Spallation Strength of Titanium at High-Speed Tension

We employ a first-principles total-energy method to investigate the theoretical tensile andshear strengths of fcc Ni systematically. The theoretical tensile strengths are shown to be36.1, 10.5 and 34.1 GPa in the [001], [110] and [111] directions, respectively. We indicatethat [110] is the weakest direction due to the formation of an instable bct ‘phase’in the tensile process. The theoretical shear strengths are, respectively, 5.1 and15.8 GPa in the ‘easy’ and ‘hard’ directions in the slip system, and 6.4 GPa in the slip system. Both the tensile and the shear strengths are consistent with eitherexperimental or theoretical values. The different shear strengths in the ‘easy’ and ‘hard’directions originate from the different charge redistribution under the shear strain. Theshear strain along the ‘easy’ direction of [112] results in a charge distributed in the which forms a directional bond, while the strain along the ‘hard’ direction of [112] makesthe charge extend to the whole {111} interlayers.

We describe two novel, practical ultrashort laser pulse measurement devices, which are also experimentally very simple. The first one is an ultra-broadband pulse characterization device that is based on FROG, but uses transient grating (TG) process. TG FROG involves forming an induced grating in a piece of glass by crossing two pulses in space and time and then diffracting a third pulse off it to create a fourth diffracted pulse. The TG process is inherently very broadband and automatically phasematched. We have implemented an ultrasimple TG FROG device, which can also operate single-shot. First, three beams are created using a simple mask. Then, a cylindrical beams line-focuses the beams horizontally, where the induced grating is generated. The variation of the relative delay is achieved by crossing the two grating-creation beams at an angle using a Fresnel biprism. Then, by detecting the diffracted pulse with spatial resolution, the TG FROG trace is captured. The second device that we present aims to measure ultrashort pulses with complex spectral and temporal structure. Spectral interferometry (SI) works perfectly for this purpose. SI simply involves measuring the spectrum of the sum of the unknown (shaped) and known (reference) light waves. Unfortunately, SI is very difficult to align and maintain aligned, as it requires that the two beams be nearly perfectly collinear . We solved this problem by utilizing optical fibers. Spectral resolution is also significantly improved by using spatial fringes, avoiding time-domain filtering.

Influence of alloying elements upon the theoretical tensile strength of Ni-based model superalloy: γ-Ni/γ′-Ni3Al multilayer

Chapter 29 - Ultrafast time reversal mirror at 800 nm based on photosensitive naphthalocyanine doped solids

Theoretical tensile stress in tungsten single crystals by full-potential first-principles calculations

Using a synchronously operating streak camera in conjunction with a 1 m monochromator the induced frequency chirp of an optical pulse on propagation over 2.2 km of single-mode optical fibre has been directly measured with picosecond resolution.

A first-principles method based on the local-density approximation using discrete variational clusters has been used to study the electronic structure of the hcp metals, Be, Mg, Sc, Y, Ti, Zr, Co, Zn and Cd. The binding energy of these metals was calculated in relation to the volume of a unit cell. The variation in the binding energy with the unit cell volume was obtained by means of a polynomial fit. The theoretical tensile strength and bulk modulus of these metals were estimated from the electronic structure and binding energy calculations. The predicted bulk moduli for these metals are in good agreement with experimental findings and other available theoretical data. A linear relationship between the calculated and the experimental strengths is observed.

Laser driven shocks have been used to investigate dynamic failure (spallation) of polycarbonate under uniaxial tensile loading at very high strain rate, of the order of 10\(^{7}\) s\(^{-1}\). First, uninstrumented recovery shots have been performed, post-test examination of the fracture damage has been carried out, and the influences of the experimental parameters (loading conditions and target thickness) have been analyzed. Then, an attempt to model the response of polycarbonate to plane shock loading has been made. On one hand, in-situ measurements have been performed in polycarbonate samples submitted to the plane detonation wave of a strong explosive, and the results have led to content with simple constitutive relations. On the other hand, piezoelectric measurements under laser shocks have provided a characterization of the loading pressure pulse, and comparisons of the measured and computed signals have confirmed the ability of the model to describe wave propagation in polycarbonate. Finally, the spallation experiments have been simulated. A spall strength has been estimated, on the basis of the experimental data, and the predictive capability of the model has been tested.

A series of plate impact experiments was performed using the single stage light gas gun facility at the Cavendish Laboratory, Cambridge University to determine the dynamic tensile, or spall strength of a hypabyssal kimberlite and a siltstone. Two distinct impact configurations were employed, over a range of impact velocities up to 185 m/s. VISAR and manganin gauges were used to monitor the spalling of the material. Thicker targets were found to be less suited for spall measurements as the signal appeared to be degraded by the increased amount of material traversed before reaching the measuring plane. A comparison with a theoretical scheme relating the spall strength to the Hugoniot elastic limit is performed, and while the agreement is not perfect it is of the same order of magnitude.

The femtosecond laser drilling for metal thin film is theoretically investigated in this paper. Femtosecond laser patterning of metal thin films is of technological significance because the fabrication of electrodes or metallization lines is a key process commonly required in the manufacturing of modern electronic devices. A femtosecond pulsed laser has a temporally short pulse that does not cause significant heat conduction in the material. This property of femtosecond laser pulse drilling makes sub-micron machining achievable with laser irradiation. Considering vaporization as the mechanism of the material removal, this paper employs two-temperature model to analyze the thermal process for femtosecond laser drilling of metal thin film. The variations of the drilling rate and squared diameter with laser fluence are compared with the available experimental data. This study also analytically validates that the drilling depth per pulse is governed by the optical penetration depth for low laser fluences and the squared crater diameter is linearly in proportion to the logarithm of laser fluence.

The femtosecond (FS) pulse laser incises soft tissues with minimal peripheral damage and is a promising cutting tool for ureteroscopic endoureterotomy of benign ureteral strictures. To evaluate the feasibility of applying the FS laser to ureteroscopic endoureterotomy. A commercial Ti:Sapphire regenerative amplifier system (Coherent, RegA 9050, USA) was used in this study. Normal saline, 5% glucose solution, 4% mannitol solution, distilled water, and a 1% (v/v) suspension of whole blood with each of these solutions were tested for their attenuation rate (AR) of the FS laser's power. Bladder specimens from Sprague-Dawley (SD) rats were used as a surrogate model. The laser incised slots of 2 mm in length at bladder samples using three power grades (5×, 10×, and 20× the threshold power) combined with five effective pulse rates (40, 20, 10, 5, and 2.5 kHz), both in air and in normal saline. After samples were processed with standard hematoxylin-eosin staining procedures, the incision depth and collateral damage range were determined microscopically. The ARs of blood suspensions with each of the three isosmotic solutions were significantly higher than the other five solutions (P < 0.001). The FS laser's cutting depth and the collateral damage were increased with the laser power or power density but the collateral damages were less than 100 µm. Microbubble formation was detected in the liquid environments tested and influenced the effective laser power. Endoscopic application of the FS laser is feasible. Microbubble formation with the laser incision, however, may influence cutting effects. Proposed methods to address these issues include increasing the irrigation rate, using distilled water as irrigation or using gas insufflation instead of irrigation. It is necessary to evaluate these methods, as well as the long-term biologic response to laser incision, on living animal models in endoscopic settings before use on humans.

Femtosecond laser induced damage threshold incubation and oxidation in AS2S3 and AS2Se3 thin films

The spall properties of rolled Al 5083-H116 plate are investigated using symmetric plate impact experiments over the stress range 1.5–6.2 GPa. Rear free surface velocity measurements made employing Velocity Interferometer System for Any Reflector interferometry reveal velocity profiles with clear signals of the Hugoniot elastic limit (HEL) and velocity pullback, indicative of a transition from elastic to plastic behavior and spalling. Experiments were performed on samples obtained both through the thickness and along all of the three principal axes of the rolled plate. For impact through the thickness, the average values of the HEL and spall strength are 0.43 GPa and 0.81 GPa, respectively. Decreasing the flyer plate and sample thicknesses resulted in an increased spall strength value of 0.95 GPa, while the HEL remained the same. The spall strength along the longitudinal (rolling) direction was 1.06 GPa versus 0.95 GPa for impact along either transverse direction. Spall damage for this impact direction often propagated away from the spall plane in the direction of impact and along the grain boundaries. For impact through the thickness, the fracture surface revealed a mixed mode of ductile and intergranular fracture that was not present for the fracture surface in the other two directions. This mixed fracture mode seems to correspond to a shoulder observed in the free surface velocity traces after the pullback. In all cases, cracked brittle inclusions were observed near the spall damage regions, indicating their role in nucleating voids during spall failure.

Results of investigations on microstructuring of steel using pico-and femtosecond laser pulses will be presented. For the experiments, lasers with high repetition rates in the 100 kHz range, but relatively small pulse energies were used. The pulse durations were 15 ps and 180 fs, respectively.Experiments showed that the volume of material ablated per pulse is always larger with femtosecond laser pulses compared to picosecond laser pulses using the same fluences and repetition rates, which suggests that the photons of femtosecond pulses are used more effectively for ablation.Moreover, the volume ablated per pulse using femtosecond laser pulses is from the beginning independent of the number of crossings used, whereas it increases at first with increasing number of crossings with picosecond pulses till a constant value is reached after 100 crossings of the laser beam over the structured area.The average roughness RA of comparatively deep structures produced with the femtosecond laser was generally found to be lower than with the picosecond laser. Thereby, the roughness increased with increasing depth of the structures, i.e. with increasing total number of laser pulses used, with both lasers.First investigations using burst mode with the picosecond laser have shown, however, that the quality of the structures with respect to surface roughness and also the ablation rates can be improved significantly by using this mode.Results of investigations on microstructuring of steel using pico-and femtosecond laser pulses will be presented. For the experiments, lasers with high repetition rates in the 100 kHz range, but relatively small pulse energies were used. The pulse durations were 15 ps and 180 fs, respectively.Experiments showed that the volume of material ablated per pulse is always larger with femtosecond laser pulses compared to picosecond laser pulses using the same fluences and repetition rates, which suggests that the photons of femtosecond pulses are used more effectively for ablation.Moreover, the volume ablated per pulse using femtosecond laser pulses is from the beginning independent of the number of crossings used, whereas it increases at first with increasing number of crossings with picosecond pulses till a constant value is reached after 100 crossings of the laser beam over the structured area.The average roughness RA of comparatively deep structures produced with the femtosecond laser was generally found to ...

The need for ultrathin fiber-based devices that can deliver light to confined places in order to perform imaging and/or laser ablation of a desired target has been a research area of significant interest. The current endoscopic devices are based on distal optics and scanning mechanisms to focus and scan the light in the end of the fiber. The distal components are limiting factors for decreasing the size of the device. However, using wavefront shaping techniques, lensless focusing and scanning of a laser focus spot through the fiber can be achieved, enabling a smaller endoscopic tool. In our case, a high power focus spot is created by wavefront shaping of the light through a multicore fiber (MCF), providing the possibility of two-photon fluorescence (TPF) imaging. Femtosecond laser ablation through the endoscopic device can be also a powerful tool for a range of applications. Therefore, we investigate limitations in the maximum peak power that can be delivered through the MCF due to nonlinear effects induced in the fiber cores in the ablation peak power regime. After characterizing the capabilities of our system, we demonstrate that femtosecond pulsed laser ablation can be performed through the MCF.