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In the present study, dynamic fracture experiments are performed on fully amorphous Liquidmetal-1 (LM-1), a Zr-based BMG, to better understand fracture initiation and propagation in notched specimens. Experiments are conducted on notched (110 μm notch radius) four-point bend specimens using an instrumented modified split-Hopkinson pressure bar apparatus. The results of these experiments suggest that the critical dynamic stress intensity factor achieved by the notched LM-1 specimens is ~110 MPa m, which is similar to the fracture toughness determined from previous quasi-static fracture experiments. This insensitivity of the fracture toughness to crack tip loading rate suggests negligible loading-rate sensitivity on the dynamic fracture initiation toughness in LM-1. In situ high-speed camera images of the notched sample during the dynamic loading process show multiple fracture initiation attempts and subsequent arrests prior to catastrophic fracture initiation. Controlled stress wave loading experiments designed to induce sub-critical levels of damage in the notched specimens show extensive deformation banding extending 150 to 200 μm outward from the notch. The deformation bands, nominally perpendicular to each other, run along the direction of the notch and perpendicular to it. They are consistent with slip-line fields in notched samples of elastic perfectly plastic materials. Subsequent loading of the damaged specimen again shows several attempts at crack initiation followed by blunting; the initial sub-critical damage in the region around the notch is understood to increase the energy required for catastrophic specimen failure and is consistent with an increase in the effective notch radius due to preexisting damage. [ABSTRACT FROM AUTHOR]

We present results of a molecular dynamics study using adaptive intermolecular reactive empirical bond order interatomic potential to analyze thermal transport in three-dimensional pillared single-walled carbon nanotube (SWCNT)–graphene superstructures comprised of unit cells with graphene floors and SWCNT pillars. The results indicate that in-plane as well as out-of-plane thermal conductivity in these superstructures can be tuned by varying the interpillar distance and/or the pillar height. The simulations also provide information on thermal interfacial resistance at the graphene–SWCNT junctions in both the in-plane and out-of-plane directions. Among the superstructures analyzed, the highest effective (based on the unit cell cross-sectional area) in-plane thermal conductivity was 40 W/(m K) with an out-of-plane thermal conductivity of 1.0 W/(m K) for unit cells with an interpillar distance Dx = 3.3 nm and pillar height Dz = 1.2 nm, while the highest out-of-plane thermal conductivity was 6.8 W/(m K) with an in-plane thermal conductivity of 6.4 W/(m K) with Dx = 2.1 nm and Dz= 4.2 nm. [ABSTRACT FROM AUTHOR]

An in vivo head impact dosimeter, termed the 'intelligent mouthguard', is under development at the Cleveland Clinic. The intelligent mouthguard facilitates correlation of acute and accumulative impact dosage with injury risk. As part of intelligent mouthguard dosimetry development for boxing and mixed martial arts participants, head and neck impact dosage was quantified in the laboratory for seven (7) punch types. Impact dosage data for punches were collected, which generated unique 'fingerprints' for use with the intelligent mouthguard computational algorithm. Additional calculations were made on parameters based on each punch 'fingerprint' and injury risk. Each of the punches had a unique 'fingerprint'. The hook imparted the most severe impact dosage, with the cross and oblique hook inducing the next most severe impact dosages, respectively. The impact dosage 'fingerprints' determined here will be used to identify punch types during intelligent mouthguard human trials in 2012. Intelligent mouthguard dosimetry will be available soon for other contact sports (American Football, Ice Hockey, Lacrosse) to monitor head-neck impact dosage for correlation to acute or accumulative injury risk. [ABSTRACT FROM AUTHOR]

This paper reports the development of an in-situ nanotensilometer that enables highly reliable mechanical tensile testing on individual micro-/nanoscale structures. The device features independent measurement of force and displacement histories in the specimen with nanoNewton force and sub-nanometer displacement resolutions, respectively. Moreover, the device is well suited for in-situ testing of free-standing micro/nanostructures within a high resolution scanning electron microscope, which permits continuous high-resolution imaging of the specimen during straining. In order to conduct the nanomechanical tests the ends of the specimen are attached to the probe tips of the device using electron-beam induced deposition. The general capabilities and features of the nanotensilometer are illustrated by presenting results of nanomechanical tensile tests on electrospun polyaniline microfibers. [ABSTRACT FROM AUTHOR]

Recently, a new generation of titanium aluminide alloy named Gamma-Met PX (GKSS, Geesthacht, Germany) has been developed with better rolling and postrolling characteristics. Previous work on this alloy has shown the material to have higher strengths at room and elevated temperatures when compared with other gamma titanium aluminides. In particular, this new alloy has shown increased ductility at elevated temperatures under both quasistatic and high-strain-rate uniaxial compressive loading. However, its high-strain-rate tensile ductility at room and elevated temperatures is limited to ~1 pct. In the present article, the results of a study investigating the effects of the loading rate and test temperature on the dynamic fracture initiation toughness in Gamma-Met PX are presented. A modified split Hopkinson pressure bar (MSHPB) was used along with high-speed photography, to determine the dynamic fracture initiation toughness. Three-point-bend fracture tests were conducted at impact speeds in the range 1 to 3.6 m/s and at test temperatures up to 1200 °C. Furthermore, the effect of long-time high-temperature air exposure on the fracture toughness was investigated. The results show that the dynamic fracture initiation toughness decreases at test temperatures beyond 600 °C. Moreover, the dynamic fracture initiation toughness was found to decrease with increasing exposure time. The reasons behind this drop are analyzed and discussed. [ABSTRACT FROM AUTHOR]

Plate impact experiments are conducted to study the dynamic fracture processes in 4340 VAR steel which occur on submicrosecond timescales. These experiments involve the plane strain loading of a planar crack by a plane tensile pulse with a duration of approximately 1 μs. The loading is achieved by impacting a precracked, disk-shaped specimen by a thin flyer plate. Motion of the rear surface of the specimen, caused by waves diffracted from the stationary crack and by waves emitted from the running crack, is monitored at four points ahead of the crack tip using a laser interferometric system. The measured rear surface motion is compared with the calculated motion using the finite element method to gain understanding of the dynamic fields that occur near the crack tip during crack initiation and propagation. For low temperature experiments, the measured rear surface particle velocity fields are in good agreement with the computed profiles obtained for a constant velocity crack propagation model. For the room temperature experiments, the experimental free surface particle velocity vs time profiles show a sharp spike, with a duration of less than 100 ns at the moment of crack initiation. The spike, which is not predicted by the inverse square root singular stress fields of linear elastic fracture mechanics, is understood to be related to the onset of crack growth. Critical values of the fracture toughness are estimated from the crack initiation times determined both from the velocity time profiles and the elastodynamic modeling of crack advance. The toughness values obtained increase with increasing impact velocity and are as large as 170 MPa√m at the highest impact velocity. Such relatively high values appear to be consistent with the ductile mode of crack initiation observed at all impact velocities used in the present study. [ABSTRACT FROM AUTHOR]

High strain-rate compression experiments were performed with a split-Hopkinson pressure bar (SHPB) at 500-4000/s on cylindrical samples of a Zr-based bulk metallic glass (LM-1) in both the fully amorphous and annealed conditions. The effects of changes to the specimen geometry (i.e., L/D ratio) and the material heat treatment [i.e., annealing versus amorphous (as-received)], on the peak stress, strain-to-failure, and failure behavior were determined with the aid of an in situ video obtained by using a high-speed digital camera in conjunction with the split-Hopkinson pressure bar (SHPB). Examination of the in situ video recordings and light optical microscopy showed that the failed samples revealed preferential failure initiating at the sample ends due to stress concentration at the sample-insert interface. A new insert design was developed using transient, elastic-plastic finite-element simulations to reduce the effects of these stress concentrations. SHPB testing, combined with in situ video, subsequently revealed that this new experimental configuration promoted failure within the gage length and away from the sample ends in the samples tested. Significant effects of specimen geometry, insert design, and annealing on the apparent values of the peak stress, strain-to-failure, and fracture behavior were exhibited. [ABSTRACT FROM AUTHOR]

Results are presented on the shock response of a zirconium-based bulk metallic glass (BMG), Zr41.25Ti13.75Ni10Cu12.5Be22.5, subjected to planar impact loading. An 82.5-mm bore single-stage gas-gun facility at Case Western Reserve University, Cleveland, OH, was used to conduct the shock experiments. The particle velocity profiles, measured at the back (free) surface of the target plate by using the velocity interferometer system for any reflector (VISAR), were analyzed to (i) better understand the structure of shock waves in BMG subjected to planar shock compression, (ii) estimate residual spall strength of the BMG after different levels of shock compression, and (iii) obtain the Hugoniot elastic limit (HEL) of the material. The spall strength was found to decrease moderately with increasing levels of the applied normal impact stress. The spall strength at a shock-induced stress of 4.4 GPa was 3.5 GPa while the spall strengths at shock-induced stresses of 5.1, 6.0, and 7.0 GPa were 2.72, 2.35, and 2.33 GPa, respectively. The HEL was estimated to be 6.15 GPa. [ABSTRACT FROM AUTHOR]