Building Materials, Third Edition

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™ IIQ

3. Modulus of elasticity N/mm^2
   9

2

3

3

4

Pal

bh

4. Horizontal shear stress on neutral N/mm^2 At centre = 0
   plane at limit of proportionality
   At ends =

3

4

P

bh

5. Horizontal shear stress at N/mm^2 At centre = 0
   maximum load
   At ends =

3

4

P

bh

6. Work to limit of proportionality Nmm/mm^32

P

lbh

9

(elastic resilience)

where l = distance between two fixed points for which deflection is recorded in mm (gauge
length).

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The specimen for impact bending test is same as that used for static bending strength test.
The test is conducted on a suitable impact bending machine. The span is 700 mm in case of
50 × 50 × 750 mm and 280 mm in case of 20 × 20 × 300 mm specimens. The hammer is 25 kg and
1.5 kg for the two specimen sizes, respectively.
Static deflection x due to the weight of the hammer is measured at the centre 2 of the specimen.
For recording of deflection, a drum is provided which can be 2 brought in contact with a stylus
attached to hammer and can be rotated on a vertical 2 axis. On the drum, a paper is fixed by
means of sticking tape, under which a carbon 2 paper is placed inverted for recording the
impressions. First, a datum line is marked 2 by placing the hammer to rest on the specimen and
rotating the drum with stylus 2 touching it. After that, the hammer is dropped from different
heights and deflection 2 recorded on the paper fixed on the drum. The first drop of hammer is
from a height 2 of 50 mm after which the height of the successive drops is increased by 25 mm
until a height of 250 mm is reached, and thereafter increment in height is 50 mm 2 until complete
failure occurs or 150 mm or 60 mm deflection is reached for the two 2 sizes, respectively.
Deflections due to successive drops are recorded. For this 2 purpose, at the drop of the hammer,
the drum is rotated as the hammer 2 rebounds.
From the tracing on the drum, record the actual deflection at each drop (that 2 is, the distance
from the lowest point to the datum line). A graph is then plotted 2 with the exact height of drop
plus maximum deflection at that drop H + (x + y) as 2 the ordinate and (y + x)^2 as the abscissa.
The point at which the curve deviates 2 from a straight line is taken as limit of proportionality.
The various characteristics should be determined by the following formulae:

S. No. Characteristic Unit Formula

1. Maximum height of drop mm H


2. Height of drop at limit of proportionality mm H


3. Fibre stress at limit of proportionality N/mm^2

(^29)
3 HWl
bh