Investigation of the transverse properties of a unidirectional carbon/epoxy laminate: Part 1—Matrix properties

The bulk properties of a commercially available high-performance epoxy resin used for continuous-fibre composites have been studied. Fracture toughness and stress/strain properties in tension and compression have been measured. The distribution of tensile strengths within the resin has been studied by fabricating novel sandwich laminates consisting of a bulk resin core with two outer (0°) glass-fibre-reinforced plastic (GRP) laminates. Sandwich laminates with core thicknesses in the range of 0.6 to 2.4 mm were tested in tension and the individual cracking events in the core were used to construct Weibull strength distribution plots. Weak-link scaling has been applied to correlate the laminate data with the results from the conventional coupon tensile test of the resin. Statistical extrapolation of the data indicates a significant size-effect. Effects of filtration of solid body impurities and inclusions from the resin have also been studied in terms of the resulting modification to the strength distribution.     

Effect of post-curing on properties of carbon fibre–epoxy composites

Abstract

The effect of post-curing on the moisture absorption characteristics of Fibredux 914/T300 carbon fibre–epoxy composites, and hence on their thermomechanical behaviour, has been examined. Laminates 1 mm thick were post-cured at 190 or 210°C for 4 or 10 h. The various cross-link densities thus established had almost no effect on the moisture absorption behaviour. Interlaminar shear strength and torsion pendulum tests gave similar results, in that the cross-link density had almost no influence on the dynamic shear modulus or the mechanical dissipation factor. From these findings, environmental degradation of the composite is shown to depend on the content of absorbed water. The behaviour of the composite in hot, humid conditions therefore cannot be improved by post-curing treatment.

MST/400     

Effect of strain rate on the fracture behaviour of epoxy–graphene nanocomposite

In this study, epoxy-based nanocomposite was fabricated by the addition of graphene nanosheet via a solution casting method. To investigate the effect of strain rate on tensile properties of epoxy, tensile tests were done on standard samples at different strain rates (0.05–1 min^?1). The role of strain rate and presence of graphene on fracture behaviour of epoxy were also studied by investigation of the fracture surfaces of some samples by scanning electron microscopy (SEM). Finally, Eyring’s model was performed to clarify the role of strain rate on activation volume and activation enthalpy of epoxy. The results of tensile tests showed a maximum strength of epoxy–graphene nanocomposite at the graphene wt% of 0.1%. Tensile strength of epoxy obviously improved with increasing strain rate, but tensile strength of epoxy/graphene nanocomposite sample was less sensitive. Fracture micrographs showed that the mirror zone of the fracture surface of epoxy diminished by increasing strain rate or addition of graphene; and final fracture zone also became rougher. Finally, by investigation of the activation enthalpies, it was showed that much higher enthalpy was needed to fracture the nanocomposite sample, as the activation enthalpy changed from 41.54 for neat epoxy to 67.34 kJ mol^?1 for EP–0.1% GNS sample.     

Effects of strain rate and temperature on the stress–strain response of solder alloys

To ensure reliable design of soldered interconnections as electronic devices become smaller, requires greater knowledge and understanding of the relevant mechanical behavior of solder alloys than are presently available. The present paper reports the findings of an investigation into the monotonic tensile properties of bulk samples of three solder alloys; a lead–tin eutectic and two lead-free solders (tin–3.5 copper and a tin–3.5 silver alloy). Temperatures between–10 and 75°C and strain rates between 10^–1 and 10^–3 s^–1 have been studied. Both temperature and strain rate may have a substantial effect on strength, producing changes well in excess of 100%. Strength is reduced by lowering strain rate and increasing temperature, and Sn–37 Pb is usually most sensitive to the latter. Expressions for strain and strain rate hardening have been developed. The Sn–0.5 Cu alloy is usually the weakest and most ductile. Sn–37 Pb is strongest at room temperature but with increasing temperature and lower strain rates it becomes inferior to Sn–3.5 Ag. Ductility changes with temperature and strain rate for all three alloys are generally small with inconsistent trends. The role of such data in stress analysis and modeling is considered and the paramount importance of employing data for conditions appropriate to service, is emphasized.     

Measurement of strain distribution by the moiré fringe multiplication method at a tip of propagating fatigue crack

The moiré fringe multiplication method which used interference of the +1st and -1st order diffracted beams from a 1000 lines/mm phase-type grating on the specimen surface was applied to the measurement of strain distribution at the tip of a propagating fatigue crack in a steel plate having a central crack. The sensitivity of the measurement was equivalent to the sensitivity obtained from a 2000 lines/mm grating on the specimen when the conventional moiré fringe method is used. The gage length was of the order of 10m. The result of the measurement of strain distribution was applied to the estimation of the fatigue crack propagation rate.

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Résumé La méthode de multiplication des franges de Moiré qui utilise les interférences d'ordre +1 et -1 de rayons lumineux diffractés par une grille de 1000 lignes/millimètre, du type à phase sur la surface de l'éprouvette, a été appliqué à la mesure de la distribution des déformations à l'extrémité d'une fissure de fatigue en cours de propagation dans une tôle d'acier présentant une fissure centrale.La sensibilité de la mesure était équivalente à la sensibilité obtenue par une grille de 2000 lignes/millimètre appliquée sur l'éprouvette et utilisant la méthode de franges de Moiré conventionnelle. La longueur de référence était de l'ordre de 10 microns. Le résultat des mesures de distribution de déformation a été appliqué à l'estimation de la vitesse de propagation d'une fissure de fatigue.

     

Fatigue residual strength of circular laminate graphite–epoxy composite plates damaged by transverse load

The research dealt with the relation between damage and tension–tension fatigue residual strength (FRS) in a quasi-isotropic carbon fibre reinforced epoxy resin laminate. The work was organized in two phases: during the first one, composite laminates were damaged by means of an out-of-plane quasi-static load that was supposed to simulate a low velocity impact; in the second phase, fatigue tests were performed on damaged and undamaged specimens obtained from the original composite laminates. During the quasi-static transverse loading phase, damage progression was monitored by means of acoustic emission (AE) technique. The measurement of the strain energy accumulated in the specimens and of the acoustic energy released by fracture events made it possible to estimate the amount of induced damage and evaluate the quasi-static residual tensile strength of the specimens. A probabilistic failure analysis of the fatigue data, reduced by the relative residual strength values, made it possible to relate the FRS of damaged specimens with the fatigue strength of undamaged ones.     

Measurement of stress in aluminum film coated on a flexible substrate by the shadow moiré method

This investigation proposes the use of the shadow moiré method (SMM) to measure stress in a thin film that is coated on a flexible substrate. The technique defines the profile of the sample by contour lines without the application of an external force, and the radius of curvature is calculated from these contour lines. The SMM is insensitive to environmental noise and has the same advantages as the interference method, such as being nondestructive and easy to use. For Al film with a thickness of 120 nm coated on a polyimide substrate by a DC magnetron sputtering system (800 W, room temperature), the stress is 0.45 +/- 0.042 GPa.     

Distributed load sensor by use of a chirped moiré fiber Bragg grating

A chirped moiré fiber Bragg grating has been demonstrated to be capable of measuring the magnitude, position, and footprint of a transverse load. The device provides an average spatial resolution of 164 microm and has a load accuracy of 0.15 N/mm, or 50 microepsilon.     

Effect of amino functionalized MWCNT on the crosslink density,fracture toughness of epoxy and mechanical properties of carbon–epoxy composites

Amino functionalized multiwalled carbon nanotubes (A-MWCNTs) reinforced two phase (A-MWNT–epoxy) and three phase (A-MWCNTs–carbon fiber–epoxy) nanocomposites were fabricated with 0.25 wt%, 0.5 wt% and 1.0 wt% loadings of A-MWCNTs. It is observed that, A-MWCNTs can improve the crosslink density of epoxy significantly. Fracture toughness of epoxy matrix is found to increase up to an optimum crosslink density improvement, indicating the role of crosslink density in imparting toughness to epoxy apart from the crack deflection contributions of A-MWCNTs. In addition to that, this study infers that, tensile, flexural properties of the three phase composites are strongly influenced by the fracture toughness changes of the matrices. This study, thus proposes additional mechanisms of toughness enhancements for two phase and mechanical properties enhancements for three phase composites imparted by A-MWCNTs.     

The influence of multiscale fillers on the rheological and mechanical properties of carbon-nanotube–silica-reinforced epoxy composite

Multiscale fillers were fabricated through synthesis of carbon nanotubes (CNTs) on silica microparticles by the use of chemical vapor deposition. Three types of catalyst precursors with different concentrations and reaction times were investigated to find the optimal conditions for CNT synthesis. The produced multiscale fillers of CNT–silica were incorporated within epoxy resin to fabricate a multiscale composite. Rheological analysis and tensile and impact tests were performed to study the effect of fillers on the structural properties of composites. The rheological results demonstrated a similar viscous behavior between CNT–silica suspensions and epoxy, which implies that there was no critical increase of viscosity. Significant improvements in the elastic modulus and tensile and impact strength were achieved for epoxy matrix filled with the optimal fraction of multiscale fillers. The reinforcing efficiency of multiscale fillers was evaluated by comparing the results of micromechanical models with experimental data.     

Impact response and damage tolerance characteristics of glass–carbon/epoxy hybrid composite plates

Impact behaviour and post impact compressive characteristics of glass–carbon/epoxy hybrid composites with alternate stacking sequences have been investigated. Plain weave E-glass and twill weave T-300 carbon have been used as reinforcing materials. For comparison, laminates containing only-carbon and only-glass reinforcements have also been studied. Experimental studies have been carried out on instrumented drop weight impact test apparatus. Post impact compressive strength has been obtained using NASA 1142 test fixture. It is observed that hybrid composites are less notch sensitive compared to only-carbon or only-glass composites. Further, carbon-outside/glass-inside clustered hybrid configuration gives lower notch sensitivity compared to the other hybrid configurations.     

Fretting fatigue studies on carbon fibre/epoxy resin laminates: I—design of a fretting fatigue test apparatus

A freeting fatigue test device was designed and built for special fretting fatigue studies with composite materials. The device is attached to the specimen grips of a servohydraulic testing machine and allows symmetrical fretting loading of the specimen surface by two freeting pads while the specimen is fatigued. Positioning of the fretting pads can be either on the edges of laminated samples or on their flat surfaces. Different fretting slip amplitudes can be specified at given fatigue conditions, and the fretting load can be exactly adjusted and controlled.     

The study of inspection on SiC coated carbon–carbon composite with subsurface defects by lock-in thermography

An investigation into the effect of size on the quantitative estimation of defect depth in a SiC coated carbon–carbon (C/C) composite has been undertaken by lock-in thermography. A dedicated 3-D thermal modeling has been introduced, and an efficient numerical algorithm based on finite-difference splitting method in time domain (FDSM-TD) is applied to solve the thermal model. The heat transfer partial differential equation (PDE) and mathematic morphological algorithms are used to filter the phase angle data noise. The diameter of a defect had an appreciable effect on the observed phase angle which consequently has significant implications with regard to estimating the defect depth. Phase angle contrast measurements for a range of defects in a 6.0 mm SiC coated C/C composite specimen indicate that an optimal excitation frequency of 0.525 Hz is available for defect detection. Results obtained with an excitation frequency of 0.525 Hz are used to discuss the limitations of determining the defect size and depth.     

Wedge-plate shearing interferometers for collimation testing: use of a moiré technique

Various optical arrangements of a double-wedge-plate shearing interferometer are presented for checking laser beam collimation. The use of moiré fringes is found to be advantageous for setting the shear fringes parallel to the direction of shear in order to obtain a well-collimated laser beam. The experimental procedure and various details of the interferometer are discussed. A brief summary of a few methods for collimation testing that use a wedge plate is also given. The accuracies achievable with shearing interferometers that use a parallel plate, a wedge plate of small angle, a double wedge having a large wedge angle, a wedge plate of large angle along with two flat mirrors, and a wedge plate having a large angle are compared and summarized.     

Effect of in situ synthesized TiB whisker on microstructure and mechanical properties of carbon–carbon composite and TiBw/Ti–6Al–4V composite joint

Carbon–carbon composite (C–C composite) and TiB whiskers reinforced Ti–6Al–4V composite (TiB_w/Ti–6Al–4V composite) were brazed by Cu–Ni + TiB₂ composite filler. TiB₂ powders have reacted with Ti which diffused from TiB_w/Ti–6Al–4V composite, leading to formation of TiB whiskers in the brazing layer. The effects of TiB₂ addition, brazing temperature, and holding time on microstructure and shear strength of the brazed joints were investigated. The results indicate that in situ synthesized TiB whiskers uniformly distributed in the joints, which not only provided reinforcing effects, but also lowered residual thermal stress of the joints. As for each brazing temperature or holding time, the joint shear strength brazed with Cu–Ni alloy was lower than that of the joints brazed with Cu–Ni + TiB₂ alloy powder. The maximum shear strengths of the joints brazed with Cu–Ni + TiB₂ alloy powder was 18.5 MPa with the brazing temperature of 1223 K for 10 min, which was 56% higher than that of the joints brazed with Cu–Ni alloy powder.     

Preparation and properties of TaC/C/TaC∼TaC composite micro-tubes by vapor phase tantalizing of the regular carbon micro-coils/micro-tubes

TaC/C/TaCTaC composite micro-tubes were prepared by the vapor phase tantalizing of the regular carbon micro-coils/micro-tubes, and the preparation conditions and some properties were examined. The carbon micro-coils with a tube-like morphology were tantalized from the surface to the core of the carbon fibers with full preservation of the tube-like morphology to form TaC/C/TaCTaC composite micro-tubes. The bulk electrical resistivity and specific surface area of the TaC/C/TaCTaC composite micro-tubes were 4 × 10^–3 to 5 × 10^–4 ·m and 5 × 10³ to 2 × 10⁴ m²/kg, respectively, depending on the tantalized ratio and the bulk density.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 1—Mechanical behaviour

Abstract

A split Hopkinson bar is used to investigate the effects of prestrain and strain rate on the dynamic mechanical behaviour of 304L stainless steel, and these results are correlated with microstructure and fracture characteristics. Annealed 304L stainless steel is prestrained to strains of 0·15, 0·3, and 0·5, then machined as cylindrical compression specimens. Dynamic mechanical tests are performed at strain rates ranging from 10² to 5 × 10³ s^-1 at room temperature, with true stains varying from 0·1 to 0·3. It was found that 304L stainless steel is sensitive to applied prestrain and strain rate, with flow stress increasing with increasing prestrain and strain rate. Work hardening rate, strain rate sensitivity, and activation volume depend strongly on the variation of prestrain, strain, and strain rate. At larger prestrain and higher strain rate, work hardening rate decreases rapidly owing to greater heat deformation enhancement of plastic flow instability at dynamic loading. Strain rate sensitivity increases with increasing prestrain and work hardening stress (σ-σ_y). However, activation volume exhibits the reverse tendency. Catastrophic fracture is found only for 0·5 prestrain, 0·3 strain, and strain rate of 4·8 × 10³ s^-1. Large prestrain increases the resistance to plastic flow but decreases fracture elongation. Optical microscopy and SEM fracture feature observations reveal adiabatic shear band formation is the dominant fracture mechanism. Adiabatic shear band void and crack formation is along the direction of maximum shear stress and induces specimen fracture.     

Effects of prestrain and strain rate on dynamic deformation characteristics of 304L stainless steel: Part 2—Microstructural study

Abstract

The morphologies and characteristics of microstructure, including dislocations, mechanical twins and α' martensite, in 304L stainless steel deformed under various strain, strain rate range from 10² to 5 × 10³ s^-1 for different prestrain levels at room temperature were examined by a split Hopkinson bar and TEM. The evolution of microstructure correlated with dynamic mechanical behaviour are presented and discussed in terms of prestrain and applied strain rate. The results show that characteristics of dislocations, mechanical twins and α' martensite varied with prestrains, strains and strain rates. They dominate the strengthening effects on the 304L stainless steel. Dislocation cell structures can be observed in all tested specimens. At larger prestrain under dynamic loading, the formation of elongated dislocation cells becomes evident. The presence of elongated dislocation cells leads to different work hardening behaviour. Twinning occurred at all testing conditions except for the 0·15 prestrain specimen deformed at 0·1 strain and 8 × 10² s^-1 strain rate. The formations of α' martensites were found to be confined to the microshear bands and were barriers of dislocation movement. As the heavy loading is imposed, irregular and blocky α' martensites were observed. Quantitative measurement revealed that dislocation and twin density, as well as the volume fraction of α' martensite increase with the prestrain, strain and applied strain rate, but a decay of twin density occurred as the prestrain of 0·5 is applied. These microstructrual changes can be related to the different work hardening stress (σσ_y and strengthening nature. The observed strengthening effect resulted from the dislocation multiplication, twin formation and α' martensite seems to reflect an enhancement of hardness. However, the increased hardness is less sensitive to the twin formation.     

The effect of carbon nanofibres on self-healing epoxy/poly(ε-caprolactone) blends

The effects of carbon nanofibres (CNFs) on the mechanical performance and healing efficiency of self-healing epoxy/poly(ε-caprolactone) (PCL) blends were examined. Through a simple polymer blending process, phase-separated epoxy/PCL blends were prepared, which showed self-healing capability upon thermal activation. The introduction of CNFs into a co-continuous phase-structured epoxy/PCL system, at the content of as low as 0.2 wt.%, has been found to yield combinational improvements in the flexural strength, tensile strength, toughness and hardness with no adverse effect on the self-healing performance. Significantly enhanced mechanical performance by low content of CNFs enables the development of epoxies and advanced polymer composites with longer service life and less maintenance.     

Parameter design of a parallax barrier based on the color moiré patterns in autostereoscopic display

We propose a parameter design of the parallax barrier (PB) based on the color moiré patterns in autostereoscopic displays. First, the display device and the PB are approximated as two corresponding binary gratings. In order to obtain different corresponding predominant Fourier low-frequency terms, the superposition of the equivalent grating for the display device and the special radial grating is analyzed, referring to the indicial equation method and Fourier theory. Moreover, the two transition regions are considered as the regions where moiré patterns vary gently. Finally, the appropriate parameter of the PB can be obtained. The validity of the proposed design is verified in the experiment.