Hongjiu Hu

Structural health monitoring of composite wind blades by fiber bragg grating

The wind turbine industry is the fastest growing market area for the use of composite materials. Fiber Bragg grating sensor can be used to monitor the mechanical behavior of composite wind blade. The internal strain of composite wind blade during a constant stress amplitude fatigue testing process was monitored with fiber Bragg gratings sensors. FBG sensors can not only be embedded in composite structures to detect fatigue damage, but also have excellent durability compared with other sensors such as electric strain gauges. After 1 million cycles, the FBG sensors can still keep good sensibility. FBGs as a fatigue indicator are a novel sensor to monitor, evaluate and give crash alert for the health state of composite wind blades during their whole service life.

Performance of the fiber Bragg grating sensors at cryogenic temperatures

The strain and temperature sensing performance of FBG are studied in the temperature range of 123K to 273K by the experiment. The temperature sensitive, strain coefficient and cross-sensitivity coefficient of the sensors are derived analytically and verified by experiments. It is found that they are nonlinear. Cross sensitivity increase with lower temperature. At a constant temperature, the Bragg wavelength shift is linear with the longitudinal strain within the range of 5000 micro strains.

Theoretical and experimental characteristics on residual stresses of advanced polymer composites

The residual stresses and mechanisms causing residual stresses in thermoset polymer composites were considered. The relative importance of the different mechanisms was analyzed. The residual stresses were determined analytically by viscoelastic model in addition to an experiment. The linear viscoelastic model was used to calculate of residual stresses in each layer of laminated composites. The fiber Bragg grating (FBG) strain sensor was used to measure the residual stresses throughout cure. The results are agreed well. The viscoelasticity of composites should be considered during calculating the residual stresses, and FBG strain sensor is shown to be a reliable for an accurate measurement of the residual stresses.

Mechanical insights into the stability of heterogeneous solid electrolyte interphase on an electrode particle

Solid electrolyte interphase (SEI) is known to be heterogeneous which comprises inorganic and organic decomposition products. To assess the effects of heterogeneity on the stability of the SEI, we establish a mathematical model in simulating the stress of heterogeneous SEI. Comparing with the analytical solution of stress in the homogeneous film, the heterogeneity of SEI is identified to be essential in the stress calculation of the inorganic layer, which is proven decisive for the stability of SEI. Further, the peak tensile stress within the inorganic layer is found in the bilayer SEI. It generates at the interface between the active material and the inorganic layer when the battery is fully charged. In addition, the impacts of the interface parameter, modulus, Poisson’s ratio, thickness of SEI and lithiation properties of active material on SEI stress are systematically investigated. Based on the simulation results, this work provides insights into the stress analysis of the heterogeneous SEI, as well as suggestions in pursuing a well-designed SEI, i.e., a functional gradient heterogeneous SEI in which the inner layer close to the active material provides sufficient mechanical performance while the outer layer near the electrolyte performs good chemical and electrochemical behaviors, to enhance the battery performance.

Revealing the Mechanical Properties of Emulsion Polymer Isocyanate Film in Humid Environments

Knowledge of the mechanical behaviors of polymer film in humid environments is of great significance for predicting the long-term performance of emulsion polymer isocyanate (EPI) as a high-performance wood adhesive. A tri-copolymer latex was cross-linked by the general polymeric methylene diisocyanate (p-MDI) and aqueous emulsified isocyanate (EMDI) at different loadings for preparing EPI. Furthermore, a series of uniaxial tension tests under different relative humidity (RH) were carried out on cured EPI samples before and after post-curing treatment, and the corresponding chemical structure, as well as the microstructure of polymers, was investigated in detail. In addition, a constitutive equation was formulated to calculate the viscoelastic characteristics of the adhesive layer. The results indicate that the EPI films reveal various kinds of intrinsic deformation as RH increases, and the tensile rupture stress and stiffness would obviously decrease, even at cross-linker weight ratios of up to 20%. Furthermore, the moisture resistance could be markedly improved by increasing the isocyanate content and post-cure. Importantly, EMDI-cross-linked film not only exhibits much better mechanical properties than that containing p-MDI at 0–80% RH, but is also more sensitive to post-cure. Finally, the derived viscoelastic model could efficiently track moisture-dependent stress-strain curves of EPI films, and the obtained relaxation time further reveals the influence mechanism of isocyanate and post-cure on the mechanical response of the cured polymer under moist conditions.

Model for temperature-dependence modulus of glass/epoxy composite

The effect of elevated temperature on the modulus of glass/epoxy composites was studied. Dynamic mechanic analysis (DMA) was carried out to investigate the storage modulus, loss modulus, loss factor and glass transition temperature. Static flexural modulus was also tested by control force mode in DMA. The effect of elevated temperature on the modulus of the composites was evaluated between 30°C and 120°C. A new temperature-dependent model both for the dynamic storage modulus and static flexural modulus was proposed. The model only needed one parameter to fit the experiment data and showed good accordance with experimental results. The physical meaning for the parameter of the model was given.