Creep and Aging Evaluation of Phenol–Formaldehyde Carbon Fiber Composites in Overhead Transmission Lines

The use of reinforced polymers as cores of transmission cables can provide significant advantages compared to traditional steel cores, such as high tensile strength, low thermal expansion coefficients, and low sag between towers. This work evaluates the applicability of pultruded rods consisting of phenol–formaldehyde resin reinforced with carbon fiber as cores of transmission cables. In this work, the samples were divided into three groups: samples without aging, and samples UV and thermally aged. At first, a dynamic mechanical analysis was performed on samples without aging in order to determine the viscoelastic properties of the material based on the application to see if it would be compatible. In addition to this test, tensile strength and Young's modulus were determined for the three groups. Since the composite cores are susceptible to creep in high temperatures, the applicability must be below the glass transition temperature. Regarding creep behavior, results showed that at a reference temperature of 100 °C, the stress level necessary to cause failure after 50 years was 89% of the ultimate strength. The results of tensile tests were favorable for application of the pultruded system as transmission cables cores and the accelerated aging affected positively in these composites.

     

Energy absorption of braiding pultrusion process composite rods

Composite body structures are now commonly used in road and rail vehicles, ships and submarines, aircraft and spacecraft due to their capability to effectively absorb high kinetic energy to weight ratio. One such structure designed as an energy device with pre-determined properties is a braided pultruded process (BPP) composite rod of either circular or square cross-section.

This paper reports the results of an investigation on circular BPP rods and unidirectional pultruded process rods in epoxy matrix subjected to compressive loading. Test results depict BPP rods to have superior properties in comparison to the unidirectional rods in terms of energy absorption capability that is manifested through well-defined progressive crushing failure mechanisms. Generally the rods' fracture and complete failure mechanisms show distinct creation of buckling zone, followed by generation of fronds as the wedge area increases with every augmentation of applied load. Fracture morphology related to overall performance characteristics is discussed through the step-by-step analysis of microphotography. The specific energy absorption property is shown to be best achieved in carbon/carbon (C/C) BPP followed by glass/carbon (G/C) rod combination and then the glass/glass (G/G) BPP rods. The latter (G/G), although worst performer of all the rods in terms of energy characteristics, still outperforms the documented best tubes made of Kevlar fibres, steel and aluminium. On average, the carbon/carbon (C/C) BPP rod's specific energy absorption is between 35% and 55% more than the nearest comparable tubes.     

CdSe/Cd(x)Zn1-xS core/shell nanocrystals: core morphology and luminescent property

CdSe cores with rod (an aspect ratio of 1.8, d-5 nm) and spherical (an aspect ratio of 1, d-5 nm) morphologies were fabricated by two kinds of organic approaches through adjusting growth processes. Because of large difference of size and morphology, two kinds of cores revealed different absorption spectra. However, these cores exhibited almost same photoluminescence (PL) spectra with a red-emitting PL peak of around 625 nm. This is ascribed that they have a similar size in diameter. A graded Cd(x)Zn1-xS shell of larger band gap was grown around CdSe rods and spheres using oleic acid as a capping agent. Based on the growth kinetics of CdS and ZnS, interfacial segregation was created to preferentially deposit CdS near the core, providing relaxation of the strain at the core/shell interface. For spherical CdSe cores, the homogeneous deposition of the Cd(x)Zn1-xS shell created spherical core/shell nanocrystals (NCs) with a size of 7.1 nm in diameter. In the case of using CdSe cores with rod morphology, the anisotropic aggregation behaviors of CdS monomers on CdSe rods led to the size (approximately 10 nm in diameter) of spherical CdSe/Cd(x)Zn1-xS core/shell NCs with a small difference to the length of the CdSe rod (approximately 8.9 nm). The resulting spherical core/shell NCs created by the rod and spherical cores exhibited almost same PL peak wavelength (652 and 653 nm for using rod and spherical cores, respectively), high PL efficiency up to 50%, and narrow PL spectra (36 and 28 nm of full with at half maximum of PL spectra for the core/shell NCs with CdSe spheres and rods, respectively). These core/shell NCs provide an opportunity for the study of the evolution of PL properties as the shape of semiconductor NCs.     

Effect of postcuring immersed in water under hydraulic pressure on fatigue performance of large‐diameter pultruded carbon/glass hybrid rod

A pultruded carbon fibre core (CFC) and glass fibre shell (GFS) hybrid fiber reinforced polymer (HFRP) rod with the diameter of 19 mm was developed. It was kept immersed in water in a self‐designed pressure chamber device with adjustable hydraulic pressure (20 MPa); after immersion, the tension‐tension fatigue performance was measured at stress levels of 41.7%, 33.4%, and 25.0%. Significant postcuring of resin was observed, resulting in the increase in T_gs for the core and shell layers. There was no significant decrease in the tensile strength of the hybrid rod. The fatigue failure of the hybrid rod was accompanied by debonding of CFC/GFS interface, redistribution of cyclic load, and catastrophic splitting or bursting of GFS. The immersion in water under hydraulic pressure led to a significant increase in fatigue life. The increase in the fatigue life was because of the improvement in interface bonding strength and toughness of the resin owing to the postcuring of the resin. After fatigue, significant degradation in the residual interface bonding strength was observed for the hybrid rods.     

Effect of filler on thermal aging of composites for next-generation power lines

The thermal aging of pultruded composite rods was investigated to determine the effects of filler on oxidation kinetics and degradation mechanisms. The unidirectional hybrid composite rods were comprised of a carbon-fiber core, a glass-fiber shell, and an epoxy matrix filled with clay particles. A reaction-diffusion model was implemented for each of the two hybrid sections to calculate the oxygen-concentration profile and the thickness of the oxidized layer (TOL) within the composite rods, and results were compared with measured oxidation kinetics. The TOL was measured for samples exposed isothermally in air and in vacuum at 200 °C for up to 13,104 h (1.5 year), and the measured values were similar to modeling predictions (within 10%). The domain validity for the reaction-diffusion model was determined from gravimetric experiments (weight-loss measurement), which showed that after prolonged thermal exposure, the degradation mechanism changed from thermal oxidation to thermal degradation. Thermogravimetric analysis (TGA) was performed to determine the thermal degradation and stability of the aged composite. In addition, the effect of thermal aging on glass transition temperature (T_g) and short beam shear (SBS) strength was determined for isothermal exposures at 180 °C and 200 °C.     

Impact response of integrated hollow core sandwich composite panels

This paper deals with an innovative integrated hollow (space) E-glass/epoxy core sandwich composite construction that possesses several multi-functional benefits in addition to the providing lightweight and bending stiffness advantages. In comparison with traditional foam and honeycomb cores, the integrated space core provides a means to route wires/rods, embed electronic assemblies, and store fuel and fire-retardant foam, among other conceivable benefits. In the current work, the low-velocity impact (LVI) response of innovative integrated sandwich core composites was investigated. Three thicknesses of integrated and functionality-embedded E-glass/epoxy sandwich cores were considered in this study—including 6, 9 and 17 mm. The low-velocity impact results indicated that the hollow and functionality-embedded integrated core suffered a localized damage state limited to a system of core members in the vicinity of the impact. The peak forces attained under static compression and LVI were in accordance with Euler's column buckling equation. Stacking of the core was an effective way of improving functionality and limiting the LVI damage in the sandwich plate. The functionality-embedded cores provided enhanced LVI resistance due to energy additional energy absorption mechanisms.     

Analysis of stress concentration during tension of round pultruded composite rods: Errata

“Analysis of stress concentration during tension of round pultruded composite rods (Composite Structures 2008;83:100–109)” had several errors without negative impact on the main results except for one. A sign error in Eq. (11) results in a tensile radial stress rather than compression in Fig. 6 according to the parameters provided in the article.     

Comparative study on tensile behavior of inorganic fibers embedded in unsaturated polyester bisphenol “A”-styrene copolymer

Due to their worldwide availability, glass fibers and aramid fibers have been the more common continuous fibers used to obtain fiber reinforced polymer (FRP) rods. These pultruded rods are used as structural materials and for reinforcements of concrete elements. The main challenge that these fibers face in concrete is their durability in high alkaline environments, therefore the fibers appropriate for this application must be alkali-resistant (AR). In the present work, a cheap Basalt fiber embedded in unsaturated polyester bisphenol “A”-styrene copolymer has been compared to “E” and AR glass fibers under unidirectional tensile strength. A pultrusion process was used to obtain FRP rods. After unidirectional tensile tests of the FRP rods along with durability tests (fibers immersed in a highly alkaline solution at constant temperature simulating the concrete pore solution), the FRP rods with basalt fibers showed to be a cheap and good alternative to be used in concrete.     

Wrapped-yarn reinforced composites: Part II—composite properties

The flexural properties of composites fabricated from yarns formed by twisting paper tapes around mono- or multifilament cores and embedded in a polyester resin matrix have been examined in standard four-point bend, notched three-point bend and Charpy impact tests. It was found that the ease of core slippage, and therefore the composite properties, depended on the yarn packing density and the resin curing conditions. The wrapped-yarn composites bent in a ductile manner with a specific bending stiffness comparable to those of steel and aluminum but showed better recovery behaviour and less notch sensitivity than these two metals. The specific impact toughness of the wrapped-yarn composite increased with sample length and compared favourably with that of a commercial pultruded glass/polyester (GRP) composite containing a much higher proportion of glass fibres.     

分布式OFBG-GFRP传感筋在粘结性能试验中的应用

提出了一种新的利用分布式OFBG-GFRP智能传感筋研究GFRP筋与混凝土之间粘结特性的方法.在OFBG-GFRP的拉挤过程中,多个串连的光栅随同母材,即GFRP筋一同拉挤成型.早期的性能试验表明OFBG-GFRP筋不仅可以作为优良的增强材料使用,还具有理想的和稳定的传感特性.对3个带有OFBG-GFRP筋的混凝土粘结试件进行了直接拔出试验,在试验中自动记录了3个试件中的14个光栅的波长变化数据,这些数据连同记录的拔出力水平一起用来研究GFRP筋与混凝土之间的粘结性能.分析结果表明,GFRP筋与混凝土之间的粘结应变沿埋置深度基本呈现倒三角分布,且随着拔出力的增大,粘结应变分布也发生较为明显的变化,记录的光栅数据也很好地印证了试验过程中发生的试验现象.试验表明,所提方法可以在不影响FRP筋与混凝土之间粘结性能的情况下展开研究.     

Mechanical repair of timber beams fractured in flexure using bonded-in reinforcements

The flexural properties of strength class C16 spruce beams have been compared to the flexural properties of the same beams repaired with bonded-in reinforcements in the form of steel or composite pultruded rods. Reinforcing materials included rectangular sections of mild steel, pultruded carbon fibre reinforced plastic (CFRP), glass fibre reinforced plastic (GFRP) and a thermoplastic matrix glass fibre reinforced polyurethane (FULCRUM). Grooves were routed into the faces of the fractured beams following straightening and the reinforcements adhesively bonded into the top, bottom or both faces of the beams. The steel and CFRP reinforcements are most effective in restoring the flexural strength which often exceeds its original value. These reinforcements are also effective in enhancing flexural strength but the CFRP reinforcement endows the greatest transformed flexural strength. The fracture mechanisms in the repaired beams depend on the placement of reinforcement and the quality of the adhesive to reinforcement bond. All properties are optimised by bonding reinforcement into both faces of the fractured beams.     

Bolted joints of pultruded sandwich composite laminates

This paper describes the tensile behavior of bolted joints of pultruded sandwich composite laminates. The pultruded sandwich laminates have a skin-core-skin structure. Joint strength of longitudinal specimens was independent of specimen width (w), whereas it increased with w in the case of transverse specimens. The joining efficiency of pultruded sandwich laminates was greater in the longitudinal direction than in the transverse direction. The core layer of longitudinal specimens failed by a combination of bearing and shear-out modes, independent of w. The failure mode of skin layers changed from net-tension to bearing mode with increasing w. In transverse specimens, the failure mode of core and skin layers changed from net-tension to bearing with increasing w. Finite element numerical analysis was carried out to predict the failure mode and joint strength. The numerical results were in good correlation with the experimental results.     

Properties of Fabric–Cement Composites made by Pultrusion

Practical manufacturing and use of thin cement-based elements composites require an industrial cost-effective production process in addition to proper reinforcement materials to improve the tensile and flexural performance. Reinforcement by means of fabric materials is an alternative to the use of short fibers. The objective of this study was to investigate use of pultrusion technique as a cost-effective method for the production of thin-sheet fabric-reinforced cement composites. Woven fabrics made from low modulus polypropylene (PP) and glass meshes were used to produce the pultruded cement composites. The influence of fabric type, PP and glass, processing method, pultrusion vs. cast and cement-based matrix modification were examined. Tensile and pullout tests as well as Scanning Electron Microscopy (SEM) observations were used to examine the mechanical, bonding and microstructure properties of the different composites. The rheology of the mix was correlated with the mechanical behavior of the pultruded composites. The tensile behavior of the pultruded fabric–cement components exhibited strain hardening behavior. The best performance was achieved for the PP pultruded composites.     

Failure modes and failure mechanisms of RC members strengthened by NSM CFRP composites – Analysis of pull-out failure mode

The aim of the experimental programme developed in this work was to study the failure modes and the pull-out failure mechanism of RC members strengthened by Near Surface Mounted reinforcement (NSM) technique. The global behaviour of reinforced concrete beams strengthened by NSM and subjected to flexure was investigated. The rods were 6- and 12-mm-diameter carbon–epoxy pultruded FRP. Two vibrated concrete compositions were tested: conventional (VC30) and high-strength (VC60), and one filling material: epoxy resin. The study was carried out up to the failure load, and focused on the failure modes of the beams. The experimental results were compared with the analytical model for the pull-out failure mode of FRP rods. Based on the concept of the anchorage length beyond the last crack, the calculation gives a good approximation of the ultimate bond stress in the cross-section located at the last crack.     

Strain measurement in a concrete beam by use of the Brillouin-scattering-based distributed fiber sensor with single-mode fibers embedded in glass fiber reinforced polymer rods and bonded to steel reinforcing bars

The strain measurement of a 1.65-m reinforced concrete beam by use of a distributed fiber strain sensor with a 50-cm spatial resolution and 5-cm readout resolution is reported. The strain-measurement accuracy is +/-15 microepsilon (microm/m) according to the system calibration in the laboratory environment with non-uniform-distributed strain and +/-5 microepsilon with uniform strain distribution. The strain distribution has been measured for one-point and two-point loading patterns for optical fibers embedded in pultruded glass fiber reinforced polymer (GFRP) rods and those bonded to steel reinforcing bars. In the one-point loading case, the strain deviations are +/-7 and +/-15 microepsilon for fibers embedded in the GFRP rods and fibers bonded to steel reinforcing bars, respectively, whereas the strain deviation is +/-20 microepsilon for the two-point loading case.     

Experimental Evaluation of Thermal Effects on the Tensile Mechanical Properties of Pultruded GFRP Rods

This work represents a preliminary study of a more articulate research project whose aim is to show the possible applications of pultruded products in civil engineering and, in particular, the substitution of FRP rods in classic concrete structures. We present an experimental procedure to first degrade FRP rods with aligned continuous glass fibres through exposure to elevated temperature and then evaluate the changes in their tensile mechanical parameters. The procedure to expose the rods to heat is defined starting from the specifications contained in ASTM D794-68, while the evaluation of the tensile mechanical parameters was set up in a previous work for analogous undamaged FRP rods. From the observation of the experimental data obtained in this work, it is possible to show the strong effects of heat on the ultimate strength and on the Young's modulus of the material that constitutes the rods. In particular, the ultimate strength shows a constant degradation when the temperature increases. By contrast, the Young's modulus is subject to different changes at different temperature exposures.     

拉挤玻璃钢型材在化工防腐中的应用

拉挤玻璃钢型材是一种新型复合材料。该型材与同截面的碳素型钢比较，强度高（比强度是碳钢的五倍），质量轻（比重为碳钢的四分之一），耐腐蚀，电绝缘性好。本文介绍了该型材的生产工艺、性能，并论述该型材在有色冶金电解车间混合酸环境条件下的应用。为腐蚀环境中的建筑物、构筑物防护提供了一条新途径     

Metallographic investigation of the damage caused to GRP by the combined action of electrical,mechanical and chemical environments

The use of glass fibre-reinforced polymers in electrical insulator components has gradually been taking place. Problems may arise where such insulators are in service at very high voltage, e.g. 200 kV and above, are under significant mechanical loads, and the environment (rain, and various pollutants) is able to gain access to the surface of the GRP. With the aid of optical and scanning electron microscope techniques, a detailed examination has been carried out on the nature of damage which has taken place in GRP pultruded rods that have operated for various periods of time in the above service conditions. These pultruded rods can receive significant levels of damage under the action of electrical fields, and the attendant environment; this takes the form of erosion, melting, burning and displacement of both glass and polymer phases. When a mechanical stress, which may be less than 10% of the breaking stress of the rod, is applied in conjunction with the above conditions a different form of insulator breakdown can take place. Instead of material displacement on the scale mentioned above, brittle failure of the GRP takes place. Such a failure mode can be compared with the process of stress corrosion which takes place when GRP is tested in 0.1 N acid solutions. It is suggested that the combined action of electrical activity and the presence of minor amounts of pollutants are able to influence the surface of glass fibres and promote stress corrosion in an analogous fashion to that described for concentrated acid solutions.     

Large inelastic response of unbonded metallic foam and honeycomb core sandwich panels to blast loading

Sandwich panels constructed from metallic face sheets with the core composed of an energy absorbing material, have shown potential as an effective blast resistant structure. In the present study, air-blast tests are conducted on sandwich panels composed steel face sheets with unbonded aluminium foam (Alporas, Cymat) or hexagonal honeycomb cores. Honeycomb cores with small and large aspect ratios are investigated. For all core materials, tests are conducted using two different face sheet thicknesses. The results show that face sheet thickness has a significant effect on the performance of the panels relative to an equivalent monolithic plate. The Alporas and honeycomb cores are found to give higher relative performance with a thicker face sheet. Under the majority of the loading conditions investigated, the thick core honeycomb panels show the greatest increase in blast resistance of the core materials. The Cymat core panels do not show any significant increase in performance over monolithic plates.     

Fatigue damage accumulation in pultruded glass/polyester rods

Stress controlled fatigue tests have been carried out on pultruded 60 vol% E-glass polyester rods. Damage evolution throughout life was monitored using stiffness decay and replication techniques. During the first 10–20% of life a rapid decay in stiffness was related to individual fibre fracture, debonding, and matrix cracking (which was also observed during monotonic testing). After this change, a more gradual decay occurred over the remainder of life when longitudinal cracks and delaminations developed, characteristic of fatigue damage. These mechanisms controlled cyclic behaviour whereas static failure was associated with bundle misorientation.