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Keywords: size effect, wrinkling, delamination.

Summary

The observed size effect follows neither the strength theory nor the linear elastic fracture mechanics, hence the delamination fracture of laminate-foam sandwiches under uniform bending moment is treated using the cohesive crack model. Both two-dimensional geometrically nonlinear finite element analysis and a one-dimensional representation of skin (or facesheet) as a beam on an elastic-softening foundation are used [1]. The use of the latter is made possible by realizing that the effective elastic foundation stiffness depends on the ratio of the critical wavelength of periodic skin wrinkles to the foam core thickness, and a simple description of the transition from shortwave to longwave wrinkling is obtained by asymptotic matching. Good agreement between both approaches is achieved. Skin imperfections (considered proportional to the the first eigenmode of wrinkling), are shown to lead to strong size dependence of the nominal strength. For large imperfections the strength reduction due to size effect can reach 50%. Dents from impact, though not the same as imperfections, might be expected to cause as a similar size effect. Using proper dimensionless variables, numerical simulations of cohesive delamination fracture covering the entire practical range are performed. Their fitting, heeding the shortwave and longwave asymptotics, leads to an approximate imperfection-dependent size effect law of an asymptotic matching type. The strong size effect on postpeak energy absorption, important for impact analysis, is also demonstrated. Finally, discrepancies among various existing formulas for critical stress at periodic elastic wrinkling are explained by their applicability to different special cases in the shortwave-longwave transition.

References

1: Z.P. Bazant and P. Grassl, "Size effect of cohesive delamination fracture triggered by sandwich skin wrinkling", Journal of Applied Mechanics, ASME, In press. doi:10.1115/1.2722778

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Front Page Browse CCP CSETS CTR IJRT Other Authors Search Purchase Guide FAQ Contact us	Civil-Comp Proceedings ISSN 1759-3433 CCP: 85 PROCEEDINGS OF THE FIFTEENTH UK CONFERENCE OF THE ASSOCIATION OF COMPUTATIONAL MECHANICS IN ENGINEERING Edited by: B.H.V. Topping Paper 63 Size Effect of Cohesive Delamination Fracture Triggered by Sandwich Skin Wrinkling Z. Bazant¹ and P. Grassl² ¹Northwestern University, Evanston IL, United States of America ²University of Glasgow, United Kingdom doi:10.4203/ccp.85.63 Full Bibliographic Reference for this paper Z. Bazant, P. Grassl, "Size Effect of Cohesive Delamination Fracture Triggered by Sandwich Skin Wrinkling", in B.H.V. Topping, (Editor), "Proceedings of the Fifteenth UK Conference of the Association of Computational Mechanics in Engineering", Civil-Comp Press, Stirlingshire, UK, Paper 63, 2007. doi:10.4203/ccp.85.63 Keywords: size effect, wrinkling, delamination. Summary The observed size effect follows neither the strength theory nor the linear elastic fracture mechanics, hence the delamination fracture of laminate-foam sandwiches under uniform bending moment is treated using the cohesive crack model. Both two-dimensional geometrically nonlinear finite element analysis and a one-dimensional representation of skin (or facesheet) as a beam on an elastic-softening foundation are used [1]. The use of the latter is made possible by realizing that the effective elastic foundation stiffness depends on the ratio of the critical wavelength of periodic skin wrinkles to the foam core thickness, and a simple description of the transition from shortwave to longwave wrinkling is obtained by asymptotic matching. Good agreement between both approaches is achieved. Skin imperfections (considered proportional to the the first eigenmode of wrinkling), are shown to lead to strong size dependence of the nominal strength. For large imperfections the strength reduction due to size effect can reach 50%. Dents from impact, though not the same as imperfections, might be expected to cause as a similar size effect. Using proper dimensionless variables, numerical simulations of cohesive delamination fracture covering the entire practical range are performed. Their fitting, heeding the shortwave and longwave asymptotics, leads to an approximate imperfection-dependent size effect law of an asymptotic matching type. The strong size effect on postpeak energy absorption, important for impact analysis, is also demonstrated. Finally, discrepancies among various existing formulas for critical stress at periodic elastic wrinkling are explained by their applicability to different special cases in the shortwave-longwave transition. References 1 Z.P. Bazant and P. Grassl, "Size effect of cohesive delamination fracture triggered by sandwich skin wrinkling", Journal of Applied Mechanics, ASME, In press. doi:10.1115/1.2722778 Electronic delivery for this paper is not yet available. However, you can request that we bring the scanning of this paper forward. We will then contact you when it available. Lodge your request here. go to the previous paper go to the next paper return to the table of contents return to the book description purchase this book (price £75 +P&P)
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