Fracture initiation at sharp notches under mode I, mode II, and mild mixed mode loading |
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| Authors: | Martin L Dunn Wan Suwito Shawn Cunningham Christian W May |
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| Affiliation: | (1) Center for Acoustics, Mechanics, and Materials, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado, 80309-0427;(2) Ford Microelectronics, Inc., Colorado Springs, Colorado, 80921;(3) Center for Acoustics, Mechanics, and Materials, Department of Mechanical Engineering, University of Colorado, Boulder, Colorado, 80309-0427 |
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| Abstract: | In the context of linear elasticity, a stress singularity of the type Knrδ(δ<0) may exist at sharp re-entrant corners, with an intensity Kn. In general the order of the stress singularity δ and the stress intensity differ for symmetric (mode I) and antisymmetric
(mode II) loading. Under general mixed-mode loadings, the magnitudes of the mode I and II intensities fully characterize the
stress state in the region of the corner. A failure criterion based on critical values of these intensities may be appropriate
in situations where the region around the corner dominated by the singular fields is large compared to intrinsic flaw sizes,
inelastic zones, and fracture process zone sizes. We determined the mode I and II stress intensities for notched mode I tensile
specimens and notched mode II flexure specimens using a combination of the Williams (1952) asymptotic method, dimensional
considerations, and detailed finite element analysis. We carried out a companion experimental study to extract critical values
of the mode I and II stress intensities for a series of notched polymethyl methacrylate (PMMA) tensile and flexure specimens
with notch angles of 90-. The data show that excellent failure correlation is obtained, in both mode I and II loading, through
the use of a single parameter, the critical stress intensity. We then analyzed and tested a series of T-shaped structures containing 90- corners. The applied tensile loading results in mixed-mode loading of the 90- corners. Failure
of the specimens is brittle and can be well-correlated with a critical mode I stress intensity criterion using the results
of the notched mode I tensile tests. This is attributed to large difference in the strength of the stress singularities in
modes I and II: δ= -0.4555 and -0.0915 for modes I and II for a 90- notch. As a result, the mode I loading dominates the failure
process for the 90- corner in the T-structure.
This revised version was published online in August 2006 with corrections to the Cover Date. |
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| Keywords: | Fracture initiation fracture toughness stress intensity factus sharp notch |
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