Michihiko Nakagaki

Residual stress due to welding and its effect on the assessment of cracks near the weld interface

Abstract An experimental/analytical hybrid-type investigation of the effects of residual stress on crack propagation due to welding has been performed. The residual stresses in the SAW welded A533B plates and electron beam welded plates that consist of HT80 and A533B steels were detected by an acoustoelastic technique. The measured residual stress was incorporated into a finite element procedure, which simulated stable crack growth in 1T compact specimens, where the effects on far-field crack parameters and on near-field crack parameters were examined. Also investigated was the effect on fatigue crack propagation with the hypothetical residual stress of the identical distribution to that in the electron beam weld. The significance of the residual stress distribution ahead and behind the crack tip in relation to the plastic zone size was identified.

Two-dimensional finite element analysis of stably growing cracks in inhomogeneous materials

Abstract The finite element method was applied to a generation phase analysis for stable crack growth in inhomogeneous materials. Experimental data on stable crack growth in bimaterial CT specimens, which were made of a weldment of the A533B Class 1 steel and HT80 steel, were numerically simulated using the node-release technique to obtain the variations of the fracture mechanics parameters such as the J-integral, T∗-integral, Ĵ-integral and CTOA with crack extension. New evaluation schemes for the integral fracture mechanics parameters were proposed as being valid for integral paths passing a fusion line of dissimilar materials. It was examined whether the simple formulae of the J-integral for a monolithic CT specimen can be applied to a bimaterial CT specimen or not. The effect of inhomogeneity on the fracture mechanics parameter is discussed in terms of the Q-factor.

Grain refinement of commercial Al-Mg alloy using severe torsion straining process

Severe plastic deformation (SPD) makes it possible to refine grain size in many metallic materials. Recently, we have developed a new SPD process designated the severe torsion straining process (STSP). This process requires no die but one side of a rod is rotated with respect to the other while producing a local heated zone in the rod and cooling both sides of the heated zone. Torsion strain is then introduced in the local heated zone. The STSP can be a continuous process because the rod is moved in the longitudinal direction while introducing torsion strain through the rotation. For grain refinement using the STSP, various factors may affect, which are the rotation speed, moving speed, straining temperature, cooling rate and diameter of the rod. In this study, the STSP is applied to grain refinement of an A5056 Al-Mg commercial alloy and the factors affecting the grain refinement are optimized. STSP was conducted at a temperature in the range from 573K to 723K. Microstructure was observed by optical microscopy, scanning electron microscopy with an orientation imaging system, and transmission electron microscopy. Microscopy observations revealed that the grain size was reduced to ~0.9 μm, when STSP was conducted at 573K with a rotation speed of 10 rpm and moving speed of 50 mm/min. There is a critical ratio of rotation speed to moving speed above which the rod breaks. The grain size tends to be finer as the straining temperature is lower, the cooling rate is faster and the ratio of rotation speed to moving speed is closer to the critical value.

Microstructures and Mechanical Properties of Mg Alloy after Severe Torsion Straining Process

Grain refinement is attempted using severe plastic deformation (SPD) through the severe torsion straining process (STSP) which we have developed recently. The STSP is a continuous process for grain refinement without requirement of any die. In this study, an AZ61 Mg alloy was subjected to STSP at a temperature of 573 K with a rotation speed of 10 rpm and a moving speed of 200 mm/min. With this process, an initial grain size of ~20 μm was reduced to ~2~3 μm. Room temperature compression tests revealed that there were no cracks after 15% of compression for the STSP sample whereas fracture occurred for a conventionally extruded sample. For compression tests at 473 K, no cracks occurred in the STSP samples after 80% compression but compression was feasible without cracking only up to 20% for an extruded sample. It is shown that the STSP can be useful for grain refinement and ductility improvement of the AZ61 Mg alloy.

Continuous Grain Refinement Using Severe Torsion Straining Process

This study presents a new rapid continuous process for grain refinement in metallic materials through severe plastic deformation (SPD). The new process, designated in this study the severe torsion straining process (STSP), is applicable to a wide range of alloys based on aluminum, magnesium and copper including carbon steel. This process consists of producing a local heated zone in a rod and cooling both sides of the heated zone by spray water while rotating one end with the other. Thus, torsion strain is introduced in the local heated zone. The STSP can be continuous because the straining is achieved while the rod is shifted along the longitudinal axis of the rod. Furthermore, the process requires no die, suggesting a potential for commercialization of grain refinement through SPD. In this study, STSP was applied to an Al-Mg alloy and a Mg-Al-Zn alloy. It is shown that STSP is effective for both alloys so that the grain size is reduced to ~1.5 μm for the Al alloy and ~0.9 μm for the Mg alloy. Tensile testing showed that the strength is increased with a minimal decrease in uniform elongation. There is a critical ratio between rotation speed and moving speed, which defines the feasibility of STSP operation without breaking the rod. The grain size tends to be lowered as the ratio is close to the critical value.

Molecular Dynamics Analysis on Crack Growth Behavior in Single and Nano-Crystalline Fe by the Use of FS-2NNMEAM Hybrid Potential

In recent years, nano-crystalline materials have attracted many researchers’ attention, but the fracture mechanism has not been fully clarified. In a molecular dynamics (MD) simulation, grain size and crystal orientation can be chosen, and their effects on the mechanical properties of nano-crystalline materials can be evaluated clearly. This research first compares the results of crack growth behavior in single crystalline Fe for three typical interatomic potentials (Embedded Atom Method (EAM), Finnis Sinclair (FS), and Second Nearest Neighbor Modified EAM (2NNMEAM) potentials) and a Hybrid potential method, which uses FS potential for bcc structure atoms and 2NNMEAM potential for non-bcc structure atoms. The 2NNMEAM potential is accurate, but the computation time is dozens of times that of FS potential, which is the simplest of the three interatomic potentials. Therefore, the 2NNMEAM potential requires too much calculation for the purpose of this research that analyzes the crack growth behavior in nano-crystalline metals. However, Hybrid potential is able to give results similar to those of the 2NNMEAM potential, and the calculation time is close to that of the FS potential. From these results, the crack extension behavior in relatively large nano-crystalline models is analyzed using the Hybrid potential, and we demonstrate the grain-size dependency of the fracture behavior.

Elastic-Plastic Constitutive Equation Accounting for Microstructure

In this study, we focus on the modeling of solid structures that include microstructures observed in particle-dispersed composites. The finite element modeling can be used to clarify how the macroscopic behaviors of solid structures are influenced by the microstructures. In such a case, if the whole structure including the microstructures is modeled by the finite elements, an enormous number of finite elements and enormous amount of computational time are required. To overcome such difficulties, we propose a new method for modeling microstructures. In this method, an explicit form of the stress-strain relation covering both elastic and elastic-plastic regions is derived from the equivalent inclusion method proposed by Eshelby that provides mathematical solutions for stress and strain at an arbitrary point inside and outside the inclusion. The derived elastic-plastic constitutive equation takes account of the microstructures, so that the effect of microstructures on the macroscopic behaviors can be obtained from the conventional finite element method by using such a constitutive equation without modeling microstructures in the finite element analysis. The effectiveness of the proposed constitutive equation is verified for a simple problem by comparing the results of the one-element finite element analyses using the proposed constitutive equation with those of the detailed finite element analyses using multi-element finite element modeling.

Molecular Dynamics Analysis on Initial Texture and Processing Route Influences on Grain Refinement Behavior of α-Fe by Equal Channel Angular Pressing

Fine-grained polycrystalline metals have a very high yield stress and excellent workability. Hence, numerous researchers are trying to develop an efficient process to obtain such materials. Our goal is to develop an efficient severe plastic deformation (SPD) process through investigating grain-refinement mechanisms in Equal Channel Angular Pressing (ECAP). In this paper, a series of molecular dynamics (MD) simulations of severe simple-shear deformations, which are ideally equivalent to SPD applied by typical ECAP processing routes, is performed using three-dimensional models that are thin and have a square shape with a periodic-boundary condition. We analyze the influences of the processing route and initial texture on the microstructural evolution. It is shown that twinning deformations are dominant under the calculated conditions, and that the structural evolution is notably affected by the relationship between the applied simple-shear direction and the characteristic crystal orientation, which can easily cause a twinning deformation. We conclude that Route A, without a rotation of the billet between processes, is the most efficient route. This is because twinning deformations along the simple-shear direction interact with the twin boundaries developed by the stress-component conjugate to the simple-shear. Furthermore, we demonstrate that the influence of the initial texture difference remains in force during multiple processes that have the same sliding plane.

Elastic-plastic constitutive equation taking account of particle size

Elastic-plastic constitutive equation taking account of particle size S. Takashima1, N. Miyazaki2, T. Ikeda3 and M. Nakagaki4 Summary Composite materials have complicated microstructures. These structures affect macroscopic deformation. In this study, we focus on the particle size effect in composite materials. For this purpose, we derived an elastic-plastic constitutive equation considering size effect by using Eshelby’s theory and the strain gradient theory of plasticity. We performed a homogenized finite element analysis using the elastic-plastic constitutive equation. The results obtained from the analysis show increase of the strength with decrease of the particle size in composite materials.

Probabilistic Modeling for Composites with Particle Interface Degrading

A popular type is of metal matrix reinforced by ceramics particles. It has been revealed that the composites are susceptible to interfacial degrading, which may be dramatically detrimental to the overall properties of composites. In present paper, an equivalent inclusion type constitutive model was developed for the composites with dispersed particles on which imperfections of the material interface are incurred. Two fundamental tensors are derived, the modified Eshelby tensor and the damage tensor of the weakened particles. By applying these tensors into a carefully schemed constitutive law, the effects of interfacial degrading on the overall properties of composite materials can be investigated. The interface degrading includes sliding and debonding. The numerical results show that even with the nil resistible sliding coefficient, its effect on the overall Young’s modulus is not notable unless the volume fraction of the particle is so high as more than 70%. For the global Poisson’s ratio, when there is the sliding on the interface, the Poisson’s ration rises irrespective of the constituent material values. It is noted that even in the elastic state, the global Poisson’s ratio rises greater than that of both the constituents. This phenomenon might indicate that even at the elastic state, the particle interfacial sliding would give somewhat a plasticity-like deformation behavior. The effect of the interfacial debonding on the overall properties of the composite is more conspicuous in comparisons of the sliding. The debonding parameter greatly affects both the properties for almost entire range of the particle volume fraction. Unlike the sliding effect case, the debonding decreases Poisson’s ratio at all cases, which represents the micro-damage effect occurring in the composite.