Papers by Author: Li Xiao

Abstract: Closed cracks, which stay in contact unless the excitation exceeds a certain threshold, exist as great menace to structures. Since nonlinear response is more sensitive to micro damage than conventional linear approaches, analyzing the nonlinear part of the collected response of structures to an input ultrasonic excitation is more promising in damage detection. In this paper, in order to image the location of a closed crack, an instantaneous baseline measurement is adopted and the nonlinear response is extracted by using scaling subtraction method. A three-dimensional finite element model of a plate with a closed crack is developed and the behavior of the closed crack is simulated with nonlinear springs at the crack interfaces. A network of actuators and sensors which constitutes of two arrays of surface-bonded piezoelectric transducers is built. The instantaneous baselines of each path are collected when the model is excited with low amplitude excitation. To diagnose the closed crack, a higher amplitude excitation over the threshold is applied to the model and the response signals of each path are recorded. The result shows that the differences caused by the crack can be observed from the scaling subtraction of these two recorded responses and the location of closed crack can be accurately imaged.

Abstract: Terrorist bombing attacks will endanger and may even destroy the target building structures, resulting in economic loss and casualties. Typical columns and floor slab systems are not designed to resist the complex blast loading. So, in recent years, the effects of blast on conventional public buildings are focused on. In this paper,a two-bay,one-story reinforced concrete frame structure which is used to model a portion of a typical reinforced concrete frame structural system is used to investigate the blast response. The experiments are conducted on two models, allowing a variation in explosives standoff and explosives charge. In each experiment,the blast pressure values are recorded and the degree of damage of the frames are studied. According to the two kinds of experiments, two numerical models are established. ALE method which considers the interaction of the explosive, the air, and the structure is applied.Structure response analyses are performed using the large deformation finite-element computer code, LS-DYNA. The numerical results are compared with the experiment results, and a good agreement is obtained. The calculating results also demonstrate that some experimental value is unreasonable.

Abstract: In this paper, a FEA model of non-normal(considering yaw angle and oblique angle) penetration into reinforced concrete is established by using software ANSYS/LS-DYNA. In order to validate the digital model which is sensitive to material parameters,mesh size,ect. Amounts of numerical simulations are performed. The numerical results of a process of peneration are compared with the experimental results, and a good agreement is obtained.In the study,the residual velocity and the ballistic attitude when a projectile penetrates into a reinforced concrete target are focused on, the relationship between positive yaw angle and negative yaw angle with the penetration capacity of the projectile is investigated. The results obtained by the numerical approach are useful for predicting the response of the penetration, and the proposed numerical simulation methods may be very beneficial to some experiment of perforation of reinforced concrete structures.

Abstract: The hysteresis dynamic behavior of metal rubber mathematically modeled with a practical method is studied, and the method of parameter separated identification is presented with details. Parameters of the model are identified with the test data of metal rubber, from which the theoretical loops are reconstructed, and the mechanism of the nonlinear damping behavior of the metal rubber is investigated. The theoretical loops and the experimental one are close to each other with satisfactory accuracy. The result shows that with the simple mathematical form and the satisfactory precision, the mixed damping model can be used effectively in practical engineering. This study provides a practical and effective method in modeling and the parameter identification of the metal rubber isolator.

Abstract: In this paper, a series of inelastic transient analysis of steel frame structures under blast loads are numerically performed. Considering the coupling actions of detonator, gases and structure, Arbitrary Lagrangian Eulerian ALE method which is suitable for large deformation systems is applied. In order to capture the effect of explosion load on members, a rate-dependent stress–strain model is used to model the material response of the affected members. The dynamic response and the structural damage of steel frame under blast loading are investigated. The damage pattern of steel frame to blast loads is studied. The results show that the permanent deformation, local buckling, and plastification reduce the resistance of the frame. The affected members may trigger the overall stability of the frame. The damage degree is related to the amount and position of explosives, shape and resistance of the building etc. The damage of the low-story columns and beams may make the structure unstable. The load might not be large enough, the damage to the entire frame may be severe, and it may cause structure collapse.

Abstract: Lamb waves are ultrasonic elastic waves that travel inside and along thin plates and is frequently used as diagnostic tools to detect damage in plate-like structures. In this paper, a transient dynamic finite element simulation of Lamb wave with piezoelectric transducers for damage detection in a stiffened plate under different temperature condition is carried out on the commercial finite element code ANSYS platform. Simulations are conducted over a temperature range. The changes in temperature-dependent material properties are used to measure the differences in the response signal’s waveform. The baseline selection method and baseline signal stretch method are used to compensate the temperature influence on Lamb wave propagation. The results of the numerical simulation demonstrate the effectiveness of the temperature compensation approach and the simulated damage on the stiffened plate can be defected effectively under elevated temperatures environment.

Abstract: Structure vibration test plays an important role in dynamic design, but in most cases the experimental data we get is always limited and insufficient for succeeding upgrade and optimization. The response prediction technique presented in this paper is the corresponding technique to deduce the full field response from the limited data obtained by simple vibration test instead of conducting the full-size test. SEREP (System Equivalent Reduction-Expansion Process) method was used to conduct the prediction research and the procedure was elaborated in detail theoretically first, then several simulations were carried on to demonstrate the prediction procedure, inspect the accuracy under various loads and investigate the factors which may influence the accuracy including the number of modes used in the prediction, the noise, and the measured points. Results show that this method has excellent accuracy and good adaptability to various loads as long as a sufficient set of orthogonal mode shapes participate in the prediction procedure.

Abstract: Structural health monitoring (SHM) is an emerging research area with multiple applications. Lamb waves are ultrasonic elastic waves that travel inside and along thin plates and is frequently used as diagnostic tools to detect damage in plate-like structures. In this paper, a transient dynamic finite element simulation of Lamb wave with piezoelectric transducers for damage detection in a composite plate is carried out. The embedded cross-shaped piezoelectric active sensor arrays were used to generate and receive guided Lamb waves propagating in the plate structure. A full-scale FEM model for the laminate was created using three-dimensional eight-node layered structural solid element and piezoelectric active sensors were created using coupled field elements on the commercial finite element code ANSYS platform. The beam forming technique of Lamb waves is used to locate damage in the plate .The results of the numerical simulation demonstrate the effectiveness of the approach.