Foreign Object Impact Research Articles

Optimized mechanical properties of the hot forged Ti–6Al–4V alloy by regulating multiscale microstructure via laser shock peening

The hot forged Ti–6Al–4V alloy demonstrates well constructed microstructure and balanced mechanical properties, which promotes its wide application in aviation field. However, its relative poor resistance to wear and foreign object impact usually leads to the cumulative damage, causing sudden failure and serious accident. Laser shock peening (LSP) is a novel surface plastic deformation technique, which could strengthen the surface layer of components through gradient grain structure. Nevertheless, the specific mechanism of microstructure evolution and mechanical properties enhancement of LSP processed hot forged Ti–6Al–4V alloy is still obscure, and its corresponding explanation would help the wide application. In the present research, the hot forged Ti–6Al–4V alloy was processed by LSP to regulate its superficial microstructure and improve mechanical properties, helping to understand the inner mechanism. The results reveal that LSP could simultaneously result in the merging of ultrafine α-Ti grains and refinement of coarse α-Ti grains, which reconstruct the dual-size grain structure. The crystal tilting and transformation promoted by the generation and movement of dislocations benefit the merging of ultrafine grains. Due to the different slip systems in dual phases, β-Ti phases exhibit much greater response to slip under surface plastic deformation, which are enforced to deform and construct the shell structure by sliding and phase transformation, while the α-Ti phases act as the core to synergistically construct ‘core-shell’ like structure. The increase of LSP impact time promotes the well wrapping of the ‘core-shell’ like structure and strengthens it by abundant dislocations, which also forms the gradient grain structure from surface to inner. Since the microstructure regulation and crystal defects engineering, the LSP improves the surface damage resistance and mechanical properties of the hot forged Ti–6Al–4V alloy obviously. Such results indicate a new technology to increase the properties of the hot forged Ti–6Al–4V alloy component further.

SHM to detect and characterize impact events in metallic aircraft structure

Aircraft structures are susceptible to foreign object impacts, which may occur during manufacturing, maintenance and in-service. Sizes of these impacted objects especially during in-service can range from a small rock to a large bird. Common ways to detect such impacts are based on flight / ground crew observations and reports leading to close examinations of structures using non-destructive evaluation (NDE) techniques. If undetected these impact damages can grow during service loading and may be detrimental to flight safety. Therefore, timely detection of any signs of impact damages are critical such that proper maintenance actions can be taken. The aim is to develop methodologies using Structural Health Monitoring (SHM) techniques to detect and characterize foreign object impact events. In this experiment, a cut-out of an aluminum panel measuring 31 x 26 inches from an out-of-service aircraft was used. The panel was instrumented with four Lead Zirconate Titanate (PZT) sensors from Acellent Inc., as well as, four Acoustic Emission (AE) sensors from Mistras Inc. Impedance and susceptance measurements were acquired to assess the proper functionality of the PZT sensors before and after the impact events. Both the PZTs and the AE sensors were directly connected to a digital oscilloscope, without any amplification and / or filtering for acquiring raw data during the impact events. An instrumented multi-use tapper was designed and developed to calibrate the system, as well as, to record the impulse (impact force and time) during the impact events. The acquired data were processed using physics-based and machine learning techniques to detect and characterize the impact events.

Fatigue behavior assessment of heavy-duty freight railway axles under different heat treatments

The safety of heavy-duty freight railway axles is threatened by defects, including inclusions, corrosion pits, fretting cracks and foreign object impact damages. Accordingly, surface strengthening techniques are applied to reduce the detrimental effect of defects on the structural integrity of railway axles. This work studied the effectiveness of heat treatment, in which the specimens machined from a heavy-duty freight railway axle were induction hardened under three types of processing parameters and tempered. The fatigue strengths of specimens with electrical discharge machined (EDM) notches and impact craters on their surfaces were tested. The fatigue behavior of induction-hardened (IH) specimens was compared with that of as-received (AR) specimens and fully-quenched (FH) specimens. The fatigue limit of smooth IH specimens is close to that of FH specimens while almost twice as much as that of AR specimens. Inclusions were found on the crack initiation sites on the surface or subsurface of AR and FH specimens. For IH specimens, the crack initiation sites moved to the zone between the case and the core. Fatigue strengths of IH and FH specimens decline with increasing EDM notch size but remain higher than AR specimens’. The influence of impact damage on fatigue properties depends on damage size and heat treatment process.

Printed PZT Transducers Network for the Structural Health Monitoring of Foreign Object Damage Composite Panel

The work presented here focuses on the structural health monitoring (SHM) of a foreign object damage (FOD) composite panel equipped with an innovative printed piezoelectric transducer network. The 3D woven composite FOD panel measures approximately 800 mm x 320 mm, is curved with a cross-sectional thickness varying from approximately 2 mm to 12 mm, and a stainless-steel leading edge is bonded at one of its sides. The core idea explored here is to rely on an innovative screen-printing technology to print a full piezoelectric transducer allowing to successfully achieve SHM on such a complex composite structure. This work is being carried out within the European project MORPHO – H2020. After printing a 25 elements PZT network, a four points bending fatigue experimental campaign using the PZT network along with other sensor technologies (embedded optical fibres with FBG sensors and acoustic emission sensors) is carried out. This unique experimental campaign allows to generate data and will help to develop diagnostic and prognostic methodologies for remaining life estimation and SHM of the FOD panel. It is demonstrated here through impedance measurements that the printing process associated with the printed PZT transducers is highly repeatable thus validating its use at a larger industrial scale. Furthermore, the printed piezoelectric transducers electromechanical behaviour is characterized and they are shown to be able to detect foreign object impact and to size and localize resulting damage using Lamb waves signals collected at different locations of the network. This innovative printing technology for PZT transducers network is thus extremely promising. It is furthermore highly advantageous to use the printed transducers for SHM instead of regular ceramic ones as this technology is non-intrusive, add negligible weight, can be printed during the manufacturing process, and arrays of transducers ensure easy availability of another transducer in case of failure of one.

Effect of impact-corrosion coupling damage on fatigue properties of 2198-T8 aluminum-lithium alloy

The peeling of aircraft surface paint due to foreign object impact can expose the aluminum–lithium alloy substrate to atmospheric corrosion, resulting in the fatigue performance being affected by the combined effect of impact damage and corrosion damage. In this work, the effects of impact damage and corrosion damage on the fatigue life of 2198-T8 aluminum–lithium alloy sheet were investigated. The strain field distribution of specimens with coupled impact and corrosion damage under cyclic loading was analyzed by Digital Image Correlation (DIC) technique, and the coupling relationship between impact and corrosion damage was analyzed based on the crack propagation path and fracture morphology. The results show that surface corrosion pits depth distribution of impact dents follows the same trend as residual stress distribution. For the coupling damage of the hemispherical punch, the coupled effect between impact damage and corrosion damage is most evident at low impact energy. At higher impact energies, the impact damage dominates the coupling damage. As the salt spray corrosion time increases, the fatigue life decrease rate gradually slows with increasing impact energy, and the impact damage has a greater effect on the fatigue life in the early corrosion stage. Compared to the salt spray corrosion environment at 35 °C, the fatigue life of the specimen at 25 °C increased by a maximum of 50.48 % as the impact energy increased from 10 J to 25 J.

Numerical simulation and experimental analysis of foreign object impact on aero-engine fan rotor blade

The detection and identification of fan rotor foreign object impact events during the working process of aero-engines are crucial to the safety and security of aircraft flight. Through the numerical simulation method based on finite element and the foreign object impact test platform to simulate the process of real engine impact by foreign objects, the rule of fan rotor blade foreign object impact and the monitoring and identification method were studied. Meanwhile, aiming at the problems of high difficulty and low accuracy in identifying the impact of foreign objects on the fan by the airborne parameters and added vibration parameters, the non-contact vibration measurement method based on blade tip timing was used to obtain the blade vibration displacement under different fan speed, foreign matter mass and impact velocity. The numerical simulation and experimental results show that the change trend of the vibration displacement of the rotor tip of the fan is consistent with that of the numerical simulation. And with the increase of the foreign object projectile mass, the vibration displacement of the blade tip caused by the impact of the foreign object on the fan blade increases. In addition, the impact velocity of the foreign object has little effect on the vibration displacement of the blade tips after impact. Furthermore, the vibration displacement of the blade tip caused by the impact of foreign objects first increases and then decreases with increasing speed.

Effects of foreign object damage on high-cycle fatigue behavior of Inconel Alloy 690TT steam generator tubes

The process of foreign object impact on Alloy 690TT steam generator (SG) tubes under high-speed water flow was simulated using quasi-static indentation method. Morphologies of foreign object damage (FOD) were detailed characterized, and the fatigue curves and fatigue strengths were acquired for smoothed and three types of FOD-affected specimens. Results indicated that the fatigue strength of the impacted specimens reduced with the increase of defect depth, and there was an exponential relationship between fatigue strength and defect depth. Fatigue cracks all initiated at the root of the defect groove, and as the depth of defect increased, the number of fatigue crack sources gradually increased. Finally, the fatigue strength was predicted by the fatigue notch factor and damage tolerance method, and the fatigue performance evaluation diagram of the SG tube damaged by foreign object based on El Haddad model was established.

A Review of Approaches for Mitigating Effects from Variable Operational Environments on Piezoelectric Transducers for Long-Term Structural Health Monitoring.

Extending the service life of ageing infrastructure, transportation structures, and processing and manufacturing plants in an era of limited resources has spurred extensive research and development in structural health monitoring systems and their integration. Even though piezoelectric transducers are not the only sensor technology for SHM, they are widely used for data acquisition from, e.g., wave-based or vibrational non-destructive test methods such as ultrasonic guided waves, acoustic emission, electromechanical impedance, vibration monitoring or modal analysis, but also provide electric power via local energy harvesting for equipment operation. Operational environments include mechanical loads, e.g., stress induced deformations and vibrations, but also stochastic events, such as impact of foreign objects, temperature and humidity changes (e.g., daily and seasonal or process-dependent), and electromagnetic interference. All operator actions, correct or erroneous, as well as unintentional interference by unauthorized people, vandalism, or even cyber-attacks, may affect the performance of the transducers. In nuclear power plants, as well as in aerospace, structures and health monitoring systems are exposed to high-energy electromagnetic or particle radiation or (micro-)meteorite impact. Even if environmental effects are not detrimental for the transducers, they may induce large amounts of non-relevant signals, i.e., coming from sources not related to changes in structural integrity. Selected issues discussed comprise the durability of piezoelectric transducers, and of their coupling and mounting, but also detection and elimination of non-relevant signals and signal de-noising. For long-term service, developing concepts for maintenance and repair, or designing robust or redundant SHM systems, are of importance for the reliable long-term operation of transducers for structural health monitoring.

A Comparison of Three Simulation Techniques for Modeling the Fan Blade–Composite Abradable Rub Strip Interaction in Turbofan Engines

Turbofan engine models for foreign object impact simulations must include a representation of fan blade interactions with surrounding components of the engine, including rubbing against the abradable lining. In this study, three numerical techniques, namely, the finite element method (FEM), smoothed particles hydrodynamics (SPH), and the adaptive (hybrid) FEM/SPH approach (ADT), were evaluated for their applicability to modeling of the blade–abradable rub strip (ARS) interaction. Models developed using these methods in the commercial code LS-DYNA were compared in terms of their computational cost, robustness, sensitivity to mesh density, and certain physical and non-physical parameters. As a result, the applicability of the models to represent the blade-ARS interaction was ranked as follows (1—most applicable, 3—least applicable): 1—SPH, 2—FEM, and 3—ADT.

Experimental investigation of metasurface-based resonator for one-to-many wireless power transfer systems in the presence of foreign objects

The wireless charging of multiple devices simultaneously is on a high demand. Metasurface-based resonators with a uniform magnetic field distribution over a large area have been recently proposed as a transmitting resonator for one-to-many WPT systems. However, the problem of the influence of foreign objects on a one-to-many WPT system characteristics has not been deeply studied. Herein, we explore the influence of different foreign objects such as a metal screen, ferrites, coins, and a smartphone on the input impedance of the WPT system utilizing a metasurface-based resonator. We experimentally investigate the impact of foreign objects on the WPT system parameters and define the most destructive foreign objects. To avoid this parasitic impact, we modify the metasurface-based resonator design by shielding the magnetic field. We numerically demonstrate that the modified design does not provide any magnetic field beneath the metasurface-based resonator.

A novel method to improve the precision of BTT under rapid speed fluctuation conditions

It is ultra-important to evaluate the turbine machine blade dynamic stress under abnormal conditions such as rapid rotating speed fluctuation result from surge, rub and foreign object impact. The blade tip timing (BTT) based non-contact dynamic stress measurement method is the most promising technology to replace rotating blade strain gauge. However, there are great challenges to precisely monitor the blade vibration as the rotating speed rapidly fluctuated. It limits the application of BTT considerably. Here, a novel BTT method is proposed to achieve high-precision blade vibration measurement under rapid speed fluctuation conditions. The influence of speed fluctuation on the theoretical arrival time is investigated based on the relationship between blade theoretical arrival time and installation angle. Further, using of the SG filter greatly reduces the computational complexity. The effectiveness of the method is verified by numerical simulation, academic test rig and large industrial turbo fan. The results indicate that the speed measurement error is not more than 0.6 rpm as the accelerating speed as high as 1200 rpm/s. The blade vibration displacement error caused by the rapid speed change can be less than 10 μm. Comparing with existing BTT methods, the average and standard deviation of the blade vibration measurement error are greatly reduced. The method has great value for the application of the blade health monitor and predictive maintenance in turbomachines.

A CFD–CSD coupling method for simulating the dynamic impact and expulsion of fragile foreign objects from the ‘inlet–bypass duct’ junction of a turboprop aircraft

Frequently appearing fragile hail, ice and other foreign objects pose great threats to advanced turboprop aircraft when they enter the intake system of the engine. However, it is difficult to simulate the effects of these objects as complicated aerodynamic and dynamic impact coupling phenomena are involved. Regarding a turboprop aircraft inlet with a bypass duct as the subject of the research reported in this article, a coupled Computational Fluid Dynamics–Computational Structural Dynamics (CFD–CSD) numerical simulation method is established based on an unstructured dynamic mesh and impact dynamics technology to solve the phenomena of ‘motion–collision–fragmentation–fragment motion’ for foreign objects such as fragile ice and hail entering the inlet. Based on airworthiness specifications, external hail and ice at the lip and inner lower wall of the inlet are modelled. Moreover, the dynamic motion of these foreign objects and their effects on the aerodynamic performance of the inlet system during the process are simulated and analysed in depth. The results show that high-speed hail breaks into small debris after collision, which may not cause a serious threat to the engine. In addition, the total pressure recovery coefficient and distortion rate are not heavily changed during the process. However, large pieces of ice at the lip of the inlet may lead to large-size fragments that impact the inner wall of the inlet, thus increasing the threat to the safety of the engine. Also, serious shielding and interference of large pieces debris with the inner flow in the dynamic process decrease the total pressure recovery coefficient while increasing the distortion rate, especially in the junction area between the inlet and the bypass duct. In particular, ice in the icing zone located in the inner lower part of the inlet should be of much more concern owing to the fact that ice in this region is more likely to fly directly into the main engine and cause a serious threat.

Formation Mechanism and Control Method of Residual Stress Profile by Laser Shock Peening in Thin Titanium Alloy Component.

In the laser shock peening process of titanium alloy thin blades, a shock wave will be repeatedly reflected and coupled in the blades, resulting in the failure of the formation of a gradient residual compressive stress layer, which is the key to improve fatigue performance and resist foreign object impact. This paper takes TC17 titanium alloy sheet as the research object to reveal the influence mechanism on residual stress-strain profile of shock wave reflection-coupling by shock wave propagation and key position dynamic response. Based on the result of influence mechanism, two wave transmission methods are proposed to regulate shock wave in order to reduce the intensity of shock wave reflection. The analysis shows that the high strength stress be formed when the shock wave is reflected and coupled in the sheet, which causes the re-plastic deformation and the decrease of transverse plastic strain. This eventually leads to residual tensile stress up to 410 MPa being formed within a 0.5 mm radial direction and 0.3 mm deep of the spot range. The use of “soft” and “hard” wave-transmitting layers greatly reduces the shock wave reflection intensity, and under the condition of the “hard” wave-transmitting layer, a better impedance matching is achieved, resulting in a residual compressive stress of about 400 MPa.

Research on the Bearing capacity of aircraft metal skin with pit damage

Pit is a kind of damage form that often occurs in the daily operation process of aircraft. It is formed by permanent plastic deformation due to the impact of foreign objects. In addition to the stress concentration caused by structural geometric changes, it will also produce local residual stress in the interior. Its impact on fatigue cannot be evaluated only through the stress concentration analysis of structural geometric changes. In this paper, the bearing capacity of the damaged metal skin dent is studied. By establishing the finite element model, the stress of the intact structure and the dent with different width depth ratio is compared and analysed.

Research Status of Aero-Engine Blade Fly-off

The research of aero engine blade out (Fan Blade Out, FBO) is an important problem that must be solved in the design stage of the reliability and safety of the whole structure. The turbofan blades are broken due to fatigue fracture or foreign object impact, and the rotor system produces large vortex under the action of huge unbalanced force, which causes the tip of the blade to collide with the casing and friction in turn. This paper analyzes the aero-engine FBO, and provides a systematic basic theory for the design, innovation and development, which has certain reference value.

Influence of foreign object impact mode on fatigue performance of 2198‐T8 Al‐Li alloy thin sheets for fuselage

AbstractThe 2198‐T8 is the advanced aircraft aluminium‐lithium alloy used for airframe construction. Based on the digital image correlation technique, the postimpact fatigue performance of 2198‐T8 aluminium‐lithium alloy sheets was analysed. The foreign object impact mode includes a single impact, continuous twice‐impacts at the same position and impacts at two parallel positions. For each mode, the collapse behaviours with different impact energies and insert dimensions were investigated. The results could show that residual fatigue life is sensitive to the impact mode, and through digital image correlation (DIC) nephogram analysis, on the surface of dented specimens, there exists the redistribution of strain field. For three types of impact modes, the concept of average damage volume is introduced to fit the effect of dent depth and width on residual fatigue life; the goodness of fit for the whole average damage volume is 89.5%. The crack initiation and propagation of the postimpact fatigue specimen will be affected by the stress concentration and microstructure, and the local multiaxial strain status around the dent will accelerate the crack propagation.

Nondestructive health monitoring techniques for composite materials: A review

Structural health monitoring is the process of acquisition and analyzing technical data obtained from structures to determine the present condition of the structure and residual life. Composites have been widely in use because of their low weight and better mechanical properties compared to conventional metals. They are more prone to damage during cyclic loading and the impact of foreign objects. So, usage of the nondestructive techniques is important to detect such damage in composites at the beginning stage itself, which further helps to avoid catastrophic failure. Many review articles are discussing a single nondestructive technique to monitor the health of the structure, but a single technique is not sufficient in most of the cases. This review is focused on the most commonly used nondestructive health monitoring techniques such as acoustic emission, vibration testing, ultrasonic testing, infrared thermography, and shearography to detect and characterize the damage in composite structures used in aerospace, automotive, and marine applications. The comparison among the techniques also has been presented in this review.

Foreign Object Impact Damage in Ceramic Matrix Composites: Experiments and Computational Predictions

Abstract Foreign object impact of ceramic matrix composite (CMC) materials and components in a gas turbine engine environment could be detrimental to engine performance and hence must be accounted for in the design of such components. This paper is concerned with experiments and computational modeling of foreign object impact phenomenon in silicon carbide (SiC)-based CMC. Controlled impact experiments were conducted on the CMC material using a gas-gun apparatus with spherical hardened steel projectile. The internal damage state within the CMC specimens was assessed using X-ray computed tomography scan technique. The computational modeling involved explicit dynamic finite element (FE) simulation of the impact process wherein either delamination mechanism is modeled or both ply damage and delamination mechanisms are modeled in a coupled manner. The delamination mechanism is modeled explicitly using cohesive-zone (CZ) fracture mechanics approach, whereas, the ply-damage mechanisms are modeled implicitly using simplified continuum damage mechanics approach. The simulation results were found to be in reasonable qualitative and quantitative agreement with the experimental results. Furthermore, it is shown that modeling both the ply damage and delamination mechanisms are essential to predict the correct delamination pattern even for intermediate velocity impacts that leads to predominantly delamination damage. The predictive nature of the modeling approach is demonstrated and approaches to enhance the models are also discussed.

Методика определения живучести и стойкости воздушных винтов летательных аппаратов к ударному воздействию посторонних предметов

The authors propose a technique for determining the survivability and resistance of the blades to impact by foreign objects located on the surface of the airfield pavement, using a set of mathematical models and experimental plants and considering the data on the layout and design of the aircraft and its propulsion system with a propeller. An example of the technique’s application to a small-sized aircraft with a propulsion system with a pushing propeller is considered. The results of theoretical studies on determining the boundaries and parameters of the impact of foreign objects with the propeller are presented. Based on the data obtained, experimental studies are conducted to determine damage to the blades made of composite materials by granite particles. The limiting value of the relative mass of a granite particle, which ensures the operability of the propeller during and after the impact is determined. Recommendations on increasing the impact resistance of the blades are given.

Foreign Object Detection in Inductive Charging Systems Based on Primary Side Measurements

A foreign object detection (FOD) method for inductive charging systems is introduced in this article. The proposed FOD only requires primary side measurements, i.e., primary resonant current and resonance frequency deviation. The proposed FOD incorporates a mechanism for effective discrimination of foreign object intrusion, horizontal and vertical alignment variations of the transmitter and receiver magnetic pads. Alignment variations of primary and secondary magnetic pads as well as intrusion of conductive/magnetic foreign object both impact electromagnetic characteristics, consequently the resonant frequency. FOD for large size objects works properly based on resonance frequency deviation, however, there is the need for discrimination between the FOD for small objects and misalignment. Thereby, both disturbances can be detected by online monitoring of the resonance frequency deviation, and be discriminated from each other by real-time measurement of the primary current. The proposed FOD technique can be added as a protection scheme to the self-tuning controllers of inductive power transfer (IPT) systems operating off its resonance frequency. Finite element analysis along with theoretical formulation of the studied IPT circuit is presented using the proposed method. Conductive FOD method based on frequency deviation and discrimination method using primary resonance current are implemented on an IPT setup, and the experimental results verify its effectiveness in both standby and full-power modes. The test results show that the frequency deviation based FOD method has a high speed in detection of even small scale objects such as a coin.