Papers by Author: Wen Lin Liu

Abstract: The sensibility analysis of the factors to crack growth life has been done. The results show that the input parameters have the following precedence ordering: fatigue crack growth threshold, fracture spectrum, initial crack, fracture toughness, the sensibility values are 11.25, 8.5417, 0.8333, 0.1125, respectively. The model parameters have the following precedence ordering: n, p, C, q. the sensibility values are 6.0417, -3.9583, 1.25, 0.1812, respectively. The reliability analysis was conducted by Monte-Carlo method, the results show that the crack growth life accord with lognormal distribution. The lives with different reliability were obtained. The reliability analysis results of the crack growth life has provided the data for a hybrid approach based on a mixture of the traditional safe-life and damage tolerance techniques which were used as an optimal strategy for ensuring the helicopter structural integrity.

Abstract: In an effort to improve the reliability and reduce the operating costs of helicopter structure, an increasing emphasis is being placed on the damage tolerant approach for life management of helicopter structure. Corrosion damage of helicopter dynamic components and life reliability prediction approach has been systematically analyzed. Based on the conclusion of fatigue life under normal environment, the model for life reliability analysis of helicopter dynamic components has been established. A hybrid approach based on a mixture of the traditional safe-life and damage tolerance techniques can be used as an optimal strategy for ensuring helicopter structural integrity; the life of a helicopter main rotor blade under corrosion environment has been obtained according to flaw-tolerance method. According to probabilistic fracture mechanics theory, the analytical model of structures with initial corner cracks has been established. Thus the service life and inspection intervals according to the request of reliability can be determined; it is very valuable in engineering for life prediction and monitoring of helicopter dynamic components under corrosion environment.

Abstract: It requires a damage tolerance assessment for all airframes and engines to set the inspection windows for safe operation. A certain rotorcraft main rotor yoke were analyzed with NASGRO model. The sensibility analysis of influence parameters in crack growth life has been done. In order to calculate the sensibility index of influence factors to crack growth life, the factors were divided into input parameters and crack growth model parameters. The results show that the input parameters have the following precedence ordering: fatigue crack growth threshold, fracture spectrum, initial crack, fracture toughness, the sensibility values are 11.25, 8.5417, 0.8333, 0.1125, respectively. The NASGRO model parameters have the following precedence ordering: n, p, C, q. the sensibility values are 6.0417, -3.9583, 1.25, 0.1812, respectively.

Abstract: According to standard test method for fatigue crack growth rates of metallic materials, the crack growth rate of 30NCD16 at three stress ratio (R=0.1, 0.3 and 0.5) were measured. Based on linear elasticity fracture mechanics theory, the fatigue crack growth rate was studied through the nonlinear least squares fitting method. The Paris model parameters at steady growth region and near threshold growth region and NASGRO model parameters were obtained. The effective stress intensity factors versus curves at three stress ratios were determined by crack closure effect. The results show that the Paris equation can preferably describe relations at steady growth region. At this region the model parameter m lies 2.5-4. This result is consistent with the known statistical facts of most metallic materials. NASGRO equation can preferably describe relations from near threshold growth region to high values region. all the test data at three stress ratio was able to correlate and . Crack closure was the major factor in correlating stress ratio and crack growth rate, the degree of crack closure weaken with increasing stress ratio.

Abstract: Three-dimensional finite element model of a cracked bolted joint has been developed in the non-linear finite element code MSC.Marc and attempts were made to validate it by comparing results with those of experiments and other finite element. Issues in modeling the contact between the joint parts, which affect the accuracy and efficiency of the model, were presented. Experimental measurements of load transfer were compared with results from finite element analysis. The results show that three-dimensional finite element model of cracked bolted joint can produce results in close agreement with experiment. Three-dimensional effects such as bolt titling, seconding and through-thickness variations in stress and strain are well represented by such models. Three-dimensional finite element analysis was also used to study the effects of hole mod and crack on the load transfer behaviour of single lap bolted joints. The results show that hole mode has big effect on load transfer of cracked bolted joint. In the whole progress of crack growth, the load transfer through bolt 1 decrease, and almost all of the load duduction of bolt 1 transfer into blot 2 rather than into bolt 3.

Abstract: A mathematical model was developed to correlate the amplitude of the pillowing deformation of lap joints to the degree of corrosion inside the joint. Based on mathematical model, finite element techniques were used to determine the effect of pillowing in the stress in fuselage lap joints. The stress caused by the internal pressure and riveting process was taken into account, and the fuselage curvature was ignored. The results show that pillowing can significantly increase the stress in a lap joint for material loss below the detection limit of current nondestructive inspection techniques, thus increasing the risk of premature cracking. In addition, the analyses show that pillowing resulted in a stress gradient through the skin thickness, which suggest that semi-elliptical cracks with a high aspect ratio can form. In the last, the stress correction curves at the critical rivet hole were used in the AFGROW crack growth rate program to predict the residual life of corroded lap joint and the analytical results were close to available test date.

Abstract: Three-dimensional finite element model of a bolted joint has been developed in the non-linear finite element code MSC.Marc and attempts were made to validate it by comparing results with those of experiments and other finite element. Issues in modeling the contact between the joint parts, which affect the accuracy and efficiency of the model, were presented. Experimental measurements of surface strains and load transfer ratio(LTR) were compared with results from finite element analysis. The results show that three-dimensional finite element model of bolted joint can produce results in close agreement with experiment. Three-dimensional effects such as bolt titling, seconding and through-thickness variations in stress and strain are well represented by such models. Three-dimensional finite element analysis was also used to study the effects of different parameters on the mechanical behaviour of single lap bolted joints. The results show that straight hole, small bolt diameter, and big hole pitch are selected first for bolted joint if other conditions allowed, and effect of bolt material on LTR of joint is small for small load. Interference and pre-stress should be strictly controlled for bolted joints in order to attain the best fatigue capability of lap joint.

Abstract: Using of a safe life approach is necessitated by the small critical crack sizes and rapid crack growth rates resulting from the severe vibratory environment of rotorcraft. However Based on the success of damage tolerance initiative for airframe structure, a crack growth based on damage tolerance approach is being examined for implementation into the design and management of dynamic components. In this paper the crack growth behavior in low cycle fatigue (LCF) and high cycle fatigue (HCF) are compared and the accuracy of several damage tolerance analysis methods in determining the crack growth life from an initial detectable crack size 1.25 mm to 12 mm for a rotorcraft main rotor yoke are investigated. The real rotorcraft materials measured by experiment and the fatigue load mean frequency spectrum based on statistical usage spectrum are adopted. The crack growth equations used in the programs are discussed to provide a basis for understanding the results. The results show that the load less than the safe fatigue limit has an important effect on crack growth life and it is reasonable for the material that the cut-off stress ratio for the threshold stress intensity factor range is set 0.7.