Static and dynamic material properties of CFRP/epoxy laminates

Carbon fiber reinforced polymer (CFRP) has been extensively used to strengthen structures owing to its outstanding mechanical properties. With an increasing threat from terrorist bombing attacks and accidental explosions, the application of CFRP has been extended to mitigate the effect of blast loading on structures. A better understanding of the dynamic material properties of CFRP/epoxy laminates at high strain rates is therefore needed for more reliable analysis and design of CFRP strengthened structures under dynamic loadings. In this study, the unidirectional tensile properties of CFRP (SikaWrap®-230C) and epoxy resin (Sikadur®-330) laminates is investigated experimentally over a wide range of strain rates. Quasi-static and low-speed tensile tests are conducted at strain rates varying from 7 × 10−5 s−1 to 0.07 s−1. Then, high-speed tensile tests are performed using a high-speed servo-hydraulic testing machine at strain rate from about 10 s−1 to 240 s−1. The testing results show that both the tensile strength and the stiffness of the CFRP/epoxy laminates are insensitive to loading speed when the strain rate is less than 50 s−1. However, when strain rate is over 50 s−1, both the tensile strength and the coupon stiffness increase with the increase of strain rate. High-speed camera images are used to assist inspecting the failure modes of CFRP/epoxy laminates. It is found that under high-strain rate tension CFRP/epoxy laminates fail differently from that at low-strain rate. The different failure mode is believed to contribute to the increment of laminate strength. The testing data are analyzed together with available testing results on CFRP/epoxy laminates at various strain rates. Empirical formulas of dynamic increase factor for CFRP material are derived for better prediction of material strength at various strain rates

A Novel Cloud Removal Method Based on Ihot and the Cloud Trajectories for Landsat Imagery

Cloud removal is significantly needed for enhancing the further utilization of Landsat imagery, since such optical remote sensing satellite images are inevitably contaminated by clouds. Clouds dynamically affect the signal transmission due to their different shapes, heights, and distribution. Generally, pixel replacement is the only and common method used to remove thick opaque clouds, and radiometric correction techniques has been widely adopted to remove the thin clouds. However, no methods can remove both thick and thin clouds at the same time. In this paper, a new method is proposed based on fitting “trajectory” of cloudy pixels with the help of IHOT spatially charactering clouds for pixel correction, which considers signal transmission including not only the additive reflectance from the clouds but also the energy attenuation when solar radiation passes through them. The experimental results show that the proposed approach performs effective removal for thick and thin clouds, and possesses the highest accuracy with the reference image, which can restore land cover information accurately

The mechanical properties of Polyvinyl Butyral (PVB) at high strain rates

Polyvinyl Butyral (PVB) has been largely used as an interlayer material for laminated glass to mitigate the hazard from shattered glass fragments, due to its excellent ductility and adhesive property with glass pane. With increasing threats from terrorist bombing and debris impact, the application of PVB laminated safety glass has been extended from quasi-static loading to impact and blast loading regimes, which has led to the requirement for a better understanding of PVB material properties at high strain rates. In this study, the mechanical properties of PVB are investigated experimentally over a wide range of strain rates. Firstly, quasi-static tensile tests is performed using conventional hydraulic machine at strain rates of 0.008–0.317 s−1. Then high-speed tensile test is carried out using a high-speed servo-hydraulic testing machine at strain rates from 8.7 s−1 to 1360 s−1. It is found that under quasi-static tensile loading, PVB behaves as a hyperelastic material and material property is influenced by loading rate. Under dynamic loading the response of PVB is characterized by a time-dependent nonlinear elastic behavior. The ductility of PVB reduces as strain rate increases. The testing results are consistent with available testing data on PVB material at various strain rates. Analysis is made on the testing data to form strain-rate dependent stress–strain curves of PVB under tension

Job burnout among primary healthcare workers during COVID-19 pandemic: cross-sectional study in China

ObjectiveThis study evaluated job burnout among primary healthcare workers (PHCWs) in China during the COVID-19 pandemic, explored its influencing factors, and examined PHCWs' preferences for reducing job burnout.MethodWe conducted a multicenter cross-sectional study in Heilongjiang, Sichuan, Anhui, Gansu, and Shandong Provinces. An electronic questionnaire survey was conducted through convenience sampling in communities from May to July 2022. We collected sociodemographic characteristics, job burnout level, job satisfaction, and preferred ways to reduce job burnout among PHCWs.ResultsThe job burnout rate among PHCWs in China was 59.87% (937/1565). Scores for each dimension of job burnout were lower among PHCWs who had a better work environment (emotional exhaustion OR: 0.60; depersonalization OR: 0.73; personal accomplishment OR: 0.76) and higher professional pride (emotional exhaustion OR: 0.63; depersonalization OR: 0.70; personal accomplishment OR: 0.44). PHCWs with higher work intensity (emotional exhaustion OR: 2.37; depersonalization OR: 1.34; personal accomplishment OR: 1.19) had higher scores in all job burnout dimensions. Improving work environments and raising salaries were the preferred ways for PHCWs to reduce job burnout.ConclusionStrategies should be developed to improve job satisfaction among PHCWs, enhance their professional identity, and alleviate burnout to ensure the effective operation of the healthcare system, especially during periods of overwork

Experimental investigation of resistance function of RC beam considering membrane effects

Membrane actions commonly exist in reinforced concrete (RC) elements under flexural deformation, which could significantly increase the ultimate flexural load-bearing capacity and potentially influence the damage mode of the RC element. Most design codes only treat membrane actions as a “hidden” safety factor without considering its influence on the resistance functions and failure modes. In this paper, an experimental investigation is conducted to study the membrane actions on the resistance behaviors of restrained RC beams. It is found that compressive membrane effect occurred at early stage of a fully restrained RC beam, which leads to amplified flexural bending resistance capacity. Diagonal shear crack is developed since the designed shear resistance is lower than the amplified flexural bending capacity. Under membrane actions, the damaged beam with shear crack could still carry the imposed load and develop further tensile membrane action until eventual failure. The resistance function of the restrained beam under combined membrane and shear damage is significantly altered as compared to the un-restrained reference beam that failed in flexural bending. A modified theoretical resistance function is proposed to consider both the membrane effects and diagonal shear damage. Comparison with testing data shows that the proposed model could accurately describe the resistance of fully restrained RC beams under combined shear and membrane actions.Membrane actions commonly exist in reinforced concrete (RC) elements under flexural deformation, which could significantly increase the ultimate flexural load-bearing capacity and potentially influence the damage mode of the RC element. Most design codes only treat membrane actions as a “hidden” safety factor without considering its influence on the resistance functions and failure modes. In this paper, an experimental investigation is conducted to study the membrane actions on the resistance behaviors of restrained RC beams. It is found that compressive membrane effect occurred at early stage of a fully restrained RC beam, which leads to amplified flexural bending resistance capacity. Diagonal shear crack is developed since the designed shear resistance is lower than the amplified flexural bending capacity. Under membrane actions, the damaged beam with shear crack could still carry the imposed load and develop further tensile membrane action until eventual failure. The resistance function of the restrained beam under combined membrane and shear damage is significantly altered as compared to the un-restrained reference beam that failed in flexural bending. A modified theoretical resistance function is proposed to consider both the membrane effects and diagonal shear damage. Comparison with testing data shows that the proposed model could accurately describe the resistance of fully restrained RC beams under combined shear and membrane actions

Improved analysis method for structural members subjected to blast loads considering strain hardening and softening effects

In analysis and design of structures subjected to blast loading, equivalent Single-Degree-of-Freedom (SDOF) method is commonly recommended in design guides. In this paper, improved analysis method based on SDOF models is proposed. Both flexural and direct shear behaviors of structures subjected to blast load are studied using equivalent SDOF systems. Methods of deriving flexural and direct shear resistance functions are introduced, of which strain hardening and softening effects are considered. To collocate with the improved SDOF models, the improved design charts accounting for strain hardening and softening are developed through systematical analysis of SDOF systems. To demonstrate the effectiveness of the proposed analysis method, a model validation is made through comparing the predictions with laboratory shock tube testing results on reinforced concrete (RC) columns. It is found that compared to the conventional approach with elastic and elastic-perfectly-plastic model, the elastic-plastic-hardening model provides more accurate predictions. Additional non-dimensional design charts considering various levels of elastic-plastic-hardening/softening resistance functions are developed to supplement those available in the design guides with elastic-perfectly-plastic resistance function only, which provide engineers with options to choose more appropriate resistance functions in design analysis

Discussion on the suitability of dynamic constitutive models for prediction of geopolymer concrete structural responses under blast and impact loading

Compared to ordinary Portland cement-based concrete (OPC), geopolymer concrete (GPC) is an environmental-friendly construction material because it is mixed by replacing Portland cement with industry wastes such as fly ash. Despite intensive researches in the last two decades, application of geopolymer concrete in construction is still rather limited. One of the reasons is the quasi-static and dynamic material properties of geopolymer concrete, which are different from those of ordinary Portland-cement concrete, are not well defined yet, and the available design guides for geopolymer concrete structures are very limited. The existing design guides and dynamic constitutive models for OPC cannot be directly employed in design analysis and numerical modelling of GPC structures subjected to blast and impact loads. This paper first summaries the available dynamic material testing data on GPC, compares them with the widely used dynamic constitutive models for OPC, and discusses the suitability of those models for modelling GPC structures. Then, based on the available GPC material testing data, new material constants for strength model, equation of state and dynamic increase factors are derived for GPC. Finally numerical modellings are carried out using these material models with modifications for GPC material to examine their accuracies in simulating the dynamic responses and damages of structural components made of GPC subjected to blast and impact loads

Improved resistance functions for RC elements accounting for compressive and tensile membrane actions

Membrane actions commonly present in reinforced concrete elements as a result of restrained boundary conditions and geometry of deformations, which could substantially improve the ultimate flexural load-resistance as compared to that using yield line theory. Nevertheless, most current design manuals do not consider membrane effect because of a short of proper analysis method. This paper proposed an improved resistance model for RC (reinforced concrete) elements which considers both compressive and tensile membrane actions. Firstly, the derivation of the proposed membrane model was presented in detail. It was then validated with available testing data, in which good agreement was found on the load-deflection relationship of RC element between the estimation using the proposed model and testing data. Combining with the equivalent SDOF (single-degree-of-freedom) analysis method, the dynamic responses of structural elements subjected to blast loads could be more accurately predicted as compared to the common elastic-perfectly-plastic resistance assumption in design guides. The proposed method was further verified with existing field blast testing results. Parametric studies were then carried out to examine the influences of critical design parameters for membrane behaviors including reinforcement ratio, span-to-depth ratio, and restraint stiffness. Last but not the least, based on the proposed analytical method a series of diagrams for modifying the design loading capacity estimated by UFC (Unified Facilities Criteria) design guides without considering the membrane effects were derived for more accurate and easy predictions of loading capacities in engineering applications

A consistent and corrected nighttime light dataset (CCNL 1992–2013) from DMSP-OLS data

Measurement(s) nighttime light intensity Technology Type(s) satellite radiomete

Free water effect on the dynamic compressive properties of mortar

This paper investigates the effect of free water on the dynamic compressive properties of mortar. Total-dried and full-saturated specimens are prepared through oven drying and water soaking processes. Common and high-porosity mortars are studied to examine the influence of porosity. Compression tests are carried out, covering strain rates between 1 × 10−6/s and 280/s. The test results show both dry and saturated mortar specimens are strain-rate sensitive. Free water results in softening effect in both common and high-porosity mortars at quasi-static and dynamic states. Retarding effect by pore water in the mortar is observed, which leads to lower longitudinal wave velocities in the saturated specimens. Water saturation increases the hydrostatic-pressure and leads to more severe water bursting; consequentially reduces the compressive strength. The experimental results demonstrate the free water deteriorates the compressive properties of mortars as a result of hydric expansion, which induces new cracks, especially in the high-porosity mortar