• PDF / 2,237,777 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 108 Downloads / 258 Views

DOWNLOAD

REPORT

---

IT

is well known that fatigue crack initiation and propagation in the earliest stage usually takes place along the persistent slip band in annealed metals. ~ The dislocation a r r a n g e m e n t s in the persistent slip bands have been studied intensively on fatigued copper by many workers. 2-a Associated with the persistent slip bands, they found so-called ladder like s t r u c t u r e s which were considerably different from that in the m a trix of the fatigued specimen. F u r t h e r m o r e some t h r e e dimensional models of this structure were also proposed, s-~ However, the correlation of fatigue c r a c k s to these s t r u c t u r e s has not been studied in d e t a i l In this paper, results of direct observation of the dislocation s t r u c t u r e s around the tips of fatigue cracks in the early stage in copper bulk specimens are p r e sented, and possible mechanisms of fatigue crack initiation and propagation in this stage a r e discussed.

EXPERIMENTAL

PROCEDURE

Polycrystalline O F H C copper (99.99 pet) supplied in the form of 3 m m thick plate was machined into specimens having cross section of 3 x 8 mm. The specimens were then annealed at 850~ [1123K] for 2 h i n v a c u o , and finally electropotished to remove the surface layer by about 30 p m before testing. The grain size was about 100 g m in diameter. Using Shimadzu UF 15 fatigue testing machine o p e r ating in plane bending at 33 Hz, the specimens were cycled with s t r e s s amplitude cr = =e 11.0 kg/mra z [108 MN/m e] corresponding to a life-time N = 1.0 x 10s cycles. The tests were terminated at 80 pet the life;

K. KATAGIRI,A, OMURA,K. KOYANAGI,and J. AWATANIare Assistant Professor, Technical Official, Research Fellow and Professor, respectively, at The hastitute of Scientific and Industrial Research, Osaka University,Japan. T. SIIIRAISH1,formerlyGraduate Student of Faculty of Engineering, Osaka University,is now Assistant Professor, Faculty of Engineering, Fukui University, Japan. It. KANESHIROis Associate Professor, Faculty of Engineering and Science, Ryukyu University, Japan. Manuscript submitted September 28, 1976. METALLURGICALTRANSACTIONSA

and observations were made for the cracks in the early stage formed in such fatigued specimens. Thin foils containing m i c r o c r a c k s were prepared as follows: F i r s t , surfaces of the fatigued specimens* *Depositdectroplatedon the speciraensurfacedarec~lyis apt to be separated fromthe specimenduringthinningprocess,so somespecnnenswereshght/yelectropohshedbeforeetectroplatmg. were protected by an etectrodeposited copper layer approximately 2 mm thick, They were then cut longitudinally into slices (1.8 mm thick) perpendicular to the specimen surface by means of a milling cutter. The slices were thinned f i r s t mechanically with e m e r y paper and then electrolytically in order to remove the damaged layer. The a r e a including c r a c k s near the interface between the specimen and electrodeposit was thinned using jet electropotishing. Finally, the area was electropolished in the