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An Experimental Investigation Of The Effects Of Stacking Sequence On Hybrid Composite Materials Response To Open-Hole
Compression Strength.
Elias Randjbaran*, Rizal Zahari, Dayang Laila Majid, Nawal Aswan Abdul Jalil, RaminVaghei, Ramin Ahmadi
Faculty of Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor Darul Ehsan. Malaysia.
* Corresponding author. Tel: +44 (0) 745 2203616, Fax: +44 (0) 844 7749975 E-mail address: [email protected]
A b s t r a c t
In the current paper, The compressive strength and failure mechanisms are investigated for hybrid composites. Static uniaxial compressive tests are performed on notched specimens made from two layers of carbon, glass, and Kevlar fibres and epoxy resin combined to give six different stacking hybrid composite materials.
Cohesive zone model is applied to estimate the open-hole compression (OHC) strength.
Keywords:
Kevlar fibre, Carbon fibre, Glass fibre, Stacking Sequence, Energy Absorption,Open-hole Compression
1. Introduction
Progressions in the use of hybrid composite materials for the structure of aircraft and automobile industries have risen considerably over the last decade. This has been driven by the need for improved performance requirements in terms of stealth, payload, range, stability and at the same time, a reduction in costs in terms of maintenance, operation and construction. Much experience in the use of hybrid composites in the aerospace industries has been achieved from the design of composite airplanes, which were designed using high stiffness requirements and not for all the parts of the current body of airplanes being planned. The stiffness of composites can be determined equitably
accurately using the particular tests and material properties from standard material characterisation tests. However, with more demanding requirements, this has changed and the minimisation of damage is something that is now required in order to satisfy higher- performance demands. This is not as simple as optimising the elastic stiffness of the structure due to the complex damage modes that can occur in hybrid composites.
Open-hole strength is an important topic as it is one of the design drivers for composite structures. There has been considerable research over the years, and the earlier work was reviewed by Awerbuch and Madhukar [1].
Scaling effects are also important, as the design of large structures is usually based on data from small coupons, and reduced scale models may be used to investigate full-scale structural behavior. There is substantial evidence of size effects in composites [2]. Reductions of strength with increasing size have been reported in tensile and flexural strength [e.g. 2-6] and in compression [2,7-12], but scaling of strength is still not well understood. In open-hole specimens scaling is accompanied by the well- known hole size effect whereby strength decreases with increasing hole size. Many researchers have investigated this [e.g. 13-19]
and a number of models have been proposed which fit the experimental trends. Most studies have kept the width constant and so specimens are not truly scaled, and the varying finite width
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correction factors can obscure the underlying scale effect.
Table 1: Mechanical properties of the fibres
Fibre Strength
(σ)(GPa)
Failure Strain
(ε)(%)Modulus
(E)(GPa)Carbon fibre
3.8 1.76 227
E- Glass
3.5 4.7 74
Kevlar KM-2, 600 denier
3.4 3.55 82.6
2. Experimental procedures 2.1. Materials
Traditionally, aerospace composites have been composed of high-stiffness carbon fibres to maintain dimensional stability under high- performance application. The stiffness property is often associated with a particular susceptibility to impact damage and a corresponding reduction of mechanical properties. However, such structures are expected to only encounter few unintentional impacts. Composite structures for military ground vehicles, on the other hand, are designed to absorb multiple high-energy impacts but have much less dimensional restrictions. Since softer materials tend to dissipate more energy during impact, a low modulus/high strength alternative would be well suited for backing panel composites. Figure 1 shows the three types of fibres, including glass, carbon, and Kevlar, which were used in fabricating the specimens.
Table 1 illustrates the mechanical properties, such as strength, failure strain, and Young's Modulus of carbon, glass, and Kevlar fibres [20].
Figure 1: Left to right; Glass, carbon, and Kevlar fibre
Kevlar KM2 fabrics are widely used to produce personnel protection systems because of their impact-resistant properties. To understand the deformation process of a fabric armour system during impact, many aspects of fabric, such as its material properties, fabric structure, projectile geometry, impact velocity, multiple ply interaction, far-field boundary conditions, and friction, must be studied [11]. The Kevlar fabric used in all composite target constructions was plain-woven Hexcel Aramid, high-performance fabric Style 706 (Kevlar KM-2, 600 denier) with a real density of 180 g/m2
2.2. Preparation of the Specimens
. Room temperature curing and the ratios of 50 parts epoxy resin (EPOKUKDO YD-128) to 50 hardener (Polyamide - Domide (A.V: 350)) by weight being cured after seven days at 20 °C [20&21].
Hand lay-up is the simplest and oldest open moulding method of the composite fabrication processes. Glass or other reinforcing mat or woven fabric or roving is positioned manually in the open mould, and resin is poured, brushed, or sprayed over and into the glass plies. Entrapped- air is removed manually with squeegees or rollers to complete the laminate structure. Room temperature curing epoxies are the most commonly used matrix resins. A catalyst initiates curing in the resin system, which hardens the fibre-reinforced resin composite without external heat, and kept them in-room temperature (19 °C, Humidity 13%). Table 2 illustrates the ordering and sequence of fibres plies in each hybrid composite materials [21].
2. 3. Experimental Testing Procedure
Open Hole Compression ASDTM D5766
measures the force required to break a
polymer composite laminate specimen with
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a centrally located hole. The hole allows for stress concentration and reduced net section while the test method calculates ultimate strength based on gross cross-sectional area, disregarding the hole.
Table 2: The fabricated composite sheets are divided into five groups
HYBRID 1
HYBRID 2 HYBRID 3 HYBRID 4 HYBRID 5
Kevlar Glass Kevlar Glass Kevlar
Carbon Carbon Glass Kevlar Carbon
Glass Kevlar Carbon Carbon Glass
Kevlar Carbon Glass Carbon Glass
Glass Kevlar Carbon Glass Carbon
Carbon Glass Kevlar Kevlar Kevlar
ASTM D5766 is commonly used in the aerospace industry as a practice to develop notched design allowable strengths. It is used to generate data where the final application of the product may require fastener holes or to simulate a flaw in a material component. The test is used for composite material forms (including tape or fabric) and limited to continuous fibre or discontinuous fibre reinforced composites with balances and symmetrical test direction. Since the physical properties of many materials can vary depending on ambient temperature, it is sometimes appropriate to test materials at temperatures that simulate the intended end use environment.
Test specimens are placed in the grips of a universal tester at a specified grip separation and pulled until failure. The test specimens and test protocol are as called out in ASTM D3039 for Compression Properties of Composite Materials. A typical test speed for standard test specimens is 1 mm/min.
Depending upon the reinforcement and type, testing in more than one orientation may be necessary.
3. Results and discussion
3.1 Compression test on open--hole specimens
Figure 3 and Table 3 illustrate the maximum compressive breakage load and compressive extension at the failure point of the open--hole specimens.
Hybrid 3 has an ultimate breakage load with about 528 N and Pure Carbon Fibre has a minimum with about 162 N, Moreover, among the Hybrids, Hybrid 1 has a reaching to the failure points.
Minimum breakage loads with about 341 N.
Figure 2 Cross-sectional, face, and back views of the open--hole specimens with various stacking sequences after compression test.
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Figure 3 Compressive force–displacement curves of open--hole specimens.
Specimen First Compressive Breakage Load, N
Compressive Extensional Failure, mm
Hybrid 1 336 0.171
Hybrid 2 294 0.203
Hybrid 3 461 0.219
Hybrid 4 489 0.142
Hybrid 5 332 0.166
Pure Carbon Fibre 150 0.184
Table 3 Comparison of maximum compressive load and compressive extension at failure point of the open--hole
specimens.
The average of the ultimate compressive breakage loads is about 406 N and the average of the compressive extension of the failure points is 0.189 mm. The results of specimens experienced much higher-than-average ultimate compressive breakage loads, including Hybrid 2, Hybrid 3, Hybrid 4, and Hybrid 5. Meanwhile, Hybrid 1 and pure carbon fibre were notably lower than the average. The compressive failure occurred in the range from 0.1 mm to 0.32 mm as the range of compressive extension points. The stiffness (the slope of the force- displacement curve) of all specimens is approximately the same as before failure.
3.2 Compressive energy absorption of the open-hole specimens
Figure 4 illustrates the amount of compressive energy absorption of the open--
hole specimens including the five groups of Hydrids and pure carbon fibre in Joules
.All six types of specimens have a central hole as has discussed in chapter 3. Hybrid 4 has a maximum strength against the compressive force with 1940.35J and pure carbon fibre has a minimum with 302.65 J. Then again, among the Hybrids, Hybrid 2 has a minimum strength against the compressive force with 775.25 J. Among the Hybrids;
average of the amount of compressive energy absorption is about 1526 J, accordingly Hybrid 1, 3, and 4 are at the top level of average, and the rest are at the bottom.
Figure 4 The amount of compressive energy absorption of the open--hole specimens including the 5 groups of Hydrids and pure
carbon fibre (in Joules).
Rank Specimen Energy Absorption, J
1 Hybrid 4 1940
2 Hybrid 3 1883
3 Hybrid 1 1689
4 Hybrid 2 1383
5 Hybrid 5 775
6 Pure Carbon 303
Table 5: Ranking of compressive energy absorption.
4. Conclusions
The results show, first, the Hybrid 4 has the
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superlative energy absorption. Second, it can be concluded that stacking the first layer with glass fibre is better than to use the Kevlar fibre. Third, using the combination of carbon and glass is more efficient than using in the central layers. Fourth, using the carbon fibre is not recommended at the last layer.
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