the authors show sem evidence of fiber failure due to tensile, twist, and flex stresses and by wear during use. the authors illustrate the failures with cotton, wool, and polyester fibers. a study is made of four types of fiber failure: type 2 is ductile crack propagation, in which a crack opening forms a v-notch. type 3 occurs at high rates of testing when the fracture surface of a fiber such as nylon changes to a mushroom shape. type 5 takes place when transverse discontinuities existing between structural units of the fiber are sufficient to stop direct crack propagation. type 6 happens to wet cotton fibers. attraction between fibrils is weaker than in dry cotton, and the break appears as a gradual thinning out due to separate breaks. 14 refs