Aircraft materials lecture 1

Mechanical Behaviour of Engineering
Materials
LECTURE 1
By: Kanu Priya Jhanji
Asst. Professor
School of Aeronautical Sciences
Hindustan University
kanupriyaj@hindustanuniv.ac.in
AIRCRAFT MATERIALS
UNIT-1

Contents
 Introduction
 Mechanical properties of materials:
Elasticity
Plasticity
Ductility
Brittleness
Hardness
Toughness
Stiffness
Resilience
Endurance
Strength
Creep
SCHOOL OF AERONAUTICAL SCIENCES
HINDUSTAN UNIVERSITY

Instructional objectives
 By the end of this lecture, the students will learn the importance of
materials and their basic mechanical properties like strength,
stiffness, ductility, hardness etc.

Introduction
 Materials are found everywhere including human body for
eg. Blood, flesh, bones, buildings, even the pen with which
you write is material.
 It is possible to study each and every material on the earth
in one semester course. This course is limited to the in-
animated solids which are used in aircraft industries.
 It means, the living things as well as fluids (liquids and
gases) are excluded from this course.

Mechanical Properties of materials
 To understand the behavior of material, it is necessary to
have knowledge about various mechanical properties of
the materials.
 It gives the idea about the suitability of the material for
making any member like machine part, structural member
etc.

Elasticity
 Elasticity is that property that enables a metal to return to its original
size and shape when the force which causes the change of shape is
removed.
 This property is extremely valuable because it would be highly
undesirable to have a part permanently distorted after an applied load
was removed.
 Each metal has a point known as the elastic limit, beyond which it
cannot be loaded without causing permanent distortion.
 In aircraft construction, members and parts are so designed that the
maximum loads to which they are subjected will not stress them
beyond their elastic limits. This desirable property is present in spring
steel.

Plasticity
 It is defined as the property of a material by virtue of which, a
permanent deformation (without fracture) takes place whenever
it is subjected to action of external deforming forces or load.
 Thus, after the elastic limit if the load is increased, the material
is no longer capable of regaining its shape and size and a
permanent set of permanent deformation occurs.
 Metals like lead, copper, zinc possess good plasticity.
 By means of this property, metals can be shaped into various
components and machine parts without fracture and cracking.

Ductility
 Ductility is the property of a metal which permits it to be permanently
drawn, bent, or twisted into various shapes without breaking.
 This property is essential for metals used in making wire and tubing.
 Ductile metals are greatly preferred for aircraft use because of their
ease of forming and resistance to failure under shock loads.
 For this reason, aluminium alloys are used for cowl rings, fuselage and
wing skin, and formed or extruded parts, such as ribs, spars, and
bulkheads.
 Chrome molybdenum steel is also easily formed into desired shapes.
Ductility is similar to malleability

Brittleness
 Brittleness is the property of a metal which allows little bending or
deformation without shattering.
 A brittle metal is apt to break or crack without change of shape.
Because structural metals are often subjected to shock loads,
brittleness is not a very desirable property.
 Cast iron, cast aluminium, and very hard steel are examples of
brittle metals.

Hardness
 Hardness refers to the ability of a material to resist abrasion,
penetration, cutting action, or permanent distortion.
 Hardness may be increased by cold working the metal and, in the
case of steel and certain aluminium alloys, by heat treatment.
 Structural parts are often formed from metals in their soft state
and are then heat treated to harden them so that the finished
shape will be retained.
 Hardness and strength are closely associated properties of
metals.

Toughness
 Toughness is the property of a material by virtue of which it can
absorb maximum energy before fracture takes place.
 Thus, it is capacity of material to withstand shock loads.
 A material which possesses toughness will withstand tearing or
shearing and may be stretched or otherwise deformed without
breaking.
 Toughness is a desirable property in aircraft metals.

Stiffness
 Stiffness is the property of material by virtue of which, it resists
deformation.
 Modulus of elasticity is a measure of stiffness of a metal.
 Materials (steels) having high stiffness are used in spring
controlled measuring instruments.

Resilience
 Resilience is the property of materials by virtue of which it stores
energy and resists shocks and impacts.
 The resilience of the material is measured by the amount of energy
that can be stored per unit volume after it is stressed upto the
elastic limit.

Endurance
 The endurance is the property of a material by virtue of which it
can withstand varying stresses or repeated application of stress.
 It is important property in the design and production of parts in a
reciprocating machine or components subjected to vibrations
 The endurance limit or fatigue strength is the maximum stress
that can be applied for indefinitely large number of times without
causing failure.
 The failure of a material under repeated loads is called fatigue
failure.

Strength
 It is the property of a material by virtue of which it resists or
withstands the application of an external force or load without
rupture.
 A metal has different types of strengths.
 Depending upon the value of stress, the strengths of a metal
may be elastic or plastic.
 Depending upon the nature of stress, the strengths of a metal
may be tensile, compressive, shear, bending and torsional.

The Elastic Strength
 It is the value of stress or strength which corresponds to
the transition from elastic range to plastic range.
 Thus, elastic limit is used to define the elastic strength of a
material.

The Plastic Strength
 It is the value of stress or strength corresponding to plastic range
and rupture, it is also called ultimate strength.
 Working stress is the greatest value of stress to which a material is
subjected to as a machine part or a part of structure during
operation or working.
 Normally working stress is kept below the elastic limit of a material.
 Safety factor = Ultimate Stress
Working Stress

Tensile Strength and Compressive Strength
 Tensile strength the maximum value of tensile stress, under a
steady load, that a material can withstand before fracture or
breaking.
Tensile stress = Maximum Tensile Load
Original cross-sectional area
 It is also called as ultimate tensile strength.
 Usually tensile strength of metals and alloys increases on cooling
and decreases on heating.
 Compressive Strength of a material is the maximum value of
compressive stress applied to break it off by crushing.
 Compressive stress = Maximum compressive load

Shear Strength
 The shear strength of a material is the maximum value of
tangential stress applied to shear it off across the resisting
section.
 Shear stress = Maximum tangential load
Original cross sectional area
 When the application of an external force on a body tends to
cause relative movement of the layers, shear stress results.

Bearing Strength and Torsional Strength
 Bending strength of a material is the maximum value of the
bending stress applied to break it off by bending across the
resisting section
 Bending stress = Maximum bending load
 Torsional strength of a material is the maximum value of stress
applied to break it off by twisting across the resisting section.
 Torsional stress = Maximum twisting load
The twisting stress is torsion.

Creep
 It is defined as the tendency of a material to slowly deform
permanently under the influence of stress
 This yielding (increase of strain without increase in load) may
continue to the point of fracture.
 Rate of deformation depends on exposure time and temperature.
 Usually creep occurs at high temperatures.
 This property is exhibited by iron, nickel, copper and their alloys at
elevated temperatures.
 But zinc, tin, lead and their alloys show creep at room
temperature.

 In metals creep is a plastic deformation caused by slip occurring
along crystallographic planes in the individual crystals together with
some deformation of the grain boundary material.
 After complete release of load, a small fraction of this plastic
deformation is recovered with time.
 Thus, most of the deformation is non-recoverable
 Creep limit is defined as the maximum static stress that will result in
creep at a rate lower than some assigned rate at a given
temperature

Stages of creep
 This stage is the most understood. The
characterized "creep strain rate"
typically refers to the rate in this
secondary stage.
 Stress dependence of this rate depends
on the creep mechanism.
 In tertiary creep, the strain rate
exponentially increases with stress
because of necking phenomena.
 Fracture always occur at the tertiary
stage.
 Creep is a very important aspect of
material science.
 In the initial stage, or primary creep, the
strain rate is relatively high, but slows with
increasing time. This is due to work
hardening.
 The strain rate eventually reaches a
minimum and becomes near constant. This
is due to the balance between work
hardening and annealing (thermal softening).
This stage is known as secondary or steady-
state creep.

Aircraft materials lecture 1

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