Page 138 - Biaxial Multiaxial Fatigue and Fracture

BiaxiaVMultiaxial Fatigue and Fracture
Andrea Carpinteri, Manuel de Freitas and Andrea Spagnoli (Eds.)
0 Elsevier Science Ltd. and =IS. All rights reserved. 123

THE INFLUENCE OF STATIC MEAN STRESSES APPLIED
NORMAL TO THE MAXIMUM SHEAR PLANES IN
MULTIAXIAL FATIGUE

Rebecca Peace KAUFMAN and Tim TOPPER
Department of Civil Engineering, University of Waterloo
200 University Ave W, Ontario, Canada

ABSTRACT
Tubular specimens of two different hardnesses of SAE 1045 steel (BHN 456 and 203) were
tested in axial tension-compression together with alternating internallexternal pressure. For the
SAE 1045 steel, BHN 456, static mean stresses ranging from -400 MPa to 740 MPa were
applied normal to the maximum shear stress amplitude planes together with alternating shear
stress amplitudes from 150 MPa to lo00 MPa. For the SAE 1045 steel, BHN 203, static mean
stresses ranging from -60 MPa to 100 MPa were applied normal to the maximum shear stress
amplitude planes together with alternating shear stress amplitudes from 125 MPa to 220 MPa.
An approximately linear relationship was found between the applied normal static mean stress
and the maximum cyclic shear stress on the critical shear planes for a given fatigue life. The
fatigue life remained constant with increasing mean stress for tests with constant shear stress
amplitude values and static tensile mean stresses larger than 500 MPa and 76 MPa for the hard
and soft steel, respectively. It was assumed that, for static mean stresses larger than these values,
the crack faces were fully separated thus allowing unhindered Mode 11 displacement, a condition
defined as interference free crack growth by Bonnen and Topper [ 11. A confocal scanning laser
microscope was used to determine the validity of this assumption. With this apparatus, the crack
depth profiles were measured as a function of the magnitude of the static mean stress applied
normal to the shear plane. The 2-D line profiles of the fracture surfaces were obtained to
investigate the impact of normal static mean stresses on asperity height and shape [2]. Current
critical plane theories were investigated to determine their ability to predict fatigue life for
alternating shear stresses and static mean stresses normal to the maximum shear planes. A
modified Findley parameter gave the best fit to the experimentally obtained data. To include the
interference free condition in the parameter, tensile static mean stresses larger than the
interference free condition were replaced with the normal static mean stress that resulted in the
interference free stress state in the modified Findley parameter.

KEYWORDS
Multiaxial fatigue, static mean stress, interference free crack growth.