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The crack growth characteristics of a 4-ply, unidirectional, titanium matrix composite, SCS-6/Ti-6Al-2Sn-4Zr-2Mo, subjected to thermomechanical fatigue were investigated. A linear summation model was developed to predict the isothermal and thermomechanical fatigue (TMF) crack growth rates of the composite. The linear summation approach assumes the total fatigue crack growth rate is a combination of a cycle-dependent and a time-dependent component. To assist the modeling effort, a series of isothermal, in-phase, and out-of-phase crack growth tests were conducted. The test temperatures ranged from 150‡C to 538‡C and the fastest thermal frequency was 0.0083 Hz. With the exception of the 150‡C isothermal test, the model was able to correlate all the baseline fatigue crack growth test data between δK of 50 to 90 MPa √m. In addition, the model was able to predict the fatigue crack growth rate of a proof test which involved a continual change in temperature range and load range to produce a constant crack growth rate. The proof test began under isothermal conditions at the maximum temperature and ended under in-phase TMF conditions.

Ultrasonic scanning acoustic microscopy is a nondestructive method useful for material elastic property quantification as well as crack size determination for surface and subsurface cracks. The advantage of the method over destructive methods for crack size determination is that the imaging technique can provide the crack sizing information while helping in the detection of interface degradation and early crack initiation so that their growth can be monitored during interrupted fatigue tests. Various metal matrix composite systems with titanium based matrix and SCS-6 fibers have been evaluated for this study [Ti-24Al-11Nb (atomic percent), Ti-6Al-2Sn-4Zr-2Mo (weight percent), and Ti-15Mo-3Nb-3Al-0.2Si (weight percent)]. The scanning acoustic microscope technique has been applied to materials subjected to both room temperature and elevated temperature fatigue cycling in addition to thermomechanical fatigue (in-phase and out-of-phase) conditions. A 50 MHz scanning acoustic microscope has been used for the imaging and evaluation of the damage initiation and growth of surface/subsurface cracks and interfacial degradation. All the images have been produced by exploiting the surface wave component of the ultrasonic signals from the scanning acoustic microscope because of the higher sensitivity of surface waves to both surface/subsurface cracks and perhaps also due to the changes in interfacial elastic properties. The results shown in this paper provide a very good understanding of the crack initiation and growth as well as interfacial degradation process of titanium based metal matrix composites when subjected to cyclical stresses at elevated temperatures and room temperature. The results indicate that the combination of high temperature and stress is very severe to the interface between the matrix and the fiber.