In 1995 Murray et al [3] highlighted the range of mechanisms for which accelerated testing would be  desirable, the technical challenges in developing a valid accelerated test methodology, and a technology  roadmap which might lead to greater understanding of the issues. In 1997 ESTL reviewed the topic and in  proposing a methodology for a more tribologically rigorous accelerated test for oil or grease lubricated  mechanisms [4] also highlighted some of the main challenges and uncertainties which could lead to a  misleading test conclusion or invalid test.   More than 20 years on, the authors have experienced many accelerated tests of different types, and this  paper seeks to highlight, both by review and presentation of new material, areas where additional  understanding has been developed and some remaining technical needs in order to further reduce the risk  that an accelerated test method may be adopted which could lead to an invalid or misleading test result –  and in the worst case an undertest of life or performance.  Uncertainties of the Usual Approach to Accelerated Test - Fluid Lubricants  Accelerated testing of fluid-lubricated mechanisms is based on the notion that if the same lubrication regime  can be maintained between the critical loaded contacts (e.g. ball and raceway) in the test as found in the  application at nominal speed then the test will be tribologically valid.   In classical EHL, the lubrication regime is usually determined based on a Stribeck curve approach using an  analysis as typically presented by Hamrock and Dowson [4]. Using this approach, a specific film thickness,  λ (the ratio of minimum lubricating film thickness to composite surface roughness of the counterfaces) is  calculated for the nominal ( λn) and accelerated conditions ( λa), the assumption being made that if  accelerated test conditions can be established in which λa≅ λ n the lubrication regimes will be similar and  therefore the risk of wear/rate of degradation of the lubricant or other tribological failures might be expected  to be similar. By this approach a valid test can in principle be achieved if the increased speed of an  accelerated test is compensated, by reduced viscosity achieved by increasing temperature, such that the  same specific film thickness and lubrication regime might be obtained in the accelerated test as in the  application at nominal operating conditions.   Whilst attractive in its simplicity it was pointed out in [5] that this approach can be inappropriate and flawed  for many reasons. Since that time publications by many authors have contributed to the up-to-date status  of these concerns for each phenomenon which is reviewed below.  Non-Newtonian Lubricant Behavior  Though the above approach assumes Newtonian lubricants, in fact for many lubricants, including widely  used space oils (and their derived greases) there ceases to be a well-defined relationship between shear  stress and velocity gradient (or in fact the fluid partially solidifies) at some stress/shear condition. For  example, high pressure rheological experiments on the oils 815Z and 2001A [6,7] suggest reversible  solidification changes at relatively modest pressures and room temperature as shown in Table 1 (and at  lower pressures if temperature is reduced). These figures suggest that at a relatively modest mean Hertzian  contact stress >~700 MPa (~1 GPa peak) visco-elastic solidification (rather than the straightforward  increase in viscosity with pressure represented by the pressure-viscosity coefficient in the Barus equation)  might start to become a factor for these lubricants causing deviations from the expected friction and film  thickness behaviors.   Table 1. Onset of Visco-elastic and Elastic-plastic Solid Behavior in Space Oils  Oil Pressure for Onset of Viscoelastic Solid Behavior Pressure for Onset of Elasticplastic Solid Behavior  815Z  [6,7] >~1.1 GPa >1.6 GPa 2001A  [6,7] >~0.95 GPa >~1.5 GPa  NASA/CP—2018-219887 514