CONSOLIDATION CHARACTERISTICS OF A SILTY CLAY: CONCERNING THE EFFECT OF SOIL DISTURBANCE

In this paper, undisturbed specimen of a silty clay constituting of Phu Bai formation (ambQ21-2 pb) was collected from boreholes in Hue city and surrounding areas. The soil, under both undisturbed and disturbed conditions, was then subjected to standard one-dimensional consolidation tests with 7 loading increments. It is shown from the experimental results that the time to the end of primary consolidation (EOP), determined by Log Time method (tLT) and 3-t method (t3T), decreases with the load increment and under the same vertical stress, the primary consolidation of disturbed silty clay finish at a shorter time than those of the undisturbed one. The coefficient of secondary consolidation (Cα) increases with the vertical stress and reaches the maximum values before decreasing. The obtained values of Cα = 0.005 - 0.020 suggest a relatively low secondary compressibility of the silty clay constituting of Phu Bai formation.

Testing the effectiveness of release valves under intensive dynamic load to the leg

Release valves are commonly used to protect hydraulic legs against overload caused by rock bursts or bumps. Due to an essential role in ensuring safety in the working, an application of a release valve is conditioned by a positive yield test results of a leg equipped with such a valve. A method of leg yield testing, used in Poland, enables a complex determination of an impact of not only a release valve but also of the parameters of the hydraulic leg, determining its stiffness such as for example a volume of the under-the-piston space, which has an impact on an observed pressure increase. The subject of this publication covers cognitive tests oriented onto a determination of an impact of a release valve exclusively on the pressure changes observed in the leg. The results of the efficiency tests of spring valves (Stoiński, 2018) on a rammer are discussed. The difference between the maximum pressure in the under-piston area of the leg with the release valve and the maximum pressure generated by the same dynamic load in the leg without this valve was the measure of the valve operation's effectiveness. Dynamic load, realized on a rammer, is characterized by a longer increase time than in the case of dynamic load acting on a powered roof support unit from the floor. The time process of the force in the leg is then characterized by a short load rising time – tn, large load increment factor – Kd and the average load growth rate –wp,n. Referring to that aspect, the features of a release valve were analyzed in relation to the parameters characterizing dynamic load acting on a powered roof support unit from the floor. Parameters characterizing the effectiveness of the release valve, i.e. change in the leg load increment index –Kd and change in the rate of load increase –wpn were defined. The test stand for generating the load of such parameters using the explosive method is described. Comparison of effects of the dynamic load generated by firing the same mass and the same type of explosive on a hydraulic leg with a release valve and the leg without this valve was the test objective. The effectiveness of the spring valve and two gas valves are analyzed. It was found that despite a very short load rising time, the release valve reduces the load acting on the leg. The positive values of the Kd and wp,n indices are the evidence.

Numerical Convergence in Wear Volume Prediction of UHMWPE Acetabular Cup Paired with cp Ti Femoral Head Hip Implants

Wear is a problem for metal on polymer (MOP) hip implants to perform lifetime endurance. Polymer excessive volumetric loss leads to implant failures. Attempts to solve this problem are usually initiated with tribological tests. The method is time-consuming because the sliding speed is low. There is a faster way to use a computational method to gather wear data. This research aims to investigate the numerical convergence of predicted wear volume with the finite element method (FEM). The model is a commercially pure titanium (cp Ti) and ultra-high molecular weight polyethylene (UHMWPE) MOP hip implant. A dynamic Paul physiological load was applied to the model. Volumetric loss of the polymer was calculated with a wear equation involved nonlinear contact load and contact area. The inputs of calculation are wear factor and the computational contact mechanic performed by FEM. The wear factor was obtained by performing biotribological experiments with a multidirectional pin on disc tribotest. Predicted wear volume was validated with hip simulator experimental data from the literature. Convergences were found at the mesh density of 1.38 elements/mm3. An acceptable numerical error was obtained in the model with 1 mm element size for femoral head and 0.3 mm for acetabular cup. This model was then used for the investigation of load increment effects. The result is that load increment variations do not affect wear volume and contact mechanic numerical outputs. The calculated stresses are below the UHMWPE yield stress limit. In this elastic region, the effects of strain rate caused by load increment are negligible.

Numerical Optimization Method for Determination of Bifurcation Points and its Application in Stability Analysis of Power System

Nonlinear bifurcation theory is very powerful and efficient to deal with multifarious nonlinear phenomena emerging from physics, biology, engineering, and economics. However, the execution of augmented system for bifurcation points will be very complicated for a system with too many variables. The purpose of this paper is to give a new optimization based method for computation of fold and Hopf bifurcation points and discuss their applications in stability analysis of power system. The validity of the proposed method has been testified by a WSCC9-bus power system in which the load increment at any bus was taken as a parameter.

An efficient multiscale modeling framework for nonlinear analyses of composite structures

Traditional multiscale modeling methods of composite structures are based on the global-local approach whereby the global analysis of structures are first performed to determine potential damage regions, followed by local analyses at those regions to identify detailed damage patterns and failure modes. Such an approach does not take into account the localized effects of critical regions on the global analysis and may become less accurate in general. To address better the behavior of local regions on multiscale analyses, homogenization-based multiscale methods are applied. For each load increment, the global problem is solved simultaneously with one Representative Volume Element (RVE) equilibrium problem for each Gauss point of the global mesh. This approach is successful to capture the local behavior at each material point; however, it is computationally expensive since the RVE is called at all the Gauss points in the global model for each load increment. We develop an efficient multiscale modeling method whereby the RVE analyses are only called at specialized locations by multiscale elements and run parallel with the global analysis. The constitutive models of multiscale elements are defined in a user-defined element subroutine (UEL) where stiffnesses of the multiscale elements are unknown at the beginning of the analysis. They can only be obtained by performing a series of RVE analyses for each set of loads received from the global analysis. The advantage of the proposed method is that the stiffnesses of the multiscale elements are directly computed from the RVE analyses and keep updated for each global load increment. The nested multiscale modeling is implemented by Python script and highly capable for nonlinear analysis of composite structures.