Acoustic emission waveform characterization of crack origin and mode in fractured and ASR damaged concrete

Different constituents of concrete can have cracking behavior that varies in terms of the acoustic waveform that is generated. Understanding the waveform may provide insight into the source and behavior of a crack that occurs in a cementitious composite. In this study, passive acoustic emission (AE) was used to investigate the waveform properties of the individual components of concrete (i.e., aggregate, paste, and interfacial transition zone (ITZ)). First, acoustic events produced by cracks generated using mechanical loading in a wedge splitting test were detected. It was observed that cracks that occurred through the aggregate have an AE frequency range between 300 kHz and 400 kHz, while cracks that propagated through the matrix (paste and ITZ) have a frequency range between 100 kHz and 300 kHz. Second, tests were performed using samples that were susceptible to alkali silica reaction; and AE and X-ray computed tomography were used to detect cracking. AE events with a frequency range between 300 kHz and 400 kHz were detected at early ages, suggesting the initiation of cracks within reactive aggregate. At later ages, AE events were detected with frequency ranges of 100–300 kHz, indicating crack development and propagation in the matrix.     

Accelerated thermal ageing of epoxy resin and 3-D carbon fiber/epoxy braided composites

This paper reports the accelerated thermal ageing behaviors of pure epoxy resin and 3-D carbon fiber/epoxy braided composites. Specimens have been aged in air at 90 °C, 110 °C, 120 °C, 130 °C and 180 °C. Microscopy observations and attenuated total reflectance Fourier transform infrared spectrometry analyses revealed that the epoxy resin oxidative degradation only occurred within the surface regions. The surface oxidized layer protects inner resin from further oxidation. Both the resin degradation and resin stiffening caused by post-curing effects will influence the compression behaviors. For the braided composite, the matrix ageing is the main ageing mode at temperatures lower than glass transition temperatures (T_g) of the pure epoxy resin, while the fiber/matrix interface debonding could be observed at the temperatures higher than T_g, such as the temperature of 180 °C. The combination of matrix degradation and fiber/resin interface cracking leads to the continuous reduction of compressive behaviors.     

Effect of stitch density on fatigue characteristics and damage mechanisms of stitched carbon/epoxy composites

The effect of stitch density (SD) on fatigue life, stiffness degradation and fatigue damage mechanisms in carbon/epoxy (T800SC/XNRH6813) stitched using Vectran thread is presented in this paper. Moderately stitched composite (SD = 0.028/mm²; ‘stitched 6 × 6’) and densely stitched composite (SD = 0.111/mm²; ‘stitched 3 × 3’) are tested and compared with composite without stitch thread (SD = 0.0; ‘unstitched’). The experiments show that the fatigue life of stitched 3 × 3 is moderately better than that of unstitched and stitched 6 × 6. Stitched 3 × 3 pattern is also able to postpone the stiffness degradation onset. The improvement of fatigue properties and postponement of stiffness degradation onset in stitched 3 × 3 is primarily due to an effective impediment of edge-delamination. Quantification of damage at various cycles and stress levels shows that stitch density primarily affects the growth rate of delamination.     

Strain and damage monitoring in carbon-nanotube-based composite under cyclic strain

The resistive behavior of multi-walled carbon nanotube (MWCNT)/epoxy resins, tested under mechanical cycles and different levels of applied strain, was investigated for specimens loaded in axial tension. The surface normalized resistivity is linear with the strain for volume fraction of MWCNTs between 2.96 × 10⁻⁴ and 2.97 × 10⁻³ (0.05 and 0.5% wt/wt). For values lower than 0.05% wt/wt, close to the electrical percolation threshold (EPT) a non-linear behavior was observed. The strain sensitivity, in the range between 0.67 and 4.45, may be specifically modified by controlling the nanotube loading, in fact the sensor sensitivity decreases with increasing the carbon nanotubes amount. Microscale damages resulted directly related to the resistance changes and hence easily detectable in a non-destructive way by means of electrical measurements. In the fatigue tests, the damage is expressed through the presence of a residual resistivity, which increases with the amount of plastic strain accumulated in the matrix.     

Fatigue properties and fracture analysis of a SiC fiber-reinforced titanium matrix composite

Tension–tension fatigue properties of SiC fiber reinforced Ti–6Al–4V matrix composite (SiC_f/Ti–6Al–4V) at room temperature were investigated. Fatigue tests were conducted under a load-controlled mode with a stress ratio 0.1 and a frequency 10 Hz under a maximum applied stress ranging from 600 to 1200 MPa. The relationship between the applied stress and fatigue life was determined and fracture surfaces were examined to study the fatigue damage and fracture failure mechanisms using SEM. The results show that, the fatigue life of the SiC_f/Ti–6Al–4V composite decreases substantially in proportion to the increase in maximum applied stress. Moreover, in the medium and high life range, the relationship between the maximum applied stress and cycles to failure in the semi-logarithmic system could be fitted as a linear equation: S_max/μ = 1.381 − 0.152 × lgN_f. Fractographic analysis revealed that fatigue fracture surfaces consist of a fatigued region and a fast fracture region. The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of the applied maximum stresses.     

A facile assembly of polyimide/graphene core–shell structured nanocomposites with both high electrical and thermal conductivities

Polyimide/reduced graphene oxide (PI/r-GO) core–shell structured microspheres were fabricated by in-situ reduction of graphene oxide (GO), which was coated on the surface of PI microspheres via hydrogen bonding and π–π stacking interaction. The highly ordered 3D core–shell structure of PI/r-GO microspheres with graphene shell thickness of 3 nm was well characterized by scanning electron microscope (SEM), transmission electron microscopy (TEM) and Raman spectra. The glass transition temperature (T_g) of PI/r-GO microspheres was slightly increased because of the interaction of r-GO and PI matrix while the temperature at 5% weight loss (T_5%) of PI/r-GO microspheres was decreased due to the side effect of reductant hydrazine hydrate. The PI/r-GO nanocomposites exhibited highly electrical conductivity with percolation threshold of 0.15 vol% and ultimate conductivity of 1.4 × 10⁻² S/m. Besides, the thermal conductivity of PI/r-GO nanocomposites with 2% weight content of r-GO could reach up to 0.26 W/m K.     

Electrical transport and magnetoresistance in La2 / 3Ca1 / 3MnO3/CuO composites

The electrical transport and magnetoresistance properties were experimentally studied for composites according to the nominal composition (1 − x)La_2 / 3Ca_1 / 3MnO₃/xCuO which were fabricated using a special chemical route (x is molar ratio). The composites display different transport behaviors for the range of x < 6% and > 6%. The introduction of CuO causes a large shift of the insulator–metal transition temperature (T_p) toward low temperature for x < 6%, while the T_p is almost independent on x for x > 6%. We also show that the magnetoresistance near T_p can be substantially increased through the introduction of CuO. The largest magnetoresistance with a value as high as ∼90% is obtained in the x = 20% composite for a rather small magnetic field (0.3 T). It is interesting to observe another transition in MR₀ vs. T curve at temperature range of 170∼200 K for the composites with CuO addition. Based on structural and microscopic analysis, unusual observations of transport and magnetoresistance are discussed.     

Effect of Fe and Co doping on electrical and thermal properties of La0.5Ce0.5Mn1−x(Fe,Co)xO3 manganites

The effect of Fe and Co doping on structural, electrical and thermal properties of half doped La_0.5Ce_0.5Mn_1−x(Fe, Co)_xO₃ is investigated. The structure of these crystallizes in to orthorhombically distorted perovskite structure. The electrical resistivity of La_0.5Ce_0.5MnO₃ exhibits metal-semiconductor transition (T_MS at ∼225 K). However, La_0.5Ce_0.5Mn_1−xTM_xO₃ (TM = Fe, Co; 0.0 ≤ x ≤ 0.1) manganites show semiconducting behavior. The thermopower measurements infer hole as charge carriers and electron–magnon as well spin wave fluctuation mechanism are effective at low temperature domain and SPC model fits the observed data at high temperature. The magnetic susceptibility measurement confirms a transition from paramagnetic to ferromagnetic phase. The observed peaks in the specific heat measurements, shifts to lower temperatures and becomes progressively broader with doping of transition metals on Mn-site. The thermal conductivity is measured in the temperature range of 10–350 K with a magnitude in between 10 and 80 mW/cm K.     

Electro-magnetic transport behavior of La0.7Ca0.3MnO3/SnO2 composites

The composites of (1 ? x)La_0.7Ca_0.3MnO₃ (LCMO) + xSnO₂ (x = 0.01, 0.05, 0.10, 0.30, 0.50, 0.60, 0.65 and 0.70) were synthesized by conventional solid-state reaction method. The results of X-ray diffraction (XRD) and scanning electronic microscopy (SEM) indicate that SnO₂ and LCMO coexist in the composites and SnO₂ mainly segregates at the grain boundaries of LCMO, which are in accordance with the results of the magnetic measurements. The detailed electrical characterizations for all the samples showed that a new metal–insulator (M–I) transition temperature (T_P2) appeared at a lower temperature compared with the intrinsic metal–insulator (M–I) transition temperature (T_P1) when x < 0.50 (T_P1 > T_P2). When x > 0.50, T_P1 disappeared, leaving only T_P2. The resistivity percolation threshold of the composites occurred at x = 0.60. Corresponding to the two M–I transition peaks, the curves of magnetoresistance against temperature also showed two peaks for all composites. These phenomena can be explained by the segregation of a new phase related to SnO₂ at the grain boundaries or surfaces of the LCMO grains.     

Comparative study on the behavior of woven-ply reinforced thermoplastic or thermosetting laminates under severe environmental conditions

This work aims at determining whether thermoplastic-based composites can be used in secondary aircraft structures to replace thermosetting-based composites or not. In order to answer this question, the mechanical behaviors of carbon fiber fabric reinforced thermoplastic (PPS or PEEK) and thermosetting (epoxy) laminates subjected to different stress states under severe environmental conditions (120 °C after hygrothermal aging) have been compared. In addition to usual mechanical tests (tensile, open hole tensile), single-bolt double lap joint and single-bolt single lap joint tests were also performed. Severe conditions help enhance the ductile behavior of the epoxy matrix, but degrade the fiber/matrix interface, resulting in lower stiffness and strength of laminates with a quasi-isotropic lay-up. In thermoplastic-based laminates, the degree of retention of mechanical properties is quite high even for PPS-based laminates when T > T_g. In laminates with a [45]₇ lay-up, severe conditions adversely affect the mechanical properties of the three composite systems. However, the combination of matrix ductile behavior, and the strain gradient near the hole, lead to an extensive plastic deformation along the ±45° oriented fibers bundles in notched A-P laminates. It results in decreasing significantly the hole-sensitivity of C/PPS and C/Epoxy under severe conditions. In bolted joints, a severe environment has a limited impact on the bearing strength of epoxy-based laminates. In the case of thermoplastic-based laminates, it increases the strength of double lap joints, but is detrimental to the strength of single lap joints.     

Selective detection of toxic Pb(II) ions based on wet-chemically prepared nanosheets integrated CuO–ZnO nanocomposites

The present study describes a selective detection methodology for hazardous metal ions based on low-dimensional nanosheets (NSs) integrated CuO–ZnO composite materials. A large-scale synthesis of NSs by wet-chemical process is performed using alkaline reducing agents at higher pH medium. The prepared NSs are characterized in terms of their morphological, structural and optical properties, and efficiently applied for the toxic metal ions detection. The detailed structural, compositional, and optical characterization of NSs are evaluated by XRD, FT-IR, XPS, EDS, and UV–vis spectroscopy, which confirmed that the obtained NSs are well-crystalline CuO–ZnO and possessed good optical properties. The CuO–ZnO NS morphology is investigated by FE-SEM, which confirmed that the NS possesses microstructure shape and growth in large-quantity. The analytical application of CuO–ZnO NSs is studied for a selective extraction of toxic lead-divalent [Pb(II)] ions prior to its determination by inductively coupled plasma-optical emission spectrometry (ICP-OES). The selectivity of doped NSs phase is investigated for eight different metal ions, including Cd(II), Cu(II), Hg(II), La(III), Mn(II), Pb(II), Pd(II), and Y(III) under similar experimental conditions. From the selectivity study, it is confirmed that the composite CuO–ZnO NS phase is the most toward Pb(II) ions according to the magnitude of distribution coefficient (K_d) values, such as Pb(II) > Y(III) > Cd(II) > La(III) > Hg(II) > Cu(II) > Mn(II) > Pd(II). The uptake capacity for Pb(II) is experimentally calculated to be ∼82.66 mg g⁻¹.     

Crack growth behavior of 7075-T6 under biaxial tension–tension fatigue

Crack growth behavior of aluminum alloy 7075-T6 was investigated under in-plane biaxial tension–tension fatigue with stress ratio of 0.5. Two biaxiality ratios, λ (=1 and 1.5) were used. Cruciform specimens with a center hole, having a notch at 45° to the specimen’s arms, were tested in a biaxial fatigue test machine. Crack initiated and propagated coplanar with the notch for λ = 1 in L–T orientation, while it was non-coplanar for λ = 1.5 between L–T and T–L orientations. Uniaxial fatigue crack growth tests in L–T and T–L orientations were also conducted. Crack growth rate in region II was practically the same for biaxial fatigue with λ = 1 in L–T orientation and for the uniaxial fatigue in L–T or T–L orientations, while it was faster for biaxial fatigue with λ = 1.5 at a given crack driving force. However, fatigue damage mechanisms were quite different in each case. In region I, crack driving force at a given crack growth rate was smallest for biaxial fatigue with λ = 1.5 and for uniaxial fatigue in T–L orientation, followed by biaxial fatigue with λ = 1 and uniaxial fatigue in L–T orientation in ascending order at a given crack growth rate.     

Crystal structure and magnetic properties of Pr- and Ti-substituted La2RuO5

Polycrystalline samples of Pr- and Ti-substituted La₂RuO₅ were prepared applying a soft-chemistry route based on the thermal decomposition of citrate-stabilized precursors. The simultaneous substitution on the La-sites by Pr and on the Ru-sites by Ti results in samples of the composition La_2−xPr_xRu_1−yTi_yO₅ with 0 ≤ x ≤ 0.75 and 0 ≤ y ≤ 0.4. The crystal structures of these compounds were analyzed by Rietveld refinement of powder X-ray diffraction patterns. For pure La₂RuO₅ a structural transition from a monoclinic room-temperature modification to a triclinic low-temperature structure was found at 161 K. This structural change is linked to a low-temperature long-range ordered spin-singlet ground state formed by Ru⁴⁺ spin-moments. Both the structural transition and the formation of the singlet ground state become progressively suppressed with higher Ti contents, while the Pr substitution has only a minor influence on the dimerization. The behavior of the Curie–Weiss temperatures can be explained assuming two almost independent magnetic sublattices corresponding to the ruthenium and the rare-earth ions, respectively. For all investigated properties, i.e. crystal structure, magnetic susceptibilities, and dimerization temperature T_d, a completely additive behavior of the effects of Pr-substitution and Ti-substitution is observed.     

Influence of dwell times on the thermomechanical fatigue behavior of a directionally solidified Ni-base superalloy

In-phase (IP) and out-of-phase (OP) thermomechanical fatigue tests with T = 100–750 °C and optional dwells of 20 min at 750 °C were carried out on directionally solidified Ni-base Alloy 247 LC DS. Introducing dwells reduced the lifetime for both phase angles to about one sixth. Specific damage mechanisms were internal carbide and carbide–matrix interface cracks in IP tests and crack propagation along {1 1 1}-microtwin planes in OP tests. Introducing dwells intensified both effects, thus contributing to the lifetime reduction. During dwells, the gauge length may exhibit transversal creep because of extensometer forces distorting the strain measurement.     

Effect of CuO nanostructure morphology on the mechanical properties of CuO/woven carbon fiber/vinyl ester composites

The effect of CuO nanostructure morphology on the mechanical properties of CuO/woven carbon fiber (WCF)/vinyl ester composites was investigated. The growth of CuO nanostructures embedded in the surface of woven carbon fibers (WCFs) was carried out by a two-step seed-mediated hydrothermal method; i.e., seeding and growth treatments with controlled chemical precursors. CuO nanostructural morphologies ranging from petal-like to cuboid-like nanorods (NRs) were obtained by controlling the thermal growth temperature in the hydrothermal process over a growth time of 12 h. The Cu²⁺/O⁻ ratio and the rate of reaction greatly influenced the formation of CuO nanostructures as self-assembled shapes on the crystal planes in the order L[0 1 0] > L[1 0 0] > L[0 0 1]. Morphological variations were analyzed by scanning electron microscopy, X-ray diffraction, and Brunauer–Emmett–Teller surface area analysis. The impact behavior, in-plane shear strength, and tensile properties of the CuO/WCF/vinyl ester composites were analyzed for different CuO NR morphologies at various growth temperatures and molar concentrations. The CuO/WCF/vinyl ester composites had improved impact energy absorption and mechanical properties because the higher specific surface area of CuO NRs grown as secondary reinforced nanomaterials on WCFs enhanced load transfer and load-bearing capacity.     

Carbon/epoxy composite delamination analysis by acoustic emission method under various environmental conditions

New multifunctional materials for aerospace industry with exceptional properties must be tested under various environmental conditions to find out possible scatter factors for evaluated properties. Delamination is a typical damage mode observed for laminated composites. Therefore, reliable information regarding the delamination growth behaviour is needed for all operational environments of an aircraft operated at cryogenic and elevated temperatures. In this paper, delamination crack growth monitoring in a climatic chamber on double-cantilever beam (DCB) specimens using optical devices and acoustic emission (AE) techniques is described. A relationship between cumulative AE energy, events localization, clusters, and crack growth in a plain-weave carbon fibre–reinforced epoxy is investigated under constant displacement rate loading at + 80 °C, and − 55 °C. Test results are evaluated for specimens with multi-walled carbon nanotubes (MWCNT) in the microstructure and for a reference material. The mechanical properties during delamination are represented by fracture toughness G_IC, and they are also correlated with the AE data. The elevated test temperature caused a decreased rate of released AE energy. The crack growth in material with more significant fibre breakage caused increase of the AE release rate.     

Mechanical behaviour of jute cloth/wool felts hybrid laminates

This experimental work is aimed at the characterization of new fibre reinforced composites based on epoxy resin with both protein (wool) and lignocellulosic (jute) natural fibres. Wool-based and hybrid (wool/jute) composites with two different stacking sequences (intercalated and sandwich) were developed. Their microstructure has been investigated through optical and scanning electron microscopy, whereas their quasi-static mechanical behaviour has been evaluated in tension and bending. In addition, the impact behaviour under low-velocity impact at three different impact energies, namely 6 J, 8 J and 9 J has been addressed. The tensile and flexural tests have been monitored using acoustic emission (AE) in order to elicit further information about failure mechanisms. AE monitoring showed that development of damage was due to nucleation of matrix microcracks and subsequent debonding and pull-out phenomena in wool fibre composites and that only in hybrid composites a sufficient stress transfer across the jute fibre/matrix interface was achieved. The results confirmed the positive role of hybridization with jute fibres in enhancing both the tensile and flexural behaviour of wool-based composites, though highlighting the need for an improved adhesion between wool fibres and epoxy matrix.     

Post-impact damage characterization of hybrid configurations of jute/glass polyester laminates using acoustic emission and IR thermography

In this work, residual post-impact properties of two configurations of E-glass/jute hybrid laminates are characterized, both manufactured using a total fibre volume of 50 ± 2% (14 glass fibre layers + 4 jute fibre layers). T-laminates included a core obtained by multiple layers of jute between two E-glass fibre reinforced skins, whilst in Q-laminates single layers of jute fibres were intercalated at different levels between E-glass fibre reinforced layers. All laminates were impacted at five levels of energy, from 5 to 15 J, and then subjected to post-impact flexural tests.The results suggest that T hybrids perform better at low impact energies (up to 10 J), which do not damage significantly the laminate core. In contrast, Q hybrids are better suited to withstand extensive damage produced by higher impact energies (12.5 and 15 J), in that they allow a more effective redistribution of impact damage in the structure. This was confirmed by acoustic emission (AE) monitoring during flexural loading, which offered indications on the maximum stress laminates can undergo after impact damage. Pulse IR thermography yielded information on their mode of failure by visualizing impact-damaged areas.     

Electrical properties of nanostructures (CoFeZr)x + (Al2O3)1−x with use of alternating current

CoFeZr–Al₂O₃ nanocomposite films of 3–5 μm thickness, containing metallic alloy nanoparticles embedded into the dielectric alumina matrix, have been deposited on a glass ceramic substrate using magnetron sputtering of composite target in Ar gas ambient. Measurements of AC conductance and lagging have been performed within the frequency range of 50 Hz–1 MHz at the temperatures from 79 K to 373 K in the initial (as-deposited) samples as well as directly after their isochronous (15 min) annealings within the temperature range from 398 K to 648 K with 25 K step.The observed variations of real part AC electrical conductivity with temperature and frequency σ_real(T, f) in the as-deposited films display transition from dielectric to metallic behaviour when crossing the percolation threshold x_C in the studied nanocomposites. After annealing of the samples below the x_C the σ_real(T, f) progress follows the hopping law of electron conductivity with sigmoidal frequency dependence. The samples being far beyond the percolation threshold revealed transition from metallic to activational σ_real(T) law after high-temperature annealing attributed to the internal oxidation of metallic nanoparticle by excess of oxygen presented in the as-deposited samples.     

Structural,optical, thermal,photoconductivity, laser damage threshold and fluorescence analysis of an organic material: β-P-amino benzoic acid single crystal

β-P-amino benzoic acid, an organic single crystal was grown by slow evaporation technique. Single crystal X-ray diffraction studies show that the grown crystal has β-polymorph of P-amino benzoic acid [β-PABA] form and the lattice parameters are a = 6.30 Å, b = 8.61 Å, c = 12.43 Å α = γ = 90° and β = 100.20°. FTIR analysis confirms that bands at 1588 cm⁻¹, 1415 cm⁻¹ are assigned to ring skeletal vibrations of title compound. The molecular structure of the grown crystal has been identified by Nuclear Magnetic Resonance spectral study. The optical absorbance spectrum from 200 to 1100 nm shows that there is an edge absorbance in UV region. Optical band gap of the crystal has been assessed from the absorbance spectrum. The thermal properties of crystals were evaluated from TG-DTA analysis, it exhibits that there is no weight loss up to 187 °C. Laser damage threshold indicates that the grown crystal has no surface damage up to 35 mJ. Photoconductivity and fluorescence spectral experiments are also carried out and the results are discussed.