Effect of carbon fiber type on monotonic and fatigue properties of orthopedic grade PEEK.

Carbon-fiber reinforced (CFR) PEEK implants are used in orthopedic applications ranging from fracture fixation plates to spinal fusion cages. Documented implant failures and increasing volume and variety of CFR PEEK implants warrant a clearer understanding of material behavior under monotonic and cyclic loading. To address this issue, we conducted monotonic and fatigue crack propagation (FCP) experiments on orthopedic grade unfilled PEEK and two formulations of CFR PEEK (PAN- and pitch-based carbon fibers). The effect of annealing on FCP behavior was also studied. Under monotonic loading, fiber type had a statistically significant effect on elastic modulus (12.5 ±â€¯1.3 versus 18.5 ±â€¯2.3 GPa, pitch versus PAN CFR PEEK, AVG ±â€¯SD) and on ultimate tensile strength (145 ±â€¯9 versus 192 ±â€¯17 MPa, pitch versus PAN CFR PEEK, AVG ±â€¯SD). Fiber type did not have a significant effect on failure strain. Under cyclic loading, PAN CFR PEEK demonstrated an increased resistance to FCP compared with unfilled and pitch CFR PEEK, and this improvement was enhanced following annealing. Pitch CFR PEEK exhibited FCP behavior similar to unfilled PEEK, and neither material was appreciably affected by annealing. The improvements in monotonic and FCP behavior of PAN CFR PEEK is attributed to a compound effect of inherent fiber properties, increased fiber number for an equivalent wt% reinforcement, and fiber aspect ratio. FCP was shown to proceed via cyclic modes during stable crack growth, which transitioned to static modes (more akin to monotonic fracture) at longer crack lengths. The mechanisms of fatigue crack propagation appear similar between carbon-fiber types.

Particle Dark Matter Searches Outside the Local Group.

If dark matter (DM) is composed by particles which are nongravitationally coupled to ordinary matter, their annihilations or decays in cosmic structures can result in detectable radiation. We show that the most powerful technique to detect a particle DM signal outside the Local Group is to study the angular cross-correlation of nongravitational signals with low-redshift gravitational probes. This method allows us to enhance the signal to noise from the regions of the Universe where the DM-induced emission is preferentially generated. We demonstrate the power of this approach by focusing on GeV-TeV DM and on the recent cross-correlation analysis between the 2MASS galaxy catalogue and the Fermi-LAT γ-ray maps. We show that this technique is more sensitive than other extragalactic γ-ray probes, such as the energy spectrum and angular autocorrelation of the extragalactic background, and emission from clusters of galaxies. Intriguingly, we find that the measured cross-correlation can be well fitted by a DM component, with a thermal annihilation cross section and mass between 10 and 100 GeV, depending on the small-scale DM properties and γ-ray production mechanism. This solicits further data collection and dedicated analyses.