Stress corrosion cracking of GRP pultruded rods in acid environments

Stress corrosion cracking of GRP pultruded rods has been investigated in 0.0001 to 5.0 N hydrochloric acid environments under bending and tensile loading modes. Crack initiation takes place at exposed glass fibres in the surface of the rod, and crack propagation is planar and at right angles to the rod axis. Leaching of calcium and aluminium from the fibres takes place during the cracking process, and time-to-failure is dependent on the acid concentration, the stress level and the ease of access of the acid to the glass fibre surface. Possible mechanisms of crack propagation through the glass fibres and resin are discussed.     

An investigation of brittle fracture of composite insulator rods in an acid environment with either static or cyclic loading

The effect of static and cyclic loading conditions on the stress corrosion cracking of unidirectional glass reinforced polymer (GRP) rods used in composite high voltage insulator has been investigated. A series of stress corrosion experiments have been performed on unidirectional E-glass/modified polyester composite rods. The rods have been subjected to mechanical tensile static and cyclic stresses in the presence of a nitric acid solution. The stress corrosion fracture process in the rods was monitored using acoustic emission techniques. The experimental loading conditions simulated possible in-service loadis for composite suspension insulators. The results obtained in this study showed that the brittle fracture process can be generated in the rods when subjected to relatively low tensile stresses in the presence of a nitric acid solution. The morphology of the experimentally generated brittle fracture cracks in the rods closely resemble those from in-service failed composite suspension. It has also been shown in this research that low frequency, low amplitude vibrations in tensile loads can significantly accelerate the fracture process. It appears that the brittle fracture cracks in the rods generated under cyclic loads are less planar in nature in comparison with the cracks formed under static conditions. It has also been found that the acoustic emission generated during the stress corrosion fracture process in the rods is sensitive to the placement of the transducers. However, reasonably good correlation between the stress corrosion crack growth rates and acoustic emission has been attained.     

Single-bolt tension joint tests on pultruded GRP plate - effects of tension direction relative to pultrusion direction

The literature on single-bolt tension joints in structural grade pultruded GRP plate is reviewed and the differences between the main investigations are highlighted. Details are given of the test setup and the joint configurations used in a series 54 such tests on single-bolt joints in which the angle between the pultrusion and tension axes (the off-axis angle) and the joint geometry are varied. Ultimate strength, intial stiffness, initial bolt slip and bolt displacement at failure data are presented as functions of the joints' principal geometric ratios. The observed joint failure modes show that, for off-axis angles ≥ 30 °, bearing failure (a relatively benign failure mode) does not arise. Instead, tension mode failure predominates and cracks tend to propagate parallel to the rovings diagonally across the width of the joint. It is concluded that the rovings play a significant role in controlling the crack propagation and this has implications for joint layout and design in pultruded GRP plate.     

Tensile failure of pultruded glass-polyester composites under superimposed hydrostatic pressure

The tensile properties of five types of pultruded 0·52 V_f glass-fibre-polyester rods were investigated by extending waisted round specimens at atmospheric and superimposed hydrostatic pressures, −H, to 300 MPa. The maximum principal stress at fracture, −700 MPa, decreased, with the superimposition of −H, approximately by its magnitude. As −H increased the failure surfaces became flatter, the amount of fibre pull-out decreased and transverse cracks became shorter or were eliminated. Glass fibres in the failure surfaces were resin free, and failure of the glass fibre bundles appeared to control the fracture process in the entire pressure range for all materials. The decrease in maximum principal tensile stress with increasing −H indicates that the glass fibre failure process is not controlled by a critical tensile stress. Failure criteria are discussed, and in the tension-compression-compression octant of stress space the relevant criteria appear to be strain energy and deviatoric tensile stress, strain and strain energy for these GRPs and glass itself.     

New method employing the electrical impedance for monitoring mechanical damage evolution in glass-reinforced: Applications to riveted joints

This paper presents a new methodology to monitor the mechanical damage progression of GLAss-REinforced (GLARE) fiber metal laminate material, complementing mechanical testing. The methodology is based on assuming GLARE as a capacitor, where two consecutive aluminum layers are separated by nonconductive glass fibers embedded in polyester cloth, acting as a dielectric. The electrical impedance of the capacitor is measured in a range of frequencies during material in service operation. The methodology was validated on riveted joints in superstructures of a naval construction. Uniaxial tensile and three point bending tests were performed while simultaneously acquiring the electric parameters of the capacitor. Merging mechanical and electrical information it was possible to identify the functional mechanical resistance of the joints which is the starting condition for galvanic corrosion occurrence and this was found to take place before mechanical fracture starts. The methodology can be applied both in Structural Health Monitoring (SHM) of riveted joints in GLARE and for analyzing the material integrity, under several working conditions, including fatigue.     

取向玻璃纤维增强塑料的应力腐蚀开裂研究

应用断裂力学的理论和方法对±30°、±45°、±60°三种取向玻璃纤维增强塑料的应力腐蚀开裂行为进行了研究,并与单向玻璃纤维增强塑料的应力腐蚀开裂行为进行了比较。结果表明,纤维取向对玻璃纤维增强塑料的应力腐蚀行为有很大影响。三种取向玻璃纤维增强塑料的耐应力腐蚀性能强弱顺序为:±30°、±45°、±60°玻璃纤维增强塑料。与单向玻璃纤维增强塑料相比,三种取向玻璃纤维增强塑料有更高的应力腐蚀临界载荷值,低于该载荷,可以认为在可接受的时间范围内不会发生应力腐蚀。进一步的讨论证实纤维/基体界面在取向玻璃纤维增强塑料的应力腐蚀中起着重要作用。随外加载荷的变化,取向玻璃纤维增强塑料与单向玻璃纤维增强塑料的应力腐蚀裂纹扩展机理不同。      

Repairing corroded RC beam with near-surface mounted CFRP rods

This experimental program investigates the possibility of using Carbon-Fiber-Reinforced Polymer (CFRP) rods to repair RC beams damaged by steel corrosion. The 6-mm-diameter carbon-epoxy pultruded FRP rods were implemented in undamaged areas of the concrete cover by using the Near Surface Mounted reinforcement (NSM) technique. The corroded beam tested was stored in a chloride environment under service loading for 23 years, which was representative of real structural conditions. The corroded beam showed longitudinal corrosion cracks more than 3 mm in width along the tension reinforcing bars. The repaired corroded beam was tested in three-point flexure up to failure. Then, concrete was removed around the reinforcing bars in order to measure the real local steel cross-section distribution. The maximum reduction in steel cross-section measured in the tension area was about 36% and was located at mid-span. In spite of this heavy corrosion, the bearing capacity measured on the repaired beam was equal to that of the uncorroded control beam. In our experimental conditions, the NSM technique was applicable for repairing corroded concrete structural members.     

Failure of pultruded GRP single-bolt tension joints under hot–wet conditions

An experimental study of the effects of hot–wet conditions on the load carrying capacity of pultruded GRP (glass reinforced plastic) single-bolt tension joints has been carried out. The bolted joints were failed in tension after being immersed in water at three temperatures for two periods of time. Two joint geometries, defined in terms of end distance and width to bolt diameter, were tested with the pultrusion and tension axes coincident. The reductions in the load capacities of the joints due to the hot–wet conditions were quantified and shown to be very large. For example, it was found that more than 60% of the load carrying capacity of a single-bolt tension joint was lost after being immersed in water for 6.5 weeks at 60 °C. This temperature is lower than the manufacturer’s recommended maximum service temperature for this type of pultruded GRP material.     

Study of the stress corrosion cracking of GFRP: effect of the toughness of the matrix resin on the fatigue damage and stress corrosion cracking of GFRP

The stress corrosion cracking of glass fibre-reinforced plastic (GFRP) accompanies a phenomenon of catastrophic failure as a result of the rapid fall in strength owing to corrosion breaking of the glass fibres. This produces a flat surface without pulling fibres out of the plane. Attack on the glass fibres can only occur by contact with an acid which must first diffuse into the matrix resin. It is confirmed, however, that no diffusion occurs or that it is too slow to be detected. The relationship between fatigue damage and stress corrosion in an acidic environment, has been investigated, focusing on the effect of matrix toughness on the resistance to stress corrosion failure of GFRP. Three types of GFRP, made from matrices with different toughness, were studied after subjecting them to fatigue damage at different levels.     

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.     

Analysis of the stress distribution of crimped pultruded composite rods subjected to traction

The stress state of crimped pultruded composite rods subjected to traction has been investigated analytically using the linear theory of elasticity of anisotropic body and the superposition principle. The theoretical solution is able to reproduce the finite element analysis results and clarify the relation between the stress state and the boundary stresses. It can be appreciated from the theoretical solution that a longitudinal compressive stress at the edge of the crimping zone is generated by the boundary shear stress induced by the flow of metal end-fitting. Thus it can be deduced that the stress concentration at the edge of crimping zone could be mitigated through appropriately increasing the extent of the flow of the metal end-fitting away from the middle of the crimping zone. Our research shows that a radial tensile stress existing at the edge of the crimping zone is corresponding to the area of the rod that axial splitting is taken place. Comparison between analytical and numerical results shows the analytical results are in good agreement with the numerical ones except for stress distribution at the edge of the loading zone. The detailed study on stress state at the edge of the crimping zone provides better understanding of the failure mechanism, the improvement possibilities on the crimping technique and the monitoring of the structural health of the composite rod.     

压水堆控制棒驱动机构CANOPY密封焊缝服役环境与失效

综述了国际上对PWR核电站控制棒驱动机构密封焊缝(CSWs)结构、载荷、应力状态、失效分析的研究。研究认为CSWs的结构设计易造成高溶解氧服役环境,在CSWs死水区空腔存在微量氯离子污染的可能,导致较大应力腐蚀倾向。目前研究确认的失效模式有穿晶应力腐蚀(TGSCC)、沿晶应力腐蚀(IGSCC)、SCC+点蚀。计算出CSWs死水区空腔理论浓度可达230×10~(-6),分析认为溶解氧和低浓度水平的氯污染是可引起上述腐蚀的环境因素;CSWs应力腐蚀裂纹扩展模式与材料敏化、服役溶液环境、初始应变、应力状态等因素相关;点蚀可能成为应力腐蚀的起源也可发展成为独立的破坏形式。     

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.     

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.

     

Diametral compression of pultruded composite rods

Diametral compression tests were performed on pultruded composite rods comprised of unidirectional glass or carbon fibers in a common matrix. During compression tests, acoustic emission (AE) activity was recorded and images were acquired from the sample for analysis by digital image correlation (DIC). In both composite systems, localized tensile strain developed in the transverse plane under the load platens prior to failure, producing non-linearity in the load–displacement curve and AE signals. In situ SEM diametral compression tests revealed the development of matrix microcracking and debonding in regions of localized strain, perpendicular to the tensile strain direction (parallel to the load axis). Comparison of linear finite element simulations and experimental results showed a deviation from linear elastic behavior in the load displacement curve. The apparent transverse modulus, in plane shear modulus, and transverse tensile strength of the GF rod was greater than that of the CF rod, and fracture surfaces indicated greater fiber/matrix adhesion in the GF system compared to the CF system. A mixed mode fracture surface showed that two failure modes were active – matrix tensile failure and matrix compression failure by shear near the loading edge.     

Thermal preconditioning study for bolted tension joints in pultruded GRP plate

Thermocouples have been used to monitor the time versus temperature rise response at the centre of four sizes of pultruded GRP plate used in bolted tension joints in order to demonstrate that all of the GRP plate is at the required temperature prior to testing the joints to failure. A simple one-dimensional finite element (FE) model has also been developed and used to predict time versus temperature rise response. The experimental and numerical results confirm that a thermal preconditioning period of 30 min duration is more than adequate for all of the pultruded GRP plates to achieve the actual test temperatures of 40, 60 and 80 °C.     

Progressive damage and nonlinear analysis of pultruded composite structures

This study combines a simple damage modeling approach with micromechanical models for the progressive damage analysis of pultruded composite materials and structures. Two micromodels are used to generate the nonlinear effective response of a pultruded composite system made up from two alternating layers reinforced with roving and continuous filaments mat (CFM). The layers have E-glass fiber and vinylester matrix constituents. The proposed constitutive and damage framework is integrated within a finite element (FE) code for a general nonlinear analysis of pultruded composite structures using layered shell or plate elements. The micromechanical models are implemented at the through-thickness Gaussian integration points of the pultruded cross-section. A layer-wise damage analysis approach is proposed. The Tsai–Wu failure criterion is calibrated separately for the CFM and roving layers using ultimate stress values from off-axis pultruded coupons under uniaxial loading. Once a failure is detected in one of the layers, the micromodel of that layer is no longer used. Instead, an elastic degrading material model is activated for the failed layer to simulate the post-ultimate response. Damage variables for in-plane modes of failure are considered in the effective anisotropic strain energy density of the layer. The degraded secant stiffness is used in the FE analysis. Examples of progressive damage analysis are carried out for notched plates under compression and tension, and a single-bolted connection under tension. Good agreement is shown when comparing the experimental results and the FE models that incorporate the combined micromechanical and damage models.     

Inconel 625合金波纹管失效分析

采用化学分析,透射电子显微镜(TEM)、X射线光电子能谱仪(XPS)和扫描电子显微镜(SEM)等微观测试手段对失效波纹管化学成分、力学性能、断口腐蚀产物及断口微观形貌进行综合分析。研究结果表明,波纹管的失效是材料在腐蚀性环境下局部高应力处首先发生高温氧化、硫化腐蚀,产生疲劳裂纹源,在交变应力的作用下发生腐蚀疲劳而造成的。通过降低腐蚀和波纹管应力可以防止或减轻腐蚀疲劳发生,延长波纹管使用寿命。     

Investigating the quasi-static axial crushing behavior of polymeric foam-filled composite pultrusion square tubes

The capability of structures to absorb large amounts of energy is a crucial factor, particularly for structural components of vehicles, in reducing injury in case of collision. In this study, an experimental investigation was conducted to study the crashworthiness of polymeric foam-filled structures to the pultruded square cross-section E-Glass fiber-reinforced polyester composite tube profiles. Quasi-static compression was applied axially to composite tubes to determine the response of the quasi-static load displacement curve during progressive damage. Three pultruded composite tube wall thicknesses at different sizes were examined, and the effects of crushing behavior and failure modes were analyzed and discussed. Experimental results indicated that the foam-filled profile is superior to the non-filled foam composite tube profile in terms of the capacity to absorb specific energy.