Fatigue Fracture of a Phosphor Bronze Spring Because of Tool Marks

Abstract

A copper alloy C51000 (phosphor bronze, 5%A) failed prematurely during life testing of several such springs. The wire used for the springs was 0.46 mm (0.018 in.) in diam and was in the spring-temper condition. The springs were revealed to be subjected to cyclic loading, in the horizontal and vertical planes during the testing. The fracture was revealed to have occurred in bend 2. An indentation, presumably caused by the bending tool during forming, at the inner surface of the bend where fracture occurred was revealed by microscopic examination. Spiral marks produced on springs during rotary straightening were observed. A crack that had originated at the surface at the inside bend and had propagated toward the outside of the bend was revealed by microscopy of a longitudinal section taken through bend 2. The small bend radius was interpreted to contribute to spring fatigue as a result of result in straining at the bend zone. The spring was concluded to have failed in fatigue. It was recommended that the springs should be made of wire free from straightener marks and the bending tool should be redesigned so as not to indent the wire.