Magnetostrictive properties of epoxy resins modified with Terfenol-D particles for detection of internal stress in CFRP. Part 1: Materials and processes

Magnetostrictive particles like Terfenol-D are investigated with respect to their ability to detect internal stress, generated in carbon fibre-reinforced polymers (CFRP) in a non-destructive way. The results are presented in two parts. The first part elucidates the ideas for the preparation of dispersions based on these particles with high density in epoxy resins. There is particular focus on the effects of particle size and concentration. Different particle sizes in a range of 0–300?μm are selected by special separation techniques. The particle size distribution is controlled in dry state by laser diffraction method. Changing of the chemical composition, particularly by the oxidation of particles, is analysed by EDX. Use of a magnetic field is identified as a suitable means for the stabilisation of these high-weighted particle fractions dispersed in epoxy resins. The particle size distribution, as well as the alignment of particles, in the cured epoxy resins is investigated by SEM and light microscopy. The second part of the study covers the magnetostrictive properties of the modified epoxy resins which are quantified by the detection of internal stress in CFRP.     

Fire behaviour of reinforced concrete beams strengthened with CFRP laminates: Protection systems with insulation of the anchorage zones

This paper presents experimental and numerical investigations about the fire behaviour of reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) laminates. The main objective was to assess the efficacy of different fire protection systems and to evaluate the viability of their use in floors of buildings. Fire resistance tests were conducted on an intermediate scale oven to investigate the behaviour under fire (ISO 834) of loaded CFRP-strengthened RC beams. The fire protection systems comprised calcium silicate boards and layers of vermiculite/perlite cement based mortar, with thicknesses of 25 mm and 40 mm, applied along the bottom soffit of the beams that was directly exposed to fire. In addition, the anchorage zones of the CFRP laminates were highly thermally insulated in order to evaluate the benefits of this particular constructive detail. Member deflection and temperatures throughout the midspan section were measured and recorded during the tests. When the strengthening system was left unprotected in the exposed length of the beam, the CFRP laminate anchorage debonded after about 23 min. When the above mentioned fire protection materials were applied in the exposed length of the beams, the strengthening system debonded after between 60–89 min (25 mm thickness) and 137-167 min (40 mm). Two-dimensional finite element thermal models of all beams tested were also developed in order to predict the evolution of temperatures in the materials. The calculated temperatures compared reasonably well with those measured in the tests.     

Controlling factors of microstructural evolution and corresponding deformation behaviour of Al–4Cu alloy in semisolid state

Abstract

The present work describes the microstructural evolution of an Al–4Cu alloy and its deformation response, as measured by the parallel plates technique. Specimens were heat treated to obtain a variety of Al₂Cu dispersions, and deformed by extrusion (1:16 ratio). Results indicated that microstructural coarsening took place initially by particle coalescence, but after 10 min soaking time Ostwald ripening became the controlling mechanism. It was also shown that texture accelerated particle growth rate by increasing the likelihood that chance encounters produced grain boundaries, and hence coalescence events. The mechanism of action of particle stimulated nucleation of recrystallisation was suggested, by observing that the particle growth kinetics of deformed specimens was much higher than that of non-deformed specimens with their larger initial particle size and slower initial growth rate. Finally, rheological experiments showed that deformation behaviour was mainly controlled by particle shape, and to a lesser extent by particle size.     

Fire behaviour of thermally insulated RC beams strengthened with NSM-CFRP strips: Experimental study

This paper presents the results of fire resistance tests on reinforced concrete (RC) beams flexurally strengthened with carbon fibre reinforced polymer (CFRP) strips installed according to the near surface mounted (NSM) technique using two different adhesives. The beams were simultaneously subjected to a service load and the ISO 834 standard fire. Different fire protection schemes were studied, comprising a thinner insulation layer along the bottom soffit of the beams and a thicker one at the CFRP anchorage zones. The main objectives of this paper were (i) to understand in further depth the fire behaviour of NSM-strengthened RC beams, in particular the structural effectiveness of the strengthening system during fire, (ii) to evaluate the efficiency of the above-mentioned fire protection strategy in extending the CFRP mechanical contribution during fire, and (iii) to compare the fire performance of the NSM-strengthening system with that of the alternative externally bonded reinforcement (EBR) technique, recently investigated under similar test conditions. The results obtained showed that using the adopted insulation schemes (i.e., thicker insulation at the anchorage zone and thinner insulation in the current zone), even after the CFRP-concrete bond is highly damaged in the central zone of the beams, the strengthening system is able to retain its structural effectiveness through a cable mechanism: for insulation thicknesses of 25 mm (current zone) and 50 mm (anchorage zones), the fire resistance of the strengthening system was extended up to 114 min. The loss of effectiveness of the CFRP system occurred when the average temperature in the adhesive at the CFRP anchorage zones attained values ranging from 2.2 to 5.6 times its glass transition temperature (T_g). The comparison with the EBR-strengthened beams confirmed the much better performance of the NSM strengthening.     

Fatigue behaviour of CFRP L-shaped plates for shear strengthening of RC T-beams

Prefabricated carbon fibre reinforced plastic (CFRP) L-shaped plates can be used to shear strengthen reinforced concrete (RC) T-beams. Previous investigations by EMPA have shown the suitability of the CFRP L-shaped plates for static shear strengthening.In this paper, a large-scale fatigue test is presented which demonstrated the suitability of the CFRP L-shaped plates for shear strengthening of RC T-beams for fatigue reasons. The test beam was subjected to 5 million load cycles at a high load level and a subsequent failure test. Its behaviour is compared with that of a similar, statically tested beam. A fatigue design proposal is presented for users of the CFRP L-shaped plates.     

Contribution to the understanding of the mechanical behaviour of CFRP-strengthened RC beams subjected to fire: Experimental and numerical assessment

Recent experimental tests and numerical simulations about the fire resistance behaviour of CFRP-strengthened RC beams proved that CFRP strengthening systems are able to attain considerable fire endurance, provided that adequate fire protection systems are used. In a fire event, even though a CFRP laminate may rapidly debond from the central part of the beam in which it is installed, if sufficiently thick insulation is applied in the anchorage zones, the laminate transforms into a “cable” fixed at the extremities, thus maintaining a considerable contribution to the mechanical response of the strengthened beam. This paper presents experimental and numerical investigations on CFRP-strengthened RC beams with the objective of understanding in further depth their fire resistance behaviour, namely the influence of the above mentioned “cable” mechanism on the mechanical response of the beams. The experimental campaign, performed at ambient temperature, comprised 4-point bending tests on RC beams strengthened with CFRP laminates according to either the EBR or the NSM techniques, in both cases fully or partially (only at the anchorages, thus simulating the cable mechanism) bonded to the soffit of the beams. For the test conditions used in this study, for both types of strengthening systems, partially bonding the CFRP laminates did not affect the stiffness of the beams and caused only a slight reduction of their strength (6–15%). The numerical study comprised the simulation of the structural response of all beams tested. Non-linear finite element models were developed in Atena commercial package, in which a smeared cracked model was adopted to simulate concrete and appropriate bond-slip constitutive relations were defined for the CFRP-concrete interfaces. A very good agreement was obtained between experimental data and numerical results, providing further validation to the “cable” mechanism and the possibility of taking it into account when designing fire protection systems for CFRP-strengthened RC beams.     

RC beams strengthened with composite material: a limit analysis approach and experimental study

This paper deals with the flexural strengthening of reinforced concrete beams by means of thin carbon fibre reinforced plastic (CFRP) plates. A simplified laminated plate model is used to describe the behaviour of three-layered plate in cylindrical bending which were subjected to third-point line loads. The upper bound theorem of limit analysis is used to approximate the ultimate load capacity for multi-layered plates and identify different collapse mechanisms. A reinforced concrete beam strengthened by CFRP is designed as a three-layered plate. Experimental results are obtained and a comparison with theoretical predictions made.     

Preparation of amorphous solid dispersions by rotary evaporation and KinetiSol Dispersing: approaches to enhance solubility of a poorly water-soluble gum extract

Acetyl-11-keto-β-boswellic acid (AKBA), a gum resin extract, possesses poor water-solubility that limits bioavailability and a high melting point making it difficult to successfully process into solid dispersions by fusion methods. The purpose of this study was to investigate solvent and thermal processing techniques for the preparation of amorphous solid dispersions (ASDs) exhibiting enhanced solubility, dissolution rates and bioavailability. Solid dispersions were successfully produced by rotary evaporation (RE) and KinetiSol® Dispersing (KSD). Solid state and chemical characterization revealed that ASD with good potency and purity were produced by both RE and KSD. Results of the RE studies demonstrated that AQOAT®-LF, AQOAT®-MF, Eudragit® L100-55 and Soluplus with the incorporation of dioctyl sulfosuccinate sodium provided substantial solubility enhancement. Non-sink dissolution analysis showed enhanced dissolution properties for KSD-processed solid dispersions in comparison to RE-processed solid dispersions. Variances in release performance were identified when different particle size fractions of KSD samples were analyzed. Selected RE samples varying in particle surface morphologies were placed under storage and exhibited crystalline growth following solid-state stability analysis at 12 months in comparison to stored KSD samples confirming amorphous instability for RE products. In vivo analysis of KSD-processed solid dispersions revealed significantly enhanced AKBA absorption in comparison to the neat, active substance.     

Micro-deformation mechanisms in thermoformed alumina trihydrate reinforced poly(methyl methacrylate)

Micro-deformation mechanisms involved in thermoforming of alumina trihydrate (ATH) reinforced poly(methyl methacrylate) (PMMA) was investigated in a new experimental method replicating industrial heavy-gage thermoforming procedure. Uniaxial tension tests under non-steady thermal conditions were carried out at different forming rates and forming temperatures. Stress–strain curves and load–displacement histories of thermoformed samples were studied in terms of specimen temperature at different forming conditions. Neat PMMA samples were stretched to 50% strain under identical thermoforming conditions as PMMA/ATH for comparison purposes. Stress whitening in thermoformed PMMA/ATH samples was monitored with optical microscope and degree of stress whitening was characterized by an index obtained from optical image histograms. Micro-deformation features on the surface of PMMA and PMMA/ATH samples were examined by scanning electron microscopy (SEM). Micro-deformation in neat PMMA was in the form of homogenous drawing and did not include any type of void formation. SEM images of PMMA/ATH samples showed that particle cracking is the dominant deformation mechanism at low-forming temperatures, while at high-forming temperatures, combined particle disintegration and interfacial failure are dominant mechanisms. Stress whitening was not observed in neat PMMA which was attributed to absence of micro-voids or craze-like structures. On the other hand, PMMA/ATH samples displayed different levels of stress whitening depending on density, size and type of micro-deformation features.     

Aluminum hydroxide filled ethylene vinyl acetate (EVA) composites: effect of the interfacial compatibilizer and the particle size

Ethylene vinyl acetate (EVA) copolymer was filled with aluminum hydroxide (ATH) with three different sizes of 1.8, 1.2 and 0.8 μm in various volume fractions. The effect of interfacial compatibilizer on the properties of the composites was studied by morphology observation, dynamic mechanical analysis, tensile and flame tests. The results illustrated that the incorporation of functionalized polyethylene combined with dicumyl peroxide (DCP) and the silane coupling agent led to a pronounced improvement in the tensile strength compared to the composites with ATH untreated or treated by silane coupling agent alone. It was found that good dispersion and interfacial adhesion between the ATH particles and the matrix can improve the flame properties of composites. The particle size has a great effect on the flammability of the EVA/ATH composites. ATH with smaller particle size can increase the LOI value and improve the UL-94 flammability of the composites.     

CFRP加固火灾后混凝土短柱抗震性能的试验研究

进行了1根未受火混凝土短柱、1根火灾后混凝土短柱、1根CFRP加固未受火混凝土短柱和3根CFRP加固火灾后混凝土短柱的拟静力试验,考察了CFRP的加固量和加固方式对火灾后混凝土短柱抗震加固效果的影响情况,建议了CFRP加固火灾后混凝土柱抗剪承载力的实用计算方法。研究结果表明:外包CFRP加固可将未受火混凝土短柱的抗剪承载力提高21.8%、可将火灾后混凝土短柱的抗剪承载力提升至未受火试件的111.2%~117.1%、可提高受火前后短柱的极限变形能力和累积滞回耗能。与未受火试件相比,外包CFRP不会提高火灾后加固混凝土短柱的割线刚度。对于相同的加固量,全包CFRP柱的抗剪承载力略优于条带包裹柱。受火前、后和外包CFRP加固前、后混凝土短柱的滞回规则均类似。     

Observations on the characteristics of a fluidized bed for the thermal shock testing of brittle ceramics

The heat transfer characteristics of a fluidized bed used as a quenching medium for the thermal stress testing of brittle ceramics, were determined by measurements of the thermal shock behaviour of rods of a soda-lime—silica glass. The heat transfer coefficient was found to be strongly dependent on the mean particle size of the powder and air flow rate, and was relatively independent of the position within the bed. The results indicate that the heat transfer coefficient during thermal shock fracture may have a value lower than that obtained under heat transfer conditions which more closely resemble steady state. The heat transfer data inferred from the quenching experiments with the glass gave excellent agreement between calculated and measured values for the thermal shock behaviour for rods of a polycrystalline aluminium oxide. It is concluded that fluidized beds are excellent inert quenching media with variable heat transfer coefficient controlled by particle size and flow rate.     

Experimental and finite element analysis of a double strap joint between steel plates and normal modulus CFRP

Strengthening of steel structures using externally-bonded carbon fibre reinforced polymers ‘CFRP’ is a rapidly developing technique. This paper describes the behaviour of axially loaded flat steel plates strengthened using carbon fibre reinforced polymer sheets. Two steel plates were joined together with adhesive and followed by the application of carbon fibre sheet double strap joint with different bond lengths. The behaviour of the specimens was further investigated by using nonlinear finite element analysis to predict the failure modes and load capacity. In this study, bond failure is the dominant failure mode for normal modulus (240 GPa) CFRP bonding which closely matched the results of finite elements. The predicted ultimate loads from the FE analysis are found to be in good agreement with experimental values.     

Experimental and Numerical Investigation of Mechanical and Thermal Residual Strains in Adhesively Bonded Joints

Abstract:  This paper describes an investigation of residual and mechanical strains in aluminium/aluminium (Al/Al) and aluminium/carbon fibre-reinforced polymer (Al/CFRP) adhesively bonded double-lap joints. Residual strains were measured inside the adherends by means of neutron diffraction (ND) and modelled using finite element analysis (FEA). In the Al/Al joints the measured residual strains were negligible, showing good agreement with FE predicted results. However, considerable strains developed in the Al/CFRP joint because of differential thermal contraction of the two materials during joint manufacture. Although considerable scatter was seen in the ND results, the measured and predicted trends showed similar behaviour and were of comparable magnitude. The paper also reports measurements of internal strains in an Al/CFRP joint under tensile load using ND and of surface strains using moiré interferometry (MI). In general, good agreement was observed between FE predictions, surface strains measured with MI and internal strains measured with ND for the loaded Al/CFRP joint.     

Production of nanosized carbon black from hydrocarbon by a thermal plasma

Thermal plasma conditions (optimal heat and radical sources for the thermal decomposition) can be used to accelerate thermodynamically favorable chemical reactions or provide the energy required for endothermic reforming processes. Direct thermal decomposition of hydrocarbon (methane, acetylene, and propane) was carried out using a thermal plasma system which is an environmentally favorable process. In case of thermal decomposition, high purity of the hydrogen and solidified nano-sized carbon can be achieved without any contaminant. The main product carbon produced by thermal decomposition can be either sequestered or used as a raw material and it can be applied for the varieties of industry fields. The morphology of the carbon was characterized by SEM and the particle size was determined by a particle size analyzer. It was observed that the carbon black particles were sphere particles with mainly several tens of nano-sized diameters, those are about 10-80 nm. It can be expected to be used as a raw material of laser printer toner which requires small sized carbon black particles; An average primary particle size of PRINTEX L (Degussa Fillers & Pigment) used in a part of printing inks is 23 nm. In case of the XRD pattern of the produced carbon black from acetylene is of higher crystalline than the commercialized carbon black used for fuel cells. Also carbon species produced were characterized by EA and TGA.     

Dimethylformamide: an effective dispersant for making ceramic-carbon nanotube composites

Carbon nanotube (CNT) and alumina dispersions were prepared separately in dimethylformamide (DMF) and ethanol by ultrasonication. The colloidal stability of the dispersions was monitored and a particle size analysis was performed to evaluate the size range of the agglomerates after different times. DMF was found to be a much more effective dispersant than ethanol for making stable, homogeneous CNT and composite dispersions. Alumina-CNT (4.65?vol%) nanocomposites were sintered in a spark plasma sintering (SPS) furnace. DMF dispersions produced homogeneously distributed and agglomerate-free CNT-alumina nanocomposites with higher electrical conductivity as compared to nanocomposites prepared using ethanol.     

Strengthening and repair of concrete foundation beams with carbon fiber composite materials

For several years Carbon Fiber Reinforced Polymer (CFRP) have been gradually replacing steel plates in the reinforcement of concrete structures that are damaged or need increased resistance. It has long been established that the addition of CFRP laminates to structures such as beams and slabs, increases their bending and shearing strength considerably. However, the behaviour of foundation beams with these reinforcements is not clear. The essential issue in the analysis of reinforced structures with composite materials is to understand the individual behaviour of each material and its interaction with the remaining ones. In this paper, bending and shearing strength of concrete foundation beams with CFRP reinforcement are analyzed through the study of their load capacity variation and beam deflections. Different height/span beam relations are considered. The numerical modeling is performed by the Finite Elements Method with the Abaqus program. Non-linear models are used for concrete and soil, and a linear elastic model is adopted for composites materials.     

40-Year-old full-scale concrete bridge girder strengthened with prestressed CFRP plates anchored using gradient method

A 17 m long internally prestressed concrete girder taken from a bridge in southern Switzerland was strengthened flexurally using prestressed CFRP plates and then tested to failure. The prestressing level in the plates was 32% of their nominal tensile strength. The load carrying behaviour of this test girder was compared to a reference girder and a girder strengthened using non-prestressed plates. For the anchorage of the prestressed CFRP plates, a method developed at Empa was used. In this method, the force in the CFRP plate is reduced to zero at the plate ends and permanent anchorage system is not required.

The experiments proved the feasibility of anchoring CFRP plates using the gradient method. Repeated loading showed stable tensile strains in the plates and shear stresses between the plates and concrete. Furthermore, prestressing resulted in decreased deflections and strains and a 45% increase in maximum recorded load.     

Modeling the performance of coated LPG tanks engulfed in fires

The improvement of passive fire protection of storage vessels is a key factor to enhance safety among the LPG distribution chain. A thermal and mechanical model based on finite elements simulations was developed to assess the behaviour of full size tanks used for LPG storage and transportation in fire engulfment scenarios. The model was validated by experimental results. A specific analysis of the performance of four different reference coating materials was then carried out, also defining specific key performance indicators (KPIs) to assess design safety margins in near-miss simulations. The results confirmed the wide influence of coating application on the expected vessel time to failure due to fire engulfment. A quite different performance of the alternative coating materials was evidenced. General correlations were developed among the vessel time to failure and the effective coating thickness in full engulfment scenarios, providing a preliminary assessment of the coating thickness required to prevent tank rupture for a given time lapse. The KPIs defined allowed the assessment of the available safety margins in the reference scenarios analyzed and of the robustness of thermal protection design.     

Flammability and tensile properties of polylactide nanocomposites with short carbon fibers

Nanocomposites of polylactide (PLA) with aluminum hydroxide (ATH), short carbon fibers (CF), and montmorillonite (MMT) were prepared via direct melt blending. The exfoliated and intercalated clay structures with some aggregations in the PLA matrix were observed. The tensile strength and elongation at break of the PLA composite caused by the high content of the retardant ATH were improved by adding modified MMT and CF to replace a portion of ATH in the PLA matrix. The thermal degradation temperatures and char residue of the PLA/ATH/MMT/CF nanocomposites as determined by thermogravimetric analysis were higher than without MMT. Furthermore, a novel method was proposed to analyze the flammability of composite using an infrared camera, which could capture the apparent thermal image of the sample during UL 94 V test. It was found that, with addition of the MMT and short CF, a more effective insulation layer could be formed on the ablating surface of the PLA/ATH composite, and the high thermal conductivity of the CF might increase the release rate of heat from the surface composite during burn, thus the PLA/ATH/MMT nanocomposite containing short carbon fibers having a V-0 rating without flaming dripping could be obtained.