1. Department of Automotive Engineer, Woosuk University, Chonbuk 565-701, Republic of Korea
2. Center for Nondestructive Evaluation, Iowa State University, Ames, IA 50011, USA
3. Department of Mechanical and Automotive Engineering, Songwon University, Gwangju 502-210, Republic of Korea
4. Department of Mechanical Design Engineering, Chosun University, Gwangju 501-759, Republic of Korea