Corrosion and corrosion fatigue are well recognized as principal causes for the aging of engineered systems and the attendant reduction in their reliability and safety. To meet the challenges of designing for a modern, competitive global market and for the life-cycle management of aging systems, it is essential to develop science-based methods for predicting the evolution of damage through corrosion and corrosion fatigue, and other time-dependent mechanisms, and incorporating them into a new paradigm for design. To this end, the use of a mechanistically based probability (versus statistically based parametric) approach is presented, and illustrated through considerations of corrosion and corrosion fatigue in airframe aluminum alloys. Its efficacy in predicting damage evolution and distribution is demonstrated through a comparison with long-term service data from an aircraft that had been in commercial service for 24 years.
This paper investigates the asymptotic transient structure of the near-tip field in an anisotropic piezoelectric bimaterial system under impact loading. By using an asymptotic methodology, the governing equation is reduced to a series of coupled partial differential equations. Using the extended version of Stroh's formulation and the method of analytical continuation, the problem is further recast into a Riemann-Hilbert problem of vector form. Upon solving the Riemann-Hilbert equation, the higher order near-tip transient asymptotic fields are obtained. For a bimaterial of PZT and PZT-4 piezoelectric ceramics, the direct and converse piezoelectric effects on the stress intensity factors and electric displacement intensity factors are clarified.
Carbon steel was friction welded and torsion fatigue tests were conducted on weld joints having an upset burr in order to reveal the effect of an upset burr on fatigue strength. It was found that the fatigue strength of joints having a constant toe radius decreased with a decrease in hardness ratio (hardness of toe/hardness of base material). Softened joints (hardness ratio≤1) and hardened joints (hardness ratio>1) fractured at the toe of the upset burr and in the base material, respectively. On the other hand, the fatigue strength of joints having constant hardness ratio increased with toe radius. Using correlation analysis, a reasonable multiple regression equation was obtained between torsional fatigue strength and the fatigue factors, that is, the hardness ratio and toe radius of the upset burr. Thus, using this regression equation, the torsional fatigue strength of friction welded joints having an upset burr could be estimated.
In this research, the effect of isothermal physical aging on creep behavior of stainless-fiber/poly-phenylene ether (PPE) composites was investigated. Metal-fiber/PPE composites are non-crystalline thermoplastics and often used in engineering plastics, which have been developed for Electro-Magnetic-Interference (EMI) shielding. The results showed creep compliance curves to shift towards both the longer physical time and lower compliance sides with an increase in pre-aging treatment time, and it was established that creep deformation is arrested due to the progression of physical aging. Thus it was clarified that pre-aged composites can withstand higher temperature and longer time. The time-aging time superposition was established in creep of stainlessfiber/PPE composite. It was also observed that within the aging-time range, the shift rate is constant at all temperatures. The results showed that the shift rate is smaller at lower temperature and greater at higher temperature, and it reaches its maximum value at a wide range of temperature.
Tensile tests are conducted for six types of quasi-isotropic carbon fiber reinforced plastics (CFRP) laminates, to investigate the influence of stacking sequence on the damage pattern and final failure. Stresses in the specimens are analyzed based on the FEM code-ANSYS, and the correlation between the numerical results and the practical observations is discussed. Both the non-damaged model and the cracked laminate model are analyzed. The FEM analysis results qualitatively agree well with the experimental observations.
Cracking damage occurs in the reinforced concrete (RC) slabs of highway bridges due to the passage of heavy vehicles. Tests were conducted to determine the ultimate flexural capacity and failure mechanism of two types of RC beam with different effective depths (Type A: d=17.2cm/Type B: d=21.2cm) modeled on RC slabs and subjected to running wheel-loads. The flexural load-carrying capacity for beams subjected to running wheel-loads decreased 16% for both Type A and Type B, as compared to beams subjected to static loads. The failure configuration for static loads and following running wheel-loads was a diagonal tension failure with a spread of approximately 50-55 deg. starting from directly beneath the wheels. For running wheel-loads of a constant strength and running wheel-loads that included vibrations, the crack that occurred at the lower edge progressed to the compression bars due to increased load strength and repeated running and became flexural tension failure. In the case of running vibration-loads, it was learned that the frequency greatly affected the cracking interval and the reduction in bending rigidity.
The In-Ti active fillers were examined for joining metallic copper (Cu) to aluminum nitride (AlN) ceramics. Brazing was carried out with In-Ti active brazing filler metals with various compositions, and the microscopic structure and mechanical strength of the Cu-filler-AlN-filler-Cu joint at various temperatures were investigated. When brazed with an In-rich filler of In-1wt%Ti, most of In atoms in the filler were diffused into the Cu plate leaving a Ti-rich reaction layer of 2-4μm thick at the Cu-AlN joint interface. The 4-point bending strength at 500°C increases linearly with the brazing temperature, whereas the strength at room temperature saturates at 40MPa. The specimen brazed at 820°C has the strength of 58MPa and 33MPa at 500°C and 600°C, respectively. On the other hand, when using Ti-rich fillers, e.g., In-7.7wt%Ti, In-15wt%Ti and In-23.8wt%Ti (the same as Ti3In4 composition), granular Ti-rich phases were formed at the interface, and the microcracks were observed in the AlN.
The transverse swelling behavior of coniferous wood cells was observed using a confocal scanning laser microscope and a digital image correlation method (DIC). Since the laser microscope has great depth of focus and high resolution, it was possible to observe the deformation of cells precisely. In order to observe the swelling behavior of specimens (Douglas-fir, Pseudotsuga menziessi) on the stage of the microscope, moisture was supplied using a humidifier. Digital image data of the cells taken by the laser microscope were used to measure the swelling behavior using DIC. The outside diameter of latewood tracheids swelled or changed almost isotropically with increasing moisture content, but the inside diameter (lumen) shrank greatly and anisotropically. Earlywood tracheids expanded a little but underwent great changes in shape. Thickness expansion of cell walls was less than circumferential expansion (perpendicular to the thickness direction), so that we concluded that the anisotropy of expansion was mainly based on thickness swelling. In latewood, circumferential expansion of radial cell walls (changes in length of the middle lamella) was great, but tangential cell walls expanded less than radial walls (in some cases the expansion was nearly zero). The angles between a radial wall and the adjacent tangential wall increased both in latewood and in earlywood by swelling.
Nickel-loaded larch chars were prepared by various methods and then molded into disk specimens to measure their electromagnetic shielding (EMS) capacities in the range of 50-800MHz. The capacity was dependent on carbonization temperature for char preparation, molding pressure, nickel loading, and the loading method. This is because the crystal structure of woody carbon and the density of a specimen were predominant factors in determining the EMS effect. The combination of carbonization at 900°C, molding at 5.9MPa, 4.5wt% loading of nickel by wet impregnation before carbonization was adequate for affording char a highly efficacious EMS capacity. These results show the feasibility of an easy and inexpensive production of an EMS material from wood by carbonization.
In order to detect material degradations in pressure vessel steel of A533B employed as a component material in light water reactors (LWRs), the leakage magnetic flux distribution on plate-type A533B specimens has been examined using a magnetic sensor of 120μm square GaAs Hall element. As reference data, the residual stress distribution has also been obtained by X-ray diffraction method where an irradiation spot was 1mm in diameter. The data of the leakage magnetic flux normal to the specimen surface (Bz) measured by the Hall sensor were converted into the form of the first derivative dBz/dx by calculating the gradient of the Bz along the axial direction of the specimen (x-axis), and the data of the axial residual stress (σx) obtained by the X-ray diffraction method were also transformed into the differential form dσx/dx. It was found that there existed a correlation between the absolute value of the dBz/dx and the dσx/dx for the tensile specimens (correlation coefficient r=0.282). Full width at half maximum (FWHM) for the frequency distribution profile of the dBz/dx was calculated. The FWHM of the tensile specimens increased remarkably at the beginning stage of the plastic deformation where the Lüder's band was generated; The FWHM of the fatigued specimens increased with stress cycling. It was suggested that the leakage magnetic flux measurement by the GaAs Hall sensor was a promising non-destructive evaluation (NDE) method to detect the amount of plastic deformation and fatigue damage.