Page 67 - Shigley's Mechanical Engineering Design

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42 Mechanical Engineering Design
Similar relationships for cast iron can be derived from data supplied by Krause. 5
The minimum strength, as defined by the ASTM, is found from these data to be

0.23H B − 12.5 kpsi
S u = (2–22)
1.58H B − 86 MPa
6
Walton shows a chart from which the SAE minimum strength can be obtained, which
is more conservative than the values obtained from Eq. (2–22).

EXAMPLE 2–2 It is necessary to ensure that a certain part supplied by a foundry always meets or
exceeds ASTM No. 20 specifications for cast iron (see Table A–24). What hardness
should be specified?

Solution From Eq. (2–22), with (S u ) min = 20 kpsi, we have

S u + 12.5 20 + 12.5
Answer H B = = = 141
0.23 0.23
If the foundry can control the hardness within 20 points, routinely, then specify
145 < H B < 165. This imposes no hardship on the foundry and assures the designer
that ASTM grade 20 will always be supplied at a predictable cost.

2–5 Impact Properties
An external force applied to a structure or part is called an impact load if the time of
application is less than one-third the lowest natural period of vibration of the part or
structure. Otherwise it is called simply a static load.
The Charpy (commonly used) and Izod (rarely used) notched-bar tests utilize bars of
specified geometries to determine brittleness and impact strength. These tests are helpful
in comparing several materials and in the determination of low-temperature brittleness. In
both tests the specimen is struck by a pendulum released from a fixed height, and the
energy absorbed by the specimen, called the impact value, can be computed from the
height of swing after fracture, but is read from a dial that essentially “computes” the result.
The effect of temperature on impact values is shown in Fig. 2–7 for a material
showing a ductile-brittle transition. Not all materials show this transition. Notice the
narrow region of critical temperatures where the impact value increases very rapidly. In
the low-temperature region the fracture appears as a brittle, shattering type, whereas the
appearance is a tough, tearing type above the critical-temperature region. The critical
temperature seems to be dependent on both the material and the geometry of the notch.
For this reason designers should not rely too heavily on the results of notched-bar tests.
The average strain rate used in obtaining the stress-strain diagram is about
0.001 in/(in · s) or less. When the strain rate is increased, as it is under impact conditions,

5 D. E. Krause, “Gray Iron—A Unique Engineering Material,” ASTM Special Publication 455, 1969,
pp. 3–29, as reported in Charles F. Walton (ed.), Iron Castings Handbook, Iron Founders Society, Inc.,
Cleveland, 1971, pp. 204, 205.
6 Ibid.