The neutron diffraction method provides a powerful nondestructive method to measure the loading and residual stresses in the interior of the materials and has been applied to the determination of the residual stress distribution in various engineering structures such as welded and brazed joints. The neutron method also gives useful information on the microscopic stress or the intergranular stress induced in single and multiphase materials by plastic deformation and fatigue. The method is also available for the determination of texture and phase transformation inside the materials. The present paper reviews the fundamentals for stress measurements and the latest applications of the neutron diffraction method in the field of materials science and engineering. Several directions of future developments of the neutron method are suggested.
Optimization of the rapid X-ray stress measurement method and its condition on large grain austenitic tube steel using a Kα line has been investigated. X-ray residual stress measurements were conducted using several Kα lines on several surface conditions of large grain austenitic tube steel. Using the VKα line, the measurement time can be reduced to one third compared to the CrKβ line. For a rapid measurement, a superimposing procedure of the X-ray diffraction profile from different incident beam angles (superimpose oscillation method) was also employed achieving a similar effect to that of incident beam oscillation. The best measurement condition was discussed using the above superimpose oscillation method that is an effective method for material with a large grain size.
Recently, the position-sensitive detector (PSD) has been becoming popular for X-ray stress measurement. However, little information is available regarding the effects of systematic errors, including mis-setting of specimen and misalignment of collimator and detector, on the stress measurement. Many factors, such as stress and X-ray diffraction broadening of the specimen, X-ray focus size, dimensions of the collimator and the PSD, and goniometer radius, are complicatedly related to the effects of the systematic errors. In this paper, a model of the Ψ assembly (the side inclination method) X-ray stress measurement using a PSD as the detector is presented, which enables us to simulate the stress measurement under various conditions of systematic errors.
X-ray diffraction was carried out in order to investigate crystal orientation and residual stress in gallium nitride (GaN) films deposited on a fused quartz substrate by radio frequency (RF) planar magnetron sputtering with a net to protect against plasma exposure. GaN films were deposited at constant gas pressure, constant input power, and various substrate temperatures. The following results were obtained: (1) GaN film of good crystal orientation can be deposited by RF sputtering; (2) in all films deposited at high substrate temperature, the c-axes of GaN crystals were oriented normal to the substrate surface; (3) crystal orientation was good in films deposited at high substrate temperature Ts>573K, but film deposited at Ts=873K peeled from the substrate; (4) good crystal orientation was attained in films deposited by sputtering with the fine mesh to protect against plasma exposure; (5) compressive residual stress was found in film deposited at low Ts below 573K; (6) compressive residual stress was found in films deposited by RF sputtering with the plasma protection net.
The effect of shot peening (SP) on stress corrosion cracking (SCC) prevention was evaluated from the viewpoints of crack initiation and propagation. It was found that the residual stress in a Type-304 stainless-steel specimen is changed-from tensile of 300MPa to compressive of -800MPa-by shot peening, and the effective SP depth is 0.35mm. It was also found that the crack initiation and propagation were prevented by shot peening. The mechanism by which the shot peening prevents these phenomena is explained according to the theory of superposition and loading history. That is, the prevention of crack initiation and propagation results from the fact that the compressive residual stress caused by SP decreases the applied load on the crack surface and prevents rupturing of the oxide film on the surface. Moreover, the effects of SCC prevention were shown to be valid when cyclic loading is applied after peening.
Several crack-free Cr plating processes using pulse-current electrolysis have been proposed for improving corrosion resistance. However, industrial applications of crack-free Cr platings are very few since these Cr layers are subjected to tensile residual stress and easily form macrocracks after plating operations, particularly at temperatures higher than 373K. The residual stress of crack-free Cr layers deposited by pulse-current electrolysis was evaluated by the X-ray diffraction method. With changing pulse conditions, various residual stresses were measured. The specimen in which initial compressive residual stress of the Cr layer exceeded-150MPa did not form macrocracks after holding at 473K for 2h, and showed a high corrosion resistance without rusting even after a 700h neutral salt spray test (NSST). The amount of change in the residual stress as a result of heat treatment correlated with the integral breadth of the diffraction profile, and became smaller and more stable with heat treatment as the integral breadth became narrower.
The residual stress distributions near the laser-irradiated line in a crystal grain of an electrical steel sheet were measured by the X-ray stress measurement method for a single crystal, in order to clarify the mechanism of the magnetic domain refinement which can be applied to reduce the core losses. A specimen containing only one grain was cut from a grain-oriented silicon steel, and was irradiated with a laser. The distributions of the residual stresses and those of the half value width (HVW) of the diffraction profile, which indicates the plastic strain level near the laserirradiated line, were measured before and after the stress-relief annealing, and their magnetic domain structures were observed. Before annealing, the HVW was increased near the laser-irradiated line, since the local plastic deformation was induced by the peening effect of the laser irradiation and the process of melting, cooling, and contraction after the laser irradiation. Therefore, the tensile residual stresses were induced near the laser-irradiated line. Next, the magnetic domains were refined more than they were before the laser irradiation. After annealing, the HVW was constant at the initial value, and the residual stresses were relieved. The magnetic domain then returned to its original width. Therefore, it was confirmed that the magnetic domain was refined due to the residual stress induced by the laser irradiation.
Four-point bending tests were conducted with smooth specimens of poled and non-poled lead zirconate titanate (PZT) ceramics with a tetragonal structure. The specimens poled in the longitudinal, transverse and shorttransverse directions were called PL, PT and PS specimens, respectively. The non-poled specimens were called NP specimens. The true stress-strain relation showed nonlinearity and was different between tension and compression. The nonlinearity of the stress and strain relation resulted from domain switching induced by external loading, and the degree of nonlinearity increased with easiness of domain switching under bending. The fracture strength expressed in terms of the nominal and true stresses followed the Weibull distribution. The shape parameter took a large value around 18 to 22 for PT and PS specimens, while it was the smallest around 7 to 8 for PL specimens. NP specimens took values in between. The mean strength was the largest for NP specimen and was the lowest for PL specimen. Domain switching during loading will relieve local stress concentrations of defects within the material, thus increases the strength and the shape parameter. The change in intensity ratio of the 002 to 200 diffractions can be used to assess the degree of domain switching caused by external straining.
Based on studies on the strain distribution in short-fiber/whisker reinforced metal matrix composites, a deformation characteristic parameter, λ is defined as a ratio of root-mean-square strain of the reinforcers identically oriented to the macro-linear strain along the same direction. Quantitative relation between λ and microstructure parameters of composites is obtained. By using λ, the stiffness moduli of composites with arbitrary reinforcer orientation density function and under arbitrary loading condition are derived. The upper-bound and lower-bound of the present prediction are the same as those from the equal-strain theory and equal-stress theory, respectively. The present theory provides a physical explanation and theoretical base for the present commonly-used empirical formulae. Compared with the microscopic mechanical theories, the present theory is competent for stiffness modulus prediction of practical engineering composites in accuracy and simplicity.
Many guide parts of Al2O3 ceramics are used as mechanical components in the hot environment of textile machines. Some guide parts are occasionally fractured via fatigue failure due to thermal stress in service. The analysis of thermal stress requires knowledge of the heat transfer coefficients between a ceramic guide and cold airflow, as well as of the hot environment in service, because a continuous string running at a high speed of 10m/s carries cold room-temperature air to the ditch bottom of guide parts in the hot environment. However, for the heat transfer coefficients in such a situation, there has been no example of practical direct measurement in the past. Therefore, the heat transfer coefficients were estimated using an inverse analysis method, in which suitable heat transfer coefficients were estimated through trial and error, considering the correspondence of the fracture mode and strength between analytical fracture and actual fracture in service. From the analytical results for the thermal stress using the estimated heat transfer coefficients, the cause of fracture of some ceramic guides was clarified through fracture mechanics of a surface crack in a thermal stress field. The lifetime of ceramic guides was estimated probabilistically also from the viewpoint of fracture lives in long-term service.
Al ion implantation was carried out with the aim of suppressing the absorption of hydrogen from the environment into the Ti-Ni shape memory alloy. A 100nm thick, high-density Al layer was formed at the surface by ion implantation. The initial stress of martensite phase accommodation did not change by ion implantation and hydrogen absorption. On the contrary, the deformation stress corresponding to a strain of 4% in the martensite phase and shape recovery stress changed by both ion implantation and hydrogen absorption. A low dose of implantation revealed that hydrogen absorbed near the surface diffused further into the specimen. The specimen with a high dose of ion implantation lost its shape memory properties due to the compositional change indicated by the shift of transformiation temperature. In all cases, ion implantation affected the shape memory properties irrespective of the presence or absence of hydrogen, although the ion-implanted layer was limited in the vicinity of the surface.
The changes in the barrier properties due to the damage imposed on the SiOx deposited nylon 6 films accompanying various deformations such as bending and elongation were examined by evaluating the corrosion rate of the copper plates by H2S kept in the pouches made of the damaged films. After application of elongational deformation of as high as 2% or less, only slight corrosion of the copper plates, almost similar to those of the copper plates kept in the pouches made of undeformed films, was observed. After application of elongational deformations of 3% or more, the corrosion of the copper plates was more distinct and proceeded significantly with time, Bending deformation given to the SiOx deposited nylon films also deteriorated the barrier property to H2S when the radius of curvature at the bent part was small. Comparison of the corrosion rates of the copper plates kept in the pouches made of films deformed in various ways and undeformed commercial films shows a clear relationship between the H2 permeation rate of the films and the corrosion rate of the copper plates by H2S.
A wicking method technique to study the sorption behavior of heavy oil into exfoliated graphite packed in a glass column was developed. The weight increase of the exfoliated graphite column during the sorption process was continuously measured. It was found that the sorption speed depended on the viscosity of heavy oils. Sorption of the A-grade heavy oil with a low kinematic viscosity was a hundred times as quick as that of the C-grade oil with a high viscosity. Packed density of the exfoliated graphite in the column affected not only sorption speed but also the sorbed amount of heavy oil, because the size of opening spaces formed among the exfoliated graphite particles and the pore volume per unit weight of exfoliated graphite decreased with increasing the packed density. The exfoliated graphite column with the higher packed density showed quicker sorption of heavy oil. Exfoliated graphite with density of 7kg/m3 showed the largest weight of sorbed oil per unit weight of the graphite. The weight of sorbed oil per unit weight of exfoliated graphite decreased with increasing the packed density. In the saturated weight of heavy oil sorbed into the column, however, the maximum value was found when the exfoliated graphite was slightly densified from 12 to 16kg/m3.
Lignocresol (LC), which is recovered from wood by adopting phase separation system with sulfuric acid/cresol, was carbonized at 900°C for 1h after loading with nickel alone or both nickel and sodium, and the crystal structure of carbon and the electromagnetic shielding (EMS) capacity at 50-800MHz were evaluated for the resulting char. For comparison, four other isolated lignins were subjected to the same treatment and measurement. LC char could produce crystallized carbon (T-component) markedly by co-loading of nickel 8-9wt% and sodium 8-10wt% to exceed a practical standard of EMS capacity, 30dB at 800MHz, although nickel alone was ineffective. The capacity of the LC char with both nickel and sodium was superior or comparable to those of the corresponding other lignin chars. Without nickel, LC char resulting from the unique structure gave less amorphous carbon with larger EMS capacity than other chars. These results showed that LC was suitable as raw lignin for converting into EMS material by nickel-catalyzed carbonization.