Damage resistance of carbon fibre reinforced epoxy laminates subjected to low velocity impact: Effects of laminate thickness and ply-stacking sequence

A major concern affecting the efficient use of composite laminates is the effect of low velocity impact damage on the structural integrity [1–3]. The aim of this study is to characterize and assess the effect of laminate thickness, ply-stacking sequence and scaling technique on the damage resistance of CFRP laminates subjected to low velocity impact. Drop-weight impact tests are carried out to determine impact response. Ultrasonic C-scanning and cross-sectional micrographs are examined to assess failure mechanisms of the different configurations.It is observed that damage resistance decreases as impact energy increases. In addition, thicker laminates show lower absorbed energy but, conversely, a more extensive delamination due to higher bending stiffness. Thinner laminates show higher failure depth. Furthermore, quasi-isotropic laminates show better performance in terms of damage resistance. Finally, the results obtained demonstrate that introducing ply clustering had a negative effect on the damage resistance and on the delamination area.     

Characteristics of kenaf fibre/epoxy composites subjected to thermal degradation

Kenaf fibres are receiving much attention in the natural fibre composite industry due to its potential as polymer reinforcements. However, like all natural fibres, kenaf fibres have lower thermal resistance as compared to synthetic fibres. In this current work, the characteristics of kenaf fibre/epoxy composites, both treated and untreated using alkalization process, exposed to high temperature were studied. Thermogravimetric analysis (TGA) was used to study the thermal decomposition behaviour of treated and untreated kenaf/epoxy composites as well as their components, kenaf fibre and neat epoxy from room temperature up to 600 °C. The weight loss and physical changes of these samples were observed through furnace pyrolysis. Surface morphology of the composites after degradation was observed using scanning electron microscopy (SEM). The results from the TGA showed that the addition of kenaf fibres into the epoxy slightly improves both the charring and thermal stability of the samples. However, it was observed that alkalization causes reduction in these behaviours for the kenaf/epoxy composite. Generally, increased exposure time causes higher weight loss of the composites only up to 150 °C. At higher temperature, duration of exposure has little influence on the weight loss. Fibre-matrix debondings were observed on degraded samples implying mechanical degradation of the composites had occurred.     

Through-thickness compression testing of fabric reinforced composite materials: Adapted design of novel compression stamps

The use of thick-walled fabric reinforced laminates in force transmission zones increased in the last years due to optimised production processes for composite structures. This enabled especially the manufacturing of thick laminates free of pores or other defects. As generally acknowledged, for a robust design and precise failure prediction of these structures reliable material property data are essential. While many test standards were analysed and modified in accordance to the specific characteristics of fibre reinforced materials, out-of-plane compression tests seem outdated. So far, some test methods are established; however, a standardised test method for compression testing in through-thickness direction of the laminate is still missing.A novel approach for through-thickness compression testing of fabric reinforced polymers with specifically designed stiffness-adapted compression stamps is presented. Comprehensive numerical analyses of the compressive load application and the dimensioning of the compression stamps in relation to the specimen size have been performed. It is shown, that the developed through-thickness compression testing device with the adapted design of the novel compression stamps is best suitable, as higher compressive strengths of fabric reinforced polymers can be experimentally determined.     

Low velocity impact and pseudo-ductile behaviour of carbon/glass/epoxy and carbon/glass/PMMA hybrid composite laminates for aircraft application at service temperature

Carbon/glass hybrid composite (CGHC) laminates are some of the most promising composites for lightweight applications. Sometimes these laminates are used in warm environment, such as aircraft frame structures, and this may affect their performance. In order to investigate this issue, the present research aims to study the effect of temperatures on the impact behavior and pseudo-ductile behaviour of CGHC in presence of different types of thermosets “epoxy” and thermoplastic “acrylic poly-methyl methacrylate-PMMA”. The experiments were started with making of CGHC laminates from different stacking sequences of unidirectional carbon and woven glass fibre layers, using a vacuum-assisted resin transfer method followed by curing treatment. In addition to CGHC laminates, four other neat batches (Carbon/epoxy, Carbon/PMMA, Glass/epoxy, Glass/PMMA) were prepared for comparison. The low velocity impact behaviour of the fabricated panels was evaluated at high temperatures (60 °C and 80 °C) according to ISO 6603-2 standard, using drop tower, while pseudo-ductile behaviour and ductility index (DI) of the specimens were estimated based on the measured total energy and elastic energy. Also, the low-velocity impact response was modeled mathematically based on a modified energy-balance model to predict the absorbed energies. Finally, the failure mechanisms were examined using optical microscope to determine the influence of these damage growth on DI of the composites under different temperatures. The results showed that the impact energy response of both hybrid composites i.e. epoxy and PMMA was stable even as the temperature rose, however, carbon/glass/PMMA exhibited better performance compared with carbon/glass/epoxy with an increase in impact energy response estimated at 50% (25 °C) and 53% (80 °C). Also, the pseudo-ductile phenomenon was strongly evident, which facilitates the predictablility of failure.     

Modified V-notched rail shear test fixture for shear characterisation of textile-reinforced composite materials

The V-notched rail shear test (VNRS) is recognised as a standard test method for the determination of shear modulus and shear strength of fibre-reinforced composite materials. This method is based on a double V-notched specimen with a large gauge section and an approximately uniform shear stress distribution between the two notches. The construction of the test device prescribed in standards was revised with the main focus of lines of force as well as precise and economic specimen clamping. Therefore, the modified test fixture has been equipped with guide units and the number of clamping bolts could be reduced. The standardised and modified VNRS fixtures were evaluated using a finite element method. It can be shown that the modified VNRS test fixture is well suited for robust and reliable material characterisation of shear property data.     

Investigation of flexural properties and failure behaviour of biaxial braided CFRP

In this research, the experimental tests of quasi-static three-point bending and three-point bending fatigue were carried out for a ±25° biaxial braided carbon fibre reinforced polymer (CFRP) manufactured using vacuum assisted resin transfer moulding (VARTM). A finite element (FE) model was also set up for quasi-static testing and the prediction results revealed that local fibre volume fraction (FVF) is a primary source affecting the mechanical properties of braided CFRP. The fatigue of the braided CFRP was defined as three different stages according to the flexural modulus results. The damage modes of the test specimens were observed via a digital microscope and scanning electron microscope (SEM) and the process-induced defects were summarised. With compiled results and observations, this study provides a better understanding of failure and fatigue behaviour of biaxial braided composites and their flexural properties which offers a good basis for any further research in fibre volume fractions, structure design and manufacturing for braided CFRP.     

Elastic properties of electrospun PVDF nanofibrous membranes: Experimental investigation and numerical modelling using pixel-based finite element method

In this paper, experimental investigation and numerical modelling of the mechanical properties of polyvinylidene fluoride (PVDF) nanofibrous membranes produced by electrospinning are addressed. Membranes with three different diameters are fabricated by adjusting the needle-collector distance during electrospinning. The fiber morphology and the physical properties of the resulting membranes are investigated using Scanning Electron Microscopy (SEM) while their elastic properties are probed using conventional tensile tests. It is found that the membrane with the largest nanofiber diameters are filled with large beads while the contrary is found in the membrane with the smallest nanofiber diameter. Consequently, the membrane with the smallest nanofiber diameter yielded the highest membrane Young's modulus thanks to better fiber packing and higher crystallinity in the nanofibers. Next, the experimental results serve as basis for a pixel-based finite element method (FEM) which is applied directly on the SEM images of the membranes. This technique has the advantage of providing estimations of mechanical properties from the real structure of the membranes. Two parameters are needed for this linear elastic analysis: the elastic modulus of a single fiber and the fiber percentage in the membrane. Results show that the model predictions are in good agreement with experimental data. These results suggest that the pixel-based FEM could be a promising nondestructive alternative to the conventional tensile tests.     

Drop-weight impact tests and finite element modeling of cast acrylic/aluminum plates

As an extension of a previous study [1], drop-weight impact tests on cast acrylic (PMMA) plates reinforced by aluminum face sheets were carried out using an instrumented drop weight impact tester. The PMMA and aluminum layers were adhered by epoxy cured at room temperature. Depending on the impact velocity and the type of top surface (acrylic or aluminum) struck by the impactor, damage caused by impact included partial or full delamination at the interface and radial cracks in the acrylic layer. The higher the impact velocity, the more damage was induced. More severe damage occurred if the bi-layer plate was impacted on the aluminum side. The ultrasonic C-scan technique was adopted to detect the damage. The pulse-echo technique with a focused transducer provided very good C-scan results for detecting damage patterns. The transducer with higher frequency gave better resolution and showed more details of damage. Finally, the finite element program, LS-DYNA, implemented with maximum strength criterion for radial cracking and mixed mode strength criterion for interfacial fracture, was used to simulate the drop weight impact tests. Impact force history, energy partition and delamination were predicted assuming various boundary conditions according to experimental results. The finite element simulations were in very good agreement with the experimental data.     

Non-linear material characterization and numerical modeling of cross-ply basalt/epoxy laminate under low velocity impact

The low velocity impact behavior of basalt/epoxy composites, seen as an eco-friendly replacement of glass-epoxy composites, has not been studied systematically so far. Here, the elastic elasto-plastic properties, strengths, intralaminar and interlaminar fracture energies were determined. The intralaminar energies were determined using compact tension and compression tests. The elasto-plastic properties needed in the plastic potential were determined using off-axis test. These properties are used in Finite Element (FE) code with an elasto-plastic damage model developed earlier to simulate the impact response of cross-ply laminates basalt/epoxy laminates. Low velocity impact (LVI) experiments at 10 J, 20 J and 30 J are performed on these composites. The FE simulation is successful in capturing force, energy, deflection histories and damage zones showing a close match to the experiments. A comparison of impact force history and damage area (ultrasonic C-scan) of basalt-epoxy laminates with glass epoxy laminates having same volume fraction shows nearly similar peak forces but the major axis of the ellipsoidal damage zone was bigger in glass/epoxy laminates.     

Experimental and numerical investigations of bi-injection moulding of PA66/LSR peel test specimens

Bi-injection moulding is a widely used process to manufacture engineering products and consumer goods. Typically, a thermoplastic is combined with rubber or another thermoplastic to create colour differences or hard and soft areas, respectively. The aim of this study was to optimise the injection parameters and processing conditions for the moulding of two-component standard peel test specimens with suitable functional properties. In this work, all parameters of thermo-rheo-kinetic behaviour were identified to predict the entire filling stage and the effect of a liquid silicone rubber cross-linking reaction during the injection moulding process. The models of Carreau-Yasuda and Isayev-Deng regarding the thermal dependence assumed by Arrhenius’ law were used. In our study, over-injection moulding is simulated and examined using finite element software (Cadmould 3D) to investigate the thermo-rheo-kinetic behaviour and the adhesion of liquid silicone rubber during the filling mould process in over-moulding. Numerical simulation results were then compared with the experimental results, and good agreement was obtained.     

Experimental and numerical investigation of yielding phenomena in a shape memory polymer subjected to cyclic tension at various strain rates

This paper presents experimental and numerical results of a polyurethane shape memory polymer (SMP) subjected to cyclic tensile loading. The goal was to investigate the polymer yielding phenomena based on the effects of thermomechanical coupling. Mechanical characteristics were obtained with a testing machine, whereas the SMP temperature accompanying its deformation process was simultaneously measured in a contactless manner with an infrared camera. The SMP glass transition temperature was approximately 45 °C; therefore, when tested at room temperature, the polymer is rigid and behaves as solid material. The stress and related temperature changes at various strain rates showed how the SMP yield limit evolved in subsequent loading-unloading cycles under various strain rates. A two-phase model of the SMP was applied to describe its mechanical response in cyclic tension. The 3D Finite Element model of a tested specimen was used in simulations. Good agreement between the model predictions and experimental results was observed for the first tension cycle.     

Finite element modelling of mode I delamination specimens by means of implicit and explicit solvers

The simulation of delamination using the Finite Element Method (FEM) is a useful tool to analyse fracture mechanics. In this paper, simulations are performed by means of two different fracture mechanics models: Two Step Extension (TSEM) and Cohesive Zone (CZM) methods, using implicit and explicit solvers, respectively.TSEM is an efficient method to determine the energy release rate components G_Ic, G_IIc and G_IIIc using the experimental critical load (P_c) as input, while CZM is the most widely used method to predict crack propagation (P_c) using the critical energy release rate as input.The two methods were compared in terms of convergence performance and accuracy to represent the material behaviour and in order to investigate their validity to predict mode I interlaminar fracture failure in unidirectional AS4/8552 carbon fibre composite laminates.The influence of increasing the loading speed and using mass scaling was studied in order to decrease computing time in CZ models.Finally, numerical simulations were compared with experimental results performed by means of Double Cantilever Beam specimens (DCB).Results showed a good agreement between both FEM models and experimental results.     

The effect of sodium benzoate and sodium 4-(phenylamino)benzenesulfonate on the corrosion behavior of low carbon steel

Abstract  The effect of sodium benzoate (SB) and sodium 4-(phenylamino)benzenesulfonate (SPABS) on the corrosion behavior of low carbon steel has been investigated using gravimetric method in the temperature range of 30–80 °C, velocity range of 1.44–2.02 m s⁻¹ and concentration range of 6.94 × 10⁻⁴ to 4.16 × 10⁻³ mol dm⁻³ SB and 3.69 × 10⁻⁴ to 2.06 × 10⁻³ mol dm⁻³ SPABS. Optimization of temperature, fluid velocity, and inhibitors concentration has been made. The obtained results indicate that the inhibition efficiency (w _IE %) at 1.56 m s⁻¹ is not in excess of 81.5% at 4.16 × 10⁻³ mol dm⁻³ SB and 84.4% at 2.06 × 10⁻³ mol dm⁻³ SPABS. The inhibitive performance of these compounds showed an improvement with increasing concentration up to critical values of SB and SPABS; beyond these concentrations no further effectiveness is observed. These inhibitors retard the anodic dissolution of low carbon steel by protective layer bonding on the metal surface. The adsorption of SB and SPABS on the low carbon steel surface was found to obey the Freundlich isotherm model. The FT-IR spectroscopy was used to analyze the surface adsorbed film. Graphical abstract  Low carbon steel corrosion in presence of sodium benzoate and sodium 4-(phenylamino)benzenesulfonate has been investigated. The adsorption mechanism obeyed Freundlich isotherm model. FT-IR was used to analyze the adsorbed film      

Optical fibre reflectance sensor for the determination and speciation analysis of iron in fresh and seawater samples coupled to a multisyringe flow injection system

A novel optical fibre reflectance sensor coupled to a multisyringe flow injection system (MSFIA) for the determination and speciation analysis of iron at trace level using chelating disks (iminodiacetic groups) is proposed. Once iron(III) has been retained onto a chelating disk, an ammonium thiocyanate stream is injected in order to form the iron(III)-thiocyanate complex which is spectrophotometrically detected at 480 nm. Iron(III) is eluted with 2 M hydrochloric acid so that the chelating disk is regenerated for subsequent experiments. The determination of total iron is achieved by the on-line oxidation of iron(II) to iron(III) with a suitable hydrogen peroxide stream.A mass calibration was feasible in the range from 0.001 to 0.25 μg. The detection limit (3s_b/S) was 0.001 μg. The repeatability (RSD), calculated from nine replicates using 1 ml injections of a 0.1 mg/l concentration, was 2.2%. The repeatability between five chelating disks was 3.6%. The applicability of the proposed methodology in fresh and seawater samples has been proved.The proposed technique has been validated by replicate analysis (n = 4) of certified reference materials of water with satisfactory results.     

A finite element method based comparative fracture assessment of carbon black and silica filled elastomers: Reinforcing efficacy of carbonaceous fillers in flexible composites

This study is focused on numerical investigation on fracture behaviors of carbon black (CB) and silica filled elastomeric composites. Finite element analysis (FEA) in compliance with multi-specimen method is used to calculate J-integral and geometry factor of the rubber composites up to a displacement of 20 mm for single edge notch in tension (SENT) and double edge notch in tension (DENT) specimens. An empirical relationship between crack tip opening displacement (CTOD) and crack advancement is established depending on notch to width ratio (NWR). The stress contours across the notches for SENT and DENT specimens is discussed briefly. It is found that fracture propagation resistance of CB filled elastomer is 125% more than that of silica filled elastomer. Although, Silica filled elastomer have good tensile strength and crosslink density but it fails to replace carbon black in terms of fracture properties. The critical J-integral for CB filled elastomer is 18.7% and 32.2% more than silica filled elastomer for SENT and DENT specimens respectively. The effect of specimen type on various fracture properties is also explored. The factor of safety is found to be significantly more in case of CB filled elastomers making them less vulnerable to crack propagation and catastrophic failure.     

Synthesis,characterization, and investigation of structure‐thermal cycloimidization relationship of novel poly(amide amic acid)s to poly(amide imide)s by thermogravimetric analysis

Novel diamic acids (DAAs) and poly(amide amic acid)s (PAAs) were prepared and their thermal cycloimidization to the corresponding imide form was investigated by thermogravimetric analysis under isothermal conditions at four different temperatures, that is, at 175, 200, 225, and 260 °C for 75 min. A general equation, 18NW/R_MW, where, the numerical 18 corresponds to the molecular weight of water, N is the number of water molecules, which would be eliminated per repeat unit of the PAA upon cycloimidization, W is the weight of PAA taken for TGA, and R is the molecular weight of the repeat unit of PAA, has been derived for the calculation of theoretical amount of weight loss of PAA upon complete cycloimidization. The degree of cycloimidization (DCI%) of PAAs to poly(amide imide)s (PAI) has been calculated from their isothermal TGA curves. The variation in DCI on temperature, time, and the structures of diamine and acid chloride, especially, with respect to meta‐ and para‐linkages and the presence of electron withdrawing groups has been discussed. Cycloimidization occurs at faster rate in the initial stages of about 20 min, curing and then proceeds in a gradual manner and reaches almost a plateau within an hour. The DCI was more at higher temperatures, and the final values were 22?60% at 175 °C, 34?78% at 200 °C, 50?96% at 225 °C, and 85?99% at 260 °C after 75 min of heating, depending on the nature of diamine and acid chloride in the PAA. The DCI of PAAs with meta‐linkages in either of diamine or diacid chloride was somewhat lower than those having para‐linkages. The DCI of PAAs containing electron withdrawing group like sulfone in the diamine is somewhat higher compared with those of others. The final DCI (%) values obtained from FT‐IR spectra and isothermal TGA curves were very close to each other. Further, the thermal and thermooxidative stabilities of the PAIs were discussed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2937–2947, 2007     

Method development for compositional analysis of low molecular weight poly(vinyl acetate) by matrix-assisted/laser desorption-mass spectrometry and its application to analysis of chewing gum

The influence of the sample preparation parameters (the choice of the solvent and of the matrix:analyte ratio) was investigated and optimal conditions were established for MALDI mass spectrometry analysis of the pristine low molecular weight polyvinyl acetate (PVAc). It was demonstrated that comparison of polymer’s and solvent’s Hansen solubility parameters could be used as a guide when choosing the solvent for MALDI sample preparation. The highest intensity PVAc signals were obtained when ethyl acetate was used as a solvent along with the lowest matrix–analyte ratio (2,5-dihydroxybenzoic acid was used as a matrix in all experiments). The structure of the PVAc was established with high accuracy using the matrix-assisted laser desorption/ionization-Fourier transform mass spectrometry (MALDI-FTMS) analysis. It was demonstrated that PVAc undergoes unimolecular decomposition by losing acetic acid molecules from its backbone under the conditions of FTMS measurements. Number and weight average molecular weights as well as polydispersity indices were determined with both MALDI-TOF and MALDI-FTMS methods. The sample preparation protocol developed was applied to the analysis of a chewing gum and the molecular weight and structure of the polyvinyl acetate present in the sample were established. Thus, it was shown that optimized MALDI mass spectrometry could be used successfully for characterization of polyvinyl acetate in commercially available chewing gum.