Yanlei Wang

Strain and damage self-sensing properties of carbon nanofibers/carbon fiber–reinforced polymer laminates

Unidirectional fiber-reinforced composites of “plain” carbon fiber–reinforced polymer laminates and carbon nanofibers modified carbon fiber–reinforced polymer laminates were prepared based on the manufacture of the epoxy resin modified with various contents of carbon nanofibers. The carbon nanofibers–modified epoxy matrix and carbon fiber–reinforced polymer laminates specimens were subject to constant amplitude cyclic tensile loading, quasi-static tension loading, and incremental cyclic tension loading while the values of their electrical resistance were monitored through electrical resistance technique. Resistance-change curves of carbon nanofibers/carbon fiber–reinforced polymer laminates indicated the changes in conductive percolation networks formed by carbon fibers or carbon nanofibers. These changes can identify the complex damage modes and the loss of mechanical integrity in laminates. The changes in resistance of specimens showed a nearly linear correlation with the strain, so the damage process of the carbon fiber–reinforced polymer laminates can be self-sensed according to the resistance-change curves. In addition, uniformly dispersed carbon nanofibers formed a network that spans the whole insulation area, which improved their self-sensing property of strain sensitivity without compromising the mechanical properties of the carbon fiber–reinforced polymer laminates. This technology can achieve the quantitative strain and damage self-sensing properties of nano-reinforced composites without any additional sensor, and it is bound to be a promising method for in situ health monitoring.

In Situ Strain and Damage Monitoring of GFRP Laminates Incorporating Carbon Nanofibers under Tension

In this study, conductive carbon nanofibers (CNFs) were dispersed into epoxy resin and then infused into glass fiber fabric to fabricate CNF/glass fiber-reinforced polymer (GFRP) laminates. The electrical resistance and strain of CNF/GFRP laminates were measured simultaneously during tensile loadings to investigate the in situ strain and damage monitoring capability of CNF/GFRP laminates. The damage evolution and conduction mechanisms of the laminates were also presented. The results indicated that the percolation threshold of CNFs content for CNF/GFRP laminates was 0.86 wt % based on a typical power law. The resistance response during monotonic tensile loading could be classified into three stages corresponding to different damage mechanisms, which demonstrated a good ability of in situ damage monitoring of the CNF/GFRP laminates. In addition, the capacity of in situ strain monitoring of the laminates during small strain stages was also confirmed according to the synchronous and reversible resistance responses to strain under constant cyclic tensile loading. Moreover, the analysis of the resistance responses during incremental amplitude cyclic tensile loading with the maximum strain of 1.5% suggested that in situ strain and damage monitoring of the CNF/GFRP laminates were feasible and stable.

Development Length of Glass Fiber Reinforced Plastic (GFRP)/Steel Wire Composite Rebar

The bond between glass fiber reinforced plastic (GFRP)/steel wire composite rebars and concrete is the key problem to the performance of concrete structures reinforced with GFRP/steel wire composite rebars. In this study, pull-out test was tested to experimentally investigate the bond strength of GFRP/steel wire composite rebars to concrete. The test variables were the nominal diameter, the embedded length, the concrete compressive strength, the concrete cover thickness and the concrete cast depth. Based on the two modification factors of 1.2 and 1.6 to account for the top rebar effect and concrete cover effect, respectively, a new formula is proposed for the calculation of development length for GFRP/steel wire composite rebars.

A Smart FRP-Concrete Composite Beam Using FBG Sensors

A new kind of smart FRP-concrete composite beam, which consists of a FRP box beam combined with a thin layer of concrete in the compression zone, was developed by using two embedded FBG sensors. The fabrication process of the smart FRP-concrete composite beam was introduced. The proposed smart composite beam was tested in 4-point bending to verify the operation of the embedded FBG sensors. The experimental results indicate the output of embedded FBG sensors in the smart beam agrees well with that of surface-bonded strain gauges over the entire load range. The proposed smart FRP-concrete composite beam can reveal the true internal strain from 0 to the failure of the beam and will have wide applications for long-term monitoring in civil engineering.

Experimental testing of a smart FRP-concrete composite bridge superstructure

A new kind of smart fiber reinforced polymer (FRP)-concrete composite bridge superstructure, which consists of two bridge decks and each bridge deck is comprised of four FRP box sections combined with a thin layer of concrete in the compression zone, was developed by using eight embedded FBG sensors in the top and bottom flanges of the FRP box sections at mid-span section of one bridge deck along longitudinal direction, respectively. The flexural behavior of the proposed smart composite bridge superstructure was experimentally studied in four-point loading. The longitudinal strains of the composite bridge superstructure were recorded using the embedded FBG sensors as well as the surfacebonded electric resistance strain gauges. Test results indicate that the FBG sensors can faithfully record the longitudinal strain of the composite bridge superstructure in tension at bottom flange of the FRP box sections or in compression at top flange over the entire loading range, as compared with the surface-bonded strain gauges. The proposed smart FRPconcrete composite bridge superstructure can monitor its longitudinal strains in serviceability limit state as well as in strength limit state, and will has wide applications for long-term monitoring in civil engineering.