Research trends – March 2013

Hygrothermal ageing of plant oil based marine composites

M. Malmstein, A.R. Chambers & J.I.R. Blake, Composite Structures (in press)

Keywords:

• marine applications
• plant oil based composites
• moisture uptake
• failure modes

Glass reinforced polyester and epoxy resin composites are used in applications operating in a harsh marine environment, e.g. boat building and offshore energy. Although material properties are known to degrade over time due to moisture uptake, these composites are still considered to perform relatively well under varying environmental conditions. Boats built 30 years ago are still functional today and until recently the environmental impact of composite materials used in marine applications has raised little concern. However, the petroleum dependence of conventional composites, especially resin systems, will no longer be acceptable in sustainability driven engineering. Although not biodegradable, plant oil-based polymers may offer a renewable alternative to conventional epoxy and polyester resins. However, the long-term performance of these novel composites needs to be understood before using them in structural applications.

This paper reports the effect of hygrothermal ageing on the flexural properties of glass/epoxy, glass/linseed oil and glass/castor oil composites. It was found that in the unaged condition, the flexural properties of glass/epoxy were significantly higher than both glass/castor oil and glass/linseed oil composites. After ageing in water at 40°C for 46 weeks, the properties of glass/castor oil were comparable to glass/epoxy while the properties of glass/linseed oil were remarkably lower. The decrease in glass/linseed oil performance is explained in terms of the changes in the failure modes caused by moisture uptake.  

Ultra-lightweight carbon fibre/thermoplastic composite material using spread tow technology

Hassan M. EL-Dessouky, Carl A. Lawrence, Composites Part B: Engineering (in press)

Keywords:

• aircraft applications
• thermoplastic composites
• spread tow technology
• carbon fibre/PPS prepreg
• mechanical properties

Continuous fibre reinforced thermoplastic (CFRTP) composites are receiving growing interest because of their fracture toughness and damage tolerance, ease of shape forming prior to consolidation, faster and lower cost manufacturing, longer shelf life of raw material, and the ability to be reshaped and reused/recycled. However, the use of these materials is mostly limited to secondary and semi-structural aircraft parts. To make thermoplastic composites more attractive for primary aircraft structures, the performance/cost ratio has to be improved.

There is also an increasing demand for lighter weight composite structures through better utilisation of the reinforcing fibre, mainly carbon fibre (CF). The state of the art is to use 3k tows for woven fabrics and conventional 12k tows for unidirectional (UD) tapes. A recent method for achieving ultra-lightweight composite material is referred to as spread tow technology, whereby a conventional 12k CF tow (i.e. comprising 12,000 filaments) is thinned by increasing the tow width from 5 mm to 25 mm, thereby reducing the weight per unit area by approximately 500%. There are clear signs that others are working on the development spread tow materials, some of which are being introduced into the composite sports sector, notably TeXtreme® by Oxeon AB (see Kiteboards perform better with TeXtreme reinforcement). Sakaiovex is developing similar products, and relatively recently Formax and G.Angeloni have exhibited spread tow materials. However, none of these developments have yet reached a performance level suitable for aerospace applications. This is largely attributed to the production of spread tow materials suitable for thermoset resin systems and the need to apply a thermoplastic resin for handle-ability, the resin being incompatible with qualified aerospace matrices.

This paper reports a study of a method for achieving ultra-lightweight thermoplastic composites whereby a conventional 12k carbon fibre (CF) tow is thinned by increasing the tow width from 5 mm to approximately 25 mm. Using the tow-spreading technology, sheets of UD and/or woven fabric may be produced. Thermoplastic film of polyphenylene sulphide (PPS) was used to stabilise and impregnate the spread tow fabric, converting it into a partially consolidated prepreg: woven 12k CF spread tow/PPS (55/45% wt.). A consolidated laminate was made from the prepreg, and for comparison, a second laminate was produced from a conventional woven prepreg of 3k CF/PPS (60/40% wt.). The spread tow laminate exhibited better fibre packing, lower level of crimp, lower void content and improved mechanical properties.  

Self-Sealing of Mechanical Damage in a Fully Cured Structural Composite

Jericho L. Moll, Henghua Jin, Chris L. Mangun, Scott R. White, Nancy R. Sottos, Composites Science and Technology (in press)

Keywords:

• microcracking
• self-healing chemistry
• glass epoxy composite
• microcapsules

Microcracking in advanced composites leads to a reduction in stiffness and an increase in permeability. In cryogenic tanks, microcracks form during thermal cycling due to the coefficient of thermal expansion mismatch between the fibres and the matrix. When a sufficient density of microcracks form a percolating network through the thickness of the composite, cryogens begin to leak through the tank wall. In sandwich structures, mechanical fatigue or low velocity impact can also induce microcracking and an increase in water absorption into the core material, which not only increases the overall weight of the composite, but can also cause delaminations between the face sheet and core.

This paper presents a thermally stable microencapsulated self-healing chemistry. The microcapsules in this dual capsule healing system are dispersed in the epoxy matrix. Crack damage ruptures the capsules releasing healing agents into the crack plane. Once the healing agents mix by diffusion, polymerisation in the crack plane bonds the crack faces together, effectively healing the matrix. In this system, one microcapsule contains a low viscosity PDMS oligomer and cross-linker while the second contains a tin catalyst. These components were incorporated in the epoxy matrix of a woven glass composite, which was cured at a temperature of 121°C. Self-healing ability was evaluated for a range of capsules sizes and concentrations and their effect on mechanical properties investigated. Complete self-healing was achieved when 42 μm diameter microcapsules at a loading of 9 vol% or 25 μm microcapsules at a loading of 11 vol% were added to the matrix.  

A review on the degradability of polymeric composites based on natural fibres

Z.N. Azwa, B.F. Yousif, A.C. Manalo & W. Karunasena, Materials & Design, Volume 47, May 2013, pages 424-442

Keywords:

• building materials
• natural fibre composites
• weathering
• chemical additives

Traditional building materials such as concrete and steel are increasingly being replaced by composite materials, e.g. fibre reinforced polymers (FRPs) and fibre reinforced cement (FRC). Fibre polymer composites are expected to find increased use due to their high strength, low weight, corrosion resistance, and low maintenance cost. Despite these advantages, engineers are being challenged to ‘go green’ and this includes finding more environmental friendly processes and the use of biodegradable or recyclable materials. One way of meeting this challenge is to replace synthetic materials with natural materials.

Fibres such as hemp, kenaf, jute and bamboo have been studied for their potential contributions in composite materials. These natural fibre reinforced composites are finding their way into the construction industry, with a projected increase in demand as high as 60% each year in the US and a predicted growth rate from 10% to 22% per year for the natural fibre industry. Their production consumes, on average, 60% less energy than the manufacture of glass fibres. On the other hand, natural fibres have some disadvantages when used as reinforcements for polymeric composites, i.e. degradability, fire resistance, interfacial adhesion, etc.

The applications of natural fibre polymer composites in civil engineering are mostly focused on non-load bearing indoor components because of their vulnerability to environmental attack. This paper evaluates the characteristics of several natural fibre composites exposed to moisture, thermal, fire, and ultraviolet (UV) degradation through an extensive literature review. The effects of chemical additives such as fibre treatments, fire retardants and UV stabilisers are addressed. Based on the evaluation conducted, optimum fibre content provides strength in a polymer composite but it also becomes an entry point for moisture attack. Several fibre treatments are being used to improve fibre/matrix interface, thereby increasing moisture durability. However, the treated fibres were found to behave poorly when exposed to weather. The addition of UV stabilisers and fire retardants is reported to enhance outdoor and fire performance of natural fibre polymer composites but compromises their strength. It is concluded that an optimum blend ratio of chemical additives must be employed to achieve a balance between strength and durability requirements for natural fibre composites.  

Flexural retrofitting of RC buildings using GFRP/CFRP – A comparative study

H.R. Ronagh, A. Eslami, Composites Part B: Engineering, Volume 46, March 2013, pages 188-196

Keywords:

• glass fibre
• carbon fibre
• seismic retrofitting

Amongst different methods suggested for repairing/upgrading of reinforced concrete (RC) buildings, the use of externally bonded fibre reinforced polymers (FRPs) has increased significantly due to their inherent advantages over traditional methods such as external bracing or steel jacketing. These include high tensile strength, low specific weight, high resistance to corrosion, and ease of application. The effectiveness of FRP in retrofitting/repairing of the (RC) components has been studied in great detail. However, the seismic performance of RC structures retrofitted using FRP composites is yet to be investigated in terms of lateral resistance, ductility, and failure mechanism. This is critical if the retrofitted structures are to withstand higher seismic ground motions than they were designed for and/or pulse-type ground motions.

This paper reports on the results of an investigation into the flexural strengthening of RC buildings using glass/carbon fibre reinforced polymers (GFRP/CFRP). An 8-storey code-compliant RC building was considered as the case study to represent medium-rise structures. Results confirm a significant increase in the lateral load carrying capacity using both composite materials. The CFRP improvement was as much as twice of the GFRP. However, the latter provides higher ductility.  

Laser direct joining of carbon fiber reinforced plastic to zinc-coated steel

K.W. Jung, Y. Kawahito, M. Takahashi, S. Katayama, Materials & Design, Volume 47, May 2013, pages 179-188

Keywords:

• laser joining
• carbon fibre reinforced plastic 
• zinc-coated steel
• automotive applications

The use of carbon fibre reinforced plastic (CFRP) is increasing in aircraft and automobiles. Zinc-coated steel (so-called galvanised steel) with excellent corrosion resistance and stable coating appearance after heating has received attention in the automotive industry for improving the durability of the vehicle body. In order to effectively apply these two materials to automotive components, the development of various manufacturing technologies such as cutting, drilling, joining and welding is required. Conventional joining of metal and CFRP or plastic is commonly performed using adhesively bonding techniques and mechanical fasteners. However, these methods give rise to several problems, including VOC emission, long bonding times, non-uniform joint strength, and the requirement for additional processes like hole drilling. As a solution, trials of new joining process technologies such as friction spot joining, ultrasonic metal welding and laser direct joining have recently been conducted.

This study was performed to investigate the joining of CFRP and zinc-coated steel using a continuous wave (CW) diode laser. Tensile shear tests showed a high strength joint of about 3300 N could be produced. The melted zone generated in the CFRP, the macro- or micro-structure near the joint interface and the fractured surfaces of the two materials after tensile shear tests were observed and specific joining mechanisms and the existence of a zinc-coated layer at the joint interface were investigated. Studies confirmed that a strong joint of CFRP to zinc-coated steel could be obtained.  

Morphing wing flexible skins with curvilinear fibre composites

Senthil Murugan, M.I. Friswell, Composite Structures, Volume 99, May 2013, pages 69-75

Keywords:

• aircraft
• morphing wings
• curvilinear fibre composites

Next generation aircraft require 'morphing wings' which can reconfigure to multiple shapes, each one optimal for a specific flight condition. A key challenge in developing a successful morphing wing is the development of a flexible skin – a continuous layer of material that would stretch over a stiff morphing structure and forms a smooth aerodynamic surface. These require composite skins with conflicting structural requirements: low in-plane stiffness to allow the skins to deform with less actuation force, and high out-of-plane bending stiffness to withstand aerodynamic loads. For conventional composite laminates with straight fibres the in-plane stiffness of the laminate is high in the fibre direction and low in the orthogonal direction. This restricts the design space of fibre reinforced composites for morphing skin applications. However, curvilinear fibre paths or spatial orientation of fibre angles can be beneficial in achieving the in-plane and out-of-plane stiffness requirements.

Studies have shown the use of curvilinear fibre paths enhances the multiple structural response of composite structures. However, no study has focused on the use of curvilinear fibre paths to minimise the in-plane stiffness and simultaneously maximise the out-of-plane bending stiffness of composite structures. In this study, composites with curvilinear fibre paths are examined to enhance these conflicting structural requirements. The numerical results show that curved fibre paths can minimise the in-plane stiffness and increase the bending stiffness simultaneously compared to a baseline plate with straight fibres. A flexibility ratio is defined to assess the in-plane and out-of-plane deformation of the plate, simultaneously. The aspect ratio of plate, laminate stacking sequence and in-plane loading direction have considerable influence on the optimal paths of the curved fibres. ♦  

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