Co-reporter:Jun He, Guijun Xian
Construction and Building Materials 2017 Volume 155(Volume 155) pp:
Publication Date(Web):30 November 2017
DOI:10.1016/j.conbuildmat.2017.08.062
•Single-lap shear tests on CFRP-to-steel joints using a linear adhesive.•A review of existing bond-slip models is conducted.•A new bond-slip model is developed.•The developed model is proved to be accurate according to numerical analysis.Fiber reinforced polymer (FRP) composites have been widely and successfully applied for externally bonded strengthening of steel structures. The bond between FRP and steel substrate plays a key role on the strengthening effectiveness of FRPs. The bond-slip relationship is essential to understand the mechanisms involved in the bond failure process. This paper experimentally studied the bond behavior of carbon fiber reinforced FRPs (CFRP)-to-steel joints through a single-shear lap testing method. The load-displacement relationship and bond-slip relationship were firstly presented. In addition, a new bond-slip model was developed based on observed experimental behavior of strengthened components. According to the developed bond-slip model, a computed result of the load-displacement relationship was obtained using numerical analysis method by ANSYS software. Comparison between computed and experimental result of the load-displacement relationship showed a good accordance, indicating the reliability of the developed bond-slip model. Finally, comparison of computed results obtained from the developed model and the existing models demonstrated that the proposed model was more accurate than the existing models.
Co-reporter:Zhongyu Lu, Guijun Xian
Construction and Building Materials 2017 Volume 150(Volume 150) pp:
Publication Date(Web):30 September 2017
DOI:10.1016/j.conbuildmat.2017.06.026
•Elevated temperature and sustained tensile loading caused irreversible interface damage.•Stress redistribution resulted in serious degradation in the tensile properties.•A design method considered exposure temperatures and sustained loading levels was proposed.Fiber reinforced polymer (FRP) composites have to face harsh environments and service loading conditions during practical use, causing their mechanical properties to degrade. Mitigating this degradation is critical for the safe design and application of FRP composites. In the present paper, the combined effects of sustained tensile loading and elevated temperatures on the mechanical properties of a pultruded basalt FRP (BFRP) plate were studied. The examined tensile stresses were set at 35%, 50% and 65% of its ultimate tensile strength (fu), and the exposure temperatures were room temperature, 80 °C, 120 °C and 160 °C. Strain was recorded during testing, and variations of the tensile and interlaminar shear (ILSS) properties were studied. The tensile strain evolution of BFRP specimens during sustained tensile loading depended largely on the exposure temperature due to the temperature sensitivity of the epoxy resin. As the exposure temperature increased above the glass transition temperature (Tg) of the composites (123.7 °C), a remarkable increase in strain was observed. Meanwhile, the residual tensile strength and tensile modulus decreased with the exposure temperature and sustained tensile loading level. The tensile strength and tensile modulus decreased by 41.8% and 30%, respectively, for the specimens exposed at 160 °C and 50% fu. Sustained tensile loading led to stress redistribution in BFRP specimens especially at elevated temperatures, leading to serious degradation of the interlaminar shear strength (ILSS). Dynamic mechanical thermal analysis was performed to investigate the interface damage. An equation for predicting the tensile strength retention of BFRP plates under elevated temperatures was also proposed.
Co-reporter:Yongming Yang, Guijun Xian, Hui Li, Lili Sui
Polymer Degradation and Stability 2015 Volume 118() pp:111-119
Publication Date(Web):August 2015
DOI:10.1016/j.polymdegradstab.2015.04.017
Fiber reinforced polymer (FRP) composites with anhydride cured epoxy resin matrices are widely used in civil engineering (e.g., pultruded FRP plates and bars), and their thermal aging behavior is a concern when they are subjected to elevated temperatures (e.g., FRP chimney). In the present article, thermal aging of an epoxy resin matrix at 130 °C–160 °C for 30 days was performed, and the effects on the flexural properties, molecular structures, free volume fraction, and mechanical properties were investigated. FTIR spectroscopy indicated that oxidation and molecular rearrangement occurred in the skin of the epoxy samples during thermal aging. Dynamic mechanical thermal analysis (DMTA) further illustrates the dominant effect of the molecular rearrangement in the sample skin with a thickness less than 100 μm, leading to a new high temperature tan δ peak. The free volume fraction of the skin and the bulk epoxy sample was characterized by positron annihilation lifetime spectroscopy (PALS). The results indicate that a noticeable reduction of the apparent free volume fraction occurred in the sample skin, while the bulk sample was only slightly affected. The flexural results indicate that thermal aging obviously reduced the break strain, while the flexural strength was only slightly affected and the modulus increased.
Co-reporter:Zhaohui Yang;Hui Li
Polymer Composites 2015 Volume 36( Issue 9) pp:1590-1596
Publication Date(Web):
DOI:10.1002/pc.23067
In this article, ramie fiber reinforced phenolic (RFRP) plates were prepared with compression molding process, and the plates were subjected to 98% humidity environment and alternating temperatures (from 25°C to 55°C in 24 h for a cycle) for 4 weeks. The resulted moisture absorption and the variation of the mechanical properties of RFRPs were studied. As found, compared to constant exposure temperatures (25°C or 60°C), alternating temperatures brought in higher moisture uptake and more serious degradation in the flexural strength, flexural modulus and short beam shear strength of the RFRP samples under the same humidity condition. The deteriorated effects of alternating temperatures is attributed to more remarkable degradation of the bonding between the fiber and resin, due to the moisture uptake and the internal cyclic stress around the ramie fibers with alternating temperatures. The flexural modulus of RFRP plates was much more susceptible to the moisture uptake than the flexural strength. After fully drying, the mechanical properties of the RFRP samples were recovered to some extent, but still less than the original values, indicating permanent damages occurred. Fiber Bragg grating sensors embedded in the RFRP plate was applied to monitor the variation of the internal strain during the exposure. As indicated, the moisture absorption and alternating temperatures bring in relaxation of the internal tension stress formed during compressing process, and decrease in the coefficient of thermal expansion of the RFRP samples. POLYM. COMPOS., 36:1590–1596, 2015. © 2014 Society of Plastics Engineers
Co-reporter:Yunfeng Pan, Guijun Xian, Manuel A.G. Silva
Construction and Building Materials 2015 Volume 101(Part 1) pp:326-337
Publication Date(Web):30 December 2015
DOI:10.1016/j.conbuildmat.2015.10.129
•The adhesive layer thickness and immersion affect the CFRP-concrete bond remarkably.•The water diffusion in multi-material systems was modeled by FEM.•The adhesive-concrete bond is susceptible to the moisture ingress.Effects of water immersion on the behavior of bond between CFRP plates and concrete substrate were experimentally investigated and analytically modeled. The finite element method was used to simulate the distribution of the moisture content in the interfacial zone. Single lap shear test results showed that water immersion decreases the maximum bond stress, bond capacity, and fracture energy as well as the maximum slip. Increasing the immersion time led to more moisture uptake in the adhesive layers, and more degradation of the bonding properties. The thickness of the adhesive layers (i.e., 0.2 mm and 1 mm) affects the bonding properties and the resistance to the water immersion. The thinner the adhesive layer, the higher moisture content is found at the adhesive/concrete interface. Water immersion altered the debonding mode from cohesive concrete fracture to adhesive separation from the concrete substrate, which was attributed to the weakening of the bonding strength between silica and epoxy adhesive due to moisture ingress.
Co-reporter:Zhongyu Lu, Guijun Xian, Hui Li
Composites Part B: Engineering 2015 Volume 77() pp:421-430
Publication Date(Web):August 2015
DOI:10.1016/j.compositesb.2015.03.066
In this article, a pultruded unidirectional basalt fiber-reinforced polymer (BFRP) plate was thermally aged at 135 °C and 300 °C for 4 h, and subsequently immersed in distilled water or strong alkaline solution (simulating concrete pore water, pH = 12.6–13) for 3 months. The variation of the tensile and interlaminar shear (ILSS) properties of the BFRP plates was studied. Thermal aging exhibited a slight effect on both the longitudinal tensile properties and the interlaminar shear strength, although thermal decomposition of the resin matrix started at 300 °C and brought in a high void content (4.8%). FTIR and DMTA results indicate that thermal aging lead to postcuring and oxidation of the resin matrix, leading to an increase of the glass transition temperatures. Thermal aging accelerated the degradation of the BFRP plates in distilled water or alkaline solution at 20, 40 and 60 °C. In the studied hash immersion conditions of 60 °C alkaline solution for 3 months, the unaged, 135 °C aged and 300 °C aged BFRP samples showed reduction in the tensile strength by 43.2%, 62.3% and 74.1%, respectively. The higher the thermal aging and immersion temperatures, the more deterioration of the mechanical properties occurred. Alkaline solution immersion showed more adverse effects compared to the distilled water. The detrimental effects of the thermal aging were attributed to the formation of voids and cracks through which water or alkaline solution tended to easily penetrate into the BFRPs. The degradation of the resin due to thermal aging and immersion was analyzed with dynamic mechanical thermal analysis and scanning electron microscopy analysis. The long term variation of the tensile strength of BFRPs was evaluated based on the Arrhenius equation.
Co-reporter:Zhongyu Lu, Guijun Xian, Hui Li
Polymer Degradation and Stability 2014 110() pp: 216-224
Publication Date(Web):
DOI:10.1016/j.polymdegradstab.2014.09.003
Co-reporter:Hui Li;Qi Lin ;Hui Zhang
Journal of Applied Polymer Science 2012 Volume 123( Issue 6) pp:3781-3788
Publication Date(Web):
DOI:10.1002/app.34870
Abstract
The freeze–thaw resistance of unidirectional glass-, carbon-, and basalt-fiber-reinforced polymer (GFRPs, CFRPs, and BFRPs, respectively) epoxy wet layups was investigated from −30 to 30°C in dry air. Embedded optic-fiber Bragg grating sensors were applied to monitor the variation of the internal strain during the freeze–thaw cycles, with which the coefficient of thermal expansion (CTE) was estimated. With the CTE values, the stresses developed in the matrix of the FRPs were calculated, and CFRPs were slightly higher than in the BFRP and GFRP cases. The freeze–thaw cycle showed a negligible effect on the tensile properties of both GFRP and BFRP but exhibited an adverse effect on CFRP, causing a reduction of 16% in the strength and 18% in the modulus after 90 freeze–thaw cycles. The susceptibility of the bonding between the carbon fibers and epoxy to the freeze–thaw cycles was assigned to the deterioration of CFRP. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012
