Resident at: NASA Langley Research Center

Durability, Damage Tolerance and Reliability Branch

Research Interests

·         Finite element analysis

·         Durability and damage tolerance of composites

·         Failure of structures and components made of composite materials

·         Computational fracture mechanics (Virtual Crack Closure Technique – VCCT)

·         Analysis of skin/stiffener disbonding

·         Development of benchmark examples for finite element analysis

Current Research

·         Analysis of Composite Delamination

The purpose of the research is to apply a methodology based on interlaminar fracture mechanics to large scale components containing delaminations.  The methodology has been used successfully in the past primarily to investigate delamination onset and disbonding in fracture toughness specimens and laboratory size coupon type specimens.  The research is performed to support the goals defined in NASA’s Aircraft Aging and Durability project as part of NASA’s Aviation Safety Program.  This research involves geometrically nonlinear finite element analyses of reinforced composite components containing delaminations at the composite skin/stringer interface.  Delamination failure criteria are compared to computed mixed-mode strain energy release rates to predict static ultimate strength. Collaborators: James Ratcliffe (NIA), Kevin O’Brien (NASA Langley), Gretchen Murri (NASA Langley)

·         Analysis Benchmarking

As new approaches for analyzing composite delamination are incorporated in finite element codes, the need for comparison and benchmarking becomes important.  Current work is focused on creating an approach that allows the assessment of delamination propagation capabilities in commercial finite element codes.

Therefore, an approach for assessing the delamination propagation simulation capabilities under static loading was developed and demonstrated. For this investigation, the Double Cantilever Beam (DCB) specimen and the Single Leg Bending (SLB) specimen were chosen for full three-dimensional finite element simulations. First, benchmark results were created for both specimens. Second, starting from an initially straight front, the delamination was allowed to propagate. The load-displacement relationship and the total strain energy obtained from the propagation analysis results and the benchmark results were compared and good agreements could be achieved by selecting the appropriate input parameters. Selecting the appropriate input parameters, however, was not straightforward and often required an iterative procedure. Qualitatively, the delamination front computed for the DCB specimen did not take the shape of a curved front as expected. However, the analysis of the SLB specimen yielded a curved front as was expected from the distribution of the energy release rate and the failure index across the width of the specimen. Overall, the results are encouraging but further assessment on a structural level is required.

An approach for assessing the delamination growth simulation capabilities under cyclic loading is under development. Collaborators: Kevin O’Brien (NASA Langley), Adrian Orifici (RMIT, Melbourne, Australia)

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Publications

·         R. Krueger, J. G. Ratcliffe, and P. J. Minguet, Panel Stiffener Debonding Analysis Using A Shell/3D Modeling Technique, Composites Science and Technology, vol. 69, pp. 2352-2362, 2009. doi:10.1016/j.compscitech.2008.12.015

·         R. Krueger, An Approach to Assess Delamination Propagation Simulation Capabilities in Commercial Finite Element Codes,  NASA/TM-2008-215123, 2008.

·         R. Krueger and P.J. Minguet, Analysis of Composite Skin-Stiffener Debond Specimens using a Shell-3D Modeling Technique, Composite Structures. Vol. 81, pp. 41-59, 2007. doi:10.1016/j.compstruct.2006.05.006

·         T.K. O’Brien and R. Krueger.  Influence of Compression and Shear on the Strength of Composite Laminates with Z-Pinned Reinforcement. Applied Composite Materials, Vol. 13, pp. 173-189, 2006. doi:10.1007/s10443-005-9005-4

·         R. Krueger, Virtual Crack Closure Technique: History, Approach and Applications, Applied Mechanics Reviews, vol. 57, pp. 109-143, 2004. DOI:10.1115/1.1595677

·         I.L. Paris, R. Krueger, T.K. O’Brien, Effect of Assumed Damage and Location on the Delamination Onset Predictions for Skin-Stiffener Debonding, AHS Journal, vol. 49, pp. 501-507, 2004.

·         T.K. O’Brien and R. Krueger, Analysis of Flexure Tests for Transverse Tensile Strength Characterization of Unidirectional Composites, Journal of Composites Technology and Research, Vol. 25, pp. 50-68, 2003. DOI: 10.1520/CTR11004J

·         R. Krueger, I. L. Paris, T. K. O'Brien, and P. J. Minguet, Fatigue Life Methodology for Bonded Composite Skin/Stringer Configurations, Journal of Composites Technology and Research, vol. 24, pp. 56-79, April 2002. doi:10.1520/CTR10561J

·         R. Krueger, I. L. Paris, T. K. O'Brien, and P. J. Minguet, Comparison of 2D Finite Element Modeling Assumptions with Results from 3D Analysis for Composite Skin-Stiffener Debonding, Composite Structures, vol. 57, pp. 161-168, 2002. doi:10.1016/S0263-8223(02)00079-X

·         R. Krueger and T. K. O'Brien, A Shell/3D Modeling Technique for the Analysis of Delaminated Composite Laminates, Composites Part A: Applied Science and Manufacturing, vol. 32, pp. 25-44, Jan-2001 2001. doi:10.1016/S1359-835X(00)00133-0

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