A. E. Segall

TRANSIENT ANALYSIS OF THICK-WALLED PIPING UNDER POLYNOMIAL THERMAL LOADING

Abstract The transient response of a thick-walled pipe subjected to a generalized excitation of temperature on the internal surface was derived using Duhamel’s relationship. Generalization of the temperature excitation was achieved by using a polynomial composed of integral- and half-order terms. In order to avoid the evaluation of recurring functions in the complex domain, Laplace transformation and a 10-term Gaver–Stehfest inversion formula were used to perform part of the necessary integrations. Excellent agreement between the derived relationships and existing analytical and finite-element solutions was seen for a thick-walled cylinder subjected to an asymptotic temperature rise on the exposed surface. As intended, the use of a smoothed polynomial allows the incorporation of empirical date not easily represented by standard functions. Moreover, the resulting relationships are easily programmed and can be used for a wide range of nuclear, piping, and cylindrical vessel applications.

Self-Lubricating Cold-Sprayed Coatings Utilizing Microscale Nickel-Encapsulated Hexagonal Boron Nitride

It is often beneficial to modify surfaces to gain desirable properties such as improved wear and friction resistance. Self-lubricating coatings can improve the performance of contacting surfaces and extend component lifetimes by reducing the coefficient of friction and/or improving resistance to specific wear modes. With these goals in mind, self-lubricating coatings of hexagonal boron nitride (hBN) particles in a deposited nickel matrix were investigated and optimized for friction and wear. These self-lubricating coatings were created via high-velocity particle consolidation or cold spray using micrometer-sized hBN powder encapsulated by nickel and nickel phosphorous alloys. Relatively thick nickel encapsulation via electrolesss Ni plating was required to aid in coating bonding/formation by “tricking” the hBN into acting as monolithic Ni during deposition. Once deposited on aluminum substrates, the coatings were analyzed and found to exhibit enhanced mechanical and tribological properties such as high bond strength and microhardness, a relatively low coefficient of friction, and improved reciprocating wear behavior relative to pure cold-sprayed Ni coatings. Furthermore, the encapsulation process was found to be both scalable and amenable to relatively small hBN particles.

Technical Note Transient analysis of internally heated tubular components with exponential thermal loading and external convection

Abstract Tubular components subjected to severe thermal conditions are widely used by industry in many applications such as radiant burners and heat exchangers. As would be expected, these applications often involve thermal transients that are severe enough to induce fatigue, and eventually failure. In order to be able to predict these failures and the thermoelastic stresses that are the underlying cause, a detailed understanding of the transient temperature distributions is essential. Although there have been numerous analytical models, they have been limited to severe and often unrealistic step or linear temperature changes [1, 2] . More recent studies [3–5] have shown the potentially complicated time dependence of the surface temperature loading and need for improved estimates using finite-element analysis along with a temperature-matching scheme when temperature dependent materials properties are involved. However, the finite element calculations were iterative and required an analytical starting point for the prescribed surface temperatures, as well as a means for verifying the numerical solution. Because of these needs, an analytical model of the thermal transients developed within tubular components subjected to a more realistic, time dependent boundary condition is ultimately required. Accordingly, this paper derives the equations for a hollow cylinder with a plausible exponential boundary condition of the form H ( t ) = V (1− e −ct ) for the internal surface with external convection to the external environment. Additionally, the unit response of a cylinder subjected to an internal step load with external convection is also derived.

Cold-Sprayed Ni-hBN Self-Lubricating Coatings

Feedstock preparation strategies were explored to produce composite admixed, milled, and precoated (encapsulated) powders of nickel–hexagonal boron nitride (Ni-hBN) for cold-sprayed self-lubricating coatings. The resulting cold-sprayed coatings were then examined for microstructural homogeneity and composition, as well as bond strength, microhardness, and relevant wear behaviors. Though admixed powders were easy to prepare and economical, milled and precoated formulations provided the advantage of aiding contact between Ni and lubricant powders prior to spraying that ultimately improved deposition and properties. The maximum amount of hBN that could be effectively built into the cold-sprayed Ni coatings was approximately 6 wt%. Results of the study also indicated that the composite coatings exhibited slightly higher hardness and reduced adhesive strength relative to a baseline of pure Ni layers. Moreover, some reductions in friction and expected decreases in bond strength and lubricant uniformity were observed when more than 4 wt% of lubricant was retained in the coatings. Given these findings, the most promising path to improve the amount, uniformity, and influence of the lubricant may be to encapsulate smaller particles with thicker levels of Ni to “trick” the composite particle to bond as pure Ni.

Evaluation of the reciprocating-wear behavior of unlubricated hypereutectic Al-Si alloys

The reduction of abrasive and scuffing wear between aluminum and cast-iron wear-pairs is an important goal for the automotive industry given the implications for improved engine performance and reliability. Hypereutectic aluminum-silicon alloys such as B390 may help lead the way towards this goal because of their potential for wear resistance and durability. Yet, despite this potential, B390 has not been evaluated under the severe reciprocating conditions typical of automotive applications. Moreover, the influence of various manufacturing and processing steps on the resulting wear resistance of the alloy has not been studied at all. To help fill this void, a series of unlubricated tests were conducted using cast, spray-formed, spray-formed then extruded, and semi-solid formed variations of B390 reciprocated against gray cast-iron under a constant contact-stress. Originally, the weight-loss per reciprocating distance was measured and converted to volume-loss to determine the steady-state wear rates for each alloy variation. However, it was determined that debris from the mating cast-iron surface was adhering to the B390 and obscuring the actual material lost to wear. To compensate for the trapped iron debris, the volume-loss was directly calculated from the changing contact area or “flat” originally measured for the loading adjustments. After this correction, the data indicated that the spray-formed, spray-formed then extruded, and semi-solid formed all experienced measurable, albeit modest decreases in their wear rates relative to the cast B390. However, there were not any significant disparities in the observed wear rates between the spray-formed, spray-formed then extruded, and semi-solid formed hypereutectic alloy despite their processing and microstructural differences.

Transient Surface Strains and the Deconvolution of Thermoelastic States and Boundary Conditions

Thermoelastic states as they pertain to thermal-shock are difficult to determine since the underlying boundary conditions must be known or measured. For direct problems where the boundary conditions such as temperature or flux, are known a priori, the procedure is mathematically tractable with many analytical solutions available. Although this is more practical from a measurement standpoint, the inverse problem where the boundary conditions must be determined from remotely determined temperature and/or flux data are ill-posed and therefore inherently sensitive to errors in the data. Moreover, the limited number of analytical solutions to the inverse problem rely on assumptions that usually restrict them to timeframes before the thermal wave reaches the natural boundaries of the structure. Fortunately, a generalized solution based on strain-histories can be used instead to determine the underlying thermal excitation via a least-squares determination of coefficients for generalized equations for strain. Once the inverse problem is solved and the unknown boundary condition on the opposing surface is determined, the resulting polynomial can then be used with the generalized direct solution to determine the thermal- and stress-states as a function of time and position. For the two geometries explored, namely a thick-walled cylinder under an internal transient with external convection and a slab with one adiabatic surface, excellent agreement was seen with various test cases. The derived solutions appear to be well suited for many thermal scenarios provided that the analysis is restricted to the time interval used to determine the polynomial and the thermophysical properties that do not vary with temperature. While polynomials were employed for the current analysis, transcendental functions and/or combinations with polynomials can also be used.

Elevated Temperature Fretting Evaluations Using a Flat-on-Flat Configuration

Traditional fretting tests rely on the high Hertzian stress contacts between a cylinder or sphere and a flat surface to generate oxide particles and an eventual wear scar. However, this configuration does not always match the stresses and wear mechanism associated with parallel surfaces where fretting may only initiate in limited regions of contacting asperities. To simulate these conditions at 175°F, fretting wear tests were used to evaluate the performance of High Velocity Oxy-Fuel (HVOF) and Plasma-sprayed Cu Ni In coatings for the reduction of gross-slip fretting (relative displacements of 100 μm) experienced between mating beta-c titanium bosses and 4340 steel lugs. Scanning electron microscopy was then used to compare the frequency and severity of fretting wear on the titanium blocks. Results of the analysis indicated the viability of the lubricious coatings for eliminating the instances of fretting. Furthermore, the tests indicated the usefulness of the flat-on-flat testing configuration for illuminating the potentially random occurrences of fretting damage between parallel surfaces.

A Critical Review of the Diametral Compression Method for Determining the Tensile Strength of Spherical Aggregates

The validity of diametral compression as an effective means of determining the tensile strength of spherical ceramic bodies has often been questioned. In this paper, a comprehensive review of the original work, as well as alternative studies that suggest shortcomings of the original method, is made. For comparative purposes, data recently collected via diametral compression on aluminosilicate aggregates is presented in the context of these latter works and compared with the original methodology. Overall, results indicate that the diametral compression test can indeed provide an accurate measure of tensile strength when several important test criteria are met.

Approximate Direct and Inverse Relationships for Thermal and Stress States in Thick-Walled Vessels Under Thermal Shock

Approximate solutions were derived for the transient through steady-state thermal-stress fields developed in thick-walled vessels subjected to a potentially arbitrary thermal shock. In order to accomplish this, Duhamel’s integral was first used to relate the arbitrary thermal loading to a previously derived unit kernel for tubular geometries. Approximate rules for direct and inverse Laplace transformations were then used to modify the resulting Volterra equation to an algebraically solvable and relatively simple form. The desired thermoelastic stress distributions were then determined using the calculated thermal states and elasticity theory. Good agreement was seen between the derived temperature and stress relationships and earlier analytical and finite-element studies of a cylinder subjected to an asymptotic exponential heating on the internal surface with convection to the outer environment. It was also demonstrated that the derived relationships can be used to approximate the more difficult inverse (deconvolution) thermal problem for both exponential (monotonic) and triangular (non-monotonic) load histories. The use of polynomial of powers tn∕2 demonstrated the feasibility of employing the method with empirical data that may not be easily represented by standard functions. For any of the direct and inverse cases explored, the resulting relationships can be used to verify, calibrate, and/or determine a starting point for finite-element or other numerical methods.

The Sliding Wear Behavior of Cobalt-Based Hardfacing Alloys Used in Steam Valve Applications©

The tribological characteristics of a low carbon cobalt-based alloy were, compared to a higher carbon cobalt-based alloy for application in a steam valve. In order to simulate the severe conditions typical for a steam valve, wear tests were conducted using a pin-on-rotating ring test configuration in an cobalt-based steam atmosphere at 260° C After the wear tests were completed, both optical and scanning electron microscopy was used to determine the mode of material removal during the wearing process. A statistical analysis of the wear data involving several parameters did not establish a clear correlation between material properties and the observed wear rates. Nevertheless, the tribological performance of the lower carbon cobalt-based alloy was similar to the higher carbon cobalt-based alloy under the test conditions of high temperature steam. In both cases, adhesive wear was found to be the dominant material removal mechanism. Presented as a Society of Tribologists and Lubrication Engineers paper at th...