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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Implementation of Sacrificial Support Structures for Hybrid Manufacturing of Thin Walls

Thin-walled features can be difficult to produce with traditional machining methods which often rely on excess stock material for stiffness. This challenge is increased in hybrid manufacturing where the feature is already near net shape before machining. Significant workpiece deflection can result in poor geometric and surface finish tolerances on the finished part. A potential solution to this problem is to implement sacrificial support structures to the as-printed geometry. The supports are then machined away during the finishing portion of the hybrid process. In the present work, several different design parameters for these sacrificial supports were evaluated to determine their impact on the quality of representative thin wall geometry samples. The angle, height, and spacing of triangular support structures were varied for each sample and then machined and examined. The addition of these supports relative to an unsupported configuration provided a deflection reduction of around 0.2 mm. Surface roughness was improved by approximately 1.5 µm. Increasing values of support height were found to correspond to reduced wall deflection. Similarly, decreasing values of support angle and support spacing improved geometric accuracy. Efficiency comparisons showed that increases in print time corresponded to rapidly diminishing gains in geometric accuracy but continued to improve surface roughness. Implications for hybrid finishing of additively manufactured thin-walled structures is briefly discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Evaluation of advanced automatic PET segmentation methods using nonspherical thin-wall inserts

Purpose: The use of positron emission tomography (PET) within radiotherapy treatment planning requires the availability of reliable and accurate segmentation tools. PET automatic segmentation (PET-AS) methods have been recommended for the delineation of tumors, but there is still a lack of thorough validation and cross-comparison of such methods using clinically relevant data. In particular, studies validating PET segmentation tools mainly use phantoms with thick plastic walls inserts of simple spherical geometry and have not specifically investigated the effect of the target object geometry on the delineation accuracy. Our work therefore aimed at generating clinically realistic data using nonspherical thin-wall plastic inserts, for the evaluation and comparison of a set of eight promising PET-AS approaches. Methods: Sixteen nonspherical inserts were manufactured with a plastic wall of 0.18 mm and scanned within a custom plastic phantom. These included ellipsoids and toroids derived with different volumes, as well as tubes, pear- and drop-shaped inserts with different aspect ratios. A set of six spheres of volumes ranging from 0.5 to 102 ml was used for a baseline study. A selection of eight PET-AS methods, written in house, was applied to the images obtained. The methods represented promising segmentation approaches such as adaptive iterative thresholding, region-growing, clustering and gradient-based schemes. The delineation accuracy was measured in terms of overlap with the computed tomography reference contour, using the dice similarity coefficient (DSC), and error in dimensions. Results: The delineation accuracy was lower for nonspherical inserts than for spheres of the same volume in 88% cases. Slice-by-slice gradient-based methods, showed particularly lower DSC for tori (DSC < 0.5), caused by a failure to recover the object geometry. The region-growing method reached high levels of accuracy for most inserts (DSC > 0.76 except for tori) but showed the largest errors in the recovery of pears and drops dimensions (higher than 10% and 30% of the true length, respectively). Large errors were visible for one of the gradient-based contouring methods when delineating drop-shaped inserts. Low DSC due to systematic underestimation of the volumes was observed for our fuzzy clustering method when using nonspherical inserts. The adaptive iterative thresholding method produced the highest DSC score on our nonspherical dataset (DSC > 0.83, except for tori) and showed robustness to the insert geometry. Conclusions: This study investigated the accuracy of eight PET-AS methods for the delineation of objects with a range of nonspherical geometries. The authors’ results confirmed the robustness of some segmentation approaches, but also showed the weaknesses of some of the other methods implemented, which were not observed with spherical inserts. This work therefore highlights the importance of using a variety of thin-wall inserts with complex geometries for the validation of PET-AS methods and provided useful information for further development of the methods tested.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

An integrated two-dimensional modeling method for predicting ductility of thin-walled die cast magnesium

In this paper, a two-dimensional (2D) microstructure-based modeling method is developed in order to predict the ductility of a thin-walled high pressure die cast magnesium (Mg) by considering the three-dimensional (3D) thru-thickness pore distributions. For this purpose, a series of 3D synthetic microstructure-based finite element models and the corresponding 2D models are first generated with various pore volume fractions, pore size distributions and pore shapes. The input material properties for the 2D models are determined based on the 3D cubic model with a spherical pore and the generalized Neuber’s rule. Based on the resulting ductility of the 3D and 2D models, a 3D/2D ductility correlation curve is developed as a function of the characteristics/shape of the input fracture strain curve used in the 3D model. The validity of the ductility correlation curve is examined with casting samples with actual microstructures. Here, the results show that the suggested 2D modeling methodology can be used together with the ductility correlation curve in predicting the ductility of thin-walled Mg castings.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Characterisation of a complex thin walled structure fabricated by selective laser melting using a ferritic oxide dispersion strengthened steel

Oxide dispersion strengthened (ODS) alloys exhibit superior mechanical and physical properties due to the presence of nanoscopic Y(Al, Ti) oxide precipitates, but their manufacturing process is complex. The present study is aimed at further investigation of the application of an alternative, Additive Manufacturing (AM) technique, Selective Laser Melting (SLM), to the production of consolidated ODS alloy components. Mechanically alloyed PM2000 (ODS-FeCrAl) powders have been consolidated and a fine dispersion of Y-containing precipitates were observed in an as built thin-walled component, but these particles were typically poly-crystalline and contained a variety of elements including O, Al, Ti, Cr and Fe. Application of post-build heat treatments resulted in the modification of particle structures and compositions; in the annealed condition most precipitates were transformed to single crystal yttrium aluminium oxides. During the annealing treatment, precipitate distributions homogenised and localised variations in number density were diminished. The resulting volume fractions of those precipitates were 25–40% lower than have been reported in conventionally processed PM2000, which was attributed to Y-rich slag-like surface features and inclusions formed during SLM. - Highlights: • A wall structure was grown from ODS steel powder using selective laser melting. • A fine dispersion of nano-precipitates was apparent in as-build material. • Precipitates were multi-phased containing several elements, e.g. O, Ti, Al, Fe, Cr, Y. • Post-build annealing changed those into typically single-crystalline Y–Al–O. • The anneal also reduced and stabilised the volume fraction of precipitates to ~ 0.006.

36 MATERIALS SCIENCE↗
NASA NTRS1989

Corrections to the thin wall approximation in general relativity

The question is considered whether the thin wall formalism of Israel applies to the gravitating domain walls of a lambda phi(exp 4) theory. The coupled Einstein-scalar equations that describe the thick gravitating wall are expanded in powers of the thickness of the wall. The solutions of the zeroth order equations reproduce the results of the usual Israel thin wall approximation for domain walls. The solutions of the first order equations provide corrections to the expressions for the stress-energy of the wall and to the Israel thin wall equations. The modified thin wall equations are then used to treat the motion of spherical and planar domain walls.

Garfinkle, David↗
NASA NTRS1945

Calculation of Centrally Loaded Thin-Walled Columns Above the Buckling Limit

When thin-walled columns formed from flanged sheet, such as used in airplane construction, are subjected to axial load, their behavior at failure varies according to the slenderness ratio. On long columns the axis deflects laterally while the cross section form is maintained; buckling results. The respective breaking load in the elastic range is computed by Euler's formula and for the plastic range by the Engesser- Karman formula. Its magnitude is essentially dependent upon the length. On intermediate length columns, especially where open sections are concerned, the cross section is distorted while the cross section form is preserved; twisting failure results. The buckling load in twisting is calculated according to Wagner and Kappus. On short columns the straight walls of low-bending resistance that form the column are deflected at the same time that the cross section form changes - buckling occurs without immediate failure. Then the buckling load of the total section computable from the buckling loads of the section walls is not the ultimate load; quite often, especially on thin-walled sections, it lies considerably higher and is secured by tests. Both loads, the buckling and the ultimate load are only in a small measure dependent upon length. The present report is an attempt to theoretically investigate the behavior of such short, thin-walled columns above the buckling load with the conventional calculating methods.

Reinitzhuber, F.↗
DOE OSTI2023

Tooling assembly and method for explosively forming features in a thin-walled cylinder

The present invention provides a method of explosively forming a helical tube from at least one thin-walled cylinder using a tooling assembly. The method includes inserting the at least one thin-walled cylinder into a die of the tooling assembly. The die surrounds the at least one thin-walled cylinder and includes an interior surface that defines a helical thread pattern. The method further includes surrounding the at least one thin-walled cylinder and the die with a casing of the tooling assembly. A cavity is defined by the casing and the thin-walled cylinder. The method further includes positioning an explosive charge within the cavity. The method additionally includes at least partially submerging the tooling assembly. The method further includes detonating the explosive charge. As a result, the at least one thin-walled cylinder is formed into a helical tube that corresponds with helical thread pattern of the interior surface of the die.

Eberl, Kurt R.↗
NASA NTRS1988

Experimental study of thin-walled composite beams

Thin-walled rectangular section composite beams are built out of graphite-epoxy prepreg tape using an autoclave molding technique. Cantilevered beams are tested under static loads using a specially-built apparatus and bending slope and elastic twist at a station are measured using a laser system. Measured bending slope and bending-induced twist distributions are correlated with predicted results obtained using a simple beam analysis as well as a refined finite element analysis. Natural vibration characteristics at different rotational speeds are determined experimentally by testing beams in a specially built 10 ft diameter vacuum chamber. The rotor is excited by piezo-electric crystals and response is determined from strain gages mounted on the surface. For composite beams with bending-twisting couplings (symmetric layup), effects of section warping and transverse shear on structural response are found to be substantial. These results were substantiated by the analyses.

Chandra, Ramesh↗
DOE OSTI2026

Fabrication of thin-walled tubes from alloy 602 CA using shear assisted processing and extrusion

Thin-walled tube is usually produced seamlessly via an expensive multi-step process or via welding of thin sheets but at reduced performance. To overcome the current process and performance inefficiencies, advanced manufacturing methods need to be investigated. In this investigation, shear assisted processing and extrusion (ShAPE) was used to fabricate thin-walled Inconel 602 tube in a single-step. Tubes measuring 0.83 m length with a 12 mm outer diameter and 1 mm wall thickness were fabricated with an average surface roughness, Ra of 1.6 µm and Rz of 15 µm. Tensile testing of tubes in the as-fabricated condition showed strength increases of 4-35% over current offerings while maintaining or improving elongation. Electron microscopy analysis revealed the recrystallized microstructure with refined inter- and intra-granular carbides. Preliminary results obtained in this investigation shows the feasibility of producing thin-wall high temperature tube in single-step via ShAPE. This achievement marks a significant step towards manufacturing larger diameter nickel alloy tubes for the U.S. Department of Energy’s Waste Treatment and Immobilization Plant.

36 MATERIALS SCIENCE↗
NASA NTRS1951

On the theory of thin and thin-walled rods

Through the work of V. Z. Vlasov a theory of thin-walled rods has been established that is widely applicable in practice. This theory was extended by A. A. Umanski to thin-walled rods of closed profile section. The authors based their work on the concepts of the modern theory of shells. An attempt is made herein to construct a theory of thin-walled rods including the classical theory of deformation of thin rods by making use of a kinematic assumption.

LOADS - CYLINDERS↗
NASA NTRS1939

The Pack Method for Compressive Tests of Thin Specimens of Materials Used in Thin-Wall Structures

The strength of modern lightweight thin-wall structures is generally limited by the strength of the compression members. An adequate design of these members requires a knowledge of the compressive stress-strain graph of the thin-wall material. The "pack" method was developed at the National Bureau of Standards with the support of the National Advisory Committee for Aeronautics to make possible a determination of compressive stress-strain graphs for such material. In the pack test an odd number of specimens are assembled into a relatively stable pack, like a "pack of cards." Additional lateral stability is obtained from lateral supports between the external sheet faces of the pack and outside reactions. The tests seems adequate for many problems in structural research.

Aitchison, C S↗
DOE OSTI2023

Correcting distortions of thin-walled machined parts by machine hammer peening

Thin-walled aerostructural components frequently get distorted after the machining process. Reworking to correct distortions or eventually rejecting parts significantly increases the cost. This paper proposes a new approach to correct distortions in thin-walled components by strategically applying hammer peening on target surfaces of a machined component. Aluminium alloy 7475-T7351 was chosen for this research. The study was divided in two stages. First, the residual stresses (RS) induced by four different pneumatic hammer peening conditions (modifying the stepover distance and initial offset) were characterised in a test coupon, and one of the conditions was selected for the next stage. In the second stage, a FEM model was used to predict distortions caused by machining in a representative workpiece. Then, the RS induced by hammer peening were included in an FEM model to define two hammer peening strategies (varying the coverage area) to analyse the capability to reduce distortions. Two workpieces were machined and then treated with the simulated hammer peening strategies for experimental validation. Results in the test coupon showed that pneumatic hammer peening can generate high compressive RS (-50 to –350 MPa) up to 800 μm depth, with their magnitude increasing with a reduced stepover distance. Application of hammer peening over 4% of the surface of the representative workpiece reduced the machining-induced distortions by 37%, and a coverage area of 100% led to and overcorrection by a factor of five. This confirms that hammer peening can be strategically applied (in target areas and changing the percentage of coverage) to correct low or severe distortions.

42 ENGINEERING↗
DOE OSTI2022

High-Temperature Tensile and Creep Test Results on Thin Wall Tube Specimens of ODS Alloys 14YWT and OFRAC

High-temperature tensile and strain rate jump (SRJ) creep tests were conducted on axial specimens fabricated from thin wall tubes of the oxide dispersion strengthened (ODS) alloys 14YWT and OFRAC. The dimensions of the 14YWT thin wall tube was 10.7 mm OD, 0.5 mm WT and 0.92 m and of the OFRAC thin wall tube was 8.5 mm OD + 0.5 mm WT and 1.78 m in length. The axial specimens fabricated from the thin wall tubes were based on a dual-gauge design with gauge dimensions of 4 mm long and 2 mm wide. The tensile tests were conducted in air at 800ºC, 900ºC and 1,000ºC and the SRJ creep tests were conducted in air at 800ºC. Two axial specimens were used in each tensile and SRJ creep tests. The results of the tensile tests showed much lower yield stress (YS) compared to ultimate tensile strength (UTS) indicative of large work hardening properties. The results showed the UTS of OFRAC was higher than that of 14YWT by ~35 MPa at 800ºC. At 900ºC, the UTS of OFRAC and 14YWT was similar at 900ºC and at 1000ºC, the UTS of 14YWT was higher than that of OFRAC by ~10 MPa. The ductility properties of OFRAC were greater than that of 14YWT with higher values of uniform elongation (UE) and total elongation (TE) at all tensile test temperatures. The creep properties of 14YWT and OFRAC were evaluated by the strain rate jump test method at 800ºC. The stress exponent (n) for creep was obtained from the SRJ test data by plotting the values of the log strain rate against the log stress, which is measured from the stress-strain curve generated during the SRJ test. The results showed the highest stress exponents of n = 19.8 and n = 35.0 were measured from the two SRJ tests on 14YWT. The stress exponent measured from one successful SRJ test on OFRAC was n = 13.3, which was lower than that of 14YWT. Nevertheless, these high values of stress exponent are consistent with ODS ferritic alloys such as 14YWT and OFRAC since they indicate dislocation-particle interactions are the dominant creep mechanism for specific ranges of temperature and stress. The results obtained from the tensile and SRJ creep tests conducted on axial specimens in this study indicate that 14YWT and OFRAC retain high strengths at temperatures up to 1,000ºC and good creep performance at 800ºC.

36 MATERIALS SCIENCE↗
NASA NTRS1966

Silver plating technique seals leaks in thin wall tubing joints

Leaks in thin wall tubing joints are sealed by cleaning and silver plating the hot gas side of the joint in the leakage area. The pressure differential across the silver during hydrostatic test and subsequent use forces the ductile silver into the leak area and seals it.

Blenderman, W. H.↗