Investigation on Dynamic Meshing Process and Factors Influencing Root Crack Propagation Trajectory of a Representative Aero-engine Gear Pair

International audienceThe gear in an aero-engine experiences alternate loads therefore the stresses on gear root change significantly in meshing process. As a consequence, fatigue crack initiates on the gear root. This paper examined meshing process and crack propagation trajectory of the aero-engine gear based on explicit dynamics and linear elastic fracture mechanics. The crack on gear root affects meshing process and causes additional noise and vibration.The meshing impact and the peak of stress distribution of the gear pair is also magnified. Furthermore, the root crack propagation can cause the rim fracture and tooth fracture of gear system. The occurrence of rim fracture increases as the backup ratio (i.e., rim thickness divided by tooth height) decreases and also increases as the initial crack location is moved down the root of the tooth; the orientation of initial crack has little influence on crack propagation trajectory

Experimental investigation of fatigue crack growth behavior of GH2036 under combined high and low cycle fatigue

International audienceFatigue crack growth rates have been experimentally determined for the superalloy GH2036 (in Chinese series) at an elevated temperature of 550°C under pure low cycle fatigue (LCF) and combined high and low cycle fatigue (CCF) loading conditions by establishing a CCF test rig and using corner-notched specimens. These studies reveal decelerated crack growth rates under CCF loading compared to pure LCF loading, and crack propagation accelerates as the dwell time prolongs. Then the mechanism of fatigue crack growth at different loadings has been discussed by using scanning electron microscope (SEM) analyses of the fracture surface

INVESTIGATION OF DESIGN PARAMETERS AND FAILURE CRITERIA OF O-RING SEAL STRUCTURE

ABSTRACT First, this paper established the seal structural 2D axisymmetric model of a certain Solid Rocket Booster (SRB) and calculated the deformation and stresses at ignition through a large displacement, incompressible, contact finite element analysis. The results show that the maximum contact stress appears at the contact area and the maximum shear stress at groove notch. Then, some typical parameters of the seal structure which might have the impact on the sealing performance, such as the gap breadth, initial compressibility, fillets of the groove notch and bottom, groove width, were analyzed. We can find that the gap breadth and initial compressibility do great contributions to the maximum contact normal stress, and the groove notch and bottom fillets act upon the maximum shear stress obviously. In order to verify the validity of the 2D axisymmetric model, 3D structural finite element analysis of the structure was conducted, and the results indicate that in service, the upper flange is inclined relative to the nether flange, which seems to mean that the gap breadth can not be considered as a constant during the 2D axisymmetric analysis. However further calculations say that if using the minimum gap breadth gotten in 3D analysis as its constant gap value, the above 2D axisymmetric model can rationally take the place of 3D model to analyze the sealing performance. Finally, the failure modes & criteria of the O-ring seals based on the maximum contact normal stress and shear stress were determined to ensure the reliability of this structure. Keywords: O-ring seal; finite element analysis; stresses; large deformation; failure criteria; contact INTRODUCTION There is a high requirement for sealing characteristics and reliabilities of O-ring seal structure in solid rocket engine. The O-rings' design greatly influences the whole engine's operation. Unreasonable design can cause seal failure, then the gas in combustion will leak out. Under such conditions, the general result is that the working pressure curves turn worse and the engine loses control and even explodes. Therefore it is essential to deeply investigate the O-ring seal structure of solid rocket booster (SRB). In this paper, the finite element (FE) model was established to calculate the stresses during the service process. Then the parameters influencing sealing performance were studied to optimize O-ring seal structure. The 2D axisymmetric and 3D structural analyses were conducted respectively and the results of 2D structure were compared with those of 3D. Meanwhile, to ensure structural reliability, sealing failure modes & criteria were obtained based on the maximum contact normal stress and shear stres

Advanced Approaches Applied to Materials Development and Design Predictions

This thematic issue on advanced simulation tools applied to materials development and design predictions gathers selected extended papers related to power generation systems, presented at the XIX International Colloquium on Mechanical Fatigue of Metals (ICMFM XIX), organized at University of Porto, Portugal, in 2018. In this issue, the limits of the current generation of materials are explored, which are continuously being reached according to the frontier of hostile environments, whether in the aerospace, nuclear, or petrochemistry industry, or in the design of gas turbines where efficiency of energy production and transformation demands increased temperatures and pressures. Thus, advanced methods and applications for theoretical, numerical, and experimental contributions that address these issues on failure mechanism modeling and simulation of materials are covered. As the Guest Editors, we would like to thank all the authors who submitted papers to this Special Issue. All the papers published were peer-reviewed by experts in the field whose comments helped to improve the quality of the edition. We also would like to thank the Editorial Board of Materials for their assistance in managing this Special Issue

An Efficient Approach for Parametric Modeling and Prediction of the Hollow Blade Manufacture Shape

In recent years, hollow fan blades have been widely used to meet the requirements of aeroengines for lightweight and better performance. However, the hollow fan blade will change from the manufacture shape to the operating shape with large deformations during operation. This deviation, if neglected, will lead to deceptive results for structural and aerodynamic analysis. However, the existing methods have low prediction accuracy or require a lot of calculation in case of large deformations. In this paper, an iterative method for parametric modeling, automatic generation of finite element model, and hot-to-cold analysis of an H-shaped hollow fan blade are proposed. The accuracy and efficiency of the traditional uncoupling (UCM) and weak coupling methods (WCS), as well as the proposed strong–weak coupling method (SWCS), are compared with the strong coupling method (SCU) as a reference. Results show that improvements in the prediction accuracy can be made by the SWCS method, and the error of the maximum blade deformation is 2.3%, while the error of the UCM method and WCS method is 30% and 12%, respectively. An excellent agreement can be observed between the SWCS and SCU methods in the whole blade height with errors of 2%~5%, and the calculation time of the SWCS method is only 2.5% of the SCU method, which is reduced from 7200 min to 180 min, making it possible to conduct the hot-to-cold analysis at the design stage

Impurity-Induced Grain Boundary Strengthening in Polycrystalline Graphene

First-principles density functional theory (DFT) calculations were performed to investigate the effects of boron (B) and nitrogen (N) doping on the fracture behaviors of a series of symmetric graphene grain boundaries (STGBs) under biaxial straining. Doping was found to generally enhance the fracture strength of STGBs, which was shown to be attributed to dopants mechanically lowering the local stretching energy or chemically strengthening the critical bond. We also showed that doping may also induce crack deflection to further improve the fracture resistance of graphene grain boundaries (GBs). Furthermore, we showed that the presence of multiple B and N dopants in the pentagon-heptagon ring(s) can contribute to promoting intergranular fracture in the presence of a small prestrain along the GBs. Our findings clarify the role of B and N dopants in GBs strengthening of polycrystalline graphene

In Situ Measurements for Plastic Zone Ahead of Crack Tip and Continuous Strain Variation under Cyclic Loading Using Digital Image Correlation Method

Fatigue crack is one of the most common damage forms for aeronautical aluminum alloy. With crack propagation, the strain fields of the whole object surface and plastic zone (PZ) ahead of the crack tip are changing continuously. For most metallic materials, the behavior of PZ around the crack tip and continuous strain variation play a vital role in crack propagation. In this work, the “continuous” strain information at and in front of the crack tip on the specimen surface was obtained quantitatively and the PZ size ahead of crack tip was in situ measured quantitatively with crack propagation by using the digital image correlation (DIC) method, which overcomes the difficulty for the in situ measurement of mechanical variables. Moreover, the method of specimen preparation was simplified by using a white matt paint with strong adhesion, but also resulted in a higher resolution being shown, even for such a large area. Furthermore, the experimental results of the PZ size from the proposed method had good agreement with the theoretical values, which overcomes the limitation that the conventional approaches only consider the quasi-static crack. Finally, the continuous strain variation behavior was analyzed from the experimental results in detail with the consideration of crack propagation