Compressive creep tests were performed on five commercially available acrylic bone cements under conditions simulating in vivo usage. Measured creep strains are quite large, generally exceeding elastic strains. Large variations in creep response were noted among the various cements, with a carbon-reinforced cement by far the most resistant to creep. The empirical model epsilon = a exp(b sigma)tn was found to predict creep strains within about 10% of the measured values. Microscopic examination of some specimens after testing revealed significant cracking, resulting from long-term loading, that could be a contributing cause of time-dependent failure.

Recent activities aimed toward improving the ASTM-type ultrasonic reference block system are described. On the aluminum block system (ASTM E127 and NBS TN 924), efforts were focused on better definition of the measurement equipment (transducer and instrument) , the implementation of a Measurement Assurance Program and Loaner Block Service, and modeling of the distance-amplitude relationship.It is shown that a large increase in the precision of reference block readings is easily achievable by implementing simple changes and controls in the measurement procedure. On steel and titanium blocks (e.g. ASTM E-428) efforts were directed toward quantifying the extent of reproducibility possible among blocks fabricated by both conventional drilling and by diffusion bonding.Reasonable reproducibility is achievable by both, with the diffusion-bonding process offering both advantages and disadvantages.

The effects of using different (nominally identical) quartz search units in the evaluation of ASTM-type standard reference blocks are determined. Various characteristics of the search units are measured and correlated with the amplitude of the ultrasonic response from reference blocks to determine which characteristics must be specified if reproducible results are to be obtained. It is shown by a series of experiments that the exact shape of the distance-amplitude curve in water (axial profile) is a primary characteristic that must be considered. When operational corrections for differences in axial profiles are made, the variability in ultrasonic responses from reference blocks, measured with different search units, is reduced from about 25 percent to 4 percent.

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