MIC Green Book

C H A P T E R T W O
RESPONSE OF METALS
12
TITANIUM
High Cycle Fatigue (HCF) - HCF of titanium is
illustrated by Figure 2-4, which compares the
capabilities of titanium alloy connecting rods
for a high performance European sports car.
The rods are manufactured using various
processes. With shot peening, the fatigue
limit was increased by approximately 20%
while weight was reduced by some 40% as
compared to steel connecting rods [Ref 2.12].
Low Cycle Fatigue (LCF) - As is typical with
other metals, the fatigue response with shot
peening increases with higher cycle fatigue.
Higher cycle fatigue would be associated with
lower stresses whereas lower cycle fatigue
would be associated with higher stress levels.
This is demonstrated graphically in the S-N
Curve (Figure 1-4) and also Figure 2-5.
Figure 2-4 Comparison of Fatigue Strengths for Polished
and Shot Peened Titanium Ti6A14V
Figure 2-5 shows the results of shot peening
titanium dovetail slots on a rotating engine
component [Ref 2.13]. There are two baseline
load curves that are not shot peened. When
shot peening is applied, the base line curve
that initially had more cycles to failure
responded significantly better. Note that
improvements in fatigue life are on an
exponential basis.
Figure 2-5 LCF Benefits from Shot Peening Notched Ti8-1-1
The most common application of LCF for
titanium is the rotating turbine engine hardware (discs, spools, and shafts) with the exception of blades. These
components are shot peened to increase durability. Each takeoff and landing is considered one load cycle.
MAGNESIUM
Magnesium alloys are not commonly used in fatigue applications. However, when used for the benefit of weight
reduction, special peening techniques can be employed to achieve 25 - 35% improvement in fatigue strength.
POWDER METALLURGY
Optimized peening parameters have been shown to raise the endurance limit of sintered steel PM alloys by
22% and the fatigue life by a factor of 10. [Ref. 2.14] Automotive components such as gears and connecting
rods are candidates for PM with shot peening. Shot peening is most effective on higher density PM parts
such as forged powder metal components.
Surface densification by shot peening can increase the fatigue limit significantly, especially in the case of
bending. The surface densification also assists in the closing of surface porosity of PM components for
sealing and other engineering applications.
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