Mikio Takemoto

Fracture mechanism characterization of cross-ply carbon-fiber composites using acoustic emission analysis

Abstract The sequence of microscopic fracture mechanisms in locally loaded cross-ply carbon-fiber composites was studied by analyzing acoustic emission (AE) signals in combination with the modal analysis of Lamb waves, using microscopic and ultrasonic examination of the specimen after load interruption. The first 70 AE events were analyzed, which were detected during the initial loading segment when the first sudden load drop and gradual load recovery were observed. Characteristics of the detected waves were compared with the S 0 - and A 0 -mode Lamb waves produced by a spot- or line-focused YAG laser. The internal damage progression of the composite specimen was determined to be the fiber fracture in the front lamina, transverse cracks in the mid-lamina, delamination and splitting.

Modal Analysis of Hollow Cylindrical Guided Waves and Applications

Dispersion behavior of guided waves in hollow cylinders (cylindrical waves) was evaluated theoretically and experimentally. Observed dispersion behavior suggests an assignment, different from the traditional one, of longitudinal (L-), flexural (F-) and torsional (T-) modes which are consistent with Lamb waves and shear-horizontal (SH) mode waves. The L- and F-modes of the cylindrical waves have characteristics which are asymptotic to Lamb waves and to waves in a solid cylinder. Experimentally, wide-band cylindrical waves in aluminum pipes were generated using a laser-ultrasonic method. Wavelet transform of the cylindrical wave signals was utilized for time-frequency analysis in order to compare them with the theoretical dispersion curves. For the L(0, 1), F(1, 1), F(2, 1), L(0, 2), F(1, 2) and F(2, 2) modes of the cylindrical waves, which were efficiently excited, theoretical and experimental dispersion curves agree with each other.

Off-diagonal elastic constant and sp2-bonded graphitic grain boundary in nanocrystalline-diamond thin films

This letter studies the relationship between the off-diagonal elastic constant C12 and bond configuration in nanocrystalline-diamond (NCD) thin films deposited by the nitrogen-doped chemical vapor deposition method. The film thickness was varied between 2.4 and 11.3μm. The elastic constants were measured by resonant-ultrasound spectroscopy coupled with laser-Doppler interferometry. The diagonal elastic constants C11 and C44, and Young’s modulus in NCD films are smaller than those of the bulk polycrystalline diamond and microcrystalline-diamond (MCD) thin films, and they decrease as the film thickness decreases. However, the off-diagonal elastic constant of the NCD films is significantly larger than that of the bulk diamond, while that of the MCD films is smaller. Micromechanics calculations revealed that this exceptional enhancement of C12 occurs when the material includes randomly distributed thin graphitic plates in the isotropic diamond matrix. Thus, this result indicates that the NCD films consist of ...

Monitoring of stress corrosion cracking in stainless steel weldments by acoustic and electrochemical measurements

A new hybrid monitoring technique for chloride stress corrosion cracking (SCC) is proposed. It uses both the acoustic emission (AE) and corrosion potential fluctuation (CPF) techniques. This paper discusses the results of SCC tests on butt-welded Type 304 stainless steel pipes. The weld pipe suffered transgranular (TG)-SCC in a concentrated magnesium chloride solution (40 mass%), but suffered intergranular (IG) attack and falling-off of grains in a heat-affected zone (HAZ) in a dilute chloride solution (35 mass%). SCC initiations in both concentrated and dilute corrodants were successfully monitored using a CPF technique. However, the CPF technique could not monitor the propagation of the SCC. This propagation could be detected using an AE technique. Secondary AE was produced by hydrogen gas evolution and by the cracking of corrosion products, and the primary AE was produced by the falling-off of grains due to the mutual actions of anodic dissolution and the mechanical fracture along a chromium-depleted zone in the grain boundary. The volume of metal loss by the dissolution was predicted from the local anodic current due to the fluctuation of the corrosion potential, and was found to correspond to the volume of the grain boundary attack. The fact that the primary AE was detected just after rapid drop (RD)-type CPF suggested that the grain boundary corrosion caused the falling-off of the grain that produced the primary AE.

Non-contact laser ultrasonics for detecting subsurface lateral defects

To evaluate subsurface lateral defects, non-contact and non-destructive laser ultrasonics was employed. Surface acoustic waves (SAW) were generated by a Q-switched YAG laser and monitored by a heterodyne laser interferometer. Both the source and monitoring lasers were scanned over the samples to monitor the location of defects and bond quality. Based on the fundamental works of the thin metallic foil and model specimen with an artificial subsurface slit, bond quality of the stainless foil brazed on the carbon steel was evaluated by the quantitative analysis of velocity dispersion of the Rayleigh and lowest order Lamb waves. Velocity dispersion of SAW over the well bond area well agreed with theoretical dispersion of the Rayleigh wave for multi-layer laminate, while that over the unbond area agreed with the lowest order anti-symmetric Lamb wave. Velocity dispersion of the SAW over the weak bond was calculated by inserting a slip layer which allowed displacement discontinuity across the interface. Analysis of the generalized Lamb wave was demonstrated to be promising in evaluating the bond quality as well as identifying the location of defects.

Estimating the diameter/thickness of a pipe using the primary wave velocity of a hollow cylindrical guided wave

A method for measuring the diameter/thickness (d∕t) of a pipe using a characteristic of a hollow cylindrical guided wave (HCGW) is presented. The HCGW is an ultrasonic guided wave propagating along a pipe. In the first part of this letter, we briefly show that the primary wave (first-arriving wave packet from an impulse source) of the HCGW achieves a faster group velocity for a larger d∕t. Experimental verifications were carried out for aluminum pipes (several different d∕t’s) using a laser ultrasonic method to generate the HCGW. The experimental results are in fairly good agreement with the theoretical prediction described.

Surface acoustic wave velocity and attenuation dispersion measurement by phase velocity scanning of laser interference fringes

We developed a noncontact, nondestructive surface acoustic waves (SAW) velocity and attenuation measurement method in the frequency range of 30-110 MHz. This method is based on the phase velocity scanning method previously proposed by the authors that uses laser interference fringes scanned at the phase velocity of SAW to generate single-mode SAW with high intensity and directivity. In order to verify the validity and accuracy of the method, we measured SAW velocity and attenuation in Al and AISI 304 steel. The error in SAW velocity measurement was less than 1% in Al. Also, the attenuation was obtained with high precision in the AISI 304 steel. Then, we applied this method to porous silicon (PS) films on Si wafers as an example of a layered medium. The SAW velocity in PS films decreased with increasing porosity. By fitting the calculated dispersion curve to the measured one, the elastic stiffness of PS films was determined with a relative accuracy of 0.2%, which is thought to be the highest using laser ultrasonic methods. The attenuation of PS films was found to be 6-80 dB/cm in the frequency range of 30-70 MHz. The frequency dependence of attenuation was obtained with high precisely.

The yttrium effect on the corrosion resistance of CO2-laser processed MCrAlY coatings

To improve the corrosion resistance and to study the effect of yttrium in the behavior of coatings produced by thermal spraying MCrAlY (M=Ni, Co) powders, CO2 laser processing was conducted. Three methods were used: (1) a combination of gas flame and plasma spraying in air followed by laser glazing in argon, (2) low-pressure plasma spraying (LPPS) and laser glazing in argon, and (3) LPPS and laser-gas (O2) alloying. Laser glazing in argon of the MCrAlY coatings sprayed in air promoted formation of weakly adherent agglomerates of Al−Y oxides and an alumina-chromia solid solution. Glazing in argon atmosphere of LPPS CoNiCrAlY and NiCrAlY coatings caused the formation of nickel aluminides besides the formation of Y−Al compounds. Gas (O2)-alloying of these coatings produces continuous and adherent (yttrium-containing) alumina and chromia layers. The effects of yttrium on the characteristics of the oxides formed in the coatings during laser glazing, laser-gas alloying, and high-temperature oxidation is discussed. This work also investigated the oxidation resistance of the laser-processed MCrAlY coatings in air and in the presence of 85 mol/o V2O5−Na2SO4 fused salt at 900°C.

An advanced method for measuring the residual stress of deposited film utilizing laser spallation technique

Abstract Residual stress of thin diamond films deposited by chemical vapor deposition (CVD) method was measured using a new method for residual stress measurement we developed. Compressive residual stresses of CVD diamond films were determined from the height and diameter of protuberance of film delaminated by pulse laser spallation technique. We produced the protuberance with different diameter and height by changing the laser energy. Compressive stresses were calculated from the momentum balance of atmospheric pressure and internal compressive force of the film. The residual stress of the well-faceted diamond film was measured as −338±22 MPa and agreed well with the stress (−348±17 MPa) measured by the X-ray diffraction method. In three other polycrystalline diamond films with different grain structure, the compressive stresses decreased from 300 to 147 MPa with decreasing the grain size from a few microns to tens of nanometers. Small compressive stresses of fine grain films are correlated with the grain boundary structure according to visible Raman spectra study conducted.

Integrity evaluation of SiOx film on polyethylene terapthalate by AE characterization and laser microscopy

Abstract An attempt is made to investigate the integrity of gas barrier SiO x film on PET (polyethylene terapthalate) by AE characterization during tensile loading. As the amplitude of AE signals is very weak due to the thinness of the SiO x film and high wave attenuation of PET, a new AE monitoring system, consisting of an AE sensor with an internal amplification of 120 dB, a broadband preamplifier, a fast A/D converter and a personal computer, was used to monitor the Lamb waves emitted by the fracture and/or exfoliation of the SiO x film. Dispersion of the group velocity obtained by the wavelet transform agreed well with the theoretical group velocity of the zeroth order symmetric Lamb mode (S o ) of the PET. The S o mode Lamb wave was monitored for the specimen with SiO x films thicker than 180 nm and usef for the identification of source location. The threshold tensile strain to cause the first fracture, estimated by the developed AE monitoring, agreed well with the strain determined by a laser microscope. The estimated source amplitude of AE waves showed a fairly good relationship with the segmental crack size in the SiO x film.