Education
- Ph.D. (Dr.-Ing.), University of Stuttgart, Germany, 1996
- M.S. (Dipl.-Ing.), University of Stuttgart, Germany, 1989
Work Experience
- National Institute of Aerospace, Associate Research Fellow, 2011-present
- National Institute of Aerospace, Senior Research Engineer, 2003-2011
- ICASE, NASA Langley Research Center, Staff Scientist, 2000-2002
- NRC Research Fellow, NASA Langley Research Center, 1997-2000
- Research and Teaching Assistant, University of Stuttgart, 1989-1996
Research Areas/Expertise
- 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
- Face sheet/core disbonding in honeycomb sandwich structures
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 Advanced Composites Project (ACP). This research involves geometrically nonlinear finite element analyses of reinforced composite components containing delaminations at the skin/stringer interface. Delamination failure criteria are compared to computed mixed-mode strain energy release rates to predict static ultimate strength.
Analysis Benchmarking
As new approaches for analyzing composite delamination are incorporated in finite element codes, the need for verification and validation (V&V) arises 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.
Face Sheet/Core Disbonding
Face sheet/core disbonding and core fracture, are typical damage modes in light honeycomb sandwich structures, which can pose a threat to the structural integrity of a component. These damage modes are of particular interest to certification authorities since several in-service occurrences such as rudder structural failure and other control surface malfunctions have been attributed to disbonding. Extensive studies have shown that face sheet/core disbonding and core fracture can lead to damage propagation caused by internal pressure changes in the core due to ground-air-ground (GAG) cycles. Two general steps are required to identify, describe and address the phenomenon associated with face sheet/core disbonding and core fracture. First, a reliable means of characterizing face sheet/core disbonding should be developed. Second, analysis methods are required to help assess the likelihood of a structure exhibiting critical disbonding.
Publications
Krueger R., Deobald L., Gu H., “A Benchmark Example for Delamination Propagation Predictions Based on the Single Leg Bending Specimen under Quasi-static and Fatigue Loading,” in Proceedings of ASC 33rd Technical Conference; 2018; Seattle, WA, USA.
Mabson G.E., Carvalho N., Krueger R., “VCCT with Progressive Nodal Release for Simulating Mixed-Mode Delamination: Formulation, Algorithmic Improvements,” in Proceedings of ASC 33rd Technical Conference; 2018; Seattle, WA, USA.
Saseendran V., Berggreen C., Krueger R., “Mode Mixity Analysis of Face/Core Debonds in a Single Cantilever Beam Sandwich Specimen,” Journal of Sandwich Structures & Materials. 2018.
Krueger R., Chen Z., “Development of a methodology to address face sheet to core separation in sandwich structures,” in Proceedings of 12th International Conference on Sandwich Structures (ICSS-12); 2018; Lausanne – Switzerland.
Krueger R., “Face sheet to core separation caused by internal core pressure: An important failure mode for aerospace structures made of honeycomb sandwich,” in Proceedings of 12th International Conference on Sandwich Structures (ICSS-12); 2018; Lausanne – Switzerland.
Krueger R., “Finite Element Analysis of Composite Joint Configurations With Gaps and Overlaps,” DOT/FAA/TC-18/4. 2018.
Carvalho N., Krueger R., Mabson G.E., Deobald L.R., “Combining Progressive Nodal Release with the Virtual Crack Closure Technique to Model Fatigue Delamination Growth Without Re-meshing”, In Proceedings of AIAA SciTech Forum – AIAA/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Kissimmee, Florida, AIAA-2018-1468, 2018.
Krueger R., Carvalho N., Sasdelli M., “Searching for Run-Time Efficient Approaches to Delamination Growth Predictions in Composites,” in Proceedings of NAFEMS World Congress 2017; Stockholm, Sweden.
Deobald L.R. et al., “Guidelines for VCCT-based Interlaminar Fatigue and Progressive Failure Finite Element Analysis,” NASA/TM–2017-219663, 2017.
Krueger R., Carvalho N., “In Search of a Time Efficient Approach to Crack and Delamination Growth Predictions in Composites,” in Proceedings of 31st ASC Technical Conference, 2016, Williamsburg, VA, USA.
Carvalho N., Krueger R., “Modeling Fatigue Damage Progression and Onset in Composites Using an Element-Based Virtual Crack Closure Technique Combined with the Floating Node Method,” in Proceedings of 31st ASC Technical Conference, 2016, Williamsburg, VA, USA.
Ratcliffe J.G., Krueger R., “Face Sheet/Core Disbonding in Sandwich Composite Components: A Road Map to Standardization and Status Report,” in Proceedings of the 11th International Conference on Sandwich Structures (ICSS-11),2016; Boca Raton, FL, USA.
Chen Z., Krueger R., Rinker M., “Face Sheet/Core Disbond Growth in Honeycomb Sandwich Panels Subjected to Ground-Air-Ground Pressurization and In-Plane Loading,” in Proceedings of the 11th International Conference on Sandwich Structures (ICSS-11),2016; Boca Raton, FL, USA.
Krueger R., “The Virtual Crack Closure Technique for Modelling Interlaminar Failure and Delamination in Advanced Composite Materials,” In: Camanho P, Hallett S, editors. Numerical Modelling of Failure in Advanced Composite Materials: Woodhead Publishing Ltd,2015, p. 3-53.
Krueger R., “The NAFEMS Composites Working Group,” In: Benchmark – The International Magazine For Engineering Designers & Analysts. July 2015.
Krueger R., “A summary of benchmark examples and their application to assess the performance of quasi-static delamination propagation prediction capabilities in finite element codes,” Journal of Composite Materials. 2015; Vol. 49(26) pp. 3297-316.
Chen Z., Krueger R., Rinker M., “Face Sheet/Core Disbond Growth in Honeycomb Sandwich Panels Subjected to Ground-Air-Ground Pressurization and In-Plane Loading,” in Proceedings of NAFEMS World Congress, 2015, San Diego, CA, USA.
Krueger R., “Finite Element Analysis of Composite Joint Configurations With Gaps and Overlaps,” NASA/CR-2014-218284, NIA report no. 2014-0619. 2014
Krueger R., “Create your own benchmark examples and learn about delamination propagation prediction capabilities in commercial finite element codes and assess their performance,” Benchmark – The International Magazine For Engineering Designers & Analysts, April 2014.
Carvalho N., Krueger R., “Combining the eXtended Finite Element Method with Virtual Crack Closure Technique and Cohesive Surface Modeling to simulate delamination migration in cross-ply laminates,” in Proceedings of SIMULIA Community Conference; 2014; Providence, RI, USA.
Rinker M., Ratcliffe J., Adams D., Krueger R., “Characterizing Facesheet/Core Disbonding in Honeycomb Core Sandwich Structure,”NASA/CR-2013-217959, NIA report no. 2013-0115, 2013.
Rinker M., Krueger R., Ratcliffe J., “Analysis of an Aircraft Honeycomb Sandwich Panel with Circular Facesheet/Core Disbond Subjected to Ground-Air Pressurization,” NASA/CR-2013-217974, NIA report no. 2013-0116, 2013.
Krueger R., Shivakumar K., Raju I.S., “Fracture Mechanics Analysis for Interface Crack Problems – A Review,” in Proceedings of 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference; 2013; Boston, MA, USA, AIAA 2013-1476.
Krueger R., “Development and Applications of Benchmark Examples for Static Delamination Propagation Predictions,” in Proceedings of 28th ASC Technical Conference, 2012, College Park, PA, USA.
Orifici A., Krueger R., “Benchmark Assessment of Automated Delamination Propagation Capabilities in Finite Element Codes For Static Loading,” Finite Elements in Analysis and Design. 2012; Vol. 54, pp. 28-36.
Krueger R., “Development of a Benchmark Example for Delamination Fatigue Growth Prediction,” In: Gibson R.F., editor. American Society for Composites Series on Advances in Composite Materials, Volume 3: Fatigue of Composite Materials: DEStech Publications, Inc.; 2012.
Krueger R., “Application of Benchmark Examples to Assess the Single and Mixed-Mode Static Delamination Propagation Capabilities in ANSYS,” NASA/CR-2012-217588, NIA report no. 2012-04. 2012.
Krueger R., “Development and Application of Benchmark Examples for Mixed-Mode I/II Quasi-Static Delamination Propagation Predictions,” in Proceedings of ASC 27th Technical Conference; 2012; Arlington, TX, USA.
Krueger R., “Development and Application of Benchmark Examples for Mixed-Mode I/II Quasi-Static Delamination Propagation Predictions,” NASA/CR-2012-217562, NIA report no. 2012-01, 2012.
Krueger R., “Development of Benchmark Examples for Quasi-Static Delamination Propagation and Fatigue Growth Predictions,” in Proceedings of SIMULIA Customer Conference; 2012; Providence, RI.
Krueger R., “Development of Benchmark Examples for Static Delamination Propagation and Fatigue Growth Prediction,” in Proceedings of NAFEMS World Congress; 2011; Boston, MA, USA.
Krueger R., “Development of Benchmark Examples for Static Delamination Propagation and Fatigue Growth Predictions,” in Proceedings of 67th American Helicopter Society Annual Forum & Technology Display; 2011; Virginia Beach, VA.
Krueger R., “Development and Application of Benchmark Examples for Mode II Static Delamination Propagation and Fatigue Growth Predictions. NASA/CR-2011-217305, NIA report no. 2011-02, 2011.
Orifici A.C., Krueger R., “Assessment of Static Delamination Propagation Capabilities in Commercial Finite Element Codes Using Benchmark Analysis,” NASA/CR-2010-216709, NIA report no. 2010-03, 2010.
Krueger R., “Development of a Benchmark Example for Delamination Fatigue Growth Prediction,” in Proceedings of 25th ASC Annual Technical Conference, 2010; Dayton, Ohio, USA.
Krueger R., “Development of a Benchmark Example for Delamination Fatigue Growth Prediction,” NASA/CR-2010-216723, NIA report no. 2010-04, 2010.
Krueger R., Ratcliffe J.G., Minguet P.J., “Panel Stiffener Debonding Analysis Using A Shell/3D Modeling Technique,” Composites Science and Technology, 2009; Vol. 69(14), pp 2352-62.
Paris I., Cvitkovich M., Krueger R., “Characterization of Fatigue Damage for Bonded Composite Skin/Stringer Configurations,” NASA/TM-2008-215308, 2008.
Krueger R., “An Approach to Assess Delamination Propagation Simulation Capabilities in Commercial Finite Element Codes, NASA/TM-2008-215123, 2008.