1. Parul Institute of Engineering & Technology
Subject Code :
Name Of Subject : Material Science And Metallurgy
Name of Unit : Metallic Materials
Topic : Fracture of Metals
Name of Studnets :
KANANI MILANKUMAR H

2. Introduction
A fracture is the separation of an object or material into two, or more, pieces under the action of stress.
Fracture is consider as the end result of plastic deformation processes which results in creation of two or more new surface.
Steps in fracture:
crack formation
crack propagation

3. Introduction (cont.)
There are five kinds of fracture in metals based on the nature of process:
Ductile,
Brittle,
Adiabatic shear,
Creep,
Fatigue fracture.
Three basics factors contribute to a brittle-cleavage type of fracture:
Triaxial state of stress
Low temperature
High strain rate or rapid rate of loading

4. Introduction (cont.)
There are circumstances under which certain ductile materials behave as brittle.
Two important cases of this type of failure (i.e. brittle failure of ductile materials) are:
Fatigue failure (which was studied previously)
Brittle fracture (which is going to be treated here).

5. Introduction (cont.)
Common examples of catastrophic failures of structures caused by brittle fracture are: Welded ships & tankers made of mild steel (during World War II), Rails of railways during cold winter periods.

6. Introduction (cont.)
Brittle fractures in steel structures usually occur without visible or audible warnings at stresses less than nominal Sy value.
Such fractures usually initiate at sharp notches and crack-like defects, and may subsequently propagate through a complete structure at faster than speed of sound.

7. Ductile fracture Ductile fracture - most metals (not too cold):
Extensive plastic deformation ahead of crack
Crack is “stable”: resists further extension unless applied stress is increased

8. Ductile fracture (cont.)
Steps:
Necking
Cavity Formation,
Cavity coalescence to form a crack,
Crack propagation,
Fracture

9. Ductile fracture (cont.)
Neck formation takes place at a point of plastic instability under tensile load.
The formation of the neck introduce a triaxial state of stress in the region. Many fine cavities form in the neck region.
Under continued straining cavities expand by plastic deformation and coalesce into acentral crack.
The crack grows perpendicular to the axial specimen until it approaches the surface of specimen.

10. Ductile fracture (cont.)
Ductile materials - extensive plastic deformation and energy absorption (“toughness”) before fracture
Brittle materials - little plastic deformation and low energy absorption before fracture

11. Ductile fracture (cont.)
Very ductile, soft metals (e.g. Pb, Au) at room temperature, other metals, polymers, glasses at high temperature.
Moderately ductile fracture, typical for ductile metals
Brittle fracture, cold metals, ceramics.
(a) (b) (c)

12. Ductile fracture (cont.)
Ductile fracture: high plastic deformation & slow crack propagation
Three steps:
Specimen forms neck and cavities within neck
Cavities form crack and crack propagates towards surface, perpendicular to stress
Direction of crack changes to 450 resulting in cup-cone fracture
Scanning electron micrograph
showing conical equaxial
features produced during the
fracture of a steel sample

13. Brittle fracture No significant plastic deformation before fracture
Common at high strain rates and low T
Three stages of brittle fracture:
Plastic deformation concentrates dislocations along slip planes
Microcracks nucleate due to shear stress where dislocations are blocked
Crack propagates to fracture
Ex.: hcp Zn single crystal under high stress along {0001} plane
Brittle fracture mostly occur due to defects like:
porosity
tears and cracks
corrosion damage
embrittlement due to atomic hydrogen
Most brittle fractures are transgranular

14. Brittle fracture (cont.)
Brittle fracture - ceramics, ice, cold metals:
Relatively little plastic deformation
Crack is “unstable”: propagates rapidly without increase in applied stress
No appreciable plastic deformation
Crack propagation is very fast
Crack propagates nearly perpendicular to the direction of the applied stress
Crack often propagates by cleavage - breaking of atomic bonds along specific crystallographic planes (cleavage planes).

15. Brittle fracture (cont.)
A great deal of attention was directed to the brittle failure of welded ships and tankers.
Failures occured during winter months and when the are in heavy seas and anchored at dock.
This fact focussed on that normally ductile mild steel can become brittle under certain conditions.
Therefore, researches aimed to understand the mechanism of brittle fracture and fracture in general.
While the brittle failure of ships concentrated great attention to brittle failure in mild steel.
Brittle failures in tanks, pressure vessels, pipelines, and bridges have been noticed.

16. Difference: Ductile & Brittle
Brittle failure
Ductile failure
It involves large plastic deformation.
It is always preceded by the localized deformation called “necking”.
Ductile fracture normally occurs in F.C.C metals.
Ductile fracture normally occurs through the grains.
A complete ductile fracture presents a rough dirty surface .it has rough dirty contour.
It occurs by slow tearing of the metal with expenditure of considerable energy.
It is associated with minimum plastic deformation.
It does not involve “necking”.
Brittle fracture is normally observed in B.C.C and H.C.P metal but not in F.C.C metal.
Brittle fracture normally follows the grain boundaries.
A complete brittle fracture shows sharp facets which reflect light.
It occurs suddenly without any warning.

17. Ductile-to-Brittle Transition (Embrittlement)
Hydrogen Embrittlement: Hydrogen produces severe embrittlement in many metals. Even very small amount of hydrogen can cause cracking in steel and titanium. It may be introduced during melting and entrapped during solidification, or it may be picked up during heat treatment, acid pickling, electroplating or welding.
Temper Embrittlement: Tempering some steels within °C results in temper brittlement, which is manifested by increase in impact transition temperature. It is due to segregation of certain elements to grain boundaries, giving local hardening to fracture.
Blue Brittleness: Low-carbon steels exhibit two types of aging which causes an increase in transition temperature: quench aging & strain aging. Strain aging is the slow increase in hardness in steels finished by cold work (mainly cold rolling). Blue brittleness is attributed to strain aging caused by heating cold worked steel to around 205 °C.

18. Ductile-to-Brittle Transition (Embrittlement)
Temperature Embrittlement:
This phenomena in which ductile materials transformer to brittle is called ductile to brittle transition (DBT). At low T, high stress levels or fast loading rates gives ductile to brittle transition. This temperature is called ductile to brittle transition temperature (DBTT)
E.g., Sinking of Titanic: Titanic was made up of steel which has low DBT temperature. On the day of sinking, sea temperature was –20 C which made the structure highly brittle and susceptible to more damage

19. Fatigue fracture
Fatigue: the phenomenon leading to fracture under repeated stresses having the maximum value less than the ultimate strength of the material
Metals often fail at much lower stress at cyclic loading compared to static loading
Crack nucleates at region of stress concentration and propagates due to cyclic loading
Failure occurs when cross sectional area of the metal too small to withstand load
Fatigue-fractures surface of steel shaft

20. Factors Affecting Type of Fracture
1. Temperature:
The notched-bar impact test has the greatest importance in determining “ductile-to-brittle transition” of a metal.
This transition occurs at a temperature below which the material is brittle and fractures with a low energy absorption & low ductility, and above which it is ductile.

21. Factors Affecting Type of Fracture (cont.)
The transition actually covers a range of temperatures in which degree of brittleness increases gradually as temperature falls.
It is very difficult to make a universal definition of transition temperature as two different materials having the same transition temperature may have different failures.
Therefore, there are many definitions of this temperature.

22. Factors Affecting Type of Fracture (cont.)
2. Composition: In metals, the impact testing is mostly applied to steel, testing of nonferrous materials is seldom.
The main factors influencing brittleness of steels are: Composition, heat treatment and section size.
The greater is the hardness, the higher is the transition temperature.
Considering the effect of composition in steels, carbon content plays important role
The optimum combination of properties in quenched and tempered low alloy steel occurs for % C.
Figure 8
The effect of other elements on impact properties can be found in the textbook.

23. Factors Affecting Type of Fracture (cont.)
3. Grain Size: As the grain size increases, transition temperature increases and fracture stress decreases. Thereby, it is possible to improve ductility and toughness of steel by obtaining ultrafine grain size.
4. Microstructure: The shape of carbide precipitates in steel has a great effect on impact toughness.
A tempered martensitic structure has the best combination of strength and fracture toughness. Tensile properties of such structures of the same carbon content and the same hardness are alike, but great variations in their impact toughness with temperature

24. Factors Affecting Type of Fracture (cont.)
5. Orientation: The orientation of test bar in a formed product affects both the impact energy and the value of Fracture Appearance Transition Temperature,FATT, as well as the tensile ductility. For rolled products, orientation does not have a great influence on FATT.
Effect of specimen orientation of Charpy transition-temperature curves

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