LOADS: DEFINITION & TYPES

ELASTIC CONSTANTS

Its types , relationship and equations.

DEFINITION OF LOADING: External mechanical resistance against which a system acts or reacts.

TYPES : LOAD can be classified on basis on the following

1)W.R.T TIME :

A) STATIC LOADS : Loads which are constant in magnitude as well as direction.

a) Dead load : A constant load acting all the time starting with some initial non zero load value (at t=0).

b) Gradually applied load (GAL) : A constant load acting for a constant time starting with some zero load value (at t=0).

B)DYNAMIC LOADS: Loads which are variable in either magnitude or direction or both.

a)Impact load : Load which acts suddenly or for a very short time interval of time.

b) Cyclic load : Load acting variably over a system in a cyclic period of time/ periodic motion. Will be discussed in detail later.

Loading in which load is suddenly applied over a system in short time interval ( Δt=0).

As per the diagram above ;

σimp   > σstatic δimp  > δstatic

EMPIRICAL RELATION : σimp= (I.F.)σstatic

Where , I.F = 1+ (1+2h/δstatic)1/2 (I.F.)MIN = 2

Therefore, impact stresses are more dangerous.

#Less impact stresses, less chances of failure for a material AND VICE VERSA.

2.W.R.T DIRECTION:

1.NORMAL AXIAL LOAD(NAL): Load acting normal to the area, along the longitudinal axis.

2.NORMAL ECCENTRIC AXIAL LOAD(NEAL): Load acting normal to the area , at a distance from longitudinal axis in a direction along longitudinal axis.

3.TRANSVERSE / DIRECT SHEAR LOAD(TSL/DSL) : Load acting along the area , perpendicular to the longitudinal axis along transverse axis.

4.ECCENTRIC TRANSVERSE / DIRECT SHEAR LOAD(ETSL/EDSL) : Load acting along the area, perpendicular to the longitudinal axis at a distance from transverse axis, in a direction along transverse axis.

3.W.R.T DISTRIBUTION:

1.CONCENTRATED LOADS : Loads and its effects that occur on a single point. It may be seen as :

a) Concentrated force

b) Concentrated moment

2.DISTRIBUTED LOADS : Loads which acts on a body over a particular span/region.

a)Uniformly distributed loads(UDL) : Load uniformly spread over a length.

b)Uniformly varying loads(UVL) : Load varying at every point over a length.

Same analogy of LOAD for MOMENT ALSO.

ELASTIC CONSTANTS :

According to HOOKE’S LAW;

” STRESS IS DIRECTLY PROPORTIONAL TO STRAIN , WITHIN ELASTIC LIMIT.”

THEREFORE,

STRESS ∝ STRAIN ⇒ STRESS = A (STRAIN)

THE CONSTANT A CALLED ELASTIC CONSTANT. These usually define the characteristics of an element under loading.

TYPES OF ELASTIC CONSTANTS:

1.FOR AXIAL LOADING : σT ∝ ε ⇒ σT = Eε

E = MODULUS OF ELASTICITY/YOUNG’S MODULUS

2.FOR SHEAR LOADING : τ ∝ γ ⇒ τ= Gγ

G = MODULUS OF RIGIDITY

3. FOR HYDRO STATIC LOADING : p∝ Δv/v ⇒ p= k (Δv/v)

k = BULK MODULUS , p = pressure stress

*POISSON RATIO (μ) μ = (ΔD/D)/(ΔL/L)

1.E=2G(1+μ)

2.E=3K(1-2μ)