Weifang Zhang

Failure Analysis of Aviation Torsional Springs

Cracks and fractures occur during the assembly process to a type of torsional springs used in the aviation mechanism. Besides visual examination, other experimental techniques used for the investigation are: 1) fracture characteristics, damage morphology and fractography by scanning electron microscopy (SEM), 2) spectrum analysis of covering, 3) metallographic observation of cracks and 4) hydrogen content testing. The results are obtained through the analysis of manufacture process and experimental data. Since no changes of microstructure are found, failures are irrelevant to the material. The cracks and fractures initiate on the inner surface, cracks initiate before the cadmium plating and after the winding. No obvious stress corrosion cracks are found near the crack source region. The opening direction of cracks is consistent with the residual tensile stress of the spring inner surface, and the springs are easy to contact hydrogen media between the spring winding and the cadmium plating. The cracks are caused by hydrogen-induced delayed cracking under the action of the residual tensile stress and hydrogen.

Failure Analysis of Cracks in the Upper Shaft of Torque Arm

In the periodic inspection of a plane having flown for 300 h, cracks were found in the upper shaft of the torque arm of the landing gear. In addition to visual examination, other experimental investigation techniques were 1) crack morphology and fracture characteristics by scanning electron microscopy, 2) metallographic observation of cracks, 3) chemical constituents and hydrogen-content testing, 4) hardness testing, 5) impact and bending simulation test, and 6) comparison of wear condition for the crack-free upper shafts with other batches. The results were obtained through the analysis of processing and experimental data. Because of higher surface frictional shear stress and tensile stress, the cracks propagated in the form of stress corrosion after the initiation, which was unrelated to the coating, matrix material, and hydrogen content.

Experimental Study of Thermal Contact Conductance Based on Heat-Transfer Theory with Variable Thermal Conductivity

To calculate the interface temperature drop ΔT during the thermal contact conductance (TCC) testing, an optimization computing method based on the one-dimensional steady heat-transfer theory with variable thermal conductance is proposed in this paper. This method takes into account the physical process of heat transfer adequately, which can calculate the temperature drop (ΔT) accurately. Besides, a practical engineering example is utilized to demonstrate the applicability of the proposed approach. By using the optimization computing method along with traditional linear fitting method, the testing values of TCC obtained from the interfaces of the structural materials GH4169 and K417 used in aero-engines are studied and compared. The results show that there are large errors and poor stability in the traditional linear fitting method, and the relative errors of these two methods vary from about 3 % to 17 %.

The Damage Investigation of Wedge-Shaped Electromagnetic Riveting Structure of CFRP/Aluminium Alloy

In the aircraft industry, electromagnetic riveting is now widely used in the riveting of wedge-shaped aluminium-alloy structure and carbon-fibre-reinforced polymer (CFRP), but damages to the CFRP often happen. To investigate the reason for the damages, methods of C-scan, x ray, Compton backscatter scanning (CBS), metallography, and thermal deply were applied to study the wedge-shaped riveting structures. The study indicates that the composites by the tail of the rivets, which are close to the edge of the structure, are almost comminuted, and those rivets expand uniformly along the shaft and interference fit with the structure, and at the same time curve toward the edge. The composites around the rivets, which are away from the edge of the structure, are slightly damaged, and these rivets deformed not so uniformly, most of them clearance fit with the structure, and in some joints interference fit and clearance fit occur alternatively. The investigation shows that, in the joints close to the edge, the balance of constraints from the aperture to the rivet is broken by the slope around the tail of the rivet, results in a rightward force working on the rivet tail. The integration of this force and the riveting force curves the rivet instantaneously, which comminutes the right-side composites of the rivet tail. For the rivets away from the edge, the deformation becomes more buckled the longer the rivet, which causes the instability of the fit situation and reduces the damage to the composites.

Investigation on Thermal Contact Conductance Based on Data Analysis Method of Reliability

Abstract The method of reliability is proposed for the investigation of thermal contact conductance (TCC) in this study. A new definition is introduced, namely reliability thermal contact conductance (RTCC), which is defined as the TCC value that meets the reliability design requirement of the structural materials under consideration. An experimental apparatus with the compensation heater to test the TCC is introduced here. A practical engineering example is utilized to demonstrate the applicability of the proposed approach. By using a statistical regression model along with experimental data obtained from the interfaces of the structural materials GH4169 and K417 used in aero-engine, the estimate values and the confidence level of TCC and RTCC values are studied and compared. The results show that the testing values of TCC increase with interface pressure and the proposed RTCC model matches the test results better at high interface pressure.

Crack Analysis of the Fourth-Stage Compressor Blades

crack morphology and fracture characteristics by video microscope and scanning electron microscopy, metallographic observation of the cracks, and energy spectrum analysis. The results were obtained through the analysis of the processing andexperimental data. The cracks are all fatigue cracks with multisource and line source, the main reason of which is concerned with higher contact stress during the assembly process after the first overhauling. The fretting wear between the tenon and the tenon-groove mating surface promotes crack initiation, which was unrelated to the material and processing.

Progress in patch repair of aerospace composite structures

With the rapid application of the composite structure in the aerospace industry, more load-bearing structures and components are used with composites instead of conventional engineering materials. However, the composite structures are inevitably suffered damages in the complex environment, the composites structures repair become more important in the airplane maintenance. This paper describes the composites patch repair progress. Firstly, the flaws and damages concerned to composite structures are concluded, and also the repair principles are presented. Secondly, the advantages and disadvantages for different repair methods are analyzed, as well as the different bonded repair and their applicability to different structures is discussed. According the recent research in theory and experiment, the scarf repair effects under different parameters are analyzed. Finally, the failure mechanisms of repair structure are discussed, and some prospects are put forward.

Investigative Method for Fatigue Crack Propagation Based on a Small Time Scale

In-situ scanning electron microscopy (SEM) testing based on a small time scale is proposed to integrally investigate the fatigue crack growth behavior and mechanisms, which is different from the widely-used, cycle-based approach due to its small time scale and comprehensive analysis of the effects of microstructure, crack closure and applied loading on crack growth. In the proposed methodology, the behavior of fatigue crack growth at any time within a loading cycle is observed by SEM to investigate the influence of microstructure on crack growth. Images with high resolution are taken to measure the crack tip opening displacement (CTOD), and the correlation between CTOD and the stress intensity factor (SIF) K is studied. A model based on experimental data is used to predict the CTOD variation. The unstable crack growth of aluminum alloy 7050-T7451 is investigated using the proposed method. Results show that this method has great potential in fatigue crack growth mechanism research compared with the traditional cycle-based approach.

Degradation Behavior of Nitrile Rubber O-rings in Hydraulic Oil

The degradation behaviors of nitrile rubber O-rings immersed in hydraulic oil were investigated by adopting the artificial accelerated aging test. The chemical structures and mechanical properties of the aged samples under different conditions were evaluated by measuring the attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, crosslinking density, weight loss, mechanical properties and fracture morphology. The ATR-FTIR results indicate that the amide groups and the hydroxyl groups were formed in the surface of the aged samples. The changes in the crosslinking density of the samples attributed to the competition between crosslinking and chain scission. The mechanical test results show that the tensile strength of samples immersion in oil changed remarkably for the different temperatures. In addition, the fracture morphology results demonstrate that the formation of the defects also led to the decrease in properties of the aged samples. Introduction Hydrogenated Nitrile rubber(NBR) are well known for their wear resistance, oil resistance and good impermeability, and have been widely used as seals, O-rings and gaskets in the sealing field of oil and gas especially in hydraulic system. However, due to the existence of unsaturated bonds and volatile components in the NBR, the severe environmental factors such as oxygen, radiation, high temperature, aggressive media and mechanical stress can accelerate the aging process of the seals, and cause the degradation in hardness, mechanical properties, molecular structure and components [1-3]. Eventually, the seals lose the ability to flexibility and resilience, resulting in the failure of sealing systems due to the leakage of gas or liquid [7,8]. The aging of NBR materials in oil mainly occur through the follow processes: the extraction of soluble components (e.g., plasticizers and antioxidant); the reactions of unsaturated bonds; thermal oxidative aging reactions [6-9]. In addition, the loss of additives weakens the antioxidant ability of NBR leading to the further degradation, and causes the worse compatibility between the matrix and additives [10]. The crosslinking and chain scission make the network structure denser, and restrict the movement of molecular chains [11]. Besides, the oil type has a direct influence on oil resistance of the seal [12,13]. In spite of these significant efforts, there are relatively less literatures focused on the degradation behaviors and mechanisms of the NBR O–rings in hydraulic oil. The objective of this study is to investigate degradation of NBR O-rings immersed in hydraulic oil at elevated temperatures (70°C, 90°C, 110°C) by the systematic accelerated thermal aging tests. Subsequently, the changes in chemical structures of the aged samples were studied by Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and solvent swelling test. The physical and mechanical properties of the aged samples were also assessed by weight loss, mechanical property and fracture morphology analysis. By comparing and analyzing the changes in physical properties, mechanical properties and molecular network structural evolution of the samples aged in oil, we concluded how the hydraulic oil and temperature affected the degradation behaviors and mechanisms of the seals during the thermal aging process.

A Multi-dimensional Experimental Method for Investigating Fatigue Crack Growth with Overloads

The comprehensive multi-dimensional experimental method is proposed to concurrently investigate the crack growth rate, microstructure effect, crack closure and crack tip plastic zone in one fatigue crack growth experiment. In the proposed methodologies, the fatigue crack growth experiment is performed to loading the specimen under constant amplitude loading, and the crack opening displacement (COD) gauge is used to measure the crack length. A self-developed micro-optical system is used to verify crack length at the same time and study the effect of microstructure on crack growth. Then the correlation between crack growth rate and stress intensity factor (SIF) range is studied to investigate the crack growth mechanism. The images near crack tip are taken to investigate the crack closure and crack tip plastic zone by using digital image correlation (DIC) measurement.