Journal of Composite Materials

This paper presents an optimization of the mechanical properties of cement–Posidonia composite by means of experimental characterization. We are interested in producing composite of cement reinforced by Posidonia raw fibers. The ratio of fibers and water-to-cement mass is varied to determine its effect on the mechanical properties of the cement–Posidonia composite, particularly, its resistance to fracture. Three point bending and compression tests were carried out to study the mechanical properties of the composite. Scanning electron microscopy was used to examine the surface of the tested samples. The experimental investigation shows an improvement of flexural strength for a ratio W/C equal to 0.5 and fiber content equal to 10 vol%.

In this work, the thermomechanical viscoelastic response of a high temperature polymer matrix composite system made up of T650-35 graphite fibers embedded in PMR-15 resin is studied through a micromechanical model based on the assumptions of simplified unit cell method within a temperature range of 250–300℃ corresponding to aerospace engine applications. The advantage of this particular micromechanical model lies in its ability to give closed-form expressions for the effective viscoelastic response of unidirectional composites as well as each of their constituents. Using the experimental data of the creep behavior of thermostable PMR-15 polyimide, the micromechanical model is first calibrated to account for the effect of temperature. The resulting elastic and viscoelastic responses are found to be in good agreement with the existing experimental data. The validated model is then used to predict the behavior of the composite material under different combinations of thermal and mechanical loadings. The results clearly demonstrate the importance of accounting for the viscoelastic effect of the matrix material as the temperature increases. Current works on modeling temperature-dependent viscoelastic behavior of polymer matrix composites are mainly based on the assumption of thermorheologically simple material. However, through the present approach where the matrix is modeled as a thermorheologically complex material, the effect of temperature on the elastic and viscoelastic response of the composite system can be individually investigated.

This investigation explores the manipulation of carbon black particles for tailoring the electrical properties of unidirectional glass fiber reinforced epoxy composites. Carbon black particles were anisotropically networked along the through-thickness direction of glass/epoxy composite plates using an alternating current electric field applied during curing of the composite, with the objective of maximizing the electrical conductivity through the thickness. Anisotropic networking was observed microscopically and was quantified by measuring the DC electrical conductivity of the cured glass/epoxy composite material in the three principal directions. The effects of carbon black amount, electric field strength, and electric field frequency on the anisotropic conductivity are elucidated using a parametric investigation. It is shown that the through-thickness conductivity can be increased by a factor of roughly 10⁴ relative to the case with no conductivity tailoring and can be of the same order of magnitude as the transverse and longitudinal in-plane conductivities, which are improvements well beyond the studies published until now. Moreover, for the first time, it is shown that the through-thickness conductivity of unidirectional glass/epoxy composites containing carbon black can exceed the in-plane transverse conductivity by selecting appropriate electric field parameters during processing.

This research aims to characterize the damping properties of fiber/epoxy composites containing different degrees of silica nanoparticles and rubber particles. Conventionally, adding rubber particles into fiber/epoxy composites would lead to dramatic reduction of stiffness although the vibration damping could be improved accordingly. In order to enhance the damping properties of the fiber composites without sacrificing the stiffness, silica nanoparticles together with rubber particles were introduced into the epoxy resin through the sonication process. The epoxy resin was then treated as matrix and impregnated into the fiber layer by means of the vacuum hand lay-up process to form the composite laminates. The vibration damping as well as the flexural stiffness of the fiber composite was measured using the forced vibration technique together with the half-power method. In addition, the vibration damping of the composite laminates, consisting of silica nanoparticles and rubber particles, was characterized using the micromechanical analysis. The repeated unit cell with the fibers displaying randomly in the matrix was employed to represent the microstructures of the unidirectional composites. The loss factor as well as the moduli obtained from the micromechanical analysis were regarded as the effective properties homogenizing within the fiber composites. In conjunction with the modal shapes, the vibration damping of the composite laminates with stacking sequence of [0]₁₀, [90]₁₀, [±45]_2s, and [90/0]_2s was calculated using the finite element analysis. Experimental results indicated that with the incorporation of the silica nanoparticles together with the rubber particles, the reduction of flexural stiffness of fiber composites, especially for the [90]₁₀ laminates, was diminished while the damping properties of laminates were improved. Moreover, it was found that the effect of the particles in the [0]₁₀ laminates is relatively minimal. The vibration damping responses of composites laminates obtained using the micromechanical analysis together with the modal analysis exhibit an agreement with the experimental data.

Conventional methods of analysis for drilling of composite materials usually study the amount of damaged area, thrust force, and effective parameters. However, these methods do not provide the investigator with sufficient information about drilling mechanisms. In the current investigation, a procedure for diagnosing different drilling mechanisms based on the analysis of the signals of acoustic emission is presented. According to the number of time domain acoustic emission parameters, using multi-variable methods of analysis is unavoidable. In this work, unsupervised pattern recognition analyses (fuzzy C-means clustering) associated with a principal component analysis are the tools that are used for the classification of the recorded acoustic emission data. After classification of acoustic emission events, the resulting classes are correlated with the different drilling stages and mechanisms. Acoustic emission signal analysis provides a better discrimination of drilling stages than mechanic-based analyses.

The paper presents a comparison of the test results for the fiber volume fraction, static bending and Charpy impact strength of glass fiber reinforced polymer laminates reinforced by 0/90 fabric and chopped strand mat, produced by the hand lay-up and the vacuum-assisted resin infusion (VARI) method. The laminates were produced under equivalent conditions, with polyester matrix and lay-up areal mass 2100 g/m². In the comparison of the obtained measurement results, similar mechanical performance was stated in the case of the hand lay-up and the VARI production. However, significantly smaller scatter of results and better uniformity of the reinforcement in the matrix with smaller amount of local structure defects was observed for the samples obtained by VARI method. The laminates obtained by VARI method show a much more advantageous coefficient of variation than that of the hand lay-up method, especially in the case of the mat reinforcement.

This study deals with the computational study of asymmetric glass reinforced plastic beams in off-axis four-point bending and the comparison of the induced results with experimental and analytical results. The measurement of the interlaminar shear strength of composite beams, an important design variable in many applications, may be successfully performed by the asymmetric bending test. A three-dimensional finite element analysis is adopted throughout the composite beams in order to, on the one hand, correlate with the experimental results and, on the other hand, to obtain the stress distributions at the supports and at the loading points where usually there is an abrupt variation due to the indentation existing because of the noses. From the finite element analysis and the experimental investigation possible crack initiation positions are determined.

Damage development due to impact needs to be understood to evaluate the consequences of impact on composite structures. This study concentrates on modelling and measuring damage development due to low velocity impact on thick industrial composites made from glass fibre epoxy by vacuum-assisted resin infusion. Cross-plied laminates were tested with different impact energy and different number of interfaces (clustering). Results were compared to a 3D finite element analysis. Interfaces and their damage development were modelled with cohesive elements. Intra ply properties were modelled by progressive failure analysis. Many elements and large memory use were needed to obtain sufficient modelling accuracy. However, all input parameters of the model were based on widely available and independently obtained material properties. Impact force and time to initiate damage and maximum force were measured and related to impact energy and clustering. Damage development was monitored optically in the translucent material for all test cases. The results show that the numerical model using only simple and independently measured material data was able to predict the impact behaviour for the different energies and different stacking sequences.

Due to the heterogeneous nature and electric anisotropy, it is challenging to establish a numerical model to analyze the electromagnetic properties of multilayer carbon fiber-reinforced polymer (CFRP) laminate. In this study, we focus on the exploration of an effective electromagnetic modeling approach for calculation of eddy currents in CFRP laminate composite, as well as eddy current testing signals due to surface cracks. In order to prove the feasibility of modeling CFRP laminate with homogeneous anisotropic layers, the electrical parameters in the three directions are measured, and eddy current path in CFRP is investigated according to the measurement results. A finite element solver based on reduced magnetic vector potential ( A _r ) formulation and edge elements is developed to enable the eddy current simulation of anisotropic CFRP material, which avoids matching the discretization of source coils with the rest of conductor mesh, and can easily solve the field continuity problem in the interface between two adjacent fiber layers of CFRP laminate. The A _r formulation and way to calculate the eddy current testing signals are described. To validate of the developed simulation code, a comparison is conducted between the calculated signals and experimental results of thin-opening cracks in a CFRP test piece, which indicates the simulation code can predict eddy current testing signals with good precision.