Non-Destructive Evaluation of Creep Damage in a Nickel Base Super-Alloy Turbine Bucket

Due to elevated temperatures, excessive stresses and severed corrosion conditions, turbine engine components are subject to creep processes that limit the components life such as a turbine bucket. The failure mechanism of a turbine bucket is related primarily to creep and corrosion and secondarily to thermal fatigue. As a result, it is desirable to assess the current condition of such turbine component. This study uses the eddy current (EC) nondestructive evaluation technique in an effort to monitor the creep damage in a nickel base super-alloy, 7FA stage 2 turbine bucket after service. The experimental results show significative electrical conductivity variations in eddy current images on the creep damage zone of nickel base super-alloy samples cut from a turbine bucket. Thermoelectric power (TEP) measurements were also conducted in order to obtain a direct correlation between the presence of material changes due to creep damage and the electrical conductivity measurements.

Application of Multifunctional Materials for Machine Tool Structures

The competition on the international markets pushes manufacturers towards shorter design cycles and decreasing manufacturing times and costs for their products. This trend generates a demand for smart, flexible and faster machining systems, easy to set up and configure, which are able to drastically reduce machining time and improve the final accuracy. This paper rises from these considerations evaluating the possible application of multifunction materials in machine tool (MT) design and building. These solutions can provide a fundamental impact on functionality and reliability of a manufacturing system. In particular, use of innovative materials in today’s technology continues to grow steadily. Numerous reasons for this growth include light weight, superior insulating abilities, energy absorbing performance, excellent strength/weight ratio and low cost. This paper aims to investigate a possible application of multifunction materials in realisation of structure components for Machine Tools. There are many aspects that affect the machining accuracy and the cutting conditions of a high performance MT. The most important issues are related to the static, dynamic, mechatronic and thermal behavior of the machines. In particular, a strict requirement that a machine tool has to fulfill in order to drastically reduce operating time while improving the final accuracy is the thermal stability. This paper shows a complete study and testing validation on prototypes (plates and beam) based on sandwiches with core made of metal foam (open and closed cells) materials impregnated by a PCM (Phase Material Change) wax. Metal foams represent a class of materials with low density and novel physical, mechanical, thermal, electrical and acoustic proprieties. They offer potential for lightweight structures, for energy absorption and thermal management. PCMs are latent heat storage materials that absorb heat keeping constant the temperature of a machine component in a defined time range. The authors have designed, realized and tested the prototypes developing thermal trials, and then evaluating the comparison between experimental data and simulative analysis (FEM). The trials consisted to process the prototypes at a variation of temperature in order to assess the PCM proprieties to absorb heat and maintain thermal stability in a defined time range. The paper shows also a simulative study on PCM material behavior and their application in MT design supported by experimental trials and data analysis. The significant advantages and perspectives that can be obtained in applying of these MT structures complete the developed study.

A Model for Deposition of Metal Vapors From Multiple Targets on a Cylindrical Substrate

The paper addresses a challenging problem of developing technology for heat exchanger tubes embedded in ceramic composite matrix. Functionally graded composite tubes, made using physical vapor deposition (PVD) process, are required to have diffusion barrier layers, withstand high temperature, and be impermeable to hydrogen. The work addresses mathematical modeling of the deposition of metallic vapors from multiple targets on a cylindrical substrate to simulate the PVD process in manufacturing such tubes. Materials used for the deposition are Molybdenum and Niobium because they have shown good formability, strength, toughness and ductility over a wide range of temperatures. Commercially available software FLUENT was used to model the process. Prediction of condensation of vapors from metal ingots occurs in varying proportion along the circumference of the tube, resulting in submicron layers of different materials of varying thicknesses being ingrained into each other. Results are presented for patterns of materials showing continuously changing relative concentration of deposited metals over a stationary and rotating cylindrical substrate.

Responsive Biosensors for Biodegradable Magnesium Implants

A biosensor is an electronic device that measures biologically important parameters. An example is a sensor that measures the chemicals and materials released during corrosion of a biodegradable magnesium implant that impact surrounding cells, tissues and organs. A responsive biosensor is a biosensor that responds to its own measurements. An example is a sensor that measures the corrosion of an implant and automatically adjusts (slows down or speeds up) the corrosion rate. The University of Cincinnati, the University of Pittsburgh, North Carolina A&T State University, and the Hannover Medical Institute are collaborators in an NSF Engineering Research Center (ERC) for Revolutionizing Metallic Biomaterials (RBM). The center will use responsive sensors in experimental test beds to develop biodegradable magnesium implants. Our goal is to develop biodegradable implants that combine novel bioengineered materials based on magnesium alloys, miniature sensor devices that monitor and control the corrosion, and coatings that slow corrosion and release biological factors and drugs that will promote healing in surrounding tissues. Responsive biosensors will monitor what is happening at the interface between the implant and tissue to ensure that the implant is effective, biosafe, and provides appropriate strength while degrading. Corrosion behavior is a critical factor in the design of the implant. The corrosion behavior of implants will be studied using biosensors and through mathematical modeling. Design guidelines will be developed to predict the degradation rate of implants, and to predict and further study toxicity arising from corrosion products (i.e., Mg ion concentrations, pH levels, and hydrogen gas evolution). Knowing the corrosion rate will allow estimations to be made of implant strength and toxicity risk throughout the degradation process.

Effect of Conductivity and Environmental Pressure on Electro-Plasma Process (EPP)

Electrolytic plasma process is an efficient surface modification method for metallic materials. With proper control of the process parameters Electro-Plasma Process (EPP) could generate unique surface morphology, which is suitable for effective cleaning of the metallic surfaces and inherently, good adhesion strength can be achieved for eventual coating the surfaces. Increasing input voltage beyond the conventional Faraday region of electrolysis, luminous discharge is observed on the surface of one of the electrodes. The electrode surface must be covered by layers of bubbles before the discharge could be set in. The discharge of energy is then taken place in an explosive way with localized high temperature. The combination of heat and kinetic impact could effectively remove the surface contaminants and could produce a unique surface morphology. In this paper, the conditions of process control parameters and the resultant surface conditions that could be achieved are studied. It has been found that elevated temperature is beneficial towards the plasma formation on electrodes; and the increase of temperature essentially increases the kinetic energy of electrons in the electrolytic solution and a high electrolyte temperature assists the boiling process and the chemical reactions that generate bubbles. The conductivity of the electrolytic solution could also affect the threshold voltage and the current density, but the total power input does not vary significantly with conductivity. Environmental pressure has been proved to be the single most critical important factor for Electro Plasma Process; and by increasing the pressure level the total breakdown energy tends to increase and more importantly, the resultant surfaces manifest that the energy consumed for surface modification increases with pressure.

Interfacial Fracture Analysis in EB-PVD/PTAL TBC Systems

Thermal barrier coatings (TBCs) have been utilized for more than four decades to increase the efficiency and durability of aircraft engines as well as land based gas turbines. Though the function of the thin TBC layer is heat insulation, it is critically important for it to maintain adherence in service. In this paper, we address some fundamental aspects of TBC debonding failure in electron beam physical vapor deposition (EB-PVD) TBC systems that had been considered in a previous study. We demonstrate that the energy release rate formulation for EB-PVD TBC systems based on as-processed conditions and a thin thermally grown oxide (TGO) layer overestimates the energy release rate for exposed TBC systems having thicker TGO layers. A modified formulation is given, applicable to exposed TBC systems. A finite element contact and fracture model is used to validate the formulation as well as study crack mode mixity. Mode II interfacial cracking is shown to be dominant for practical TGO thicknesses seen in exposed EB-PVD TBC systems.

Mechanical Properties of NiAl Nanocomposite Intermetallic Alloys With Varying Porosity

Recent advancements in the field of nano-technology focused attention on developing materials with new and useful characteristics. In particular, there is interest in designing nanocomposite thermites for combustion synthesis applications. The composite material consists of nano-scale particles that are in nearly atomic scale proximity but constrained from reaction until triggered. Once initiated, the reaction will become self-sustaining and a new intermetallic alloy product will be produced. An example of this type of reaction is between Ni and Al such that a nickel-aluminide alloy is produced (Eq. (1) [1].

A Comparison of the Processing Routes on the Properties of Mg Reinforced With Nanosize MgO

Powder metallurgy and liquid metallurgy techniques were used to fabricate Mg reinforced with different volume fraction of nano-size MgO particles. A higher volume fraction of MgO particles could be added into the Mg matrix when the liquid metallurgy technique was used. Microstructural analysis was carried out to examine the distribution of the nanoparticles in the Mg matrix when different processing routes were chosen. Individual particles together with sparsely distributed agglomerations could be discerned in the Mg matrix for both processing routes. Mechanical properties results revealed that a more substantial improvement in macrohardness and tensile properties could be achieved by using the liquid metallurgy route where a higher amount of nano-size MgO particles could be incorporated.

Corrosion Detection/Quantification on Thin-Wall Structures Using Multi-Mode Sensing Combined With Statistical and Time-Frequency Analysis

In this paper, we present a multiple mode sensing methodology to detect active corrosion in aluminum structure utilizing the broadband piezoelectric wafer active sensors. This method uses ultrasonic Lamb wave complemented with the electromechanical impedance measurement to detect, quantify, and localize the corrosion progression in plate-like structures. The ultimate objective of this research is to develop in-situ multimode sensing system for the monitoring and prediction of critical aerospace structures that can be used during in-service period, recording and monitoring the changes over time. The test experiments were conducted on an aluminum plate installed with a five sensor network using 7-mm piezoelectric wafer active sensors. The corrosion was emulated as material loss of an area of 50mm 38mm on the other surface of the plate. Detection of corrosion and its growth was first conducted using the Lamb wave method in pitch-catch mode. The corroded area resulted in a thickness loss on the Lamb wave propagation and caused the amplitude and phase changes in the structural responses. The experimental data was first evaluated by the statistics-based damage indicator using root mean square deviation. Though the damage indicator is able to detect the presence of the corrosion and identify the corrosion location quantitatively, it failed in giving the right indication of corrosion development. A more corrosion signal processing based method, the cross time-frequency analysis, was proposed and used to analyze the phase characteristics of the data set. This cross time-frequency analysis was found more reliable and precise for detecting the corrosion progression compared with the damage indicator method.

Atomistic Modeling of Fatigue Crack Growth in Magnesium Single Crystals Under Cyclic Loading

The fatigue crack propagation behavior of magnesium single crystal was analyzed using molecular dynamics simulation. The inter-atomic potential used in this investigation is Embedded Atom Method (EAM) potentials. The studies of the influences of crystal orientation and strain rate were perfomred using CC (center crack) and EC (edge crack) specimen. For CC specimen, the periodic boundary conditions were assigned in the x and z direction, while for EC specimen, only z direction was allowed periodic boundary conditions. In order to study the orientation dependence of fatigue crack growth mechanism, ten crystal orientations of initial crack, namely, orientation A-(12¯10) [101¯0], orientation B-(101¯0)[12¯10], orientation C-(101¯0)[0001], orientation D-(12¯10)[0001], orientation E-(0001)[101¯0], orientation F-(0001)[12¯10], orientation G (101¯1)[1¯012], orientation H (101¯1)[12¯10], orientation I (101¯2)[101¯1], and orientaton J (101¯2)[12¯10] were analyzed and the simulation results reveal that the fatigue crack growth rate and the crack path vary significantly with the crystallographic orientations of initial crack. The growth rate of orientaton G is the highest and the resistance of fatigue crack growth of orientation B is the highest. A CC specimen was employed to demonstrate the fatigue damage caused by pyramidal slip band under increased maximum strain cyclic loading in the specimen of orientation E. The analysis of the influences of strain rate was carried out on the orientation C, D, F, and G and the results revealed that the growth rate of fatigue crack decreasing with increasig strain rate. Fatigue growth rate can be expressed by da/dN = cΔCTOD, where the constant c was determined by the present atomistic simulations. The values of the constant c are large and vary widely from on orientation to another.