Aviation Plexiglass Fatigue Crack Characteristics Pilot Study

Polymethyl Methacrylate (PMMA) has been widely used as the transparent material of the aircraft canopies because of its properties of excellent transparency and strength. The understanding to its fatigue behaviour is one of the key parts of the fatigue life prediction of aircraft structures.YB-MD-3 PMMA has been used in several Chinese aircraft types as the canopy transparent structural materials. In order for using the material safely and economically, it is of engineering significance to study the response of the kind of damage-contained materials and structures withstanding service loading quantitatively, and to develop corresponding analytical techniques. The main purpose of this dissertation is to characterize its fatigue crack propagation behaviour experimentally, and thereby, to offer basis to the evaluation of the residual life and the maintenance guideline of canopy structures.An experimental study of stable fatigue crack propagation behavior was carried out at room temperature. During the preparation of initial through fatigue cracks for M(T) specimens, load cycles with compressive portion demonstrates better quality than usual tension-tension loading. The fatigue crack propagation rate in Paris's region for certain load ratio shows good correlation with a straight line in double logarithmic daldN- AK coordinate. The daldN and the slope of the straight line increases with the increase of load ratio. For the negative load ratios, however, the compressive portion in load cycles accelerates the crack growth. A proper formulation to these effects is unavailable. Then, the Walker's equation was drawn up.For the positive load ratios, a modified Walker's equation is developed and gives a good regression to the behavior of the PMMA material. While for the negative load ratios, the compressive portion in load cycles accelerates the crack growth of the PMMA. The influence of the compressive portion of the load cycles on the fatigue crack propagation rate (FCPR) may be interpreted by the plasticity-induced crack closure phenomenon and a proper formulation is so far unavailable.The measurement of fatigue crack propagation thresholds of YB-MD-3 PMMA plates was carried out in ambient air. The single-edge-notch tension (ESE(T)) specimen was adopted .The fatigue crack propagation thresholds were attained at R = 0.06, R = 0.3, R = 0.5. And the rules of fatigue crack propagation were gained in range of thresholds.The cyclic stress-strain relation testing of YB-MD-3 PMMA material has been made. The hysteresis loops were measured. The tension-compression cyclic characteristic is very different from metal material. The unloading rule for the common metals is not suitable to the YB-MD-3 PMMA. The material is cyclic softening and the stress-strain cycles tend to stable, but the low-cycle fatigue life is poor.Finally, the fatigue crack closure of PMMA were also studied. The crack mouth opening displacement was measured using a 'clip on' gage. And a series of strain measuring device were used. During load cycles at given crack lengths, the load-displacement and the load-strain curves were recorded. Such data were analyzed with reference to the cyclic stress-strain characteristic of the material to figure out the crack openning/closure forces.