Strength of Materials

In engineering point of view the terms Strength of materials may be defined as one of the major parts in engineering and science.Generally this engineering methodology deals with nature of behavior  of various materials of failure under various action and forces.

Definition of Stress

Whenever any external forces applies on a body or mass , on that same time equal and opposite forces try to react and resist at various section of the body.This internal forces per unit area at any section of the body is defined as Stress.

Basics of  STRENGTH OF MATERIAL

MECHANICAL PROPERTY OF MATERIAL

  • Strength

The capacity of material to withstand load is called Strength.

Ability to sustain under loading condition without undue distortion, rupture or any collapse.

Maximum stress that any material can withstand is called Ultimate Strength.

  • Elasticity

On material, external load is applied, it undergoes deformation and on removal of load, it returns back to it’s original shape. This property of material is called Elasticity.

  • Plasticity

If material does not regain its original shape, on removal of the external load.

Ability of material to deform without breaking is called Plasticity.

  • Ductility

If material can undergo a considerable deformation without rupture, it is called Ductility.

Ductile material is most suitable material for tension member.

Tension test performed on ductile material.

Stainless steel is most ductile material & Grey cast iron is least ductile material.

Higher the percentage of elongation, more ductile is material.

As temperature increase, ductility increase.

Metals having elongation more than 15% are ductile.

  • Brittleness

If material can not undergo any deformation and it fails by rupture.

Stronger in compression and weak in tension.

Materials having less than 5% elongation are considered as brittle.

Compression test performed on brittle material.

  • Malleability

Property by which material can be converted in to thin sheets by hammering.

Material can easily rolled & forged without cracking.

Ex.:- Gold, Silver, Copper, Aluminium, Tin, Lead, Steel etc.

Gold is most malleable material while C.I is the least.

  • Toughness

Resistance to impact or shock loading.

Capacity of material to absorb energy before rupture.

Work required to cause rupture.

Ability to withstand large stress and strains without fracture.

Ex.:- Mild steel, W.I., Manganese etc.

Steel used for cutting tools = High carbon steel.

  • Hardness

Resistance to abrasion, wear or scratches.

C.I. is hardest material.

Magnesium alloy is softest material.

  • Stiffness

Resistance to deformation or strain.

Force required to produce unit deformation.

Stiffness measured by modulus of elasticity.

Higher E, more stiffness.

Steel is stiffer material.

  • Creep

Increase in strain under sustained load.

Inelastic deformation due to sustained load.

Creep phenomena used for steel cables, nuclear reactor field etc.

Slow extension of material with time under load.

  • Fatigue Strength

Maximum stress which material can withstand under repeated stress cycles without fracture is called Fatigue Strength.

  • Endurance Limit :- Stress at which a material factures under large number of reversals of stress is called endurance limit.

 

  • Homogeneous material :- Material of the member is of the same kind through its length.

 

  • Isotropic material :- It possesses the same elastic properties in all the direction.
  • STRESS

On a body when external force is applied, it undergoes some deformation and internal resisting forces are set up. This resistance to force per unit area is called Stress.

Stress = Force/ Area = P/A = ς

S.I. Unit = N/mm2 (MPa) 1 Pa = 1 N/m2

  • STRAIN

Ratio of change in length to the original length.

Strain = Change in length /Original length = δl/l = ε

No unit.

Direct or Normal Stress :- The stresses which act normal to plane on which the forces act are called Normal or Direct Stress.

  • TENSION STRESS

Body subjected to two equal & opposite pulls, the stress produced is Tensile stress.

Stress is tensile, corresponding strain is Tensile Strain.

  • COMPRESSIVE STRESS

Body subjected to two equal & opposite pushs, the stress produced is Compressive Stress.

Stress is compressive, corresponding strain is Compressive Strain.

  • Elastic Limit :- Maximum value of force up to and within which to deformation entirely disappears on removal of force is elastic limit.
  • Hooke’s Law :- “Within elastic limit, Stress is proportional to Strain.”

MODULUS OF ELASTICITY

Also known as Young’s Modulus.

Ratio of Direct stress to Direct strain.

M.O.E. = Stress/Strain = ς/ε

S.I. Unit = N/mm2

For Mild Steel E = 2 x 105 N/mm2

Modulus of elasticity is Highest for Steel.

Modulus of elasticity is least for Wood.

For Perfectly rigid body, M.O.E. = Infinity.

Reciprocal of M.O.E is called Coefficient of Compressibility (StrainStress).

Relation between δl & E is equal to δl = Pl/AE

Deformation of body due to self weight for uniform prismatic bar = δl = wl2/2AE

Deformation of body due to self weight for uniform conical bar = δl = wl2/6AE

Stresses in bars of uniformly tapering circular section = δl = 4Pl/πEd1d2

Modular ratio :- Modular ratio of two material is the ratio of theirs modulus of elasticises.

The direction of force = Linear Direction.

The direction perpendicular to linear direction = Lateral Direction.

LINEAR STRAIN (ε)

Linear strain = Change in lengthoriginal length = δl/l

Strain has No unit.

Also called Longitudinal strain.

Strain rosetters are used to measure linear strain.

LATERAL STRAIN (ε’)

Lateral strain = Change in diameter/original diameter = δd/d

Lateral strain is always less than linear strain.

POISSON’S RATIO (μ)

Poisson’s ratio = Lateral strain/Linear strain = 1/m

For steel μ = 0.23 to 0.27.

For cast iron μ = 0.25 to 0.33.

For concrete μ = 0.08 to 0.18.

This ratio is not greater than 0.5 for any material.

Poisson’s ratio is Highest for Rubber.

If modulus of elasticity = 2 x modulus of rigidity, then μ=0.

VOLUMETRIC STRAIN (εv)

Volumetric strain = Change in volume/Original volume = δv/V = εv

Equation to find change in volume = ε (1 – 2μ) = ε (1 – 2/m)

BULK MODULUS (k)

When body is subjected to three mutually perpendicular stresses of equal intensity, the ratio of direct stress to volumetric strain is Bulk modulus.

k = Direct stress/Volumetric strain=ς/εv

Unit = N/mm2

MODULUS OF RIGIDITY (G or N or C)

Also known as Shear Modulus.

C = Shear stress/Shear strain = τ/∅

Unit = N/mm2

Bar fixed at ends is cooled, it will develop Tensile Stress.

Bar fixed at ends is heated, it will develop Compressive Stress.

RELATION BETWEEN K & E & G

Between E & K

k = mE/3 (m−2)

Between E & G

G = mE/2 (m+1)

Between G, K & E

E = 9GK/G+3K

SHEAR STRESS (τ)

When body is subjected to two equal and opposite force acting tangentially, body tends to shear off across the section, then the stress induced is called Shear stress.

Shear stress = Shear force/Shearing area = FAs

Unit = N/mm2

THERMAL STRESS (ς)

With increase in temperature material expands, With decrease in temperature material contracts. If this free expansion or contraction is provided, stress will be generated in the material.

Allow expand or contract freely, no stress will be produced in material.

If temperature increased & expansion prevented, compressive stress will be provided.

If temperature decreased & contraction prevented, tensile stress will be provided.

Thermal stress for support do not yield = ς = αtE.

Thermal stress for supports are yielding = ς = αt− (δ/l) E.

End of body yield, magnitude of thermal stress decrease.

Thermal stress in bar of tapering circular section

Maximum stress = ς = αtE (d1/d2) Where, d1 = Larger dia

Minimum stress = ς = αtE (d2/d1),  d2 = Smaller dia.

THERMAL STRAIN (ε)

Thermal strain = Change in length due to change in temperature/Original length = αt

Thermal strain for support do not yield = ε = αt.

Thermal strain for supports are yielding = ε = αt – δ/l

.

STRAIN ENERGY

When body is strained within elastic limit, energy stored in it is called strain energy.

Work done on body = u = (ς2/2E) x V

Unit = N.m.

RESILIENCE

Total strain energy stored in body within elastic limit.

Resilience = u = (ς2/2E) x V

PROOF RESILIENCE (up)

Maximum strain energy that can be stored in body at elastic limit.

Proof resilience = up = (ςE 2/2E) x V

ςE = Stress at elastic limit

MODULUS OF RESILIENCE (um)

Maximum strain energy that can be stored in body per unit volume at elastic limit is called M.O.R.

M.O.R = (ςE )2/2E

Unit = N.mm/mm3

Instantaneous Stress :- When body is subjected to sudden load or impact load, the stress produced.

Gradual Load :- Load increasing gradually from zero to P.

ς = P/A

Sudden Load :- Body is subjected to total load P at a time without small increment.

ς = 2P/A

Impact Load :- Load fall on a body from some height.

STRESS-STRAIN CURVE

Area under curve is indicate energy required to cause failure.

Points on curve

A = Proportional Limit

B = Elastic Limit

C = Upper Yield Point (Yield stress)

D = Lower Yield Point

E = Ultimate Stress (Tensile stress)

F = Breaking Point (nominal)

G = Breaking Point (actual)

 Different stage in curve

OA = Elastic stage

CD = Yield stage

DE = Strain Hardening

EF = Necking (waisting)

At proportional limit, stress is proportional to strain.

At elastic limit, the deformation entirely disappear on removal of the force.

Beyond elastic limit, strain increase more rapidly than stress.

Factor of safety = Ultimate stress

Working stress

F.O.S is always greater than 1.

Yield stress = Yield load/Original c.s area

Ultimate stress = Ultimate stress/Original c.s area

Breaking stress = Breaking stress/Original c.s area

Actual stress = Breaking stress /Final cross sectional area

For mild steel, percentage elongation = 23 to 25%.

For mild steel, percentage reduction in area = 40 to 65%.

GAUGE LENGTH (L0)

Distance between point A & B on stress-strain curve is called Gauge Length.

L0 = 5.65 √A0

BENDING MOMENT

Unit = N.m or kN.m.

B.M is zero at free end of cantilever & at hinged support.

POINT OF CONTRAFLEXURE

The point in B.M diagram at which B.M change sign from +ve to –ve or –ve to +ve.

At point of contra flexure, B.M is zero.

Rate of change of S.F with respect to distance = Intensity of loading.

Rate of change of B.M with respect to distance = S.F at that section.

The point at which S.F changes sign or zero, B.M will maximum.

Slope = θ = Unit =Radian or Degree. ( 1 degree = π/180 )

Deflection = y = Unit = mm or cm.

Flexural Rigidity = Modulus of elasticity x Moment of inertia = EI

SLOPE-DEFLECTION EQUATION

Beam Slope Deflection
Cantilever beam with a point load at the free end Wl2/2EI Wl3/3EI
Cantilever beam with a uniformly distributed load wl3/6EI wl4/8EI
Simply supported beam with central point load Wl2/16EI Wl3/48EI
Simply supported beam with uniformly distributed load wl3/24EI (5/384)(wl3/EI )
Fixed beam with central point load Wl3/192 EI
Fixed beam with uniformly distributed load wl4/384 EI
Cantilever beam with moment at the free end Ml/EI Ml2/2EI
Cantilever beam with partially u.d.l. (7/384)(wl4/EI )
Cantilever beam with gradually varying load (wl4/30EI)
Simply supported beam with eccentric load (Wa2b2/3EIl3 )
Fixed beam with eccentric load

BENDING EQUATION ( Flexure Equation)

M/I= f/y= E/R

MAXIMUM BENDING MOMENT

Beam Maximum Bending Moment
Simply supported beam with central point load Wl/4
Simply supported beam with eccentric load Wab/l
Simply supported beam with uniformly distributed load wl2/8
Cantilever beam with a point load at the free end WL
Cantilever beam with a uniformly distributed load wl2/2

EQUATION OF SHEAR STRESS

τ = FAȳ/Ib

STRUT

Member subjected to axial compressive force.

Used in roof truss and bridge trusses.

C/s area is small.

COLUMN

Strut is vertical, it is known as column.

Used in concrete and steel buildings.

C/s area is large.

RADIUS OF GYRATION

Radius of gyration = k =√ I/A

SLENDERNESS RATIO (λ)

Slenderness ratio = λ

= Effective length of column/Minimum radius of gyration = le/kmin

For column, λ is more then load carrying capacity is less.

For column, λ is less then load carrying capacity is more.

 

LONG COLUMN

Le/d ≥ 12.

Le/kmin ≥ 50.

For mild steel, λ ≥ 80 is called Long Column.

SHORT COLUMN

le/d < 12.

le/kmin < 50.

For mild steel, λ< 80 is called Short Column.

Crushing Load :- The load at which short column fails by crushing is called Crushing Load.

Crippling Load :- The load at which long column starts buckling is called Critical Load or Buckling Load.

Buckling Load is always less than Crushing Load.

COLUMN END CONDITIONS & EFFECTIVE LENGTH

Column ends condition Effective Length
Both ends Hinged le = l
Both ends Fixed le = l/2
One end Fixed & Other end Hinged le = l/√2
One end Fixed & Other end Free le = 2l

Rankine constant = 1/7500 for mild steel column.

 

PRINCIPAL PLANE

Plane on which only direct stress is acting is called Principal Plane.

Shear stress on principal plane is zero.

Major principal plane

Carry maximum direct stress.

Minor principal plane

Carry minimum direct stress.\

ANGLE OF OBLIQUITY (∅)

Angle made by resultant stress with normal stress.

tan ∅ = ςt/ςn

∅ = tan−1 (𝜎𝑡/𝜎𝑛)

Axial Load :- Load acting on the longitudinal axis of column.(e = 0)

ECCENTRIC LOAD

Load acting away from the longitudinal axis of column.

Eccentric load produces both direct stress(ςo) and bending stress(ςb).

Maximum stress = Direct stress + Bending stress

Maximum stress is always Compressive.

Minimum stress = Direct stress – Bending stress

Minimum stress is Compressive if ςo > ςb.

Minimum stress is Tensile if ςo < ςb.

Eccentricity (e)

Horizontal distance between longitudinal axis of column and line of action of load.

Limit of Eccentricity (ςo = ςb)

Maximum distance of load from the centre of column, up to which there is no tensile stress in column is called Limit of eccentricity.

For No Tension condition in column, e ≤ ZA

For Rectangular Section, e ≤ b/6 & e ≤ d/6

For Circular section, e ≤ d/8

PRESSURE AT BASE OF DAM

Weight of dam

w = (a + b) ρ (H/2)

ρ = Density of water = 10 kN/m3 = 1000 kg/m3

Total Water Pressure

P = wh2/2

Eccentricity

e = (d – b/2 )

Maximum pressure

ςmax = (W/b) (1+ 6e/b)

Minimum pressure

ςmin = (W/b) (1− 6e/b )

For No sliding, Resisting force > Sliding force (μW > P)

For No crushing, Maximum pressure < Crushing pressure (ςmax < ςc)

THIN CYLINDRICAL SHELL

If t ≤ d10, then it’s Thin Cylindrical Shell.

If t > d10, then it’s Thick Cylindrical Shell.

Hoop or Circumferential Tensile Stress

Design of thin cylindrical shell is based on Hoop stress.

ςc = pd/2t

Longitudinal Stress

ςl = pd/4t

Circumferential Strain

εc = (pd/2tE) (1 – 1/2m)

Longitudinal Strain

εl = (pd/2tE) (1/2 −1/m)

Volumetric Strain

δvV = (pd/4tE)( 5− 4/m )

Maximum Shear Stress

τmax = pd/8t

THIN SPHERICAL SHELL

Hoop Stress

ςc = pd/4t

Circumferential Strain

εc = (pd/4tE) (1− 1/m)

Volumetric Strain

Δv/V = (pd/2tE)( 1− 1/m)

THICK CYLINDRICAL SHELL

Tangential Stress is Maximum at inner surface & Minimum at outer surface.

Radial Stress is Maximum at inner surface & Zero at outer surface.

Torsion :- When turning force is applied on pulley mounted on the shaft, deformation is produced in the shaft. This deformation is called Torsion.

TORQUE OR TURNING MOMENT OR TWISTING MOMENT (T)

Due to turning force, moment is produced in the shaft, This moment is called Torque.

Unit = N.m or kN.m.

For Circular Shaft, T = (π/16) τ D3

Shear Stress in Shaft :- Shaft is subjected to torque, stress resisting torque is produced in material of shaft. This stress is called Shear stress in shaft.

POLAR MOMENT OF INERTIA (J)

M.I of plane area respect to axis perpendicular to plane of lamina.

Unit = mm4

For Circular Shaft, J = (π/32) D4

POLAR SECTION MODULUS (Zp)

Zp = J/R

Unit = mm3

For Circular Shaft, Zp = (π/16) D3

 

TORSIONAL RIGIDITY

Torsional Rigidity = Modulus of Rigidity x Polar Moment of Inertia = C x T.

Unit = N.mm2

EQUATION OF MOTION

T/J=Cθ/l=τ/R

Tθ = Torsional Rigidity

Bending Stress = ς = 32 M/πD3

Shear Stress = τ = 16 T/πD3

POWER IN H.P

P = 2πNT/4500 h.p

POWER IN WATT

P = 2πNT/60 watt

Elongation of conical bar/ Elongation of prismatic bar = 1/3

In composite bar, Stress produced in part with higher coefficient of thermal expansion is Tensile.

In composite bar, Stress produced in part with lower coefficient of thermal expansion is Compressive.

There are four elastic constant E, k, G & μ.

E & μ are independent constant.

Castellated Beam used for Light Construction.

In triangular section, Maximum shear stress occurs at Mid of the height.

For Rectangular section, τmax = 1.5 τavg

For Circular section, τmax = 1.33( 4/3 ) τavg

EULER’S FORMULA

PE = π2EI /le 2

Beam Euler’s Equation
Both ends Hinged π2EI/l2
Both ends Fixed 2EI/l2
One end Fixed & Other end Free π2EI/4l2
One end Hinged & Other end Fixed 2EI/l2

In engineering material, Modulus of rigidity is always less than half the young’s modulus.

The purpose of flitched beam is to improve moment of resistance over the section & equalise the strength in tension and compression.

SPRING

Spring absorbs shock and vibration is called Leaf spring.

Spring Stiffness

Load required to produce unit deflection in spring is called Spring Stiffness.

Spring in Instrument Use
In spring balance To measure force
In watches Absorb shock and vibration
In car Store strain energy
In break & clutches To apply forces

Radius of Mohr circle of stress represent Shear stress.

The extremities of any diameter on Mohr’s circle represent normal stresses on planes at 45°.

Reaction in Propped Cantilever Beam = 4.

The Self Weight of Beam will be taken as Uniformly Distributed Load.

Example of Shearing Failure = Making hole in paper using punch.

Shear centre of section is which the load must applied to produce zero twisting moment on the section.

 SHAPE OF S.F. & B.M. DIAGRAM

Beam S.F B.M
Cantilever beam with point load on free end Rectangle Right angle triangle
Cantilever beam with u.d.l Right angle triangle Parabolic
Cantilever beam with gradually varying load Right angle triangle Parabolic
Simply supported beam with central point load Two same rectangle Triangle
Simply supported beam with u.d.l Two right angle triangle Parabolic
Simply supported beam with eccentric load Two rectangle Triangle

Strain energy due to the Shear stress = τ2/2C x V

Strain energy due to the Torsion = τ2/4C x V

 RIVETS

Rivets made of ductile type  material.

Tearing of plate at an edge can be avoided by keeping the margin atleast = 1.5 d.

Tearing of the plate across the row of rivets, required resistance or pull Pt = p.d.t.ςt.

Shearing of rivets, pull required Ps = n.(π/4) d2 τ for single shear &

Ps = (π/2) d2 τ for double shear.

Crushing of rivets, pull required Pc = n.d.t.ςc

Strength of rivet joint is equal to least of Pc, Ps & Pt .

Efficiency = least of Pc Ps Pt /ptdt

 Chain rivet

Rivets in the various rows are opposite to each other is called chain rivet.

 Zigzag rivet

Rivets in adjacent row are staggered in such a way that every rivet is in middle of two rivets of opposite row then joint is called zigzag rivet.

 Diamond rivet

Number of rivets decreases from inner most row to outer most row.

 Pitch

Distance from the centre of one rivet to centre of the next rivet measured parallel to the seam.

 Back pitch

Perpendicular distance between the centre lines of the successive rows.

 Diagonal pitch

Distance between the centres of the rivets in adjacent rows.

 Margin

Distance between the centre of rivet hole to the nearest edge of the plate.

For Riveting, size of hole drilled is greater than shank diameter of rivet.

The object of caulking in riveted joint is to make joint leak proof.

Transverse fillet welded joints are designed for Tensile strength.

Parallel fillet welded joints are designed for Shear or Bending strength.

The problem of thick cylinder is complex and it solved by Lame’s theory.

As per Rankine’s theory the horizontal thrust offered by the retaining wall

P = wh2/2((1−sin∅)/(1+sin∅ ))

FOR RECTANGULAR BEAM

If the width is double, then the strength is 2 times greater.

If the depth is double, then strength is 4 times greater.

If the  length is double, then strength is ½ times greater.

In case of beam, Compressive stress is on Bottom layer.

In case of Beam, Tensile stress is on Top layer.

Unwin formula, d = 6√t