1. Wear, 174 (1994) 235-237 235
Short Communication
Rise in fatigue strength of Ti by
extremely mild abrasive processing
Seiki Watanabe*, Jun Amanob
and Hiroshi Furuichi’
“Matsue Na:ional Coltege of Technology, Matsue 690 (Japan)
bMatsue National College of Technology, Matsue 690 (Japan)
%epartment of Mechanical @stem Engineekng, Faculty of
Engineetig Yamanashi Um.~e~i~, Kofu 400 (Japan)
(Received October 12, 1993; accepted January 13, 1994)
Titanium is frequently used because of its high
strength-to-weight ratio and high corrosion resistance.
Fine cracks and tensile residual stresses readily arise
in Ti from abrasive processing; they markedly lower
the fatigue strength of Ti [l-3]. Adequate cooling and
low abrasive speed etc. are required to avoid the lowering
of fatigue strength [4] and therefore abrasive processing
has not been recommended for Ti [SJ.
It has been reported that mild or to and fro grinding
raises the fatigue limit of steels [6-8). This leads to
the idea that mild abrasive processing could raise the
fatigue strength of Ti.
specimens were annealed in vacuum (about 13 FPa)
for 30 min at 700 “C. Table 1 shows the mechanical
properties of annealed specimens. After the annealing,
the specimens were treated in three ways: chemical
polishing in a solution (10 vol. 46% extra pure HF, 60
vol. 30% extra pure HzO, and 30 vol. distilled water)
at 20 “C to remove 20 ,um, dry lapping and wet (tap
water) lapping with Sic abrasive papers (at first average
grain size 18 pm removed 10 pm and the next average
grain size 15 pm removed 10 pm), using a lapping
machine (Pedemax-2, Struers Co., Ltd.). Lapping is
considered to prevent abrasive grains, which may cause
fatigue cracks [7,9], from being embedded in the spec-
imens, owing to the change in the abrasive direction.
In every lapping process, including that before the
annealing, the lapping pressure was 30 Pa and the
lapping speed was less than 0.35 m s-l.
The main purpose of this paper is to present evidence
of the remarkable rise in endurance limit of Ti by
extremely mild lapping with SiC papers.
Co~ercially pure Ti (0.~2 wt.% H, 0.074 wt.%
0, 0.0035 wt.% N, 0.064 wt.% Fe} was shaped into
specimens (Fig. 1) and the surfaces were wet lapped
with Sic papers (average grain size, 22 pm). These
The edges of all the specimens were mildly rounded
off to a radius of curvature of 0.2 mm with abrasive
papers in running tap water by hand: for chemically
polished specimens and some of the wet-lapped spec-
imens before the annealing (average grain size of the
abrasive paper 22 pm) and for the other lapped spec-
imens after the lapping (average grain size of the paper,
initially 18 pm and later 15 pm). Fatigue tests were
made using a Shenck-type plane bending tester at
20 f 2 “C and the endurance limit was determined by
the staircase method [lo].
Residual stresses were obtained by the sin’ $ method
[ll]: specific X-ray, diffraction plane and the stress
constant were Co I&, (114) and -2.86 MPa rad-’ [12],
respectively.
The surface roughness of the specimens was measured
by a stylus method. The fracture stress vs. the number
of cycles is shown in Fig. 2. Table 2 shows the endurance
limit, the residual stress, the hardness near the specimen
surfaces and the surface roughness before the fatigue
test. The endurance limit of lapped specimens whose
edges were rounded off with Sic abrasive papers were
higher by about 55% than the others: statistical analysis
shows no significant difference between the endurance
limit of dry-lapped specimens and that of wet-lapped
TABLE 1. Mechanical properties of annealed titanium
L 90
J
Fig. 1. Size and dimensions of specimens: 0, diameter of round
hole; R, radius of curvature. Dimensions in millimetres.
Proof stress Tensile strength Elongation
(MPa) (MPa) (“ro)
Reduction
of area
(%)
328 385 31.9 76.2
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2. 236 S. Wotanobe et al. i Rise in fatigue strength of Ti by abrasive pmcesfing
TABLE 2. Treatment and properties of specimens
Treatment
Chemically polished
Wet lapped
Dry lapped
Wet lapped (edges,
chemically polished)
Endurance limit
(MPa)
194*21
305 Itr34
3OOi16
203 f21
Residual stress
(MPa)
-123f24
-83k2.8
-123524
Hardness Roughness
Q-IV:0.49 N) JG,, (cLm)
138f5 2.7
158*5 1.0
161*6 1.3
158*5 1.0
*.0 0
oO@ OO
A
OA
o A a
100104 lo5 106 107
Number of cycles
Fig. 2. Fracture stress vs. number of cycles: 0 chemically polished;
m wet lapped; 0 dry lapped; A wet lapped (edges chemically
polished).
0.1 mm .
Fig. 3. Typical example of fatigue cracks on lapped surface: wet
lapped specimen; stress amplitude, 382 MPa; 2x1@ cycles.
specimens and between the endurance limit of the other
specimens (level of significance 5%).
For the lapped specimens, all, the fatigue cracks
originate only at the edges of the specimens, as shown
in Fig. 3. In chemically polished specimens, however,
they originate not only at slip bands on the specimen
surfaces, but also at the edges, and these cracks with
different birth place connect themselves and propagate
to fracture. The cracks originating at the slip bands
do not occur in every crystal (Fig. 4), like those in
Fig. 4. Typical example of fatigue cracks in chemically polished
specimens: stress amplitude, 304 MPa; 2.5 X 104 cycles. (a) Edge
initiation; (b) surface initiation.
copper [13]. In annealed mild steels [14] and aluminium
alloys [15], fatigue cracks originate in every crystal.
Lapping brings about high compressive residual stress
and high hardness as shown in Table 2, though not
higher than those arising in polished or ground steels
16-93. It has been reported that compressive residual
stress and high hardness arising from mechanical pol-
ishing raises the fatigue strength of steels and aluminium
alloys, to an extent less than 20% [16]. The structure
of Ti is h.c.p. with a single slip system, which restricts
freedom of plastic deformation to form cracks. In work-
hardened Ti, extremely mild lapping should avoid crack
formation, and the embedding of abrasive grains and
hence fatigue crack formation ought to be more difficult
than in other abrasive-processed metals and alloys with
different structures. This may be the cause for the
greater rise in fatigue strength for Ti than for steels
and aluminium alloys. Less binding to the grains at
the edges of the specimens by other grains, compared
with the grains inside the specimens, lowers the yield
stress [17]. This may cause fatigue crack formation at
the lapped edges. It can therefore be said that the
process of rounding off of the edges is an important
factor in determining the endurance limit of Ti.
To sum up, including the rounding of edges, extremely
mild lapping is effective in raising the endurance limit
of Ti.

3. S. Watanabe et al. I Rise in fatigue strength of Ti by abrasive processing 237
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