Impact stress - BE Mech

2. • An electric motor weighing 500 N is mounted
on a short cantilever beam of uniform
rectangular cross section. The weight of motor
acts at a distance of 300 mm from the
support. The depth of the section is twice the
width. Determine the cross section of the
beam. The allowable stress in the beam is
40 N/mm^2.

3. IMPACT STRESSES

4. • An impact is a high force or shock applied over
a short time period when two or more bodies
collide, that may either be in motion or at
rest.
• Such a force or acceleration usually has a
greater effect than a lower force applied over
a proportionally longer period.Why?

5. EXAMPLES?

12. • W = falling weight
• h = Height through which the weight falls
• δ = displacement of the point of load
application (mm)
• l = length of the bar
• A = Cross section area of the bar
• P = impact forces which produce deflections
• E = Modulus of elasticity of bar material
• σi = impact stress in the bar

13. • The quantity (P/W) is called shock factor that
indicates the magnification of load into impact
force P during impact
• Impact Forces acting instantaneously
• h = 0
• σi = 2 (P/A)

14. • University question

15. An unknown weight falls through 10 mm on a
collar rigidly attached to the lower end of a
vertical bar 3 m long and 600 mm2 in section. If
the maximum instantaneous extension is known
to be 2 mm, what is the corresponding stress
and the value of unknown weight? Take E = 200
kN/mm2

16. • A mass of 50 kg drops through 25 mm at the
center of a 250 mm long simply supported
beam. The beam has square cross – section. It
is made of steel 30C8 and the factor of safety
is 2. The modulus of elasticity is 207 000
N/mm^2. Determine the dimension of the
cross section of the beam.