The fatigue crack initiation and propagation behaviors were examined for pure copper films of 100 μm thickness. The cold rolled films were annealed and adhered to cover a circular through hole in a base plate. The base plate was subjected to constant cyclic stress with a stress ratio of R = 0. In order to discuss correlation between fatigue crack propagation and change of crystal orientation, crystal orientation on the surface of the copper film was measured before and after fatigue testing. The crystallographic information of these films was analyzed using the Electron Back-scatter Diffraction (EBSD) system and the quantitative evaluation method for the crystal rotation angle and the rotation axis, the rotation direction with fatigue was developed. As a result, the crystal rotation angle near the fatigue crack is larger than that apart from the crack but that is quantitatively small around the crack propagated on the annealing twin boundary.
In this study, we develop a homogenization theory and its finite element discretization based on the nonlocal crystal plasticity theory of Gurtin (J. Mech. Phys. Solids 48 (2000) ; J. Mech. Phys. Solids 50 (2002)), which includes slip gradients and additional slip boundary conditions with respect to microforce balances for each slip system. Although it is difficult to determine slip boundary conditions, the development of the homogenization theory for the nonlocal crystal plasticity makes it possible to obtain the unified treatment of slip boundary conditions on the periodic boundary in periodic materials. Furthermore, by applying a tangent modulus method to the finite element discretization of rate-dependent small deformation, a strong coupling boundary value problem is derived from homogenization equations which consist of the stress balance, the microforce balances and the macroscopic relation. Finally, the effectiveness of the proposed theory and method is confirmed through finite element analyses of a double slip single crystal model with periodic obstacles.
We examined applicability of highly non-equilibrium states in a polymer glass to its cold working. To study the cold-workability of a polymer glass, we first deformed poly (methyl methacrylate) (PMMA) in uniaxial compression up to the yield point at 30°C to have its highly non-equilibrium state and subsequently stretched it. The tensile fracture strain of such PMMA specimens was nearly ten times as large as that for specimens simply stretched without pre-compression. When the specimens compressed up to the yield point (εc = 0.2) were left under the strained state for various time periods, the tensile fracture strain increased with length of the time period. The pre-compression up to the yield point was confirmed as a quite effective procedure for cold working of brittle plastics.
The present study is concerned with the static and dynamic characteristics of laser welded joints related to the tailored blanks technique for vehicles. Three kinds of laser welded butt joints made of different steel sheets for vehicles, a mild steel HR270, high tensile strength steels HR590 and HR780, were used in the static and impact tensile tests. In the static test, all welded butt joint specimens showed that the tensile strength was similar to that of the base metal specimens corresponding to them, but the strain at fracture decreased. The measurement of the impact tensile strength of laser welded butt joints was performed by means of the modified split Hopkinson bar method for impact tensile test. The impact tensile strength of the laser welded butt joint of mild steel HR270 tended to increase comparing with the static one due to the effect of strain rate. On the other hand, regarding the high tensile strength steels of HR590 and HR780, the effect of strain rate was not recognized at about 320s-1. Thus the impact tensile strength and the fracture strain were almost same as the results in the static tests.
In high strength steel sheets, it seems to be possible to improve the fatigue strength of joints welded by using laser welding due to the lower heat input of welding and the flatter weld beads compared with conventional welding methods such as arc welding or resistance spot welding. Therefore, in this paper, fatigue tests were conducted for the specimens with the butt joints of 780 MPa grade steel sheets welded by using laser welding, plasma welding and MAG welding and for the specimens flattened the weld beads to investigate the effect of welding methods on the fatigue properties of welded joints of high strength steel sheets for automobiles. And the macrostructure, the microstructure and the hardness distribution around the weld metal were observed to study the mechanism for the difference of the fatigue strength by the welding methods. It was found that the fatigue strength in arc-welded joints and plasma-welded joints is lower than that in laser-welded joints mainly due to the occurrence of softening in the heat affected zone. On the other hand, it was found that the stress concentration at the toe of the weld bead significantly deteriorates the fatigue strength even in the laser-welded joints.
Relation between microstructures in the heat affected zones (HAZ) and impact strength was investigated for SAF2205 duplex stainless steel on the basis of three-dimensional fractography. Simulation of thermal cycles occurring in the HAZ was performed. The specimens were heated at various peak temperatures from 1373 K to 1623 K using Gleeble simulation tester, and then impact-tested in the temperature range from 77 K to 273 K. The morphologies of the fracture surfaces tested at 77 K were like shear fracture in case of the base metal specimen and the specimen heated at 1473 K. The impact strengths were relatively high, even at liquid nitrogen temperature, because of the presence of austenite. However, the morphologies of the fracture surfaces with the microstructures heated to higher than 1573 K such as the bond region showed cleavage fracture of ferrite. The impact strengths were much lower than those of the as-received metals, because of the grain growth of ferrite, the dissolution of austenite and precipitation of chromium nitrides. The orientations of cleavage facets observed in the specimen heated at 1623 K and tested at 77 K were measured using three-dimensional topography. As a result, it was found that there were groups of facts having almost same orientations. It is considered that the group of facets is corresponding to one ferrite grain and each facet is separated by intragranular austenite. It was found that intragranular austenite subdivided a ferrite grain into smaller facets and suppressed the degradation of bond toughness of duplex stainless steel.
Titanium and its alloys are currently in use as implant materials for orthopaedic surgery. These alloys own outstanding corrosion resistance due to a dense and passive oxide film of a few nanometers' thickness on their surfaces. When these alloys are implanted in a living body, implanted surface material removal takes place because of mechanical wear and corrosion. Material degradation, due to simultaneous chemical and mechanical effects, limits long-term use of metallic biomaterials, such as stainless steels and titanium alloys. Thus, it is very important to investigate synergistic interaction between wear and corrosion. The aim of the present study is to clarify the damage accumulation mechanism of Ti-6Al-4V alloys under the simultaneous reaction of corrosion and wear. For this purpose, first, we developed a new tribocorrosion system and then we estimated corrosion wear characteristics of the alloys with special attention focused on the effects scratching rate on repassivation behavior. Two types of corrosion wear tests were carried out. One was a free corrosion potential measurement with simultaneous application of wear damage and the other was potentiostatic polarization testing with wear damage. To discuss the corrosion wear mechanism, the damaged surfaces were carefully observed by a scanning electron microscope. It was concluded that in corrosion wear environment, a transition of wear type from abrasive to adhesive was observed with an increase in the scratching rates. A higher scratching rate prevented regeneration of passive films on worn surfaces, resulting in the generation of relatively large corrosion pits and changes in wear form. These results suggested that the scratching rate was an important factor that affected the damage accumulation process.
In this study, one set of stress function is shown on crack problem in an infinite isotropic plate. Features of obtained stress function are the plane elasticity problem that are common to both of an interface crack between two dissimilar elastic half planes and an infinite isotropic plate containing a central straight crack, and it is linearly independent at the crack tip. In some numerical examples, energy release rate for Young modulus ratio (E1/E2) has been obtained in an interface crack problem. Besides, the energy component of shearing stress has also been shown in these figures.
Polyurethane/silica nanocomposites were innovated by sol-gel method from polyurethane and tetraethylorthosilicate under TEOS and some specified conditions. Then, the properties of the obtained polyurethane/silica nanocomposites were investigated from the viewpoints of mechanical and thermal properties. It is shown that the hardness, elasticity modulus, thermal properties and UV irradiation character depend deeply on the silica content in the nanocomposie materials. These results indicated that by changing the amount of silica we could control the material properties. Even for the silica content of 40 wt%, the fracture elongation of over 300% is obtained. The excellent ductile property of polyurethane resin is maintained in the developed materials even for high silica content. Based on TEM analysis, it is found that the size of silica in developed nanocomposites is about 5 nm.
This paper deals with the development of polymer-modified mortar with high flexural strength, which is used in place of ordinary cement concrete for the manufacture of precast products with thin thickness. A polymer-modified mortar using a polymethacrylate emulsion and an ordinary cement concrete are prepared, and tested for the fresh properties and the mechanical properties, length change and durability after steam curing. The results showed excellent fresh properties and strength development, as well as improved mechanical properties and durability such as resistance to freeze-thaw, neutralization, chloride ion penetration, hydrochloric acid and sulfuric acid. The physical properties obtained were further considered from the stand point of the characteristics of the material used and the composition and structure of hardened mortar. After steam curing, the flexural, compressive and tensile strengths of this mortar were 15.3 MPa, 78.0 MPa and 6.7 MPa at 14 days, respectively; thus, the flexural strength was 3.1 times higher and the tensile strength was 2.1 times higher compared with ordinary concrete under the same curing conditions. These flexural strength and flexural/compressive strength ratio of this mortar are higher than those of conventional ultra-high-strength concrete. Therefore, this mortar can be effectively used in thin wall concrete products which are to be produced without mechanical vibration (self-compacting).
Recently, an indentation (ID) method using a Vickers hardness tester has been specified as a standard by the Society of Materials Science, Japan (JSMS-SD-4-01) as the method for practical use. This method determined a base value (KC) while measuring residual stress by the ID method. Porosity and other factors have been reported to cause problems when using the ID method on Al2O3, Si3N4 and ZrO2 as structural ceramics, a fact which motivated the establishment of this new method. The value measured by this method might change not only in response to microstructural factors such as grain size and aspect ratio, but also with different types of materials. However, this problem has received little attention to date. In this study, a prescribed bending moment was loaded on Al2O3 ceramics that differed in microstructure (grain size and aspect ratio). The bending stress value was considered to establish the level of residual stress, and was compared to the value obtained by the ID method. The variation of that relationship was examined as a function of the aspect ratio. The stress ratio φ' was defined as the ratio of the value determined by the ID method and the way that tensile stress σT or compressive stress σC varied with the aspect ratio. This alteration appears commonly in the case of tensile stress. It was thought that the origin of changing φ' (stress ratio) values was interaction across crack faces by grain bridging during cracking, which varied with the aspect ratio, making the crack propagation resistance value different in various materials.