The bauschinger effect in tungsten lamp wire
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resented above the abscissa, ]~'rI, for easy comparison. In all stress-strain curves there was a significant lack of convergence between the absolute values of forward stress and reverse stress. This observation is presented graphically in Fig. 2 where the stress ratio is plotted as a function of reverse strain. It is apparent that the ratio will not reach the value 1. This is the general effect which Orowan has termed "permanent softening": No matter where on the strain axis the comparison is made, reverse stress is always smaller than forward stress. At reverse strains higher than about ~/r 0.02 the ratio increased relatively little. Figure 2 also shows the smaller magnitude of the Bauschinger effect for structure modified wire. The magnitude of the Bauschinger effect has been expressed in a variety of ways. One way is to assess the flow stress difference (rB.02) due to reverse strain at some constant strain such as 2 pct. 6 These parameters for the wire tested are given in Table II. Another way of evaluating the Bauschinger effect has been proposed to serve for the comparison of many metals and alloys. 1~A generally applicable formula is =
rB = k y p ,
[1]
where % is the decrease in flow stress due to the Bauschinger effect after plastic strain 7p; k and m are constants called the Bauschinger effect coefficient and exponent respectively. Values for the lamp wire tested are compared with those of other materials in Table III. One explanation of the Bauschinger effect has dislocations piling up on barriers during prestressing with excessive movement on unloading prevented by weaker barriers behind them which were created by or superseded by the original deformation process. 4,5 The reverse stress required to overcome the weaker obstacles is understandably lower, giving rise to the Bauschinger effect. The Bauschinger effect has also been described
SHEAR STRESS
Table I. Tensile Properties of Lamp Wire Wire
Description
s y*
Sy*
e*
A A SSS
As-received Reverse-bend rolled-high-tension As-received Reverse-bend rolled-low tension Reverse-bend rolled-high tension
2013 1862
2482 2503
2.9 3.1
2289 1731
2627 2592
2.5 4.4
2013
2689
3.0
B B SS B SSS
!
/
SHEAR STRAIN
*g-y= average yield strength at 0.2 pct offset, MPa.sy = average ultimate tensile strength, M P a . E = average pct elongation. Data were obtained using a 50 m m gage length and a 0.01 per min strain rate.
J. W. P U G H is Research Advisor, L a m p Metals Laboratory, Lighting Research a n d Technical Services Operation, General Electric Company, Cleveland, O H 44112. Manuscript submitted October 10, 1979.
T Fig. 1--Schematic torsion stress strain curve.
ISSN 0360-2133/80/0811-1487500.75/0 M E T A L L U R G I C A L T R A N S A C T I O N S A 9 1980 A M E R I C A N SOCIETY FOR M E T A L S A N D T H E M E T A L L U R G I C A L SOCIETY O F A I M E
V O L U M E 11A, A U G U S T 1980--1487
REVERSED FLOW
STRESS
RATIO
Table III. Bauschinger Effect Constant and Exponent for a Variety o! Metals
FOR ,~p = 0 . 0 3 4
I.O
Material, Grain Size,
Data Loading...
