Mixed-mode fracture of ductile thin-sheet materials under combined in-plane and out-of-plane loading

Abstract

Cracks in thin structures often are subjected to combined in-plane and out-of-plane loading conditions leading to complex mixed mode conditions in the crack tip region. When applied to ductile materials, large out-of-plane displacements make both experimentation and modeling difficult. In this work, the mixed-mode behavior of thin, ductile materials containing cracks undergoing combined in-plane tension (mode I) and out-of-plane shear (mode III) deformation is investigated experimentally. Mixed-mode fracture experiments are performed and full, threedimensional (3D) surface deformations of thin-sheet specimens from aluminum alloy and steel are acquired using 3D digital image correlation. General characteristics of the fracture process are described and quantitative results are presented, including (a) the fracture surface, (b) crack path, (c) load-displacement response, (d) 3D full-field surface displacement and strain fields prior to crack growth, (e) radial and angular distributions of the crack-tip strain fields prior to crack growth and (f) singularity analysis of the crack-tip strains prior to crack growth. Results indicate that the introduction of a mode III component to the loading process (a) alters the crack tip fields relative to those measured during nominally mode I loading and (b) significantly increases the initial and stable critical crack-openingdisplacement. The data on strain fields in both AL6061-T6 aluminum and GM6208 steel consistently show that for a given strain component, the normalized angular and radial strains at all load levels can be reasonably represented by a single functional form over the range of loading considered, confirming that the strain fields in highly ductile, thin-sheet material undergoing combined in-plane tension and out-of-plane shear loading can be expressed in terms of separable angular and radial functions. For both materials, the displacement and strain fields are (a) similar for both mixed-mode loading angles = 30 • and = 60 • and (b) different from the fields measured for Mode I loading angle = 0 • . Relative to the radial distribution, results indicate that the in-plane strain components do not uniformly exhibit the singularity trends implicit in the HRR theory.

Mixed-mode crack growth in ductile thin-sheet materials under combined in-plane and out-of-plane loading

“…17 are consistent with our previously reported results Yan et al 2007). The mixed mode I/III average stable COD for AL6061-T6 (GM6208 steel) is 4×(3×) greater than the Mode I and mixed-mode I/II stable COD.…”

“…(5) The loading fixture with the specimen is removed, the two cameras are synchronized, and the stereovision system is calibrated according to the procedures outlined in Sutton et al (2007) and Yan et al (2007). From the calibration results, the measurement precision is estimated to 0.0012 mm for the in-plane displacement measurement and 0.0097 mm for the out-of-plane displacement measurement for a typical mixed mode I/III experiment setup with the image scale factor being about 25 pixel/ mm ).…”

“…Remote Mode I Mode III loading cases correspond to loading angles = 0 • and = 90 • , respectively. Detailed information regarding the loading fixture can be found in Yan et al (2007). It should be pointed out that the extension bar (item 10 in Fig.…”

“…Mixed-mode I/III fracture has been the focus of many investigations (Pan 1990;Pan and Shih 1992;Paterson and Gungor 1997;Gao and Shih 1998;Jones et al 2001;Kamat and Hirth 1995;Kamat et al 1998;Liu et al 2004;Zucchini et al 2000;Sutton et al 2001;Yan et al 2007;Wei et al 2005;Wei 2008), with most of the work focused on fracture toughness measurements and evaluation (Jones et al 2001;Kamat and Hirth 1995;Kamat et al 1998;Li et al 1998) instead of on stable tearing of thin sheet materials under mixed mode I/III loading. Mixed mode I/III fracture in ductile thin sheet materials is quite complex, including large scale yielding, significant warping of the specimen (Wei et al 2005;Sutton et al 2001;Yan et al 2007;Sutton et al 2007) and a highly three-dimensional (3D) response of the flawed component, resulting in complex stress and strain fields near the crack tip (Wei et al 2005;Zucchini et al 2000;Sutton et al 2001;Pan 1990;Pan and Shih 1992).…”

“…Mixed mode I/III fracture in ductile thin sheet materials is quite complex, including large scale yielding, significant warping of the specimen (Wei et al 2005;Sutton et al 2001;Yan et al 2007;Sutton et al 2007) and a highly three-dimensional (3D) response of the flawed component, resulting in complex stress and strain fields near the crack tip (Wei et al 2005;Zucchini et al 2000;Sutton et al 2001;Pan 1990;Pan and Shih 1992). This situation has led to limited reports of experimental investigations and numerical simulations regarding crack growth under mixed mode I/III loading in the literature (Paterson and Gungor 1997;Wei et al 2005;Sutton et al 2001Sutton et al , 2007Yan et al 2007;Wei 2008).…”