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Experimental Mechanics

, Volume 8, Issue 12, pp 529–538 | Cite as

Spin tests to determine brittle fracture under plane strain

Paper describes a procedure developed for propagating a fatigue crack in a test-rotor blank
  • Gale O. Sankey
Article

Abstract

The feasibility of using cyclic thermal stress or hydrostatic pressure to generate a fatigue crack in a large test-rotor blank is demonstrated. Test rotors, having test notches with fatigue-crack terminations, were spun to fracture to determine optimum test-notch design. It was found that, for the other test-notch dimensions held, it was necessary to extend the fatigue crack a minimum of 0.1 in. from the machined portion of the test notch to obtain a most effectively notched test rotor. In another series of tests, the influence of temperature on brittle-fracture strength of a Cr-Mo-V steel under plane strain was evaluated. It was found that, although there is a significant increase in fracture strength with increasing temperature, no knee in the curve is apparent in the vicinity of the conventionally measured transition temperatures ofNDT andT50. Also, design against brittle fracture is still required at temperatures aboveNDT andT50.

Keywords

Fatigue Mechanical Engineer Brittle Transition Temperature Fluid Dynamics 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Nomenclature

C

notch length of test rotor (total), in

E

modulus of elasticity, psi

e

elongation in 2-in. gage length, percent

GC

fracture toughness of material,\(\begin{gathered} \hfill \\(\pi \sigma _o ^2 C/2E) (1 - \nu ^2 ) \hfill \\ \end{gathered} \), in. lb/in.2

GIC

fracture toughness of material under plane strain,\(G_c /[1 + (1.4/\alpha _1 ^2 )]\), in. lb/in.2

h

length of test rotor, in

KIC

stress-intensity factor under plane strain,\(10^{ - 3} [EG_{IC} /(1 - \nu ^2 )]^{1/2} ,ksi\sqrt {in.} \)

l

length of fatigue crack forming one of the test-notch terminations, in

N

fracture speed of test rotor, rpm; ω=2πN/60, rad/sec

NDT

nil-ductility transition temperature, °F

Ri

inside radius of test rotor, in.; ID=2R i , in

Ri

outside radius of test rotor, in.; OD=2R o , in

RA

reduction of area, percent

T

test temperature, °F

ΔT

temperature difference between that at the hole wall and that remote from the hole, °F

T50

Charpy Vee-notch 50-percent shear-fracture appearance transition temperature, °F

α

thermal coefficient of expansion, (in./in.)/°F

α1

constraint index,\(h\sigma _{y^2 } /EG_C \), dimensionless

ε

strain at the hole wall, 102 σ/E, percent

ν

Poisson's ratio=0.3 (for steel)

ρ

mass density of material, lb sec2/in.4=0.283/386

σ

tangential stress at the hole wall,EαΔT/(1−ν), psi

σaν

average net-section tangential stress corresponding to the fracture speed of the test rotor,\(\frac{{\rho \omega ^2 }}{3}(\frac{{R_o ^3- R_i ^3 }}{{R_o- C/2}}),psi\)

σo

nominal fracture stress. Tangential stress at center of solid rotor corresponding to the fracture speed of the test rotor,\(\frac{{(3 - 2\nu )}}{{8(1 - \nu )}}\rho \omega ^2 R_o ^2 \), psi

σ4

nominal stress correction for uniformC of 4.0 in.,\(\sigma _o (C/4)^{1/2} \)

σIO

nominal fracture stress for conditions of complete plane strain andC=4 in.,\(\alpha _4 /(1 + \frac{{1.4}}{{\alpha _1 ^2 }})^{1/2} \), psi

σy

yield strength of material, psi (0.2-percent offset)

σu

ultimate strength of material, psi

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References

  1. 1.
    Irwin, G. R., “Fracture Mode Transition for a Crack Traversing a Plate,” Met. Eng. Conf., ASME, Albany, N. Y. (April 29–May 1, 1959).Google Scholar
  2. 2.
    Cooper, G., Sankey, G. O. and Wessel, E. T., Discussion of paper by Brothers, A. J., Newhouse, D. L. and Wundt, B. M., “Results of Bursting Tests of Alloy Steel Disks and Their Application to Design Against Brittle Fracture,” ASTM Annual Meeting, Purdue University (June 13–18, 1965).Google Scholar
  3. 3.
    Wessel, E. T., “The Influence of Pre-Existing Sharp Cracks on Brittle Fracture of a Nickel-Molybdenum-Vanadium Forging Steel,”ASTM Proc 60,721–732 (1960).Google Scholar
  4. 5.
    Winne, D. H. and Wundt, B. M., “Application of the Griffith-Irwin Theory of Crack Propagation to the Bursting Behavior of Disks, Including Analytical and Experimental Studies,” Trans. ASME,80, (1958).Google Scholar
  5. 6.
    Clark, W. G., Jr., and Ceschini, L. J., “Fatigue Precracking of Spin-Burst Toughness Specimens,” presented at SESA Spring Meeting, Paper No. 1357A, Albany, N. Y. (May 7–10).Google Scholar

Copyright information

© Society for Experimental Mechanics, Inc. 1968

Authors and Affiliations

  • Gale O. Sankey
    • 1
  1. 1.Westinghouse R & D CenterPittsburgh

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