Dynamic Laboratory Tests Research Articles

Three-dimensional discrete element simulation on degradation of air voids in double-layer porous asphalt pavement under traffic loading

Porous asphalt pavement is credited with a variety of environmental benefits, including improvement of stormwater management, enhancement of underground water quality, mitigation of heat-island effect, and reduction of traffic noise. However, the sustainability of these functions is a major practical concern as they are seriously affected by clogging in the porous pavement. This study proposed three-dimensional discrete element simulation to evaluate degradation of air voids in the double-layer porous pavement under traffic loading. Double-layer porous asphalt pavement having 25 mm upper layer with 20% porosity and 40 mm lower layer with 22% porosity was designed. Temperature-dependent micromechanical parameters of contact models in asphalt mastic were determined by conducting laboratory dynamic creep test. The virtual wheel tracking test was verified and performed on the virtual double-layer porous asphalt pavement specimen. It has demonstrated that the connected voids content is decreased by 19.8%, 30.2%, and 1.7% in the whole specimen, within the loading and non-loading areas of the specimen, respectively as strain is increased from 0.01 to 0.12 during the wheel tracking test. Furthermore, the long-term degradation model of air voids in the double-layer porous asphalt pavement under traffic loading was formulated. At service life of 20 years, the connected voids content is decreased by 9.9%, 16.2%, and 0.7% in the whole specimen, within the loading and non-loading areas of the specimen under light traffic, respectively. When subjected to medium, heavy, and extra heavy traffic for 20 years, the connected voids content within the loading region of the specimen is reduced by 18.2%, 19.5%, and 20.4%, respectively. The long-term degradation of air voids in the double-layer porous asphalt pavement has important implications on its performance and functionalities. This study develops a methodology to predict air voids degradation of the double-layer porous asphalt pavement under traffic loading, which provides useful information for pavement engineers in the design of porous asphalt pavement.

Effects of Clay Mineral Composition on the Dynamic Properties and Fabric of Artificial Marine Clay

Marine clays are easily affected by different mineral composition in cyclic load-based geological hazards. Therefore, based on analyzing the mineral composition of natural marine clay, it is the key to predict the dynamic properties of natural materials under cyclic loading by using quantitated artificial marine clay. In this study, the marine clay found in the South China Sea deltas was investigated. Based on the results of geological conditions and mineral composition analyses, raw non-clay minerals (such as quartz, albite) and clay minerals (such as Na-montmorillonite and kaolinite) were used to produce artificial marine clay, the dynamic properties of which were studied from the impact of mineral composition. Dynamic triaxial laboratory testing for artificial marine clay comprising various clay minerals was performed under identical test conditions. The artificial marine clay with high montmorillonite content exhibited slower development of strain, more sluggish growth in pore water pressure, more rounded hysteresis curves, greater stiffness, and more prolonged viscous energy growth than the clay with low montmorillonite content. In addition, the flocculated fabric of the artificial marine clay with high montmorillonite content demonstrated sufficient pore space changes, more uniform pore distribution, and larger specific surface area than the dispersed fabric of the clay with low montmorillonite content. The factors arising from the influence of montmorillonite may lead to microstructural and fabric changes, hinder the development of pore water, and increase intergranular contact stiffness as well as delay the cyclic strain amplitude at the breakpoint of viscous energy dissipation. In general, the results presented in this study confirm that clay minerals, especially montmorillonite, have significant influence on the dynamic properties of large strain.

Development of an assistive device for adaptive cycling

Abstract Background Biomechanics literature has widely recognised the importance of creating modifying conditions for Sports for Persons with Disabilities, such as assistive devices. In this context, it is key that the motor function capacity and physical limitations of each person be taken into due account, leading to highly personalised equipment. This project optimised an assistive device for an adaptive cyclist that rides only with the right leg due to a transtibial amputation on the left leg, which remains supported by the assistive device, classifying him as C2 category for competitions, according to Union Cycliste Internationale (UCI). The purpose was to improve the comfort, ergonomics, and safety of the cyclist. Methods Reverse engineering, 3D modelling and manufacturing were the methodology applied to design the device with a crank-arm, a quick step-in pedal type clamp system, a carbon fibre composite shell, adapting the pedal position to support the cyclist stump, and an aluminium alloy 7075-T6 component to anchor to the seat post, providing stability to his body. Results The device which provided stability to the athlete's body was submitted to static and dynamic laboratory tests and real-life (in-track) try-outs. The developed clump system guarantees the athlete’s comfort and safety by automatically detaching him from the bicycle in case of fall. Conclusions The assistive device proved to be determinant for the athlete’s performance due the improvement of the comfort, ergonomics, and safety provided to the cyclist. Thus, qualifying him for the Tokyo 2021 Paralympic Games.

STUDY OF NEGATIVE INFLUENCE OF PETROLEUM PRODUCTS ON METAL ELEMENTS OF RAILWAY INFRASTRUCTURE

Purpose. The main purpose of the work is to improve the safety of railway transport operation. The implementation of this purpose is provided by assessing and preventing the negative influence of petroleum products on the structural elements of the railway infrastructure. Methodology. The main criterion for assessing the residual life of technical elements of equipment is their strength characteristics. One of the key factors affecting the suitability of technical devices during operation is corrosion damage to the metal. Therefore, the influence of various combinations of the mineralization degree and concentration of petroleum products on the corrosion rate of metals most often used in the design of cooling systems for diesel engines and other transport objects was studied. Thus, for carrying out static and dynamic laboratory tests, model working solutions with different salinity (demineralization model) and different concentrations of dissolved petroleum products (model of standard solutions with organic impurities) were selected. Dynamic and static studies of corrosion damage were carried out by the gravimetric method and the method of polarization resistance using standard samples and solutions. Findings. Based on the results of experimental studies of the authors and analytical data processing, generalizing model dependences of the corrosion rate on the mineralization degree of the working solution of the fractional composition of petroleum products were obtained. A study was carried out and the influence of the water demineralization degree on the corrosion rate of materials of diesel cooling systems was established. Originality. Based on a wide range of author's experimental data, the presence of generalizing dependences of the corrosion rate on temperature, the mineralization degree of the working solution and the fraction of the petroleum products composition has been shown and proven. The obtained dependences form the basis of the developed mathematical model of corrosion, which is a set of relationships linking the characteristics of the corrosion process with various factors influencing its development. Practical value. The obtained experimental and analytical data can be widely used for non-destructive testing procedures, detailed prediction of the state of structural elements and the selection of effective inhibitors to reduce the corrosive aggressiveness of the environment and protect structures.

Dynamic Identification of Tensile Force in Tie-Rods by Interferometric Radar Measurements

An experimental investigation on the accuracy of dynamically determined tensile force in tie-rods by applying the interferometric radar technique was performed. Tie-rods were used in historical masonry constructions for absorbing thrusts of arches and vaults, and the radar interferometry may represent a fast and easy non-destructive approach for the tensile force identification in the occasion of structural assessments. Laboratory dynamic tests on a cable under a known tensile force show that, provided that a suitable dynamic identification model is used, tensile force evaluations made stating from interferometric radar measurements were characterized by a very good accuracy (mean error in the tensile force estimation less than 2%), comparable with evaluations made starting from accelerometric measurements. In particular, the dynamic identification model considered is a modified version of a model proposed in the literature. The influence on the accuracy in the determination of the tensile force of some features of the experimental setup, like, e.g., the employ of corner reflectors, is discussed.

Investigation of interlocking effect of crushed stone ballast during dynamic loading

The present paper deals with the experimental investigation of interlocking effect of crushed stone ballast material, assessing it as the relationship with the residual and dynamic stresses under the ballast layer during laboratory dynamic tests with the consideration of different boundary conditions. The laboratory experiments were executed with a scaled model of ballast under the sleeper. The measured pressure at the bottom surface of the ballast has two parts: dynamic and residual. The dynamic part depends on the external loading; the residual part remains after unloading. The measured residual stress was observed up to 3 times higher than the stress due to cyclic external loading. The relationship of the residual stress and interlocking effect to ballast particles angularity is analyzed. A simple interpretation of the distribution of residual stress is proposed, that depends on the measured cyclic stress and the elasticity of bounding walls. The study of interlocking effect of ballast could be potentially useful for many practical problems of railway track design as well as for the track maintenance issues.

Advanced full-scale laboratory dynamic load testing of a ballasted high-speed railway track

Understanding ballast layer dynamic response and long-term behavior under moving train loads is necessary for optimization of railway track performance. This paper discussed an advanced testing effort undertaken in a controlled laboratory environment for the measurement of dynamic load responses via multitude of installed sensors and the development of validated computer models and simulation tools suitable for analyzing full-scale ballasted track under dynamic loading. Constructed in a large rectangular metal frame, the laboratory model consisted of a full-scale track with eight ties, ballast, subballast, and embankment. Three different speeds and axle load configurations were applied sequentially onto the full-scale ballasted track using eight actuators, which realistically captured both slow moving freight and high-speed passenger train loads. Vibration velocities were captured at different locations on ballast surfaces and ties. Dynamic soil stresses were recorded at the bottom of the ballast and subballast layers. Both transient and permanent deformations due to repeated dynamic loads were measured in the track substructure layers and for all eight ties. “SmartRock” advanced sensors were used for the first time to track ballast particle movements under dynamic loading. Further, the test data from the experiments were compared with preliminary Discrete Element Method (DEM) simulations.

HYDROPHOBIZATION OF BASALT FIBER AND ITS INFLUENCE ON THE MECHANICAL CHARACTERISTICS OF SAND CONCRETE

Abstract. The materials of the proposed article are devoted to the study of mechanical properties of sand concrete with the addition of hydrophobized basalt fiber and polycarboxylate superplasticizer Relaxol-Super PC. Adding hydrophobic properties to the basalt fiber causes a decrease of water consumption of fine-grained concrete mixture, which leads to improved mechanical properties of concrete. The aim of the work was to increase the mechanical characteristics of sand concrete by introducing hydrophobized basalt fiber into its composition. The objective of the research is to study the effect of hydrophobized basalt fiber on the mechanical characteristics of sand concrete. The polycarboxylate superplasticizer Relaxol-Super PC (Budindustriya, Zaporozhye) was used to increase the mobility of the concrete mixture. Basalt fiber Bauson-basalt 12 mm long and 18 ± 2μm in diameter was used as a fibrous filler. Sand concrete mixture was prepared in a laboratory forced-action mixer. Dosing of Portland cement, quartz sand and basalt fiber was carried out by weight, water and water-reducing additive ‒ by volume, taking into account the density of the additive. The fiber was introduced into a dry cement-sand mixture. After mixing for 120 ... 150 seconds, water with a dosed amount of additive was introduced into the mixture. The hardening of samples concrete took place under normal conditions in a chamber with a temperature of 20 ± 20C and a relative humidity of at least 95%. The compressive strength of concrete was determined by testing the halves of the samples – beams 4×4×16 cm in size at 28 days of age. The abrasion of the investigated concrete was determined by testing cube specimens with an edge of 7.07 cm on an LKI-3 device in accordance with the procedure set forth in DSTU B.V.2.7-212: 2009 “Building materials. Concrete. Methods for determining abrasion “. The impact resistance of concrete was determined from the results of testing cubic specimens with an edge of 7.07 cm on a vertical dynamic laboratory test machine. Especially effective is manifested positive role hydrophobization basalt fiber in combination with the water-reducing additive Relaxol-Super PC. The introduction of hydrophobic fiber (2 kg/m3) and Relaxol – Super PC (1.2% by weight of cement) into the sand concrete mix provides an increase in the strength of sand concrete by 45 ... 48%, impact resistance by 45 ... 50%. The abrasion of concrete is reduced by 36 ... 48% compared to the control.

Research Concerning the Impact of Geometrical Characteristics of Natural Quarry Aggregates Used in the Composition of Asphalt Concrete

The paper presents studies and laboratory tests concerning the influence of the way the natural quarry aggregates are processed upon the composition of an asphaltic concrete with a maximum grain size of 16 mm (BA 16). The research included the realization of grading curves on certain aggregates, curves lately compared to the maximum density line. This approach aims at classifying certain aspects concerning some of the physical-mechanical characteristics of the asphalt concretes determined through static and dynamic laboratory tests. The research resulted from the fact that in Romania, the elementary gradings on granular fractions from the same source, entering the composition of a certain asphalt mix, differ most of the times depending on the manipulation and the processing manner, respectively. This finding is based on the experience of the laboratory testing, where different values concerning the volume weight, apparent volume weight, voids and rigidity module were obtained on the same asphalt mix dosage prepared according to the prescription in two separate samples.Thus, in order to emphasize the impact of the processing manner (crushing) of the natural aggregates, the authors determined the percentage distribution on intermediate granular fractions of the gradings entering the composition of an asphalt concrete. Since the Romanian Standard for asphalt mixtures stipulates a large grading envelope between the standard sizes corresponding to each grading, the authors considered that there is no uniform distribution ratio depending on the size of the grains entering its composition. The study started from different research methods found in the specialized literature dealing with the calibration of the optimal grading curve. The results of the laboratory tests performed on the asphalt mixture confirm the importance of the particle size analysis in obtaining high quality physical-mechanical characteristics.

Evaluation of Mathematical Solutions for the Determination of Buckling of Columns Under Self-weight

Investigation of mathematical solutions for including the self-weight of a column for calculating buckling critical load. The complexity of including self-weight resides in the fact that the mathematical formulation depends on elliptical integral solutions related to differential equations of the problem. Comparing the analytical solutions with computation modeling. The obtained results with these computational models are approximate since they depend on the discretization of the domain of the problem while analytical solutions tend to be exact. An experimental activity was carried out for analyzing mathematical accuracy. The investigation of buckling was performed through static as well as dynamic analysis. The results found shown differences smaller than 5% among the mathematical investigated solution. Besides that, all solutions represented well the problem in comparison with a laboratory test. It has been evaluated solutions for stability analyses of bars under self-weight, using Euler–Greenhill analytical approach, solutions with the eigenvalues from FEM analysis, and also a dynamic solution based on the Rayleigh’s method, comparing them with dynamic laboratory tests. Can be concluded that the mathematical ways and the experimental investigation had a good agreement. IV, evidence obtained from multiple mathematical methods, static and dynamic, is experimentally confirmed.

Poro-mechanical model of strain hysteresis due to cyclic water freezing in partially saturated porous media

The paper presents a novel mathematical model of coupled hygro-thermo-mechanical processes in a porous material, partially saturated with liquid water and exposed to temperatures below the freezing point of pore water. Water – ice phase change is modelled by means of a non-equilibrium approach considering both water supercooling and a hysteresis of ice content during freezing and thawing of moist porous materials. The hysteresis results in different crystallization pressure and material strains during freezing and thawing processes at a given temperature. The latter effect is modelled by means of the effective stress principle, considering crystallization pressure of ice in the material pores. Methods used for discretization of the model equations and their numerical solution are described.The model is applied for solving the numerical example dealing with laboratory Dynamic Mechanical Analysis test of two different cement mortars saturated with water and exposed to temperatures below the freezing point of water (down to −15 °C) where hysteresis of strains was observed. The results are used for experimental validation of the proposed model. Then, for a 1-D case concerning water freezing-thawing of a wall, the effects on the simulation results of the phase-change model parameters, of the material hygro-thermal state, of the supercooling phenomenon and finally of the rate of temperature variation, are analyzed and discussed.

Super-selective transarterial chemoembolization of hepatocellular carcinoma with doxorubicin-eluting beads sized 40–75 microns: assessment of efficacy and safety

We aimed to evaluate the effectiveness and safety of super-selective transarterial chemoembolization (TACE) with doxorubicin-loaded drug-eluting beads (DEB) sized 40-75 μm for hepatocellular carcinoma (HCC) in early and intermediate stages according to Barcelona Clinic Liver Cancer (BCLC) staging system. This single-center retrospective study was conducted with 45 consecutive HCC patients treated by 72 sessions of DEB-TACE during the 2012-2017 period. Thirty-seven patients (82.2%) had single tumor staged BCLC A and B (53.3% and 46.7%, respectively). All procedures were performed by super-selective approach using 1.7-2.0 F microcatheters. Cone beam CT was performed to detect all tumor-feeding arteries and assess the treatment results immediately. Dynamic MRI and laboratory tests were obtained at 1-month follow-up and every 3 months thereafter. Response to treatment according to modified Response Evaluation Criteria in Solid Tumors, demographic and clinical status, laboratory tests, time-to-event durations and rates, complications according to the National Cancer Institute Criteria for Adverse Events were evaluated. A total of 45 patients (median age, 65.6 years; range, 35-88 years; 33 men, 73.4%) were included. Eight patients (17.7%) underwent liver transplantation after DEB-TACE, and 20 (44.4%) died during the follow-up period. Median follow-up was 22 months (range, 13-31), and 42 (93.3%) patients were followed up for more than 1 year. Overall complete response, partial response, and progressive disease rates were 53.3%, 33.3%, and 13.4% at one year and 22.2%, 26.7%, and 13.3% at three years, respectively. For target lesions, these rates were 60.0%, 26.7%, and 13.3% at one year and 28.9%, 6.7%, and 4.4% at three years, respectively. Median overall survival (OS) duration was 24 months (95% CI, 20.9-31.9 months). At one year and three years, OS rates were 71.0% and 44.4%, respectively. The only statistically significant relationship with OS was presence of chronic liver disease, which worsened the OS rate (P = 0.031). Time-to-progression (TTP) was 23 months (95% CI, 15.1-40.0), and progression-free survival (PFS) was 28 months (95% CI, 6.2-39.8). Post-embolization syndrome occurred in 10 patients (22.2%). Transient grade I/II bilirubin and aminotransferase elevation was observed in 26 (57.7%) and 18 (40%) patients, respectively. Super-selective DEB-TACE with doxorubicin-loaded beads sized 40-75 μm is an effective and safe treatment method with prolonged TTP and PFS in early and intermediate stages of HCC. Presence of chronic liver disease is the only significant factor that worsened OS ratios after DEB-TACE.

Dynamic performance of energy-absorbing rockbolts based on laboratory test results. Part II: Role of inherent features on dynamic performance of rockbolts

Deep mining and the related dynamic loading are the critical challenges confronting the reinforcement system like energy-absorbing rockbolts, which plays a decisive role in the dynamic response of underground mine excavations. Over the past decades, various types of novel energy-absorbing rockbolts with different deformation mechanisms and dynamic characteristics have been invented. Meanwhile, the test methods and facilities to quantify the dynamic performance of rockbolts have been proposed, and a lot of test results are obtained. In the companion Part II, with comparative analysis of test results along with available data from literature, some inherent features related to the dynamic performance of bolts have been revealed. The research indicates that, for both shank stretching and ploughing/extrusion bolts, the energy absorption is approximately proportional to the displacement. For the shank stretching bolts, only when the impact energy is higher than a threshold value, yield and strain hardening occurs; in the case of strain hardening, even a low impact energy can roughly predict the dynamic response curve. The statistical analysis of test results indicates that, in laboratory dynamic tests, the strain rate of bolts varies between 0.5/s and 3/s, and the increments of dynamic capacity enhancement factor (DCEF) is 3.4%-10.5%. For a particular bolt, DCEF can be roughly viewed as a constant which is mainly dependent on the yield strength level of the shank. Therefore, dynamic impact load can be approximated as a constant; the dynamic load–displacement curve can be simplified into a unique polyline segment comprising two line segments; and the impact duration is linearly related to the input momentum. These findings could be used for the pre-evaluation of the dynamic performance of the candidate bolts before laboratory test and support scheme design.

Effect of laboratory long-term oven aging on the stiffness of asphalt mixtures

This study evaluates the effect of laboratory long-term oven aging (LTOA) on the stiffness of asphalt mixture using laboratory dynamic modulus test. A large variety of materials was subjected to diverse LTOA conditions including different aging temperatures and durations with loose mixture and compacted samples. Dynamic modulus master curve and dynamic modulus ratio (DMR) data shows that the aging of loose mixture at 95 °C could be an ideal LTOA procedure with the efficiency and capability to characterize the long-term aging in the field. The variety of materials including the use of recycled asphalt pavement, warm mix asphalt technologies and asphalt binder performance grade was also found to have appreciable effect on the LTOA properties.

Experimental and molecular dynamics simulation study on thermal, transport, and rheological properties of asphalt

The purpose of this study was to evaluate the thermal (glass transition temperature), transport (self-diffusion), and rheological (viscosity, storage and loss modulus) properties of asphalt via laboratory experiments and molecular dynamics simulations. A 12-component asphalt model was employed in the molecular dynamics simulations with the modified Amber-Cornell force field. The asphalt model with the specific force field was validated against experimental and literature density data at various temperatures. The transition from the glassy state to the viscoelastic regime was explored by calculating the glass transition temperature. The results were compared with results from the differential scanning calorimeter (DSC) experiments. The self-diffusivity was calculated at a broad temperature range. The viscosity at the viscoelastic liquid regime was measured in the laboratory and calculated with reverse non-equilibrium molecular dynamic (rNEMD) simulations at various temperatures. Laboratory dynamic shear rheological testing was conducted for a wide frequency range at 70 °C. Oscillatory shear was applied in the asphalt model for calculating the storage and loss moduli within the experimental frequency range.

Subgrade soils characterization data, for correlation of geotechnical variables on urban roads in northern Colombia.

Within the variables relevant to the design of a pavement structure, the subgrade soils should be considered, which must be characterized in order to determine their mechanical properties and their bearing capacity. However, in developing countries such as Colombia, where the resources available for the design phase of a road project are very scarce, simplified and low-cost techniques should be used while delivering fast results, in order to be able to determine the geotechnical characteristics of soils. Therefore, it is necessary to look for correlations between the different geotechnical variables of subgrade soils. This document contains a database with the main physical characteristics of the soils. To collect these data, 46 geotechnical survey were carried out through several urban road sectors located in the city of Sincelejo, northern Colombia. Field tests were carried out with the Dynamic Cone Penetrometer and laboratory tests from undisturbed samples, for the realization of the California Bearing Ratio. Additionally, from disturbed samples, standard soil tests were conduct. The dataset obtained from the characterization of the soils, helps to create correlations between different variables, in such a way that it is possible to obtain bearing capacity parameters, such as CBR and Resilient Modulus, required for pavement design, based on simpler and faster tests such as the Dynamic Cone Penetrometer test, soil particle size analysis, the Atterberg limits or soil moisture content. In addition, these data can be supplemented by future researches in geographical regions with socioeconomic characteristics similar to those of Colombia.

An archive of data from resonant column and cyclic torsional shear tests performed on Italian clays

Measurement of soil properties under cyclic and dynamic loading conditions is a critical task in the solution of most geotechnical earthquake engineering problems. The main dynamic properties of soils are usually expressed in terms of shear modulus, G, and damping ratio, D, and they are generally obtained from laboratory tests at different strain levels. Dynamic geotechnical problems often require a site-level or territorial approach involving a considerable number of dynamic laboratory tests that might be too expensive and time-consuming. Thus, it is a common practice to use empirical relationships between dynamic parameters and measurements from routine geotechnical tests. Therefore, the availability of a large and reliable archive of multiple testing results constitutes a fundamental step for geotechnical earthquake engineers and researchers. To this aim, a large data-set of the index and dynamic parameters measured from 170 undisturbed clay samples obtained from 90 sites in Central and Northern Italy is made available, and its use and application are further described and discussed.

Comparison of Lifetime of the PVD Coatings in Laboratory Dynamic Impact Test and Industrial Fine Blanking Process.

Protective hard PVD coatings are used to improve the endurance of the tools exposed to repeated impact load, e.g., fine blanking punches. During the fine blanking process, a coated punch repeatedly impacts sheet metal. Thus, the coating which protects the punch surface is exposed to the dynamic impact load. On the other hand, the laboratory method of dynamic impact testing is well known and used for the development and optimization of protective coatings. This paper is focused on the comparison of tool life and lifetime of the industrial prepared PVD coatings exposed to repeated dynamic impact load in the industrial fine blanking process and the laboratory dynamic impact testing. Three different types of protective coatings were tested and the results were discussed. It was shown that the lifetime of coated specimens in both the fine blanking and the dynamic impact processes was influenced by similar mechanical properties of the protective coatings. The qualitative comparison shows that the lifetime obtained by the dynamic impact test was the same as the lifetime obtained by the industrial fine blanking process. The laboratory impact test appears to be a suitable alternative for the optimisation and development of protective PVD coatings for punches used in the industrial fine blanking process.

Mechanical and Volumetric Fracturing Behaviour of Three-Dimensional Printing Rock-like Samples Under Dynamic Loading

Heterogeneous rock contains numerous pre-existing three-dimensional (3D) cracks, which control its mechanical and fracturing properties. Considerable effort has been devoted to studying the volumetric fracturing behaviour of rock under static loading conditions. Although rock masses are often subject to dynamic impacts such as earthquakes and blasting, the mechanical and volumetric fracturing behaviour of rock under dynamic loading is still poorly understood. In this paper, dynamic laboratory tests were performed on 3D-printed artificial rock samples with 3D embedded flaws created during three-dimensional printing (3DP), with the aim of studying the volumetric fracturing and mechanical properties of these samples under impact with high strain rate. The results show that the dynamic compressive strength and the tangent modulus decrease with an increasing number of flaws, but have very limited effects on the ratio of the fracture initiation stress of the first crack to the peak stress of the sample, the maximum axial strain of the sample and the volumetric fracturing behaviour of the sample. The tensile failure of a sample is caused by the continuous extension of wing cracks from the outer flaw tips. The mechanical and volumetric fracturing behaviour of samples with 3D embedded flaws are strain rate dependent. The tangential modulus and the ratio of the fracture initiation stress of the crack to the peak stress increase significantly when the loading type changes from static compression to dynamic compression. Under dynamic compression, wing cracks can continuously extend to the sample ends, whereas under static compression, wing cracks can intermittently extend only a limited distance. Moreover, the fracturing behaviour of 3D flaw differs from that of 2D flaws under dynamic loading. Under high strain rate loading, wing cracks generated at 3D flaw tips lead to splitting failure of the sample, while shear cracks formed at 2D flaw tips result predominant shear failure of the sample. The findings in this paper could facilitate a better understanding of rock failure subjected to dynamic loading conditions.

Research on Variation Law of Back-Calculation Modulus of Asphalt Pavement Layer

Based on in-situ FWD testing and laboratory dynamic modulus test, the influence of loading level, position and pavement temperature on FWD back calculated modulus and the relation between bacalculated modulus and dynamic modulus was analysed. The structure combination of asphalt layer was determined based on pavement temperature profile and the composite modulus was calculated based on laboratory modulus test and in-situ measured pavement temperature. The results show that loading level had little influence on bacalculated modulus and testing position had much more influence on bacalculated modulus than loading level. The backalculated modulus and composite modulus decreased with temperature increase, when temperature was low backalculated modulus was greater than composite modulus, when temperature was in room temperature backalculated modulus was equal to composite modulus and when temperature was high backalculated modulus was less than composite modulus. For relation between bacalculation modulus and temperature, temperature correction coefficient for bacalculation modulus was developed and for relation between backcalculation modulus and composite dynamic modulus, correction coefficient model between them was developed.