Construction of constant fatigue life diagram for a carbon fiber composite

Using the constant amplitude fatigue data at various stress ratios, the constant fatigue life (CFL) diagram was constructed for the CFC material which is useful in prediction of fatigue life under variable amplitude fatigue loads. A quasi-isotropic lay-up sequenced carbon fiber-epoxy composite (CFC) laminate was fabricated by resin infusion technique. The tensile and compression tests were carried out to determine the static strength of the material. About 175 mm long, constant rectangular cross-sectioned fatigue test specimens were cut and prepared from the laminate. The stress-controlled, constant-amplitude fatigue tests were conducted in a 100 kN servo-hydraulic test machine, at room temperature and in lab air atmosphere. All the fatigue tests were performed with a sinusoidal waveform and a frequency of 1-3 Hz. The fatigue tests were conducted at four different stress ratio, R = �min/�max, Viz., 0.7,0.5 (tension-tension), -1.0 (tension-compression), and 4.0 (compression-compression). Anti-buckling guide was employed during the fatigue tests which involved compressive load cycles . The CFL Diagrams are material specific with predictive capability for any designed levels from selective experimental data

Effect of ply-drop on fatigue life of a carbon fiber composite under a fighter aircraft spectrum load sequence

Carbon fiber composite laminate containing symmetric internal ply-drop simulating thickness variation was fabricated and tested to determine fatigue life under a standard mini-FALSTAFF spectrum load sequence. The fatigue life of ply-drop composite was significantly lower than that of plain composite due mainly to initiation and growth of delamination near the ply-drop location. The spectrum fatigue life was also predicted by empirical method. For this purpose, static and constant amplitude fatigue data was generated to construct constant life diagrams (CLDs). The spectrum fatigue life predicted using CLDs was in good agreement with experimental results for both plain and ply-drop composites

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

A modified piecewise linear constant life diagram for fatigue life prediction of carbon fiber/polymer multidirectional laminates

Prediction accuracy of constant life diagrams (CLDs) for fiber-reinforced plastics varies with the stacking, tensile/compression dominated failure, input S–N curves, the complexity of the plot, and so forth. The article's authors propose a modified piecewise linear CLD and compare its prediction accuracy with traditional piecewise and 2-segment anisomorphic CLD. The constant amplitude fatigue life data for three different carbon fiber reinforced polymer composites multidirectional laminates that is (+45, −45, +45, and −45)2S, (+45, −45, 0, and 90)2S, and (0, 90, 0, and 90)2S were generated at five different stress ratios (R = σmin/σmax) that is, R = 0.1 and 0.5 (T–T), −1 and critical stress ratio (T–C), and 5 (C–C). The proposed modified Piecewise linear CLD considers the critical stress ratio (UCS/UTS) as a definite input node. The prediction accuracy of the proposed CLD and others was evaluated using additional fatigue testing at random stress ratios and stress levels. The proposed CLD showed the highest prediction accuracy compared to Piecewise or Anisomorphic CLD. The non-linear CLD plotted using single S-N curve data may save time and effort but achieving absolute accuracy will ultimately depend on the stacking sequence

Fatigue life of a carbon fiber composite T-joint under a standard fighter aircraft spectrum load sequence

The carbon fiber/epoxy composite (CFC) T-joints were fabricated and tested to determine their fatigue life under a standard fighter aircraft spectrum load sequence, mini-FALSTAFF with various reference loads. Further, the fatigue life of T-joint subjected to the same spectrum load sequence was predicted. For this purpose, the static pull-out (tension) and push-in (compression) tests were conducted on T-joints. Then, the constant amplitude fatigue tests were conducted to determine the load-life (P–N) curves at three different load ratio, R = Pmin/Pmax of 0.1, −1.0 and 10.0.The static and fatigue test data generated was then employed to construct constant life diagram (CLD) for T-joint. The fatigue damage for each of the rain flow counted load cycle of mini-FALSTAFF spectrum load sequence was determined using CLD. Using linear damage accumulation theory, the fatigue life of T-joint was estimated. The predicted fatigue life was observed to agree quite well with experimental results