Most structural health monitoring systems adopt parametric methods based on modeling or non-parametric methods such as artificial neural networks. The former methods require modeling of each structure, and the latter methods require a large number of data for training. These methods demand high costs, and it is impossible to obtain training data of the damaged state of an in-service structure. By the present method, damage is detected by judging the statistical difference between data of the intact state and the current state. The method requires data of the undamaged state, but does not require complicated modeling or data for training. As an example, the present study deals with the detection of delamination of a composite beam. Damage is detected from the change of strain data using statistical tools such as the response surface and F-statistics. As a result, the new method successfully diagnoses the damage without the need to use modeling or data of the damaged state.
Off-axis stress relaxation behavior of unidirectional T800H/3631 carbon/epoxy composite exposed to high temperature is examined at relatively high tensile strain levels, and a phenomenological viscoplasticity model is tested on the capability to describe the time-dependent response observed. First, stress relaxation tests are performed at 100°C on plain coupon specimens with different fiber orientations, θ=0, 10, 30, 45, and 90°. For each of the fiber orientations, in principle, stress relaxation tests are carried out at three different strain levels. The relaxation of axial stress in the unidirectional composite is clearly observed, regardless of the fiber orientation. Just after the total strain hold, the axial stress quickly relaxes with time in a short period. The stress relaxation rate of the composite tends to become zero, irrespective of the fiber orientation. The associated relaxation modulus depends on the level of strain. The entire process of the prior instantaneous tensile response and the subsequent off-axis stress relaxation behavior is simulated using a macromechanical viscoplasticity model based on an overstress concept. It is demonstrated that the model succeeds in adequately reproducing the off-axis stress relaxation behavior of the unidirectional composite laminate.
A new concept of pseudo-continuum model is proposed to analyze the complicated nonlinear behavior of plain-weave fabrics in in-plane problem. The deformation of plain-weave fabric is categorized into three types in view of thread deformations, that is, skewing, straightening and extension. In this categorization, it is assumed that the bending and shear of threads caused by heterogeneity and friction can be neglected. The skewing and straightening give rise to in-plane transverse compressive strain, and the extension means axial tensile strain. The warp and weft are individually subject to these two strains with in-plane and anti-plane finite rotation. The geometrical nonlinearity, therefore, arises from the finite rotation of threads, while these strains are defined in the sense of infinitesimal strain. The framework of this pseudo-continuum model, which consists of the equilibrium equation, the strain-displacement relationship and the constitutive law, is newly constructed. The nonlinear behavior under biaxial extension, that is a characteristic mechanical behavior of the plain-weave fabric, is analyzed for the proof of validity of the proposed model.
A new breed of finite element is developed to analyze the nonlinear behavior of plain-weave fabrics in the in-plane problems of arbitrary boundary condition. A nondimensional parameter called crimp parameter is introduced as an unknown to represent the crimp condition of warp and weft, and handled as a component of the displacement vector. The plain-weave fabric is homogenized by means of a newly defined strain-displacement relationship including the crimp parameter for the sake of consistent dealing with the three types of thread deformations, that is, skewing, straightening and extension. This homogenized model called pseudo-continuum model induces the geometrical nonlinearity with respect to the finite rotation of the threads, and its finite element is formulated by the principle of virtual work in the total Lagrangian description. The mechanism of nonlinear behavior of the plain-weave fabrics is elucidated through several examples by the proposed finite element.
A determination procedure for element elimination criterion in finite element simulation of high-strain-rate impact and penetration phenomena, occurring between tungsten heavy alloy long-rod penetrators and steel targets, has been presented with some demonstrations for the validity of the established criterion. The element elimination criterion for the two types of materials have been determined by comparing the simulated depth of penetration (DOP) and deformed shape of the penetrator with previously available experimental results. Although the criterion affects the simulated DOP significantly at the studied impact velocity of 1500m/s, once established, they are shown to be valid in predicting the DOP in the impact velocity range between 1100 and 1750m/s. The events of partial penetration with severe material deformation such as critical ricochet angle and ricochet phenomenology have also been successfully predicted using the established criterion in the similar impact velocity range. Thus it is suggested that the determination procedure for the suitable element erosion criterion is prerequisite in simulating high-strain-rate impact/penetration phenomena and the criterion established by the procedure is useful in fairly broad range of the velocity and for other similar high-strain-rate events.
Acoustic intensity is usually estimated from the cross-spectrum of acoustic pressures measured by two adjacent microphones. The cross-spectrum, calculated by the digital Fourier transform technique, has unavoidable leakage error because the signal period does not usually coincide with the record length. Therefore, acoustic intensity, estimated by a conventional FFT analyzer, has distorted values. In our research, the expression of Fourier-transformed data of a harmonic signal with a single frequency was formulated when there was leakage error. A method to eliminate the effect of leakage error from contaminated data was also investigated. Some numerical examples show the validation of the proposed method.
Using highly ductile acrylic adhesive, the present authors proposed a new technique of plastic bending of adhesively bonded sheet metals. In this process, the suppression of large transverse shear deformation occurring in the adhesive layer, which in some cases would induce the geometrical imperfection (so-called ‘gull-wing bend') and the delamination of the sheet, is one of the most important technical issues. In the present work, the effect of forming speed on bending deformation was investigated. From experimental observations in V-bending experiments of adhesively bonded aluminium sheets, as well as the corresponding numerical simulations which consider the viscoplasticity nature of the adhesive resin, it was found that the large shear deformation and ‘gull-wing bend' are successfully suppressed by high-speed forming since the deformation resistance of the adhesive resin becomes higher at a high strain rate.
Martin's method, which is used to determine the internal displacement of atomic systems and elastic constants, is applied to the Tersoff potential. The potential is modified to provide an accurate description of the high-temperature elastic properties of silicon. The elastic constants of crystalline silicon were investigated at both low and high temperatures. Results were verified using the statistical thermodynamic method, i. e., ‘Fluctuation formula’. It was found that values of elastic constants and the influence of the internal displacement are valid. However, at high temperatures the gap becomes larger owing to the thermal fluctuation. Since the convergence of the Martin's method is faster by about two orders, it is the more effective method. It was also found that the fluctuation term includes the effects of the internal displacement and thermal fluctuation.
The effect of stress on the magnetic performance of a giant magneto-resistive (GMR) head was investigated. The stress in the GMR head consists of residual stress caused by thin-film processing and thermal stress due to heat dissipation in the GMR element. When the difference between the in-plane stresses in the GMR element is large, the output voltage of the head changes; thus, the read characteristic of the hard-disk drive is deteriorated. The stress in the GMR head was analyzed by a three-step finite-element analysis, called zooming FEA, and the magnetic performance of the head under the effect of the stress was calculated. Accordingly, it was found that to reduce the effect of the stress on magnetic performance, the intrinsic stress in the protective layer on the head must be optimized and the temperature rise of the GMR element must be decreased during operation.
In this paper, a fatigue damage estimation procedure is implemented by integrating the results of an EPRI and a GE testing reports as well as a shareware developed by the Oslo University, which is incorporated with a verified transient simulation program developed by the Aberdeen University to study the effects of power system unbalance on turbine blade damaging. Based on the Weibull distribution in the negative sequence current (I2) and the operational environment containing 22% NaCl, the probability level of fatigue life as well as the reliability against fatigue failure for the long blades of low-pressure (LP) turbine are evaluated. It is shown that even though the blades could withstand the most serious impact arising from three-phase-to-ground fault, still it cannot guarantee adequate long-term reliability in the normal operational condition.