Semi-analytical thermoelastic stress solutions for a single phase, homogeneous, and finite-width slab or thick cylinder with a constant-velocity growing or receding boundary under unit-loading were derived. Initially, a semi-analytical solution for the heat equation for a slab with a growing or receding boundary was derived in the Laplace domain, and a series representation was then used to approximate the inverse Laplace transform in the time domain. Conformal mapping was then used to transform the slab solution to an annulus. The resulting semi-analytical solutions were used with elasticity relationships to determine the resulting transient thermoelastic stresses. All solutions allow for convection on the fixed boundary that is the opposite side for a plate and outer radius for the cylinder. Once derived, the semi-analytical stress predictions were compared to finite element simulations with excellent agreement. Given the changing thickness, both the thermal and stress states cannot reach true steady-state equilibrium, especially for faster growth or recession rates. Indeed, the temperature states and resulting stresses become somewhat linear with respect to time, reflecting the constant velocity of growth or recession. In practice, the resulting solutions can be used to determine transient stresses during machining, wear, erosion, corrosion, and/or additive manufacturing, especially for lower temperature solid-state methods such as cold-spray.

Thermal conduction considerations of a solid media with moving boundaries are of great interest in many research areas. Unfortunately, it is very difficult to find analytical or semi-analytical solutions for the single-phase heat equation in real-time with a growing or receding boundary. While non-numerical solutions for infinite and semi-infinite domains are available, these cannot accurately model many common situations. In order to overcome this shortcoming, an approximate semi-analytical solution for the heat equation for a single phase, homogeneous, and finite-slab with a growing or receding boundary under unit loading was derived using the Laplace transform method and Zakian series representation of the inverse Laplace transform. Predictions were compared to finite element solutions with good agreement obtained for low to moderate growth or recession rates with improvements shown to be possible by using a heuristic approach. Applications of this work could include direct or inverse prediction of temperatures during machining, wear, corrosion, and/or additive manufacturing via cold spray.

A semi-analytical solution for the thermal conduction of a single phase, homogeneous, circular, hollow-cylinder with a growing or receding inner radius at a constant rate under unit-loading was derived using conformal mapping, Laplace transformation, and a Zakian series representation of the inverse Laplace transform. All solutions allow for convection on the fixed outer radius. Predictions were compared to finite element simulations with excellent agreement observed. Given the changing thickness, thermal transients could not reach true steady-state equilibrium, especially for faster growth or recession rates. Indeed, the temperature states become somewhat linear with respect to time, reflecting the constant velocity of growth or recession. In practice, the resulting solutions can be used to determine temperatures during machining, wear, erosion, corrosion, and/or additive manufacturing, especially for lower temperature solid-state methods such as cold-spray.

Cold-Spray is a low-temperature method that can both modify and coat a surface via a high-velocity jet of solid-phase particles. The method been shown to be capable of both preparing and coating the lower surface of a nuclear reactor to enhance the formation of vapor microjets and improve the Critical Heat Flux (CHF), thus avoiding boiling crises during emergency external cooling. Microporous coatings were created by spraying a binary mixture with the sacrificial element then etched away. Quenching experiments on uncoated and coated surfaces showed that CHF values for the coated vessel developed faster and were consistently higher.

Quantitative fractography has been hindered by the lack of tools capable of accurately characterizing fracture modes and crack initiation locations. An easily implementable, non-destructive image analysis-based tool utilizing fractal and topological techniques was developed to overcome these traditional shortcomings. With this tool, feature complexity, heterogeneity, and connectivity are quantified through the calculation of fractal dimensions and lacunarity and topology measurements, respectively. Validation of this tool was performed on complex fatigue fractures in additively manufactured Ti-6Al-4V, with fracture initiation sites at near surface, sub-surface, and internal defects and fatigue fracture modes being easily differentiated.

Advances in numerical algorithms for Inverse problems has led to significant leaps in our ability to model complex situations of importance. Nonetheless, there is still a strong need for analytical approaches that can be used to calibrate/validate numerical simulations since the old adage of Trust, but Verify is sound advice. Moreover, access to Inverse codes is often limited despite a strong need. Fortunately, generalized Inverse formulations capable of good approximations can be obtained for linear problems by using a known Direct solution. Duhamel’s Integral (convolution) and a systems Unit Response are first employed to derive a Direct solution, with generality maintained via an arbitrary function with coefficients such as polynomials to described the time-dependent boundary condition. For the ensuing Inverse problem, the Direct solution in the form of a more complex polynomial with its coefficients now considered unknown are then used to enforce remotely measured data using the Levenberg–Marquardt algorithm. Once the coefficients are determined, the original function (polynomial or alternative) now describes the unknown boundary condition. Intriguingly, an approximate Inverse Laplace Transform method applied to the Convolution Integral allows for relatively simple formulations for both the Direct and Inverse problems; for this approximation, only the remotely measured data along with the Unit Response as the solitary system information are required. However, the predicted boundary temperature history tends to be shifted forward in time indicating the influence of the thermal thickness and will also reflect any errors in the data. Accuracy can be enhanced by correcting (back-shifting) for the penetration time to the measurement depth and/or using least-squares smoothing. When the various generalized solutions were used for a plate and a thick-cylinder with a time-dependent thermal boundary-condition on one surface and convection on the other, good-to-excellent Inverse predictions were observed; the methods can be adapted so that remotely measured and near instantaneous surface-strains can also be used to determine an unknown surface temperature history. Finally, the convolution-based approach can easily be adapted for 1-degree of freedom vibration problems.

Graphene has gained prominence owing to its excellent mechanical, thermal, and electrical properties. Indeed, polymer-graphene composites have demonstrated superior mechanical properties over monoliths and are well suited for seals and bearings. The objective of this work was to gain a fundamental understanding of the friction and wear mechanics of epoxy reinforced by graphene-based fillers. Epoxy composites were prepared with three commercially available graphene/graphite fillers identified as KS6, synthetic graphite powder; xGNP, graphene platelets; and iGO, industrial graphite oxide and tribologically evaluated in a ball-on-disc configuration at sliding velocities from 0.047 to 1.61 m/s. It was found that only the epoxy +5 wt% iGO demonstrated a lower wear rate among other fillers, with 1 wt% being the best for a range of iGO content. The addition of iGO improved the friction and wear resistance due to functional groups on the surface and the resulting enhanced adhesion; it was the nature of the surface film rather than the bulk polymer that dictated the tribological response. Although further studies are warranted, there are indications that harder particles such as iGO may have limited the rate of detachment and wear by arresting crack propagation in the matrix and filler dispersion. The wrinkled morphology of iGO led to better interlocking with the matrix, thereby limiting the detachment of particles; the filler-matrix interaction between functional groups on the graphene oxide surface may also play an important role. These results have important implications for improving the manufacture of composites with superior tribological and mechanical properties.

Thick plates that are thermally loaded on one surface with convection on the other are often encountered in engineering practice. Given this wide utility and the limitations of most existing solutions to an adiabatic boundary condition, generalized direct thermal solutions were first derived for an arbitrary surface loading as modeled by a polynomial and its coefficients on the loaded surface with convection on the other. Once formulated, the temperature solutions were then used with elasticity relationships to determine the resulting thermal stresses. Additionally, the inverse thermal problem was solved using a least-squares type determination of the aforementioned polynomial coefficients based on the direct-solution and temperatures measured at the surface with convection. Previously published relationships for a thick-walled cylinder with internal heating/cooling and external convection are also included for comparison. Given the versatility of the polynomial solutions advocated, the method appears well suited for complicated thermal scenarios provided the analysis is restricted to the time interval used to determine the polynomial and the thermophysical properties do not vary with temperature.

Plasma-spray forming has been used to fabricate thick-wall tubes of MoSi2 and MoSi2 containing concentric layers of Al2O3. This process is being investigated as a potential fabrication method for producing tubular components of MoSi2 and MoSi2 composites for use in high temperature fuel-burner applications. Results will be reported on the spray forming method used to produce tubes of various sizes. The room temperature strength of pure MoSi2 tubes in the as-deposited condition, and after heat-treating at 1500 °C for 2 hours in vacuum, will also be reported. The strength of plasma sprayed MoSi2 tubes were measured via diametral compression of O-ring and C-ring sections in air at room temperature. Qualification of the strength distribution was based on Weibull statistical theory.

Directed Energy Deposition is a near net-shape, additive manufacturing process that uses high-energy lasers for powder melting and consolidation. While a detailed knowledge of the thermal histories of the process can help understand and ultimately predict the resulting microstructure, residual-stresses, and/or material properties of the component, experimental limitations usually restrict all temperature measurements to far-field locations. When fixed, these measurements become increasingly removed from the laser/material interactions as the build process unfolds. To help offset this limitation, a relatively straightforward method using finite-elements and a fixed far-field measurement was developed that considers experimental processing conditions such as a moving heat source and relevant (and evolving) boundary conditions to generate more complete thermal histories. In essence, an inverse problem was iteratively solved using a direct computational approach. Once validated, the model was then used over multiple depositions with the outcome discussed relative to the agreement and disparities in peak temperatures, heating, and cooling rates. The increasing importance of the growing surface area and evolving radiative and convective boundary conditions with each layer was clearly demonstrated

Titanium and its alloys possess several attractive properties that include a high strength-to-weight ratio, biocompatibility, and good corrosion resistance. However, due to their poor wear resistance, titanium components need to undergo surface hardening treatments before being used in applications involving high contact stresses. Laser nitriding is a thermochemical method of enhancing the surface hardness and wear resistance of titanium. This technique entails scanning the titanium substrate under a laser beam near its focal plane in the presence of nitrogen gas flow. At processing conditions characterized by low scan speeds, high laser powers, and small off-focal distances, a nitrogen plasma can be struck near the surface of the titanium substrate. When the substrate is removed, this plasma can be sustained indefinitely and away from any potentially interacting surfaces, by the laser power and a cascade ionization process. This paper presents a critical review of the literature pertaining to the laser nitriding of titanium in the presence of a laser-sustained plasma, with the ultimate objective of forming wide-area, deep, crack-free, wear-resistant nitrided cases on commercially pure titanium substrates.

In this paper, a method of forming hard, wide-area, crack-free, and wear-resistant nitrided cases on commercially-pure titanium using a 3.5kW CO2 laser-sustained plasma is described. This surface hardening method was comprised of two steps: (1) a laser-sustained nitrogen plasma was first used to nitride the titanium substrate; and (2) a laser-sustained argon plasma was then employed to remelt the nitrided layer deposited in the first step. Previous research using single laser trail experiments had shown that the (second) remelting step can eliminate cracks formed during the (first) nitriding step and homogenize the nitrided layer. In this work, the two-step nitriding-remelting process was extended to wider surface areas by depositing multiple overlapping trails at four different nitriding speeds and a constant remelting speed. The hardened layer was characterized using x-ray diffraction (XRD), optical metallography, and hardness testing. Reciprocating ball-on-flat wear tests were conducted to assess the wear resistance of the nitrided case, with the wear scar being characterized using scanning electron microscopy (SEM) and optical profilometry. Crack-free, hard cases of depths up to 600μm and average hardness values up to 641±86HV0.3 were observed. The LSP nitriding-remelting treatment was found to improve the wear resistance of the base metal (CP-Ti) by up to 80%.

During a reaction-initiated accident (RIA) or loss of coolant accident (LOCA), passive external-cooling of the reactor lower head is a viable approach for the in-vessel retention (IVR) of Corium; while this concept can certainly be applied to new constructions, it may also be viable for operational systems with existing cavities below the reactor. However, a boiling crisis will inevitably develop on the reactor lower head owing to the occurrence of critical heat flux (CHF) that could reduce the decay heat removal capability as the vapor phase impedes continuous boiling. Fortunately, this effect can be minimized for both new and existing reactors through the use of a cold-spray-delivered, microporous coating that facilitates the formation of vapor microjets from the reactor surface. The microporous coatings were created by first spraying a binary mixture with the sacrificial material then removed via etching. Subsequent quenching experiments on uncoated and coated hemispherical surfaces showed that local CHF values for the coated vessel were consistently higher relative to the bare surface. Moreover, it was observed for both coated and uncoated surfaces that the local rate of boiling and local CHF limit varied appreciably along the outer surface. Nevertheless, the results of this intriguing study clearly show that the use of cold spray coatings could enhance the local CHF limit for downward facing boiling by more than 88%. Moreover, the cold-spray process is amenable to coating the lower heads of operating reactors.

During a Reaction Initiated Accident (RIA) or Loss of Coolant Accident (LOCA), passive external-cooling of the reactor lower head is a viable approach for the in-vessel retention of Corium; while this concept can certainly be applied to new constructions, it may also be viable for operational systems with existing cavities below the reactor. However, a boiling crisis will inevitably develop on the reactor lower head owing to the occurrence of Critical Heat Flux or CHF that could reduce the decay heat removal capability as the vapor phase impedes continuous boiling. Fortunately, this effect can be minimized for both new and existing reactors through the use of a Cold-Spray delivered, micro-porous coating that facilitates the formation of vapor micro-jets from the reactor surface. The micro-porous coatings were created by first spraying a binary mixture with the sacrificial material then removed via etching. Subsequent quenching experiments on uncoated and coated hemispherical surfaces showed that local CHF values for the coated vessel were consistently higher relative to the bare surface. Moreover, it was observed for both coated and uncoated surfaces that the local rate of boiling and local CHF limit varied appreciably along the outer surface. Nevertheless, the results of this intriguing study clearly show that the use of Cold Spray coatings could enhance the local CHF limit for downward facing boiling by more than 88%. Moreover, the Cold-Spray process is amenable to coating the lower heads of operating reactors.

An inverse vibration solution was derived for single DOF systems (undamped and damped) using a method based on the least-squares determination of polynomial coefficients representing the unknown force excitation. Initially, a closed-form and generalized direct-solution to the Vibration Equation was derived using standard polynomials to represent the excitation; while simple polynomials and a single DOF system were chosen to demonstrate the method analytically, more sophisticated functions such as Splines, B-Splines, Basis Functions, and/or Finite-Element analysis can be used with the advocated method. In order to solve the inverse problem, the generalized solutions were then used to enforce measured displacement data and the Levenberg-Marquardt algorithm employed to determine the unknown polynomial coefficients representing the base excitation. Results including random errors added to the data (up to ±5%) indicated that the method could be used to determine a time varying excitation; indeed, good agreement was seen between exact and polynomial solution (undamped), as well as an inversely predicted excitation for both damped and undamped systems subjected to a half-sine force history. Given the results, the method appears well suited for complicated scenarios provided the response can be approximated by a polynomial (or more sophisticated functions) and inverse calculations are restricted to the time interval bounding the measured data. In addition, the least-squares inverse approach should be adaptable to more complicated system and excitations by using general purpose finite-element codes. For the current analysis, the least-squares smoothing inherent in the process appears to have helped minimize the influence of measurement errors.

Bond strength and the lubrication potential of coatings made of 7 µm Hexagonal Boron Nitride particles encapsulated with nickel (hBN-Ni), and deposited onto aluminum 6061 substrates via cold spray were examined; for all tests, N2 was used as the carrier gas at a temperature of 480 °C and pressure of 2.4 MPa. Results showed significant improvement in both wear resistance and reduced surface friction. Coated samples also demonstrated unexpected high bond strength, which was much greater than pure nickel cold sprayed onto aluminum. However, while the results were truly promising, the primary reason for the observed high bond strength could not be explained using existing cold spray theories which were primarily developed for pure metal particles. Based on the present findings compared to cold-sprayed layers of composite nickel-nickel (nickel particles encapsulated with nickel), a mechanism for bonding of hBN-Ni particles to aluminum based on the level of plastic deformation and hardenability is proposed. Indeed, the high bond strength between the coating and substrate is related to the relatively high initial ductility of the nickel encapsulation, compliance of the hBN, as well as the ensuing significant plastic deformation of the composite particles during cold spray deposition.

In-vessel retention by passive external reactor vessel cooling under severe accident conditions is a viable approach for retention of radioactive core melt within the reactor vessel. In this study, a new and versatile coating technique known as “cold spray” that can readily be applied to operating and advanced reactors was developed to form a microporous coating on the outer surface of a simulated reactor lower head. Quenching experiments were performed under simulated in-vessel retention by passive external reactor vessel cooling conditions using test vessels with and without cold spray coatings. Quantitative measurements show that for all angular locations on the vessel outer surface, the local critical heat flux (CHF) values for the coated vessel were consistently higher than the corresponding CHF values for the bare vessel. However, it was also observed for both coated and uncoated surfaces that the local rate of boiling and local CHF limit vary appreciably along the outer surface of the test vessel. Nonetheless, results of this intriguing study clearly show that the use of cold spray coatings could enhance the local CHF limit for downward-facing boiling by > 88%.

The effects of severe thermal and pressure transients on coated substrates with indentation-induced, blister defects were analyzed by experimental and finite element methods. Cohesive zone properties evaluated in a previous study were first used in an implicit indentation simulation. Indentation simulation results then served as the initial conditions for explicit modeling of interfacial flaw evolution due to the already determined thermal and pressure transients that included interstitial pressure in the defect. The thermal structural model was used to assess the transient thermal- and stress-states and the propensity for fracture related damage and evolution while undergoing severe convective heating and pressure loading analogous to gun tube conditions. Results illustrated the overall benefits of the in-phase applied pressure in terms of suppressing crack growth except when delayed interstitial loading was considered. Thermal capacitance was also studied and it was found that crack growth decreased significantly with higher specific heat and demonstrates the potential importance of coating thermophysical properties.

Although there are several techniques available for the evaluation of various interfacial cohesive zone properties of coatings, each has difficulties and limitations. For instance, the four-point bend method is often plagued by excessive deformation and plasticity without any coating delamination, and the button test is limited by the constant stress distribution assumption. Given such issues, a new hybrid numerical/experimental technique has been developed that is based on ball indentations, which can usually induce delamination regardless of the materials used. Using this method, indentations were first made on coated samples (nickel-on-steel and aluminum-on-aluminum for the current study) to intentionally create localized, circular delaminations, the initiation and dimensions of which were functions of the applied loads. Numerical models using finite element analysis were then used with the known indentation loads to inversely evaluate the cohesive zone properties, which reproduced the experimental results. The technique was validated based on the successful prediction of the indentation results evaluated using properties from four-point bend results.