereas in the stitched specimens, stitching plays an important role in improving the GIc and suppresses the influence of fibre orientation; degree of suppression increases with increasing stitch density. When the value of θ is above 45°, transverse cracks are observed in the delamination interface surrounded by UD layers; while in the delamination interface surrounded by WRM layers, transverse cracks are not initiated irrespective of the fibre orientation angle. © 2008 Elsevier Ltd. All rights reserved.

Influence of in-plane fibre orientation on mode I interlaminar fracture toughness of stitched glass/polyester composites

The energy absorbing capability of FRP composite cylindrical tubes used as energy absorbers, by destroying itself progressively, depends on the way in which the tube material is crushed i.e., trend of petalling. This paper investigates the influence of fibre orientation and stacking sequence on the petal formation and specific energy absorption (SEA) of four and six-ply, 0°/90° glass/polyester composite cylindrical shells under axial compression. Number of petals formed and the trend of petalling are changed with proportion of axial (0°) and circumferential (90°) fibre content and stacking sequence in the tubes. In the tubes undergo petalling, presence of axial fibres close to inner surface and the proper proportion of circumferential fibres close to outer surface of the tube wall lead to higher energy dissipation. The axial fibres placed nearer to outer surface leads to more number of petal formation, leading to a stable crushing mechanism. The contribution of mode I strain energy release rate (GIc) to the energy dissipation in the form of circumferential delamination is also studied with double cantilever beam (DCB) tests. Analytical model which considers petalling is developed, and used to predict the mean crush load and SEA of cylindrical composite shells under axial compression. Results from the analytical model agree well with experimental results and are presented. © 2007 Elsevier Ltd. All rights reserved.

Fulltext

International Journal of Solids and Structures

Influence of fibre orientation and stacking sequence on petalling of glass/polyester composite cylindrical shells under axial compression

This paper examines the effect of mode I interlaminar fracture toughness (GIc) on the specific energy absorption of stitched glass/polyester composite cylindrical shells under axial compression. The laminated composite cylindrical shells used as energy absorbers, absorb large amount of impact energy during collision. Since mode I delamination in the thin wall of axially collapsed shell is one of the major energy absorbing modes, contribution of GIc to specific energy absorption (SEA) of tubes is significant during collision. The GIc values are determined through double cantilever beam (DCB) test with stitched and unstitched planar specimens. The four and six-layered cylindrical tubes of D/t ratios 29.27 and 20, respectively, with GIc values ranging from 1.68 to 8.09 kJ/m2 are prepared by stitching and are subjected to quasi-static axial compression. Increasing GIc up to certain value leads to controlled progressive crushing, which is a good energy absorbing mechanism, beyond which failure is uncontrolled. Cylindrical tubes having GIc up to 6.34 kJ/m2 leads to 40% increase in SEA for four-layered tubes and 6.6% for six-layered tubes comparing with the corresponding unstitched tubes. When the tubes have GIc of 8.09 kJ/m2, four-layered tubes undergo unstable failure, but six-layered tubes undergo stable progressive crushing with 22% increase in SEA. Transition from stable to unstable failure depends upon the thickness of tubes. An analytical model is developed based on energy approach to predetermine the steady state mean crush load of cylindrical composite shells under axial compression. The model results are validated by experimental results, and show good agreement. © 2006 Elsevier Ltd. All rights reserved.

Progressive crushing of stitched glass/polyester composite cylindrical shells

FRP composites have usually poor through-thickness mechanical properties and, therefore, are able to sustain more loads in tension than in compression and fail as a consequence of buckling. The through-thickness reinforcement is carried out by stitching to improve the delamination strength and to reduce the in-plane crack growth rate. Experiments were performed on both stitched and unstitched laminated plates which were prepared by using woven roving glass fibre mat and chopped strand glass fibre mat with polyester resin, and the effect of stitching was studied. It is observed that stitching increases delamination strength to a great extent. There are losses in the in-plane mechanical properties due to in-plane fibre damage and creation of resin-rich pockets. Various energy-absorbing modes observed during progressive crushing of both stitched and unstitched FRP cylindrical shells under axial compression were studied both experimentally and theoretically. Analytical expressions for the calculation of energy absorption in various modes and average crush stress were derived, and the results thus obtained were compared with the experiments. © 2004 Elsevier Ltd. All rights reserved.

International Journal of Impact Engineering

The effect of stitching on FRP cylindrical shells under axial compression

European Journal of Mechanics - A/Solids

Buckling analysis of two layer delaminated beams with bridging

Composites Part A: Applied Science and Manufacturing

Analysis of the fracture behaviour of a stitched single-lap joint

Finite element analyses of mode I interlaminar delamination in z-fibre reinforced composite laminates

Delamination properties of translaminar-reinforced composites

Improvements in Mode I interlaminar fracture toughness and in-plane mechanical properties of stitched glass/polyester composites

Journal	Composites Science and Technology
ISSN	02663538
Open Access	No