Although the contact stress level was the same for both tests, this contact stress level was achieved by  applying a higher preload during the material selection test. Due to the higher preload, and the different contact geometry, the normal load distribution around the contact line was higher for the material selection test coupons. In addition, the material selection test was performed quasi-statically, and the random-vibration test applied fretting motions at roughly 80 Hertz. Based on these conclusions, the following guidelines are proposed.  Flight-Like Wear Test Guidelines 1. Shear tie wear testing should employ the same wear mode as the flight application.  2. Shear tie wear testing should occur under the same contact stress level, and with the same  normal load distribution around the contact line.  3. It is recommended that test specimens employ the same contact geometry as the flight design  (Same spherical radius and contact angle, at the very least).  4. Although the frequency that wear cycles occur is considered to be less important than other  parameters, it is good to get as close as possible to the flight wear cycle frequency.  Payload Wing Shear Tie Wear Environment  Due to the large mass and relatively small surface area of the Payload Wing, the bulk of wear that occurs  during launch is due to the coupled loads between the launch vehicle and spacecraft. This results in a wear mode characterized by relatively large shear tie displacements (rocking about the instant-center of the spherical surface) at a relatively low frequency (~20 Hz). Shear tie rocking angles are found by performing Finite Element Analysis of the next-higher-assembly. Although the coatings used for shear tie wear surfaces often provide low-friction surfaces, the interface cannot be accurately modeled as a frictionless ball joint. Unlike a frictionless ball joint, when a torque is applied to a shear tie joint, it will not  begin to slip until friction is overcome. Conversely, the joint cannot be modeled as a rigid joint, because  once slip occurs, the stiffness of the joint drops significantly. However, it is this blend of characteristics  that does allow for some bounding assumptions.  Rather than attempting to determine the exact rotation angle that would occur (which would be highly  sensitive to the friction coefficient of the coating), the problem was bounded by finding the maximum  possible rotation angle and the maximum available overturning moment. The maximum possible rotation angle was found by modeling the shear tie as a frictionless joint, and the maximum available overturning moment was found by modeling the shear tie as a rigid joint. In this case, design load factors were used to ensure test conservatism. One factor that is important in creating a truly flight-like wear test is the  random nature of shear tie loads. Because the forcing function that drives the motions of the shear ties is  random, the maximum fretting distance is not achieved during the entire launch sequence. Typically, the  rocking angle that occurs throughout a launch event will follow a bell-curve. The test was designed with this in mind – 1, 2, and 3-sigma rocking angles were applied with a corresponding number of cycles to match the expected statistical distribution.  For the rigid case, the maximum (3-sigma) moments are:  /g16 M x: 623 N-m (5866 in-lb)  /g16 M y: 333 N-m (2949 in-lb)  /g16 M z: 787 N-m (6966 in-lb)  For the zero rotational stiffness case, the maximum (3-sigma) rotations are:  /g16/g84 x: 0.51 deg  /g16/g84 y: 0.26 deg  /g16/g84 z: 1.75 deg  Note that the coordinate system used above places the centerline of the shear ties along the X axis, per  Figure 1. 161