The microscopic plastic deformation behavior of aluminum sheet during uniaxial tension is experimentally investigated by a confocal laser-scanning microscope. The surface roughening is closely related to the inhomogeneity of polycrystalline metals, that is, to the inhomogeneous plastic deformation of respective grains. In order to clarify the deformation behavior of polycrystalline metals, new measuring methods and 4 types of strains of grains are introduced and inhomogeneous of strain of grains are investigated. Digital image processing technique is applied to the surface image taken by the CCD camera for measurement. It is shown that the grain-area strain, the maximum dimension and mean dimension strain, the grain-boundary length strain and the standard deviations of these parameters statistically increase in proportion to the applied strain. Discussions are made on the differences between these measuring methods of strain parameters of grains, variations of measured statistical results including comparison with equilateral polygon model during tensile deformation.
It is well known that a tensile overload applied during constant amplitude cycling at a positive stress ratio R can lead to retardation in the rate of subsequent crack propagation. However, in a case of a negative stress ratio R, the fatigue crack growth rate actually accelerated after a tensile overload. This crack propagation behavior is related to the local bulging of specimen thickness, the blunting behavior of crack tips and the residual stress distributions. When the crack tips were blunted during tensile overload and the succeeding compression stress with a high value was applied after the overload, the crack propagation rate got higher level than that before the overload application. In this case, a tensile residual stress was created in front of the crack. To confirm the relationship between the crack blunting behavior and the crack propagation behavior, the effect of two overloads' application (first one was tensile overload and second one was compression overload) on crack propagation was investigated. It was found from this that the acceleration of the crack propagation occurred when the range of crack tip opening displacement became large level during overload stage.
Fatigue strength and crack initiation depth were investigated in carburized SCM415 steel under rolling contact fatigue (RCF). RCF tests were carried out until the number of cycles reached 1.2×108. The fatigue strength of carburized SCM415 was found to be more than 4000MPa. Observation of a flaking surface showed that the crack initiated around an inclusion or a defect, the depth of which was about 50μm. Based on the observations, we concluded that the subsurface crack was propagated when its shape remained circular. We evaluated the crack growth behavior in terms of contact stresses and stress intensity factors. The contact stresses and the stress intensity factors were calculated by using Hanson and Fabrikant's analytical method. It is found that the absolute values of KIImax and KIImin were larger than other stress intensity factors and the subsurface crack was dominantly propagated in mode II. The flaking depths vary depending on the KII peak curbing slightly. The friction between crack faces causes the flaking failure which is shallower than predicted by the maximum shear stress.
In order to study fatigue crack growth characteristics in the components used in liquid sodium, fatigue tests were carried out at 550°C. This is near the system temperature used for sodium coolant in fast breeder reactors (FBRs). The factors influencing fatigue lifetime in sodium compared with that in air were investigated by observation of surface cracks in 316FR steel. Furthermore, the effects of sodium environment on fatigue were investigated based on examining the results of thermal striping tests, etc., obtained up to now. The results of the fatigue tests show that many micro cracks in the shearing direction were produced by the midlifetime, and micro cracks connected quickly after that. This is because an oxidation film was not formed, since sodium is of a reductive nature, and strain of the material surface tends to distribute equally. During crack progression there is no oxide formed on broken surfaces. Therefore re-combination between broken surfaces takes place, and crack progression rate falls. Furthermore, in non-propagating crack, the wedge effect by oxide between broken surfaces at the time of compression is small. Therefore, the crack closure angle is small, compression strain generated in the crack tip becomes large, and the crack cannot stop easily. As mentioned above, the main sodium influence on the fatigue characteristics are because of its reductive nature. In summary, in sodium environment, it is hard to form a crack and to get it to grow. Once started, however, it is hard to stop the crack in sodium compared with in the case of the air.
This study was carried out in a serial research on spider thread. The thread is supposed to consist of three layers, outer coat resistant to water, middle layer with oriented micro-fibers and inner core with soft substance. It is desirable to observe its longitudinal and perpendicular cross-sections directly to confirm its hierarchical structure. This paper aims to tear natural and synthetic fibers with diameters 20 to 100μm and observe their micro structures in a simple manner, while the final goal is to inspect those of structures of more fine fibers with diameter of a few micrometers like spider thread. It was shown that 1. several natural fibers such as horse mane, human hair, rabbit fur, fox tail grass etc. and synthetic fibers of aramid and nylon were torn and their hierarchical structures were observed easily through a SEM and 2. the forces required to tear fibers along their longitudinal directions were measured by a laboratory-made tearing machine and their fracture toughness were calculated on the basis of the fracture mechanics.
Free-edge stress singularity develops near the intersection of the interface and the free-surface of bonded dissimilar materials. Therefore, fracture of bonded dissimilar materials may occur at the intersection of the interface and the free-surface. Free-edge stress singularity depends on the combination of material properties and shape of bonded dissimilar materials. In this study, the relation between the stress intensity factor of a small edge crack on interface and free edge stress singularity of bonded dissimilar materials under external loading was investigated numerically by using the boundary element method with changing the combination of wedge angle of bonded materials. The relationship was also investigated theoretically by using the principle of superposition. The results of numerical analyses were compared with those of theoretical analyses. It was found that stress intensity factors of small edge crack on interface K1, K2 were proportional to the intensity of free-edge stress singularity without crack Kσ. The numerically determined proportional coefficient between K1 and Kσ agreed well with the theoretical one, and was not affected by crack length when proper normalization was applied.
The relationship between the deformation heat input in the upset stage or the upset burn-off length and joint strength was examined on the friction-welded SUS304 stainless steel joints. Joint strength was evaluated by tensile test and fatigue test. It was found that the deformation heat input in the upset stage or the upset burn-off length was correlated well with tensile strength, and a stable tensile strength was obtained when the deformation heat input in the upset stage or the upset burn-off length exceeded a certain value. In the Ono's rotary bending fatigue testing, the fatigue strength of sound joints was a little less than that of SUS304 base metal. While, in the cantilever rotary bending fatigue testing, the fatigue strength of sound joints was a little more than that of SUS304 base metal. This difference probably resulted from the fatigue fracture occurred in the softened area in the Ono's type testing and at the weld interface in the cantilever type one. Judging from the fatigue limit obtained, sound joints can be produced when the deformation heat input in the upset stage or the upset burn-off length exceeds a certain value.
A laminated beam containing an interface crack subject to thermal gradient is analyzed on the basis of the classical beam theory. The axial forces are induced in the parts of the constituent beams above and below the interface crack. For the case where crack faces are open, a nonlinear equation for determining the in-plane forces is derived by modeling the cracked part as two lapped beams hinged at both ends, and by imposing the compatibility condition of the deformations of the two beams. Numerical solutions are obtained for some model beams. It is shown that the relative displacement at the center of the crack increases gradually with the increase in temperature gradient, However, at a critical temperature gradient, the relative displacement begins to increase very rapidly, i. e., local delamination buckling occurs. Energy release rate is small for temperature gradient below the critical value, but it takes a large value when the temperature gradient is increased beyond the critical value. The model where the two crack faces are contacting one another is also analyzed. In this case it is shown that the energy release rate is identically zero if the temperature gap between the crack faces is zero.
Small fatigue crack initiation and growth behavior were investigated for two kinds of ultra fine-grained P/M (Powder Metallurgy) aluminum alloys of which grain size was from 200 to 500nm. Those two materials were denoted as Types M and S according to the chemical composition and manufacturing process. Type M without Si addition was extruded, while Type S with Si was forged. The shape of powders changed through manufacturing processes, where the aspect ratio of elongated powders in Type M was larger than that in Type S. Reversed plane bending fatigue tests were conducted and small fatigue cracks were observed by means of plastic replicas technique. The effect of specimen orientation on fatigue properties in those two materials was also investigated. The specimen orientations evaluated were C-C and L-C for Type M, and C-C and C-L for Type S. It was found that small fatigue cracks initiated at the inclusions for C-C orientation and at the boundary between powders for C-L orientation in Type M, while they initiated at the boundary regardless of specimen orientation in Type S. S-N curves were dependent on the crack initiation mode, and the crack initiation life was well estimated by Kmax calculated with stress amplitude and the size of inclusions or cluster of powders. The fatigue crack growth behavior was almost irrelevant to the materials and specimen orientations.
Aluminum alloys to S25C carbon steel were friction welded, and the friction weldability and intermetallic formations at the weld interface of joints were investigated. Joints of A1050, A5052, A6061 and A6063 to S25C showed good-weldability, and A5056/S25C joint was somewhat poor than former one. An intermetallic compound of Al13Fe4 was formed at the weld interface of A5056/S25C and A6061/S25C joints, and their intermetallic formations were tend to increase with friction pressure, friction time and rotation speed. The joint strength of these joints was high when thin instermetallic conpound layer was formed under high upset pressure. Somewhat poor-weldability of A5056/S25C joint compared with A6061/S25C joint seemed a cause of the intermetallic formation accelated by high content of magnesium in A5056. Joints of A2011, A2017, A2024 and A7075 to S25C showed poor-weldability. Brittle intermetallic compounds of CuAl2 and Al7Cu2Fe were formed at the weld interface of A2011/S25C joint containing a copper.
A method of molding composite materials with using loosing technique was developed, and the tensile properties of the composite materials were investigated. This molding method was developed to obtain higher fiber content and to increase the interfacial area between fiber and resin. Polypropylene/polyethylene (PP/PE) fibrous resin and cotton fibers were used as raw materials, and the composite materials were made by an injection molding and a hotpress molding with loosing cotton fibers and loosing PP/PE fibrous resin. It was clarified that the tensile strength of loosing fiber reinforced composite materials was drastically improved by the vacuum hot-press molding. It was also shown that the tensile strength of the composite material was 75MPa at the fiber weight content 70wt.% and its value was nearly equal to that of GF/PP composite material (30% fiber weight content) made by injection molding.
When mechanical components of ceramics are machined, some extent of residual stress is necessarily introduced in the products. In the reliability evaluation of the ceramic components, it is important to know the residual stress introduced in the fabrication process. A simple method to measure the residual stress based on the indentation fracture (IF method) was standardized by the Society of Materials Science, Japan, in 2001. In order to examine the applicability of the above standard method, stress measurements were carried out for some ceramics such as alumina, mullite and silicon nitride. In the experiments, a definite residual stress was introduced by applying the static load through a vice, after which the residual stress was measured by the IF method. In addition, the residual stress introduced in the machining process was measured by the IF method and X-ray diffraction method, and the respective results were compared to each other. Typical results in this study were summarized as follows; (1) load criterion to transit the crack types to median from Palmqvist was clarified, (2) experimental results of residual stress were in good agreement with the stress level artificially given in advance and (3) residual stress in the surface layer measured by the IF method well corresponded to the value obtained by the X-ray diffraction method.
In the current manufacture of multi-layer printed wiring boards, AFRP (Aramid Fiber Reinforced Plastices) has been attracted attention, because of its low coefficient of linear thermal expansion in the x-y plane and of good machinability of CO2 laser. However, there are few reports dealing with drilling of AFRP printed wiring boards. Therefore, this paper describes the small diameter drilling of AFRP printed wiring boards. First, the temperature of drill tools was monitored by infrared thermography. Second, the drilled hole walls were observed by SEM and optical microscope. Comparing with them, it is clear that the drilled hole quality depend on the drill tool temperature in drilling. Additionally, it is confirmed that the increase of drill temperature is due to the heat caused by friction between the drill tool and drilled hole wall.
The Split Hopkinson pressure bar (SHPB) method has been used for impact tests such as compression and shearing, and is currently modified for tensile test, since the various kinds of high strength sheet metals are greatly concerned with the safety and the energy saving demands for high velocity vehicles. Evaluation of mechanical properties by a tensile test using the SHPB, however, has not been clarified, since the testing conditions such as specimen geometry and settings are quite different from the usually operated compression version. In the present paper, we try to identify the characteristics of the method, particularly such as a yielding phenomena, and have clarified the effect of three dimensional stress wave propagation in the tensile test where a large ratio of sectional areas between a specimen and a loading bar is usually inevitable.
In order to examine the distribution of creep properties in a 316FR weld metal, a narrow gap welded joint was prepared from 50mm thick 316FR steel plates by gas tungsten arc welding (GTAW) process using type 316 filler wire, and constant-load creep rupture tests were conducted at 823K in air using full thickness large welded joint specimens and pure weld metal miniature specimens. The latter specimens were taken parallel to the weld line of 316FR weld metal in the welded joint. Specimen sampling locations were the plate surface, the 1/4 and the 1/2 plate thickness positions from the surface. By interrupting the creep test for the full thickness welded joint specimen, we observed the change in creep strain distribution in the weld metal in the thickness direction of the plate. The last layer of weld metal showed considerably larger creep strain than the central layers of the weld metal. Further, the creep strength of weld center is larger than that of the top of the weld. The cause of this difference in the creep strength in the weld metal is that due to large thermal expansion coefficient and small thermal conductivity of austenitic stainless steel strain introduced through thermal histories was larger in the central part than in the vicinity of the surface of the welded joint. Hardness variation in the weld metal and the results of transmission electron microscopic examination revealing dislocation configuration support this speculation. From the results above, it is concluded that the creep properties of weld metal in multi-pass, multi-layer welded joints strongly depend on the location of specimen sampling.
Time-dependent-change of flowability of fresh concrete is very important for developping concrete practice design method. In this study, in order to develop a predicting method of the change of viscosity for cement paste used polycarboxylate based superplasticizer referencing Hattori-Izumi theory, time-depending-change test of cement paste was carried out. The main factors of this experiment was type of powder. For lime powder which is not active powder, viscosity kept steady with time. It was considered that this result was what the theory of potential had expected. On the other hand, viscosity changed with time for ordinary portland cement which was chemically active powder. A prediction model of time-dependent-change of viscosity for cement paste was developped assuming apparatus potential energy between two cement particle from experimental test result. It was considered that the model could predict viscosity-change for temperature effect and for using high early strength cement on the whole.