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Welcome to Dr. L. Roy Xu's homepage

Research Professor University of New Mexico

emails: l.roy.xu@alumni.caltech.edu or lrxu@unm.edu

Education

Ph.D. in Aeronautics and Material Science, California Institute of Technology Thesis: Dynamic Failure Characteristics in Layered Materials and Structures. Advisor: Prof. Ares J. Rosakis , Members of the National Academy of Engineering, National Academy of Science.

Honor and award

Research interests

  • Biologically Inspired Designs and Mechanics of Biomaterials

  • Composite Materials and Structures-- Designs and Analysis

  • Dynamic Failure Mechanics for Protecting Structures /Personnel during Extreme Events (blast, impact, hurricane, earthquake)

  • Damage Tolerance Design, Experimental Mechanics, Interface Mechanics, Fracture and Failure Mechanics

  • Environmental Effects/Durability of Composite Structures and Sustainability

  • Nanocomposite Materials and Nanoindentation Technique

My research is focused on the mechanical response and failure of advanced materials and structures at different time-scales (static to dynamic) and length-scales (nano-scale to macro-scale). Typical applications include nano/micro-scale engineering, defense, and homeland security engineering. My work is supported by the US National Science Foundation, the Office of Naval Research and the Army Research Office, and the Air Force Office of Scientific Research.

Actually, my major research work is to “torture” all kinds of advanced materials (pull them, bend them and hit them…) till they are broken before they could be used safely in modern structures and devices.

Here is our most recent result of nanoindentation: a simple yet accurate approach to measure the Young's moduli of polymers and composites---"Measurements of Through-

Thickness Young’s Moduli of Fibrous Composite Laminates Using Nanoindentation" (PDF file)

YouTube videos about our previous research work could be found from my YouTube Channel

1. High-speed photography showed a bullet was penetrating a brittle polymer layer bonded to a steel layer. Details could be found: L. R. Xu and A. J. Rosakis, “Impact Damage Visualization of Heterogeneous Two-Layer Materials Subjected to Low-speed Impact,” International Journal of Damage Mechanics, Vol. 1, pp.215-233, 2005 ( PDF file)

2. Delamination crack propagation of an impacted composite laminates. Details could be found:

L. R. Xu; Arun Krishnan; Haibin Ning; Uday Vaidya, “A seawater tank approach to evaluate the dynamic failure and durability of E-glass/vinyl ester marine composites”, Composites Part B. Vol. 43, pp. 2480–2486, 2012.(PDF file)

3. Interfacial crack initiation and propagation from a notch formed by two bonded polymer plates. Details could be found: A. Krishnan and L. R. Xu, “Experimental Studies on the Interaction among Cracks, Notches and Interfaces of Bonded Polymers”, International Journal of Solids and Structures, 2013. Vol. 50, pp. 1583–1596 (PDF file)

4. High-speed photography showed stress wave interaction and crack propagation and arrest in two bonded brittle polymer with a thin interface. Details could be found: L. R. Xu and A. J. Rosakis, “An Experimental Study on Dynamic Failure Events in Homogeneous Layered Materials Using Dynamic Photoelasticity and High-speed Photography,” Optics and Laser in Engineering, Vol. 40, pp. 263-288,2003. ( PDF file)

Major research projects and funding (total $1.75 M federal funding as the PI)

1. "A Multi-Scale Approach of Combining Nano-indentation with Computational Mechanics to Predict Impact Behavior of Structural Composite Materials," Air Force Office of Scientific Research, PI: L. R. Xu,

co-PI, Prof. Ertugrul Taciroglu (UCLA). The project summary is shown in the illustration below.

2. “Mechanisms of Enhancing Impact Resistance of Layered Materials Using Thin Polymeric Interfaces,” Army Research Office, PI: L. R. Xu. The project summary is shown in the illustration below.

3. “I-Corps: Hybrid Material Carry-On Shields for Civilians,” National Science Foundation, PI. L. R. Xu

More information of this project was reported by Local Newspaper Las Cruces Sun-New and could be found here (web like) . A new personal protection concept is shown here :

A prototype of the carry-on shield

4.“Combined investigation on durability and dynamic failure of advanced naval materials,” Office of Naval Research, PI

5.“GOALI and collaborative research: a combined materials and mechanics study on failure and improvement of coating/substrate interfaces,” National Science Foundation, PI.

6.“Intrinsic Impact and Fatigue Property Degradation of Composite Materials in Seawater,” Office of Naval Research, PI.

7. “SGER: Application of Dynamic Failure Mechanics in Material and Interface Selections for Protecting Critical Structures,” National Science Foundation, Structural Systems and Hazards Mitigation of Structures Program, PI.

8. “Novel Designs for Interfacial Mechanical Property Enhancement and Characterization of Dissimilar Materials and Structures,” National Science Foundation, Surface Engineering and Material Design Program, PI.

9. “Enhancing the Interfacial and Dynamic Failure Behavior of Advanced Hybrid Structures Using Nanocomposite Materials,” Office of Naval Research, Young Investigator Program, PI.

Teaching

    • Graduate “Advanced Mechanics of Solids, I & II" (Theory of Elasticity, Plates and Shells, Buckling, Plasticity, Stress Wave and Fracture Mechanics)

  • Graduate "Mechanics of Composite Materials"

    • Graduate "Nano-scale Science and Engineering", co-instructor

      • Undergraduate "Mechanics of Materials"

      • Undergraduate "Statics"

Publications (46 journal papers, h-index 19, total citations>1,000)

Here is the Google scholar citations (click this link)

A. Book chapter

L. R. Xu and A. J. Rosakis, “Real-time Experimental Investigation on Dynamic Failure of Sandwich Structures and Layered Materials”, Composite Materials and Sandwich Structures, An Anthology of ONR Sponsored Research (editors: E.E. Gdoutos, I.M. Daniel and Y.D.S. Rajapakse), pp. 571-604, Springer publisher, 2009. (PDF file)

B. Peer-reviewed publication (journal papers)

1. L. Xu,"Exploration on Constants Independent of Materials and Layups in Transverse Matrix Cracking of Cross-ply Laminates,"Advanced Composites Letter, 1993, Vol.2, No.1, pp.25-29.

2. L. Xu, "Interaction between Matrix Cracking and Edge Delamination in Composite Laminates, " Composites Science and Technology, 1994, Vol. 50, No.4, pp.469-478 (Full paper in PDF file).

3. L. Xu, C.H. Kou "Effect of Interfacial Interleaf to the Interlaminar Fracture and Intralaminar Fracture in A New Bismaleimide MatrixHistry Composites System ,"Journal of Reinforced Plastics and Composites, 1994, Vol.13, No.6, pp.509-540.(Full paper in PDF file).

4. L. Xu, "Influence of Stacking Sequence on the Transverse Matrix Cracking in Continuous Fiber Crossply Laminates," Journal of Composite Materials, 1995,Vol. 29, No.10, pp.1337-1339 (Full paper in PDF file)..

5. L. Xu, "Study on the Characteristic Curve of Stiffness Degradation Caused by Matrix Cracking in Multi-directional Composite Laminates," Journal of Composite Materials, 1996, Vol.30, No.7, pp.820-838 (Full paper in PDF file).

6. L. Xu, "On the Law of the Transverse Matrix Cracking Growth in Brittle Polymeric Matrix Composite Laminates under Static Loadings," Journal of Composite Materials,1996,Vol.30, No.3, pp.384-405.(Full paper in PDF file).

7. L. Xu, "Modifying Stacking Sequence Design to Delay Delamination and Matrix Crack in Laminated Composites---The Multi-Delamination-Interface Design," Journal of Reinforced Plastics and Composites , 1996 , Vol. 15, No.3, pp.230-248

8. L. Xu, " Probabilistic Character of the Microcrack Growth in Brittle Layered Materials," Theoretical and Applied Fracture Mechanics, 1997, Vol.27 ,pp.167-174 (Full paper in PDF file).

9. L. Xu, " Some Techniques to Establish the Characteristic Curve for Microcrack Growth in Layered Materials," Engineering Fracture Mechanics, 1997, Vol. 56, No.6, pp.827-837

10. L. Xu, "Study on the Characteristic Stress-Life Curve of Different Materials and Failure Modes," Journal of Reinforced Plastics and Composites,1998, Vol.16, No.17, pp.1450-1462.(Full paper in PDF file).

11. L. R. Xu and A. J. Rosakis, “Impact Failure Characteristics in Sandwich Structures; Part I: Basic Failure Mode Selections,”International Journal of Solids and Structures, 2002, Vol.39, pp. 4215-35 ( PDF file)

12. L. R. Xu and A. J. Rosakis, “Impact Failure Characteristics in Sandwich Structures; Part II: Effects of Impact Speed and Interfacial Strength,” International Journal of Solids and Structures, 2002, Vol.39, pp. 4237-48 ( PDF file)

13. L. R. Xu and A. J. Rosakis, “An Experimental Study on Dynamic Failure Events in Homogeneous Layered Materials Using Dynamic Photoelasticity and High-speed Photography,” Optics and Laser in Engineering, Vol. 40, pp. 263-288,2003. ( PDF file)

14. L. R. Xu, Y. Y. Huang and A. J. Rosakis, “Dynamic Crack Deflection and Penetration at Interfaces in Homogeneous Materials: Experimental Studies and Model Predictions,” Journal of the Mechanics and Physics of Solids, Vol. 51, pp.425-460, 2003. ( PDF file)

15. L. R. Xu and A. J. Rosakis, “Real-time Experimental Investigation of Dynamic Crack Branching Using High-speed Optical Diagnostics,” SEM Experimental Techniques, Vol. 27,pp.23-26, 2003. (PDF file)

16. L. R. Xu and S. Sengupta and H. Kuai “An Experimental and Numerical Investigation on Adhesive Bonding Strengths of Polymer Materials,” International Journal of Adhesion and Adhesives, Vol. 24 , pp. 455–460, 2004 ((PDF file)

17. Zhong, W.H., Li, J., Xu, L.R ., Michel, J.A, Sullivan, L.M. and Lukehart, C.M. “Processing of Polymeric Nanocomposites Reinforced with Derivatized Graphitic Carbon Nanofibers with Linker Molecules”. Journal of Nanoscience and Nanotechnology,Vol. 4, pp.794-802, 2004.(PDF file)

18. L. R. Xu, V. Bhamidipati , W.-H. Zhong, J., Li, C. M. Lukehart, E. Lara-Curzio, K. C. Liu and M. J. Lance “Mechanical Property Characterization of A Polymeric Nanocomposite Reinforced by Graphitic Nanofibers with Reactive Linkers, Journal of Composite Materials, Vol. 38, pp. 1563-1582, 2004.( PDF file)-----Top 5 in 2005; and top 10 in 2006 of the Most-Frequently-Read Articles in Journal of Composite Materials.

19. L. R. Xu , H. Kuai and S. Sengupta “ Dissimilar Material Joints with and without Free-edge Stress Singularities; Part I: A Biologically Inspired Design,” Experimental Mechanics, Vol. 44, pp. 608-615, 2004 (PDF file)

20 . L. R. Xu , and S. Sengupta “ Dissimilar Material Joints with and without Free-edge Stress Singularities; Part II: An Integrated Numerical Analysis,” Experimental Mechanics, Vol. 44, pp. 616-621, 2004 ( PDF file)

21. Zhong, W.H., Li, J., Xu, L. R . and Lukehart, C.M. “GCNF/Epoxy Monoliths Using Reactive Oxydianiline Linker Molecules and the Effect of Nanofiber Reinforcement on Curing Conditions,” Polymer Composites, Vol. 26, pp. 128-135. 2005. (PDF file)

22. L. R. Xu and S. Sengupta, “Interfacial Stress Transfer and Property Mismatch in Discontinuous Nanofiber/nanotube Composite Materials,” Journal of Nanoscience and Nanotechnology, Vol. 5, pp.620-626, 2005 ( PDF file)

23 . L. R. Xu and A. J. Rosakis, “Impact Damage Visualization of Heterogeneous Two-Layer Materials Subjected to Low-speed Impact,” International Journal of Damage Mechanics, Vol. 1, pp.215-233, 2005 ( PDF file)

24 . L. R. Xu , H. Kuai and S. Sengupta“ Free-Edge Stress Singularities and Edge Modifications for Fiber Pushout Experiments,”Journal of Composite Materials, Vol. 39, pp.1103-1125, 2005. ( PDF file)

25. X.F. Li, and L. R. Xu “Transient Response of a Finite Bimaterial Plate Containing a Crack Perpendicular to and Terminating at the Interface,” ASME Journal of Applied Mechanics, 2006, Vol.73, pp.544-554 ( PDF file)

26. P. Wang and L. R. Xu “Convex Interfacial Joints with Least Stress Singularities In Dissimilar Materials and Structures,”Mechanics of Materials, 2006, Vol.38, pp.1001-1011.( PDF file)

27. L. R. Xu, V.Bhamidipati and S. Mahadevan “Shear-lag Analysis of Constrained Shrinkage Cracking in Reinforced Concrete Slabs,” Journal of Failure Analysis and Prevention, 2006, Vol. 6, pp. 57-66.

28. L. R. Xu and P. Wang “Dynamic Fracture Mechanics Analysis of Failure Mode Transitions along Weaken Interfaces in Elastic Solids,” Engineering Fracture Mechanics, 2006, Vol. 73, 1597-1614.( PDF file)

29. P. Wang and L. R. Xu “Dynamic Interfacial Debonding Initiation Induced by An Incident Crack,” International Journal of Solids and Structures, 2006, Vol. 43, pp.6535-6550.( PDF file)

30.X.F. Li, and L. R. Xu “T-stresses across Static Crack Kinking,” ASME Journal of Applied Mechanics, 2007, Vol. 74, pp. 181-190 ( PDF file)

31. L. R. Xu, L. Li, C. M. Lukehart and H. Kuai “Mechanical Property Characterization Nanofiber-Reinforced Composite Adhesives,” Journal of Nanoscience and Nanotechnology, Vol. 7 , pp.1-3, 2007 ( PDF file).

32. Z. Li, C. Ji, Y. Li and L.R. Xu, “The brittle fracture criterion based on the maximum tensile stress on the surface of blunt crack tip,” Mechanics Research Communications, 2007, vol.34, pp.472-479.

33. L. R. Xu, “In-class Paper Demonstration and Experiment for Mechanics of Materials and Advanced Solid Mechanics Courses,”International Journal of Engineering Education, Vol. 24, pp. 540-544. 2008 ( PDF file)

34. A. Krishnan and L. R. Xu, “Effect of the interfacial stress distribution on the material interfacial shear strength measurement,”Experimental Mechanics, Vol. 50. pp. 293-288. 2010 (PDF file)

35. H. Yan, C. Oskay, A. Krishnan and L.R. Xu, “Compression After Impact Response of Woven Fiber-Reinforced Composites,”Composite Science and Technology, Vol. 70, pp.2128-2136, 2010 (PDF file).

36. P. Cesar, V. Rosa, M. Pinto, H. Yoshimura and L. R. Xu, “Effect of ion exchange on R-curve behavior of a dental porcelain,” Journal of Materials Science, Vol. 46, pp. 117-122, 2011 (PDF file)

37. A. Krishnan and L. R. Xu, “Systematic evaluation of bonding strengths and fracture toughnesses of adhesive joints,” Journal of Adhesion, Vol. 87, pp.53-71, 2011.(PDF file)

38. A Krishnan and L.R. Xu, A short-beam shear fracture approach to measure the mode II fracture toughness of materials with preferred interfaces, International Journal of Fracture, Vol. 169, pp. 15-25, 2011(PDF file)

39. A. Krishnan* and L. R. Xu, “A Simple Effective Flaw Model on Analyzing the Nanofiller Agglomeration Effect of Nanocomposite Materials,” Journal of Nanomaterials, 2012. DOI:10.1155/2012/483093. (PDF file)

40. L. R. Xu; Arun Krishnan*; Haibin Ning; Uday Vaidya, “A seawater tank approach to evaluate the dynamic failure and durability of E-glass/vinyl ester marine composites”, Composites Part B. Vol. 43, pp. 2480–2486, 2012.(PDF file)

41. V.-X. Tran, D. Leguillon, A. Krishnan* and L. R. Xu “Interface Crack Initiation at V-Notches along Adhesive Bonding in Weakly Bonded Polymers Subjected to Mixed-Mode Loading,” International Journal of Fracture, Vol.176, pp.65-79. 2012. (PDF file)

42. A. Krishnan* and L. R. Xu, “Experimental Studies on the Interaction among Cracks, Notches and Interfaces of Bonded Polymers”, International Journal of Solids and Structures, 2013. Vol. 50, pp. 1583–1596 (PDF file)

43. M. Flores* and L. R Xu, “An Efficient Mixed-Mode Brittle Fracture Experiment Using Paper,” International Journal of Fracture, 2013, Vol. 183, pp. 267-273 (PDF file)

44. K. Zhao* and L R Xu “Size Effects of the Adhesive Bonding Strengths of Metal/Polymer Similar and Dissimilar Material Joints,” Journal of Adhesion, in press

45. R. Martinez* and L R Xu, “Comparison of the Young’s Moduli of Polymers Measured from Nanoindentation and Bending Experiments,” MRS Communications, Vol. 4, pp. 89–93. 2014 (PDF file)

46. A. Krishnan* and L. R. Xu, “An Experimental Study on the Crack Initiation from Notches Connected to Interfaces of Bonded Bi-materials,” Engineering Fracture Mechanics, 2013, Vol. 111, 65- (PDF file)

Professional Activities

Professional Society

        • American Society of Mechanical Engineers (ASME)---- Chair of the Fracture and Failure Mechanics Technical Committee 2012-2015.

        • American Institute of Aeronautics and Astronautics (AIAA)

        • American Society for Composites (ASC)

    • Materials Research Society (MRS)

        • Society for Experimental Mechanics (SEM)--- Chair of the Composite Material Technical Committee 2006-2008

Journal Referee

  • ASME Journal of Applied Mechanics

  • Composite Science and Technology

  • Experimental Mechanics

  • Mechanics of Materials

  • Polymers

  • Engineering Fracture Mechanics

  • International Journal of Fracture

  • Journal of Composite Materials and more

Personal Hobbies

Modeling Aircraft-- See F-18 Hornet aircraft model -A souvenir (I spent one month by using three simple tools ) as a player of the University Aero-model team at the Beijing University of Aeronautics and Astronautics. An exhibit at the Beijing Aerospace Museum for eight years

Research Lab

The Multi-scale Mechanical Testing Lab (MMTL) has one fatigue test machine, one impact tester and one nano-indentation system with high-temperature application to measure mechanical responses of advanced materials and structures across broad time scales (static loading to impact), and length scales (nanometer measurements to structural components).

Movie Time

After extensive impact and fracture experiments of composite and layered materials using high-speed photography, I stacked all these photos for making QuickTime movies. What you will see is what the high-speed camera "saw" during the impact experiment. Enter here to watch vivid QuickTime movies. The copyright of these movies belongs to L. R. Xu, A. Krishnan and A. J. Rosakis at California Institute of Technology.

Contact Information

Dr. L. Roy Xu Email: l.roy.xu (AT) alumni.caltech.edu