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Understanding how aging population and low fertility affect household energy consumption is important for optimizing household energy consumption and reaching effective policies. This paper studies the impacts of demographic transition on household energy consumption based on panel data of 30 provinces in China from 2005 to 2016. Child-age dependency rate (CDR) and old-age dependency rate (ODR) are selected to track the shifts in age structure. They are introduced into a STIRPAT model to measure their impacts on household energy consumption. Besides, 8 representative regions are additionally chosen and investigated to find some regional characteristics. The results show that current demographic transition to aging population expands household energy consumption. The aging population and low fertility cause additional challenges for energy saving and emission reduction. Household energy consumption in less developed areas is more likely to be affected by CDR and ODR. Regions with large population are also more easily influenced by demographic transitions especially CDR. This study emphases the effects of demographic elements on household energy consumption. It indicates that continuous optimization of household energy consumption structures should be based on population dynamics.

In this work, a constitutive model able to capture the strain rate dependency, small strain effects and the inherent anisotropy is proposed considering the influence of the overconsolidation ratio (OCR). Small strain effects are captured by using an extended ISA plasticity formulation (Fuentes and Triantafyllidis in Int J Numer Anal Methods Geomech 39(11):1235–1254, 2015). The strain rate dependency is reproduced by incorporating a third strain rate mechanism (in addition to the elastic and hypoplastic strain rate). A loading surface has been incorporated to define a three-dimensional (3D) overconsolidation ratio and to account for its effects on the simulations. Experimental investigations using Kaolin Clay and Lower Rhine Clay with horizontal bedding plane have shown that under undrained cycles of small strain amplitudes (<10-4), the effective stress path in the p–q space is significantly inclined towards the left upper corner of the p-q plane. Consequently, a transversely (hypo)elastic stiffness has been successfully formulated to capture this behaviour. The performance of the model has been inspected by simulating the database of approximately 50 cyclic undrained triaxial (CUT) tests on low-plasticity Kaolin Clay (Wichtmann and Triantafyllidis) considering different deviatoric stress amplitudes, initial stress ratios, displacement rate, overconsolidation ratio and cutting direction. Furthermore, 4 CUT tests conducted on high-plasticity Lower Rhine Clay were simulated, whereby the influence of the displacement rate, as well as the deviatoric stress amplitude, has been analysed. The simulations showed a good congruence with the experimental observations.

Background Polyurethane foams have many uses ranging from comfort fitting seats and shoes to protective inserts in helmets and sports equipment. Current military helmet designs employ foam pads of varying densities and bulk material properties to help absorb energy from impacts ranging from quasi-static to ballistic level strain-rates. Objective This study aims to analyze the thermomechanical uniaxial compression behavior of a high density liner foam pad and a low density liner foam pad used in the Advanced Combat Helmet. These experiments were conducted under strain-rates of 10 2  s - 1 and under temperature conditions ranging from -20 to 40 °C. This temperature range was chosen to simulate desert and arctic conditions, with a strain-rate regime chosen to represent loads that would occur often throughout the life of the helmet, such as drops, bumps from riding in a vehicle, or heavy collisions from falling. Method Multiple experimental apparatuses were used in this study, including a Shimadzu TCE-N300 thermostatic chamber (used to create the varying temperature environments) and a custom-built drop-test system (used to induce intermediate strain-rates). Every experiment was paired with two accelerometers and a high speed camera used for Digital Image Correlation (DIC) to analyze sample deformation and resultant acceleration. The foam’s mechanical response and energy absorption properties were investigated from the measured stress-strain curves. Additionally, each foam composition was analyzed with X-ray computed micro-tomography (XCT) to investigate microstructure properties pre and post-mortem. Results Results show that temperature decreased the energy absorption of the low density composition by 48% ± 5% as temperature changed from -20 °C to 40 °C, while energy absorption increased by 53% ± 16% for the high density composition over the same temperature. Conclusion A comparison between the loading response and the material’s density characteristics revealed that the foam’s mechanical properties are heavily dependent on strain-rate applications, as well as environmental factors including temperature. Several important characteristics surrounding each foam composition’s deformation mechanics and damage tolerance as a result of temperature are discussed.

This work deals with the strain-rate dependent characterization of paper under uniaxial tension at high strain-rates. Experiments were performed involving a Split Hopkinson bar for high strain-rate testing, comparing the results with conventional quasi-static tests. Tests were conducted in a strain-rate range between 0.0083 and 212 s −1 , which is equivalent to testing velocities between 0.0003 and roughly 13.6 m/s. For the first time the change in tensile behaviour of paper is comprehensively characterized and modelled, using the Cowper-Symonds model for strain-rate hardening. The experimental tests showed that the tensile strength as well as the initial stiffness were gradually increasing with increasing strain-rate. The increase in tensile strength between the lowest and the highest strain-rate was 58% on average whereas the mean increase in stiffness between these two strain-rates was almost 115%. Regarding the fracture strain, it was observed that it significantly decreases with increasing strain-rate. While the average fracture strain of the quasi-static tests was at roughly 6% it was close to 3% for the dynamic tests. In case of the Split Hopkinson bar tests, high-speed videos of the samples were made to determine their elongation via target tracking and digital image correlation (DIC). We found that strain localization, which is a highly relevant mechanism for quasi-static tensile failure, is likely related to short term plastic creep of the material as strain localization nearly entirely disappears at high loading rates of paper.

Developed countries implement migration policies to mitigate the negative effects of aging populations. While such policies can have positive economic impacts for the receiving countries, migrants often face difficulties in social and cultural adaptation. The migration policies of G8 countries illustrate this dilemma in various ways. Although countries that consider migration an effective policy tool have greater potential to manage the challenges of aging populations, it is crucial to maintain social and political balance during this process. Efforts to alleviate the effects of population aging, along with the socioeconomic costs borne by migrants, lead to non-linear changes in migration patterns. Given these non-linear dynamics, evaluating migration thresholds becomes particularly important. This study aims to estimate the optimal migration level that can help manage this dilemma. Specifically, it seeks to determine the maximum number of migrants that aging G8 countries can absorb using TAR models. One key finding is that migration flows below a certain threshold can positively influence the country’s absorption capacity and enhance labor productivity as younger migrants enter the workforce. The policy recommendations derived from these results are expected to provide valuable guidance to policymakers. Implementing these recommendations can help maintain economic stability while leveraging the benefits of migration to address the challenges posed by an aging population.

The strain rate-dependent mechanical behavior of shale is characterized using triaxial compression tests under a constant confining pressure of 50 MPa and axial strain rates \\[ _1\\] ranging from 5 × 10−6 s−1 to 1 × 10−3 s−1. This study is conducted on the Longmaxi shale from Dayou in China, which is predominantly composed of brittle minerals including quartz (55%), albite (15%) and cristobalite (3%). The experimental results show that higher axial loading strain rates \\[ _1\\] lead to higher elastic modulus and higher peak shear strength, both following exponential relationships with \\[ _1\\]. When \\[ _1 1 10^ - 5 s^ - 1\\], failure results in a single linear fracture, whereas a more complex multiple crisscrossing fracture network is formed when \\[ _1 1 10^ - 4 s^ - 1\\]. Failure in shale specimens can be described by a damage parameter \\[D\\], which is strongly affected by the axial strain \\[_1s\\]. In addition, the strain rate \\[ _1\\] had different effects on \\[D\\], which also depends on axial strain \\[_1s\\]. Energy accumulation and dissipation are also closely related to \\[ _1\\] with the total absorbed energy \\[U_A\\], the recoverable elastic strain energy \\[U_A^e\\] and the dissipated energy \\[U_A^d\\] at the peak stress increasing with \\[ _1\\]. As for the total energy accumulation \\[U_A\\], the recoverable elastic energy \\[U_A^e\\] decreases while the dissipated energy \\[U_A^d\\] increases with increasing strain rate.

Asperities within pre-existing fractures of coals can experience local damage during the fracture closure due to external loading. Previous research postulates that this local asperity damage can lead to strain rate-dependency without causing permanent deformation to the bulk of the coal specimens. This study aims to comprehensively investigate this behavior by developing a theoretical model that characterizes the strain rate-dependency driven by fracture asperity damage in coal. To achieve this objective, an initial series of micro-scale mechanical tests are conducted on joint specimens to establish a model for effective stress acting on asperities. Building upon this model, a theoretical foundation is further developed to describe the strain rate-dependent asperity damage evolution and resulting energy dissipation. These frameworks are subsequently incorporated into elasticity and damage mechanics to capture the strain rate-dependent stress–strain relationships. To validate the proposed model across multiple scales, additional triaxial tests on core-scale specimen and micro-scale mechanical tests on joint specimens are performed. The experimentally measured strain rate-dependency aligns well with the predictions of the proposed model, indicating a successful development of a robust model. The results of the model developed in this study reveal that the strain rate-dependency in fractured coals is governed by several factors, including asperity damage, mechanical properties of the coal specimens and effective stress acting on asperities of pre-existing fractures within the bulk of coal. Moreover, it is shown that the effective stress acting on asperities is significantly affected by both applied normal stress and joint roughness coefficient (JRC). The insights derived from this study demonstrate that the strain rate-dependency induced by micro-scale asperity damage of pre-existing fractures leads to observable strain rate-dependency in bulk specimens at core-scale and the proposed model can adequately capture this behavior.HighlightsMicro-scale mechanical tests are conducted on coal joint specimens to establish the equation for effective stress acting on joint asperities.A multi-scale theoretical model is developed for the strain rate dependency within coal resulting from fracture asperity damage.The proposed model is validated by experimental data obtained from multi-scale experiments.The effective stress acting on joint (fracture) asperities is governed by both the applied normal stress and JRC.The theoretical model indicates that micro-scale asperity damage leads to the strain rate dependency at core-scale level.

Owing to its excellent crack resistance and durability, High-Performance Fiber-Reinforced Concrete (HPFRC) has been extensively applied in engineering structures exposed to extreme loading conditions. The Mode I dynamic fracture strength of HPFRC under high-strain-rate conditions exhibits significant strain-rate sensitivity and nonlinear response characteristics. However, existing experimental methods for strength measurement are limited by high costs and the absence of standardized testing protocols. Meanwhile, conventional data-driven models for strength prediction struggle to achieve both high-precision prediction and physical interpretability. To address this, this study introduces a dynamic fracture strength prediction method based on a feature-weighted linear ensemble (FWL) mechanism. A comprehensive database comprising 161 sets of high-strain-rate test data on HPFRC fracture strength was first constructed. Key modeling variables were then identified through correlation analysis and an error-driven feature selection approach. Subsequently, six representative machine learning models (KNN, RF, SVR, LGBM, XGBoost, MLPNN) were employed as base learners to construct two types of ensemble models, FWL and Voting, enabling a systematic comparison of their performance. Finally, the predictive mechanisms of the models were analyzed for interpretability at both global and local scales using SHAP (SHapley Additive exPlanations) and LIME (Local Interpretable Model-agnostic Explanations) methods. The results demonstrate that the FWL model achieved optimal predictive performance on the test set (R2 = 0.908, RMSE = 2.632), significantly outperforming both individual models and the conventional ensemble method. Interpretability analysis revealed that strain rate and fiber volume fraction are the primary factors influencing dynamic fracture strength, with strain rate demonstrating a highly nonlinear response mechanism across different ranges. The integrated prediction framework developed in this study offers the combined advantages of high accuracy, robustness, and interpretability, providing a novel and effective approach for predicting the fracture behavior of HPFRC under high-strain-rate conditions.

Compression testing is the common approach to determining the properties of lattices. This study discusses various factors required to design a robust test protocol. A significant number of tests were conducted to evaluate sample geometry and test procedure, as well as data collection and analysis. Several well-known criteria including characteristic diagrams, collapse mechanism, and properties such as modulus of elasticity, yield and plateau stresses, and volumetric energy were considered for investigation. In addition to the macro-level study of size effect (number of cells) with these parameters, digital image correlation (DIC) is used to study convergence at the micro-level by analysis of cell deformation and strain. This is used to determine the size of the lattice sample which optimally represents it as a cellular material. It was demonstrated that the influence of face-plates (boundary effect) on the measured properties is significant. However, their presence helps the repeatability and consistency of the results. At the limited range of strain rates achievable by ordinary hydraulic test machines, the test condition remains quasi-static, and the rate dependency of measured property for this range is generally lower than 10%. In addition, limitations of using DIC were discussed as a suitable tool to analyze the deformation of lattices as cellular material. Some modifications for test set-up and data analysis were presented. This led to a reduction in time and cost by using 2D DIC instead of 3D DIC.

The mechanical properties and failure modes of concrete are affected by strain rate. Therefore, various experimental methods have been used to quantify the strain-rate dependency of concrete properties, such as the spalling test by using a Hopkinson bar. In the test, pullback velocity at the free-end surface of the specimen is usually measured to evaluate the dynamic tensile strength of concrete, instead of directly measuring the critical stresses at the damaged location(s) due to experimental constraints. Herein, such indirect measurements of tensile strength are compared with direct determinations of tensile strength based on lattice models of the spalling tests. To represent rate-dependent material behavior, rheological units are introduced within the lattice elements. The parameters of the rheological units are calibrated through comparisons with experimental data. The calibrated values remain unchanged in subsequent simulations, which can be regarded as virtual spalling tests at various high strain rates of loading. The separation of viscous and inertial contributions to apparent tensile strength provides insights into the dependence of actual tensile strength on high strain rates. The simulation results indicate indirectly measured dynamic tensile strength, as commonly done in practice, is sufficiently close to directly measured strength.