Date of Graduation

2000

Document Type

Thesis

Degree Type

MS

Committee Chair

Roger H. L. Chen

Abstract

The Armored Vehicle Launch Bridge (AVLB) is subjected to cyclic loading during its launching as well as tank crossing. The cyclic loading causes crack to initiate in critical bridge components, and to propagate. Unless these cracks are detected and repaired before they rapidly grow to reach their critical stage of propagation, the failure of bridge components can occur. According to the Preventive Maintenance Checks and Services (PMCS) from the U.S. Army, three AVLB components, the splice doubler angle, the splice plate, and the bottom chord, are susceptible to fatigue damage. In the present study, the laboratory fatigue tests on the materials used for the components, aluminum 2014-T6, aluminum 7050-T765, and ASTM A36 steel, were conducted along with the acoustic emission (AE) fatigue crack monitoring technique. A total of fourteen compact-tension specimens were prepared in this study. These specimens were six aluminum 2014-T6, four aluminum 7050-T76511, and four ASTM A36 steel specimens. The aluminum 2014-T6 specimens were subjected to four combination sets of loading frequency and ratio (R), the aluminum 7050-T76511 specimens to two combination sets, and the ASTM A36 steel specimens to two combination sets. The experimental results from both fracture mechanic approach and AE monitoring provide essential information to understand the fatigue crack growth in the critical components. Especially, the crack growth rate (da/dn), the AE count rate (dN/dn), and the stress intensity factor (K) can be used to indicate the stress levels of the components. Finite Element Method (FEM) was used to obtain the stress intensity factor (K) of the components with cracks. Because of the complexity of loading conditions and component geometry, several assumptions and simplifications are made in the FEM modeling. The FEM results, along with the results obtained from laboratory fatigue tests, are then utilized to evaluate the approximate critical crack length and the remaining life of the components. Linear Elastic Fracture Mechanics (LEFM) is applied in the FEM analysis to obtain the stress intensity factor. This study provides the experimental information of material dependent constants C and m in Paris Law for the materials and the loading sets considered. Provided for the materials considered are also the characteristics of the AE signals that may be used in future AE monitoring work. In addition, the stress intensity factor (K) as a function of crack growth in the components has been generated numerically. From the experimental and numerical results, the critical crack length and the remaining life of the critical AVLB components are estimated to help establish the damage assessment for the AVLB.

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