Rate dependent constitutive modelling of laminated FRP composites degraded by fire

A constitutive theory is proposed for FRP laminated composite materials that is designed to account for both temperature and strain rate dependent response, such as would occur during, and after, exposure to the elevated temperatures due to fire. The theory is physically based, and in particular, is based on a kinematical framework fixed on the fabric laminates; full accounting of laminate reorientation and anisotropic response is thus achieved. The theory is numerically implemented and FEM analysis of compressive deformation of a sandwich panel, given as an example, demonstrates how common material failure modes such as kinking are naturally included in the theory and in analyses using the theory. The theory accounts for finite strains and thus the extent of deformation is arbitrary. The theory and its numerical implementation are designed specifically to perform numerical analysis of structural response of FRP structures subject to fire degradation.     

Modeling of thermo-physical properties for FRP composites under elevated and high temperature

A decomposition model for resin in glass fiber-reinforced polymer composites (GFRP) under elevated and high temperature was derived from chemical kinetics. Kinetic parameters were determined by four different methods using thermal gravimetric data at different heating rates or only one heating rate. Temperature-dependent mass transfer was obtained based on the decomposition model of resin. Considering that FRP composites are constituted by two phases – undecomposed and decomposed material – temperature-dependent thermal conductivity was obtained based on a series model and the specific heat capacity was obtained based on the Einstein model and mixture approach. The content of each phase was directly obtained from the decomposition model and mass transfer model. The effects of endothermic decomposition of the resin on the specific heat capacity and the shielding effect of evolving voids in the resin on thermal conductivity are dependent on the rate of decomposition. They were also described by the decomposition model; the effective specific heat capacity and thermal conductivity models were subsequently obtained. Each model was compared with experimental data or previous models, and good agreements were found.     

The visco-damage mechanical response of swirl-mat composites

This article concerns the characterization of the mechanical response of composite materials made of continuous, randomly oriented strands (‘swirl-mat’) of E-glass fibers embedded in a urethane matrix. The purpose of this work is to identify and quantify the various parameters that govern the material response in order to develop a predictive model. The characterization is based upon creep data collected during the application of constant load of several amplitudes and various durations, and recovery data collected after load release. The glass-fiber/urethane matrix swirl-mat composites were observed to develop distributed damage in the form of profuse internal micro-cracks, they exhibit time-dependent response and develop permanent deformation upon load removal. A one dimensional model is presented, which incorporates the foregoing features of creep, continuum damage, and permanent deformation. The model is employed to represent the characterization data as well as to predict the response under two-step load histories. This revised version was published online in July 2006 with corrections to the Cover Date.     

Peculiarities of mechanical response of heavily filled polypropylene composites

Temperature dependences of dynamic moduli of polypropylene filled with flame retardant Mg(OH)₂ or with CaCO₃ were investigated. The filler content varied from O to 50 vol% and both storage (G) and loss (G) shear moduli were measured from + 50 to + 300 °C at frequencies 1 and 90 Hz. The influence of the filler particle shape was dominant for the origin of the physical network. Chemical adhesion aid was superfluous in composites with subcritical particle size. Two step decrease of G and two peaks on G temperature dependences were observed above the critical filler concentration. The explanation of this phenomenon was based on the proposal that polypropylene immobilized on the filler surface creates a new continuous phase, while primary polypropylene exists only in separated domains. Destruction of the physical network was explained using results of thermoanalysis, providing the catalytic effect of Mg(OH)₂ on the thermal oxidation of polypropylene in the interlayer above 240 °C.     

Probabilistic failure prediction for FRP composites

Results are presented from a theoretical study on failure-locus prediction for unidirectional FRP laminae under complex in-plane loading, taking into account statistical aspects of the basic strengths of the material. The purpose of the analysis is to establish simple rules and methodologies for failure prediction under specific reliability requirements. Therefore, the problem one is faced with is the definition of the cumulative distribution function (CDF) of the failure condition if the respective CDFs of the basic material strengths are known by experiment. To this end, two analytical approaches, namely a functional expansion technique and the introduction of Pearson’s semi-empirical distribution function, were developed and implemented in software. Both methods were shown to predict satisfactory results compared with Monte Carlo simulated ones and experimental data, wherever available. Finally, a semi-deterministic approach was examined according to which the failure criterion is used in the usual deterministic way but with basic strength values of a certain reliability level. Results from this simple and fast method were found in good agreement with those derived by expensive pure statistical methods or numerically simulated ones and experimental data.     

Modeling and mechanical performance of carbon nanotube/epoxy resin composites

The effect of multi-walled carbon nanotube (MWCNT) addition on mechanical properties of epoxy resin was investigated to obtain the tensile strength, compressive strength and Young’s modulus from load versus displacement graphs. The result shows that the tensile strength, compressive strength and Young’s modulus of epoxy resin were increased with the addition of MWCNT fillers. The significant improvements in tensile strength, compressive strength and Young’s modulus were obtained due to the excellent dispersion of MWCNT fillers in the epoxy resin. The dispersion of MWCNT fillers in epoxy resin was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis.Also, Halpin–Tsai model was modified by considering the average diameter of internal/external of multi-walled nanotube and orientation factor (α) to calculate the Young’s modulus of multi-walled carbon nanotubes (MWCNTs)/epoxy resin composite. There was a good correlation between the experimentally obtained Young’s modulus and modified Halpin–Tsai model.     

Fracture behaviour of fly-ash filled FRP composites

The major objective of this study was to determine the fracture toughness and fracture surface energy of epoxy, epoxy/fly-ash, epoxy/carbon fibre, epoxy/carbon fibre/fly-ash, epoxy/glass fibre and epoxy/glass fibre/fly-ash composites. The quality of composite specimens was evaluated by the ultrasonic method. The results show that a fly-ash particle can arrest the crack path and thus improve the fracture properties of fibre reinforced plastic (FRP) composites. The results of this study have further significance in view of the fact that fly-ash powder is far cheaper than carbon fibre, glass fibre and epoxy resin.     

On the mechanical response of short fibre reinforced polymer composites

In this paper, a generalized Mori-Tanaka scheme is applied to evaluate the elastic response of short fibre composite materials. Numerical predictions are compared with experimental measurements performed on short fibreglass reinforced thermoplastics with a wide range of fibre concentrations, including the range characteristic of usual industrial applications with non-trivial fibre orientation distribution.     

Acoustic fatigue and damping technology in FRP composites

There is considerable interest in the use of composite materials in aerospace structures. One important area is to develop a stiff, lightweight composite material with a highly damped, high-temperature polymer matrix material. This paper concerns the application of such material, in the form used in thin skin panels of aircraft, and investigates of its fatigue properties at both room and high temperature. Flexural fatigue tests have been carried out at two different temperatures and harmonic three-dimensional finite-element (FE) analyses were performed in order to understand the dynamic behaviour of plates. Random acoustic excitation tests using a progressive wave tube, up to an overall sound pressure level of 162 dB, at room temperature and high temperatures were also performed in order to investigate the dynamic behaviour of panels made of the materials. Parameter studies were carried out in order to examine various methods for including damping in the structure, and conclusions have been drawn concerning optimal incorporation of a highly damped matrix material into a high-performance structure.     

Application of FRP composites for underwater piles repair

The lightweight, high strength and corrosion resistance of fiber reinforced polymers (FRP) make them ideally suited for quick and effective structural repairs. As a result, they have been favoured for conducting emergency bridge repairs where speed is of essence. The availability of resins that can cure under water has made it possible to similarly extend its application to substructure elements such as partially submerged damaged piles. Such repairs can be carried out using the same strategies that were successfully used in recent demonstration projects in which FRP was used to repair and rehabilitate corrosion-damaged piles. In the projects two disparate FRP systems – a pre-preg and a wet layup – were used and both carbon and glass evaluated. Access to the piles in the deep waters was provided by a custom-designed, lightweight modular scaffolding system that was assembled around the piles. An overview of the project is provided with particular emphasis on changes that would allow its adoption for emergency repairs.     

Quantification of uncertainty modelling in stochastic analysis of FRP composites

The extensive use of FRP composite materials in a wide range of industries, and their inherent variability, has prompted many researchers to assess their performance from a probabilistic perspective. This paper attempts to quantify the uncertainty in FRP composites and to summarise the different stochastic modelling approaches suggested in the literature. Researchers have considered uncertainties starting at a constituent (fibre/matrix) level, at the ply level or at a coupon or component level. The constituent based approach could be further classified as a random variable based stochastic computational mechanics approach (whose usage is comparatively limited due to complex test data requirements and possible uncertainty propagation errors) and the more widely used morphology based random composite modelling which has been recommended for exploring local damage and failure characteristics. The ply level analysis using either stiffness/strength or fracture mechanics based models is suggested when the ply characteristics influence the composite properties significantly, or as a way to check the propagation of uncertainties across length scales. On the other hand, a coupon or component level based uncertainty modelling is suggested when global response characteristics govern the design objectives. Though relatively unexplored, appropriate cross-fertilisation between these approaches in a multi-scale modelling framework seems to be a promising avenue for stochastic analysis of composite structures. It is hoped that this review paper could facilitate and strengthen this process.     

Effects of weave architecture on mechanical response of 2D ceramic composites

A meso-scale finite element model is developed to investigate effects of weave architecture on strain and stress evolution in an eight harness-satin SiC/SiCN composite. Fiber tows are modeled explicitly using elastic rebar layers embedded within elastic/plastic effective medium elements. Effects of through-thickness constraint are investigated using several idealized test geometries, ranging from a single (unconstrained) ply to a fully-constrained two-ply lay-up with periodic boundary conditions in the through-thickness direction. A parallel experimental study of surface strain evolution in a representative SiC/SiCN composite is used to assess the model predictions. The results indicate that, because of bending and straightening of wavy tow segments at the locations of tow cross-overs, strain and stress concentrations arise. The effects are exacerbated by reductions in the constraints on bending and straightening caused by matrix damage, especially in surface plies. The implications of the results in the fracture process and on potential mitigation strategies are discussed.     

Epoxy/iron alginate composites with improved fire resistance,smoke suppression and mechanical properties

Journal of Materials Science - A balance of sustainability and high fire resistance and smoke suppression is important for the preparation of epoxy resins (EPs). Herein, bio-based iron alginate was...     

The dynamic mechanical response of SiC particulate reinforced 2024 aluminum matrix composites

The compression properties of the aluminum alloy 2024 metal matrix composites reinforced with 50 vol.% SiC particles were investigated using Instron testing machine and split Hopkinson pressure bar (SHPB) in this paper. The compression stress–strain curves were obtained at the strain rates ranging from 1 × 10^− 3 to 2.5 × 10³/s. The fracture surfaces were characterized by scanning electron microscopy. The results showed that SiCp/2024 Al composites exhibited high strain-rate sensitivity. The strength of composites tended to increase–decrease with increasing of strain rates. The effect of the strain rate on elongation was also discussed.     

FRP复合材料剩余刚度退化复合模型

为建立剩余刚度与材料损伤量及剩余寿命的关系, 将纤维增强树脂复合材料(FRP)层合板在拉-拉疲劳载荷作用下的失效模式划分为纤维间破坏、纤维随机断裂与分层3种类型, 分析不同失效模式与剩余刚度退化量的定量关系, 提出一个集成各失效模式影响的剩余刚度退化复合模型。该模型适用于占寿命绝大多数比例的Ⅰ、Ⅱ阶段, 避免了Ⅲ阶段刚度降不确定性的影响。剩余刚度退化曲线按时间尺度归一化, 消除了试件个体分散性影响, 分散性显著降低。对4种E-glass/Epoxy玻璃纤维复合材料层压板与3种AS-4/聚醚醚酮(PEEK)碳纤维复合材料层压板的疲劳试验结果进行了统计分析, 表明本文模型适于精确描述复合材料的剩余刚度下降规律。      

Comparative behaviour of hollow columns confined with FRP composites

Confining columns with fibre reinforced polymer (FRP) composites have been investigated in the last few decades to address the problem of upgrading and retrofitting reinforced concrete (RC) columns; however, most studies have concentrated on solid columns. This paper investigates the comparative behaviour of FRP confined hollow RC columns subjected to concentric loading. A total of twelve RC columns made from high strength concrete (HSC) were cast and tested. Six of the columns had a circular cross section (two solid columns, two hollow columns each having a circular hole, and two hollow columns each having a square hole) and the remainder columns had a square cross section (two solid columns, two hollow columns each having a circular hole, and two hollow columns each having a square hole). Six columns in total, three from each configuration were left unconfined as control specimens, while the others were confined with FRP. It was found that FRP confinement increased hollow RC columns’ axial load and ductility capacities; and hollow columns having circular holes had better performance compared to hollow columns having square holes.     

The effect of temperature on fatigue damage of FRP composites

The present study intends to investigate the effect of temperature on cumulative fatigue damage (D) of laminated fibre-reinforced polymer (FRP) composites. The effect of temperature on fatigue damage is formulated based on Ramkrishnan–Jayaraman and Varvani-Farahani–Shirazi residual stiffness fatigue damage models. The models are further developed to assess the fatigue damage of FRP composites at various temperatures (T). This task is fulfilled by formulating the temperature dependency of Young’s modulus (E) and ultimate tensile strength (σ_ult) as the inputs of the models. Temperature-dependant parameters of Young’s modulus and ultimate tensile strength are found to be in good agreement with the experimentally obtained data when used for unidirectional, cross-ply and quasi-isotropic FRP laminates. The proposed fatigue damage model is evaluated using six sets of fatigue damage data. The proposed temperature-dependent model was also found promising to predict the fatigue damage of unidirectional (UD) and orthogonal woven FRP composites at different temperatures.     

Influence of reinforcement and processing on steel-based composites: Microstructure and mechanical response

Steel matrix composite reinforced with 2–4 vol.% titanium diboride particles was fabricated successfully by powder metallurgy route through hot pressing method. Influence of sintering parameters on densification was investigated by measurement of density of resultant composites. Microstructural analysis of hot-pressed materials was performed. Hardness and deformation behavior under constant load were evaluated by conducting microhardness and nanoindentation tests. The addition of titanium diboride proved to be effective for enhancement of hardness and strength. Composite with 4 vol.% titanium diboride sintered at 1100°C resulted in improved hardness and elastic modulus which could be related to Orowan strengthening resulting from homogeneous distribution of fine titanium diboride particles in steel matrix. The results indicate that proposed method is economically feasible to process steel matrix composites with improved properties. A comparatively lower temperature and pressure offers better control of interface kinetics and microstructure.     

Repair of damaged RC columns using fast curing FRP composites

In this study, two fast curing resins were used to repair pre-damaged RC columns. They were 1.5-hour heat activated curing prepreg and 20-min ultraviolet curing resin. A 24-hour curing epoxy was also used for comparison purposes. A total of 24 steel reinforced φ 152.4 mm×609.6 mm small-scale concrete columns were designed, cast, cured, surface prepared, and pre-damaged. The damaged samples were repaired using the three types of E-glass fabric reinforced resins. An accelerated conditioning using boiling seawater and ultraviolet radiation was also conducted to investigate the hygrothermal durability of the repaired samples. Uniaxial compression test was conducted on both control samples and conditioned samples. The test results and cost/benefit analysis results show that the two fast curing resins can replace the currently used long-time curing resins in repairing damaged RC columns.