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Positron-lifetime measurements have been performed on austenitic stainless steel during ͑i͒ stress-and ͑ii͒ strain-controlled fatigue experiments for different applied stress and strain amplitudes, respectively. For this purpose a generator-detector assembly with a 72 Se/ 72 As positron generator ͓maximum activity 25 Ci ͑0.9 MBq͔͒ has been mounted on mechanical testing machines in order to measure the positron lifetime without removing the specimens from the load train. The average positron lifetime has been determined by a ␤ ϩ Ϫ␥ coincidence. The feasibility to use the average positron lifetime for monitoring the evolution of fatigue damage and to predict early failure has been examined. In strain-and stress-controlled experiments the average positron lifetime shows a pronounced increase within the first 10% and 40% of the fatigue life, respectively. In stress-controlled experiments the average positron lifetime at failure depends significantly on the applied stress amplitude. In strain-controlled experiments significantly different positron lifetimes for different applied plastic strain amplitudes are obtained within the first 1.000 fatigue cycles, whereas differences get wiped out during further cycling until failure.

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Stem modularity in total hip replacement introduces an additional taper joint between Ti-6Al-4V stem components with the potential for fretting corrosion processes. One possible way to reduce the susceptibility of the Ti-6Al-4V/Ti-6Al-4V interface to fretting is the surface modification of the Ti-6Al-4V alloy. Among the tested, industrially available surface treatments, a combination of two deep anodic spark deposition treatments followed by barrel polishing resulted in a four times lower material release with respect to untreated, machined fretting pad surfaces. The fretting release has been quantified by means of radiotracers introduced in the alloy surface by proton irradiation. In a simple sphere on flat geometry, the semispherical fretting pads were pressed against flat, dog-bone shaped Ti-6Al-4V fatigue samples cyclically loaded at 4 Hz. In this way a cyclic displacement amplitude along the surfaces of 20 mum has been achieved. A further simplification consisted in the use of deionized water as lubricant. A comparison of the radiotracer results with an electrochemical material characterization after selected treatments by potentiostatic tests of modular stems in 0.9% NaCl at 40 degrees C for 10 days confirmed the benefit of deep anodic spark deposition and subsequent barrel polishing for improving the fretting behavior of Ti-6Al-4V.

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A piezo-electrically driven fretting testing device has been constructed and fretting release and release rates have been determined with highest accuracy, using a radiotracer technique. First results on the fretting release and release rate of titanium alloy fretting pads against cobalt-chrome alloy fatigue specimens are reported. The frequency dependency of fretting release has been determined between 1 and 8 Hz and shows higher release rates for low frequencies, thus indicating that accelerated testing of materials and components of artificial joints must be analyzed extremely carefully. The present experiments under simple conditions present a base-line study for step-wise applying more complex and realistic testing conditions and for using radiotracer methods to quantify fretting release in simulated testing of artificial hip- and knee-prostheses.