Fracture Toughness and Fatigue Crack Propagation Rate of Short Fiber Reinforced Epoxy Composites for Analogue Cortical Bone

[+] Author and Article Information
Alexander C. M. Chong

Department of Mechanical Engineering, University of Kansas, Lawrence, KS 66045cmchong@ku.edu

Forrest Miller

 Pacific Research Laboratories, Inc., Vashon, WA 98070foss@pacific-research.com

McKee Buxton

 Pacific Research Laboratories, Inc., Vashon, WA 98070mckee@pacific-research.com

Elizabeth A. Friis1

Department of Mechanical Engineering, University of Kansas, Lawrence, KS 66045lfriis@ku.edu


Corresponding author.

J Biomech Eng 129(4), 487-493 (Jan 19, 2007) (7 pages) doi:10.1115/1.2746369 History: Received May 31, 2006; Revised January 19, 2007

Third-generation mechanical analogue bone models and synthetic analogue cortical bone materials manufactured by Pacific Research Laboratories, Inc. (PRL) are popular tools for use in mechanical testing of various orthopedic implants and biomaterials. A major issue with these models is that the current third-generation epoxy–short fiberglass based composite used as the cortical bone substitute is prone to crack formation and failure in fatigue or repeated quasistatic loading of the model. The purpose of the present study was to compare the tensile and fracture mechanics properties of the current baseline (established PRL “third-generation” E-glass–fiber–epoxy) composite analogue for cortical bone to a new composite material formulation proposed for use as an enhanced fourth-generation cortical bone analogue material. Standard tensile, plane strain fracture toughness, and fatigue crack propagation rate tests were performed on both the third- and fourth-generation composite material formulations using standard ASTM test techniques. Injection molding techniques were used to create random fiber orientation in all test specimens. Standard dog-bone style tensile specimens were tested to obtain ultimate tensile strength and stiffness. Compact tension fracture toughness specimens were utilized to determine plane strain fracture toughness values. Reduced thickness compact tension specimens were also used to determine fatigue crack propagation rate behavior for the two material groups. Literature values for the same parameters for human cortical bone were compared to results from the third- and fourth-generation cortical analogue bone materials. Tensile properties of the fourth-generation material were closer to that of average human cortical bone than the third-generation material. Fracture toughness was significantly increased by 48% in the fourth-generation composite as compared to the third-generation analogue bone. The threshold stress intensity to propagate the crack was much higher for the fourth-generation material than for the third-generation composite. Even at the higher stress intensity threshold, the fatigue crack propagation rate was significantly decreased in the fourth-generation composite compared to the third-generation composite. These results indicate that the bone analogue models made from the fourth-generation analogue cortical bone material may exhibit better performance in fracture and longer fatigue lives than similar models made of third-generation analogue cortical bone material. Further fatigue testing of the new composite material in clinically relevant use of bone models is still required for verification of these results. Biomechanical test models using the superior fourth-generation cortical analogue material are currently in development.

Copyright © 2007 by American Society of Mechanical Engineers
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Figure 1

Third-generation femur implanted with a cemented femoral stem that had been fatigue loaded in single legged stance (6). The arrow points to the location of the early crack formation in the third-generation analogue bone material; cracks formed due to stress concentrations at the tip of the implant and low fatigue resistance of the third-generation material.

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Figure 2

Composite analogue vertebrae made from third-generation experience fracture after use in testing with pedicle screws. Fatigue cracks developed in the analogue vertebrae around the pedicle screw insertion sites.

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Figure 3

Dimensioned drawing of the standard single edge-notched compact specimen for KIc and FCPR testing, based on ASTM E1820-01 and E647-00 standards (13-14). (a) Detailed dimensions of the compact test specimen for both KIC and FCPR test specimen; (b) dimensions for the KIc test specimen thickness; and (c) dimensions for the FCPR test specimen thickness. The specimen width, W=63.50mm(2.5in.), was determined based on the above mentioned ASTM standards.

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Figure 4

Photograph of the KIc experimental setup. Note that a thin layer of whiteout on the surface of the test specimen was for crack visualization; the whiteout had no effect on the composite material. The light source was also used for crack visualization during testing.

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Figure 5

Log-log scale graphical presentation of da∕dN versus ΔK fatigue crack propagation pooled specimen results for the third- and fourth-generation bone analogue materials. The lines drawn on the graph are the boundaries of the fatigue crack growth response regimes (14).



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