Residually Stressed Bimaterial Beam Specimen for Measuring Environmentally Assisted Crack Growth
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RESEARCH PAPER
Residually Stressed Bimaterial Beam Specimen for Measuring Environmentally Assisted Crack Growth S.J. Grutzik1
· S. Aduloju2 · T. Truster2 · E.D. Reedy1
Received: 2 April 2020 / Accepted: 10 August 2020 © Sandia National Laboratories 2020
Abstract Background: Subcritical crack growth can occur in a brittle material when the stress intensity factor is smaller than the fracture toughness if an oxidizing agent (such as water) is present at the crack tip. Objective: We present a novel bi-material beam specimen which can measure environmentally assisted crack growth rates. The specimen is “self-loaded” by residual stress and requires no external loading. Methods: Two materials with different coefficient of thermal expansion are diffusion bonded at high temperature. After cooling to room temperature a subcritical crack is driven by thermal residual stresses. A finite element model is used to design the specimen geometry in terms of material properties in order to achieve the desired crack tip driving force. Results: The specimen is designed so that the crack driving force decreases as the crack extends, thus enabling the measurement of the crack velocity versus driving force relationship with a single test. The method is demonstrated by measuring slow crack growth data in soda lime silicate glass and validated by comparison to previously published data. Conclusions: The self-loaded nature of the specimen makes it ideal for measuring the very low crack velocities needed to predict brittle failure at long lifetimes. Keywords Slow crack growth · Glass fracture · Environmentally assisted crack growth · Crack propagation
Introduction Under inert conditions, a crack in a brittle material will only grow when the stress intensity factor at the crack tip equals the fracture toughness. If water or another oxidizing agent is present (as it is in almost all practical environments), oxidation at the crack tip to cause slow crack propagation at lower stresses [1, 2]. Relative humidity [3, 4] and temperature [4, 5] both enhance this phenomenon. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. S.J. Grutzik
[email protected] 1
Materials and Failure Modeling, Sandia National Laboratories, Albuquerque, NM 87185, USA
2
Department of Civil and Environmental Engineering, University of Tennessee, Knoxville, TN 37996, USA
Environmentally assisted crack growth (EACG) is a type of sub-critical crack growth and can cause flaws that would otherwise be stable to grow to a point where they become critical. As such, EACG is an important factor in determining service life or shelf life of a component containing brittle materials. In the past, EACG has been studied under a variety of conditions but specimens have typically required a sustained mechanical load to be externally applied. The most common experimental approach requires a double
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