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2009 SFB Workshop Presentation: Fatigue Crack Initiation and Propagation Behavior of Neat PEEK under Notched Conditions

The objective of this study was to investigate the tensile fatigue behavior of PEEK with stress concentrations of varying severity and to determine the relative time spent in crack initiation vs crack growth. PEEK OPTIMA LT1™ material was used. Three geometries of circumferentially grooved dogbone specimens were machined and tested. Prior to testing, specimens were pre-conditioned in a 37°C PBS bath for 8 weeks. Fatigue testing to failure (fracture) was performed in a PBS bath at 37°C on an Instron 8511 load frame. This work found that crack propagation lifetime was a minor part of the total lifetime for the moderate and deep notch geometry conditions.

Tailoring of PEEK surface properties via surface-initiated atom transfer radical polymerization

The interfacial properties of commercial poly(ether ether ketone) (PEEK) have been tailored by tethering polymeric brushes to the PEEK surface via surface-initiated atom transfer radical polymerization (SI-ATRP). The immobilization of an ATRP initiator on the PEEK surface was achieved by an unprecedented simple two-step wet chemical method. Atomic force microscopy, scanning electron microscopy, attenuated total reflection infrared spectroscopy, water contact angle measurements, and X-ray photoelectron spectroscopy ascertained the successful grafting of these polymer brushes at the PEEK surface. These brush-modified PEEK surfaces exhibited fully the physiochemical properties of the respective polymer brush: the surface with polyMPS brush showed selective staining by electrostatic interaction, while the polyMeOEGMA-modified surface was biorepellent.

Static and Cyclic Behavior of PEEK in the Presence of Mild Stress Risers

This Ph.D. Dissertation by Michael Sobieraj from the Case Western Reserve University describes the static and fatigue behavior of medical grade PEEK under smooth and notched conditions. Virgin, unfilled medical grade PEEK (Optima LT1) was investigated. Monotonic stress-strain behavior in the presence of a notch was examined using a custom developed video based system, under physiologically relevant conditions. The fatigue behavior of PEEK was also investigated. Using a total life approach to fatigue, it was shown that the majority of the fatigue lifetime is spent in the initiation phase.

Measurements of Medical Grade PEEK Using R-FTIR

Characterizing the crystallinity in PEEK is critical, since it dictates the mechanical behavior of the polymer. Wide angle X-ray scattering (WAXS) has been typically employed for this purpose; yet, it is impractical for the measurement of some composite materials. Differential scanning calorimetry (DSC) has also been used but with limited accuracy. By comparing FTIR, WAXS and DSC for a range of medical PEEK grades, researchers from Drexel University demonstrate that FTIR can detect changes in crystallinity due to annealing treatments and incorporation of carbon fibers or radiopaque compounds.

Wear Studies on the Likely Performance of CFR-PEEK/CoCrMo for Use as Artificial Joint Bearing Materials.

Pin-on-plate wear tests were performed on various combinations of PEEK-OPTIMA and carbon fibre reinforced PEEK-OPTIMA (CFR-PEEK) against various CoCrMo alloys to assess the potential of this material combination for use in orthopaedic implants. The PEEK/low carbon CoCrMo produced the highest wear. CFR-PEEK against high carbon or low carbon CoCrMo provided low wear factors. Pin-on-plate tests performed on ultra-high molecular weight polyethylene (UHMWPE) against CoCrMo (using comparable test conditions) have shown similar or higher wear than that found for CFR-PEEK/CoCrMo. This study gives confidence in the likelihood of this material combination performing well in orthopaedic applications.

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