Kaoshan Dai

A Low-Cost Energy-Efficient Cableless Geophone Unit for Passive Surface Wave Surveys

The passive surface wave survey is a practical, non-invasive seismic exploration method that has increasingly been used in geotechnical engineering. However, in situ deployment of traditional wired geophones is labor intensive for a dense sensor array. Alternatively, stand-alone seismometers can be used, but they are bulky, heavy, and expensive because they are usually designed for long-term monitoring. To better facilitate field applications of the passive surface wave survey, a low-cost energy-efficient geophone system was developed in this study. The hardware design is presented in this paper. To validate the system’s functionality, both laboratory and field experiments were conducted. The unique feature of this newly-developed cableless geophone system allows for rapid field applications of the passive surface wave survey with dense array measurements.

Application of the Surface Wave Survey Method on Multi-Scale Engineering Problems: Laboratory and Field Testing Case Studies

The geometric dispersion of surface waves offers the feature to infer the properties of a medium. The surface wave survey method is under continuous evolution and its applications include the noninvasive characterization of a medium at a small scale and the earthquake geotechnical survey at a large scale. Despite the significant difference in scales, these applications generally utilize the propagation of the surface waves along the boundary of a layered medium. This paper discusses standard steps involved in surface wave surveys. Two application case studies are conducted and presented in detail. In the small scale laboratory application case, a nondestructive testing (NDT) methodology is proposed for the coating investigation using laser interferometric measurements of surface waves. In the large scale field application, an accelerometer-based passive surface wave survey is designed for earthquake geotechnical engineering site characterization. Finally, recommendations for practical application of surface waves in multi-scale engineering problems are presented.

Introducing Case Studies in the Civil Engineering Curriculum of the Tongji University, China

Engineering college students are educated to be able to apply mathematics, science and engineering knowledge for design, construction, and solving engineering problems. Civil Engineering programs in China usually introduce students various mechanics and detailed design principles through the instructor-centered method. Although these knowledge is developed based on lessons learned by engineers of past generations, it is more effective to convey engineering philosophies if casebased teaching can be followed. This paper investigates the feasibility of improving educational functionality of case studies at the College of Civil Engineering of Tongji University, China. A sample course was introduced and suggestions for using research results to improve teaching were presented. Class designs for promoting failure case studies and involving students in laboratory research were described. INTRODUCATION

Investigation of the stochastic subspace identification method for on-line wind turbine tower monitoring

Structural health monitoring (SHM) of wind turbines has been applied in the wind energy industry to obtain their real-time vibration parameters and to ensure their optimum performance. For SHM, the accuracy of its results and the efficiency of its measurement methodology and data processing algorithm are the two major concerns. Selection of proper measurement parameters could improve such accuracy and efficiency. The Stochastic Subspace Identification (SSI) is a widely used data processing algorithm for SHM. This research discussed the accuracy and efficiency of SHM using SSI method to identify vibration parameters of on-line wind turbine towers. Proper measurement parameters, such as optimum measurement duration, are recommended.

Dynamic behaviors of historical wrought iron truss bridges: a field testing case study

The U.S. transportation infrastructure has many wrought iron truss bridges that are more than a century old and still remain in use. Understanding the structural properties and identifying the health conditions of these historical bridges are essential to deciding the maintenance or rebuild plan of the bridges. This research involved an on-site full-scale system identification test case study on the historical Old Alton Bridge (a wrought iron truss bridge built in 1884 in Denton, Texas) using a wireless sensor network. The study results demonstrate a practical and convenient experimental system identification method for historical bridge structures. The method includes the basic steps of the in-situ experiment and in-house data analysis. Various excitation methods are studied for field testing, including ambient vibration by wind load, forced vibration by human jumping load, and forced vibration by human pulling load. Structural responses of the bridge under these different excitation approaches were analyzed and compared with numerical analysis results.

Development of a low-cost cableless geophone and its application in a micro-seismic survey at an abandoned underground coal mine

Due to the urbanization in China, some building construction sites are planned on areas above abandoned underground mines, which pose a concern for the stability of these sites and a critical need for the use of reliable site investigations. The array-based surface wave method has the potential for conducting large-scale field surveys at areas above underground mines. However, the dense deployment of conventional geophones requires heavy digital cables. On the other hand, the bulky and expensive standard stand-alone seismometers limit the number of stations for the array-based surface wave measurements. Therefore, this study developed a low-cost cableless geophone system for the array-based surface wave survey. A field case study using this novel cableless geophone system was conducted at an abandoned underground mine site in China to validate its functionality.

Theory on the air-couple wave propagation in layered composite materials with laminate defect

It has been known for many years that defects exist along interfaces between two dissimilar materials, and conventional contact ultrasonic techniques are not always enable to detect the defects, so more advanced methods are still necessary. In this paper, the spring model for the layered composite materials is built to simulate wave propagation. A stiffness matrix-based method is proposed for quantifying the transmission coefficients. The solutions of the characteristic equations are numerically efficient and stable. This work has shown that the spring models are well adapted to evaluate the quality and integrity of complex composite structures.

SH wave propagation in a layered magneto-electric hollow cylinder

In this paper, the SH wave propagation in a layered magneto-electric hollow cylinder is studied. The wave equations and system matrices for state vector are obtained for SH wave propagating along the circumferential direction. It is found that the propagation of SH wave is coupled to the electric and magnetic field. The zero-order mode SH0 is non-dispersive under the electrically open boundary, while it is dispersive under the electrically closed boundary, and it is the same for the piezomagnetic material.

Depth profiling of coating by laser ultrasonic technique

Functionally graded materials (FGMs) show great potential, and the development of new FGMs will lead to materials that have properties not achievable with homogeneous materials. The dispersion curves of surface acoustic wave propagating in coating structures are investigated using the reflection and transmission coefficient method. Its found that the dispersion curves are sensitive to the transverse wave velocity, but they are not very sensitive to the longitudinal wave velocity and density. Genetic algorithms adopted to handle the inverse problem. The object function is established based on experimental phase velocity and the theoretical one. The reconstructed depth-profiling of material properties agrees well with the experimental ones.