Relationship between spinal structural damage on radiography and bone fragility on CT in ankylosing spondylitis patients

To evaluate whether the risk of bone fragility on computed tomography (CT) (scanographic bone attenuation coefficient of the first lumbar vertebra (SBAC-L1)) is associated with the severity of spine structural involvement (mSASSS) in patients with ankylosing spondylitis (AS). This retrospective study included AS patients, followed from 2009 to 2017, who fulfilled the New York criteria and who underwent thoraco-abdomino-pelvic CT and radiography (spine, pelvis). The structural involvement was retained for mSASSS ≥ 2. The SBAC-L1 was measured in Hounsfield units (HU). A SBAC-L1 ≤ 145 HU was used to define patients at risk of vertebral fracture (VF). A total of 73 AS patients were included (mean age: 60.3 (± 10.7) years, 8 women (11%), mean disease duration: 24.6 years (± 13.9)). Sixty patients (82.2%) had a mSASSS ≥ 2 (mean score 20.7 (± 21.2)). The mean SBAC-L1 was 141.1 HU (± 45), 138.1 HU (± 44.8) and 154.8 HU (± 44.9) in the total, mSASSS ≥ 2 and mSASSS < 2 populations, respectively. Patients with bone bridges had lower SBAC-L1 than mSASSS ≥ 2 patients without ankylosis (p = 0.02) and more often SBAC-L1 ≤ 145 HU (73% vs 41.9%, p = 0.006). A SBAC-L1 ≤ 145 HU was not associated with structural spine involvement, but patients with bone bridges had significantly decreased SBAC-L1 and an increased probability of being under the fracture threshold.

DEFORMATION ANALYSIS OF H62 Cu ALLOY FOIL SUBJECTED TO MULTI-PULSED LASER DYNAMIC FORMING: SIMULATIONS

By utilizing the ABAQUS software, this paper simulates and analyzes the forming of the H62 foil subjected to multi-pulse laser dynamic forming (LDF). The Johnson–Cook failure mode is adopted to predict the fracture threshold value of the H62 foil. Compared with the single-pulsed LDF, the multi-pulsed LDF improves the limit depth and forming quality of the foil effectively. With the increase of impact number, the uniformity of foil is effectively improved. Appropriate peak pressure and impact number are important to increase limit forming depth and improve forming quality.

Modelling the Fracture of High-Hardness Armour Steel in Taylor Rod-on-Anvil Experiments

A series of Taylor rod-on-anvil experiments have been performed to validate the predicted impact velocity fracture threshold and fracture mode of a high hardness armour steel (HHA) obtained through explicit finite element simulations. Experimentally, the rods exhibited principal shear failure, a condition that can be closely linked to adiabatic shear band (ASB) formation in high strength steel. Using a stress triaxiality and Lode angle dependent failure strain criterion (Basaran 3D fracture locus), calibrated from quasi-static mechanical characterisation tests, the simulations were unable to predict the onset of fracture observed in experiments. As such, a strength-fading criterion is proposed using a phenomenological description to capture the loss of load-carrying capacity resulting from ASB formation. The ASB criterion is based on an exponential fit to experimentally-observed instability strains measured at different average stress triaxialities in a series of tests on inclined cylindrical and modified flat-hat specimens. With the prediction of ASB formation the material strength is reduced to model the thermal softening experienced in the shear band, and fracture of the material (in the form of element erosion) remains controlled by the Basaran fracture model. Incorporating the ASB-based criterion, the numerical models were found to accurately predict both the impact velocity fracture threshold, as well as the general appearance of the observed principal shear fracture. The proposed criterion enables the effects of ASB formation to be captured in an impact simulation with little increase in computational cost.

Graphene-based bimorphs for micron-sized, autonomous origami machines

Origami-inspired fabrication presents an attractive platform for miniaturizing machines: thinner layers of folding material lead to smaller devices, provided that key functional aspects, such as conductivity, stiffness, and flexibility, are persevered. Here, we show origami fabrication at its ultimate limit by using 2D atomic membranes as a folding material. As a prototype, we bond graphene sheets to nanometer-thick layers of glass to make ultrathin bimorph actuators that bend to micrometer radii of curvature in response to small strain differentials. These strains are two orders of magnitude lower than the fracture threshold for the device, thus maintaining conductivity across the structure. By patterning 2-𝝁m-thick rigid panels on top of bimorphs, we localize bending to the unpatterned regions to produce folds. Although the graphene bimorphs are only nanometers thick, they can lift these panels, the weight equivalent of a 500-nm-thick silicon chip. Using panels and bimorphs, we can scale down existing origami patterns to produce a wide range of machines. These machines change shape in fractions of a second when crossing a tunable pH threshold, showing that they sense their environments, respond, and perform useful functions on time and length scales comparable with microscale biological organisms. With the incorporation of electronic, photonic, and chemical payloads, these basic elements will become a powerful platform for robotics at the micrometer scale.

Temperature Effects on Fracture Thresholds of Hydrogen Precharged Stainless Steel Welds

Austenitic stainless steels are typically used in hydrogen environments due to their resistance to hydrogen embrittlement; however, the behavior of welds is not as well understood and can vary from wrought base materials due to chemical composition differences and the presence of ferrite in the fusion zone of the weld. Applications of welded austenitic stainless steels exposed to hydrogen are not limited to room temperature but also include sub-ambient environments, which can have an additional effect on the degradation. In this study, fracture thresholds were measured of three different austenitic stainless steel welds in the hydrogen-precharged condition. Forged 304L, 316L, and 21Cr-6Ni-9Mn stainless steels were gas tungsten arc welded with 308L filler metal and machined into 3-pt bend bars for fracture testing. Crack growth resistance (J-R) curves were measured of the three welds in the hydrogen-precharged condition at ambient (293 K) and sub-ambient (223 K) temperatures to determine the effects of temperature on fracture threshold. Fracture thresholds were determined using elastic-plastic fracture mechanics through development of J-R curves to determine the stress intensity factor following standard practice for determination of fracture toughness. Fracture threshold tests for the welds revealed significant susceptibility to subcritical cracking when tested in the hydrogen-precharged condition. The 21-6-9/308L and 304L/308L welds exhibited some variability in fracture thresholds that did not appear to trend with temperature, while the 316L/308L weld exhibited a reduction of over 50% in fracture threshold at the lower temperature compared to room temperature. In addition to fracture testing, mini-tensile specimens were extracted from the weld region and tested at 293 K and 223 K in the hydrogen-precharged condition. Hydrogen-precharging slightly increased the yield strength relative to the as-welded condition for all three welds at both temperatures. For all three welds, hydrogen reduced the total elongation by 3–11% at 293 K, whereas reductions in total elongation from 50–64% were observed at 223 K (relative to room temperature without hydrogen). The role of slip planarity on hydrogen-induced degradation of ductility and fracture resistance is discussed as a function of temperature, nickel content, and hydrogen. The fracture surfaces were examined to elucidate the observed differences and similarities in mechanical properties.