For conical indentation, the strain energy is a function of the semi-vertical cone angle, the indentation depth and the stress-strain relation. According to equivalent energy principle of ...representative volume elements (RVE) and the classical cavity assumption for material deformation region, the function with dual-parameters about volume and deformation is theoretically derived in the present study. This original equivalent-energy indentation model (EIM) is capable of forward-predicting load-depth relation and reverse-predicting uniaxial stress-strain relation for ductile materials only based on loading part of indentation. Further analyses show that the forward and reverse predicted results from EIM method are in excellent agreement with those by finite element analyses (FEA). Macro conical indentation experiments on five types of metals have been conducted using conventional indenters which are similar to Rockwell sclerometer. Consequently, the stress-strain relations predicted by EIM are quite close to those from standard tensile tests.
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•A strain energy equivalence principle is originally proposed.•A unified model relating load, displacement, and material properties is proposed.•For the eight SCs analyzed in this study, the unified ...model predictions agree well with the FEA results.•By using this model, required material properties could be easily obtained by self-designed SCs.
A unified elastoplastic model was proposed to describe the relation among load, displacement, and uniaxial constitutive parameters of ductile materials according to the von Mises energy equivalence principle at a special location or energy center in the deformed region of a structural component (SC). Two pairs of parameters were considered in the model: one was related to the volume of deformed region and the other to the Mises equivalent strain at the energy center. In addition, they are easily determined by finite element analysis (FEA). For eight kinds of SCs under proportional loading, the load–displacement behaviors of various materials predicted by the unified model were highly consistent with the results of FEA.
The EIM (equivalent-energy indentation method) based on equivalent energy principle is proposed to determine the stress-strain relations of materials via spherical indentation. For various materials ...in power hardening law, plenty of finite element simulations were conducted to verify this novel method. The numerical verification shows the forward-predicted load-depth curves and the reverse-predicted stress-strain curves by EIM agree with the results from FEA (finite element analysis). For different light alloys, the stress-strain curves predicted by the method are consistent with the tensile results of the alloys. Additionally, a comparison between EIM and the existing ABI (automatic ball indentation) is conducted as well.
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•Based on EEP, the EIM correlating strain energy, depth, tip diameter and Hollomon parameters is proposed.•For wide range of imaginary materials, forward-predictions and reverse-predictions from EIM agree well with FEA results.•For various light alloys, tensile properties predicted by EIM are closed to those from tensile tests.
Based on the power-law stress–strain relation and equivalent energy principle, theoretical equations for converting between Brinell hardness (HB), Rockwell hardness (HR), and Vickers hardness (HV) ...were established. Combining the pre-existing relation between the tensile strength (
σ
b
) and Hollomon parameters (
K
,
N
), theoretical conversions between hardness (HB/HR/HV) and tensile strength (
σ
b
) were obtained as well. In addition, to confirm the pre-existing
σ
b
-(
K
,
N
) relation, a large number of uniaxial tensile tests were conducted in various ductile materials. Finally, to verify the theoretical conversions, plenty of statistical data listed in ASTM and ISO standards were adopted to test the robustness of the converting equations with various hardness and tensile strength. The results show that both hardness conversions and hardness-strength conversions calculated from the theoretical equations accord well with the standard data.
Fiber reinforced composite laminates have been increasingly replacing conventional materials in various manufacturing sectors due to their extremely superior mechanical properties. Usually, ...mechanical drilling is an important final manufacturing process for composite laminates, whereas drilling of high-strength composite laminates is very challenging and difficult. As the most undesirable damage and challenging failure mode, drilling-induced delamination for fiber reinforced composite laminates is a hot research area of immerse engineering importance. A review on the path towards delamination-free drilling for composite laminates can significantly help researchers improve currently-available cost-effective drilling process and develop high performance drilling process. This review paper summarizes an up-to-date progress in drilling-induced delamination for composite laminates reported in the literature. It covers delamination formation mechanism, delamination quantification methodologies and measurement technologies, delamination suppression strategies (including tool design optimization, drilling conditions optimization and high performance drilling methods). This general review of drilling-induced delamination for composite laminates can be referenced as not only a summary of the current results from literature survey but also future work possibilities, giving the researchers the opportunity to deepen specific aspects and explore new aspects for reaching delamination-free drilling for composite laminates.
•An elastoplastic energy model about energy, load, displacement and materials properties is proposed.•For thirteen SCs, the model predictions agree well with the FEA results under linear elastic, ...fully plastic and elastoplastic conditions.•For cracked SCs, the J- integral vs. load relation is derived and verified based on the EEM.•The model has been applied into materials testing like indentation, small punch test and ring compression etc.
Elastoplastic load-displacement relation is widely concerned in material testing. For isotropic-homogeneous and power-law hardening ductile materials, an elastoplastic energy model (EEM) correlating energy, load, displacement and uniaxial constitutive parameters is derived based on equivalent energy principle. An elastoplastic factor λ for engineering superposition of elastic displacement and plastic displacement is introduced and discussed, which makes the model applicable with more structural components (SCs). The model is verified with thirteen SCs used in materials testing and the results show a good agreement between model predictions and calculations from finite element analysis (FEA) under linear elastic, fully plastic and elastoplastic conditions. Some experimental results for ring-compression, spherical indentation and funnel tension are conducted to verify the model and a valid accordance is presented. Additionally, an explicit J-integral – load relation for classic cracked SCs is also derived based on the EEM and a good coincidence is observed during a comparison with results directly from FEA.
The bulk morphology of the active layer of organic solar cells (OSCs) is known to be crucial to the device performance. The thin film device structure breaks the symmetry into the in-plane direction ...and out-of-plane direction with respect to the substrate, leading to an intrinsic anisotropy in the bulk morphology. However, the characterization of out-of-plane nanomorphology within the active layer remains a grand challenge. Here, we utilized an X-ray scattering technique, Grazing-incident Transmission Small-angle X-ray Scattering (GTSAXS), to uncover this new morphology dimension. This technique was implemented on the model systems based on fullerene derivative (P3HT:PC
BM) and non-fullerene systems (PBDBT:ITIC, PM6:Y6), which demonstrated the successful extraction of the quantitative out-of-plane acceptor domain size of OSC systems. The detected in-plane and out-of-plane domain sizes show strong correlations with the device performance, particularly in terms of exciton dissociation and charge transfer. With the help of GTSAXS, one could obtain a more fundamental perception about the three-dimensional nanomorphology and new angles for morphology control strategies towards highly efficient photovoltaic devices.
Urban boundaries, an essential property of cities, are widely used in many urban studies. However, extracting urban boundaries from satellite images is still a great challenge, especially at a global ...scale and a fine resolution. In this study, we developed an automatic delineation framework to generate a multi-temporal dataset of global urban boundaries (GUB) using 30 m global artificial impervious area (GAIA) data. First, we delineated an initial urban boundary by filling inner non-urban areas of each city. A kernel density estimation approach and cellular-automata based urban growth modeling were jointly used in this step. Second, we improved the initial urban boundaries around urban fringe areas, using a morphological approach by dilating and eroding the derived urban extent. We implemented this delineation on the Google Earth Engine platform and generated a 30 m resolution global urban boundary dataset in seven representative years (i.e. 1990, 1995, 2000, 2005, 2010, 2015, and 2018). Our extracted urban boundaries show a good agreement with results derived from nighttime light data and human interpretation, and they can well delineate the urban extent of cities when compared with high-resolution Google Earth images. The total area of 65 582 GUBs, each of which exceeds 1 km2, is 809 664 km2 in 2018. The impervious surface areas account for approximately 60% of the total. From 1990 to 2018, the proportion of impervious areas in delineated boundaries increased from 53% to 60%, suggesting a compact urban growth over the past decades. We found that the United States has the highest per capita urban area (i.e. more than 900 m2) among the top 10 most urbanized nations in 2018. This dataset provides a physical boundary of urban areas that can be used to study the impact of urbanization on food security, biodiversity, climate change, and urban health. The GUB dataset can be accessed from http://data.ess.tsinghua.edu.cn.
•Proposed a novel model for solving the C*-integral of cracked specimens.•The models are verified using FEA and ASTM methods.•New model promotes the development of creep crack growth rate testing ...methods.
The energy rate line integral C* is an essential fracture parameter that characterizes the stress and strain rate field intensity at a crack tip for materials undergoing creep deformation, and is also a crucial parameter describing the creep crack growth rate. This study proposes a method for determining the characteristic parameters of the creep displacement rate model for specimens with mode-I crack under various constraints. Based on the displacement rate model, a semi-analytical model of the C*-integral related to the displacement rate, load, specimen sizes, and Norton's law parameters of materials is proposed. Furthermore, finite element analysis (FEA) models were established for compact tension (CT), single-edged notched bending (SEB), C-shaped tension (CST), and single-edge notched tension (SET) specimens, and the proposed models were verified using FEA results and those predicted by the ASTM method. The explicit correlation between the creep displacement rate and crack length a in the model can contribute to developing novel testing methods for real-time crack length and creep crack growth rate.
An energy‐based spherical indentation (ESI) model according to equivalent energy principle is developed to determine the stress–strain relation, tensile strength and hardness of steels. Several ...parameters in the model are determined with finite element analysis (FEA). For a wide range of homogeneous and isotropic materials in power stress–strain law, a large number of FEA calculations are carried out to verify the ESI model. Results show that both the forward‐predicted load‐depth relations and the reverse‐predicted stress–strain relations from the model agree well with the results from FEA. For 12 steels, the tensile properties, Rockwell and Brinell hardness predicted by the ESI model are close to the standard testing results.
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