Development of a universal accelerated test for alkali-silica and alkali-carbonate reactivity of concrete aggregates

A universal accelerated test for both alkali-silica and alkali-carbonate reactivity was proposed based on extensive comparative studies on existed Accelerated Mortar Bar Test (AMBT), (e.g., ASTM C1260, CSA A23. 2–25A, RILEM TC191-ARP-AAR02) and Chinese accelerated procedures. A single size fraction of 2.5–5.0 mm aggregate particles is used in the test instead of five-graded requirements in the AMBT, and 0.15–0.80 mm fine particles for ASR, 5–10 mm particles for ACR in existed Chinese accelerated tests. Three short-fat bars, 40 × 40 × 160 mm, made at fixed cement-aggregate ratio of 1:1, and water-cement ratio of 0.33 are used and the length change of the bars is monitored till 28 days in 1 M NaOH solution at 80°C after being soaked in 80°C water for 24 h. Over 40 kinds of aggregates from various origins, which include both ASR and ACR aggregates and show a broad range of reactivity levels in the concrete prism test (CPT), were used to evaluate the reliability of the new test in this study. Experimental results indicate that, for ASR aggregates, the new test gives a better indication than the AMBT does of both the reactive/nonreactive characteristic and reactive levels of almost all tested aggregates based on an acceptance criteria of 0.093% at 14 days, although some very highly reactive aggregates show low expansions relative to the CPT. The “abnormal” low expansion of some highly reactive aggregate in the test is mainly due to the rapid formation and loss of fair amount of low viscosity ASR product into the soaking alkali solution. The results on some typical ACR aggregates usually undetected by the AMBT show that the new test gives the same outcome as using 5–10 mm particles in the Chinese Accelerated Concrete Microbar Test for ACR aggregates and is in agreement with the CPT, which suggests that it has good potentials to be used for ACR aggregate when an expansion criteria of 0.1% after 28 days is used.     

Impact behaviour of preloaded glass/polyester woven plates

In this work, the effect of a biaxial preload in the behaviour of glass/polyester woven-laminate plates subjected to high-velocity transversal impact was studied. For this, an analytic model based on energy considerations that include the presence of an in-plane preload was used. The results of the analytic model for the biaxial preload state were compared with those found for a non-preload plate, the difference between them being minimal for the pre-stressed level reached in the tests (31% of the static UTS). Therefore, numerical simulations were made in order to study the effect of the preload in greater detail; furthermore, experimental tests were conducted, validating the analytic and numerical model. In general, the two methods revealed minimal differences between the values of the ballistic limit and those of the residual velocity.     

Dielectric behaviour of glasses and glass ceramics in the system BaO-PbO-TiO2-B2O3-SiO2

Glasses with varying molar ratios of PbO/BaO in the system BaO-PbO-TiO₂-B₂O₃-SiO₂ were prepared keeping (BaO + PbO)/TiO₂ ratio equal to one. The glasses were ceramized by two-stage heat treatment. X-ray diffraction indicates that PbTiO₃ crystallizes in lead-rich glasses while BaTiO₃ precipitates in barium-rich compositions. Solid solution (Ba, Pb)TiO₃ does not seem to crystallize over the entire range of compositions. Simultaneous presence of PbO and BaO in the initial glass composition reduces the yield of ferroelectric phase. Dielectric properties have been interpreted in terms of microstructural features.     

Anisotropic behaviour of the glass transition temperature in thin films of Langmuir-Blodgett deposited side chain polymers

Langmuir-Blodgett (LB) films of side chain polyamic acids were formed, deposited onto optical waveguides and imidized via heating to the corresponding polyimides on the optical waveguides. The LB film formation of the side chain polyamic acids, the imidization reaction and the glass transition behaviour of the polyimide thin films were investigated. Polarisation dependent waveguide mode spectroscopy was applied therefore. It was found that the glass transition temperature (Tg) of bulk and LB thin film samples of these polymers are identical and only depend on the spacer length between the side chain and backbone, not on film thickness. Our investigation shows that in the case of the ultrathin film geometry, the glass transition temperature exhibits an anisotropic behaviour. We find the classical kink in the temperature scan (a change in slope of the free volume vs. temperature plot) due to the thermal expansion change at Tg, in p-polarisation only, which is perpendicular to the main chain orientation. In the s-polarisation measurements (parallel to the main chains) no kink was found, only an offset in the vicinity of Tg. We attribute the anisotropic thermal behaviour to a transition, the freezing of segmental motion, in the side chains only.     

Characterization of the dynamic failure behaviour of a glass-fiber/vinyl-ester at different temperatures by means of instrumented Charpy impact testing

Instrumented Charpy impact testing is used to investigate the strength and failure properties of a glass-fiber/vinyl-ester composite. The test technique, originally developed for testing of steel specimens, is presented in its basic aspects; reported are the conventional procedures for determining load, displacement and energy absorption that a specimen experiences, over the entire phase of loading and subsequent failure of the specimen. Techniques are described for generating data of sufficient accuracy when applying the test to composites. In particular, the necessity of utilizing measurement chains of sufficiently high frequency response and striker tups of sufficiently high sensitivity is emphasized. Tests are performed with glass-fiber/vinyl-ester specimens, provided with notches oriented in two different directions with respect to the plies of woven glass fiber rovings. Two different types of failure result: fiber breakage ahead of the notch due to tensile stresses, and delaminations of the interface planes between the plies of woven glass fiber rovings due to shear stresses. Specifically, energies absorbed by the specimen over the entire failure process and values of maximum load occurring during the impact process are measured over a large range of temperatures. The data are correlated with the observed failure phenomena. The high level of information obtained in characterizing the failure behaviour by means of a test which requires limited technical effort proves the instrumented Charpy impact test to be a simple but effective tool for quantifying the quality of a composite in practical applications, as e.g. in surveillance programs for controlling processes such as manufacturing or aging of the material.     

Neural network approach for estimating the residual tensile strength after drilling in uni-directional glass fiber reinforced plastic laminates

The drilling of fiber reinforced plastics (FRP) often results in damage around the drilled hole. The drilling induced damage often serves to impair the long-term performance of the composite products with drilled holes. The present research investigation focuses on developing a predictive model for the residual tensile strength of uni-directional glass fiber reinforced plastic (UD-GFRP) laminates with drilled hole which has not been developed worldwide till now. Artificial neural network (ANN) predictive approach has been used. The drill point geometry, the feed rate and the spindle speed have been used as the input variables and the residual tensile strength as the output. The results of the predictive model are in close agreement with the training and the testing data.     

Controlled oxidative synthesis of Bi nanoparticles and emission centers in bismuth glass nanocomposites for photonic application

Here we demonstrate an oxidative process to control metallic bismuth (Bi⁰) nanoparticles (NPs) creation in bismuth glass nanocomposites by using K₂S₂O₈ as oxidant and enhanced transparency of bismuth glasses. Formation of Bi⁰ NPs has been monitored by their distinct surface plasmon resonance (SPR) band at 460 nm in the UV-visible absorption spectra. It is further confirmed by the transmission electron microscopy (TEM) images which disclose the formation of spherical Bi⁰ NPs whereas the selected area electron diffraction (SAED) pattern reveals their crystalline rhombohedral phase. These glasses are found to exhibit visible and near infrared (NIR) luminescence bands at 630 and 843 nm respectively on excitation at 460 nm of the SPR band. It is realized that the luminescence center of bismuth species is an uncertain issue, however, it is reasonable to consider that the emission band at 630 nm is due to the combination of ²D_5/2 → ⁴S_3/2 of Bi⁰ and ²P_3/2 (1) → ²P_1/2 of Bi²⁺ transitions, and that of NIR emission band at 843 nm is attributed to the ²D_3/2 → ⁴S_3/2 of Bi⁰ transition.     

Investigation of combined stress state failure criterion for glass fiber/epoxy interface by the cruciform specimen method

The interfacial failure criterion under combined stress state in a glass fiber/epoxy composite is investigated by the cruciform specimen method. Experiments were conducted by using specimens with a fiber whose angle from the loading direction is varied in order to make various stress state of normal and shear at the interface. Finite element analysis is performed to calculate the interfacial stress distribution. By combining the experimental measurement of the specimen stress at the interfacial debonding initiation and the finite element stress analysis, it is possible to obtain the interfacial stress state at interfacial failure. A method to determine the interfacial failure criterion and the interfacial failure initiation location simultaneously is proposed in the present study. We conclude the value of the interfacial shear strength is higher than that of the interfacial normal strength for the material system used in the present study.     

Implementation of a constitutive micromechanical model for damage analysis in glass mat reinforced composite structures

A micromechanical model based on a probabilistic approach is implemented in the finite element code CASTEM 2000 to develop numerical simulations that efficiently predict the overall damaged behaviour of random oriented fibre composites. The proposed damage constitutive model is based upon the generalised Mori and Tanaka scheme and Eshelby's equivalence theory. Damage mechanisms occurring at each composite constituent (fibres, matrix and interface) are associated to Weibull probabilistic functions to model their onset and progressive growth at the microscopic scale level. It is obvious that the damaged behaviour of the composite material depends widely on the microscopic material parameters (fibre length, fibre volume fraction, fibre orientation, …). On one hand, the micromechanical model uses homogenisation techniques which enabled us to link these microscopic parameters to the material behaviour and to evaluate explicitly their influences. On the other hand, the implementation of the derived behaviour law into a finite element code enabled us to reflect on the effect of these microscopic parameters on the overall response of a simple composite structure presenting heterogeneous stress fields. In fact, the damage evolution in each constituent (local scale) and the related stiffness reduction are estimated at any material point (integration point) or node of the considered structure subject to a specific loading. Numerical simulations of a composite plate with a hole under in-plane tension were performed to validate the implementation of the behaviour law. Numerical results have been compared to experimental curves and damage evolutions monitored by acoustic emission techniques. Simulations agree well with experimental results in terms of damage onset and growth.     

Mechanisms of Yb sensitization to Tm for blue upconversion luminescence in fluorophosphate glass

The sensitization mechanisms of Yb³⁺ to Tm³⁺ for the blue upconversion luminescence in fluorophosphate glass were studied. Two different mechanisms exist in the sensitization. One is the sequential sensitization that Tm³⁺ is excited from ³H₆ to ¹G₄ through absorbing three photons transferred from Yb³⁺ one by one. Another is the cooperative sensitization that two Yb³⁺ ions form a couple cluster firstly, and then the couple cluster Yb³⁺ ions transfer their energy to Tm³⁺ and excite it to ¹G₄. With the increment of the concentration of Yb³⁺ ions, the sequential sensitization becomes weak and the cooperative sensitization becomes intense, and the transformation trend of sensitization mechanism with the increment of Yb³⁺ concentration can be clarified by the introduction of Tb³⁺ ions in the glass.     

Comparison of different failure criteria for fiber-reinforced plastics in terms of fracture curves for arbitrary stress combinations

For fiber-reinforced plastics exists a big number of different criteria for the failure prediction. The intention of this paper is to compare the TSAI-HIL-, the LaRC04- and PUCK’s criterion in terms of their fracture curves for a unidirectional glass-fiber reinforced composite layer. Therefore after the implementation of these three criteria, the two-dimensional fracture curves for all possible stress combinations, which can be derived from a general spatial stress tensor, are computed. In this way, the characteristics of the criteria, similarities and differences and possible weak points become obvious.