1. MECHANICAL PROPERTIES AND TESTING

2. Elastic Deformation 1. Initial 2. Small load 3. Unload
Elastic means reversible.

3. Plastic Deformation (Metals)
1. Initial
2. Small load
3. Unload
Plastic means permanent.

4. Stress-Strain Diagram
ultimate
tensile strength 3
necking
Slope=E
Strain
Hardening
yield
strength
Fracture 5 2
Elastic region
slope =Young’s (elastic) modulus
yield strength
Plastic region
ultimate tensile strength
strain hardening
fracture
Plastic
Region
Stress (F/A)
Elastic
Region 4 1
Strain ( ) (DL/Lo)

5. Mechanisms of plastic deformation
Two prominent mechanisms of plastic deformation
slip
Twinning

6. Slip
Slip is the prominent mechanism of plastic deformation in metals. It involves sliding of blocks of crystal over one other along definite crystallographic planes, called slip planes.
It is analogous to a deck of cards when it is pushed from one end. Slip occurs when shear stress applied exceeds a critical value.
During slip each atom usually moves same integral number of atomic distances along the slip plane producing a step, but the orientation of the crystal remains the same.
Generally slip plane is the plane of greatest atomic density, and the slip direction is the close packed direction within the slip plane.

7. Slipping Mechanism Slip System
Slip plane – plane along which the dis. line traverses
plane allowing easiest slippage
highest planar densities
Slip direction - direction of movement - Highest linear densities
FCC Slip occurs on {111} planes (close-packed) in <110> directions (close-packed)
=> total of 12 slip systems in FCC
in BCC & HCP other slip systems occur

8. Stress and Dislocation Motion
• Crystals slip due to a resolved shear stress, tR.
• Applied tension can produce such a stress.
Applied tensile
stress:
= F/A s
direction
slip F A
slip plane
normal, ns
Resolved shear
stress: tR = F s /A
direction
slip AS FS
direction
slip
Relation between s
and tR = FS
/AS F
cos l A / f nS AS

10. Twinning
Portion of crystal takes up an orientation that is related to the orientation of the rest of the untwined lattice in a definite, symmetrical way.
The twinned portion of the crystal is a mirror image of the parent crystal.
The plane of symmetry is called twinning plane.
The important role of twinning in plastic deformation is that it causes changes in plane orientation so that further slip can occur.

11. c07f25

12. Comparison between slip and twinning mechanisms
Slipping
Twinning
The atoms in one side of the slip plane all move equal distances
The atoms move distances proportional to their distance from the twinning plane
Slip Leaves a series of
steps (lines)
2. Twinning leaves small but well defined regions of the crystal deformed
3. Most for FCC and BCC structure, they have more slip systems
3. Is most important for HCP structure , because its small number of slip system
4. Normally slip results in relatively large deformations
4. Only small deformations result for twinning

13. Fracture
Fracture is the separation of a specimen into two or more parts by an applied stress.
Fracture takes place in two stages:
(i) initial formation of crack and
(ii) spreading of crack.
Depend upon the type of materials, the applied load, state of stress and temperature metals have different types of fracture.
Types of fracture
Brittle Fracture
Ductile Fracture
Fatigue Fracture
Creep Fracture

14. Types of fracture Ductile Fracture Brittle Temperature
Factors affecting fracture
Strain rate
State of stress

15. Tension
Torsion
Fatigue
Conditions of fracture
Creep
Low temperature Brittle fracture
Temper embrittlement
Hydrogen embrittlement

16. Brittle Fracture
Brittle fracture is the failure of a material with minimum of plastic deformation. If the broken pieces of a brittle fracture are fitted together, the original shape & dimensions of the specimen are restored.
Brittle fracture is defined as fracture which occurs at or below the elastic limit of a material.
The brittle fracture increases with
Increasing strain rate
Decreasing temperature
Stress concentration conditions produced by a notch.

17. Salient Features of Brittle Fracture
Brittle fracture occurs when a small crackle in materials grows. Growth continues until fracture occurs.
The atoms at the surfaces do not have as many neighbors as those in the interior of a solid and therefore they form fever bonds. That implies, surface atoms are at a higher energy than a plane of interior atom. As a result of Brittle fracture destroying the inter atomic bonds by normal stresses.
In metals brittle fracture is characterized by rate of crack propagation with minimum energy of absorption.
In brittle fracture, adjacent parts of the metal are separated by stresses normal to the fracture surface.
Brittle fracture occurs along characteristics crystallographic planes called as cleavage planes. The fracture is termed as cleavage fracture.
Brittle fracture does not produce plastic deformation, so that it requires less energy than a ductile failure.

18. Ductile Fracture
Ductile fracture is defined as the fracture which takes place by a slow propagation of crack with considerable amount of plastic deformation.
An important characteristic of ductile fracture is that it occurs through a slow tearing of the metal with the expenditure of considerable energy.
The fracture of ductile materials can also explained in terms of work-hardening coupled with crack-nucleation and growth.
The initial cavities are often observed to form at foreign inclusions where gliding dislocations can pile up and produce sufficient stress to form a void or micro-crack.
Consider a specimen subjected to slow increasing tensile load. When the elastic limit is exceeded, the material beings to work harden.
Increasing the load, increasing the permanent elongation and simultaneously decrease the cross sectional area.
The decrease in area leads to the formation of a neck in the specimen, as illustrated earlier.

19. Ductile Fracture
The neck region has a high dislocation density and the material is subjected to a complex stress.
The dislocations are separated from each other because of the repulsive inter atomic forces.
As the resolved shear stress on the slip plane increase, the dislocation comes closed together.
The crack forms due to high shear stress and the presence of low angle grain boundaries.
Once a crack is formed, it can grow or elongated by means of dislocations which slip.
Crack propagation is along the slip plane for this mechanism.
Once crack grows at the expense of others and finally cracks growth results in failure.

20. Ductile Fracture
various stages in ductile fracture

21. Ductile fracture Brittle fracture
Material fractures after plastic deformation and slow propagation of crack
Material fractures with very little or no plastic deformation.
Surface obtained at the fracture is dull or fibrous in appearance
Surface obtained at the fracture is shining and crystalling appearance
It occurs when the material is in plastic condition.
It occurs when the material is in elastic condition.
It is characterized by the formation of cup and cone
It is characterized by separation of normal to tensile stress.
The tendency of ductile fracture is increased by dislocations and other defects in metals.
The tendency brittle fracture is increased by decreasing temperature, and increasing strain rate.
There is reduction in cross – sectional area of the specimen
There is no change in the cross – sectional area.

22. Fatigue Fracture
Fatigue fracture is defined as the fracture which takes place under repeatedly applied stresses.
It will occur at stresses well before the tensile strength of the materials.
The tendency of fatigue fracture increases with the increase in temperature and higher rate of straining.
The fatigue fracture takes place due to the micro cracks at the surface of the materials.
It results in, to and fro motion of dislocations near the surface.
The micro cracks act as the points of stress concentration.
For every cycle of stress application the excessive stress helps to propagate the crack.
In ductile materials, the crack grows slowly and the fracture takes place rapidly.
But in brittle materials, the crack grows to a critical size and propagates rapidly through the material.

23. Creep Fracture
Creep fracture is defined as the fracture which takes place due to creeping of materials under steady loading.
It occurs in metals like iron, copper & nickel at high temperatures. The tendency of creep fracture increases with the increase in temperature and higher rate of straining.
The creep fracture takes place due to shearing of grain boundary at moderate stresses and temperatures and movement of dislocation from one slip to another at higher stresses and temperatures.
The movement of whole grains relation of each other causes cracks along the grain boundaries, which act as point of high stress concentration.
When one crack becomes larger it spreads slowly across the member until fracture takes place.
This type of fracture usually occurs when small stresses are applied for a longer period.
The creep fracture is affected by grain size, strain hardening, heat treatment and alloying.

24. ROCKWELL HARDNESS TEST
The Rockwell hardness test method consists of indenting the test material with a diamond cone or hardened steel ball indenter.
The indenter is forced into the test material under a preliminary minor load usually 10 kg. When equilibrium has been reached, an indicating device, which follows the movements of the indenter and so responds to changes in depth of penetration of the indenter is set to a datum position.
While the preliminary minor load is still applied an additional major load is applied with resulting increase in penetration.
When equilibrium has again been reach, the additional major load is removed but the preliminary minor load is still maintained. Removal of the additional major load allows a partial recovery, so reducing the depth of penetration.
The permanent increase in depth of penetration, resulting from the application and removal of the additional major load is used to calculate the Rockwell hardness number.

29. BRINELL HARDNESS TEST
The Brinell hardness test method consists of indenting the test material with a 10 mm diameter hardened steel or carbide ball subjected to a load of 3000 kg.
For softer materials the load can be reduced to 1500 kg or 500 kg to avoid excessive indentation.
The full load is normally applied for 10 to 15 seconds in the case of iron and steel and for at least 30 seconds in the case of other metals.
The diameter of the indentation left in the test material is measured with a low powered microscope.
The Brinell harness number is calculated by dividing the load applied by the surface area of the indentation.

30. VICKERS HARDNESS TEST
The Vickers hardness test method consists of indenting the test material with a diamond indenter, in the form of a right pyramid with a square base and an angle of 136 degrees between opposite faces subjected to a load of 1 to 150 kg.
The full load is normally applied for 10 to 15 seconds. The two diagonals of the indentation left in the surface of the material after removal of the load are measured using a microscope and their average calculated.
The area of the sloping surface of the indentation is calculated. The Vickers hardness is the quotient obtained by dividing the kg load by the square mm area of indentation.

31. F= Load in kgf d = Arithmetic mean of the two diagonals, d1 and d2 in mm HV = Vickers hardness

34. Tensile Test
The tensile test can be conducted with either a round bar or sheet specimen.
The round bar specimen used for the current test complies with the ASTM standards.
A 2 inch gage length is marked on the specimen prior to testing.
The specimen is held in the clamps at either end. Load and movement are applied to the bottom clamp.

35. Tensile Test
Step 1: Original shape and size of the specimen with no load.
Step 2: Specimen undergoing uniform elongation.
Step 3: Point of maximum load and ultimate tensile strength.
Step 4: The onset of necking (plastic instability).
Step 5: Specimen fractures.
Step 6: Final length.

36. Tensile Test Primary Test Output:
The primary output from a tensile test is the load vs. elongation curve of the specimen, which is recorded in real-time using a load cell and an extensometer. This curve is then used to determine two types of stress-strain curves:
Engineering stress-strain.
True stress-strain. Lu Lf

37. Tensile Test Machine

38. Tensile Test
Mark a 2 inch gage length on the tensile test specimen using the dial calipers and marker.
Measure the diameter of the specimen using dial calipers.
Load specimen in the machine grips and remove most of the slack by moving the lower crosshead.
Attach and zero the extensometer; secure it with a lanyard so it will not fall and break if specimen fracture occurs before the extensometer can be removed.
Zero the load indicator and open the right side hydraulic valve about ½ turn.

39. Tensile Test
As the sample is loaded, close the valve and record the load and elongation at regular load intervals (e.g. every 1000 pounds) up to the yield point (when the load starts increasing more slowly and the strain starts increasing more rapidly).
Continue to load the sample until the extensometer range is exceeded, then remove the extensometer.
Continue to load the sample until it breaks; pay close attention to the load indicator and record the load at failure.
Observe and record the maximum load on the follower needle.
Using the dial calipers, measure the final gage length and gage diameter of the fractured specimen (note: when you calculate the fracture strength, use the fracture area calculated from the measured final diameter).

40. Compressive stress
Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume).
Usually compressive stress is applied to bars, columns, etc.

41. Shear stress
Shear stress is a stress state where the shape of a material tends to change without particular volume change.
Shear testing involves an applied force or load that acts in a direction parallel to the plane in which the load is applied. Shear loads act differently than, say, tensile or compressive loads that act normal or perpendicular to the axis of loading. Direct shear and torsional shear are important forces used to determine shear properties. Direct or torsional loading depends on the forces a material is expected to be subjected to during service.

47. Impact Testing
A specimen under test will exhibit different properties, depending on the rate at which the load is applied. For example, most materials will exhibit greater strength is the load is applied in a slower, gentler manner (static loading) than suddenly (dynamic). Because properties are strain-rate dependent, tests have been standardized to determine the energy required to break materials used sudden blows. These are termed impact tests.
Impact tests generally involve sudden shock loading that results in breakage of the specimen. The result is calculated based on the energy required to break the specimen and the resultant loss of momentum. This can be calculated if one knows the initial energy and final energy or the initial angle and final angle of the object used to break the specimen. The Izod and Charpy tests are commonly used to measure impact strength. They differ only slightly, the configuration and specifications of the test specimen.

48. Izod Impact Test& Charpy Impact Test
In the Izod impact test, the test piece is a cantilever, clamped upright in an anvil, with a V-notch at the level of the top of the clamp.
The test piece is hit by a striker carried on a pendulum which is allowed to fall freely from a fixed height,
After fracturing the test piece, the height to which the pendulum rises is recorded by a slave friction pointer mounted on the dial, from which the absorbed energy amount is read.
Charpy Impact Test:
The principle of the test differs from that of the Izod test in that the test piece is tested as a beam supported at each end; a notch is cut across the middle of one face, and the striker hits the opposite face directly behind the notch.

49. The pendulum continues its swing, rising a maximum height h ' which should be lower than h naturally.
The energy absorbed at fracture E can be obtained by simply calculating the difference in potential energy of the pendulum before and after the test such as,
E = m.g.(h-h ')
where m is the mass of pendulum and g is the gravitational acceleration.
The geometry of 55mm long, standard Charpy test specimen.

50. CHARPY IMPACT TEST
The Charpy impact test, also known as the Charpy v-notch test, is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture.
This absorbed energy is a measure of a given material's toughness and acts as a tool to study temperature-dependent brittle-ductile transition. It is widely applied in industry, since it is easy to prepare and conduct and results can be obtained quickly and cheaply.
But a major disadvantage is that all results are only comparative. The apparatus consists of a pendulum axe swinging at a notched sample of material. The energy transferred to the material can be inferred by comparing the difference in the height of the hammer before and after a big fracture.
The notch in the sample affects the results of the impact test, thus it is necessary for the notch to be of regular dimensions and geometry.
The size of the sample can also affect results, since the dimensions determine whether or not the material is in plane strain. This difference can greatly affect conclusions made.

53. Impact Testing (a) Charpy impact testing machine.
(b) Charpy impact test specimen.
(c) Izod impact test specimen.

54. Creep Testing machine

55. Creep Test
The slow and progressive deformation of a material with time at constant stress
Creep is found to occur at higher temperature than at lower temp.
Therefore the study of creep is very important for those materials which are used at high temp like components of gas turbines, furnaces, rockets, missiles etc.

56. Stages of creep Instantaneous deformation, mainly elastic.
Primary/transient creep. Slope of strain vs. time decreases with time: work-hardening
Secondary/steady-state creep. Rate of straining constant: work-hardening and recovery.
Tertiary. Rapidly accelerating strain rate up to failure: formation of internal cracks, voids, grain boundary separation, necking, etc.

58. Failure under fluctuating stress
Fatigue
Failure under fluctuating stress
Under fluctuating / cyclic stresses, failure can occur at lower loads than under a static load.
90% of all failures of metallic structures (bridges, aircraft, machine components, etc.)
Fatigue failure is brittle-like –
even in normally ductile materials. Thus sudden and catastrophic!

59. Fatigue: Cyclic Stresses
Characterized by maximum, minimum and mean
Range of stress, stress amplitude, and stress ratio
Mean stress m = (max + min) / 2
Range of stress r = (max - min)
Stress amplitude a = r/2 = (max - min) / 2
Stress ratio R = min / max
Convention: tensile stresses  positive
compressive stresses  negative

60. Fatigue: S—N curves (III)
Most alloys: S decreases with N.
Fatigue strength: Stress at which fracture occurs after specified number of cycles (e.g. 107)
Fatigue life: Number of cycles to fail at specified stress level

61. Fatigue: Crack initiation+ propagation (I)
Three stages:
crack initiation in the areas of stress concentration (near stress raisers)
incremental crack propagation
rapid crack propagation after crack reaches critical size
The total number of cycles to failure is the sum of cycles at the first and the second stages:
Nf = Ni + Np
Nf : Number of cycles to failure
Ni : Number of cycles for crack initiation
Np : Number of cycles for crack propagation
High cycle fatigue (low loads): Ni is relatively high. With increasing stress level, Ni decreases and Np dominates

62. Fatigue: Crack initiation and propagation (II)
Crack initiation: Quality of surface and sites of stress concentration
(microcracks, scratches, indents, interior corners, dislocation slip steps, etc.).
Crack propagation
I: Slow propagation along crystal planes with high resolved shear stress. Involves a few grains.
Flat fracture surface
II: Fast propagation perpendicular to applied stress.
Crack grows by repetitive blunting and sharpening process at crack tip. Rough fracture surface.
Crack eventually reaches critical dimension and propagates very rapidly

63. Factors that affect fatigue life
Magnitude of stress
Quality of the surface
Solutions:
Polish surface
Introduce compressive stresses (compensate for applied tensile stresses) into surface layer.
Shot Peening -- fire small shot into surface
High-tech - ion implantation, laser peening.
Case Hardening: Steel - create C- or N- rich outer layer by atomic diffusion from surface
Harder outer layer introduces compressive stresses
Optimize geometry
Avoid internal corners, notches etc.