Tomoo Okinaka

A constitutive model for fcc crystals with application to polycrystalline OFHC copper

Based on the results of a series of experiments on commercially pure OFHC copper (an fcc polycrystal), a physically based, rate- and temperature-dependent constitutive model is proposed for fcc single crystals. Using this constitutive model and the Taylor averaging method, numerical calculations are performed to simulate the experimental results for polycrystalline OFHC copper. The model calculation is based on a new eAcient algorithm which has been successfully used to simulate the flow stress of polycrystalline tantalum over broad ranges of temperature, strain rate, and strain (Nemat-Nasser, S., Okinaka, T., Ni, L., 1998. J. Mech. Phys. Solids 46, 1009). The model eAectively simulates a large body of experimental data, over a broad range of strain rates (0.001‐8000 s ˇ1 ), and temperatures (77‐1096 K), with strains close to 100%. Few adjustable constitutive parameters of the model are fixed at the outset for a given material. All other involved constitutive parameters are estimated based on the crystal structure and the physics of the plastic flow. ” 1998 Published by Elsevier Science Ltd. All rights reserved.

A physically-based constitutive model for bcc crystals with application to polycrystalline tantalum

Abstract Based on the results of an extensive series of systematic experiments on commercially pure tantalum (bcc crystals), a physically-based, rate- and temperature-dependent constitutive model is proposed for bcc single crystals and is applied to simulate the experimental results, using the Taylor averaging method. The model calculation is based on a new efficient algorithm for the numerical solution of the finite deformation of bcc single crystals, involving up to 48 potentially active slip systems. The accuracy and efficiency of the proposed algorithm are checked through comparison with the results of the conventional explicit Euler time-integration scheme, using a very large number of timesteps. The model effectively simulates a large body of experimental data, over a broad range of strain rates (10−3 − 4 × 104/s), and temperatures (77 to 1300 K), with strains exceeding 100%, using very few adjustable parameters whose values are fixed at the outset for a given material. All other involved constitutive parameters are estimated based on the crystal structure and the physics of plastic flow.

Dynamic void collapse in crystals: computational modelling and experiments

Localization of inelasticow and crack initiation in fcc single crystals are studied experimentally and by numerical simulations, focusing on the anisotropic inelastic response of the crystal, and the mechanism of possible crack initiation and growth, produced upon unloading by the residual inhomogeneous plastic strains. Hollow circular cylinders of single-crystal copper are subjected to externally applied explosive loads which cause the collapse of the cylinder; this procedure is called the thick-walled cylinder (TWC) method. Then, numerical simulations are performed to understand the deformation process which leads to localized deformation, and tensile cracking when partial collapse is followed by unloading. Various loads and initial orientations of the lattice are examined in these numerical simulations in order to study their eA ects on theow localization and crack initiation phenomena.

A new computational approach to crystal plasticity: fcc single crystal

Abstract An algorithm is proposed for the calculation of the finite deformation of fcc single crystals, using a rate-dependent slip model. The method also applies to bcc and hcp crystals. The history of the deformation is divided into three regimes, depending on the number of active slip systems, and a computational strategy is proposed for each regime. The proposed algorithm uses a combination of the forward-gradient and the plastic-predictor elastic-corrector methods. The efficiency and the accuracy of the proposed algorithm is demonstrated by comparing the results with those of the conventional method.

A 16 Mfps 165kpixel backside-illuminated CCD

In 2002, we reported a CCD image sensor with 260×312 pixels capable of capturing 103 consecutive images at 1,000,000 frames per second (1Mfps) [1]. We named the sensor “ISIS-V2”, for In-situStorage Image Sensor Version 2. 103 memory elements are attached to every pixel; generated image signals were instantly and continuously stored in the in-situstorage without being read out of the sensor. The ultimate high-speed recording was enabled by this parallel recording at all pixels. In 2006, the color version, ISIS-V4, was reported [2]. In 2009, we developed ISIS-V12, a backside-illuminated image sensor mounting the ISIS structure and the CCM, charge-carrier multiplication, on the front side [3]. The CCM is a CCD-specific efficient signal-amplification device. CCM, combined with the BSI structure and cooling, achieved very high sensitivity. The ISIS-V12 was a test sensor intended to prove the technical feasibility of the structure. The maximum frame rate was 250kfps for a charge-handling capacity of Qmax=10,000e− and 1Mfps for a reduced Qmax. The pixel count was 489×400 pixels. For backside-illuminated (BSI) image sensors, metal wires can be placed on the front surface to increase the frame rate without reducing fill factor or violating uniformity of the pixel configuration. It has been proved by simulations that 100Mfps is achievable by introducing innovative technologies including a special wiring method [4]. We now report on ISIS-V16, developed by incorporating technologies to increase the frame rate with those to achieve very high sensitivity, which was confirmed by evaluation of ISIS-V12. The performance specification of ISIS-V16 is summarized in Fig. 23.4.1.

Development of high-speed video cameras

Presented in this paper is an outline of the R and D activities on high-speed video cameras, which have been done in Kinki University since more than ten years ago, and are currently proceeded as an international cooperative project with University of Applied Sciences Osnabruck and other organizations. Extensive marketing researches have been done, (1) on users requirements on high-speed multi-framing and video cameras by questionnaires and hearings, and (2) on current availability of the cameras of this sort by search of journals and websites. Both of them support necessity of development of a high-speed video camera of more than 1 million fps. A video camera of 4,500 fps with parallel readout was developed in 1991. A video camera with triple sensors was developed in 1996. The sensor is the same one as developed for the previous camera. The frame rate is 50 million fps for triple-framing and 4,500 fps for triple-light-wave framing, including color image capturing. Idea on a video camera of 1 million fps with an ISIS, In-situ Storage Image Sensor, was proposed in 1993 at first, and has been continuously improved. A test sensor was developed in early 2000, and successfully captured images at 62,500 fps. Currently, design of a prototype ISIS is going on, and, hopefully, will be fabricated in near future. Epoch-making cameras in history of development of high-speed video cameras by other persons are also briefly reviewed.

An image sensor of 1,000,000 fps, 300,000 pixels, and 144 consecutive frames

An image sensor for an ultra-high-speed video camera was developed. The maximum frame rate, the pixel count and the number of consecutive frames are 1,000,000 fps, 720 x 410 (= 295,200) pixels, and 144 frames. A micro lens array will be attached on the chip, which increases the fill factor to about 50%. In addition to the ultra-high-speed image capturing operation to store image signals in the in-situ storage area adjacent to each pixel, standard parallel readout operation at 1,000 fps for full frame readout is also introduced with sixteen readout taps, for which the image signals are transferred to and stored in a storage device with a large capacity equipped outside the sensor. The aspect ratio of the frame is about 16 : 9, which is equal to that of the HDTV format. Therefore, a video camera with four sensors of the ISIS-V4, which are arranged to form the Bayer’s color filter array, realizes an ultra-high-speed video camera of a semi-HDTV format.

Crack propagation imaging by the ISIS camera and a video trigger system

An ultra-high speed camera of 1Mfps was applied to visualize the crack propagation. Change of stress field around the propagating crack tip was captured as a change of the fringe pattern by means of the photo-elastic imaging technique. Newly developed video trigger system is employed to detect the occurrence of the crack propagation as a trigger in the experiment. The trigger successfully perceived the initiation of the crack propagation stably. Also its response time was fast enough even for the image capturing with 1Mfps. As a result, it is revealed that the elastic wave, propagating in the continuous body, has a significant effect on the velocity and kinking behavior of the propagating crack.

Experimental observation and computational simulation of dynamic void collapse in single crystal cooper

Hollow circular cylinders of single crystal copper are subjected to externally applied explosive loads that cause the collapse of the cylinder. Then numerical simulations are performed to understand the deformation process that leads to localized deformation and tensile cracking, observed in partially collapsed cylinders of fcc single crystals. The results of experiments and numerical simulations are in good agreement.

Improved design of an ISIS for a video camera of 1,000,000 pps

The ISIS, In-situ Storage Image Sensor, may achieve the frame rate higher than 1,000,000 pps. Technical targets in development of the ISIS are listed up. A layout of the ISIS is presented, which covers the major targets, by employing slanted CCD storage and amplified CMOS readout. The layout has two different sets of orthogonal axis systems: one is mechanical and the other functional. Photodiodes, CCD registers and all the gates are designed parallel to the mechanical axis systems. The squares on which pixels are placed form the functional axis system. The axis systems are inclined to each other. To reproduce a moving image, at least fifty consecutive images are necessary for ten-second replay at 5 pps. The inclined design inlays the straight CCD storage registers for more than fifty images in the photo- receptive area of the sensor. The amplified CMOS readout circuits built in all the pixels eliminate line defects in reproduced images, which are inherent to CCD image sensors. FPN (Fixed Pattern Noise) introduced by the individual amplification is easily suppressed by digital post image processing, which is commonly employed in scientific and engineering applications. The yield rate is significantly improved by the elimination of the line defects.