Fracture

MATERIAL SCIENCE

1. FRACTURE BY Dr. K. SENTHILARASAN ASSISTANT PROFESSOR DEPARTMENT OF PHYSICS E.G.S.PILLAY ARTS & SCIENCE COLLEGE NAGAPATTINAM-611002

2. Fracture Fracture refers to failure of a solid body under load by breakage into two or more pieces. Thus the separation or fragmentation of a metal into two or more parts under the action of load is called fracture.

3. F our Types F ractures Brittle fracture Ductile fracture Fatigue fracture Creep fracture

4. Brittle fracture: A brittle fracture may be defined as a fracture which takes place by the rapid propagation of crack with a quite negligible plastic deformation. Ductile fracture: It is defined as the fracture which takes place by a slow propagation of crack with appreciable plastic deformation. Fatigue fracture: Fatigue fracture is defined as the fracture which takes place under repeatedly applied fatigue stress. It occurs at stress well below the tensile strength of the materials. The fatigue stresses are found in the machine parts such as axles, shafts and crank shafts . Creep fracture: Creep fracture is defined as the fracture which takes place due to excessive creeping of materials under steady loading.

5.   Brittle fracture Ductile fracture 1 Brittle fracture is the one which has the movement of crack with a negligible plastic deformation adjacent to crack. Ductile fracture is the one which is accompanied with large plastic deformation and is a result of intense localised plastic deformation adjacent to crack. 2 Rapid rate of crack propagation. Slow rate of crack propagation. 3 Failure is on account of direct stress. Failure is on account of shear stress developed at 45° 4 Surface obtained at the fracture is dull and accompanied with hills and valleys. Surface obtained at the fracture is shifting and accompanied with the formation of slip planes. 5 It is characterised by separation of normal to tensile stress. It is characterised by the formation of cap and cone. 6 It occurs when the material is in elastic condition. It occurs when the material is in plastic condition. 7 The tendency of brittle fracture is increased by decreasing temperature, increasing strain rate and work hardening. The tendency of ductile fracture is increased by dislocations and other defects in metals.

6. Mechanism of brittle fracture: Griffth’s theory in glass. Normally the stress at which a material fracture is far lower than the value of the ideal breaking stress calculated from the atomic bond strength. The ideal or theoretical fracture strengths of engineering materials are very high. But actual fracture strengths observed in these materials are much lower. Griffth postulated that in a brittle material there are always presences of micro cracks which act to concentrate the stress the stress at their tips. There is a High stress concentration at the tip of the cracks even when the applied stress is quite low. When the applied stress is below a critical level, it is impossible for a micro crack of the given size to grow. The value of the critical stress for the growth of a micro crack decreases with increasing crack size is reached; the growth of the crack is likely to be very rapid. Thus the fracture strength is inversely proportional to the square root of the crack length. For ductile materials there is always some plastic deformation before fracture. This involves an additional energy term.

7. Fracture strength The graphite flakes in cast irons act as micro cracks and lower the fracture strength. From the above formula, one can get the size of the largest flaw or crack which can be allowed in service.

8. OA is fairly along straight line indicates to stress to strain is constant and Hooke’s law holds well from O to A. A is called proportional limit. AB indicates stress to strain not constant so metal continues to behave perfectly elastic. B is called elastic limit C is called lower yield point D is called upper yield point. E is tendency of the material or ultimate tensile strength. DE upward curve. EF downward curve F Fracture.

9. Mechanism of ductile fracture The tensile stress applied across the specimen is increased beyond the elastic limit, there is a uniform reduction in its cross sectional area. The process continues till the ultimate tensile strength is reached. It observed that during the formation of neck small micro cracks are formed due to the combination of dislocations which are formed during the manufacture of the specimen. The continued plastic deformation increases the size of the cracks into bigger cracks

10. Fatigue Fatigue: The failure of a material under repeated applied stress is called fatigue. Total number of cycles required to bring about final fracture under a given condition of use is called fatigue life. The maximum stress withstand by a material undergoing repeated cycle of loading before fracture is called fatigue strength.

11. Mechanism of fatigue growth Fatigue fracture is not taking place in an instantaneous manner. It is followed by three stages: Nucleation of crack Crack growth Fracture The material undergoing repeated cycles of loading and unloading, tensile strength and compressive stress are developed in alternating manner. The cycles load continue, the micro cracks is grown in its size. These types of micro cracks create stress concentration at some areas. Finally the growth of micro cracks leads to fracture it is called fatigue fracture.

12. Creep fracture Creep fracture is defined as the fracture which takes place due to excessive creeping of materials under steady loading. Mechanism of creep fracture: Creep fracture takes place due to sliding of grain boundaries at moderate stresses and temperature and movement of dislocation from one slip to another by climbing at higher stresses and temperature. The crack initiates along the grain boundaries which act as points of high stress concentration or due to growth of voids along the grain boundaries.

13. Mechanism of creep fracture Creep fracture takes place due to sliding of grain boundaries at moderate stresses and temperature and movement of dislocation from one slip to another by climbing at higher stresses and temperature. The crack initiates along the grain boundaries which act as points of high stress concentration or due to growth of voids along the grain boundaries.