The results of an investigation of thermomechanical fatigue (TMF) on a superalloy specimen, with an applied thermal barrier coating (TBC) and a circular through hole, are presented. Tensile loads were applied in phase with increasing temperature. Damage evolution in the form of cracks develops in the TBC adjacent to the hole. These cracks run perpendicular to the loading axis. Stress mapping of the thermally grown oxide (TGO) using luminescence spectroscopy determined an increase in compressive residual stress with increasing TMF cycling. Scanning electron microscopy examination, following cross sectioning, determined the TBC cracks to be vertical separations of the columnar yttria‐stabilized zirconia (YSZ) top coat. Microscopic damage mechanisms in the form of plasticity (bending of YSZ columns) and TGO cracking were observed. Imperfections in the bond coat are associated with these vertical separations. Energy‐dispersive element mapping of these imperfections indicated a composition of alumina and mixed Cr, Co, and Ni oxides.
A flexural test for the in situ measurement of the delamination toughness of the interface between a thermally grown oxide and a bond coat in the presence of a thermal barrier coating (TBC) has been implemented. To accomplish the testing, a section of the substrate was removed by microelectro‐discharge machining and a precrack introduced through the TBC by center point loading. This was followed by...
The finite element method is used to analyze the elastodynamic response of a columnar thermal barrier coating due to normal impact and oblique impact by an erosive particle. An assessment is made of the erosion by crack growth from preexisting flaws at the edge of each column: it is demonstrated that particle impacts can be sufficiently severe to give rise to columnar cracking. First, the transient stress state induced by the normal impact of a circular cylinder or a sphere is calculated in order to assess whether a 2D calculation adequately captures the more realistic 3D behavior. It is found that the transient stress states for the plane strain and axisymmetric models are similar. The sensitivity of response to particle diameter and to impact velocity is determined for both the cylinder and the sphere. Second, the transient stress state is explored for 2D oblique impact by a circular cylindrical particle and by an angular cylindrical particle. The sensitivity of transient tensile stress within the columns to particle shape (circular and angular), impact angle, impact location, orientation of the angular particle, and to the level of friction is explored in turn. The paper concludes with an evaluation of the effect of inclining the thermal barrier coating columns upon their erosion resistance.
Evidence has accumulated recently that a high‐capacity electrode of a lithium‐ion battery may not recover its initial shape after a cycle of charge and discharge. Such a plastic behavior is studied here by formulating a theory that couples large amounts of lithiation and deformation. The homogeneous lithiation and deformation in a small element of an electrode under stresses is analyzed within nonequilibrium thermodynamics, permitting a discussion of equilibrium with respect to some processes, but not others. The element is assumed to undergo plastic deformation when the stresses reach a yield condition. The theory is combined with a diffusion equation to analyze a spherical particle of an electrode being charged and discharged at a constant rate. When the charging rate is low, the distribution of lithium in the particle is nearly homogeneous, the stress in the particle is low, and no plastic deformation occurs. When the charging rate is high, the distribution of lithium in the particle is inhomogeneous, and the stress in the particle is high, possibly leading to fracture and cavitation.
In engineered systems where thermal strains and stresses are limiting, the ability to tailor the thermal expansion of the constituent materials independently from other properties is desirable. It is possible to combine two materials and space in such a way that the net coefficient of thermal expansion (CTE) of the structure is significantly different from the constituents, including the possibility of zero and negative thermal expansion. Bimaterial lattices that combine low, negative, or an otherwise tailored CTE with high stiffness, when carefully designed, have theoretical properties that are unmatched by other known material systems. Of known lattice configurations with tailorable CTE, only one geometry, a pin‐jointed lattice, has been shown to be stretch dominated and thus capable of having stiffness that approaches its theoretical upper bound. A related lattice with bonded joints, more amenable to fabrication, is developed that has a stiffness and CTE similar to the pinned structure. Analytical models for this rigid‐jointed lattice's CTE and stiffness are developed and compared successfully with numerical results. A near space‐filling, negative thermal expansion version of this lattice is devised and fabricated from titanium and aluminum. CTE measurements on this lattice are made and are well predicted by the analytical and numerical models. These insights guide the design of a family of bonded lattices with low areal density, low or negative CTE, and high stiffness to density ratio. Such lattices are shown to have a thermomechanical response that converges on pin‐jointed behavior when the lattice elements are long and slender.
The development of ceramic composites with three‐dimensional fiber reinforcement architectures formed by textile methods has led to the potential for active shape‐morphing surfaces that can operate in high temperature and variable pressure environments. This technology is of particular interest for hypersonic applications, where SCRAM jet engines require variable inlet geometry to achieve efficient flight over realistic flight profiles and variable flight conditions. The experiments reported here show that significant shape morphing can be achieved and good control of the shape sustained even in the presence of large temperature and pressure gradients. Experiments were carried out using a subscale morphing hypersonic inlet with rectangular cross‐section in a Mach 8 wind tunnel facility with a total temperature of 800 K. The upper surface of the inlet consisted of a C–SiC composite plate (0.7 mm thick, 37.5 cm long, and 11 cm wide) connected to five actuators through a triangular truss support structure. The lower surface was a flat plate instrumented with an array of pressure taps along the flow centerline. As the shape varied, the surface contour was reliably controlled for high efficiency, low loss compression. A factor of six inlet area ratio variation was achieved and good agreement with model predictions was observed.
A simple method is described for measuring material erosion by reaction with water vapor under high‐speed flow conditions, with H2O partial pressures, velocities, temperatures, and erosion rates representative of those experienced in gas turbine engines. A water vapor jet is formed by the feeding water at a controlled rate into a capillary inside a tube furnace, where the large expansion of vaporization within the confines of the capillary accelerates the jet. With modest flow rates of liquid water, steam jets with temperatures up to ∼1400°C and velocities in the range 100–300 m/s have been achieved. The partial pressure of water vapor in the 100% steam jet is the same as in an industrial turbine operating at 10 atm total pressure with 10% water vapor. In preliminary experiments with SiC, erosion rates of the order of 1 μm/h have been observed.
The compression response of extruded aluminum 6061‐T6 corrugated core sandwich columns is investigated. Analytical equations that predict the collapse load are used to generate failure mechanism maps. From these maps dominant failure mechanisms can be identified as a function of various geometric parameters and material properties. Experimental testing and numerical simulations are performed to test...
Cellular materials with periodic architectures have been extensively investigated over the past decade for their potential to provide multifunctional solutions for a variety of applications, including lightweight thermo‐structural panels, blast resistant structures, and high‐authority morphing components. Stiffer and stronger than stochastic foams, periodic cellular materials lend themselves well...
This paper investigates the effects of thin plastically deformable layers embedded in an elastic coating upon debonding of a multilayer from its substrate. Such coatings are normally deposited at high temperature and cooled to ambient, resulting in significant stresses from thermal expansion mismatch. Other stresses can develop during subsequent thermal cycling if volumetric changes such as phase changes or oxidation occur in the system. We present an elastic–plastic model to calculate these stresses in the adhered state (after deposition and cooling but before debonding) and the released state (after delamination). These results are used to calculate the steady‐state energy release rate (ERR) that drives debonding at the interface between the multilayer and the substrate. It is shown that plastic straining in the ductile layers can lead to significant reductions in crack driving force by dissipating energy both before and during coating release. Regime maps are developed to illustrate reductions in ERR in terms of the yield strength and volume fraction of the metal layers. As an example, the model is used to predict the impact of embedded platinum layers within an yttria‐stabilized zirconica coating; the crack ERR for the composite coating is shown to be 30% lower than that for a uniform ceramic coating.
Atmospheric cruise hypersonic vehicles are subject to high viscous heating over large surface areas. Acreage thermal protection systems (TPSs) must be stiff, strong, and light while withstanding large thermal gradients and protecting the cool interior of the vehicle. It is a challenge to design thermal protection to minimize the thermal stresses caused by thermal expansion mismatch. This paper uses a recent concept for low‐thermal‐expansion periodic lattices to propose a sandwich configuration for acreage TPSs. A key aspect of these concepts is that they can be attached to coll structures without inducing thermal stresses during heating. Sandwich TPSs are analyzed and optimized for minimum mass for required performance characteristics, and compared with an optimized baseline system. For performance requirements relevant to atmospheric hypersonic flight, the sandwich TPSs using low‐thermal‐expansion periodic lattices are superior to the baseline system for a large range of operating conditions.
The success of Si‐based ceramics as high‐temperature structural materials for gas turbine applications relies on the use of environmental barrier coatings (EBCs) with low silica activity, such as Ba1−xSrxAl2Si2O8 (BSAS), which protect the underlying components from oxidation and corrosion in combustion environments containing water vapor. One of the current challenges concerning EBC lifetime is the effect of sandy deposits of calcium–magnesium–aluminosilicate (CMAS) glass that melt during engine operation and react with the EBC, changing both its composition and stress state. In this work, we study the effect of CMAS exposure at 1300°C on the residual stress state and composition in BSAS–mullite–Si–SiC multilayers. Residual stresses were measured in BSAS multilayers exposed to CMAS for different times using high‐energy X‐ray diffraction. Their microstructure was studied using a combination of scanning electron microscopy and transmission electron microscopy techniques. Our results show that CMAS dissolves the BSAS topcoat preferentially through the grain boundaries, dislodging the grains and changing the residual stress state in the topcoat to a nonuniform and increasingly compressive stress state with increasing exposure time. The presence of CMAS accelerates the hexacelsian‐to‐celsian phase transformation kinetics in BSAS, which reacts with the glass by a solution–reprecipitation mechanism. Precipitates have crystallographic structures consistent with Ca‐doped celsian and Ba‐doped anorthite.
The materials we use today for mechanical design are the outcome of at least 3000 years of development, much of it empirical but much the outcome of systematic science. Both approaches have been motivated by the desire for stiffer, stronger, more durable, and lighter structures, progressively populating material property “space”. We first examine the extent to which this space is now filled and estimate...
First‐principles studies of metal/α‐Al2O3 interfaces have revealed strong interfacial stoichiometry effects on adhesion. The metals included Al, Ni, Cu, Au, Ag, Rh, Ir, Pd, Pt, Nb, and β‐NiAl. Metallic and ionic‐covalent adhesive bonding effects were found in varying amounts depending on whether the interfacial stoichiometry is stoichiometric, oxygen‐rich, or aluminum‐rich in a qualitative way. A...
The role of oxidation‐induced layers in the failure process of aluminide‐coated nickel base single crystals subject to high‐temperature fatigue cycling has been investigated experimentally and via finite element analysis. Isothermal strain‐controlled compressive fatigue experiments (R=−∞) with 120 s holds in compression were conducted at 982° and 1093°C. Surface‐initiated cracks containing a layer of alumina progressively grew through the coating layers into the superalloy substrate, ultimately causing failure. Growth stresses in the oxide provided a driving force for extension of the oxide into the softer coating and substrate layers. Finite element modeling shows the rate of growth of the oxide‐filled cracks is sensitive to the strength of the constituent layers and the magnitude of the oxide growth strains. Implications for design of failure‐resistant coating–substrate systems are discussed.
Oxidation of Al2O3 scale‐forming alloys is of immense technological significance and has been a subject of much scientific inquiry for decades. The oxidation reaction is remarkably complex, involving issues of alloy composition, kinetics, thermodynamics, microstructure, mechanics and mechanical properties, crystallography, etc. A brief overview of the formation of passivating, thermally grown oxide...
Previous studies have shown that while stainless‐steel sandwich panels with pyramidal truss cores have a superior blast resistance to monolithic plates of equal mass per unit area, their ballistic performance is similar to their monolithic counterparts. Here, we explore concepts to enhance the ballistic resistance without changing the volumetric efficiency of the panels by filling the spaces within the core with combinations of polyurethane, alumina prisms, and aramid fiber textiles. The addition of the polyurethane does not enhance the ballistic limit compared with the equivalent monolithic steel plate, even when aramids are added. This poor performance occurs because the polymer is penetrated by a hole enlargement mechanism which does not result in significant projectile deformation or load spreading and engagement of the steel face sheets. By contrast, ceramic inserts deform and erode the projectile and also comminute the ceramic. The ceramic communition (and resultant dilation) results in stretching of both steel face sheets and leads to significant energy dissipation. The ballistic limit of this hybrid is about twice that of the equivalent monolithic steel plate. The addition of a Kevlar fabric to the ceramic hybrid is shown to not significantly change the ballistic limit but does reduce the residual velocities of the debris.
The objective of the present study is to assess the capabilities of a recently developed mechanism‐based model for inelastic deformation and damage in structural ceramics. In addition to conventional lattice plasticity, the model accounts for microcrack growth and coalescence as well as granular flow following comminution. The assessment is made through a coupled experimental/computational study of the indentation response of a commercial armor ceramic. The experiments include examinations of subsurface damage zones along with measurements of residual surface profiles and residual near‐surface stresses. Extensive finite element computations are conducted in parallel. Comparisons between experiment and simulation indicate that the most discriminating metric in the assessment is the spatial extent of subsurface damage following indentation. Residual stresses provide additional validation. In contrast, surface profiles of indents are dictated largely by lattice plasticity and thus provide minimal additional insight into the inelastic deformation resulting from microcracking or granular flow. A satisfactory level of correlation is obtained using property values that are either measured directly or estimated from physically based arguments, without undue reliance on adjustable (nonphysical) parameters.
Thermal barrier coatings typically fail on cooling after prolonged thermal cycling or isothermal exposure. The mechanics of spalling requires that first a critical sized portion of the coating separates from the underlying material, then buckles and finally spalls away. The critical size for buckling depends on the thickness of the coating but is several millimeters for typical zirconia coatings 150 μm thick. As‐deposited coatings do not have interface separations but they form on thermal cycling as described in this work based on observations of coating cross‐sections combined with the stress redistribution in the thermally grown oxide imaged using a piezospectroscopic luminescence method. Analysis of the images reveals that small, isolated regions of damage initially form and then grow, linking up and coalescing to form percolating structures across the coating until the buckling condition is attained, the buckle extends and failure occurs by spallation. The piezospectroscopic imaging of the stresses in the thermally grown oxide formed by oxidation beneath thermal barrier coatings provides a form of “stress tomography” enabling the subcritical separations to be monitored.
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