ASM Metals Handbook Volume 01 - Properties and Selection Irons, Steels, and High-Performance Allo

on this property. Fatigue
life curves at room temperature for a gray iron under completely reversed cycles of bending stress are shown in Fig. 13 (a), in
which each point represents the data from one specimen. The effects of temperature on fatigue limit and tensile strength are
shown in Fig. 13 (b) and 13 (c), respectively.
Fig. 13 Effect of temperature on fatigue behavior and tensile strength of a gray iron (2.84% C, 1.52% Si, 1.05% Mn,
0.07% P, 0.12% S, 0.31% Cr, 0.20% Ni, 0.37% Cu). (a) Reversed bending fatigue life at room temperature. (b) Reversed
bending fatigue limit at elevated temperatures. (c)Tensile strength at elevated temperatures. Source: Ref 8
ASM Handbook,Volume 1 Gray Iron 01 Sep 2005
Copyright ASM International. All Rights Reserved. Page 47
Axial loading or torsional loading cycles are frequently encountered in designing parts of cast iron, and in many instances
these are not completely reversed loads. Types of regularly repeated stress variation can usually be expressed as a function of a
mean stress and a stress range. Wherever possible, the designer should use actual data from the limited information available.
Without precisely applicable test data, an estimate of the reversed bending fatigue limit of machined parts may be made by using
about 35% of the minimum specified tensile strength of the particular grade of gray iron being considered. This value is probably
more conservative than an average of the few data available on the fatigue limit for gray iron.
An approximation of the effect of range of stress on fatigue limit may be obtained from diagrams such as Fig. 14 . Tensile
strength is plotted on the horizontal axis to represent fracture strength under static load (which corresponds to a 0 stress range).
Reversed bending fatigue limit is plotted on the ordinate for 0 mean stress, and the two points are joined by a straight line. The
resulting diagram yields a fatigue limit (maximum value of alternating stress) for any value of mean stress.
Fig. 14 Diagram showing safe and unsafe fatigue zones for cast iron subjected to ranges of alternating stress
superimposed on a mean stress. Example point P shows conditions of tensile (positive) mean stress; P′ shows compressive
(negative) mean stress. The safety factor is represented by the ratio of OF to OP or OF′ to OP′. For conditions of constant
mean tensile stress, DK/DP is the safety factor.
Few data available are applicable to design problems involving dynamic loading where the stress cycle is predominantly
compressive rather than tensile. Some work done on aluminum and steel indicates that for compressive (negative) mean stress,
the behavior of these materials could be represented by a horizontal line beginning at the fatigue limit in reversed bending, as
indicated in Fig. 14 . Gray iron is probably at least as strong as this for loading cycles resulting in negative mean stress, because it
is much stronger in static compression than in static tension. It is therefore a natural assumption that the parallel behavior shown
in Fig. 14 is conservative.
If, prior to design, the real stress cycle can be predicted with confidence and enough data are available for a reliable S-N
diagram for the gray iron proposed, the casting might be dimensioned to obtain a minimum safety factor of two based on fatigue
strength. (Some uses may require more conservative or more liberal loading.) The approximate safety factor is best illustrated by
point P in Fig. 14 . The safety factor is determined by the distance from the origin to the fatigue limit line along a ray through the
cyclic-stress point, divided by the distance from the origin to that point. In Fig. 14 this is OF/OP.
On this diagram, point P′ represents a stress cycle having a negative mean stress. In other words, the maximum compressive
stress is greater than the tensile stress reached during the loading cycle. In this instance, the safety factor is the distance OF′/OP′.
However, this analysis assumes that overloads will increase the mean stress and alternating stress in the same proportion. This
may not always be true, particularly in systems with mechanical vibration in which the mean stress may remain constant. For this
condition, the vertical line through P would be used; that is, DK/DP would be the factor of safety.
Most engineers use diagrams such as Fig. 14 mainly to determine whether a given condition of mean stress and cyclic stress
results in a design safe for infinite life. The designer can also determine whether variations in the mean stress and the alternating
stress that he anticipates will place his design in the unsafe zone. Usually the data required to analyze a particular set of
conditions are obtained experimentally. It is emphasized that the number of cycles of alternating stress implied in Fig. 14 is the
number normally used to determine fatigue limits, that is, approximately 10 million. Fewer cycles, as encountered in infrequent
overloads, will be safer than indicated by a particular point plotted on a diagram for infinite life. Too few data are available to
draw a diagram for less than infinite life.
Fatigue Notch Sensitivity. In general, very little allowance need be made for a reduction in fatigue strength caused by
notches or abrupt changes of section in gray iron members. The low-strength irons exhibit only a slight reduction in strength in
the presence of fillets and holes. That is, the notch sensitivity index approaches 0; in other words, the effective stress
concentration factor for these notches approaches 1. This characteristic can be explained by considering the graphite flakes in
gray iron to be internal notches. Thus, gray iron can be thought of as a material that is already full of notches and that therefore
has little or no sensitivity to the presence of additional notches resulting from design features. The strength-reducing effect of the
internal notches is included in the fatigue limit values determined by conventional laboratory tests with smooth bars.
Axial loading or torsional loading cycles are frequently encountered in designing parts of cast iron, and in many instances
these are not completely reversed loads. Types of regularly repeated stress variation can usually be expressed as a function of a
mean stress and a stress range. Wherever possible, the designer should use actual data from the limited information available.
Without precisely applicable test data, an estimate of the reversed bending fatigue limit of machined parts may be made by using
about 35% of the minimum specified tensile strength of the particular grade of gray iron being considered. This value is probably
more conservative than an average of the few data available on the fatigue limit for gray iron.
An approximation of the effect of range of stress on fatigue limit may be obtained from diagrams such as Fig. 14 . Tensile
strength is plotted on the horizontal axis to represent fracture strength under static load (which corresponds to a 0 stress range).
Reversed bending fatigue limit is plotted on the ordinate for 0 mean stress, and the two points are joined by a straight line. The
resulting diagram yields a fatigue limit (maximum value of alternating stress) for any value of mean stress.
Fig. 14 Diagram showing safe and unsafe fatigue zones for cast iron subjected to ranges of alternating stress
superimposed on a mean stress. Example point P shows conditions of tensile (positive) mean stress; P′ shows compressive
(negative) mean stress. The safety factor is represented by the ratio of OF to OP or OF′ to OP′. For conditions of constant
mean tensile stress, DK/DP is the safety factor.
Few data available are applicable to design problems involving dynamic loading where the stress cycle is predominantly
compressive rather than tensile. Some work done on aluminum and steel indicates that for compressive (negative) mean stress,
the behavior of these materials could be