(Machines, structures) are often subjected to forces/ deformations, resulting in stresses/strains, the properties of materials under the action of forces and deformations becomes an important engineering consideration.

The properties of materials when subjected to stresses and strains are called mechanical properties. In other words the properties that determine the behavior of engineering mats under applied forces are called mechanical properties.

The response of a material to applied forces depends on the type and nature of the bond and the structural arrangement of atoms, molecules or ions.

Basic deformation types for load carrying materials are:Elastic deformation (deformations are instantaneously recoverable)Plastic deformation (non-recoverable)Viscous deformation (time dependent deformation)

Elastic means reversible!1. Initial2. Load3. UnloadFdbonds stretchreturn to initial shapeReturn to the original shape when the applied load is removed.

Plastic means permanent!1. Initial2. Load3. Unloadplanes still shearedFdelastic + plasticbonds stretch & planes sheardplasticFdlinear elasticlinear elasticdplasticdelasticCould not return to the original shape when the applied load is removed.

Plastic deformations in noncrystalline solids (as well as liquids) occurs by a viscous flow mechanism. Usually attributed to fluids. But solids may also behave like viscous materials under high temperature and pressure. Viscous materials deform steadily under stress. Deformations are time dependent.

Based on the abovementioned deformation characteristics, several material idealizations could be made. Such as:Elastic MaterialsPlastic Materials Elastoplastic MaterialsViscoelastic Materials

Return to the their original shape when the applied load is removed.UnloadingdPLoading

No deformation is observed up to a certain limit. Once the load passes this limit, permanent deformartions are observed. PLimitPlastic deformationUnloadingLoading

Up to a limit shows elastic properties. Within this limit if the load is removed, returns to its original shape. If the load passes the limit, plastic deformations are observed.Plastic deformationElastic deformationPElastic Limit

Deformations are time-dependent.PSlowLoading-UnloadingFast Loading-Unloading

The physical properties of some substances depend on the crystallographic direction in which the measurements are taken.This directionality of properties is termed as anisotropy, and it is associated with the variance of atomic or ionic spacing with crystallographic direction. Substances in which the measured properties are independent of the direction of measurement are called isotropic.

Isotropic materials have the same mechanical properties in all directions.Anisotropic materials show different behavior in different directions.Isotropic Materials (METALS)12Anisotropic Materials (WOOD)1= 21 2

Modulus of Elasticity, E: Hooke's Law: For elastic materials, stress is linearly proportional to strain and is independent of time.

s = E esLinear- elasticEeFFsimple tension test

ElasticityPlasticityFractureFatigueMechanical BehaviourCreepElongation at constant load at High temperaturesNote: above is a broad classification for convenience. E.g. Creep is also leads to plastic deformation!Recoverable deformationPermanent deformationPropagation of cracks in a materialOscillatory loading

ElasticPlasticDeformationInstantaneousTime dependentInstantaneousTime dependentRecoverablePermanentAnelasticityViscoelasticity

ElasticityElasticityLinearNon-linearE.g. Al deformed at small strainsE.g. deformation of an elastomer like rubberElastic deformation is reversible deformation- i.e. when load/forces/constraints are released the body returns to its original configuration (shape and size).Elastic deformation can be caused by tension/compression or shear forces.Usually in metals and ceramics elastic deformation is seen at low strains (less than ~103).The elastic behaviour of metals and ceramics is usually linear.

r Force r0For displacements around r0 Force-displacement curve is approximately linear THE LINEAR ELASTIC REGIONNear r0 the red line (tangent to the F-r curve at r = r0) coincides with the blue line (F-r) curve Elastic modulus is the slope of the Force-interatomic spacing curve (F-r curve), at the

Stress strain CompressionTensionStress-strain curve for an elastomerDue to efficient filling of spaceT due to uncoiling of polymer chainsCTCT>

ELASTIC MODULII = E.E Youngs modulus = G.G Shear modulus hydrodynami = K.volumetric strainK Bulk modulus

Anisotropy in the Elastic modulus In a crystal the interatomic distance varies with direction elastic anisotropy

Elastic anisotropy is especially pronounced in materials with two kinds of bonds Two kinds of ordering along two directions

Stiffness of a material is its ability to resist elastic deformation of deflection on loading depends on the geometry of the component. High modulus in conjunction with good ductility should be chosen (good ductility avoids catastrophic failure in case of accidental overloading) Covalently bonded materials- e.g. diamond have high E (1140 GPa) BUT brittle Ionic solids are also very brittle Elastic modulus in design

Ionic solids NaClMgOAl2O3TiCSilica glassYoungs Modulus (GN / m2)3731040230870

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