Key Engineering Materials Vols. 353-358

Abstract: The experiment of acoustic emission (AE) on concrete specimens under uniaxial cyclic loading was conducted. The Kaiser effect of acoustic emission in concrete and the Felicity effect, which manifest the memorizing ability to the maximum previous stress level of Kaiser effect, were validated by the experiment. The mechanism of Felicity effect was analyzed based on the theory of statistical damage mechanics and a tentative AE factor constitutive model of brittle material under uniaxial cyclic loading was suggested. The curve of constitutive model is in good coincidence with the curve from the experiment. The experimental results showed that the Felicity effect became clearer along with the increasing of stress level. Each loading cycle would cause new damage inside the material, and the response of material to the new loading cycle is different from the previous cycle.

Abstract: Statistical diagnosis using electrical resistance changes is performed to detect a delamination crack in a CFRP beam. This method enables to reduce data required for damage identification. First, a new measuring method of multiple electrical resistance changes is proposed to perform statistical diagnosis. The proposed method measures electrical resistance changes of multiple segments in a CFRP beam although electrical interference must be considered when multiple voltages are charged at once. Next, statistical diagnosis is performed on loading to the CFRP beam. A delamination crack is detected by the change of relative relationship between multiple electrical resistance changes due to damage occurring. As a result, the monitored states of the CFRP beam are diagnosed exactly by the proposed damage detection system.

Abstract: Water damage of asphalt concrete pavements has been one of the major distresses in highway engineering. Engineering experiences and research results indicate that this damage is due to the extortionate air voids of bitumen mixtures. Paranormal porosity of asphalt mixture is largely the result of the poor controlling of compact degree during paving process. The quality of asphalt pavement is subjected to the controlling methods of compacting of bitumen mixtures. Ultrasonic is an excellent method for non-destructive testing used in various engineering fields. It is also a potentially effective tool to be applied in the control and evaluation of the compaction quality of bitumen mixtures. This paper presents the results of a study aimed at the applicability of ultrasonic technology for evaluation of porosity and compaction quality of bitumen mixtures. It is shown that this technology, which has been successfully used for many years for evaluation of Portland concrete structures, might be used in asphalt pavement engineering to provide a simple, quick, and objective methodology for control and evaluation of the compact quality of bitumen mixtures. The results of laboratory testing demonstrate the potential of this technology.

Abstract: The diffraction of Lamb waves by a finite delamination situated on a random plane parallel to the symmetric plane of a laminated plate is taken into account. Two imaginary planes are introduced to solve the problem by adopting the method of mode matching. By taking an appropriate finite number of terms of the infinite expansion series and some selected points on the two imaginary planes through convergence and precision tests, a matrix equation is obtained to evaluate the expansion coefficients numerically. Reflection coefficients versus the normalized length a/h and the relative location h1/h of the delamination are calculated in the low-frequency domain, the relative errors are found to be less than 1%. Results obtained indicate that the theory developed in this paper is meaningful for the detection of both the size and the location of a delamination in a laminated plate by cooperating with experimental techniques for the non-destructive evaluation (NDE) technology.

Abstract: An acoustic resonant spectroscopy technique for measuring the acoustic impedance of micron-scale polymer films without knowing any values of ultrasonic velocity, thickness and density has been 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. The acoustic impedances of poly(vinyl chloride), poly(vinyl alcohol) and nylon films are determined by the spectroscopy technique. Moreover, the values of ultrasonic velocity and density of the films are also determined from the resonance frequencies of the films.

Abstract: A number of instability problems in rock engineering projects are caused by crack propagation. However, crack growth mechanisms from 3-dimentional flaw are not fully understood, in particular for 3-D flaw case with varied dipping angle. This study focuses on 3-D surface flaw using real rock specimens containing a flaw with varied inclination angle α from axial loading and dipping angle γ from specimen surface under uniaxial compression. Acoustic emission technique was used for tracing the initiation and growth of micro-cracks inside of specimen. It was found that crack growth process is affected by the dipping angle γ of the 3-D flaw. When dipping angle γ ≠ 90º, the thickness of rock above the flaw plane is thinner than that of below the flaw plane. As a result, compressive crack and wing crack initiated easily from the thinner flaw tips. And, the normalized stress for crack initiation σi /σc, AE events and the AE energy for crack growth decreases with the dipping angle γ. However, for γ = 90º, the thickness of rock above and below of the flaw tips is the same, it was observed that anti-wing crack (crack growth direction opposite to wing crack) initiated first at a certain place away from the flaw tips, then wing crack and compressive crack emerged at the late stage. For this case, the stress σi /σc, AE events and the AE energy for crack initiation and propagation are at a high value. Thus, for rock mass contains flaws geometry with small dipping angle, some problems of crack propagation may be induced easily during excavation.

Abstract: This paper presents the crack growth mechanism from a 3-D surface flaw on gabbro specimens using strain measurement and acoustic emission (AE) technique. Based on the results of strain and AE measurement, microcracks initiated inside the rock and extend to the surface of the specimen. With the observation from the measurements, four types of crack patterns initiate wing crack, anti-wing crack (opposite direction of wing crack), petal crack and compressive crack. The strain values of anti-wing cracks are larger 1 to 2 times than that of wing crack. The AE energy release from anti-wing crack is higher 2.5 times than that of wing crack, while the energy release form wing crack is the least but the compressive crack is the highest. Thus, the appearance of initiation and propagation of the anti-wing crack and compressive crack are very actively than that of the wing crack. The strain and AE measurement is not only to provide a clear concept on the mechanisms of crack growth form a 3-D surface flaw but also to provide useful knowledge on the AE property of the crack patterns.

Abstract: Quantitative evaluation of small cracks in stainless steel under water, which simulated the environment of unclear reactor, by using microwave, was demonstrated. The crack depth was evaluated by means of the microwave dual frequency technique, and the crack closure stress was estimated based on the compliance technique.

Abstract: The ESPI (Electronic Speckle Pattern Interferometry) is a real-time, full-field, non-destructive optical measurement technique. In this study, ESPI was proposed for the purpose of vibration analysis for new and composite materials. Composite materials have various complicated characteristics according to the materials, orientations, stacking sequences of the ply and boundary conditions. Therefore, it was difficult to analyze composite materials. For efficient use of composite materials in engineering applications the dynamic behavior (i.e., natural frequencies and nodal patterns) should be informed. With the use of Time-Average ESPI, one could analyze vibration characteristics of composite material by real time easily. We manufactured two kinds of laminated composites (i.e., symmetry and asymmetry) which were consisted of CFRP (Carbon Fiber Reinforced Plastics) and the shape of the test piece was of rectangular form.

Abstract: Nondestructive testing using magnetic field is useful for detection of a crack on ferromagnetic material. The magnetic field distribution has to be obtained for quantitative evaluation of crack direction, size, and shape. Also, a crack can be evaluated by using the inverse problem analysis. However, an analysis method using a dipole model can be used to analyze the magnetic field distribution around a crack at a higher speed than the finite element method (FEM). Therefore, a dipole model simulation can provide useful information which can be used for the inverse problem analysis. However, the magnetic charge per unit area, m, and the permeability, μ, has been treated as constants. Therefore, analyzed results have been different from experimental results in most cases. This paper proposes the improved dipole model simulation method, which assumes that the magnetic charges per unit area exist at the section areas, edge lines and summits of a crack. Also, the magnetic charges per unit area were assumed to depend on the square of the crack depth. The improved method is validated by comparing its results with the experiment results obtained with the use of the magnetic camera.