Study of damage behavior and repair effectiveness of patch repaired carbon fiber laminate under quasi-static indentation loading

Damage caused due to low-velocity impacts in composites leads to substantial deterioration in their residual strength and eventually provokes structural failure.This work presents an experimental investigation on the effects of different patch and parent laminate stacking sequences on the enhancement of impact strength of Carbon Fiber Reinforced Polymers(CFRP)composites by utilising the adhesively bonded external patch repair technique.Damage evolution study is also performed with the aid of Acoustic Emission(AE).Two different quasi-isotropic configurations were selected for the parent laminate,viz.,[45°/45°/0°/0°]s and[45°/0°/45°/0°]s.Quasi Static Indentation(QSI)test was performed on both the pristine laminates,and damage areas were detected by using the C-scan inspection technique.Damaged laminates were repaired by using a single-sided patch of two different configurations,viz.,[45°/45°/45°/45°]and[45°/0°/0°/45°],and employing a circular plug to fill the damaged hole.Four different combinations of repaired laminates with two configurations of each parent and patch laminate were produced,which were further subjected to the QSI test.The results reveal the effectiveness of the repair method,as all the repaired laminates show higher impact resistance compared to the respective pristine laminates.Patches of[45°/0°/0°/45°]configuration when repaired by taking[45°/45°/0°/0°]s and[45°/0°/45°/0°]s as parents exhibited 68%and 73%higher peak loads,respectively,than the respective pristine laminates.Furthermore,parent and patch of configuration[45°/0°/45°/0°]s and[45°/0°/0°/45°],respectively,attain the highest peak load,whereas[45°/45°/0°/0°]s and[45°/45°/45°/45°]combinations possess the most gradual decrease in the load.

High-sensitivity phase imaging eddy current magneto-optical system for carbon fiber reinforced polymers detection

This paper proposed a high-sensitivity phase imaging eddy current magneto-optical (PI-ECMO) system for carbon fiber reinforced polymer (CFRP) defect detection. In contrast to other eddy current-based detection systems, the proposed system employs a fixed position excitation coil while enabling the detection point to move within the detection region. This configuration effectively mitigates the interference caused by the lift-off effect, which is commonly observed in systems with moving excitation coils. Correspondingly, the relationship between the defect characteristics (orientation and position) and the surface vertical magnetic field distribution (amplitude and phase) is studied in detail by theoretical analysis and numerical simulations. Experiments conducted on woven CFRP plates demonstrate that the designed PI-ECMO system is capable of effectively detecting both surface and internal cracks, as well as impact defects. The excitation current is significantly reduced compared with traditional eddy current magneto-optical (ECMO) systems.

Flexure-Compression Testing of Bridge Timber Piles Retrofitted with Fiber Reinforced Polymers

The adequacy of using Fiber Reinforced Polymer (FRP) retrofit technique to restore the flexure-compression behavior of deteriorated bridge timber piles is examined experimentally in this paper. Sixteen specimens are tested monotonically under eccentric compressive loading. The specimens are first tested in their unretrofitted condition to determine their elastic properties. Each specimen is then cut and connected (posted) using the proposed FRP retrofit technique, and retested. The results show that the retrofitted specimens are capable of reaching same or higher strengths than that of the unretrofitted specimens with minimal reduction in their stiffness. Based on the experimental results, a design equation is presented to compute the volumetric ratio of FRP needed for retrofitting bridge timber piles under eccentric load.

Mechanical joint performances of friction self-piercing riveted carbon fiber reinforced polymer and AZ31B Mg alloy

Carbon fiber reinforced polymer(CFRP) and AZ31B Mg alloy were joined by the friction self-piercing riveting(F-SPR) with different steel rivet shank sizes. With the increase of rivet shank size, lap shear fracture load and mechanical interlock distance increased. Ultrafine grains were formed at the joint in AZ31B as a result of dynamic recrystallization, which contributed to the higher hardness. Fatigue life of the CFRP-AZ31B joint was studied at various peak loads of 0.5, 1, 2, and 3 kN and compared with the resistance spot welded AZ31B-AZ31B from the open literature. The fatigue performance was better at higher peak load(>2 kN) and comparable to that of resistance spot welding of AZ31B to AZ31B at lower peak loads(<1 kN). From fractography, the crack initiation for lower peak load(<1 kN) case was observed at the fretting positions on the top and bottom surfaces of AZ31B sheet. When peak load was increased, fretting between the rivet and the top of AZ31B became more dominant to initiate a crack during fatigue testing.

Study of galvanic corrosion and mechanical joint properties of AZ31B and carbon-fiber–reinforced polymer joined by friction self-piercing riveting

A new testing methodology was developed to quantitively study galvanic corrosion of AZ31B and thermoset carbon-fiber–reinforced polymer spot-joined by a friction self-piercing riveting process.Pre-defined areas of AZ31B in the joint were exposed in 0.1 M NaCl solution over time.Massive galvanic corrosion of AZ31B was observed as exposure time increased.The measured volume loss was converted into corrosion current that was at least 48 times greater than the corrosion current of AZ31B without galvanic coupling.Ninety percent of the mechanical joint integrity was retained for corroded F-SPR joints to 200 h and then decreased because of the massive volume loss of AZ31B。

Parameters of static response of carbon fiber reinforced polymer(CFRP) suspension cables

The feasibility of longer spans relies on the successful implementation of new high-strength light weight materials such as carbon fiber reinforced polymer(CFRP). First, a dimensionless equilibrium equation and the corresponding compatibility equation are established to develop the cable force equation and cable displacement governing equation for suspension cables, respectively. Subsequently, the inextensible cable case is introduced. The formula of the Irvine parameter is considered and its physical interpretation as well as its relationship with the chord gravity stiffness is presented. The influences on the increment of cable force and displacement by λ2 and load ratio p′ are analyzed, respectively. Based on these assumptions and the analytical formulations, a 2000 m span suspension cable is utilized as an example to verify the proposed formulation and the responses of the relative increment of cable force and cable displacement under symmetrical and asymmetrical loads are studied and presented. In each case, the deflections resulting from elastic elongation or solely due to geometrical displacement are analyzed for the lower elastic modulus CFRP. Finally, in comparison with steel cables, the influences on the cable force equation and the governing displacement equation by span and rise span ratio are analyzed. Moreover, the influences on the static performance of suspension bridge by span and sag ratios are also analyzed. The substantive characteristics of the static performance of super span CFRP suspension bridges are clarified and the superiority and the characteristics of CFRP cable structure are demonstrated analytically.

Shear strengthening of pre-damaged reinforced concrete beams with carbon fiber reinforced polymer sheet strips

This paper presents the results of an experimental investigation on the response of pre-damaged reinforced concrete （RC） beam strengthened in shear using applied-epoxy unidirectional carbon fiber reinforced polymer （CFRP） sheet. The reasearch included four test rectangular simply supported RC beams in shear capacity. One is the control beam, two RC beams are damaged to a predetermined degree from ultimate shear capacity of the control beam, and the last beam is left without pre-damaged and then strengthened with using externally bonded carbon fiber reinforced polymer to upgrade their shear capacity. We focused on the damage degree to beams during strengthening, therefore, only the beams with sidebonded CFRPs strips and horizontal anchored strips were used. The results show the feasibility of using CFRPs to restore or increase the load-carrying capacity in the shear of damaged RC beams. The failure mode of all the CFRP-strengthened beams is debonding of CFRP vertical strips. Two prediction available models in AC1-440 and fib European code were compared with the experimental results.

Investigating Some Parameters Affecting Flexural Behavior of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Laminate

In this paper,the influence of some important parameters affecting the flexural behavior of reinforced concrete beams strengthened with one layer of carbon fiber reinforced polymer(CFRP)laminate has been studied.For this purpose,six reinforced concrete beams were cast and tested in the laboratory.Based on the obtained data,when CFRP laminate is applied to the tension face,too close to the steel rebar,the flexural strength of the strengthened beam is reduced.In general,the performance of the beam strengthened with one wide CFRP strip is better than that strengthened with two equivalent narrow strips.Ultimate load capacity of each strengthened beam was calculated based on the method given by the ACI 440.2R and compared with the test one.It is concluded that,to avoid the steel rebar-CFRP laminate interaction effect,the CFRP laminate depth-to-the effective depth ratio(df/d)should not be smaller than about 1.17.

STUDY ON ULTRASONIC VIBRATION DRILLING IN CARBON FIBER REINFORCED POLYMERS

This paper researches ultrasonic vibration drilling of carbon fiber reinforced polymers composites that are hard, brittle, and have low shear strength between layers. An experiment plan has been developed to reduce the axial force. Experimental studies have been done on the influence of process parameters, tool structures on the drilling axial force. The drilling mechanism is specially investigated. Thus an effective method is presented to reduce the drilling axial force. The authors suppose that ultrasonic vibration drilling is feasible for carbon fiber reinforced polymers composites.

A low-cost and high-strength basalt/carbon fiber reinforced polymer improved by imitating tree-root micro/nano aramid short fiber

The high-strength Basalt Carbon Fiber Reinforced Polymer(BCFRP)composites had been manufactured by guiding Imitating Tree-root Micro/Nano Aramid Short Fiber(ITMNASF)into the interlayer of Basalt Fiber(BF)and Carbon Fiber(CF)plies to form thin interleaving,and various mass proportions of IT-MNASF were designed to discuss the reinforcing effect on the BCFRP heterogeneous composites.The results of three points bending tests showed that flexural strength and energy absorption of 4wt%IT-MNASF reinforced BCFRP heterogeneous composites had been improved by 32.4%and 134.4%respectively compared with that of unreinforced specimens.The 4wt%IT-MNASF reinforced BCFRP specimens showed both a greater strength and a lower cost(reduced by 31%around)than that of plain CFRP composites.X-ray micro-computed tomography scanning results exhibited that the delamination-dominated failure of plain BCFRP composites was changed into multi-layer BF and CF fabrics damage.The reinforcing mechanism revealed that the introduced IT-MNASF could construct quasi-vertical fiber bridging,and it was used as"mechanical claws"to grasp adjacent fiber layers for creating a stronger mechanical interlocking,and this effectively improved resin-rich region and interfacial transition region at the interlayers.The simple and effective IT-MNASF interleaving technique was very successful in low-cost and high-strength development of BCFRP heterogeneous composites.

Micro Model of Carbon Fiber/Cyanate Ester Composites and Analysis of Machining Damage Mechanism

Machining damage occurs on the surface of carbon fiber reinforced polymer (CFRP) composites during processing. In the current simulation model of CFRP, the initial defects on the carbon fiber and the periodic random distribution of the reinforcement phase in the matrix are not considered in detail, which makes the characteristics of the cutting model significantly different from the actual processing conditions. In this paper, a novel three-phase model of carbon fiber/cyanate ester composites is proposed to simulate the machining damage of the composites. The periodic random distribution of the carbon fiber reinforced phase in the matrix was realized using a double perturbation algorithm. To achieve the stochastic distribution of the strength of a single carbon fiber, a novel method that combines the Weibull intensity distribution theory with the Monte Carlo method is presented. The mechanical properties of the cyanate matrix were characterized by fitting the stress-strain curves, and the cohesive zone model was employed to simulate the interface. Based on the model, the machining damage mechanism of the composites was revealed using finite element simulations and by conducting a theoretical analysis. Furthermore, the milling surfaces of the composites were observed using a scanning electron microscope, to verify the accuracy of the simulation results. In this study, the simulations and theoretical analysis of the carbon fiber/cyanate ester composite processing were carried out based on a novel three-phase model, which revealed the material failure and machining damage mechanism more accurately.

Earthquake simulation test of circular reinforced concrete bridge column under multidirectional seismic excitation

Structures behave multi-directionally when subjected to earthquake excitation. Thus, it is essential to evaluate the effect of multidirectional loading on the dynamic response and seismic performance of reinforced concrete bridge columns in order to develop more advanced and reliable design procedures. To investigate such effects, a 1/4 scaled circular reinforced concrete bridge column specimen was tested under two horizontal and one vertical components of a strong motion that has long duration with several strong pulses. Damage progress of reinforced concrete columns subjected to strong excitation was evaluated from the test. The test results demonstrate that the lateral force response in the principal directions become smaller than computed flexural capacity due to the bilateral flexural loading effects, and that the lateral response is not significantly affected by the fluctuation of the axial force because the horizontal response and axial force barely reached the maximum simultaneously due to difference of the predominant natural periods between the vertical and the horizontal directions. Accuracy of fiber analyses is discussed using the test results.

Effect of neat and reinforced polyacrylonitrile nanofibers incorporation on interlaminar fracture toughness of carbon/epoxy composite

This paper presents an experimental investigation on fracture behavior of epoxy resin-carbon fibers composites interleaved with both neat polyacrylonitrile （PAN） nanofibers and A1203-PAN nanofibers. In particular, the paper focuses on the effect of adding Al2O3 nanopartiles in PAN nanofibers, which were incorporated in unidirectional （UD） laminates. The effectiveness of adding a thin film made of Al2O3-PAN on the fracture behavior of the carbon fiber reinforced polymer （CFRP） has been addressed by comparing the energy release rates, obtained by testing double cantilever beam （DCB） samples under mode I loading condition. A general improvement in interlaminar fracture energy of the CFRP is observed when the both neat PAN nanofibers and Al2O3-PAN nanofibers are interleaved. However, higher interlaminar strength has been observed for the samples with a thin film of Al2O3-PAN nanofibers, suggesting a better stress distribution and stress transformation from resin-rich area to reinforcement phase of hybrid composites.

Anisotropic Thermal Diffusivity Measurements in High-Thermal-Conductive Carbon-Fiber-Reinforced Plastic Composites

This paper presents the temperature dependence of in-plane thermal diffusivity and anisotropy distribution for pitch-based carbon-fiber-reinforced polymers (CFRPs). The measurement was performed using the laser-spot periodic heating method. The samples were unidirectional (UD) and crossply (CP) CFRPs. All carbon fibers of the UD samples ran in one direction, while those of the CP samples ran in two directions. In both UD and CP CFRPs, from -80&deg;C to +80&deg;C, temperature dependence of thermal diffusivity values increased as temperature decreased. In this temperature range, the anisotropic ratio between the fiber direction and its perpendicular direction of the UD CFRP was 106 - 124. During the anisotropy distribution measurement, it was found that thermal anisotropy can be visualized by scanning the laser in a circle on the sample. The thermal diffusivity of the UD CFRP in the fiber direction was 17 times larger than that in the 15&deg;direction, and the thermal diffusivity in the other directions was lower than that in the 15&deg;direction. The anisotropy distribution for the CP CFRP reflected its inhomogeneous structure.

Computational and Experimental Analyses of Detachment Force at the Interface between Carbon Fibers and Epoxy Resin

Herein, we used theoretical and experimental methods to investigate the shear fracture strengths of carbon fiber/epoxy resin interfaces. The shear strengths of carbon fiber and epoxy resin were measured using the microdroplet test, whereas interaction and binding energies were estimated using?Ab initio?and molecular dynamics methods. However, binding energies did not impact the shear strength volumes determined by microdroplet tests,?i.e., bonds between functional groups of the carbon filer and the epoxy resin were difficult to break. On the other hand, the interaction energies calculated for epoxy monomers were in good agreement with experimental data. Moreover, we determined the relationship between the simulated interaction energy and the shear fracture strength volume obtained using the microdroplet test.

Ductility of Timber Beams Strengthened Using Fiber Reinforced Polymer

This research was conducted to investigate the ductility behavior of timber beams strengthened with CFRP （carbon fiber reinforced polymer） plates. The surface to be bonded was spiked by punching small holes of 2 mm in diameter with 10 mm spacing. The aim is to increase bonding capacity by having small studs. Five beams with the dimension of 100 mm x 200 mm x 3,000 mm were tested where one of the beams was used as control beam （unstrengthened）. The remaining beams were strengthened with different configurations before tested to failure under four-point loading. The results showed that the ductility was increased as the percentage of CFRP increased. The ductility was dramatically improved where the highest ductility index based on deflection method was 2.2 where the percentage increase was 37.5%, whereas the highest ductility index based on energy method was 3.2 where the percentage increase was 88.2%. From this study, it was found that 0.3% is the optimum value of CFRP area to achieve maximum ductility index. Ductility index obtained from energy method gives higher values when compared to deflection method. All beams in this study did not fail due to peel off or debonding. It was also proved that the spikes that have been made at the wood surface were very effective for bonding.

CFRP及EWSS复合加固震损双层高架桥框架式桥墩恢复力模型研究

为研究碳纤维布(Carbon Fiber Reinforced Polymer,CFRP)及外包型钢(Externally Wrapped Steel Section,EWSS)复合加固震损双层高架桥框架式桥墩的恢复力模型,对1榀原型对比试件、1榀无预损加固试件和2榀遭受不同地震损伤加固试件进行了低周往复加载破坏试验,获取了滞回曲线并提取骨架曲线,分析其滞回特性,提出一种弹性段和强化段为双折线、下降段为指数函数且考虑初始损伤的骨架曲线模型;采用试验数据回归拟合方法,定量描述了试件滞回曲线卸载刚度的退化规律,考虑了同级加载承载力退化和定点指向特征,建立了恢复力模型。研究结果表明:复合加固试件滞回曲线捏缩现象明显,呈倒S型,各滞回环分别相交于骨架曲线上正向、负向荷载为屈服荷载0.25倍的点,峰值荷载后EWSS产生包辛格效应;所提出的骨架曲线模型对中度震损和重度震损加固试件下降段指数函数的参数k建议取值分别为3.6和3.4;所建立的骨架曲线模型和恢复力模型计算结果与试验实测结果吻合较好,可为该类结构弹塑性地震反应分析提供依据。

玄武岩纤维增强复合材料拉挤圆管抗弯性能研究

为使玄武岩纤维增强复合材料(Basalt Fiber Reinforced Polymer,BFRP)拉挤圆管在桥梁工程中得到更广泛地应用,对其抗弯性能进行试验及有限元分析。对2根长度为750 mm的BFRP拉挤圆管进行三点抗弯试验;采用LS-DYNA有限元软件对试验过程进行模拟,验证有限元模拟的正确性;在保持BFRP拉挤圆管总厚度不变的条件下,研究内、外层双向纤维布的铺设厚度和角度对抗弯性能的影响。结果表明:试件从加载至破坏分为线弹性阶段、下降阶段和残余阶段,试件破坏后残余强度较大且卸载后迅速回弹恢复几何形状;有限元模拟试件的荷载~滑移曲线、失效现象和试验结果吻合较好,有限元模拟正确;布置适量厚度的内、外层双向纤维布可以提高试件的抗弯承载力;将内、外层双向纤维布铺设方向由90°变为45°后,抗弯承载力进一步提高,随着外层45°纤维布厚度增加,破坏时的挠度下降。建议制作BFRP拉挤圆管时,布置适量厚度的双向纤维布,角度以45°为宜。

BFRP筋回收轮胎钢纤维混凝土黏结性能分析

对玄武岩纤维增强复合材料(BFRP)筋在回收轮胎钢纤维(RTSF)混凝土中的黏结性能进行了试验和分析研究.对不同RTSF体积掺量、BFRP筋直径和嵌入长度的试件进行了拉拔试验,研究其对黏结破坏模式、黏结滑移响应和黏结强度的影响,并提出了BFRP筋RTSF混凝土的黏结滑移理论模型.结果表明:BFRP筋RTSF混凝土黏结破坏类型包括拔出破坏、劈裂破坏以及拔出且劈裂破坏;BFRP筋直径对黏结破坏模式无显著影响,而RTSF体积掺量、BFRP筋直径和嵌入长度均对黏结强度有较大影响,且RTSF体积掺量与极限黏结强度呈正相关,BFRP筋直径和嵌入长度对其影响与之相反;当RTSF体积掺量从0增大至1.5%时,其极限黏结强度最大可提高约23.87%.与已有模型相比,所提出的黏结滑移理论模型对BFRP筋RTSF混凝土的黏结滑移曲线具有更高的拟合精度.

抢险车对92m跨度复合材料桁架桥动态响应的影响

为研究大跨径桁架式纤维增强复合材料(FRP)应急抢修桥梁成桥后的动力性能,基于导梁推送施工92 m跨度的FRP桁架桥模型,编制FRP桁架桥的车桥耦合算法,研究抢险车辆的车速、质量及路面不平顺对FRP桁架桥的跨中竖向位移、竖向加速度和关键杆件轴向应力等指标的影响.结果表明:路面越不平顺,车辆对桥梁结构的冲击作用越大,且路面等级对竖向位移的影响大于对关键杆件轴向应力的影响;车速为20~50 km/h时,车速变化对冲击系数影响较小,但车速超过50 km/h时,车速的影响逐渐增大;车辆质量对桥梁各项指标动力响应的峰值曲线呈线性增大趋势.当车辆质量为2.80×10^(4)kg,车速30 km/h时,最大跨中竖向位移为167 mm,未超过抢修桥梁位移限值(L/120),表明该桥梁的设计可以满足抢险的通行要求.