Discussion of Heritage Data  light heritage is often considered to be the highest level of design validation. Although this is a  he failure mode for shear ties is coating wear-through followed by galling, cold-welding, or adhesion of  lthough the LLA shear ties are officially qualified during spacecraft level vibration and acoustic tests, the  Flight-Like Wear Testing Background  he wear life of a shear tie is sensitive its subjected wear mode, as has been shown in past testing. This  Table 1. Material selection life test results F reasonable statement, care must be taken in using heritage designs. The use of shear ties in launch locks is a good example of this. Although the LLA shear ties are similar in overall geometry to heritage shear ties (sphere-in-cone contact geometry), the spherical radius, cone half angle, and overall size deviate from heritage designs. In addition, the wear environment for this shear tie differs from heritage environments. Similarly, the coating used on the these shear ties (Tiodize type 2 over-coated with Tiolube 460), has significant heritage as a solid lubricant for space environments, but had not been used on previous Lockheed Martin shear tie designs. It was chosen for the LLA because it performed well when  subjected to large-displacement wear under heavy loads in the material selection test described below.  T the base metal. In order to qualify a shear tie design by similarity, several requirements should be met. First, the same coating and substrate must be used in both applications. Second, the shear tie geometry should have the same overall configuration with a contact stress level that is enveloped by the heritage design. Finally, the wear environment for the new design or application must be enveloped by the wear  environment for the heritage design. A application was considered to deviate from heritage enough to justify a development level risk-reduction wear life test. Design changes resulting from a qualification test failure (especially at the spacecraft level),  have heavy cost and schedule impacts, which can often be prevented during the design phase by  performing appropriate development testing.  T conclusion was drawn because of the difference between the results of two prior tests. The first test was a material/coating selection test. During this test, test specimens with shear tie contact geometry were loaded into an Instron machine and cycled axially (see Figure 2). Relative motion was achieved by varying the load on the shear ties – as load increases, the contact angle causes hoop deformations which allow axial motion. A fretting distance of approximately 0.6 mm (per cycle) was achieved. A number of  shear tie material and coating combinations were tested using this method to evaluate the relative performance of each coating. Tiodize type 2 over-coated with Tiolube 460 was the highest performing  shear tie coating in this test. The second highest performing coating is referred to as coating B. The life  test data is shown in Table 1. As shown in the table, the wear life of the Tiodize coating was significantly  longer and much more consistent under this wear mode than coating B.  Specimens Max Load Failure Coating kN cycles Coating B 35.6 10250 Coating B 35.6 6250 Tiodize/Tiolube 35.6 15250 Tiodize/Tiolube 35.6 15250 Tiodize/Tiolube 35.6 14250 t the time the material selection was coming to a close, issues with shear tie coatings for another A program required that a risk-reduction flight-like validation test be performed. The material selection test had been designed to compare the performance of a number of shear tie material/coating combinations, not to simulate launch. Because shear tie wear during ground test and launch is driven by random inputs (such as acoustic loads), comparing the wear life observed during the material selection test to the actual wear environment could not be done with a sufficiently high level of confidence. It was determined that a test should be designed that simulated the flight wear environment as closely as possible. Because the application for these shear ties involved high-frequency rocking motions between the shear ties (due to  158