Fracture Origin Research Articles

Towards a Fracture Mechanics-Based Assessment for Fatigue Life Prediction of Ni–Ti Stents

The fatigue failure of Ni–Ti peripheral stents still represents an open issue of major concern due to the non-linear material behavior, the complex loads acting in vivo, and the manufacturing process. The fatigue assessment currently exploits total-life methodologies devoted to preventing crack nucleation. This work investigates a complementary fracture mechanics-based approach accounting for crack propagation from pre-existing manufacturing defects. Fatigue crack growth tests were performed on rolled Ni–Ti samples with a thickness and microstructure comparable to that of stents. A fracture mechanics-based assessment was implemented to predict the fatigue durability of surrogate samples tested at different mean and alternate strains. The fracture surfaces of the samples were inspected to determine a statistical distribution of defect size at the fracture origin. The cyclic J-integral was adopted as the crack driving force parameter, and it allowed to account for the complex response of the material, undergoing energy dissipations during phase transformation. Encouraging fatigue life predictions conforming to experimental data were obtained in the finite-life regime, whereas conservative estimates were computed below the fatigue threshold. This approach can be reverted to determine the maximum acceptable material defects for specific applications, providing a useful tool to manufacturing companies.

Effects of laser shock peening on silicon nitride ceramic with varying sintering additive ratios

Silicon nitride ceramic (Si3N4) for industrial applications is conventionally manufactured with sintering additives, and the properties of Si3N4 change significantly based on the contents of these additives. In this study, we investigate the effects of laser shock peening (LSP) on Si3N4 sintered with varying ratios of sintering additives.The Si3N4 samples were sintered with a mixture of Y2O3, MgO, and SiO2 sintering additives at 5, 7, 9, and 11 wt%. A Nd:YAG laser (wavelength = 532 nm, maximum pulse energy = 1.4 J, repetition rate = 10 Hz, pulse duration = 8 ns, beam diameter = 11 mm, top-hat profile) was used to irradiate the Si3N4 samples. The samples were coated with a protective layer (100 μm thick aluminum foil) and a water layer to confine plasma. LSP of Si3N4 with 5% sintering additives resulted in a slight change in surface hardness but a 77% decrease in surface compressive residual stress. In contrast, LSP of Si3N4 with 11% sintering additives led to a simultaneous increase in surface hardness (7.3%) and surface compressive residual stress (67%), indicating a significant difference in the effectiveness of LSP depending on the ratio of sintering additives. Additionally, the surface of Si3N4 with 11% sintering additives showed evidence of grain refinement after LSP. It was demonstrated that the bending strength of Si3N4 with 11% sintering additives increased by 15.2%, and the depth of the fracture origin was significantly deepened.

Zirconia specimens printed by vat photopolymerization: Mechanical properties, fatigue properties, and fractography analysis.

The mechanical and fatigue properties of zirconia specimens printed by vat photopolymerization (VPP) were evaluated and compared with those of zirconia specimens milled by computer numerical control (CNC). Bar-shaped specimens were printed by stereolithography (SL) and digital light processing (DLP). CNC-milled specimens were used as control samples. The fracture toughness, hardness, and flexural strength properties of the zirconia specimens were evaluated via single edge V-notch beam tests, Vickers hardness tests, and 3-point bending tests. Dynamic fatigue tests were carried out in distilled water using a step-stress test. After static bending and dynamic step-stress testing, fractography analysis was performed. Statistical analysis was carried out to compare the fracture toughness, hardness, flexural strength, and fatigue cycle results of each group (α = 0.05). The fracture toughness values did not significantly differ among the groups (p>0.05). The flexural strength was 894.10MPa for SL, 831.46MPa for DLP, and 1140.39MPa for CNC. The flexural strength of CNC was greater than that of SL and DLP (p<0.01). The mean fatigue cycles were 23498.07 for SL, 19858.60 for DLP, and 31566.80 for CNC. The mean fatigue failure strength was 643.13MPa for SL, 530.63MPa for DLP, and 903.75MPa for CNC. The fatigue failure strength of CNC was greater than that of SL and DLP (p<0.05). Fractography analysis revealed material defects at the fracture origin for each group. A partially fused structure of the incompletely debonded resin could be observed in SL, and a porous region of incompletely sintered zirconia grains could be observed in CNC. The fracture toughness and hardness of zirconia printed by VPP are comparable to those of zirconia milled by CNC. However, zirconia milled by CNC has superior static flexural strength and dynamic fatigue resistance. Further studies are needed to explore the clinical applications of VPP-printed zirconia.

Evaluation of colour change, marginal adaptation, fracture strength, and failure type in maxillary and mandibular premolar zirconia endo-crowns

ObjectivesThe objective of this in vitro study was to evaluate the effects of different preparation designs on the mean colour change (ΔE*), marginal adaptation, fracture resistance, and fracture types of maxillary and mandibular premolar endocrowns (ECs).MethodologyA total of 40 extracted maxillary and mandibular premolars were treated endodontically, and each type was subdivided according to the remaining axial height (remaining walls on all surfaces; 2–4 mm) and 2 mm inside the pulp chamber. Specimens were immersed in coffee for 14 days, ΔE* was determined, marginal adaptation was observed, fracture forces test was conducted, and the samples were examined visually at 10× magnification to evaluate failure type and identify fracture origin. The data were entered and analyzed using Statistical Package for Social Sciences, and significance between and within groups was evaluated through ANOVA. The p-value ≤ 0.05 was considered statistically significant.ResultsThe ΔE* values of the maxillary premolar with 2 mm axial height were the highest (6.8 ± 0.89 units), whereas the lowest value was observed in the mandibular premolar with 4 mm axial height (2.9 ± 0.53 units). Significant differences (p < 0.05) in teeth and design were observed. The marginal adaptation of the mandibular premolar with 4 mm axial height was the highest (30.20 ± 1.53 μm), whereas the lowest marginal adaptation was observed in the maxillary premolar with 2 mm axial height (14.38 ± 0.99 μm), and the difference was statistically significant (p < 0.05). The maximum fracture force was observed in maxillary premolars with 2 mm axial height (2248.15 ± 134.74 N), and no statistically significant difference (p = 0.07) was observed between maxillary and mandibular premolars at 4 mm axial height.ConclusionThe recorded ΔE* values of the ECs were within clinically acceptable values or slightly higher, and the marginal adaption values were within acceptable and recommended clinical values in µm. EC preparation with 2 mm axial height in both arches recorded the highest fracture forces. Type III (split fracture) failure was recorded as the highest in the maxillary and mandibular premolar ECs with different axial wall heights.

Exploration of an atomic‐scale boron additive in SiAlON ceramics

AbstractSiAlON ceramics are of interest for high‐risk applications such as biomedical implants and combustion engine turbines. This work is part of a larger study aimed at leveraging atomic‐ or molecular‐scale additives to control the densification, microstructures, and ultimate structural properties of SiAlONs. Here, we investigate the effects of boron in silicon nitride‐based ceramics. The present work demonstrates a possible chemical method for controlling the microstructural development of SiAlONs by incorporating boric acid (H3BO3) into the starting powder blend. Raman spectroscopy and 11B solid‐state magic angle spinning nuclear magnetic resonance cooperatively indicate that after sintering, boron exists in threefold coordination with nitrogen in the turbostratic boron nitride (t‐BN) structure. The results of this work indicate that the incorporation of boron and generation of t‐BN bonding in the SiAlON system result in a narrower grain size distribution, a suppression of second phases such as yttrium aluminosilicates, and ultimately, increased flexure strength. A separate fractographic study showed that SiAlONs fabricated with 3 wt% boric acid exhibited fracture origins such as subtle surface flaws or cracks, while lower dopant levels and undoped SiAlONs typically failed from flaws such as inclusions or large grains. It is argued that the modification of the intergranular glass chemistry and resulting generation of t‐BN reduces atomic diffusion through the grain boundary phase and inhibits the crystallization of second phases as well as exaggerated grain growth that often characterizes the development of β′‐SiAlON.

Influence of surface condition, cycling frequency and ferritic zones on the high and very high cycle fatigue properties of a pearlitic steel

In the present work, the high and very high cycle fatigue (HCF/VHCF) properties of pearlitic steel R350HT are investigated. Servo-hydraulic (cycling frequency between 15 Hz and 60 Hz) and ultrasonic (19 kHz) fatigue tests were performed with different surface conditions (fine turned, mechanically polished and electropolished). At a load ratio of R = −1, solely failures in the HCF regime were found for all surface conditions, although runout specimens were loaded with more than 1010 cycles, and crack initiation was exclusively at the surface. At R = 0.1, in contrast, mechanically polished specimens showed internal failures for lifetimes between 106 and 109 cycles. The origin of fracture in the interior was identified as distributed ferritic phases embedded in the harder pearlitic microstructure. Crack initiation from ferritic zones could be explained by their larger size relative to inherent defects such as nonmetallic inclusions and precipitates. The absence of turning grooves in combination with compressive residual stresses near the surface of mechanically polished specimens led to an increase in HCF strength compared with fine-turned and electropolished specimens and facilitated VHCF failure from the interior. No frequency effect was observed when crack initiation originated from turning grooves or artificial surface defects, but slightly higher fatigue strengths were determined for electropolished specimens at 19 kHz than at conventional cycling frequency. This can be explained by the strain-rate sensitivity of ferrite where fatigue cracks preferentially initiate. Additional tests with specimens containing artificially introduced micro-holes were performed to systematically study material's defect tolerance and to quantify the mean-stress sensitivity.

Tensile strength of girth weld joint of linepipe with softened HAZ

In seismic-active area, the buried pipeline is subjected to the axial tensile load exceeding the yield stress of pipe material due to the lateral flow induced by soil liquefaction. Designing a girth weld joint that does not fracture at girth weld but base metal which has high elongation capacity is feasible to ensure adequate ground displacement absorbing capacity of the girth weld joint. However, as for fine grain steel, the heataffected zone (HAZ) is softened by welding heat input, and then, the zone of softened HAZ can be the origin of fracture under tensile load. The apparent strength of softened HAZ is affected by the plastic constraint from the surrounding weld metal and base metal during tensile loading, and the fracture location depends on the apparent strength of HAZ. This study investigates the tensile strength and fracture location of girth weld joints with softened HAZ in the static tensile test. The effect of geometric and mechanical heterogeneity of HAZ, strength overmatching of weld metal, and the width-thickness ratio of tensile test specimen on tensile strength are elucidated using parametric finite element analysis. Subsequently, an equation is proposed to predict the tensile strength and fracture location of the welded joint. Using the proposed equation, the condition of geometric and mechanical heterogeneity of the weld to confirm the girth weld joints that fracture at base metal under tensile load is clarified. The proposed equation can be used to design a girth weld joint with liquefaction earthquake resistance.

Deformation Behavior and Fracture Strength of Single‐Crystal 4H‐SiC Determined by Microcantilever Bending Tests

In this study, the deformation behaviors and mechanical properties of 4H‐SiC single crystals are investigated using microcantilever beam specimens with two different sizes, A and B (A &lt; B). Tensile stress is applied along &lt;20&gt; direction. Plastic deformation, or nonlinearity, is observed in the stress–strain curves, and yield stress, or proportional limit, coincides between the two specimens at ≈25 ± 2 GPa. Scanning transmission electron microscopy and transmission electron microscopy studies show that the plastic deformation is due to dislocation activities; multiple‐dislocation pileup areas are observed in both the specimens. Assuming {100}/&lt;110&gt; prismatic slip which most plausibly occurs in the &lt;20&gt; stress application, the critical resolved shar stress is estimated to be 10.9 GPa, which agrees well with the previous studies. Measured fracture strength is 41.9 ± 2.8 and 33.5 ± 2.4 GPa for the A and B, respectively. Dislocation–fracture relationship is discussed on the basis of dislocation‐based fracture mechanics, etc. It is suggested that cracks form within the multiple‐dislocation pileup area, by interaction with dislocation pileups, and act as fracture origins. A's strength is close to an ideal tensile strength of 4H‐SiC in the &lt;110&gt; direction, 47–55 GPa.

Analysis of the effect of deep-profile compressive residual stress induced by laser peening on fatigue strength of 7075 aluminum alloy

The effect of laser peening-induced compressive residual stress on the fatigue strength improvement of A7075 specimens was investigated. Rotary bending fatigue tests and residual stress distribution measurements using the sequential polishing method were conducted on untreated, shot-peened, and laser-peened specimens. The results revealed that laser peening provides a deeper compressive stress and a deeper fracture origin, resulting in higher fatigue strength. Furthermore, controlling the laser parameters could extend the depth of the compressive stress layer by approximately 5 mm, indicating the possibility of applying laser peening to thicker materials to improve the fatigue strength.

Mechanical response in tension of twin-induced Mg alloys in wide strain rate regimes

Effect of twin boundary with/without segregation of solute atoms on deformation behavior in tension is examined in Mg–3Al–1Zn (AZ31) alloys from low strain rates to high strain rates. Solute atoms (Al and/or Zn atoms) are segregated at {10 1‾ 2} twin boundaries by static annealing at a temperature of 423 K with a holding time of 150 min. The room-temperature tensile mechanical response of the specimens, which have pre-induced twin boundaries, shows detwinning behavior, regardless of the strain rate between 10−5/s and 100/s. The yield strength of these specimens is also unlikely to be affected by strain rate, because twin boundary mobility, i.e., the shrinkage, is an athermal process. On the other hand, the specimen with segregating of solute atoms at twin boundaries exhibits an increased strength and a decreased strain hardening as compared with those in the specimen having non-segregated twin boundaries. Newly formed deformation twins of several types, i.e., the {10 1‾ 2} twins, {101‾ 1} twins and {101‾ 2}-{101‾ 1} double twins, are the origin of fracture in these specimens.

Micro‐scale mechanical properties of surface layer in ion‐exchanged glass

AbstractToday, ion‐exchanged or chemically strengthened glass is ubiquitously used all over the world, typically for scratch‐free display cover of personal mobile phones. It has been known that the strengthening is due to developments of high compressive stress in the surface layer by its internally constrained dilation . We investigated micro‐scale mechanical properties of the surface layer itself of a Na+‐to‐K+ ion‐exchanged soda lime silicate glass, revealing that the strength of the layer increased nearly 40% by the ion‐exchange with little compression effect from the interior. The fracture toughness, however, was comparable before and after the ion‐exchange, indicating that the ion‐exchange reduces the crack size of the fracture origin. Theoretical estimates of fracture surface energies verified the measured fracture toughness. It was presumed that the observed crack‐size reduction is due to crack healing effects enhanced by the ion‐exchange.

Comparison of color changes, fracture strengths, and failure modes of conventional endocrowns and endocrowns with different design modifications.

Endocrowns (ECs) are alternatives for rebuilding severely damaged teeth and show superior efficacy in molars over premolars. The objective of this in vitro study is to evaluate the effects of different preparation designs with short pulp chambers on the mean color change (Δ⁢E), fracture resistance, and failure types of mandibular molar ECs. A total of 40 extracted mandibular molars were treated endodontically and divided into four groups. Samples in groups 1, 2, 3, and 4 had occlusal preparation depths of 5 mm, 3 mm, 3 mm with ferrule, and 3 mm with boxes, respectively. The samples were immersed in coffee and their Δ⁢E values were measured by using the Commission Internationale de l'Eclairage color system. They were also subjected to a fracture test. Next, all specimens were examined visually under a stereomicroscope to evaluate their failure modes and identify their fracture origins. Data were entered and analyzed by using Statistical Package for Social Sciences. Among all groups, group 4 (3 mm + boxes) presented the highest Δ⁢E (4.15) after immersion in coffee. Moreover, ANOVA revealed that the Δ⁢E of group 4 (occlusal preparation depth of 3 mm with boxes) was significantly different (p< 0.05) from that of group 2 (3 mm + ferrule, 3.07). The EC with a 3 mm chamfer and ferrule showed the highest maximum load of 2847.68 ± 693.27 N, whereas that with a 5 mm chamfer finish line had a marginally reduced load at fracture of 2831.52 ± 881.83 N. The EC with a 3 mm chamber and boxes had a slightly increased maximum load of 2700.75 ± 436.40 N, whereas that with the 3 mm chamber had the lowest maximum load at fracture of 2385.97 ± 465.61 N. One-way ANOVA showed that different EC preparation designs had no effect on maximum fracture load (F [3,16] = 0.550, p= 0.6). The recorded Δ⁢E values of ECs in all groups were equal or marginally higher than the acceptable values. The EC with a 3 mm chamfer and ferrule displayed the highest mean maximum load. The EC with a 5 mm chamfer finish line had a marginally lower maximum load at fracture than other ECs. Failures, such as ceramic fracture, split fracture, and ceramic and tooth splitting above the cemento-enamel junction (CEJ) or vertically were predominant in samples with occlusal preparation depths of 3 mm with ferrule and 5 mm.

Mechanical and optical properties of a borosilicate glass used to improve the finishing of 3Y-TZP restorations.

Borosilicate glass was developed to enhance the mechanical behavior and smoothness of dental zirconia as an alternative to conventional glaze. This study assessed the mechanical and optical properties of 3 mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) coated with borosilicate glass or a commercial glaze fired for an extended period of time. Disc-shaped 3Y-TZP zirconia specimens (Zpex, Tosoh) were sintered at 1550°C for 2 hours. The specimens were divided into three groups: as-sintered (control, C); commercial glaze (G); and borosilicate glass (SL). The glaze and borosilicate glass were applied over the zirconia and fired for 20 minutes at 950°C and 1200°C, respectively. Biaxial flexural strength, fractography, X-ray diffraction (XRD), roughness (Ra and Rz), fracture toughness (Vickers indentation method), color difference (∆E00), and translucency (TP00) analyses were conducted. The t-test or the one-way ANOVA and Tukey's tests were used to analyze the data (α = 0.05). Flexural strength data were subjected to the Weibull analysis. The SL group exhibited the highest flexural strength (1025.8 MPa), whereas the C (859.41 MPa) and G (816.0 MPa) groups exhibited similar values. The SL group also had the highest characteristic strength. The fracture origin in all groups was on the zirconia surface. XRD analysis revealed that the specimens from the SL group contained tetragonal, cubic, and monoclinic phases. The SL group presented the lowest surface roughness. Fracture toughness in the SL group was lower than in the C group, but similar to that observed in the G group. The translucency and color differences observed in the G and SL groups were similar. Borosilicate glass enhanced the flexural strength of 3Y-TZP, promoted the smoothest surface, and exhibited optical properties similar to those of the glaze.

Borosilicate glass as a surface finishing alternative for improving the mechanical properties of third-generation zirconia

ObjectiveThis study evaluated the effect of an experimental borosilicate glass on the mechanical and optical behavior of 5Y-PSZ zirconia and comparing it to commercial glaze and as-sintered. MethodsDisc-shaped specimens of a 5Y-PSZ (Zpex Smile) were prepared and sintered (1550 °C, 2 h). The zirconia discs were randomly divided according to the surface treatment: as-sintered (C), commercial glaze (G), and experimental borosilicate glass (SL). Glaze and experimental glass powders were mixed with building liquids and applied to zirconia with a brush. G specimens were fired at 950 °C and SL at 1200 °C. An extended dwell time of 20 min was applied to both groups. Biaxial flexural strength, roughness (Ra and Rz), translucency (TP00), color alteration (ΔE00), Vickers hardness, fracture toughness, residual stresses, and x-ray diffraction analyses were conducted. Statistical analyses were performed with Weibull statistics, Kruskal-Wallis, or ANOVA tests (α = 5%). ResultsSL yielded the highest flexural strength (799.35 MPa), followed by G (662.34 MPa), and C (485.38 MPa). The fracture origin of SL specimens was in the bulk zirconia, while G and C showed fractures starting at the surface. As-sintered reached the highest fracture toughness and hardness. Glaze and borosilicate glass provided surface compressive stresses. Borosilicate glass application led to phase transformation (t→m). SL and G showed the lowest roughness. TP00 and ΔE00 were similar among groups. SignificanceBorosilicate glass improved strength without harming the optical properties of third-generation zirconia. Toughness and roughness provided by the experimental glass were similar to those from commercial glaze.

Effect of thermal exposure on microstructure, orientation relationship, residual stress and failure mechanism of ultrasonic-assisted Kovar/SnSb10/Kovar joints

Thermal exposure is often accompanied by the accumulation of inhomogeneous element diffusion, continued solid-state phase transformation, and occurrence of residual stress, causing the failure of the joint directly. In this work, we focused on the orientation relationship (OR) alternation, residual stress distribution, and failure mechanism of the reflow/ultrasonic-assisted Kovar/SnSb10/Kovar joint after thermal exposure. Especially, the role of Ni element during thermal exposure was analyzed in detail. It was noticed that the thickness of interfacial intermetallic was merely lower than 4 μm, even after thermal exposure for 45 d. The interfacial ORs of IMC/Kovar, whether ultrasound was applied or not, changed to an incoherent state. However, it did not deteriorate the ultimate shear strength, which reached over 50 MPa even after thermal exposure at 160 °C for 45 d. Compared to the reflow process after thermal exposure, the joints with ultrasonic improves the strength by about 10%. However, it dramatically decreased to only 20 MPa after thermal exposure at 200 °C for 45 d. The rationale was that ultrasonic-induced supersaturated Ni elements in solder and the Ni elements from the Kovar alloy diffused into the IMC region simultaneously, forming the (Fe, Ni)Sn2 IMC layer with relatively high residual stress and acting as the fracture origin region. The current findings demonstrated that the Kovar/SnSb10 interface proposed outstanding microstructure stability and mechanical reliability and was a promising alternative for a more extensive range of high-temperature applications.

Minimally processed recycled yttria-stabilized tetragonal zirconia for dental applications: Effect of sintering temperature on glass infiltration

This study aimed to develop a recycling process for the remnants of milled 3Y-TZP and enhance their properties using glass infiltration. 3Y-TZP powder was gathered from the vacuum system of CAD–CAM milling equipment, calcined and sieved (x < 75 μm). One hundred twenty discs were fabricated and pre-sintered at 1000 °C/h. These specimens were then divided into four groups, categorized by glass infiltration (non-infiltrated [Zr] or glass-infiltrated [Zr-G]) and sintering temperature (1450 °C [Zr-1450] or 1550 °C [Zr-1550]/2h). After sintering, the specimens were characterized by X-Ray Diffraction (XRD), relative density measurement, and scanning electron microscopy and energy dispersive spectroscopy (SEM-EDS). The biaxial flexural strength test was performed according to the ISO 6872 and followed by fractographic analysis. Subsequent results were analyzed using Weibull statistics. Relative density values of the sintered specimens from Zr-1450 and Zr-1550 groups were 86.7 ± 1.5% and 92.2 ± 1.7%, respectively. Particle size distribution revealed particles within the range of 0.1–100 μm. XRD analysis highlighted the presence of the ZrO2-tetragonal in both the Zr-1450 and Zr-1550 groups. Glass infiltration, however, led to the formation of the ZrO2-monoclinic of 9.84% (Zr-1450-G) and 18.34% (Zr-1550-G). SEM micrographs demonstrated similar microstructural characteristics for Zr-1450 and Zr-1550, whereas the glass-infiltrated groups exhibited comparable infiltration patterns. The highest characteristic strength was observed in the glass-infiltrated groups. Fractographic analyses suggested that fracture origins were related to defects on the tensile side, which propagated to the compression side of the samples. Both the sintering temperature and glass infiltration significantly influenced the mechanical properties of the 3Y-TZP recycled.

Equibiaxial flexural strength determination of UO2 using a ball-on-ring test

To increase nuclear fuel performance and reliability, their mechanical properties require an accurate and statistically relevant assessment. Biaxial flexural strength tests provide an alternative to bend bar techniques for assessing mechanical behavior; namely, the transverse rupture strength (TRS) of ceramic samples. Biaxial test samples require simple geometries and minimal surface preparation, reducing fabrication costs and handling hazards. This study investigated the TRS of polycrystalline UO2 fuel forms at room temperature using a ball-on-ring test fixture. Pellets were fabricated from UO2 powder using conventional powder processing and sintering techniques. The TRS and Weibull parameters were obtained through Weibull statistics on over 60 UO2 samples tested under equibiaxial flexure. The larger sample size in this study enabled a more robust Weibull statistical analysis than alternative test methods, which may not capture the stochastic failure of typical ceramics. Furthermore, two different loading ball diameters were employed to assess the impact of contact damage on fracture strength. While Hertzian contact damage was observed with the smaller loading ball, the fracture strength remained unaffected. A fracture analysis of the tested UO2 samples indicated a mixture of intergranular and transgranular fracture that transitions to transgranular fracture with increasing distance from the fracture origin. The characteristic strength of the combined data sets was determined to be 148 MPa, and the Weibull modulus was determined to be 9.1. The TRS values and Weibull parameters were close to values found in the literature for alternative testing techniques using samples with similar microstructure and density. The findings in this study validate the ball-on-ring method used to obtain the TRS of UO2 with a sample geometry more representative of nuclear fuels. Additionally, experimental TRS results from this study can be implemented in modeling codes to predict fuel performance, which is critical to fuel burnup extension and advanced nuclear fuel technologies.

Influence of vacuum annealing on mechanical characteristics of focused ion beam fabricated silicon nanowires

This paper describes the influence of vacuum annealing on the mechanical characteristics of silicon (Si) nanowires (NWs) fabricated using focused ion beam (FIB) technologies. Two types of Si NWs having a cross-sectional one-side length or diameter ranging from 19 to 447 nm are prepared using the direct milling and Ga ion doping functions of FIB. The Si NWs prepared are annealed at 400–700 °C in high vacuum for 10 min, followed by quasi-static uniaxial tensile testing using a microelectromechanical system based tensile test system in a scanning electron microscope. All the Si NWs fracture in a brittle manner. Young's modulus of submicrometer-sized Si NWs shows both annealing and specimen size influences in the range from 120 to 170 GPa, whereas that of nano-sized Si NWs shows only annealing influence in the range from 60 to 110 GPa. Tensile strength scatters greatly, ranging from 1.0 to 7.2 GPa, which increases with increasing the NW size. A transmission electron microscope and an atomic force microscope suggest that, by annealing, recrystallization happens in the damaged layer introduced by FIB milling and the NW surface morphology changes due to its recrystallization and gallium (Ga) ion evaporation. Fracture origin is discussed through the comparison between surface roughness and crack length estimated by the Griffith theory of brittle fracture.

Effects of input energy and impactor shape on cranial fracture patterns

This study documents relationships between input energy, impactor shape, and the formation of fractures in human crania. Parietal impact experiments (n = 12) were performed at 67% higher input energy compared to previously reported experiments. Fracture origins, characteristics, and locations were compared at two input energy levels with three impactor shapes (focal “hammer”, flat “brick”, and curved “bat”). Impacts with all three impactors at both energy levels produced fractures originating at and remote to the impact site, indicating both mechanisms are typical in temporoparietal blunt force impacts. Higher energy impacts generally produced more impact site fractures, depression, and comminution than lower energy impacts. A small, focal impactor produced cone cracks, depression, and fractures localized near the impact site. A broad, curved impactor produced circumferential fractures and linear fractures extending into adjacent bones. A broad, flat impactor produced fracture patterns ranging from linear fractures to large depressed and comminuted defects.

Thermal conductivity improvement in silicon nitride ceramics via grain purification

Abstractβ‐silicon nitride (β‐Si3N4) ceramics with an additive oxide system of 1 wt% MgO and 3 wt% Re2O3 (Re = Gd, La, Y, and Yb) were fabricated by liquid phase sintering at 1950°C under nitrogen pressure of 748 kPa. Starting α‐Si3N4 powder with 10 vol% rod‐like β‐Si3N4 seed crystallites and an extended sintering time from 5 to 40 h resulted in the formation of bimodal microstructure composed of fine matrix grains and large grains. The 40 h‐sintered specimens of pseudo ternary β‐Si3N4‐MgO‐Gd2O3 system exhibited enhanced thermal conductivity of 127.2 ± 2.5 W m−1 K−1 associated with a degradation of the fracture strength from 1008.0 ± 38.0 to 491.7 ± 32.0 MPa, which was due to the formation of coarse‐grained aggregates that acted as both fracture origin as well as a thermal conductive pathway. The theoretical thermal conductivity was predicted for the sintered specimens by using equations based on a mean‐field micromechanics model to estimate the effective thermal conductivity of each component of binary composites. The calculation results suggested that the thermal conductivity of the large β‐Si3N4 grains (≥ 2 μm in diameter) was relatively high and estimated to be in the range of 175 to 191 Wm−1 K−1. The improved thermal conductivity of the 40 h‐sintered specimens was further discussed for the series of β‐Si3N4‐MgO‐Re2O3 systems based on the nanostructure characterization results obtained by the high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐energy dispersive X‐ray spectrometry analyses.