Copyright Goodheart-Willcox Co., Inc. Heavy Equipment Power Trains and Systems 594 overloaded can fracture into multiple pieces. A spiral bevel gear set may have multiple frac- tures on the concave or convex side of the teeth. Fractures vary in appearance based on their type. Fracture types include the following: • Fatigue fracture. • Impact fracture. • Ductile fracture. • Brittle fracture. • Combination fracture. A fatigue fracture occurs due to repeated stress cycles. A common example of a fatigue fracture is a piece of metal that is bent back and forth until it breaks. Curved lines called beach marks radiate from one or more origin points of a slowly occurring fatigue fracture. See Figure 17-36. A forged component may have both beach marks and chevron marks. A forged component is formed using heat and force. See Figure 17-37. Ratchet marks are sometimes located at the origin of the failure. These short abrupt lines indicate a very high stress load. The final fracture of a component is sometimes a brit- tle fracture, as evidenced by a dark woody appearance. See Figure 17-38 and Figure 17-39. An impact fracture is caused by an abrupt, extreme force (shock load). The fracture can produce an immediate failure or a delayed failure. Different forces, such as a bending stress or torsional stress, can cause an impact fracture. A torsional stress fracture is a fracture that occurs due to applied torque or twisting action. This type of fracture leaves a 45° break that indicates the torsional twisting that occurred during the failure. See Figure 17-40. A ductile fracture is a slow type of fracture that results in plastic deformation, mean- ing the component stretches and elongates before it fails. This type of fracture may cause bending or narrowing of the component. See Figure 17-41. A brittle fracture is a fast type of fracture. The material does not elongate, but breaks apart quickly. A brittle fracture in wrought metals often has chevron-shaped marks that point to the origin of the fracture. A wrought metal is a metal that is worked during its formation, such as a forged component. A cast component (formed by pouring molten Beach marks Chevron marks Goodheart-Willcox Publisher Figure 17-36. This spiral bevel pinion gear has a fatigue fracture. The wavy (curved-shaped) beach marks radiate from the origin of the initial fracture. Goodheart-Willcox Publisher Figure 17-37. A failure started at the center of this forged track link. The beach marks radiate away from the center toward the top of the link. The final brittle fracture has chevron-shaped marks that point to the origin of the fracture. Ratchet marks Ratchet marks Final brittle fracture Beach marks