Associate Research Fellow

Ronald Krueger

Ronald Krueger
757-864-3482
NIA

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
  • Face sheet/core disbonding in honeycomb sandwich structures

Education

  • Ph.D. (Dr.-Ing.), Aerospace Engineering, University of Stuttgart, Stuttgart, Germany (1996)
  • M.Eng. (Dipl.-Ing.) Aerospace Engineering, University of Stuttgart, Stuttgart, Germany (1989)

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. Collaborators: Nelson Carvalho (NIA), James Ratcliffe (NASA), 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 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.

Initially, an approach for assessing the delamination propagation simulation capabilities under static loading was developed and demonstrated. For these investigations, the Double Cantilever Beam (DCB) specimen, the End-Notched Flexure (ENF) specimen, the Mixed-Mode Bending (MMB) specimen and the Single Leg Bending (SLB) specimen were chosen for two-dimensional planar and full three-dimensional finite element simulations. First, benchmark results were created for all the 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.

Later, the approach was extended to allow for the assessment of delamination fatigue growth prediction capabilities in commercial finite element codes. As for the static case, benchmark results were created manually first for the mode I DCB and the mode II ENF specimens. Second, the delamination was allowed to grow under cyclic loading in a finite element model of a commercial code. For all cases, input control parameters were varied to study the effect on the computed delamination propagation and growth. The benchmarking procedure proved valuable by highlighting the issues associated with choosing the input parameters of the particular implementation. Consequently, the benchmark enabled the selection of the appropriate input parameters that yielded good agreement between the results obtained from the growth analysis and the benchmark results. Overall, the results are encouraging but further assessment on a structural level is required.

Collaborators: Kevin O’Brien (NASA Langley), Adrian Orifici (RMIT, Melbourne, Australia)

  • 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.

To address the analysis step, a honeycomb sandwich panel containing a circular disbond at one facesheet/core interface was modeled with 3D solid finite elements. The disbond was modeled as a discrete discontinuity and the strain energy release rate along the disbond front was computed using the Virtual Crack Closure Technique. Special attention was paid to the pressure-deformation coupling which can decrease the pressure load within the disbonded sandwich section significantly when the structure is highly deformed. The recursive pressure-deformation coupling was solved by defining fluid filled cavities in Abaqus/Standard® to represent the entrapped air in the honeycomb cells. The results show that disbond size, face sheet thickness and core thickness are important parameters that determine crack tip loading at the disbond front.

Collaborators: Zhi Chen (FAA), James Ratcliffe (NASA)

 

Selected Publications

  • R. Krueger, 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, Published online before print doi: 10.1177/0021998314561812, 2015.
  • Z. M. Chen, R. Krueger, and M. Rinker, Face Sheet/Core Disbond Growth in Honeycomb Sandwich Panels Subjected to Ground-Air-Ground Pressurization and In-Plane Loading, proceedings of NAFEMS World Congress, 2015.
  • N. V. D. Carvalho and R. Krueger, Combining the eXtended Finite Element Method with Virtual Crack Closure Technique and Cohesive Surface Modeling to simulate delamination migration in cross-ply laminates,” proceedings of 2014 SIMULIA Community Conference, 2014.
  • M. Rinker, R. Krueger, and J. Ratcliffe, 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.
  • R. Krueger, K. Shivakumar, and I. S. Raju, Fracture Mechanics Analysis for Interface Crack Problems – A Review, proceedings of 54th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, AIAA 2013-1476, 2013.
  • R. Krueger, 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.
  • A. Orifici and R. Krueger, Benchmark Assessment of Automated Delamination Propagation Capabilities in Finite Element Codes For Static Loading, Finite Elements in Analysis and Design, Vol. 54, pp. 28-36, 2012.
  • R. Krueger, Development and Application of Benchmark Examples for Mode II Static Delamination Propagation and Fatigue Growth Predictions, NASA/CR-2011-217194, NIA report no. 2011-02, 2011.
  • R. Krueger, Development of a Benchmark Example for Delamination Fatigue Growth Prediction, NASA/CR-2010-216723, NIA report no. 2010-04, 2010.
  • 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.
  • 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.
  • 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.
  • R. Krueger, Virtual Crack Closure Technique: History, Approach and Applications, Applied Mechanics Reviews, vol. 57, pp. 109-143, 2004.
  • 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.
  • 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, 2002.
  • 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.
  • 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, 2001.