Masumi Saka

Improvement of Fatigue Strength of Aluminum Alloy by Cavitation Shotless Peening

Cavitation impact, which normally produces severe damage in hydraulic machinery, can be used to modify surfaces in the same way as shot peening. Cavitation impact enables the surface of a material to be peened without the use of shot, thus it is called cavitation shotless peening. As there are no solid body collisions occurring in this peening process. the roughness of the peened surface should be less than that produced by shot peening. This characteristic makes it suitable for peening soft metals. In order to demonstrate the improvement of the fatigue strength of aluminum alloy be this process, specimens were subjected to the process, and then tested in a rotating bending fatigue test. Cavitation impacts were produced and controlled by using a submerged high speed water jet with cavitation, i.e., a cavitating jet. It was revealed that the fatigue strength of an aluminum alloy specimen treated by this peening process was 50% stronger than that of a specimen without peening.

Use of Cavitating Jet for Introducing Compressive Residual Stress

In an attempt to strengthen the surface of materials, the potential of using a cavitating jet to form compressive residual stress has been investigated. Introducing compressive residual stress to a material surface provides improvement of the fatigue strength and resistance to stress corrosion cracking. In general, cavitation causes damage to hydraulic machinery. However, cavitation impact can be used to form compressive residual stress in the same way as shot peening. In the initial stage, when cavitation erosion progresses, only plastic deformation, without mass loss, takes place on the material surface. Thus, it is possible to form compressive residual stress without any damage by considering the intensity and exposure time of the cavitation attack. Cavitation is also induced by ultrasonic, high-speed water tunnel and high-speed submerged water jet, i.e., a cavitating jet. The great advantage of a cavitating jet is that the jet causes the cavitation wherever the cavitation impact is required. To obtain the optimum condition for the formation of compressive residual stress by using a cavitating jet, the residual stresses on stainless steel (JIS SUS304 and SUS316) and also copper (JIS C1100) have been examined by changing the exposure time of the cavitating jet. The in-plane normal stresses were measured in three different directions on the surface plane using the X-ray diffraction method, allowing for the principal stresses to be calculated. Both of the principal stresses are found changing from tension to compression within a 10 s exposure to the cavitating jet. The compressive residual stress as a result of the cavitating jet was found to be saturated after a certain time, but it starts decreasing, and finally, it approaches zero asymptotically. It could be verified in the present study that it was possible to form compressive residual stress by using a cavitating jet, and the optimum processing time could also he realized. The great difference between the water jet in water and air has also been shown in this regard.

Dry and semi-dry machining using finely crystallized diamond coating cutting tools

Finely crystallized diamond coatings with smooth surfaces have recently been developed by the authors. In the present study, shaft tools with these new coatings were applied to dry and semi-dry machining of highly adherent aluminum alloys whose efficient means of dry machining have not yet been established for. In comparison to the case of conventional diamond coatings with rough surfaces, more than four times longer durability of drills was realized in perfectly dry condition. In the case of minimum quantity of lubricant condition, the lifetime was found again to be more than four times longer and successfully achieved the largest improvement of diamond-coated drills ever reported in semi-dry condition.

A new systematic method of characterization for the strength of thin films on substrates—evaluation of mechanical properties by means of ‘film projection’

Abstract The strength of a thin film deposited on a substrate should be recognized as the strength of a materials system which is made of a film, a substrate and an interface. The macroscopic fracture of this system should be controlled by the strength of the film and the interface, as well as the residual stress and elastic properties in/of the film. We have developed a systematic method of measurement for all these properties by means of a specimen with ‘film projection’. By applying external load to the film projecting a little out of the edge of the substrate, cracks can be introduced exclusively in the film or along the interface. The strength of film and interface is independently evaluated in terms of toughness on the basis of crack extension resistance. Deformation of the film projection gives its Youngs modulus, and the buckling behavior is used to estimate the residual stress. As an application example of the method, those properties mentioned above were evaluated for the case of a diamond film deposited on a cobalt-cemented tungsten carbide cutting tool.

Acoustic resonant spectroscopy for characterization of thin polymer films

An acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity, and density of micron-scale polymer films is developed. The method, which is based on spectral analysis, observes the acoustic resonance between water, the film, and a tungsten plate with high acoustic impedance in the frequency range of 20–70MHz. The interface between the film being examined and the plate is vacuum sealed, enabling us to characterize the low-density polyethylene film with acoustic impedances as low as about 1.9MNm−3s and the poly(vinyl chloride) film as thin as about 8μm. The error in the film density measurements is found to be less than 1%, and the validity of the technique is verified.An acoustic resonant spectroscopy technique for measuring the acoustic impedance, ultrasonic velocity, and density of micron-scale polymer films is developed. The method, which is based on spectral analysis, observes the acoustic resonance between water, the film, and a tungsten plate with high acoustic impedance in the frequency range of 20–70MHz. The interface between the film being examined and the plate is vacuum sealed, enabling us to characterize the low-density polyethylene film with acoustic impedances as low as about 1.9MNm−3s and the poly(vinyl chloride) film as thin as about 8μm. The error in the film density measurements is found to be less than 1%, and the validity of the technique is verified.

Development of high performance diamond-coated drills for cutting high silicon aluminum alloy

In machining aluminum alloy that contains a large amount of silicon, highly wear-resistant cutting tools such as diamond coated ones are needed because silicon particles in the matrix of aluminum quickly wear the tools. However, diamond coatings on cutting edges actually debond off easily. In this paper, we examined two possibility in order to improve the adhesion of diamond coatings on cutting edges. One is to optimize the construction of coated drills by changing the shape of cutting edges as well as coating thickness. Another is improvement of the toughness of diamond coatings. By putting together the results obtained, the diamond-coated drills with optimized structure were newly developed. Their performance appeared to be satisfactory in cutting high silicon aluminum alloy.

Numerical analysis of blunting of a crack tip in a ductile material under small-scale yielding and mixed mode loading

The blunting of the tip of a crack in a ductile material is analysed under the conditions of plane strain, small-scale yielding, and mixed mode loading of Modes I and II. The material is assumed to be an elastic-perfectly plastic solid with Poissons ratio being 1/2. The stress and strain fields for a sharp crack under mixed mode loading are first determined by means of elastic-plastic finite element analysis. It is shown that only one elastic sector exists around the crack tip, in contrast with the possibility of existence of two elastic sectors as discussed by Gao. The results obtained for a sharp crack are used as the boundary conditions for the subsequent numerical analysis of crack tip blunting under mixed mode loading, based on slip line theory. The characteristic shapes of the blunted crack tip are obtained for a wide range of Mode I and Mode II combinations, and found to resemble the tip of Japanese sword. Also the stress field around the blunted crack tip is determined.

Microwave nondestructive detection of delamination in IC packages utilizing open-ended coaxial line sensor

A new method of microwave nondestructive testing which utilizes an open-ended coaxial line sensor is developed in an attempt to increase the spatial resolution. With the aid of this technique, the delamination in IC packages is inspected. An open-ended coaxial line sensor with inner and outer conductors in smaller dimension than the wavelength is used to incident and receive the test signal that interacts with the detected objects. The magnitude of effective reflection coefficient, which is proportional to the total reflection from different interfaces, is measured as a characteristic signal to distinguish the delamination. A phenomenon of magnitude coherent resonance is observed in detail, by which the measurement sensitivity is enhanced significantly. Four IC packages were used as samples, and the measurement results indicate that the microwave technique using an open-ended coaxial line sensor has a bright prospect to evaluate the delamination in IC packages nondestructively.

Residual stress distribution in the direction of the film normal in thin diamond films

The residual stress distribution in the direction of the film normal in thin diamond films deposited on Si substrate has been evaluated together with the distribution of Young’s modulus. The films were deposited on the substrate by the microwave chemical vapor deposition method. It has been observed that the curvature of the diamond films delaminated from the Si substrate is functionally dependent on the film thickness. Young’s modulus, which has been estimated by the film bending test in conjunction with a finite element method of analysis, appears to be gradually decreasing towards the adhesion interface. On the basis of detailed measurement of curvature and with the aid of Raman spectroscopy, the residual strain distribution in the film has been evaluated. Although the average intrinsic stress was tensile as reported earlier, we have found that a huge compression concentrates in the very small region near the adhesion interface. This finding shows evidence that something happens on the interface, which...

Design and performance of a thin, solid layer for high-resolution, dry-contact acoustic imaging

Compared to the usual water immersion case, more effective transmission and reception of high-frequency ultrasound through a thin, solid layer are reported. A theoretical model is presented to perform the signal amplification and the signal modulation toward the higher frequency components for getting the high-quality acoustic images without immersing the object to be imaged. Also, the thin, solid layers are designed from the theoretical model, and the transmission of high-frequency ultrasound is carried out through the layer/silicon interfaces with an applied pressure of about 0.1 MPa. The spectral intensity in the frequency range of 20 to 70 MHz remarkably improves compared with water immersion, and the peak frequencies of the spectra modulate the higher than water immersion. Furthermore, the solder joint inspection of a package is performed. The present dry-contact technique achieves the higher spatial resolution and the higher signal-to-noise ratio (SNR) than the usual water immersion technique, and clearly detects the defective joint without getting the package wet.