Unloading Tests Research Articles

Viscous Property of Clay

Viscous property of saturated, wet, air-dried and oven-dried clay was evaluated by performing a series of one dimensional compression tests including primary loading, creep, global unloading and reloading tests. In the tests, axial strain rate was changed stepwise many times during monotonic primary loading at a constant strain rate. Test results show that viscous property of clay with different water content under different test condition is similar. In order to evaluate the viscous property of clay, parameter β was introduced basing on the present test results of clay and author’s previous study. β value of clay with different water content under different test condition varied from 0.034 to 0.064. Test results show that β value of saturated clay is generally larger than that of dried clay.

Strength Attenuation and Post Failure Behaviour of Fine Sandstone

Three types of physical tests were designed to study post failure behaviour and strength attenuation of fine sandstone. Tests include rock compression test, rock unloading test and rock block shear test. The stress-strain curves at different confining pressures had been obtained as well as axial strain-lateral strain curves. The differences of axial strain-lateral strain curves exhibit that stress peak point and the point of fault formation are apparently different for the rock. After the fault is generated, fractured rock slides on the fault surfaces, while confining pressure, material strength and structure effect decide the post failure behaviour. Damaged rock specimens were poured in concrete specimens as well as irregular blocks. Material strength of rock blocks was attained. Shear strength gradually attenuates with unloading points close to the peak. Past the stress peak, rock blocks from fractured rock still have considerable material strength. It is shown that there are differences of strength parameters obtained from direct shear test and triaxial test.

Strength Attenuation and Post Failure Behaviour of Fine Sandstone

Three types of physical tests were designed to study post failure behaviour and strength attenuation of fine sandstone. Tests include rock compression test, rock unloading test and rock block shear test. The stress-strain curves at different confining pressures had been obtained as well as axial strain-lateral strain curves. The differences of axial strain-lateral strain curves exhibit that stress peak point and the point of fault formation are apparently different for the rock. After the fault is generated, fractured rock slides on the fault surfaces, while confining pressure, material strength and structure effect decide the post failure behaviour. Damaged rock specimens were poured in concrete specimens as well as irregular blocks. Material strength of rock blocks was attained. Shear strength gradually attenuates with unloading points close to the peak. Past the stress peak, rock blocks from fractured rock still have considerable material strength. It is shown that there are differences of strength parameters obtained from direct shear test and triaxial test.

Force Measurements in the Medial Meniscus Posterior Horn Attachment

Background: Tears of the medial meniscus posterior horn attachment (PHA) occur clinically, and an anterior cruciate ligament (ACL)–deficient knee may be more vulnerable to this injury. Hypothesis: The PHA forces from applied knee loadings will increase after removal of the ACL. Study Design: Controlled laboratory study. Methods: A cap of bone containing the medial meniscus PHA was attached to a load cell that measured PHA tensile force. Posterior horn attachment forces were recorded before and after ACL removal during anteroposterior (AP) laxity testing at ±200 N and during passive knee extension tests with 5 N·m tibial torque and varus-valgus moment. Selected tests were also performed with 500 N joint load. Results: For AP tests with no joint load, ACL removal increased laxity between 0° and 90° and increased PHA force generated by applied anterior tibial force between 30° and 90°. For AP tests with an intact ACL, application of joint load approximately doubled PHA forces. Anteroposterior testing of ACL-deficient knees was not possible with joint load because of bone cap failures from high PHA forces. Removal of the ACL during knee extension tests under joint load significantly increased PHA forces between 20° and 90° of flexion. For unloaded tests with applied tibial torque and varus-valgus moment, ACL removal had no significant effect on PHA forces. Conclusion: Applied anterior tibial force and external tibial torque were loading modes that produced relatively high PHA forces, presumably by impingement of the medial femoral condyle against the medial meniscus posterior horn rim. Under joint load, an ACL-deficient knee was particularly susceptible to PHA injury from applied anterior tibial force. Clinical Relevance: Because tensile forces developed in the PHA are also borne by meniscus tissue near the attachment site, loading mechanisms that produce high PHA forces could also produce complete or partial radial tears near the posterior horn, a relatively common clinical observation.

Mechanical Deformation Characteristics of Hard Rock in Deep Excavation

In the process of deep tunnel excavation, the surrounding rock mass, which often endures high initial geo-stress, is in the state of typical unloading. As the adjustment of stress induced by excavation, rock mass behaves a peculiar kind of mechanical deformation characteristics, different from those in loading conditions. As thus, a series of representative unloading tests on hard granite from DaGangShan power station in China, which is being excavation at 1500 meters deep below earth surface, are carried out to simulate dynamically excavation process for studying the peculiar mechanical deformation characteristics. On the basis of unloading tests, a series of research results are acquired as follows： (1) Under unloading condition, lateral deformation occurs in the unloading direction and volumetric strain changes from compression deformation to dilation deformation. (2) Under unloading condition, hard granites behave brittle failure and Mogi-coulomb criterion can well describe the failure characteristics. (3) According to curves of unloading tests, the constitutive model is deduced for simulation analysis.

Investigation of the liquid in‐ and outflow parameters of the human eye using a modified strategy of the mechanical loading test

AbstractPurpose To obtain an additional information about the status of the inflow and outflow systems of the eye from ophthalmologic procedures based on static loading, with further application to diagnostics.Methods The standard method, based on static load application, for the estimation of parameters characterizing the aqueous humor dynamics was supplemented with an unloading test. After a four‐minute stationary mechanical loading by a Schiøtz tonometer, the load was removed and IOP was measured in a discrete regime for up to 20 min using the same tonometer. Clinical measurements were carried out on glaucoma (13) and non‐glaucoma (4) patients. The data obtained were processed using our mathematical model that represents the eyeball as an elastic water‐filled shell whose inner volume is regulated by dynamic balance between the liquid inflow and outflow and is assumed to be a function of IOP and the elastic and geometric characteristics of the system.Results The most important result is a strong difference between two characteristic times: the time that characterizes the rate of pressure fall under load and the time that characterizes the rate of pressure recovery after removing the load. The characteristic time is directly related with the outflow facility. The difference varies in different groups of patients. The results can be understood within the assumption of nonlinear IOP dependence of the outflow facility coefficient.Conclusion The modified mechanical loading test is a useful tool for investigating the biomechanical behavior of glaucoma and non‐glaucoma eyes. Our preliminary results give grounds to expect that the effect revealed may be applied to diagnostics.

The effect of sintering temperature on the microstructure and mechanical properties of a bioceramic bone scaffold

Micro and nanostructural properties are believed to play a critical role in the osteoinductive capacity of bioceramic bone scaffolds. Physical characteristics also play an important role for optimum biological performance, including osteoconductivity and strength. In this study microstructural and nano-mechanical properties of a bioceramic bone scaffold were investigated as a function of the sintering temperature in the range of 950–1150 °C, through the use of scanning electron microscopy (SEM), X-ray diffraction (XRD) and nanoindentation testing. Although the samples presented the same crystallographic phase, an increase in sintering temperature resulted in increased grain size, density and crystallite size. The intrinsic mechanical properties were measured by nanoindentation testing and analyzed with the Oliver–Pharr method. The nanoindentation tests consisted of a series of fourteen partial unload tests (n=14 per treatment) of twelve steps ranging from 1 to 12 mN. Statistically significant increases in hardness and elastic modulus were measured for increasing sintering temperature. These results support the development of clinically successful bioceramic scaffolds with mechanical properties that encourage bone ingrowth and provide structural integrity.

Research of Rock Mass Unloading Constitutive Model and its Engineering Application

Based on the results of triaxial unloading tests，the stress-strain characteristics and failure characteristics of rock mass have been discussed. Considering unloading degree as an important parameter，an elastic-brittle-plastic mechanical model has been established according to the unloading stress condition，which considers that unloading yield function varies linearly between Griffith with Hoek-Brown criterion after rock mass receives plastic deformation caused by unloading. Taking jinshajiang excavation slope of a hydropower station as engineering background, the three-dimensional continua fast Lagrangian analysis software FLAC3D is employed in the slope stability analysis under the excavation, and the distribution characteristics of the slop deformation is studied.

Study on Rock Unloading Failure and its Effects on Rock Burst

The mechanical parameters and failure characteristics were obtained based on the results of triaxial unloading tests of limestone. Results show that: The evolution of volumetric strain is determined by lateral strain under unloading condition. Failure modes of rock can be classified into three types. Deformation modulus decreases and Poisson ratio increases in the process of unloading. Mohr-Coulomb criterion can not describe rock unloading failure strength property. Rock burst occur during tunnel excavation are closely related to the rock unloading failure characteristic.

Unloading Triaxial Experimental Study on Stress Path of Excavated Soil Slope

The current research was implemented to study the practical unloading stress path that the slope mass experienced during the excavation process, which is very important in the stress and strain numerical analysis. Series of unloading tests were carried out under different confining pressure. During the test process, the minor principal stress was kept decreasing, while the major principal stress was kept unchanged to simulate the stress path in some locations of the soil slope, such as at the top of the slope. The corresponding conventional triaxial compression tests were also carried out as comparison. It is shown that there are many differences between the unloading and loading tests. Through analyzing, the tests results could be applied in the development of unloading constitutive models of excavation soil mass. Also, such unloading tests data are valuable in calibration and verification of the current existing popularly used models.

Unloading Characteristics of Hard Rock at Depth

DaGangShan power station in China is being excavation at 1500 meters deep below earth surface, where the surrounding rock is affected by high initial geo-stress. Deep excavation of rock tunnels causes release of high initial geo-stress and produces unloading action. As thus, a series of representative unloading tests on hard granite from DaGangShan power station are carried out to simulate dynamically excavation process, and in unloading tests, the stress path of decreasing confining pressure along with increasing axial pressure is accepted. The results of tests show as follows: (1) Under unloading condition, lateral deformation occurs in the unloading direction and volumetric strain changes from compression deformation to dilation deformation. (2) Under unloading condition, crack deformation appears, which results in Poisson’s ration rise and deformation modulus decrease. (3) Under unloading condition, hard granites behave brittle failure and Mogi-coulomb criterion can well describe the failure characteristics.

Study on the Hoek-Brown Parameters under Loading and Unloading Triaxial Laboratory Tests

Based on the brittle fracture theory of composite crack, and then overlapping the stresses at the end region of a crack under tension-shear or compression-shear failure conditions, the relationships of parameters in rock macroscopic and microscopic are determined by combining the Hoek-Brown failure criterion. There are many valuable conclusions through a series of loading and unloading triaxial laboratory tests on rock samples. The tension-shear fracture under the unloading triaxial tests at pre-peak is different from the compression-shear fracture under the loading triaxial tests. The Hoek-Brown criterion for rock is applicable and validity under the condition of loading triaxial tests. Because the uniaxial compressive strength is not the fundamental characteristic of rock, the virtual uniaxial compressive strength is proposed for the first time considering the unloading problem. Comparing with the loading tests, the Hoek-Brown material parameters of rock samples change greatly under unloading at pre-peak, and the virtual uniaxial compressive strength improves and Hoek-Brown material constant m reduces.

Ringlike failure experiment of thick-walled limestone cylinder specimens in triaxial unloading tests

In order to study the failure of surrounding rock under high in situ stress in deep underground engineering projects, disturbed by excavation unloading, we carried out triaxial unloading experiments using thick-walled cylinder specimens on a TATW-2000 rock servo-controlled triaxial testing machine in a laboratory. The specimens were made of limestone material, taken from Tongshan county, Xuzhou city, Jiangsu province, China. In our experiments, rock deformation and failure behavior was studied through loading and unloading of inner hole pressure of thick-walled cylinder specimens. At first, the axial stress, confining pressure and inner pressure were increased simultaneously to a specified designed state of stress. Then, keeping the axial stress and confining pressure stable, the pressure on the inner hole was decreased until the specimen was fractured. When the inner pressure was released completely but the specimen did not fracture, the confining pressure was decreased subsequently until complete failure occurred. Our experimental results suggest that traces of major circular ringlike fractures with a number of radial cracks often appear in thick cylinder walls. This type of ringlike failure phenomenon, similar to intermittent zonal fracturing characteristics of deep exploitation, has, so far, not been published. Our experimental results show that rock deformation and failure behavior of thick-walled limestone cylinders vary under different stress paths between loading and unloading. Tensile failure and orderly failure surfaces occur under unloading conditions while irregular damaged rock blocks are produced during loading failure. This type of triaxial unloading experiment provides for new research methodology and approach for thorough investigations on intermittent zonal fracturing in deep underground excavations.

Research on Rock Burst Loading Test and PFC<sup>3D</sup> Numerical Simulation

During the tunnel excavation in Zhang-Shi Expressway 2nd-Stage Project, rock burst phenomenon has occurred. Limy dolomite obtained from tunnel rock burst area is used to conduct laboratory loading rock burst test with simulation system of deep rock burst. Simultaneously, based on Particle Flow Theory and PFC3D program, a load-unload code is developed by fish language embedded in PFC3D to carry out simulation of laboratory loading rock burst test. Laboratory test result shows that the rock burst type and intensity of specimen are lagged and severe, respectively; During the test process, a spot of particles eject from the exposed surface; Obvious slabbing phenomenon can be seen on the exposed surface and the separate face of rock slab is parallel to the direction of maximal load σ1 as consistent with the actual phenomenon of engineering rock burst. In the loading rock burst simulation, the load grade, stress-step curve and failure mode of rock burst are almost consistent with the laboratory test. This simulation method describes the failure process actually and subtly, and reveals that the specimen fracture mechanism of loading rock burst is tensile type. Finally, some differences between loading and unloading tests of rock burst are discussed.

Markers of Postmatch Fatigue in Professional Rugby League Players

The aim of the present study was to identify neuromuscular, biochemical, and endocrine markers of fatigue after Rugby League match play. Seventeen elite Rugby League players were monitored for a single match. Peak rate of force development (PRFD), peak power (PP), and peak force (PF) were measured during a countermovement jump (CMJ) on a force plate pre and postmatch play. Saliva and blood samples were collected 24 hours prematch, 30 minutes prematch, 30 minutes postmatch, and then at 24-hour intervals for a period of 120 hours to determine plasma creatine kinase concentration ([CK]) and salivary cortisol concentration ([sCort]). There were significant (p < 0.05) decreases in PRFD and PP up to 24 hours postmatch with PF significantly (p < 0.05) decreased immediately postmatch. The [sCort] significantly (p < 0.05) increased from 24 hours prematch to 30 minutes prematch and up to 24 hours postmatch compared with 24 hours prematch. Plasma [CK] significantly (p < 0.05) increased 30 minutes postmatch with a peak occurring 24 hours postmatch and remained elevated above 24 hours prematch for at least 120 hours postmatch. There were significant (p < 0.05) correlations between the increase in [CK] and reduction in PRFD 30 minutes postmatch and 24 hours postmatch. The [sCort] was significantly (p < 0.05) correlated with the reduction in PF 30 minutes postmatch. Results demonstrate that neuromuscular function is compromised for up to 48 hours after match play. Elevated [CK] despite 120-hour recovery indicate that damage to muscle tissue after Rugby League match play may persist for at least 5 days postmatch. Despite the prolonged presence of elevated [CK] postmatch, strength training 48 hours postmatch may have resulted in a compensatory increase in PRFD supporting the inclusion of strength training during the short-term postmatch recovery period.

Calculation of the Mechanical Properties of FeB Layer's by Finite Element Method

Surface treatments of engineering materials are important for serviceable engineering components. One of the thermo-chemical surface treatments of steel based materials is the boriding process. In this study, low alloy steel substrates were borided by pack boriding process at 900 °C. Experimental indentation tests were conducted on Dynamic Ultra-micro Hardness test machine, under applied peak loads of 800 mN, 1000 mN, 1200 mN and 1400 mN. To get the mechanical properties of FeB layers, the resulting load–unload test data of the samples obtained from the experimental indentation tests were analyzed and curve-fitted in Kick’s and Meyer’s law for the loading and the unloading part of the load-unload curve respectively. Then, a set of analytical functions that take the pile-up and sink-in effects into account during instrumented sharp indentation were solved using numerical methods. These analytical functions were defined within an identified representative plastic strain, εr, for the Vickers indenter geometry as a strain level that allows for the description of the indentation loading response independent of strain hardening exponent, n. The mechanical characterization of samples, finite element modeling was applied to simulate the mechanical response of FeB layer on low alloy steel substrate by using ABAQUS software package program.

A Model for the Calculation of Mechanical Properties of Hydroxyapatite Coatings on Ti6Al4V Substrate Using Finite Element Methods

The scope of this study is to find out yield curve on the coating-substrate system of hydroxyapatite (HA) by finite element modeling. With this regard, Hydroxyapatite (HA) coatings were successfully formed on Ti6Al4V substrates by using biomimetic technique. Experimental indentation tests were conducted on Dynamic Ultra-micro Hardness test machine. In finite element modeling (FEM), mechanical properties of coatings were determined by using ABAQUS software package. This numerical study was carried out using an axisymmetric FEM model. To calculate the mechanical properties of coating, the resulting load–unload test data of the samples obtained from the experimental indentation tests were analyzed and curvefitted in Kick’s and Meyer’s law for the loading and the unloading part of the loadunload curve respectively. Then, a set of analytical functions that take the pile-up and sink-in effects into account during instrumented sharp indentation were solved using numerical methods These analytical functions were defined within an identified representative plastic strain, εr, for the Vickers indenter geometry as a strain level that allows for the description of the indentation loading response independent of strain hardening exponent, n.

Nonlinear Unloading Model for Hard Rock Under High Geo-Stress

The strength feature of excavated rock mass has the nature of non-linearity under high geo-stress action. To simulate the brittle failure of underground engineering, a new mechanical model was put forward based on true triaxial unloading test. At first, triaxial unloading mechanical test for hard rock was performed under low, middle, very high confining stress. The axial failure stress of unloading test is quite different from that of loading test, the strength envelop of unloading is nonlinear and S-typed. It is an important discovery in the unloading test. Then based on the concept that brittle damage is also a progressive process, a new unloading model is presented to simulate the brittle failure of hard rock by using nonlinear Hoek-Brown failure criterion. The parameter m increases and s decreases in the unloading model, m and s are the functions of plastic strain. At last, the new unloading model was applied to Mine-by experimental tunnel, and the test result shows that the new model can reflect the brittle and tensile failure of rock mass. The case study proves the nonlinear brittle model is reasonable.

Sandstone Unloading Rheological Test and Model Parameters Study

Rock rheology is common in engineering practice, which has an effect on long-term security and stability of the project. In order to understand the rheological mechanical properties under the condition of unloading sand, automatic servo apparatus of triaxial creep of rock was used to have a rheological mechanical unloading triaxial test on sandstone, which got typical flow complete curves of rock. Rheological test results showed that rheological phenomena occured in sandstone, and its rheological properties were significant with large rheological deformation, especially in high-stress conditions, When the external load exceeded the long-term strength of rock samples, specimen deformation went through three typical rheological stages with time increasing, and ultimately speed up the flow breakdown. Based on the experimental results, analyzed and studied the rheological parameters that were obtained, and discussed unsteady character law of rheological parameters. defined rheological damage of rock in the accelerating flow process as the unloading capacity and time-related exponential function, and proposed corresponding damage evolution equation, brought damage evolution equation into Burgers rheological model, and obtained nonlinear unloading model of rheological damage of rock, used accelerating rheological test curve of sandstone samples under the condition of 15MPa confining pressure and 135MPa stress level to verify nonlinear unloading model of rheological damage of rock, and the results showed that the fitting results of rheological test data and the improved rheological model had good consistency, and the rheological model can reflect the three stages of nonlinear rheology of rock, which can reflect the rheological characteristics under the condition of unloading sandstone more accurately.

Experimental Investigation of Dilatancy-Bulking Behavior of Weak Sandstone

The process of stress-induced fracturing around underground excavations is associated with dilatancy-bulking behavior of the fractured rock. Due to the importance in earth science and engineering design, this paper presents an experimental investigation of rock dilatancy-bulking behavior based on triaxial loading and unloading tests carried out on weak sandstone samples from the Gubei colliery. Through experimental and theoretical analyses, it is found that the volume increase of stress-fractured rock can be divided into two groups: (1) pre-peak dilatancy due to crack opening and propagation, (2) post-peak bulking by the sliding, rotate or detachment of rock blocks. And the post-peak bulking deformation is the main cause of the rock volume expansion. Moreover, the evolution of characteristic stresses and dilation angle show strong pressure dependence, which means that confining pressure strongly inhibits dilatancy-bulking behavior. It also indicates that the rock response under loading and unloading conditions is substantially different.