The Experts below are selected from a list of 147 Experts worldwide ranked by ideXlab platform
Per Ståhle - One of the best experts on this subject based on the ideXlab platform.
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Modelling of Fracture Processes During Fast Crack Growth
2020Co-Authors: Per StåhleAbstract:The role of the fracture Process Region during fast fracture is discussed. Reviewed experiments on modern pressure vessel steels show that already at crack tip speeds of one or two hundred mls fracture Processes are dominantly brittle. Modelling of the Process Region is discussed in the light of asymptotic elastic plastic fields and Process Region autonomy. It is suggested that a fraction of the crack surface (0.1 to 0.2) is created by ductile tearing of ligaments remaining after most of the crack surface has failed through cleavage. Ligament like test specimens of sub mm size are used to detennine the constants of an elastic viscoplastic model assumed for both the surrounding continuum and the crack bridging ligaments, The latter are replaced with a cohesive zone model embedded by an elastic viscoplastic continuum. The Process Region model captures cleavage in a foremost patt with very high cohesive stresses and rupture of ligaments in a trailing part where stresses are assumed to be rate dependent. In the surrounding continua the plastic deformation work is calculated considering high strain rate effects. The energy release rate in the Process Region is estimated. A balance between elastic energy release rate, energy dissipated at plastic deformation work in the plastic zone and energy release rate in the Process Region is examined. The energy released in the Process Region is consumed a) at cleavage and b) during rupture of ligaments. A cohesive zone model is used for the Process Region. The conclusion is that the experimental results can be explained only if the ligaments are considered. Crack growth rates down to one to two hundred mls are predicted by the model. Energy balance is possible at lower speeds but the crack immediately switches to a higher speed or comes to instant arrest as has been observed in experiments. (Less)
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CRACK TIP ENERGY RELEASE RATE DURING FAST FRACTURE
Mechanical Behaviour of Materials VI, 1992Co-Authors: Per StåhleAbstract:ABSTRACT The energy release rate in the Process Region is studied for a mode III crack growing in a steady state under dynamic conditions. The material is assumed to be viscoplastic and high strain rates are considered. A perturbation solution for a sharp crack tip is compared with the solution for a Barenblatt model of the Process Region. A advantageous simplification made when the extension of the Process Region is ignored but this limits the analysis. The validity for this simplification as regards actual Process Region length is studied.
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On the Autonomy of the Process Region
International Journal of Fracture, 1992Co-Authors: F. Sjöberg, Per StåhleAbstract:An investigation of the autonomy of the Process Region at the tip of a crack is carried out. An elastic plastic linearly hardening material is assumed. A cohesive zone model with constant cohesive stress is chosen. The cohesive zone length is observed to decrease with decreasing hardening rates and increasing cohesive stresses. For a cohesive stress larger than about three times the yield stress the zone becomes extremely small for hardening rates relevant for e.g. structural steels, and for perfectly plastic materials it did not develop at all. The autonomy of the cohesive zone was found to extend to at least 1.65 times the load specified for linear fracture mechanics. A very accurate description of the state of the Process Region is given by the near Region J-integral. Only a slight improvement is obtained by using the J-integral taken for a path outside the inelastic Region instead of using the stress intensity factor.
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Process Region Characteristics During Fast Crack Growth
1991Co-Authors: Per StåhleAbstract:The conditions for crack arrest at speeds down to below 0.1 of the Rayleigh-wave speed are examined. The elastic energy release rate is assumed to be balanced by the deformation work in a viscoplastic continua and the energy release rate in a fracture Process Region. Fractographic show that a small fraction of the surface is created through ductile rupture. Even though the fraction represents only 10- 25 of the surface, it is likely to consume much more energy than the brittle Processes. The ductile Processes, leading to rupture are assumed to be viscous. A rate dependence is therefore assumed for the Process Region.
K. B. Broberg - One of the best experts on this subject based on the ideXlab platform.
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Differences Between Mode I and Mode II Crack Propagation
Pure and Applied Geophysics, 2006Co-Authors: K. B. BrobergAbstract:Although mode I and mode II crack propagation show many similarities, in particular when analysed by linear elastic fracture mechanics, they differ significantly in the micro-structural behaviour. These differences, which are clearly noticeable in the fracture surface morphology, lead to fundamental differences in the macroscopic behaviour. Thus, mode II crack expansion under remote loading, appears to obey micro-structural scaling laws, implying that the dimensions of the Process Region stay essentially constant during crack expansion, rather than increasing with crack length. Therefore, expanding mode II cracks can almost reach the Rayleigh velocity, and actually also intersonic velocities. An expanding mode I crack, on the other hand, seems to obey continuum scaling laws, implying that the dimensions of the Process Region increase in proportion to crack length, leading to self-similar crack expansion at a velocity significantly below the Rayleigh speed and dependent on the remote load.
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Influence of T-stress, cohesive strength and yield strength on the competition between decohesion and plastic flow in a crack edge vicinity
International Journal of Fracture, 1999Co-Authors: K. B. BrobergAbstract:The discussion is based on the cell model of materials. Each cell contains one dominant kernel of micro-separation, for instance a particle. A cell is either in a cohesive or a decohesive state, the latter implying instability at load control. The Process Region consists of cells which have reached the decohesive state. The cells are characterized by their linear size and their cohesion-decohesion relation. The Process Region develops either in an elastic or in a plastic environment. In the latter case, it may be more or less deeply embedded in the plastic Region. In some cases there will be no Process Region, only plastic flow. The ratio between the cohesive strength and the yield strength is an important parameter for describing this competition between decohesion and plastic flow, but the T-stress also plays a part. The fracture toughness depends on the area under the cohesion-decohesion curve and on the embedment of the Process Region.
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Intersonic Crack Propagation in an Orthotropic Material
International Journal of Fracture, 1999Co-Authors: K. B. BrobergAbstract:Intersonic crack propagation is found to exhibit essentially the same features in orthotropic and isotropic materials, provided that the crack propagates along a plane of elastic symmetry. Thus the stress and strain singularity at the crack edge is weaker than the inverse square root singularity in the sub-Rayleigh case, except at one distinct velocity. The energy flux into the Process Region is determined by using the Barenblatt model. It depends on the crack velocity and on the size of the Process Region, approaching zero with this size.
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Loss of localization at high crack speeds
Constitutive Relation in High Very High Strain Rates, 1996Co-Authors: K. B. BrobergAbstract:At the IUTAM Symposium on “High Velocity Deformation of Solids” in Tokyo 1977, I argued that localization of the Process Region at a crack edge to a size determined by an intrinsic length parameter of the material was lost at a very high crack speeds [1]. A few years later several experimental results appeared, giving indirect support for such a loss by showing lack of a unique relation between stress intensity factor or surface roughness and crack velocity [2],[3],[4],[5]. Some of the experiments also showed a remarkable preference for constant crack velocity in the high velocity Region. These observations were later parallelled by numerical simulations by Johnson [6],[7]. The constant velocity reached did not seem to be a material property. Here it is argued that the preference for a constant velocity is in accordance with a perception of the Process Region as a continuum at very high crack velocities, a consequence of the lost significance of an intrinsic length parameter.
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Intersonic Bilateral Slip
Geophysical Journal International, 1994Co-Authors: K. B. BrobergAbstract:SUMMARY Symmetric "bilateral slip at constant intersonic velocity is investigated. Linear isotropic elasticity is assumed, and the idealization of a point-sized Process Region is adopted. The energy release rate is calculated for a slip propagation velocity equal to lh times the S-wave speed, which is the intersonic velocity for which stresses and strains are square-root singular in this idealization. The result shows a smaller energy release rate than at low subRayleigh velocities, but in general not much smaller: it equals approximately the energy release rate at about 95 per cent of the Rayleigh wave velocity for Poisson's ratio around 0.25. For other intersonic velocities the results were modified to include a finite Process Region, by using previously obtained results concerning steady-state propagation. In this way the energy flow to the Process Region can be obtained as a function of the slip propagation velocity in the whole intersonic Region for each set of Process Region characteristics assumed. Numerical calculations, assuming a Barenblatt Process Region model, indicate little sensitivity to Process Region size and a somewhat flat maximum close to fi times the S-wave speed.
Hiroyoshi Sakurai - One of the best experts on this subject based on the ideXlab platform.
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Rare-RI ring project at RIKEN RI beam factory
Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2020Co-Authors: Yoshitaka Yamaguchi, Akira Ozawa, Akira Goto, I. Arai, T. Fujinawa, Nobuhisa Fukunishi, Takashi Kikuchi, Tetsuya Ohnishi, Takashi Ohtsubo, Hiroyoshi SakuraiAbstract:[著者版]A new apparatus named "Rare-RI Ring", for precisely measuring the masses of short-lived rare nuclei including the r-Process Region is proposed. The Rare-RI Ring is one of the major experimental installations at the RI KEN RI beam Factory. It consists of three main parts: a long injection beam line, a fast kicker system and a cyclotron-like storage ring. The combination of a long injection beam line and a feast kicker system enables Lis to inject short-lived rare nuclei into the ring one by one. The revolution time of each nucleus in the ring is measured with all accuracy of better than 10(-6). Overall, the masses of short-lived rare nuclei including the r-Process Region can be determined with an order of 10(-6) precision. (c) 2008 Elsevier B.V. All rights reserved
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Rare-RI ring project at RIKEN RI beam factory
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2008Co-Authors: Yoshitaka Yamaguchi, Akira Ozawa, Akira Goto, I. Arai, T. Fujinawa, Nobuhisa Fukunishi, Takashi Kikuchi, Tetsuya Ohnishi, Takashi Ohtsubo, Hiroyoshi SakuraiAbstract:Abstract A new apparatus named “Rare-RI Ring”, for precisely measuring the masses of short-lived rare nuclei including the r-Process Region is proposed. The Rare-RI Ring is one of the major experimental installations at the RIKEN RI beam factory. It consists of three main parts: a long injection beam line, a fast kicker system and a cyclotron-like storage ring. The combination of a long injection beam line and a fast kicker system enables us to inject short-lived rare nuclei into the ring one by one. The revolution time of each nucleus in the ring is measured with an accuracy of better than 10 - 6 . Overall, the masses of short-lived rare nuclei including the r-Process Region can be determined with an order of 10 - 6 precision.
Frank Brueckner - One of the best experts on this subject based on the ideXlab platform.
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High-performance laser cladding with combined energy sources
Journal of Laser Applications J. Laser Appl J. Laser Appl, 2015Co-Authors: Steffen Nowotny, Sebastian Thieme, Frank Brueckner, Christoph Leyens, Eckhard BeyerAbstract:Articles you may be interested in Advanced laser welding of high-performance thermoplastic composites J. Laser Appl. 27, S29004 (2015); 10.2351/1.4906379 High-performance laser Processing using manipulated ultrafast laser pulses AIP Conf. Proc. 1464, 57 (2012); 10.1063/1.4739860 Melt pool temperature and its effect on clad formation in pulsed Nd:yttrium-aluminum-garnet laser cladding of Stellite 6 J. Laser Appl. 19, 32 (2007); 10.2351/1.2402524 Aluminum alloy welding by using a high power direct diode laser In the field of laser additive manufacturing, modern hybrid technologies offer advantageous solutions for combining the high quality level of laser surface claddings with the industry's economical requirements regarding productivity and energy efficiency. The technical approach is to supply energy sources in two fundamentally different ways. First, welding material in the form of wire is directly heated, i.e., by electrical resistance, almost to its melting point and is fed to the Process Region simultaneously with the laser beam. A newly developed coaxial wire head allows for omni-directional welding operation and, thus, the use of wire even for complex surface claddings as well as layer-by-layer fabrication of metallic parts. Second, an additional energy source is used to heat the substrate in order to compensate for the heat conduction losses. This technical variant is suitable for use with both wire and powder as deposition materials. Additionally, heating and cooling gradients can be precisely adjusted, thus improving the build-up of structures of hard and brittle metallic super-alloys. Currently, deposition rates up to 18 kg/h of metal alloys and metal matrix composites have been demonstrated using these laser hybrid techniques. Industrial applications are, among others, large hydraulic cylinders and tools of the oil and mining industries. V C 2014 Laser Institute of America.
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High-performance laser cladding with combined energy sources
Journal of Laser Applications, 2014Co-Authors: Christoph Leyens, Sebastian Thieme, Steffen Nowotny, Eckhard Beyer, Frank BruecknerAbstract:Articles you may be interested in Advanced laser welding of high-performance thermoplastic composites J. Laser Appl. 27, S29004 (2015); 10.2351/1.4906379 High-performance laser Processing using manipulated ultrafast laser pulses AIP Conf. Proc. 1464, 57 (2012); 10.1063/1.4739860 Melt pool temperature and its effect on clad formation in pulsed Nd:yttrium-aluminum-garnet laser cladding of Stellite 6 J. Laser Appl. 19, 32 (2007); 10.2351/1.2402524 Aluminum alloy welding by using a high power direct diode laser In the field of laser additive manufacturing, modern hybrid technologies offer advantageous solutions for combining the high quality level of laser surface claddings with the industry's economical requirements regarding productivity and energy efficiency. The technical approach is to supply energy sources in two fundamentally different ways. First, welding material in the form of wire is directly heated, i.e., by electrical resistance, almost to its melting point and is fed to the Process Region simultaneously with the laser beam. A newly developed coaxial wire head allows for omni-directional welding operation and, thus, the use of wire even for complex surface claddings as well as layer-by-layer fabrication of metallic parts. Second, an additional energy source is used to heat the substrate in order to compensate for the heat conduction losses. This technical variant is suitable for use with both wire and powder as deposition materials. Additionally, heating and cooling gradients can be precisely adjusted, thus improving the build-up of structures of hard and brittle metallic super-alloys. Currently, deposition rates up to 18 kg/h of metal alloys and metal matrix composites have been demonstrated using these laser hybrid techniques. Industrial applications are, among others, large hydraulic cylinders and tools of the oil and mining industries. V C 2014 Laser Institute of America.
Eckhard Beyer - One of the best experts on this subject based on the ideXlab platform.
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High-performance laser cladding with combined energy sources
Journal of Laser Applications J. Laser Appl J. Laser Appl, 2015Co-Authors: Steffen Nowotny, Sebastian Thieme, Frank Brueckner, Christoph Leyens, Eckhard BeyerAbstract:Articles you may be interested in Advanced laser welding of high-performance thermoplastic composites J. Laser Appl. 27, S29004 (2015); 10.2351/1.4906379 High-performance laser Processing using manipulated ultrafast laser pulses AIP Conf. Proc. 1464, 57 (2012); 10.1063/1.4739860 Melt pool temperature and its effect on clad formation in pulsed Nd:yttrium-aluminum-garnet laser cladding of Stellite 6 J. Laser Appl. 19, 32 (2007); 10.2351/1.2402524 Aluminum alloy welding by using a high power direct diode laser In the field of laser additive manufacturing, modern hybrid technologies offer advantageous solutions for combining the high quality level of laser surface claddings with the industry's economical requirements regarding productivity and energy efficiency. The technical approach is to supply energy sources in two fundamentally different ways. First, welding material in the form of wire is directly heated, i.e., by electrical resistance, almost to its melting point and is fed to the Process Region simultaneously with the laser beam. A newly developed coaxial wire head allows for omni-directional welding operation and, thus, the use of wire even for complex surface claddings as well as layer-by-layer fabrication of metallic parts. Second, an additional energy source is used to heat the substrate in order to compensate for the heat conduction losses. This technical variant is suitable for use with both wire and powder as deposition materials. Additionally, heating and cooling gradients can be precisely adjusted, thus improving the build-up of structures of hard and brittle metallic super-alloys. Currently, deposition rates up to 18 kg/h of metal alloys and metal matrix composites have been demonstrated using these laser hybrid techniques. Industrial applications are, among others, large hydraulic cylinders and tools of the oil and mining industries. V C 2014 Laser Institute of America.
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High-performance laser cladding with combined energy sources
Journal of Laser Applications, 2014Co-Authors: Christoph Leyens, Sebastian Thieme, Steffen Nowotny, Eckhard Beyer, Frank BruecknerAbstract:Articles you may be interested in Advanced laser welding of high-performance thermoplastic composites J. Laser Appl. 27, S29004 (2015); 10.2351/1.4906379 High-performance laser Processing using manipulated ultrafast laser pulses AIP Conf. Proc. 1464, 57 (2012); 10.1063/1.4739860 Melt pool temperature and its effect on clad formation in pulsed Nd:yttrium-aluminum-garnet laser cladding of Stellite 6 J. Laser Appl. 19, 32 (2007); 10.2351/1.2402524 Aluminum alloy welding by using a high power direct diode laser In the field of laser additive manufacturing, modern hybrid technologies offer advantageous solutions for combining the high quality level of laser surface claddings with the industry's economical requirements regarding productivity and energy efficiency. The technical approach is to supply energy sources in two fundamentally different ways. First, welding material in the form of wire is directly heated, i.e., by electrical resistance, almost to its melting point and is fed to the Process Region simultaneously with the laser beam. A newly developed coaxial wire head allows for omni-directional welding operation and, thus, the use of wire even for complex surface claddings as well as layer-by-layer fabrication of metallic parts. Second, an additional energy source is used to heat the substrate in order to compensate for the heat conduction losses. This technical variant is suitable for use with both wire and powder as deposition materials. Additionally, heating and cooling gradients can be precisely adjusted, thus improving the build-up of structures of hard and brittle metallic super-alloys. Currently, deposition rates up to 18 kg/h of metal alloys and metal matrix composites have been demonstrated using these laser hybrid techniques. Industrial applications are, among others, large hydraulic cylinders and tools of the oil and mining industries. V C 2014 Laser Institute of America.