Quasi-static and dynamic mechanical elastic moduli of alkaline aged pultruded fibre reinforced polymer composite rebar

Due to the increased use of glass fibre reinforced polymer composite (GFRP) rebar in concrete structures, the durability performance of GFRP rebar has been an important research topic in recent years. This paper presents elastic modulus of alkaline environment (pH ≈ 13) aged pultruded GFRP rebar as evaluated by three different methods, namely, quasi-static tensile, quasi-static flexural and dynamic mechanical thermal tests. It was found that elastic modulus of the GFRP rebar samples did not change significantly due to exposure in alkaline environment at 60 °C for 1, 2, 3, 4, 6 and 14 months when compared with that of control sample. Elastic modulus was found to be in the range of 52.5–56.5 GPa irrespective to testing methods and ageing time. In addition, it was estimated from the long time projected results that quasi-static tensile, quasi-static flexural and dynamic mechanical moduli will be retained by about 93%, 95% and 85%, respectively, after 100 years in alkaline environment at 60 °C. Microscopic analysis indicated that quasi-static tensile and flexural failure was mainly due to matrix cracking and shear failure of fibre/matrix interface.     

Influence of defects on fatigue strength and failure mechanisms in the VHCF-region for quenched and tempered steel and nodular cast iron

Investigations are presented in this paper on quenched and tempered steel 42CrMoS4 from two batches, with two different tensile strengths (R_m = 1100 MPa, 1350 MPa) but with similar microstructure, and a nodular cast iron EN-GJS-900-2 (R_m = 930 MPa). Fatigue tests with smooth (K_t = 1) and notched (K_t = 1.75) specimens were performed at R = −1 and R = 0 up to the number of cycles N = 2·10⁹ in order to determine the fatigue strength behaviour and failure mechanisms, especially in the VHCF-region. Failure in smooth specimens often initiated at material defects such as oxides in the quenched and tempered steel and shrinkage holes in the nodular cast iron. Firstly, a fatigue strength analysis was performed that did not consider these defects. A possibility of analysis of experimental data including VHCF-results has been discussed. Next, a linear elastic fracture mechanics analysis was performed in order to describe the defect behaviour, assuming that the defects act like cracks. The results showed that there are lower limit or threshold values of the stress intensity factor range ΔK for crack propagation in both materials. Analysis of defects and defect distribution in run-out specimens confirmed this conclusion. From the comparison of the results with an S–N curve from the design code FKM-Guideline Analytical strength assessment of components, recommendations for design and assessment of components have been derived.     

Behaviour of reinforced concrete rectangular columns strengthened using GFRP

Rectangular columns are often used in bridge pier design, and they make up the majority of building columns. Columns in need of strengthening and retrofit are very common. This paper presents results of a comprehensive experimental investigation on the behavior of axially loaded rectangular columns that have been strengthened with glass fibre reinforced polymer (GFRP) wrap. This paper is intended to examine several aspects related to the use of glass FRP fabrics for strengthening rectangular columns subjected to axial compression. The objectives of the study are as follows: (1) to evaluate the effectiveness of external GFRP strengthening for rectangular Concrete Columns (2) to evaluate the effect of number of GFRP layers on the ultimate load and ductility of confined concrete and (3) to evaluate the effect of the aspect ratio of the column on the effectively confined cross-section. To cover a wide range of cross-sectional dimension ratios, three aspect ratios (a/b, where a and b are, respectively, the longer and shorter sides of the cross-section) were studied: a/b = 1.0, a/b = 1.25, and a/b = 1.66. Specimens with zero, one, and two layers of GFRP wrap were investigated. Totally nine specimens were subjected to axial compression which includes three control specimens. All the test specimens were loaded to failure in axial compression and the behavior of the specimens in the axial and transverse directions was investigated.     

Effect of aging on bond of GFRP bars embedded in concrete

This paper presents an experimental investigation of the durability of the bond between GFRP bars and concrete, specifically as it relates to degradation of the GFRP-bar surface and behavior of the bar–concrete interface. The GFRP bars were embedded in concrete and exposed to tap water at 23 °C, 40 °C, and 50 °C to accelerate potential degradation. The bond strengths before and after exposure were considered as a measure of the durability of the bond between the GFRP bars and concrete. In addition, Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM) were used to characterize how bar aging affected the bond between the GFRP bars and the concrete. The results showed that aging did not significantly affect the durability of the bar–concrete interface under the conditions used in this study.     

Glass transitions of hygrothermal aged pultruded glass fibre reinforced polymer rebar by dynamic mechanical thermal analysis

The changes that can occur in glass fibre reinforced polymer (GFRP) composites with ageing can affect its application, performance and lifetime. Hygrothermal ageing (i.e. accelerated ageing by moisture absorption and temperature change) is a very useful technique to evaluate durability as well as development of GFRP composites in a reasonable timeframe. Dynamic mechanical thermal analysis (DMTA) is essentially able to detect all changes in the state of molecular motion in polymer composites as temperature is scanned. In this work, pultruded GFRP rebars were accelerated aged in an alkaline aqueous environment at 60 °C for 1, 2, 3, 4 and 6 months to evaluate the changes in glass transition of viscoelastic GFRP rebars by DMTA. Five different glass transitions in an average temperate range from 110 to 165 °C were observed at storage modulus, loss modulus and damping factor traces of DMTA. It was also found that glass transition temperature (T_g) of the aged samples changed up to maximum 6 °C compared with that of controlled sample. This change in T_g with ageing time was believed to be due to moisture absorption by rebars.     

Experimental study on the fire resistance of GFRP pultruded tubular columns

This paper presents experimental investigations about the fire behaviour of GFRP pultruded columns with square tubular cross-section. The objectives of the study were to evaluate (i) the efficacy of passive (calcium silicate (CS) boards) and active (water-cooling) systems in providing fire protection to GFRP pultruded columns; and the effects of (ii) the number of sides exposed to fire; and (iii) the service load level on the GFRP columns’ fire response. To this end, fire resistance tests were performed on 1.5 m long GFRP columns, subjected to two different load levels corresponding to axial shortenings of L/1500 and L/750 (L being the span), and simultaneously exposed to fire, in either one or three sides, according to the time–temperature curve defined in ISO 834. Results obtained were analysed regarding the thermal and mechanical responses of the GFRP columns (unprotected and protected), namely in terms of the evolution of axial and transverse displacements, the failure modes and the fire resistance. It was possible to conclude that for one-side exposure, water-cooling is the most effective protection, particularly with flowing water, providing more than 120 min of fire resistance. For three-sides exposure, the fire resistance of the different solutions tested was severely reduced, namely that of the water-cooling systems, which provided fire resistances of only about 20 min, regardless of using flowing water; for this type of exposure, the best performance was provided by the CS board protection, with about 40 min of fire resistance. The results obtained in this study draw the attention to the technical advantages of adopting a building architecture in which the GFRP columns are integrated in the façades and embedded in partition walls, preventing the columns from being exposed to fire in three sides. As expected, the load level increase caused a reduction of fire resistance, due to the higher stresses developed in the sections’ walls and to the GFRP strength decrease with temperature.     

Environmental effects on fatigue behavior of adhesively-bonded pultruded structural joints

The fatigue response of adhesively-bonded pultruded GFRP double-lap joints has been investigated under different environmental conditions. Tests were performed at ?35 °C, 23 °C and 40 °C. A fourth set of fatigue data was collected from tests on preconditioned specimens in warm (40 °C) water. The tests were performed at 40 °C and at 90% relative humidity. Specimens were instrumented with strain and crack gages to record fatigue data. In addition to the S–N curves, stiffness fluctuations and crack initiation and propagation during fatigue were monitored. The dominant failure mode was a fiber-tear failure that occurred in the mat layers of the GFRP laminates. In the presence of high humidity, the failure shifted to the adhesive/composite interface. Although the testing temperature was lower than the glass transition temperature of the adhesive, its influence on the fatigue life and fracture behavior of the examined joints was apparent and was aggravated by the presence of humidity.     

Blast loading response of reinforced concrete panels reinforced with externally bonded GFRP laminates

The behavior of reinforced concrete panels, or slabs, retrofitted with glass fiber reinforced polymer (GFRP) composite, and subjected to blast load is investigated. Eight 1000 × 1000 × 70 mm panels were made of 40 MPa concrete and reinforced with top and bottom steel meshes. Five of the panels were used as control while the remaining four were retrofitted with adhesively bonded 500 mm wide GFRP laminate strips on both faces, one in each direction parallel to the panel edges. The panels were subjected to blast loads generated by the detonation of either 22.4 kg or 33.4 kg ANFO explosive charge located at a 3-m standoff. Blast wave characteristics, including incident and reflected pressures and impulses, as well as panel central deflection and strain in steel and on concrete/FRP surfaces were measured. The post-blast damage and mode of failure of each panel was observed, and those panels that were not completely damaged by the blast were subsequently statically tested to find their residual strength. It was determined that overall the GFRP retrofitted panels performed better than the companion control panels while one retrofitted panel experienced severe damage and could not be tested statically after the blast. The latter finding is consistent with previous reports which have shown that at relatively close range the blast pressure due to nominally similar charges and standoff distance can vary significantly, thus producing different levels of damage.     

A complex approach to the development of the method and equipment for thermal nondestructive testing of CFRP cylindrical parts

Composite materials are being increasingly used in high-tech industries, such as aerospace, automotive manufacture and building inspection. Thermal nondestructive testing (TNDT) has become an accepted method for composite inspection. However, the majority of investigations have dealt with flat or slightly-curved composite components with a thickness of up to 5 mm. Particular studies have been devoted either to NDT modeling with an emphasis on some theoretical issues, or they have been based exclusively on experimental results. There has been some recent interest in the use of composite materials in the nuclear industry. Some critical parts, including centrifuge components, have been made of carbon fiber reinforced polymer (CFRP) composites. The working conditions in a centrifuge include radioactivity and high rotational speed, and the composites used in centrifuges must have very uniform thermal properties and must be free of defects.This paper describes a complex approach to the TNDT of cylindrical parts made of CFRP by starting from thermal properties measurement, theoretical modeling and preliminary experiments, and finishing with the technical requirements for the development of practical equipment capable of operating in both laboratory and industrial conditions.The objects tested were CFRP cylinders with a diameter of 150 mm and a wall thickness of 4–6 mm, and they contained some artificial defects of varying size and depth. Both one- and two-sided test procedures have been analyzed for spot, line and uniform heating. Ultrasonic excitation has also been used as an alternative stimulation technique.In a one-sided test, the depth detection limit has been about 4 mm. Similar results have been observed in the case of ultrasonic stimulation, but the practical implementation of ultrasonic IR thermography to the inspection of cylindrical parts requires further exploration.In a two-sided test, even fairly mild heating resulted in the reliable detection of all defects independent of their size and depth.In all test cases, the highest signal-to-noise ratio occurred after applying the technique of principal component analysis.     

A methodology for hydrocode analysis of ultra-high molecular weight polyethylene composite under ballistic impact

Ballistic performance analysis of ultra-high molecular weight polyethylene (UHMW-PE) is critical for the design of armour systems against ballistic threats. However, no validated modelling strategy has been published in literature for UHMW-PE composite that captures the penetration and damage mechanisms of thick targets impacted between 900 m/s and 2000 m/s. Here we propose a mechanistically-based and extensively validated methodology for the ballistic impact analysis of thick UHMW-PE composite. The methodology uses a non-linear orthotropic continuum model that describes the composite response using a non-linear equation of state (EoS), orthotropic elastic–plastic strength with directional hardening and orthotropic failure criteria. A new sub-laminate discretisation approach is proposed that allows the model to more accurately capture out-of-plane failure. The model is extensively validated using experimental ballistic data for a wide range of UHMW-PE target thicknesses up to 102 mm against 12.7 mm and 20 mm calibre fragment simulating projectiles (FSPs) with impact velocities between 400 m/s and 2000 m/s. Very good overall agreement with experimental results is seen for depth of penetration, ballistic limit and residual velocity, while the penetration mechanisms and target bulge behaviour are accurately predicted. The model can be used to reduce the volume of testing typically required to design and assess thick UHMW-PE composite in ballistic impact applications.     

The influence of binder tow density on the mechanical properties of spatially reinforced composites. Part 1 – Impact resistance

This paper examines the influence of binder tow stitch density on the impact performance of advanced composite structures. Spatially reinforced composite reinforcements with multi-axis, multi-layer structures were woven on a specially developed loom. The binder tow stitch density, which was used to consolidate the structure, was varied in the range of 1–4 binder tow stitches/cm² (10 × 10 mm to 5 × 5 mm binder tow stitch spacing). A drop weight impact test (6.7 J/mm of composite thickness) was used to damage the samples. Both the depth of penetration and the damage area were measured after impact. The analysis of the results has shown that as the binder tow stitch density was increased the extent of damage decreased. The weave architecture, in terms of the relative position of the ±45° tows, was also shown to be a significant factor, the nearer the off-axis tows are to the impact surface the greater was the damage area.     

Fire protection systems for building floors made of pultruded GFRP profiles: Part 1: Experimental investigations

This paper presents results of experimental investigations on the behaviour of GFRP pultruded profiles exposed to fire, in order to study the viability of their structural use in floors of buildings, taking into account the fulfilment of fire resistance requirements. The feasibility and efficacy of using three different protective coatings/layers, often used to protect structural steel, and a water cooling system to provide fire protection to GFRP pultruded profiles were investigated. The experimental programme included dynamic mechanical analyses (DMA), thermogravimetric and differential scanning calorimetry (TGA/DSC) experiments and fire resistance tests on GFRP tubular loaded beams. The unprotected GFRP beam failed after about 38 min, the three different passive protection systems provided a fire resistance between 65–76 min and the water cooling system provided a fire resistance of at least 120 min. Failure occurred in the upper part of the beams, due to compression and shear stresses. Results of these experiments allowed defining the field of application of each investigated solution, according to building code requirements.     

Load and resistance factor design of fiber reinforced polymer composite bridge deck

The majority of our bridges were constructed with conventional civil engineering materials of steel and concrete in a typical slab on girder or truss construction. Reinforced concrete bridge decks have approximately 40% life of the steel girders that support these structures. In order to support the use of alternative materials to replace deteriorating concrete decks, this paper outlines the Load and Resistance Factor Design (LRFD) of Fiber Reinforced Polymer composite (FRP) panel highway bridge deck. The deck would be of a sandwich construction where 152.4 mm × 152.4 mm × 9.5 mm square pultruded glass FRP (GFRP) tubes are joined and sandwiched between two 9.5 mm GFRP plates. The deck would be designed by Allowable Stress Design (ASD) and LRFD to support AASHTO design truckload HL-93. There are currently no US standards and specifications for the design of FRP pultruded shapes including a deck panel therefore international codes and references related to FRP profiles will be examined and AASHTO-LRFD specifications will be used as the basis for the final design. Overall, years of research and laboratory and field tests have proven FRP decks to be a viable alternative to conventional concrete deck. Therefore, conceptualizing the design of FRP bridge decks using basic structural analysis and mechanics would increase awareness and engineering confidence in the use of this innovative material.     

Impact behavior of entangled steel wire material

Entangled steel wire (Q195F) with total porosity of 36.3 ± 1.3 to 61.8 ± 2.4% and pore sizes of 15–825 µm have been investigated in terms of the porous morphologies, impact deformation and failure behavior. The results reveal that the impact toughness increases with the decrease of the porosity. The sintered entangled steel wire materials with 61.8 ± 2.4% porosity exhibit an average of 11.8 J/cm² impact toughness. With 36.3 ± 1.3% porosity, the sintered materials have an average of 45.5 J/cm² impact toughness. Impact absorbing energy and impact toughness have been obtained by Charpy impact testing. Essential impact deformation and failure mechanisms such as pore edges (i.e. fibers) bending, bulking, rotating, yielding, densification and fracture, as well as break (or avulsion) of sintering points in the steel wire framework contribute to the excellent energy-absorbing characteristics under impact loading condition.     

Enhanced fatigue behavior of a glass fiber reinforced hybrid particles modified epoxy nanocomposite under WISPERX spectrum load sequence

Two types of glass fiber reinforced plastic (GFRP) composites were fabricated viz., GFRP with neat epoxy matrix (GFRP-neat) and GFRP with hybrid modified epoxy matrix (GFRP-hybrid) containing 9 wt.% of rubber microparticles and 10 wt.% of silica nanoparticles. Fatigue tests were conducted on both the composites under WISPERX load sequence. The fatigue life of the GFRP-hybrid composite was about 4–5 times higher than that of GFRP-neat composite. The underlying mechanisms for improved fatigue performance are discussed. A reasonably good correlation was observed between the experimental fatigue life and the fatigue life predicted under spectrum loads.     

Time-dependent and residual behavior of pultruded GFRP beams subjected to sustained intensities and cold temperature

This paper presents the time-dependent response and residual behavior of pultruded glass fiber reinforced polymer (GFRP) beams. A total of nine beams are tested in four-point bending. One beam is served as control and eight beams are loaded to failure after exposing to three levels of sustained intensities (20%, 40%, and 60% of the static capacity) at room (25 °C) and cold (− 30 °C) temperatures for 2000 h. Time-dependent material parameters are obtained from the test. Analytical approaches are used to predict the behavior of test beams, based on mechanics-based failure criteria and Findley's creep theory. Three-dimensional finite element models are also developed, based on the experimentally obtained material parameters. The GFRP beams demonstrate time-dependent material degradation due to the sustained load. Cold temperature alters the load-carrying capacity and creep response of the beams. Brittleness of the GFRP is accelerated when the beams are exposed to sustained intensities and cold temperature. The contribution of shear deformation to the deflection of the beams increases with sustained load. Although the proposed modeling approaches agree with the experiment, further development is recommended to account for micro-level material deterioration characteristics.     

The effect of interfacial state on the thermal conductivity of functionalized Al2O3 filled glass fibers reinforced polymer composites

Rapidly increasing packaging density of electronic devices puts forward higher requirements for thermal conductivity of glass fibers reinforced polymer (GFRP) composites, which are commonly used as substrates in printed circuit board. Interface between fillers and polymer matrix has long been playing an important role in affecting thermal conductivity. In this paper, the effect of interfacial state on the thermal conductivity of functionalized Al₂O₃ filled GFRP composites was evaluated. The results indicated that amino groups-Al₂O₃ was demonstrated to be effective filler to fabricate thermally conductive GFPR composite (1.07 W/m K), compared with epoxy group and graphene oxide functionalized Al₂O₃. It was determined that the strong adhesion at the interface and homogeneous dispersion of filler particles were the key factors. Moreover, the effect of interfacial state on dielectric and thermomechanical properties of GFRP composites was also discussed. This research provides an efficient way to develop high-performance GFRP composites with high thermal conductivity for integrated circuit packaging applications.     

Photodegradation of pyrene on soil surfaces under UV light irradiation

The rates of photodegradation of pyrene (Pyr) on soil surfaces under UV light have been studied. Different parameters such as temperature, soil particle sizes, soil depth, and humic acid (HA) concentration responsible for photodegradation have been monitored. The results obtained indicated that Pyr photodegradation follows pseudo-first-order kinetics. Pyr degradation was fastest at 30 °C, while the photodegradation rate was increased with the temperature from 20 to 30 °C. The degradation loss was about 35% at 30 °C, 30% at 25 °C and 25% at 20 °C, respectively. The rates of Pyr photodegradation at different soil particle sizes followed the order: less than 1 mm > less than 0.45 mm > less than 0.25 mm. The results showed that the relationship of Pyr half-life with soil depth was linear (significant correlation, p < 0.01). When the soil depth increased from 1 to 4 mm, the half-life increased from 19.80 to 37.46 d. HA treatments significantly increased the photodegradation of Pyr on soil surfaces under UV light. However the concentration of HA increased from 5 to 40 mg kg^?1, the pseudo-first-order rates of Pyr decreased due to the shielding effect of HA.     

Effects of dynamic mechanical properties on the ballistic performance of a new near-β titanium alloy Ti684

A comprehensive study of the newly developed near-β titanium alloy Ti684 has been carried out to determine the influence of the dynamic strength, dynamic hardness and critical failure strain on the ballistic impact properties. Two heat treatments of Ti684, namely β solution-treatment and α + β solution-treatment followed by aging, were carried out and the results were compared with Ti–6Al–4V. Ballistic impact tests were conducted on 7 mm thick front plates with a 20 mm thick A3 steel backing plate, using 7.62 mm armor piercing projectiles. The ballistic performance was evaluated by measuring the residual depth of penetration (DOP) in the A3 steel backing plates. It was found that the DOP values did not show obvious corresponding relation with both dynamic strength and dynamic hardness. The 800 °C solution +550 °C aged Ti684, which had the maximal dynamic strength, presented the worst ballistic performance, with a maximum DOP of 12.5 mm. In addition, the Ti–6Al–4V plate in the study with highest dynamic hardness did not show the best ballistic performance, having a DOP of 11.86 mm. However, as the critical failure strain increased, the DOP of the A3 steel backings were observed to decrease. This relationship was revealed from post ballistic microstructural observations.     

Variation of strength and stiffness of fibre reinforced polymer reinforcing bars with temperature

This paper presents the results of tensile mechanical properties of FRP reinforcement bars, used as internal reinforcement in concrete structures, at elevated temperatures. Detailed experimental studies were conducted to determine the strength and stiffness properties of FRP bars at elevated temperatures. Two types of FRP bars namely: carbon fibre reinforced polyester bars of 9.5 mm diameter and glass fibre reinforced polyester bars of 9.5 mm and 12.7 mm diameter were considered. For comparison, conventional steel reinforcement bars of 10 mm and 15 mm diameter were also tested. Data from the experiments was used to illustrate the comparative variation of tensile strength and stiffness of different types of FRP reinforcing bars with traditional steel reinforcing bars. Also, results from the strength tests were used to show that temperatures of about 325 °C and 250 °C appear to be critical (in terms of strength) for GFRP and CFRP reinforcing bars, respectively. A case study is presented to illustrate the application of critical temperatures for evaluating the fire performance of FRP-reinforced concrete slabs.