“Fracture” is the Breaking of a Metal to Yield an Irregular Surface:
The topography of the fractured surface is an important characteristics intimately connected with the crystalline form and in some cases it may give an indication of the purity of the metal.
In a polycrystalline material there can be three distinct modes of fracture as given below:
1. Inter-granular fracture by separation of crystals at the grain boundaries.
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2. Fracture due to shear between the crystallographic planes.
3. Cleavage fracture through pulling apart of crystallographic planes.
Ductile and Brittle Fractures:
Depending on the fracture mechanisms the following two distinct types of fractures are usually encountered:
1. Ductile fractures
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2. Brittle fractures.
Most metals fail by a combination of the two, but there are instances of complete brittle and ductile fractures.
In the case of “ductile fracture” there occurs an appreciable plastic deformation prior to failure and the fractured surfaces give cup and cone appearance. The fracture is found to start only after a necked portion shows up on the test piece. The first formed micro-cracks and cavities grow larger and finally join together to form a crack in the centre of the necked portion.
The cavity then spreads in a direction inclined at 45° to the tensile axis. During this stage hardening occurs. The size of the cup depends on the relative shear and cleavage strength values. Metal with a high yield strength gives a smaller cup. The fracture faces are dull, irregular and fibrous in appearance.
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In the case of “brittle fracture” failure of the metal occurs when the fracture crack propagates through the cross-section without an appreciable plastic deformation. The fracture crack may start from any location where there are stress raisers. The surface condition of the metal can be critical and markings on it can initiate cracks. Such a fracture is more likely to occur in metal with poor plasticity and low temperatures.
The engineering structures usually fail due to tensile stress and designs based on tensile properties. This is due to the fact that under tensile stress the crack once formed has no chance of healing and during strain the cross- sectional area of the stressed component is decreased causing an increase in the true stress level.
There is also a stress concentration effect due to localised necking. The effect of compressive stress is first the opposite of tensile stress and this type of loading is likely to cause fracture.
Comparison between Ductile and Fracture and Brittle Fracture:
Ductile Fracture:
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1. Ductile fracture is accompanied with large plastic deformation and is the result of intense localised plastic deformation adjacent to crack.
2. Slow rate of crack propagation.
3. Ductile fracture is characterised by the formation of cup and cone.
4. Surface obtained at the fracture is shining and accompanied with the formation of slip planes.
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5. Failure is on account of shear stress developed at 45°.
Brittle Fracture:
1. Brittle fracture is one in which the movement of the crack involves very little plastic deformation of the metal adjacent to the crack.
2. Rapid rate of crack propagation.
3. Brittle fracture is characterised by separation normal to tensile stress.
4. Surface obtained at the fracture is dull and accompanied with hills and valleys.
5. Failure is on account of direct stress.
Effect of Structure on Fracture:
a. The grain size of the metal greatly affects the breaking strength and mode of fracture. The breaking strength normally increases and the possibility of local straining is decreased with a decrease of grain size. Also intergranular fracture becomes less likely on grain refinement.
b. The existence of preferred orientation in cold worked metals and columnar structure in cast medals can modify the mode of fracture.
c. In dirty steels the excessive quantity of inclusions cause failure by the brittle fracture.
Ductile-Brittle Transition Temperature:
It has been observed that the mode of fracture is greatly affected by temperature.
Many metals show a ductile-brittle or tough brittle transition and this affect appears at low temperature in the form of increased yield stress or flow stress.
The “transition temperature” is a narrow temperature interval in which there is a drastic decrease in the percentage reduction of area and elongation and the energy absorbed during impact loading.
Metals with BCC structure are more succeptible to ductile-brittle transition than those of FCC structure.
The existence of the transition temperature is attributed to the change of resistance of the metals to shear and cleavage failures as the temperature changes. The relative values of the respective critical shear and cleavage stresses determine whether the fracture is ductile or brittle. According to one theory the ductile-brittle transition occurs when the temperature is so low that yield stress exceeds the fracture stress.
The following are some other engineering and metallurgical factors that increase the possibility of brittle fracture in metals, in addition to stress concentration at cracks, low temperature transition and grain size:
(i) High residual stress.
(ii) Sudden loading of a metal.
(iii) Nonmetallic inclusions and dissolved nonmetallic elements (e.g., carbon, hydrogen, nitrogen, sulphur and phosphorus).
(iv) Strain ageing, precipitation ageing and work hardening.
Griffith’s Theory of Fracture:
For an ideal solid the fracture strength of a real material is much lower than the theoretical minimum value. The first explanation for the discrepancy was offered by Griffith’s (1920). According to his theory there are many fine elliptical cracks and there are high concentrations of stress at the lips of such cracks. With such stress concentration the theoretical cohesive strength can be obtained in this localised region when the body of a material is under a fairly low applied tensile stress.
Griffith’s fracture or crack theory, in its simplest form, can be described as follows:
Refer to Fig. 2.66. Consider a rectangular slab of unit thickness subjected to a uniaxial stress a. Due to this stress a certain amount of elastic strain energy is stored in the slab. Now consider the formation of a crack of length 2C and of elliptical cross-section running from the front to the back of the slab.
The elastic strain energy that is released by formation of this crack is given by – 2σC2 / E (negative sign indicates that the elastic energy stored in the material is released as the crack forms) where E is the Young’s modulus of the material. Also the crack formation produces two new surfaces of total surface energy 4 γ C where γ is the specific surface energy.
The energy change during the formation of the crack is given by:
When the critical stress is applied to a brittle material, the free-existing crack propagates spontaneously with a decrease in energy, culminating in fracture. Thus eqn. (2.4) gives the Griffith’s energy balance criterion for crack propagation.
Modified Griffith’s Theory:
When the surface of a brittle fracture in metals is examined by X-ray diffraction a thin layer of plastically deformed metal is observed. This means that whenever brittle fracture in metal occurs, there is always small amount of plastic deformation.
Griffith theory is modified accordingly as:
Fracture Toughness:
In all engineering materials defects, cracks or flaws are inevitably present (which may be introduced during solidification, fabrication or heat treatment stages of the material). It is therefore, very necessary to evaluate the fracture resisting capability of a machine component or an engineering structure. The fracture resistance of a material in the presence of cracks or discontinuities is known as its fracture toughness.
The fracture toughness, from the Griffith type of approach, is defined by the critical value of a parameter Gc, which gives the value of strain energy release per unit area of the crack surface when unstable crack extension (leading to fracture) takes place.
i. In an ideally brittle material such as silicate glass, Gc can be equal to 2γ.
ii. In materials where plastic deformation occurs during crack imitation, Gc can be much larger than 2γ.
Fracture toughness is also described by another more commonly used factor, known as the critical stress intensity factor K1C. Its values for a sharp crack in an infinitely wide plate (when the applied tensile stress is perpendicular to the crack faces) is given by
Factors Affecting Fracture Toughness:
Fracture toughness of a material is affected by the following factors:
(i) Thickness of the material;
(ii) Composition of the material-
a. Different alloying elements have different fracture toughness.
b. Aluminium alloys have lower values of plane strain toughness (K1C) than many steels.
c. Phosphorus, sulphur in steels reduce the toughness.
(iii) Heat treatment.
(iv) Service conditions.
Methods of Protection against Fracture:
As per Griffith’s theory surface cracks are more effective than internal cracks and cause brittle fracture. Fatigue cracks often propagate from the surface inwards.
Protection against fracture or failure by crack propagation, in many cases, involves the following precautions and prevention steps:
1. Avoid sharp corners and notches in the parts to eliminate points of stress concentration.
2. Provide better finish to the surface, by polishing (as it removes some of the cracks from the surface).
3. Introduce compressive stresses on the surface to counteract the tensile stress causing cracks to propagate.
4. Stresses applied should be parallel to the direction of elongation.
5. Use composites/ reinforced materials to provide protection against tensile loads.
6. Use fine grained polycrystalline materials which offer good protection against brittle fracture.
7. Avoid impurities in the base metal as well as during welding process.
8. Ensure proper distribution of solute atoms in the metals to prevent the formation of brittle phases at grain boundaries.