Before studying the manner of application of load , we know various sources of load.
(a) Self Weight
(b) Supported weight
(c) Force due to fluid pressure
(d) Inertia force = mass x acceleration
(e) Centrifugal force
(f) Frictional force
(g) Wind load
(h) Snow load
(i) Thermal load
(j) Seismic load
(k) Tensile force
(l) Compressive force
(m) Shear force
(n) BENDING Moment
(p) Buckling load
Gradually Applied Load
Apply a total load in equal installments.
Let us apply a total load of 100 N.
First apply a load of 5 N, then 10 N, 15 N, 20 N ——-100 N. Increase load in equal instalments. Overall it causes less stress and less strain as compared to suddenly applied load.
Mostly find gradually applied load during testing of materials . However, gradually applied load applications are very few. In the study of strength of materials, assumed gradually applied load in all the derivations. Gradually applied load and extensions form a triangle. As and when apply load suddenly or impact, find its equivalent gradually applied load.
(i) Place 20 books, one by one on a table (gradually applied load)
(ii) Unloading bricks from a truck. Then placing bricks one by one over the previous brick ( gradually applied load).
Stress = σGradually applied load = F/A
Strain = ε = σGradually applied load/E
Where E is Young’s Modulus
In this, stress strain curve will be a triangle.
Suddenly Applied Load
Apply total force in one installment. Apply a force of 100 N at once. Stress produced is 2 times the stress under a 100 N gradual load.
Example: Place a weight of 50 kg at once in a weighing balance.
A person sitting on a chair
Placing a television on a table.
Placing a bundle of books on a table
A laborer carrying six bricks at a time
σsudden;ly applied load = 2F/A
(In this case stress strain curve is a rectangle. It is because one load one extension)
Stress & strain due to suddenly applied load is two times when the same load is applied gradually. Material fails at half load when suddenly applied.
Impact load is moving load. The moving body striking another body is an impact load. It causes many times the stress produced by same load applied gradually. This type of load is extremely dangerous.
(i) Force applied in an accident is an impact load
(ii) Hammering is impact load
(iii) Lahti charge is an impact load.
(iv) Striking ball with the bat is impact load
(v) Jumping on the chair causes impact load.
(vi)Falling of an object from the hand and striking the foot is impact load.
σimpct load stress = (F/A) [ 1+ (1+2hAE/F)0.5]
where h is the height from which load F is falling
Impact load helps to find toughness of a material. Under impact load, energy required to fracture is toughness. Its unit is Joules.
Fig. Gradually Fig. Suddenly
Applied load Applied load
Fig. Impact load
Fig. Suddenly applied load (placing a box gently on a table)
Fig. Impact load (striking a hammer on a nail)
Table: Gradually, suddenly & impact loads
Gradually applied load,
Suddenly applied load, Ps
Impact load, Pi
Equivalent gradually applied load, Pge
Pge =2 Ps
σi = Pi[1+(1+2hAE/L)0.5]/A
€i = Pi[1+(1+2hAE/L)0.5
Strain energy, U
(σ2i /2E) Volume
Pgs equivalent gradual load for the sudden load Ps
Pgi equivalent gradual load for the impact load Pi