Fibre/polymer interface integrity on structural strength and stability

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Sanghamitra Sethi1 and Bankim Chandra Ray2
1. Department of Mechanical Engineering,MITAOE,Pune,India
2. Composites Materials Group, Department of Metallurgical and Materials Engineering, National Institute of Technology, Rourkela-769008, India
 
Biography
Sanghamitra Sethi completed her Ph.D in 2016, in the field of Metallurgical and Materials Engineering from NIT, Rourkela. Currently working as an Assistant Professor in the department of Mechanical Engineering at MITAOE, Pune, India, her research includes polymer matrix composites with a focus on interface regions.
 
Abstract
The geometry and topography of the fracture surface are often dictated by the micro structural features and the failure mechanisms that govern the fracture. Thus, the study of the fracture surfaces, that is fractography, provides an important input for understanding the fracture processes. The aim of the current investigation is to present the variation of mechanical properties of glass fiber and carbon fiber/epoxy composites under the synergistic effect of temperature and rate of loading. The composite specimens were subjected to elevated temperatures. A 3-Point short beam shear bend test was conducted in order to characterize the mechanical behavior of laminated composite and to determine the influence of loading rate on interlaminar shear strength. To understand the interactions between various failure mechanisms in the fiber, matrix and fiber/matrix interface, microscopic analyses were conducted. Following the test, the fracture surfaces of the samples were scanned under SEM and AFM to understand the dominating failure modes. Micro structural assessments can also reveal the response of each constituent viz. fibre, matrix resin and the interface/interphase.
 
Acknowledgements
The authors are heartily thankful to National Institute of Technology Rourkela for providing infrastructural and financial support for carrying out the present research work.
 
Corresponding Author
Dr Sanghamitra Sethi, Assistant Professor, Mechanical, Engineering, MITAOE, Pune, India
 
INTRODUCTION
 
A recent review1 of composite interfaces has highlighted their importance, however, assessed that there are challenges due to the lack of comprehensive understanding in the field and that this is of critical concern to materials scientists of today.
 
The performance of fibre-reinforced composites is, to a large extent, controlled by the properties of fibre-matrix interfaces. Interface chemistry and character are vital to a composite material2-6. Good interfacial properties are essential to ensure efficient load transfer from matrix to reinforcement, which helps to reduce stress concentrations and improves the overall sustainability of mechanical properties1,4,5.
 
The strength of composite materials depends not only on the substrate but also on the interface strength. The interface here does not have a unique fracture energy unlike homogeneous materials. Consequently, there is great interest in developing new concepts for tailoring the strength of fibre-matrix interface7,8.
 
Some researchers have reported that the mechanism responsible for improved fibre-matrix interface adhesion is removing weak boundary layer, and thereby improving wettability9-11. However, a high performance composite functions because a weaker interface or matrix stops a crack running continuously between the strong brittle reinforcements.
 
There are still scientific arguments about whether the interface should be weak in shear or in tension. Whichever is correct, the situation is not quantitative and we do not know how weak we can make the interface. Improved interfacial properties reduce the effects of fatigue; however, failure is inevitable. If reversible covalent bonds can form between the polymer network and the reinforcement material, then the interphase will be capable of healing, resulting in improved durability of the composite12.
 
The interface/interphase (Fig. 1) comprises the functional interlayer and the part of the matrix affected by the presence of the coated fibre. The coated interlayer should improve compatibility between the fibre and the matrix by forming a strong but tough link between both phases. The interphase thickness has been evaluated as the thickness of the transition zone, where the matrix hardness increased and the friction coefficient decreases close to the fibre surface.
 
Figure 1. Schematic diagram of fibre/matrix interface and interphase
 
Weather and radiation factors that contribute to degradation in plastics include temperature variations, moisture ingression, sunlight exposure, oxidation, microbiological attack, and other environmental elements13-15.
 
At high temperatures, there is sufficient thermal energy available to the molecules of polymer to allow easy rotation, movement, and disentanglements. At low temperatures, there is little thermal energy available and molecular motion is inhibited16. Thus fibre/matrix interface degrades the interfacial bond strength, resulting in loss of microstructural integrity. 
 
Consequently, the relaxation and creep rates of polymers are highly sensitive to temperature. The moisture diffusion rate along the fibre direction is significantly larger than that transverse to the fibre direction, which is attributed to the fact that the fibre-matrix interface offers a preferential path-way for moisture ingress17.
 
An aging-induced new zone at the interphase is more mobile than the matrix because it has its own reduced glass transition temperature. Humid aging is recognized as one of the main causes of short as well as long-term failures of organic matrix composite. Ray18 has cited the role of temperature during humid ageing in polymer composites is critically decisive about the degree of degradation, not solely by the amount of absorbed moisture as reported by many literature in last few decades.
 
There are several modes of humid aging. Plasticization of the matrix, differential swelling, embrittlement of the macromolecular skeleton by hydrolysis; osmotic cracking, hygrothermic shock and also localized damage at the fibre/matrix interface have been reported so far. The interphase material tends to have lower glass transition temperature (Tg), higher modulus and tensile strength than the bulk matrix19.
 
Liang Li et al have proposed that absorbed water molecules forming double hydrogen bonds with polymer will cause an increase of Tg. Here Tg depends not only on double hydrogen bound water molecules but also on the plasticizing effect20.The fibrous composite is full of holes and channels. To obtain a clear picture of micro characterization of interface, various techniques have been employed, as attenuated transform infrared spectroscopy (ATR-FTIR), positron annihilation lifetime spectroscopy (EIS), and solid-state nuclear magnetic resonance (NMR)20.
 
Concerning the micromechanical analysis of fibre-matrix interfaces in composites, various linear relationships has been proposed as interlaminar shear strength (ILSS), interfacial shear strength (IFSS) or the capacity of the interface to transfer the stress from the matrix to the fibre.
 
The chemical composition of the fibre surface consists of weakly adsorbed materials that are removable by heat treatments as well as strongly adsorbed materials that are chemically attached with strong covalent bonds. 
 
The adhesive fibres elongate in a stepwise manner as folded domains are pulled open. The elongation events occur for forces of a few hundred pico-newtons. These are smaller than the forces of over a nano-newton, which are required to break the polymer backbone. When the force rises to a significant fraction of the force required to break a strong bond and threatens to break the backbone of the molecule, a domain unfolds. Thus, it can avoid the breaking of a strong bond in the backbone21.
 
These may represent the cumulative effect of multiple intra- and inter-chain bonds acting in concert22,23. The chemical makeup of a polymer chain has a direct influence on the strength of bonding available between the molecules. The stronger this is, the higher the glass transition temperature will be. The stiffness of the polymer chain is also determined by the nature of the chemical repeat groups, and thus, also has a profound effect on Tg. Stiff chains lead to high Tg values. Increasing densities of chemical cross-links reduce the freedom of motion of the segments of the polymer chains between the segments and thus increase Tg. Another effect of cross-links is that they decrease the ability of a polymer to crystallize, and thus highly cross-linked polymers are almost universally polymer glasses in solid state. The smaller the size of noncrystalline regions, the greater the effect of the constraint and the more Tg is increased.
 
Figure 2. Scanning electron micrographs show different fibre/matrix interface failure modeS
 
MATERIALS AND METHODS
 
Fibres: In this work there are three types of fibres reinforcement are used for the fabrication of laminate for fibre reinforced polymer matrix composites. These fibres are E-Glass fibre, Carbon fibre and Kevlar fibres and they were selected for their potential use in structural engineering application. E-Glass fibres with nominal diameter of 14µm, supplied from Saint Gobin Bangalore, India, were used. High modulus carbon fibres with nominal diameter of 7µm supplied from Nikunj Bangalore, India, were used.
 
Matrix: The epoxy resin used is diglycidyl ether of Bisphenol A (DGEBA) and the hardener is triethylene tetra amine (TETA) supplied by Atul Industries Ltd, Gujarat, India under the trade name Lapox, L-12 and K-6 respectively. Some properties of these reinforcements and epoxy resin used in the study are provided in the Table 1. The volume fraction of fibres is 60%. The ratio of epoxy and hardener is taken as 10:1.
 
Table 1.
 
Environmental Conditions: Fibre reinforced polymer matrix composites were subjected to different environmental conditions during their service life.
 
Mechanical Testing: Experimental Methods
 
Flexural Test (Short beam shear test): The flexural methods are applicable to polymeric composite materials. A testing machine with controllable crosshead speed is used in conjunction with a loading fixture. It is a three point flexural test on a specimen with a small span, which promotes failure by inter-laminar shear. The shear stress induced in a beam subjected to a bending load, is directly proportional to the magnitude of the applied load and independent of the span length. Thus the support span of the short beam shear specimen is kept short so that an inter-laminar shear failure occurs before a bending failure.
 
This test method is defined by ASTM D 2344, which specifies a span length to specimen thickness ratio of five for low stiffness composites and four for higher stiffness composite. This test has an inherent problem associated with the stress concentration and the non-linear plastic deformation induced by the loading nose of small diameter. 
 
The effects of stress concentration in a thin specimen are compared with those in a thick specimen. Both specimens have the same span-to-depth ratio (SDR). The stress state is much more complex than the pure shear stress state predicted by the simple beam theory. Effect of stress concentrations on short beam shear specimens: (a) thin specimen; (b) thick specimen. 
 
Scanning Electron Microscope (SEM): The scanning electron microscope (SEM) has been a well accepted tool for many years in the examination of fracture surfaces. The prominent imaging advantages are the great depth of field and high spatial resolution and the image is relatively easy to interpret visually. To analyse the prevailing failure modes of all the composites at various temperatures, their fracture surfaces were observed under Scanning Electron Microscope (SEM) with JEOL-JSM 6480 LVSEM operated at 20KV or a Nova NanoSEM (FEI) at 5 kV. For better identification of failure modes the fracture surfaces are tilt around 15°-20°. Prior to SEM, the top surface of the specimens were coated with platinum using a sputter coater. The coating is used to make the surface conductive for scanning and prevents the accumulation of static electric charge for clear images during the microscopy.
 
A critical point in understanding the formation of SEM images of fracture surfaces, and their interpretation, is an appreciation of the factors that affect this excited volume of electrons in the specimen. To understand the different failure mechanisms in FRP composites, photomicrographs were taken using a SEM. There is a dramatic change in the structure and properties of the composite when exposed to high and low temperatures.
 
Atomic Force Microscope (AFM): The AFM consists of a cantilever with a sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hookes law. AFM has become a useful tool for characterizing the topography and properties of solid materials since its advent. The specimens were scanned by AFM in contact mode with a conducting p-n doped silicon tip using a SPMLab-programmed Vecco Innova multimode scanning probe microscope. The scans were taken at scan rates of 1 Hz.
 
Figure 3. (a) SEM micrograph showing riverline marking on glass fibre/epoxy polymer composites at -80°C temperature (b) AFM 3D-images of fibre/matrix interfaces
 
Figure 4. SEM micrographs (a) Cusps generated under static loading (b) Cusps generated under dynamic loading (c) brittle fracture of carbon fibre11
 
RESULTS AND DISCUSSIONS
 
From a macroscopic point of view flexure is one of the most common failure modes observed in polymer composites due to its large surface area compared with the thickness. The influence of fibre type on interlaminar fracture morphology is highly dependent on the fibre-matrix interface strength. To analyse this concept clearly we investigate the interfacial behaviour of polymeric composites by considering three different cases. Carbon fibre-epoxy composites shown (Fig. 2A) the interfacial strength is usually good at ambient (27°C) temperature when subjected to short beam shear test (ILSS) under static condition. The carboxyl group present on the functionalized carbon fibre surface form ester bonds with epoxy matrix thus produce a good interfacial adhesion24. Fig. 2B shows glass fibre-epoxy matrix composites subjected to short beam shear test at ambient (27°C) temperature under dynamic loading. Here dominant fractography feature was cusps formation in intermixing spacing between fibres. Cusps lean towards right; indicating the opposing fracture surface has moved from right to left (negative shear)25. Fig. 2C represents the glass fibre-epoxy composites when subjected to low temperature under various loading speed. Different failure modes observed including fibre pull-out, fibre-matrix debonding and matrix cracking. When multiple microcracks are formed and begin to propagate in several planes, and they subsequently converge onto one plane. Ambient temperature signifies the beginning of the steps and welts as well as macromatrix cracking. Both the features are parallel to the fracture propagation direction. This failure mode was observed at low loading speed.
 
Optimisation of the stress transfer capability of the fibre–matrix interface region is critical to achieving the required composite performance level. The coefficients of thermal expansions of matrix polymers are much greater than that of the fibre reinforcement. During curing stage compressive radial stresses build-up at the interface region. Assuming that the coefficient of static friction at the interface is non-zero, these compressive stresses will contribute a frictional component to the apparent shear strength of the interface1-3. Enormous efforts have been conducted to acquire a comprehensive understanding of the interphase properties to produce advantageous interactions and maximize potential performances of the polymer matrix composites4-7.
 
Although advances in interfacial improvement have been made, in-depth knowledge of the interphase, especially quantitative characterization of CFRP interphase has not been clearly understood. The main reasons are attributed to its nanoscale thickness8,9 and mutual diffusion of the organic components, which are quite similar in chemical compositions.
 
This research revealed a number of key challenges regarding interface issues in producing polymer nanocomposites that exhibit a desired behaviour. The greatest stumbling block to the large-scale production and commercialization of nanocomposites is the dearth of cost effective methods for controlling the dispersion of the nanoparticles in polymeric matrix26. Dispersed nanoparticles swell the linear polymer chains, resulting in a polymer radius of gyration that grows with the nanoparticle volume fraction. This entropically unfavourable process is offset by an enthalpy gain due to an increase in molecular contacts at dispersed nanoparticle surface as compared with the surfaces of phase-separated nanoparticles27. The enhancement of polymer properties by the addition of inorganic nanoparticles is a complex function of interfacial interactions, interfacial area and the distribution of inter-nano-fillers distances. The chain relaxation dynamics and glass transition of polymer nano-composites (PNCs) are profoundly influenced by the relative strength of the chain–particle interactions and the morphology, particularly the particle dispersion and the inter-particle spacing, 1D28. Sufficiently strong particle–chain enthalpic interactions lead to permanent attachment of chain segments to the nanoparticles28. Under these conditions, PNCs have been shown to exhibit two glass-transition temperatures, Tg: one associated with polymer chains far from the nano-particles, and a second, larger Tg, associated with chains in the vicinity of the particles29,30.The ability to control the exact location of the position of the nano-particles within a polymeric matrix, would allow the properties of the resulting hybrid material to be tailored and greatly enhanced, increasing the number of possible applications.
 
SUMMARY
 
Different micro-characterization techniques and precise modelling are to be ascertained to predict the durability of the materials in the long-term applications. In this review, some of the fundamental knowledge governing the mechanical behavior response and subsequent load-bearing properties of these materials has been identified. An explicit scientific explanation to better comprehend the theories of environmental degradation is the need of the hour to utilize the promises and prospects of fibrous polymeric composites to the fullest potential with minimum risk factors and maximum environmental durability. The unprecedented failures of these materials during service conditions necessitate a holistic approach to be more comprehensively conclusive on the environmental damage and degradation of polymeric composites. The present review has emphasised this challenging and also contradictory understanding in reaching to a concise and convergent point. Considering interlaminar shear strength and delaminations behavior, the tested laminate characterized by SEM to reveal various failure modes. The present study may possibly reveal the following conclusions: Delaminations is the life limiting failure process in a composite material. It induces great loss of stiffness, local stress concentration, and buckling failures of composite materials. At -100°C temperature glass fibre/epoxy laminates shows better ILSS value but decreases with increasing loading speed.
 
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