Syntactic foam core metal matrix sandwich composite under bending conditions

The present work aims at characterizing a metal matrix syntactic foam core sandwich composite under three-point bending conditions. The sandwich comprises alumina hollow particle reinforced A356 alloy syntactic foam with carbon fabric skins. Crack initiation in the tensile side of the specimen causing failure of the skin, followed by rapid failure of the core in the direction applied load, is observed as the failure mechanism. Crack propagation through the alumina particles is observed in the failed specimens instead of interfacial failure. The average maximum strength, flexural strain and stiffness were measured as 91.2 ± 5.6 MPa, 0.49 ± 0.06% and 20.6 ± 0.7 GPa respectively. The collapse load is theoretically predicted using mechanics of sandwich beams. Experimental values show good agreement with theoretical predictions.     

Post-impact compressive strength of curved GFRP laminates

Low velocity impacts to fibre reinforced plastic composites cause a pattern of damage consisting in general of delamination, fibre breakage and matrix cracking. Such damage is accidental and may go unnoticed; therefore composite structures must be designed assuming impact damage exists. Previous work on flat composite laminates has resulted in a reasonable understanding of the mechanisms of compressive strength reduction. There are, however, many instances where curved laminates are used in structures where impact is likely. Furthermore, due to the mechanisms of strength reduction, it may be expected that curvature would have a significant effect on the behaviour of the laminates.The work described here consists of experimental measurement of the post-impact compressive strength of curved GFRP laminates. The laminates were of 8 plies of 0.3 mm thick pre-impregnated glass fibre/epoxy tape in a (0, ±45, 0°)_s lay-up. Each laminate was 200 mm in length by 50 mm wide with the plane of curvature normal to the length. Laminates were impacted on the convex surface of the laminate by dropping a steel mass from 1 m vertically above it.Impacted laminates were loaded in compression and the out-of-plane displacements of the top and bottom surfaces were recorded. Final failure was typically due to fibre breakage occurring through the centre of the impacted area of the laminate. Possible differences in the impact response, and measurable differences in the sizes of the impact damage area, were found to arise from these curvatures, and differences were observed in their post-impact buckling behaviour. However, perhaps unexpectedly, the post-impact compressive strength for a curved laminate was found to be similar to that for a flat laminate. The failure loads for the impact damage laminates are shown to be comparable with those for laminates containing artificial delaminations.     

Structural limits of FRP-balsa sandwich decks in bridge construction

The span limits of two glass fiber-reinforced polymer (GFRP) bridge concepts involving GFRP-balsa sandwich plates are discussed. The sandwich plates were either used directly as slab bridges or as decks of a hybrid sandwich-steel girder bridges. In the latter case, the potential of the sandwich decks to replace reinforced concrete (RC) decks was also evaluated. Taking the limits of manufacturing into account (800 mm slab thickness), maximum bridge spans of approximately 19 m can be reached with FRP-balsa sandwich slab bridges, if a carbon-FRP (CFRP) arch is integrated into the balsa core. Above this limit, hybrid sandwich-steel girder bridges can be used up to spans of 30 m. RC deck replacement requires timber and steel plate inserts into the balsa core above the steel girders. GFRP-balsa sandwich slabs or decks exhibit full composite action between lower and upper face sheets. Stress concentrations occur at the joints between balsa core and timber inserts which however can effectively be reduced by changing from butt to scarf joints.     

Strengthening mechanics of thin and thick composite T-joints reinforced with z-pins

This paper presents an experimental and analytical study into the importance of the skin–flange thickness on the strengthening mechanics and fracture modes of z-pinned composite T-joints. The structural properties of unpinned and z-pinned carbon fibre–epoxy T-joints that had skin–flange thickness values between 2 mm (thin) and 8 mm (thick) were determined under tension (stiffener pull-off) loading. Experimental testing revealed that the capacity of z-pins to improve the structural properties was strongly dependent on the T-joint thickness. The joint properties increased at a quasi-linear rate with the skin–flange thickness, and z-pin pull-out tests showed that this was due to the increased crack bridging traction load and traction energy. The increase to the structural properties of the z-pinned T-joints with increasing thickness is explained using the bridging traction laws for z-pinned laminates.     

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.     

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.     

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.     

Ultra-light asymmetric photovoltaic sandwich structures

This work evaluated the possibility of using silicon solar cells as load-carrying elements in composite sandwich structures. Such an ultra-light multifunctional structure is a new concept enabling weight, and thus energy, to be saved in high-tech applications such as solar cars, solar planes or satellites. Composite sandwich structures with a weight of ～800 g/m² were developed, based on one 140 μm thick skin made of 0/90° carbon fiber-reinforced plastic (CFRP), one skin made of 130 μm thick mono-crystalline silicon solar cells, thin stress transfer ribbons between the cells, and a 29 kg/m³ honeycomb core. Particular attention was paid to investigating the strength of the solar cells under bending and tensile loads, and studying the influence of sandwich processing on their failure statistics. Two prototype multi-cell modules were produced to validate the concept. The asymmetric sandwich structure showed balanced mechanical strength; i.e. the solar cells, reinforcing ribbons, and 0/90° CFRP skin were each of comparable strength, thus confirming the potential of this concept for producing stiff and ultra-lightweight solar panels.     

Effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of unidirectional hemp/epoxy composites

The objective of this study was to investigate the effect of pectin and hemicellulose removal from hemp fibres on the mechanical properties of hemp fibre/epoxy composites. Pectin removal by EDTA and endo-polygalacturonase (EPG) removed epidermal and parenchyma cells from hemp fibres and improved fibre separation. Hemicellulose removal by NaOH further improved fibre surface cleanliness. Removal of epidermal and parenchyma cells combined with improved fibre separation decreased composite porosity factor. As a result, pectin removal increased composite stiffness and ultimate tensile strength (UTS). Hemicellulose removal increased composite stiffness, but decreased composite UTS due to removal of xyloglucans. In comparison of all fibre treatments, composites with 0.5% EDTA + 0.2% EPG treated fibres had the highest tensile strength of 327 MPa at fibre volume content of 50%. Composites with 0.5% EDTA + 0.2% EPG → 10% NaOH treated fibres had the highest stiffness of 43 GPa and the lowest porosity factor of 0.04.     

Behavior of full-scale reinforced concrete beams retrofitted for shear and flexural with FRP laminates

Four full-scale reinforced concrete beams were replicated from an existing bridge. The original beams were substantially deficient in shear strength, particularly for projected increase of traffic loads. Of the four replicate beams, one served as a control and the remaining three were implemented with varying configurations of carbon fiber reinforced polymers (CFRP) and glass FRP (GFRP) composites to simulate the retrofit of the existing structure. CFRP unidirectional sheets were placed to increase flexural capacity and GFRP unidirectional sheets were utilized to mitigate shear failure. Four-point bending tests were conducted. Load, deflection and strain data were collected. Fiber optic gauges were utilized in high flexural and shear regions and conventional resistive gauges were placed in eighteen locations to provide behavioral understanding of the composite material strengthening. Fiber optic readings were compared to conventional gauges.Results from this study show that the use of fiber reinforced polymers (FRP) composites for structural strengthening provides significant static capacity increases approximately 150% when compared to unstrengthened sections. Load at first crack and post cracking stiffness of all beams was increased primarily due to flexural CFRP. Test results suggest that beams retrofit with both the designed GFRP and CFRP should well exceed the static demand of 658 kN m sustaining up to 868 kN m applied moment. The addition of GFRP alone for shear was sufficient to offset the lack of steel stirrups and allow conventional RC beam failure by yielding of the tension steel. This allowed ultimate deflections to be 200% higher than the pre-existing shear deficient beam. If bridge beams were retrofit with only the designed CFRP failure would still result from diagonal tension cracks, albeit at a 31% greater load. Beams retrofit with only the designed shear GFRP would fail in flexure at the mid-span at an equivalent 31% gain over the control specimen, failing mechanism in this case being yielding of the tension steel. Successful monitoring of strain using fiber optics was achieved. However, careful planning tempered by engineering judgement is necessary as the location and gauge length of the fiber optic gauge will determine the usefulness of the collected data.     

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 role of delamination in strength,failure mechanism and hole size effect in open hole tensile tests on quasi-isotropic laminates

Results of several different series of open hole tension tests on quasi-isotropic IM7/8552 carbon fibre/epoxy laminates with the same stacking sequence but different ply block thicknesses and numbers of sublaminates are summarised. Specimens with single 0.125 mm thick plies failed by fibre fracture, with the strength decreasing with increasing hole size. Ones with 0.5 mm thick blocks of plies all delaminated, with the failure stress increasing with increasing hole diameter, the opposite to the usual hole size effect. Specimens with 0.25 mm thick ply blocks showed intermediate response, with small ones failing by delamination, and large ones by fibre failure, and constant strength over a range of hole sizes from 1.6 to 12.7 mm diameter. The crucial role of delamination in the strength and failure mechanism in open hole tension is examined in order to explain these results and show why conventional hole size corrections may not always be applicable.     

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.     

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 geometry on crashworthiness parameters of natural kenaf fibre reinforced composite hexagonal tubes

The effect of geometry on energy absorption capability and load-carrying capacity of natural kenaf fibre reinforced composite hexagonal tubes had been investigated experimentally. A series of experiments were carried out for composite hexagonal tubes with different angles from a range of 40–60° in 5° steps. This range is suitable for obtaining a regular hexagonal shape. Kenaf fibre mat form was used in this work due to several advantages such as low cost, no health risk, light weight and availability. The kenaf density was usage 0.17 g/cm³ with thickness of 4 mm. Results demonstrated that structures failed in few distinct failure modes. Precisely in progressive failure mode and fragmentation failure associated with longitudinal cracks. The composite tube with β = 60° exhibited local buckling failure mode and displayed the highest specific energy absorption capability equal to 9.2 J/g. On the other hand, new crashworthiness parameter has been introduced as catastrophic failure mode indicator (CFMI). Furthermore, typical load–deformation histories were presented and discussed.     

High temperature effects on concrete members reinforced with GFRP rebars

GFRP rebars are often used for the internal reinforcement of concrete structures, such as bridge deck slabs, to improve the corrosion resistance. Several studies were conducted to evaluate the static and fatigue behaviour of these elements but the fire resistance still needs further investigation. This paper presents an experimental investigation aimed at understanding the static behaviour of concrete beams reinforced with GFRP rebars exposed to localized elevated temperatures. Two parameters were varied: the maximum temperature imposed on the bottom side of the specimens (230 °C and 550 °C) and the lapping scheme of the rebars, including rebars with hooks and laps of different lengths. The mechanical response was investigated by quasi-static three-points bending tests at room temperature and after heating. The results show that the geometry of the reinforcement has a more relevant influence on the ultimate load than on the initial stiffness of the specimens. The localized heating temperature generates damage in concrete and partial evaporation of the matrix in the GFRP rebars without causing the collapse of the element. The reduction of the load carrying capacity mainly depends on the reinforcement geometry in the overlapping areas.     

Strengthening steel bridge sections using CFRP laminates

This paper presents results from an experimental investigation to determine the feasibility of using carbon fiber reinforced polymer (CFRP) epoxy laminates to repair steel composite bridge members. Six specimens, each consisting of a 6.1 m long W8×24 wide flange A36 steel beam acting compositely with a 0.114 m thick by 0.71 m wide reinforced concrete slab, were first loaded past yield of the tension flange to simulate severe service distress. The damaged specimens were then repaired using 3.65 m lengths of 2 or 5 mm thick CFRP laminates bonded to the tension flange and tested to failure. The results indicated significant ultimate strength gains but more modest improvement in the elastic response. Non-linear finite element analyses were in good agreement with the experimental results. The study suggests that it is feasible to strengthen steel composite members using CFRP laminates.     

Impact strength of a few natural fibre reinforced cement mortar slabs: a comparative study

This paper presents the experimental investigations of the resistance to impact loading of cement mortar slabs (1:3, size: 300 mm × 300 mm × 20 mm) reinforced with four natural fibres, coir, sisal, jute, hibiscus cannebinus and subjected to impact loading using a simple projectile test. Four different fibre contents (0.5%, 1.0%, 1.5% and 2.5%—by weight of cement) and three fibre lengths (20 mm, 30 mm and 40 mm) were considered. The results obtained have shown that the addition of the above natural fibres has increased the impact resistance by 3–18 times than that of the reference (i.e. plain) mortar slab. Of the four fibres, coir fibre reinforced mortar slab specimens have shown the best performance based on the set of chosen indicators, i.e. the impact resistance (R_u), residual impact strength ratio (I_rs), impact crack-resistance ratio (C_r) and the condition of fibre at ultimate failure.     

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.     

A new method for the measurement of static and dynamic Young’s moduli of long fibres

This article describes a new laser systems to measure the Young’s modulus of a long fibre based on the diffraction of light. Two setups are presented. The first setup has been used to measure the static Young’s modulus of a thin basalt fibre (10 μm diameter, 93 mm length). The second setup called “impulse mechanical spectrometer” was used to measure the dynamic Young’s modulus of a long polyamide fibre (128 μm diameter, 371 mm length). A change of the vibration frequency is achieved by changing the length of the fibre or the load. The damping coefficient was also estimated in the dynamic characterisation. The presented experimental method does not require calibration.