Instrumented indentation performed at room temperature with a Berkovich and 10 μm radius sphere has been used to measure the stress exponent for creep before and after the strain burst observed in ...well-annealed, high-purity indium. Before the strain burst, the measured values are successfully rationalized using a new model based on stress directed diffusional flow along the interface between the indenter tip and test specimen. After the strain burst, the measured stress exponents are found to be representative of dislocation glide and climb assisted glide. These results are compared and contrasted to the previous experimental investigations and modeling efforts of Feng et al., Lucas et al., and Li et al. Collectively, the experimental observations and rationalization presented here provide significant new insight into the mechanisms of action that control the competition for stress relief in small, constrained volumes of crystalline metals subjected to high homologous temperatures.
Graphical abstract
Creep–fatigue testing of Ni-based superalloy Inconel 617 and Haynes 230 were conducted in the air at 850°C. Tests were performed with fully reversed axial strain control at a total strain range of ...0.5%, 1.0% or 1.5% and hold time at maximum tensile strain for 3, 10 or 30min. In addition, two creep–fatigue life prediction methods, i.e. linear damage summation and frequency-modified tensile hysteresis energy modeling, were evaluated and compared with experimental results. Under all creep–fatigue tests, Haynes 230 performed better than Inconel 617. Compared to the low cycle fatigue life, the cycles to failure for both materials decreased under creep–fatigue test conditions. Longer hold time at maximum tensile strain would cause a further reduction in both material creep–fatigue life. The linear damage summation could predict the creep–fatigue life of Inconel 617 for limited test conditions, but considerably underestimated the creep–fatigue life of Haynes 230. In contrast, frequency-modified tensile hysteresis energy modeling showed promising creep–fatigue life prediction results for both materials.
Low cycle fatigue (LCF) and creep–fatigue testing of Ni-based alloy 617 was carried out at 850°C. Compared with its LCF life, the material’s creep–fatigue life decreases to different extents ...depending on test conditions. To elucidate the microstructure-fatigue property relationship for alloy 617 and the effect of creep and oxidation on its fatigue life, systematic microstructural investigations were carried out using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction (EBSD). In LCF tests, as the total strain range increased, deformations concentrated near high angle grain boundaries (HAGBs). The strain hold period in the creep–fatigue tests introduced additional creep damage to the material, which revealed the detrimental effect of the strain hold time on the material fatigue life in two ways. First, the strain hold time enhanced the localized deformation near HAGBs, resulting in the promotion of intergranular cracking of alloy 617. Second, the strain hold time encouraged grain boundary sliding, which resulted in interior intergranular cracking of the material. Oxidation accelerated the initiation of intergranular cracking in alloy 617. In the crack propagation stage, if oxidation was promoted and the cyclic oxidation damage was greater than the fatigue damage, oxidation-assisted intergranular crack growth resulted in a significant reduction in the material’s fatigue life.
Pellet-cladding mechanical interaction due to thermal expansion of nuclear fuel pellets during a reactivity-initiated accident (RIA) is a potential mechanism for failure of nuclear fuel cladding. To ...investigate the mechanical behavior of cladding during an RIA, we developed a mechanical pulse-controlled modified-burst test instrument that simulates transient events with a pulse width from 10 to 300ms. This paper includes validation tests of unirradiated and pre-hydrided ZIRLO™ cladding tubes. A ZIRLO™ cladding sample with 168wt.ppm of hydrogen showed ductile behavior and failed at the maximum limits of the mechanical test setup; hoop strain to failure was greater than 9.2%. ZIRLO™ samples showed high resistance to failure even at very high hydrogen contents (1466wt.ppm). When the hydrogen content was increased to 1554wt.ppm, “brittle-like” behavior was observed at a hoop strain of 2.5%. Preliminary scoping tests at room temperature of tubes fabricated from FeCrAl, a candidate material for accident-tolerant cladding were conducted to reproduce the pulse behavior of transient test reactors during integral tests. The preliminary FeCrAl tests are informative from the perspective of characterizing the test rig and supporting the design of RIA integral tests for current cladding materials and for candidate accident-tolerant cladding materials.
Strain-controlled low cycle fatigue (LCF) and creep-fatigue testing of Ni-based alloy 230 were carried out at 850°C. The material creep-fatigue life decreased compared with its low cycle fatigue life ...at the same total strain range. Longer hold time at peak tensile strain further reduced the material creep-fatigue life. Based on the electron backscatter diffraction, a novel material deformation characterization method was applied, which revealed that in low cycle fatigue testing as the total strain range increased, the deformation was segregated to grain boundaries since the test temperature was higher than the material equicohesive temperature and grain boundaries became weaker regions compared with grains. Creep-fatigue tests enhanced the localized deformation, resulting in material interior intergranular cracking, and accelerated material damage. Precipitation in alloy 230 helped slip dispersion, favorable for fatigue property, but grain boundary cellular precipitates formed after material exposure to the elevated temperature had a deleterious effect on the material low cycle fatigue and creep-fatigue property.
Residential clothes drying consume about 650 TBtu of primary energy per year in the United States, equivalent to 3% of primary residential energy consumption. There is a strong impetus to reduce the ...energy consumption of clothes dryers by both improving existing technology and developing alternatives that use fundamentally different drying mechanisms. Clothes drying technologies are broadly classified into either evaporative or mechanical drying. The focus of this paper is on mechanical drying, including vibrational, centrifugal, and press-based methods. In this work, the physical processes involved in these mechanical fabric drying processes were analyzed to develop general theories of mechanical cloth drying energy efficiency. Quantitative evaluation of the theories requires measured fabric properties. To accomplish this, a set of experiments was conducted on samples of a standard test fabric. The fabric was a cotton-polyester blend specified by the US Department of Energy to evaluate the standardized efficiency of all residential clothes dryers in the US. Mercury porosimetry experiments were conducted to determine the fabric pore size distribution, apparent density, and porosity. Elasticity experiments were conducted to determine the fabric's Young's modulus. Isostatic press experiments were conducted to establish a relationship between compression force and fabric moisture content. The data resulting from these experiments were combined with mathematical models developed in this work to calculate the theoretical maximum performance limits for mechanical drying of the standard fabric. The results of the analysis are used to make recommendations for the most promising technologies that offer the greatest potential energy savings for residential clothes drying.
Purpose
This paper aims to investigate the deposited structure and mechanical performance of printed materials obtained during initial development of the Big Area Additive Manufacturing (BAAM) system ...at Oak Ridge National Laboratory. Issues unique to large-scale polymer deposition are identified and presented to reduce the learning curve for the development of similar systems.
Design/methodology/approach
Although the BAAM’s individual extruded bead is 10-20× larger (∼9 mm) than the typical small-scale systems, the overall characteristics of the deposited material are very similar. This study relates the structure of BAAM materials to the material composition, deposition parameters and resulting mechanical performance.
Findings
Materials investigated during initial trials are suitable for stiffness-limited applications. The strength of printed materials can be significantly reduced by voids and imperfect fusion between layers. Deposited material was found to have voids between adjacent beads and micro-porosity within a given bead. Failure generally occurs at interfaces between adjacent beads and successive layers, indicating imperfect contact area and polymer fusion.
Practical implications
The incorporation of second-phase reinforcement in printed materials can significantly improve stiffness but can result in notable anisotropy that needs to be accounted for in the design of BAAM-printed structures.
Originality/value
This initial evaluation of BAAM-deposited structures and mechanical performance will guide the current research effort for improving interlaminar strength and process control.
•Manufacturing methods for 3D printing of poplar wood-filled PLA were investigated.•20% poplar in PLA; extrusion at 210 °C and printing at 230 °C were the best conditions.•The statistical ...distribution of tensile properties using 70 poplar trees was studied.
Fabrication of renewable materials through additive manufacturing using wood-filled polylactic acid (PLA) is an emerging field of study. The variability in the tensile properties of 3D-printed materials due to the variability in woody biomass properties has not been studied. Biomass size reduction, filament extrusion, and 3D-printing methods were investigated to determine the conditions that resulted in consistent printing. Using 20% ball-milled poplar reinforcement in PLA, 210 °C filament extrusion temperature and 230 °C printing temperature were found to be the best conditions. Subsequently, seventy poplar samples from a common garden were used to test the tensile properties of the printed materials. The median tensile strength at yield was 50 MPa, with 5–95 percentiles ranged in 37–54 MPa. Strain% at break had a median value of 2.1%, and 5–95 percentiles were 1.7–2.7%. The median Young's modulus was 3.65 GPa, and 5–95 percentiles ranged in 2.9–4.1 GPa. Biomass density was correlated to composite density. Median particle size of ball-milled poplar was negatively correlated with tensile strength. Composite density affected tensile strength of the composite. β-glycosidic bond of polysaccharides in biomass affected Young's modulus of the composite. These data show that genotypic variation among Populus trichocarpa have substantial effect on tensile properties of 3D printed PLA-poplar materials.
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Quasi-static and intermediate rate axial crush tests were conducted on tubular specimens of Carbon/Epoxy (Toray T700/G83C) and Glass/Polypropylene (Twintex). The quasi-static tests were conducted at ...10 mm/min (1.67 × 10—4 m/s); five different crush initiators were used. Tests at intermediate rates were performed at speeds of 0.25, 0.5, 0.75, 1, 2, and 4m/s. Modes of failure and specific energy absorption (SEA) values were studied. The highest SEA measured was 86 kJ/kg. This value was observed using Carbon/Epoxy samples at quasi static rates with a 45° chamfer initiator. The highest energy absorption for Twintex tubes was observed to be 57.56kJ/kg during 45° chamfer initiated tests at 0.25m/s. Compared with steel and aluminium, SEA values of 15 and 30 kJ/kg, respectively, the benefits of using composite materials in crash structures become apparent.