Processing of Aerospace
Materials – Part I (ME-772)
Lecture 2:
Semi-products, materials and tensile properties
7 January 2022
Ref:
Amol A. Gokhale, Professor, Mechanical Engineering 1. Pradip K Saha book: Chapter 3
Room S05 ME Department, extension 7399 2. A P Mouritz book: 5.1, 5.2
gokhale@iitb.ac.in
Semi-finished product forms used
, Plate
(Ribs are along the chord, spars along web)
*only external components are
mentioned above. We will deal
with internal components later
in the course.
Aircraft structure consists of
Materials selection is based on the following criteria:
• Higher tensile/compressive properties (strength,
stiffness)
• Tolerance to flaws under static loading (high fracture
toughness) and under fluctuating loading (high fatigue
crack growth resistance)
• Corrosion resistance and long fatigue life- cycles-to-
failure
• Lower density
• Producibility - sheet, plate, extrusion, forging, and tube
Which properties need to be balanced
1. Elastic modulus (stiffness): resistance to elastic stretching, bending and buckling
2. Tensile, compression, and shear yield strength: resistance to plastic (permanent)
deformation under monotonic loading
3. Fracture toughness: resistance to catastrophic failure in monotonic or cyclic
(fatigue) loads in structures containing flaws
damage tolerance
4. Fatigue crack initiation resistance: resistance to the initiation of cracks in an
Durability and
otherwise defect free material/component under cyclic (fatigue) loading under
elastic or near-plastic loads
5. Fatigue crack propagation resistance: resistance to propagation of existing cracks
under cyclic elastic or near- plastic loading
6. Resistance to degradation in service environments by various types of corrosion:
Galvanic corrosion, Exfoliation corrosion and Stress corrosion cracking
7. There may be other properties important in materials selection
Major aircraft
materials
Matrix:
Non-ferrous metals continuous
• Aluminum alloys phase
• Titanium alloys
• Nickel alloys
Ferrous metals
• High strength alloy steel
• Stainless steel
Non-metals
• Composite materials
(90-95 C)
Tensile test
It gives the following properties
• Modulus of elasticity (stiffness) (resistance to elastic deformation)
• Yield strength (0.2% Proof Strength) (minimum stress to initiate
plastic deformation)
• Ultimate Tensile Strength (stress where plastic instability occurs)
• Elongation to failure (%ductility) (total plastic / permanent strain
before failure)
• Strain hardening coefficient (a measure of hardening caused by
increase in strain. As we strain a material, it becomes more resistant
to further straining)
• Lower wing skin and fuselage frames, bulkheads and stringers experience
tensile loading
• Mainly applied in case of metals, alloys and polymers
• Rarely used for ceramics, since they are brittle and may fail during
alignment of the specimens
Tensile Testing: Universal Testing Machine
Load Cell
Usually done at strain rates of 10-4 to 10-2 s-1
Known as ‘quasi static’ conditions
Definitions
Engineering stress, where F is the
applied load and A0 is the original
cross sectional area.
Engineering strain
True stress, where F is the applied
load and Ai is the instantaneous
cross sectional area.
True strain
F: Newtons, N • In tensile tests, true and engineering strains do not vary much.
A: m2 But in metal working operations (usually under compressive
: N/m2 or Pa or MPa loading to large strains), these strains can differ significantly,
: unit-less or in % and true strain gives more representative picture of the strain.
• True strains are additive in nature and hence are used to
describe metal working operations which involve a series of
successive strains
Elastic modulus
• A measure of stiffness
• Usually high stiffness materials are preferred for structural
applications
• (For seals, low stiffness rubbers are preferred)
• Hooke’s law: Law: The strain in a solid is proportional to the
applied stress within the elastic limit of that solid.
• is closely linked to binding energy (force of attraction) between
atoms
• Elastic modulus is material dependent, and does not depend on
heat treatment or mechanical working, grain size, dislocation
density etc. It can change by alloying additions
Yield strength (= proof strength)
• The point when a material changes from elastic to plastic behaviour is
called the yield point (also known as the elastic limit).
• In metals: yielding signifies beginning of crystallographic slip (to be taught)
• In polymers, it indicates permanent disentanglement and sliding of the
polymer chains
All materials do not show sharp change in slope at the yield point.
Hence the yield strength is defined as the stress at which a fixed plastic strain (conventionally
decided as 0.2%) occurs (called as the ‘Proof Strength’). See the construction to arrive at the
proof strength.
In mild steels, a yield point phenomenon occurs which allows determination of upper and
lower yield points
Al-alloys
Mild steels
Brittle vs ductile behaviour Geometric
softening region
Brittle Ductile
Strain hardening region
0.2% PS
e.g. ceramic
e.g. metal, plastic
Total
%El: ductility, lf = final length, Plastic elongation to failure =
l0 = original length ductility
• In general, under service conditions, acting stress must be less than 0.2%PS
• During metal working operations, applied stress should be higher than 0.2%PS
• Due to strain hardening, increasing stress is required to cause plastic deformation
beyond YS (i.e. PS)
Elastic recovery and progress of necking
Another (less common
• Elastic strain recovers after unloading from measure of ductility) is %
plastic region. reduction in area (%RA):
• Reloading retraces the unloading curve in the
elastic region and then catches up with the
original stress strain curve in the plastic
region
• No volume change in plastic deformation A0 is original and Af is the cross
sectional area at fracture
Necking and failure
Localised deformation due to imperfections leads to
necking. Neck requires less load to deform than the
surrounding (because of less cross sectional area).
Hence further deformation takes place
preferentially at the neck, thus leading to localised
deformation and failure.
Materials which show high strain hardening resist
deformation in the neck triggering deformation in
other areas, thus spreading deformation over
different parts of the gage length. This increases
elongation to failure
Stress vs Strain for engineering materials
Materials have a range of yield strengths and elastic modulus
345
0.06
Stress concentration at notches
Flaws may be
generated due to
processing or during
service
• The ratio m/0 is denoted as the stress concentration factor Kt
• Longer and sharper notches give higher stress concentration
• If the notch is a sharp crack, t 0, stress concentration can approach
theoretical cohesive stress (the strength of bonding between the atoms or
surfaces that make up that material), leading to catastrophic failure of the
material
= the surface energy per unit area of the surface
Example (W D Callister’s book)
Assignment 2
1. Show that the true stress 𝜎 = 𝜎 1 + 𝜀 under plastic deformation. Note that there is
no volume change in plastic deformation.
2. In rolling, a metal slab is passed between two rolls such that the thickness reduces and
the length increases correspondingly, without any change in width. If a 300 mm thick
slab is rolled to 10 mm thick plate. Calculate the total engineering and true strains.
Now, roll the same slab in three stages i.e. 300 to 100 mm, 100 to 50 mm and 50 to 10
mm thickness. Calculate engineering and true strains in each stage. Compare the
total engineering and true strain values obtained in rolling directly from 300 to 10 mm
with the corresponding values obtained by adding strains in multiple stages of rolling.
Are they different? Similar?
