A Study on the Fatigue Life of a Composite T-Joint Subjected to Transport Aircraft Spectrum Loads

In this study, an experimental study was carried out on the fatigue life of a carbon fiber composite (CFC) T-joint under a standard transport aircraft spectrum load sequence and compared with predictions based on empirical methods. The co-cured CFC T-joints of size 175 mm × 150 mm × 2.7 mm skin (flange) and centrally placed 2 mm thick, 120 mm high web, were fabricated by autoclave process. The fatigue life in terms of number of load blocks required to fail was experimentally determined under a standard transport aircraft spectrum load sequence, mini-TWIST at various reference loads. All the tests were conducted in a 100 kN servo hydraulic test machine using a specially designed test fixture. The stiffness degradation of the T-joint was monitored during the spectrum fatigue test at regular intervals from the load–displacement data. The fatigue life was observed to increase with decreasing reference load of the spectrum load block. The fatigue damage was observed to initiate as cracks in the fillet region and grow in both flange and web directions. The stiffness was observed to gradually decrease with load blocks indicating the initiation and growth of fatigue damage in the T-joint. Further, the fatigue life under mini-TWIST load sequence was predicted by empirical method using constant life diagram of the material and was observed to correlate reasonably well with experiments.