Key Engineering Materials Vols. 297-300

Abstract: A new diagnosis equipment is designed to examine the strength of large scale bridge. It is composed of three accelerometers, radio modem, and related electronics parts. Two accelerometers with high accuracy are used for measuring the change of inclination angle of the structure base and two horizontal components of structure vibration. The third accelerometer measures a vertical component of vibration. The equipment is installed in the sidewalk of some Moscow bridges as an application. Several factors such as traffic load, wind, water wave, and etc. cause the vibration of bridge. By means of appropriate processing, the spectral characteristics of the bridge itself are determined. Several month’s test shows that the spectral characteristics remains constant if the bridge state keeps constant, and is changed by the change of the bridge state (for example, before and after repair). If the measured inclination angle of the base is increased to a particular orientation, or the spectrum of bridge is changed, it is assumed that the sagging of the structure base or deformation of its structural members might happen. Periodic or continuous monitoring of the structure state under in-service condition with the help of this diagnosis equipment makes it possible to reveal the beginning of destructive changes in structural members and to prevent the occurrence of an emergency. The study result could be applied to other large structures such as towers and buildings.

Abstract: Until now, surface defects of continuous casting slab have been removed by the enforced surface scarfing to produce high quality steel materials. An evaluation technique for surface and internal defects of slab is required to enhance the production of medium carbon steels and acquire defect-map. Accordingly as a preliminary step, longitudinal wave testing and Rayleigh wave testing were carried out on slab specimens of medium carbon steel to get basic transmission characteristics of ultrasonic waves. This research provides as basic data for on-line defect estimation using a laser ultrasonic or EMAT in non-contact ultrasonic detecting techniques in future.

Abstract: Large welded structures, including ships and offshore structures, are normally in operation under cyclic fatigue loadings. These structures include many geometric as well as material discontinuities due to weld joints, and the fatigue strength at these hot spots is very important for the structural performance. In the past, various Non-Destructive Evaluation (NDE) techniques have been developed to detect fatigue cracks and to estimate their location and size. However, an important limitation of most of the existing NDE methods is that they are off-line; the normal operation of the structure has to be interrupted and the device often has to be disassembled. In this study, a new impedance-based structural health monitoring system employing piezoceramic transducers is developed with a special interest in applying the technique for welded structural members in ship and offshore structures. In particular, the impedance-based structural health monitoring technique that employs the coupling effect of piezoceramic (PZT) materials and structures is investigated.

Abstract: The elastic waves in the isotropic plate are dispersive waves with the characteristics of Lamb wave, however, S0 symmetric mode is less dispersive in the frequency region less than the first cut-off frequency. In the anisotropic plates such as CFRP plates, the propagation velocities vary with the directions as well as the dispersion of the Lamb wave, and the phase velocity direction does not accord with the group velocity direction. The phase velocity direction is equivalent the wave vector direction, while the group velocity direction is equivalent the energy flow direction. In this work, the group velocity dispersion curves were obtained by the dispersion relation of the Lamb wave in unidirectional CFRP plate with an orthotropic structure. The group velocities of the S0 symmetric mode in the frequency region less than the first cut-off frequency were corrected by applying the slowness surface. The propagation velocities of Lamb wave were decided by measuring the arrival time of the Lamb wave signals received with the two pinducers varying the propagating direction in the laminated unidirectional CFRP plates of 8, 16 and 24 plies having a volume fraction of 67%. The measured velocities are better agreement with corrected group velocity curve, except near the fiber direction at the cusp region. When the propagating direction is not accorded with the principal axis, the direction of the group velocities inclines toward the fiber direction in the unidirectional CFRP plates, suggesting that the energy propagates preferentially toward fiber direction.

Abstract: Eddy current testing (ECT) is widely used in in-service inspection as well as pre-service inspection of the steam generator (SG) tubes in nuclear power plant of pressurized water reactor type. The interpretation of ECT signals, however, is truly a difficult task so that the reliability enhancement of signal interpretation is strongly desired. An enhanced interpretation tools for ECT signals have been developed by the novel combination of neural networks and finite element modeling for quantitative flaw characterization SG tubes. A database was constructed using synthetic ECT signals generated by the finite element models and principal component analysis (PCA) was adopted in order to optimize the feature set of ECT signals. The improvement in the performances by the features with PCA and the excellent performance for the experimental ECT signals demonstrate the high potential of the developed inversion tools for reliable interpretation of eddy current signals. To explore the possibility of applying the developed approach in practical inspection, we developed an automated system (laboratory prototype) that can acquire experimental ECT signals from SG tubes and carry out the quantitative flaw characterization in a real time fashion by applying the approach developed in the present work.

Abstract: In the analysis of materials containing regularly placed in-homogeneities, the evaluation of homogenized properties is an important issue. Common procedure for the homogenization is to define a unit cell, load it somehow, and investigate its structural response. A periodic variation of strain appears when the periodically heterogeneous material is under uniform macro-stress, and therefore, the implementation of this characteristic behavior would be essential to the material characterization based on a unit cell analysis. With the method proposed here, conventional finite element analysis tools can be used with no modification, and it is applicable to the skew-arrayed in-homogeneity problems. The orthotropic elastic and creep properties of materials with voids are examined, and the accuracy and effectiveness are demonstrated.

Abstract: In this paper, the boundary element analysis of viscoelastic strain energy release rate ) (t G for the cracked linear viscoelastic solids has been attempted. The ) (t G has been defined as the derivative of the viscoelastic potential energy ) (t P with respect to crack length a . Numerical results show the applicability of the proposed method to the analysis of the cracked linear viscoelastic structures.

Abstract: Nanoindentation tests on a folded chain crystal of polyethylene are implemented with the molecular dynamics simulation. The orthorhombic crystal is made of the planar zig-zag chains and has the thickness of about 10nm. The ideal Berkovich indenter is plunged into upper surface of the crystal down to 2nm with the constant loading rate of 200m/s or 2000m/s. After the holding time of 1000fs at the maximum depth, the indenter is then pulled up with the same speed. The results are summarized as follows; a) The indentation of 2000m/s remains the residual depression while that of 200m/s recovers the hollow, b) No elastic component is found in the deformation under the both rate of 200m/s and 2000m/s, c) The crystal deforms statically under the indentation of 200m/s while that of 2000m/s shows delayed response.

Abstract: The purpose of this study is to estimate structural characteristic for air inlet part of ramjet in condition of flight at 12-15km height. Because air inlet part of ramjet is heated up to 700oK, high temperature properties such as Young’s modulus, thermal expansion coefficients and thermal conductivity coefficients are applied to the analysis. The analysis of transient temperature and thermal stress was performed by the finite element method with nonlinear code ABAQUS. The analysis was performed using several thermal coefficient and material properties to select preliminary design model for flight condition. Thermal boundary conditions were applied at ramjet inlet part until 0.01sec. The results of the analysis recommend tendency of thermal stress and temperature distribution. These are useful to select the structural material and to determine shape of air inlet part which satisfy the structural design safety.