Friction and Wear Characteristics of Haynes 25, 188, and 214 Superalloys Against Hastelloy X up to 540 <Emphasis Type=Bold>°</Emphasis>C

As demand for more power increases, compression ratios, and operating temperatures keep rising. High speeds combined with high temperatures make turbomachinery sealing applications even more challenging. In order to confirm sufficient service life material pairs should be tested under conditions similar to engine operating conditions. This study presents high temperature friction and wear characteristics of cobalt/nickel superalloys, Haynes 25 (51Co–10Ni–20Cr–15W), Haynes 188 (39Co–22Ni–22Cr–14W), and Haynes 214 (75Ni–16Cr–3Fe–0.5Mn) sheets when rubbed against Hastelloy X (47Ni–22Cr–18Fe–9Mo) pins. Tests are conducted at 25, 200, 400, and 540 °C with a validated custom design linear reciprocating tribometer. Sliding speed and sliding distance are 1 Hz and 1.2 km, respectively. Friction coefficients are calculated with friction force data acquired from a load cell. Wear coefficients are calculated through weight loss measurements. Results indicate that Haynes 25 (H25) has the lowest friction coefficients at all test temperatures. Above 400 °C, H25 and Haynes 188 (H188) exhibit the best wear resistance. Protective cobalt oxide layers are formed on the H25 and H188 at 540 °C in addition to nickel, chrome, and tungsten oxides. Although, it has better oxidation resistance, Haynes 214 has relatively higher wear rates than other tested materials especially at low temperatures. However, its wear performance improves beyond 200 °C.     

An improved methodology for determining tensile design strengths of Alloy 617

This paper presents an improved methodology for determining high-temperature tensile design strengths of Alloy 617, which is regarded as one of main structural materials for very high temperature reactor (VHTR) system. In establishing time-independent allowable stress values, an existing ASME standard procedure is preliminarily analyzed and their limitations are pointed out. Then, an improved methodology, which has a consistent and quantifiable design margin at low and high temperatures for tensile strengths, is proposed and compared with the ASME method. To find suitable curves of temperature trend to the tensile strength data, three fitting methods are demonstrated, and a statistical technique is adopted for design use. The results will be utilized to reasonably determine the tensile design strengths of Alloy 617 for application in the VHTR system.     

Laminar and turbulent channel flow simulation using residual based variational multi-scale method

We present a residual-based isogeometric variational multiscale method to solve laminar and turbulent channel flow. Residual-based variational multiscale method is a new finite element formulation for solving turbulent flows using a large-eddy simulation type modeling. Isogeometric analysis, a new finite element method using CAD blending functions as its basis functions, is employed for higher order approximation of the solution. First, laminar flow with Re _τ 0.55 = through flat channel is considered and linear, quadratic and cubic basis functions, which are C ⁰, C ¹, and C ²-continuous across element boundaries, respectively are employed and their accuracy is presented by the comparison with analytical result. Next, same methodology is applied to the turbulent channel flow with Re_r = 180. Current results are validated by the comparison of turbulence statistics using available DNS data.     

Inspection method of cable-stayed bridge using magnetic flux leakage detection: principle,sensor design,and signal processing

A nondestructive testing technique based on magnetic flux leakage is presented to inspect automatically the stay cables with large diameters of a cable-stayed bridge. Using the proposed inspection method, an online nondestructive testing (NDT) modular sensor is developed. The wreath-like sensor is composed of several sensor units that embrace the cable at equal angles. Each sensor unit consists of two permanent magnets and a hall sensor to detect the magnetic flux density. The modular sensor can be installed conveniently on cables with various diameters by increasing the number of sensor units and adjusting the relative distances between adjacent sensor units. Results of the experiments performed on a man-made cable with faults prove that the proposed sensor can inspect the status signals of the inner wires of the cables. To filter the interfering signals, three processing algorithms are discussed, including the moving average method, improved detrending algorithm, and signal processing based on a digital filter. Results show that the developed NDT sensor carried by a cable inspection robot can move along the cable and monitor the state of the stay cables.     

A study to maximize the crash energy absorption efficiency within the limits of crash space

The design of an engine room is important to protect the passenger from a crash impact by improving the absorption of the crash impact energy. The side member in the engine room absorbs most of the crash impact energy when the vehicle experiences a frontal crash. The side member is of two types: hat and ‘U.’ Analysis of the extent of energy absorption and the mechanism of the side member are necessary through a collapse mode in various load conditions. In this study, the design of experiments was used for evaluating the characteristics of the absorption of crash energy by side members through design variables. First, crash analysis was performed by experiment number extracted from the design of the experiment. Then, using the results of crash analysis, multiple regressions were conducted and sensitivity analysis performed for each design variable. Finally, the optimum design was developed for maximizing the absorption energy per unit weight considering various boundary conditions. In the present study, as a basic step for modeling the fatigue behavior of an extruded Al alloy cylinder, the fatigue crack growth data of the alloy was collected in two orientations. Microstructural analysis revealed that the material had recrystallized grains and clusters of constituent particles aligned in the direction of extrusion. Fatigue life of the samples revealed a shorter fatigue life representing a higher fatigue crack growth rate in the transverse direction.     

Anti-plane moving crack in a functionally graded piezoelectric layer between two dissimilar piezoelectric strips

The dynamic propagation of a crack in a functionally graded piezoelectric material (FGPM) interface layer between two dissimilar piezoelectric layers under anti-plane shear is analyzed using integral transform approaches. The properties of the FGPM layers vary continuously along the thickness. The FGPM layer and two homogeneous piezoelectric layers are connected weak-discontinuously. A constant velocity Yoffe-type moving crack is considered. The Fourier transform is used to reduce the problem to two sets of dual integral equations, which are then expressed to the Fredholm integral equations of the second kind. Numerical values on the dynamic energy release rate (DERR) are presented for the FGPM to show the effects on electric loading, gradient of the material properties, crack moving velocity, and thickness of the layers. The following are helpful to increase resistance to crack propagation in the FGPM interface layer: (a) certain direction and magnitude of the electric loading, (b) increasing the thickness of the FGPM interface layer, and (c) increasing the thickness of the homogeneous piezoelectric layer to have larger material properties than those of the crack plane in the FGPM interface layer. The DERR always increases with the increase of crack moving velocity and the gradient of the material properties.     

Output power measurement of a blood pump considering pulsating flows

A blood flow simulator is one of the most useful tools for investigating the dynamic properties of a cardiovascular system. The blood pump in a blood flow simulator generates pulsating flows by oscillating a membrane mechanically at different pulse rates. To evaluate the performance of the blood pump properly, the flow characteristics of the pulsating flows should be considered. In this paper, two basic indicators for evaluating the blood pump characteristics, the output power and the overall efficiency, were determined with consideration of the pulsating flows, based on a phase-averaged analysis. These indicators were obtained by integrating closed contours in a P − Q diagram. The measured output power was expected to be meaningful for understanding the dynamic effects caused by pulsating flows.     

A numerical simulation of train-induced unsteady airflow in a tunnel of Seoul subway

This study has been conducted to investigate numerically the characteristics of train-induced unsteady airflow in a subway tunnel. A three-dimensional numerical model using the dynamic layering method for the moving boundary of a train is applied. The validation of the present study has been carried out against the experimental data obtained by Kim and Kim [1] in a model tunnel. After this, for the geometries of the tunnel and subway train which are very similar to those of the Seoul subway, a three-dimensional unsteady tunnel flow is simulated. The predicted distributions of pressure and air velocity in the tunnel as well as the time series of mass flow rate at natural ventilation ducts reveal that the maximum exhaust mass flow rate of air through the duct occurs just before the frontal face of a train reaches the ventilation duct, while the suction mass flow rate through the duct reaches the maximum value just after the rear face of a train passes the ventilation duct. The results of this study can be utilized as basic data for optimizing the design of tunnel ventilation systems.     

Improved dynamic equations for the generally configured Stewart platform manipulator

In this paper, a Newton-Euler approach is utilized to generate the improved dynamic equations of the generally configured Stewart platform. Using the kinematic model of the universal joint, the rotational degree of freedom of the pods around the axial direction is taken into account in the formulation. The justifiable direction of the reaction moment on each pod is specified and considered in deriving the dynamic equations. Considering the theorem of parallel axes, the inertia tensors for different elements of the manipulator are obtained in this study. From a theoretical point, the improved formulation is more accurate in comparison with previous ones, and the necessity of the improvement is clear evident from significant differences in the simulation results for the improved model and the model without improvement. In addition to more feasibility of the structure and higher accuracy, the model is highly compatible with computer arithmetic and suitable for online applications for loop control problems in hardware.     

Ultrasonic Velocity as a Tool for Physical and Mechanical Parameters Prediction within Geo-Materials: Application on Cement Mortar

This paper discusses the ability of ultrasonic wave velocity to evaluate some physical parameters within mortar. The behavior of ultrasonic pulse velocity within mortar subjected to incremetal stress was also studied. For experimentation, we carried out ultrasonic measurements on mortar samples before and during uniaxial compressive strength, perpendicularly to the stress direction. The water/cement ratios were varied in order to contribute certain specific characteristics. A set of expressions was obtained linking the initial velocities of longitudinal ultrasonic waves with compressive strength, density, porosity and load at elastic limit.The evolution of ultrasonic velocity through mortar samples under continuous incremental uniaxial stress were also investigated. The results illustrated the behavior of ultrasonic pulse velocity as a function of the applied stress. It was observed that velocity did not decrease under initial loading and until about 70% of the ultimate stress, where sudden decrease occurred, followed by the failure of the material.