Different Criteria For Design With Fracture Mechanics
INTRODUCTION
Fracture is understood to be separation of a body of material into two or more pieces, whereby the load carrying capacity is reduced to zero. The phenomenon of fracture has been associated with man’s life, the better or worse, since the beginning of human existence. In prehistoric days man use brittle fracture in shaping of stone tools & carving of house out of rocks, even though he felt its less fortunate consequences when he got his bones broken. Quarrying, shaping and structural fitting of stones, the essential processes of ancient construction all depended upon controlled manipulation of brittle fracture. Likewise, today many industrial manufacturing and constructional processes, machining of materials for example, involve brittle fracture at some point.
Most of the understanding about fracture in engineering aspects, however, is due to the less beneficient aspects of brittle fracture, particularly the sudden catastrophic failure of structures occurring as a result of unexpected brittle fracture of component materials. The history of technology is full of such examples, dating from 10th century. One of the earliest on record was falling of a mill in Oldham, England, in 1888, an accident in which failure by fracture of cast iron beam led to the death of 20 factory workers. Yet earlier accident occurred in 1830, when Montrose suspension bridge spanning the river in Scotland, partially collapsed with a great loss of life. 12 persons died and 40 others were injured when a molasses tank suddenly fractured in Boston, USA in 1919. Yet another storage tank fracture in Cleveland USA in 1973.
When a methane storage tank fractured in Bellview, USA, in 1944, 128 persons were killed and property damage totalled almost 7 million dollars. The most spectacular failure by fractured occurred in 1943, when a T-2 tanker lying quietly at her fitting out pier at Portland, USA, suddenly cracked in the middle and separated into two with a report that could be heard for a mile. In 1972, an oil barrage fractured in the middle in New York. There are several other examples of fractures of bridge structures, boilers, cranes, naptha conduits, large and small guns, and rotors of steam and gas turbines and electrical machines. During World War II, out of 4694 welded liberty ships as many as 1289 had structural failures.
The ship failures in particular gave a great impetus to researches in fracture. Soon it was concluded that three factors could be responsible for brittle failure of a material, which normally in laboratory would behave as ductile.
These factors are:
(a) Presence of defect which could be of mechanical type, created during machining, forming or welding or of metallurgical nature like slag, other inclusions or even microscopic derangement.
(b) Low temperature which generally tends to increase the yield strength and decrease the fracture strength of material.
(c) Rate of straining – higher the strain rate, less ductile the material. (b) and (c) have already been discussed under impact properties.
Unitl 1960, the factors (b) and (c) were generally regarded to be important in controlling material fracture and consequently Charpy V-notch energy value was the major criterion for material selection for structures and machines. Difficulties have arisen when quantitative description of these factors in design processes could not be incorporated. Further, the experimental observation on service failures and laboratory tests clearly indicated that failure had always initiated at some defect or site of stress concentration and a crack bad generally propagated, mostly under fatigue loading. Failure by fracture eventually took place due to crack becoming unstable. Thus, there were reasons to concentrate attention on crack like defects. Thus, a new body of knowledge, known as fracture Mechanics, came into existence. Now fracture mechanics is being increasingly applied to design structures and machine parts so as to control sudden or brittle fracture.