FAILURE ANALYSIS OF STEPLADDERS MANUFACTURED FROM EXTRUDED ALUMINUM

摘要

Despite advances in materials and manufacturing techniques for extruded aluminum and fiberglass reinforced composite structures, accidents involving ladders cause an estimated 300 deaths and 130,000 injuries annually. Although the origins of accidents are rarely known, the causes are usually attributed to the actions of the user and not the failure of the atructure. To better understand the material and structural characteristics of common, commercially available aluminum stepladders, and the potential role they play in accidents, a Structural analysis was performed on several typical designs. Stress analysis of common structural geometries revealed that (he potential modes of failure are complex, and might often be the result of a combination of compression, bending, and torsion in the front side rails. In addition, some loading scenarios suggest failure can result from elastic buckling in either the front or rear side rails. Measurements to quantify variations in the material and geometric properties of the ladder components revealed that significant variations can exist and are attributable to several different aspects of the manufacturing process. In some instances, actual rail strength and section properties did not meet the specifications of the ladder manufacturer. Overall, the susceptibility of these structures to Mure under certain loading scenarios, coupled with the inherent variations associated with extrusion manufacturing, indicate that aluminum stepladders, especially Type Ⅱ and Type Ⅲ, are not adequately designed for strength, and that current designs do not demonstrate a factor of safety sufficient to meet the standards required by the ladder industry.