Class: BE210
Group: T10
Members:

Kevin Cronk
Jack Kent
Deep Maji
Chris Walstad

Date: April 2004

Full Text

Abstract: 

Fatigue failure occurs in structures that have undergone dynamic and repetitive stresses. After such loading, the material may fail at a considerably lower tensile strength than normal. Repetitive stresses cause a greater amount of stress raisers within the wood which increase the susceptibility of the material to crack initiation. These cracks propagate with each stress cycle until a critical crack size is reached and failure occurs. Fatigue testing is important because a material needs to maintain its structural integrity long after its initial use. Airplane wings, bridges and other structures that undergo millions of cycles of low level stress are manufactured to have a lifetime well over their usable life. In this experiment, wooden samples will be cyclically tested to determine how the wood reacts to various loading rates, load magnitudes, and weight percent water. The structure of wood consists of a layered composite of cellulose microfibrils embedded in a hemicelluloses and lignin matrix. Wood cells are composed of four layers surrounding the lumen and are connected to one another via the middle lamella. Heckes et al. determined that the mechanical response of the individual cells was the same as the entire tissue, and thus the mechanical properties are governed by individual cells rather than the middle lamella and connections between cells. While the matrix ultimately determines the wood’s fatigue characteristics, the distribution of lignin plays an important role. The mechanical characteristics of wood vary greatly with water content. When dry the wood is brittle, but at high moisture contents it behaves like a ductile metal. Heckes et al. found that the reason for this variation in mechanical properties of wood with changing water contents is a “stick-slip” mechanism that allows would to plastically deform when wet. The stiffness of wood comes from the semi-crystalline polymeric cellulose microfibrils. The lignin should be the first to fail in fatigue testing, as it is a more brittle and weaker material compared with cellulose.