Aircraft materials lecture 1

AIRCRAFT MATERIALS UNIT-1

Mechanical Behaviour of Engineering MaterialsLECTURE 1By: Kanu Priya JhanjiAsst. ProfessorSchool of Aeronautical SciencesHindustan [email protected] MATERIALSUNIT-1

Contents IntroductionMechanical properties of materials:ElasticityPlasticityDuctilityBrittlenessHardness ToughnessStiffnessResilienceEnduranceStrengthCreep

SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

Instructional objectivesBy the end of this lecture, the students will learn the importance of materials and their basic mechanical properties like strength, stiffness, ductility, hardness etc.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

IntroductionMaterials 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 materialsTo 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.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

ElasticityElasticity 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.


PlasticityIt 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.


DuctilityDuctility 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 malleabilitySCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

BrittlenessBrittleness 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.


HardnessHardness 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.


ToughnessToughness 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.


StiffnessStiffness 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.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

ResilienceResilience 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.


EnduranceThe 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 vibrationsThe 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.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

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.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

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.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

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 StressSCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

Tensile Strength and Compressive StrengthTensile 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 areaIt 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 Original cross-sectional area


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 areaWhen the application of an external force on a body tends to cause relative movement of the layers, shear stress results.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

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 loadOriginal cross-sectional area

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 Original cross-sectional areaThe twisting stress is torsion.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

CreepIt is defined as the tendency of a material to slowly deform permanently under the influence of stressThis 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.SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITY

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-recoverableCreep 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 SCHOOL OF AERONAUTICAL SCIENCESHINDUSTAN UNIVERSITYThis 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 ofneckingphenomena.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 towork 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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