Mdyb-3 Aviation The Plexiglass Fatigue Crack Characteristics Of The Pilot Study

Due to its excellent performance, MDYB-3 PMMA (polymethylmethacrylate)has been used in several Chinese aircraft types as the canopy transparent structural materials. In order for using MDYB-3 PMMA safely and economically, it is of engineering significance to study the response of the kind of damage-contained canopy materials and structures withstanding service loading quantitatively, and to develop corresponding analytical techniques. The main purpose of this dissertation is to determine some of the fracture parameters of MDYB-3 PMMA and characterize its fatigue crack propagation behavior experimentally, and thereby, to offer basis to the residual life evaluation and the maintenance guideline of MDYB-3 PMMA canopy structures.The fatigue crack propagation testing of MDYB-3 PMMA material under constant amplitude cyclic loading has been made. The effect of different stress ratios, room temperatures and loading frequencies were investigated in terms of the Paris law presentations of the testing results. It appears that fatigue crack propagation rates increases with the increase of stress ratios and room temperatures, and decreases with the increase of loading frequencies even within the very low range of frequencies. At the meanwhile the slopes of the fitted straight lines of \og{dal dN) ~ log(A/0 varied from case to case. It is also found that fatigue crack propagation behavior of the material under compression controlled cyclic loading can not be properly denoted by simply truncating the compression portion from the load cycles.The mechanisms of fatigue fracture failure of PMMA were also studied. It is believed that the existence and advance of "silver-bright" crazes are the prerequisite for crack initiation and propagation. The microscopic plastic deformation of the local crazes cause the change of crack opening loads. Therefore, the fatigue crack closure concept could be introduced. A crack closure model of MDYB-3 PMMA were recommended and phenomenologically formulated for positive and negative stress ratios separately, according to the testing results of fatigue crack propagation.According to our experimental investigations of varying load amplitude, it was found that the phenomena of fatigue crack growth retardation occurred for PMMA material following a high overload cycle exerted among the regular constant amplitude load cycles, The ratio of peak value of overload to that of the regular cycles should be over at least 1.35 to make the retardation taking effect. The experiment scheme for measuring the shutting-off ratio of overload (defined as the overload ratio when da/dN<10-7/cyc.) was proposed and the shutting-off ratio has been figured out to be 2.18. As for the mechanism of overload retardation effect, the unique microscopic characteristic of PMMA fractography has been observed. The significant plastic blunting and irregularity of the crack fronting occurred during the overloading and the reduced propagation rate is the result of fatigue crack reinitiating along the blunted crack fronting edge.The fatigue crack propagation threshold of MDYB-3 PMMA at R=0, △KR=0th, has been determined by means of experiments. The effect of stressratio to the threshold value was discussed, based on the concepts of crack closure and short crack.Finally, the fracture toughness, Kc, for given MDYB-3 PMMA material with nominal thickness of 9mm, was measured according to R-curve method. It is inferred during the processing of testing data that due to the extreme sensitivity of the material to notch-like defects or cracks, the fracture toughness could not been determined by the usual tangent-point strategy. The fracture toughness was then determined using the critical values of load and crack length.