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High-speed ballistic impact tests were conducted at room temperature and 500 °C on nickel-based superalloy GH4169 simulated blade specimens containing leading-edge features. The microscopic characteristics of the impact notch at room temperature versus 500 °C were observed by electron backscatter diffraction (EBSD), and it was found that the grains on the notched subsurface were ruined, while in more distant regions, the impact energy was mainly absorbed by grain boundaries. Internal damage is more concentrated in the notched subsurface region at 500 °C compared to room temperature. The high cycle fatigue strength of the damaged specimens under different conditions was tested. The results showed that the high cycle fatigue strength of the damaged specimens increased with the increase in the notch depth, and the fatigue strength of the damaged specimens at 500 °C was higher than the fatigue strength at room temperature. Both the 48 h post-impact holding time at 500 °C and the preload during impact at 500 °C increased the fatigue strength of the damaged specimens. [ABSTRACT FROM AUTHOR]

In the present study, based on kinetic energy theorem and Newton's second law, the velocity equation of the hard foreign object was derived by taking the engine inlet as reference frame. Taking a certain type of aircraft as an example, the factors that influence the impact velocity of foreign objects were discussed at 100% design rotating speed of the engine. It is observed that the velocity of the foreign object is not only related to the dimension, shape, material, and impact location of the foreign object, but also related to the length of the engine inlet as well as the working state of the engine. The proposed method can be used for estimating the impact velocity in FOD tests by taking into account working state of the engine. [ABSTRACT FROM AUTHOR]

High-speed ballistic impact tests were conducted at room temperature and 500 °C on nickel-based superalloy GH4169 simulated blade specimens containing leading-edge features. The microscopic characteristics of the impact notch at room temperature versus 500 °C were observed by electron backscatter diffraction (EBSD), and it was found that the grains on the notched subsurface were ruined, while in more distant regions, the impact energy was mainly absorbed by grain boundaries. Internal damage is more concentrated in the notched subsurface region at 500 °C compared to room temperature. The high cycle fatigue strength of the damaged specimens under different conditions was tested. The results showed that the high cycle fatigue strength of the damaged specimens increased with the increase in the notch depth, and the fatigue strength of the damaged specimens at 500 °C was higher than the fatigue strength at room temperature. Both the 48 h post-impact holding time at 500 °C and the preload during impact at 500 °C increased the fatigue strength of the damaged specimens.

Copyright of Journal of Ordnance Equipment Engineering is the property of Chongqing University of Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The fracture toughness of thermal barrier coatings (TBC) is a critical mechanical property that governs damage resistance. Catastrophic delamination of TBC under erosion conditions occurs in TBC with low toughness. Prior research has explored indirect and complex experiments to measure TBC toughness, but the miniaturized nature of the multi-layered coating makes it difficult to quantify its intrinsic toughness. This paper integrates computational modeling and experimental approaches to estimate the TBC toughness and the substrate delamination strength. The results show that a typical newly fabricated yttrium stabilized zirconia coating under service conditions has a toughness estimated in the range of 0.1–0.5 MPa m1/2 and a toughness of thermally grown oxide layer in between 1.5 and 1.7 MPa m1/2. The analysis also determined that a thermally grown oxide with a fracture toughness above 2.0 MPa m1/2 would not delaminate under the service conditions. Overall, the approach demonstrates the value of integrated computational material approaches, which can save time and enhance predictive power. [ABSTRACT FROM AUTHOR]

The present paper aimed at investigating the effect of foreign object damage (FOD) on the high-cycle fatigue resistance and remaining life of high-speed railway axle. The commercial compressed gas was used to prefabricate the impact surface damage on the specimen taken from the actual axle surface. The probabilistic fatigue curves and fatigue limits of smoothed and FODed specimens were obtained according to the Owen tolerance limit method. Due to the variations of loads, surface qualities and sizes between the small-sized specimens and full-scale axles, the fatigue limits of damaged specimens were modified for assessing the service performance of full-scale axles. In addition, theoretical fatigue limits evaluated by the Peterson formula under two impact velocities (100 and 138 m/s) were more conservative than the experimental values by the effects of residual stress and microdamage. Finally, the service life of impact-damaged axles was calculated by the modified Miner theoretical formula to determine a reasonable inspection period for axles. [ABSTRACT FROM AUTHOR]

Foreign object damage (FOD) is a common mode of failure in high-speed rotating machinery, such as aircraft engines. Therefore, research on FOD is crucial for ensuring blade integrity. FOD induces residual stress on the surface and within the blade, impacting its fatigue strength and service life. Therefore, this paper utilizes material parameters determined by existing experiments, based on the Johnson–Cook (J-C) constitutive model, to numerically simulate impact damage inflicted on specimens, compare and analyze the residual stress distribution of impact pits, and investigate the influence law of foreign object characteristics on blade residual stress. TC4 titanium alloy, 2A12 aluminum alloy, and Q235 steel were selected as foreign objects, and dynamic numerical simulations of the blade impact process were performed to explore the effects of different types of metal foreign objects. This study analyzes the influence of different materials and foreign objects on the residual stress generated by blade impact through numerical simulation, examining the distribution of residual stress in different directions. The findings indicate that the generated residual stress increases with the density of the materials. Additionally, the geometry of the impact notch is also influenced by the density difference between the impact material and the blade. The distribution of the residual stress field reveals that the maximum residual tensile stress in the blade is related to the density ratio, and the residual tensile stress in the axial and circumferential direction is relatively large. It is important to note that a significant residual tensile stress has a detrimental effect on the fatigue strength. [ABSTRACT FROM AUTHOR]

Kaplan turbines, as one of the well-known hydraulic turbines, are generally utilized worldwide for low-head and high-flow conditions. Any failure in each of the turbine components can result in long-term downtime and high repair costs. In a particular case, if other parts are damaged due to the impact of the broken blades (e.g., the main shaft of the turbine), the whole power plant may be shut down. On the other hand, further research on the primary causes of failures in turbines can help improve the present failure evaluation methodologies in power plants. Hence, the main objective of this paper is to present the major causes of Kaplan turbine failures to prevent excessive damage to the equipment and provide practical solutions for them. In general, turbines are mainly subjected to both Internal Object Damage (IOD) and Foreign Object Damage (FOD). Accordingly, this paper presents a state-of-the-art review of Kaplan turbine failures related to material and physical defects, deficiencies in design, deficits in manufacturing and assembly processes, corrosion failures, fatigue failure, cavitation wear, types of cavitation in hydro turbines, hydro-abrasive problems, and hydro-erosion problems. Eventually, the authors have attempted to discuss practical hints (e.g., nanostructured coatings) to prevent damages and improve the performance of Kaplan turbines. [ABSTRACT FROM AUTHOR]

The inside-out ceramic turbine (ICT), a novel microturbine rotor architecture, uses monolithic ceramic turbine blades held in compression by a rotating structural shroud instead of traditional superalloy turbine blades in order to increase its maximum turbine inlet temperature (TIT) and cycle efficiency. Previous work has shown that this microturbine architecture has potential to be viable for long-term operation in normal operating conditions. However, foreign object damage (FOD) has been a major concern for ceramic turbine rotor past development efforts, and its effects on the ICT ceramic turbine blades lifespan have not been studied thus far. An experimental approach was used to characterize the effect of the ICT configuration on ceramic turbine blade impact resistance: spherical steel impactors were fired toward silicon nitride specimens simulating blade boundary conditions for both traditional and inside-out turbine layouts. Preload levels ranging from 100 MPa to 400 MPa were applied on the inside-out layout specimens to determine its effect on impact resistance. Results indicate that the inside-out turbine configuration increases the resistance to FOD over a traditional turbine configuration. For the specific conditions of this study, the specimens resisted to over five times the impactor energy when fixed at both ends. Also, results show that applying a higher compression preload on the specimens helps to resist higher energy impacts by about 50% when far from the buckling limit. [ABSTRACT FROM AUTHOR]

• The pitting distribution matches the trend of the residual stress distribution. • The coupled fatigue life tends to increase and then decrease with increasing impact energy. • The coupled effects of impact and corrosion are more pronounced at lower impact energies. • The effect of coupled corrosion time on fatigue life is the same as corrosion alone. 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. [ABSTRACT FROM AUTHOR]

• An easy-to-use apparatus is developed for foreign object damage (FOD) tests of materials under tunable high temperature, impact speed and angle. • Morphology and microstructure of a thin nickel-based superalloy plate after high-speed impact show significant difference between room and elevated temperatures. • Theoretical deduction of FOD crater geometry based on Hertz theory is proposed. • Residual stress distribution under the small impact crater is revealed for different temperatures and impact angles via both experimental and numerical methods. Turbine blades with thin-walled structures usually works in harsh environments, and foreign object damage (FOD) is one of the conditions of special concern. In this paper, the FOD characteristics of thin nickel-based superalloy plates are studied by a combination of experimental, numerical and analytical methods, considering room and high temperatures, different impact conditions and plate thicknesses. An easy-to-use test system is developed to realize high speed impact of the thin nickel-based superalloy plate under elevated temperature. Crater morphologies, internal microstructure, and residual stress are analyzed after impact with different conditions. Numerical simulation of the impact process is performed by using Johnson-Cook (J-C) constitutive model. Based on Hertz theory, an analytical method for calculating the crater length and depth is proposed considering the deformation of the impact steel sphere. Results shows that the FOD characteristics at high temperature is significantly different from that at room temperature. The crater has lager dimensions under high speed and elevated temperature. Moreover, significant grain refinement is obvious and the dislocation layer is also thicker at higher speed and higher temperature. Due to the effect of high temperature softening, hardness and residual stress after impact with elevated temperature is lower than that at room temperature. Besides, non-normal impact mainly influences Goss texture and distribution of residual stress after high temperature impact. In addition, it is found that thickness have a significant effect on the FOD characteristics especially when the plate is thinner. The validity of the numerical model and analytical method is proved by comparing with the experimental results. The present study can provide data foundation and numerical analysis support for the damage assessment and maintenance of turbine blades. [ABSTRACT FROM AUTHOR]

Foreign object damage (FOD) is a common mode of failure in high-speed rotating machinery, such as aircraft engines. Therefore, research on FOD is crucial for ensuring blade integrity. FOD induces residual stress on the surface and within the blade, impacting its fatigue strength and service life. Therefore, this paper utilizes material parameters determined by existing experiments, based on the Johnson–Cook (J-C) constitutive model, to numerically simulate impact damage inflicted on specimens, compare and analyze the residual stress distribution of impact pits, and investigate the influence law of foreign object characteristics on blade residual stress. TC4 titanium alloy, 2A12 aluminum alloy, and Q235 steel were selected as foreign objects, and dynamic numerical simulations of the blade impact process were performed to explore the effects of different types of metal foreign objects. This study analyzes the influence of different materials and foreign objects on the residual stress generated by blade impact through numerical simulation, examining the distribution of residual stress in different directions. The findings indicate that the generated residual stress increases with the density of the materials. Additionally, the geometry of the impact notch is also influenced by the density difference between the impact material and the blade. The distribution of the residual stress field reveals that the maximum residual tensile stress in the blade is related to the density ratio, and the residual tensile stress in the axial and circumferential direction is relatively large. It is important to note that a significant residual tensile stress has a detrimental effect on the fatigue strength.

• The simulation tests of foreign object damage with different impact depths under different centrifugal load conditions were carried out. • The contribution of centrifugal load to microscopic failure during foreign object damage was found. • The influence of centrifugal load on the residual stress distribution of foreign object damage was revealed by numerical simulation. To systematically reveal effects of the centrifugal force on macro and micro characteristics and residual stress distributions of notch-type foreign object damages (FODs) on the leading edge of rotating titanium alloy fan/compressor blades, four series of notch-type damages with different depths were obtained under the same impact condition but four kinds of preloading conditions by using a light gas gun equipped with a 4-axial adjustable fixture. Macro and micro characteristics, stress concentrations and predicted numerical residual stress distributions of the notch-type damage impacted under the different preloading conditions were observed, calculated and compared. Finally, a qualitative analysis of fatigue crack initiation locations of FOD specimens was conducted by the comprehensive comparison of locations of stress concentrations, maximum residual stresses, micro damages such as micro cracks. [ABSTRACT FROM AUTHOR]

Kaplan turbines, as one of the well-known hydraulic turbines, are generally utilized worldwide for low-head and high-flow conditions. Any failure in each of the turbine components can result in long-term downtime and high repair costs. In a particular case, if other parts are damaged due to the impact of the broken blades (e.g., the main shaft of the turbine), the whole power plant may be shut down. On the other hand, further research on the primary causes of failures in turbines can help improve the present failure evaluation methodologies in power plants. Hence, the main objective of this paper is to present the major causes of Kaplan turbine failures to prevent excessive damage to the equipment and provide practical solutions for them. In general, turbines are mainly subjected to both Internal Object Damage (IOD) and Foreign Object Damage (FOD). Accordingly, this paper presents a state-of-the-art review of Kaplan turbine failures related to material and physical defects, deficiencies in design, deficits in manufacturing and assembly processes, corrosion failures, fatigue failure, cavitation wear, types of cavitation in hydro turbines, hydro-abrasive problems, and hydro-erosion problems. Eventually, the authors have attempted to discuss practical hints (e.g., nanostructured coatings) to prevent damages and improve the performance of Kaplan turbines.

To investigate the effects of notch size and residual stress on fatigue strength of different metal materials, based on the concept of fatigue notch coefficient, combined with the impact test data of different material specimens, the relationship between fatigue notch coefficient, fatigue strength and damage notch size, residual stress of different metal materials damaged specimens was studied by using Peterson's theoretical stress concentration coefficient calculation formula. The main conclusions are that fatigue strength will decrease due to the impact damage, and this decrease is mainly due to the size of the notch and the distribution of the residual stress near the notch. The ratio of notch depth to notch radius determines the reduction of fatigue strength. The larger the ratio is, the greater the degree of fatigue strength reduction is. The residual stress caused by foreign object damage in specimens of different materials may be tensile or compressive stresses. Whether it is tensile or compressive depends on the ratio of Young's moduli of the two impinging materials. The tensile stress is beneficial for the fatigue strength, while the compressive stress is detrimental. The conclusions can not only guide the design of anti-foreign damage of blades, but also have theoretical significance for the establishment of criteria for judging and repairing damaged blades. [ABSTRACT FROM AUTHOR]

Thermal barrier coatings (TBCs) applied to turbine blades in jet engines or gas turbines are at risk of high-velocity impingement from different foreign objects, which can cause severe damage from delamination of the TBC. In this study, a high-velocity impingement test system is developed under high-temperature conditions to understand the delamination mechanism of TBCs under actual operation conditions. In this system, a spherical impactor with a diameter of 18mm can be impinged onto a TBC specimen at different temperatures up to 900°C. Highvelocity impingement tests were conducted on atmospheric plasma-sprayed TBC specimens with 8 mass% yttria-stabilized zirconia topcoat (TC) under room temperature (RT) and 900°C conditions. The results indicate that a hemispherical indentation was formed at 900°C, which indicates plastic deformation of the TC, unlike the brittle deformation observed at RT. In addition, vertical and interfacial cracks formed under both RT and 900°C conditions. Cross-sectional observation revealed that the formation process of the interfacial crack at 900°C was different from that at RT. In particular, the interfacial crack tended to become significantly longer at more than 170m/s at 900°C. [ABSTRACT FROM AUTHOR]

Two-dimensional carbon fiber reinforced silicon carbide (2D-C/SiC) composites are well-known for the high-temperature performance in aeronautical and aerospace fields, but they are susceptible to foreign object damage in service. Therefore, the high-temperature performance of pre-impacted 2D-C/SiC is closely connected with flight security. In this study, the pre-impact testing was conducted using a stress-reversal split Hopkinson bar, and the pre-impact energies were set from 0.1J to 0.9J. Subsequently, the pre-impacted specimens were heated for a period in the temperature-keeping procedure from 20 °C to 1600 °C. Oxidation kinetics of the pre-impacted 2D-C/SiC specimens were obtained based on the weight loss-temperature trends. Finally, the specimens were compressed under high temperatures at the strain rates from 10−4/s to 1000/s, so the residual compressive strength (RCS)-temperature trends were achieved. Hence, the correlation between the weight loss and the RCS was established. Interestingly, the pre-impact strengthening effect on the compressive strength was found. The results revealed that the RCS of 2D-C/SiC is affected by various parameters, providing new sights into the high-temperature damage tolerance of ceramic matrix composites. [Display omitted] • High-temperature damage tolerance of 2D-C/SiC composites. • Oxidation mechanism is not only controlled by the temperature. • Pre-impact gives rise to the increase in strength to some extent. • Strain rate effect on the strength is reduced by the increasing temperature-keeping duration. [ABSTRACT FROM AUTHOR]

Failures affecting the compressor components of jet engines are critical during their operation because they compromise flight reliability. Blades are one of the most critical components. They are subjected to high dynamic loads resulting from high rotational speeds and aerodynamic forces. A better understanding of the failure mechanisms that affect these components could provide a safer strategy in the operation and maintenance of jet engines. A fighter jet engine compressor blade failure was analyzed in this work. Mechanical, macroscopic, and microscopic observations were performed to identify deterioration of properties. SEM and EDS analysis were performed to assess crack propagation and to identify possible surface treatments performed during the manufacturing of the blade. The material of the blades was found to be martensitic stainless steel (AISI 403) and the blades presented an aluminized coating. A crack was identified to have started on the high-pressure side of the blade. Examination of the fractured surface indicates that the failure mechanism was fatigue. The presence of a dent on an edge of the fractured surface suggests that the cause of failure might have been foreign object damage (FOD) due to high-speed impact of one or more particles on the blade surface. The axial location of the failed blade inside the engine was such that the probability of suffering high-speed impact of hard particles ingested by the engine was not negligible. An analytical model is presented to determine if the stresses expected at the FOD location were high enough to cause fatigue failure of the blade. [ABSTRACT FROM AUTHOR]

Copyright of China Sciencepaper is the property of China Sciencepaper and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Flying ballast impact damage on railway axles poses a threat to structural integrity and safety of high-speed trains. A comprehensive evaluation on the fatigue strength debit of axles with foreign object damage (FOD) is vital for their safe operation and maintenance under damage tolerance philosophy. In this study, FODs were simulated by shooting tungsten steel balls to the specimens extracted from surface induction hardened S38C axles. Specifically, tests were performed under three impact velocities, i.e. 200, 300 and 400 m/s, and five incident angles for each impact velocity. Morphologies of impact damage were observed by scanning electron microscope (SEM) as well as the cross-sections. Fatigue strength of impacted specimens were then evaluated. Results show that depth and volume of damaged zone increases with the incident angle at the velocity of 200 m/s. Fatigue strength losses of specimens impacted at velocities of 300 and 400 m/s is around 50% compared with that of smooth ones. Fatigue crack originated from the exit rim for 200 m/s glanced impacted specimens, while multiple cracks propagated from both exit rim and bottom of crater impacted at 300 and 400 m/s. Finally, fatigue strength of damaged specimens was evaluated based on the defect depth concept and Kitagawa–Takahashi (K–T) diagram.

The 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. [ABSTRACT FROM AUTHOR]

Foreign object damage (FOD) is one of the main factors reducing the service life of wings, engine blades, aircraft compressors, and highly loaded machine components. This article presents a detailed study of the peculiarities of the growth rates of fatigue cracks upon FOD in the vicinity of the leading edge of a Ti-6Al−4V titanium alloy blade exposed to preliminary laser shock processing treatment. The FOD and the laser shock processing treatment were modeled using 3D numerical analysis by the finite-element method. The resulting range of changes in the effective stress intensity ratios takes into account both the fields of residual stresses caused by FOD and the crack closure effect due to residual compression stresses. The derived effective stress intensity ratios are used to analyze the growth rates of fatigue cracks in complex residual stress fields. Such actual operational loading modes are considered as low-cycle and multicycle fatigue, separately and in combination. [ABSTRACT FROM AUTHOR]

Modern railway axles frequently experience external damages, which results in deteriorated fatigue performance and a need for careful maintenance. Herein, the shot peening (SP)-induced strengthening of high-speed railway axle EA4T alloy or 25CrMo4 steel subjected to foreign object damage (FOD) and the effect of this strengthening on fatigue strength were elucidated using the electron backscattered diffraction, transmission electron microscopy, X-ray diffraction and nanoindentation techniques. An improved backward statistical inference method was employed to construct probabilistic fatigue S-N curves based on the results of high cycle fatigue test and thus derive reliable endurance limits for unFODed and FODed specimens subjected or not subjected to SP treatment.  Eventually, El-Haddad model and the Wöhler curve were combined with the modified Miner damage accumulation theory to develop a three-dimensional Kitagawa-Takahashi diagram in the nominal stress framework. The above diagram was shown to well correlate the fatigue damage accumulation and the critical FOD defect size of externally impacted railway axles with knee-point life and fatigue S-N curve slope.

Environmental barrier coatings (EBCs) protect advanced ceramics and ceramic matrix composites (CMCs) from oxidation and corrosion in gas turbine engine environments. Foreign object damage (FOD), where debris impact the protective coatings, is a critical hazard which limits the turbine durability. Despite previous efforts to understand FOD in EBCs, a detailed understanding of the fundamental transient damage mechanisms is still lacking. In the current work, the real‐time FOD behavior of a mullite/ silicon EBC was visualized via a dynamic synchrotron X‐ray source in phase contrast imaging (PCI) configuration. Prior to the in situ FOD experiments, the microstructure and composition of the coating were, respectively, characterized using a scanning electron microscope (SEM) and X‐ray diffraction (XRD). The variation in the properties of the debris was modeled by~1.5mm diameter partially stabilized zirconia (PSZ) and silicon nitride (Si3N4) spheres. A modified light‐gas gun setup, synchronized with the X‐ray beam, was used to propel the projectiles at velocities ranging between 300 and 355ms−1. Coated samples were impacted under a fully backed support configuration and at normal incidence. Coating penetration and delamination, as well as projectile deformation at the bond coat resulted for FOD by PSZ spheres. Comparatively, projectile fracture, with subsequent rebound of fragments, as well as complete coating penetration and delamination at the substrate interface occurred for FOD by Si3N4 spheres. Only cone cracking was observed for FOD by PSZ spheres, whereas back surface cracking was present for both projectile types. The driving forces for the observed damage mechanisms are qualitatively assessed. [ABSTRACT FROM AUTHOR]

2D-C/SiC composites have widely been used in aeronautical and aerospace engineering, but their mechanical behaviors under small-mass and high-speed impact have not been thoroughly studied yet. In this paper, 2D-C/SiC beam specimens were impacted by a single-stage light-gas gun and the fracture processes were captured by a high-speed camera. Post-impact internal and surface damage morphologies were scanned by a CT and a SEM, respectively. Similar damage modes were revealed by high-speed images. Subsequently, quasi-static post-impact tension tests were conducted to understand the residual mechanical properties. Acoustic emission (AE) signals of specimens were detected during the tests and then classified by the K-means algorithm. Therefore, evolutions of matrix damage, interfacial delamination and fiber fracture were recognized. At the same time, strain value was obtained by digital image correlation (DIC) method and main crack propagations were obvious in strain contours. A combination of the AE and DIC methods very well monitored the real-time damage during post-impact testing, which further revealed the damage during impact phase. [ABSTRACT FROM AUTHOR]

• A total of 94 impact tests were conducted to assess foreign object damage (FOD). • FOD showed a transformation of dent→E-notch→P-notch with increasing impact energy. • Functions of FOD with impact velocity and impact energy were obtained. • FOD was modelled with a spring-mass system on Winkler's elastic-plastic foundation. • Response surfaces of FOD size were developed using the theoretical model. Aeroengines in operational service were susceptible to the ingestion of small, hard particles, resulting in foreign object damage (FOD) on rotating blades. Such particles impacting the blade edge at a velocity up to about 350 m/s yielded millimeter-sized damages, which might result in crack initiation and became the primary life-limiting factors. In this study, experimental and analytical efforts were undertaken respectively to examine the effect of spherical foreign impact on simulated blade edge. For the experiments, a total of 94 laboratory air gun tests were conducted under 17 different conditions, with impact energy ranging from 0.05 J to 16.01 J. The resulting distribution zone of FOD, which continuously characterized the damage sizes, showed that the raised impact energy would boost FOD size. The macroscopic morphology of FOD exhibited three distinct types of transformations. Additionally, FOD prediction models about impact velocity and impact energy were developed using the linear and power formulas, respectively. These models demonstrated good accuracy, with the linear method achieving a maximum error of 11.7 %. For theoretical analysis, a spring-mass system based on Winkler's elastic-plastic foundation theory was employed to model FOD. It was found that the predicted results from this model were in good agreement with the experimental measurements. The predicted results could serve as the upper and lower bounds within the damage space to envelope the test results, providing a reference for fatigue life assessment of the aeroengine blades. [Display omitted] [ABSTRACT FROM AUTHOR]

• Split-Hopkinson pressure bar tests and quasi-static tests with different triaxialities and temperature were carried out to investigate the quasi-static and dynamic mechanical properties of AM355. • A Johnson-cook model with failure criterion of AM355 was developed and validated by finite element method. • Foreign object damage (FOD) of AM355 blade was investigated by numerical model and validated by the laboratory simulated tests. • FOD was modelled via a spring-mass system based on Winkler's elastic-plastic foundation theory. Foreign object damage (FOD) of aeroengine blade was a critical concern in aeroengine maintenance due to its potential impact on airworthiness. This study investigated FOD of simulated blades made of high-strength steel AM355 through a combination of experiments, finite element (FE) simulations, and theoretical analysis. Firstly, mechanical tests were conducted, including the quasi-static tensile tests at different triaxialities and temperatures, as well as Split-Hopkinson pressure bar (SHPB) tests. Experimental results showed that AM355 was sensitive to strain rate, with a 30.7% increase in yield strength observed at a strain rate of 3864 s−1. Then, a Johnson–Cook (J-C) model with failure criterion was developed and validated through FE analysis. The mechanical performance of AM355 was compared to that of TC4. Lastly, FOD tests accompanied by corresponding numerical simulations were conducted via the laboratory air gun device and the developed J-C model. The predicted notch morphologies and sizes agreed well with experimental observations, with the notch depth linearly increased with the increase of the projectile velocity. Finally, a FOD theory prediction model was developed through a spring-mass model based on Winkler's elastic-plastic foundation. The theoretical predictions were in good agreement with the numerical and experimental results, providing valuable insights for FOD assessment of real aeroengine blades. [Display omitted] [ABSTRACT FROM AUTHOR]

Foreign object damage (FOD) is a common accident that happens in aeroengines, especially in fan and compressor blades. Accurate prediction of the fatigue strength of the fan/compressor blade materials following FOD is important for the repair and maintenance of aeroengines. This paper was based on the experiment part of Ref. [13], and fatigue strength prediction of TC4 alloy following simulated FOD was carried out by the theory of critical distances (TCD). The FOD notch bottom morphology was characterized by structured light scanning, and a superimposed notch model containing the characteristic of FOD contour was proposed for fatigue prediction. The results show that the prediction based on the FOD macro size overrates the fatigue strength. This overrated prediction is mainly caused by ignorance of the irregular local defects at the FOD notch bottom. By taking the local defects into consideration, the superimposed notch model can effectively improve the prediction accuracy of fatigue strength, and the relative error is within ±30 %. [ABSTRACT FROM AUTHOR]

Abstract Foreign object damage (FOD) phenomenon of flat and curved Hi Nicalon MI SiC/SiC ceramic matrix composite (CMC) was determined at ambient temperature subject to impact of 1.59 mm steel ball projectiles at a velocity of approximately 340 m/s. The differences in damage morphology between the flat and curved specimens was investigated through the use of electrical resistance (ER), micro-computed tomography (micro-CT), and optical microscopy. The experimental test data was then simulated with GENOA MS-PFA and LS-Dyna using rebound velocity/energy absorbed and damage size as a means to correlate experimental and simulation. The effect of curvature has a profound effect on the impact damage morphology, and must be accurately accounted for when designing full scale CMC components. [ABSTRACT FROM AUTHOR]

Abstract The ingestion of runway debris during takeoff, landing and low-flying of aircraft has been a major concern for gas turbine engines. The small craters or notches introduced by particles impacting blades are commonly known as foreign object damage (FOD). The objective of this paper is to identify the initiation and propagation behavior of small fatigue crack originating from FOD crater in titanium alloy TC4. FOD was simulated by firing steel sphere projectile onto the surface of flat specimen at 30°, 60° and 90°. Replication technique based on a two-part silicon mixture was used to observe the initiation and growth process of FOD-initiated crack. The results reveal that all the cracks nucleate from the micronotches at the material pileup of FOD crater rim. The growth process of FOD-initiated crack consists of two stages. In the first stage, the crack propagates merely inside the crater. Once exceeding ~500 µm in length, the crack will grow into the second stage, in which the crack extends rapidly both inside and outside the FOD crater. Moreover, the growth rates of FOD-initiated cracks are lower than naturally-initiated cracks. Of all the three tested impact angles, the 60° impact seemed to lead to the most crucial decline in fatigue resistance. A linear relationship exhibits between the fatigue lifetime and crater cross-section area of FODed specimens. [ABSTRACT FROM AUTHOR]

• Foreign object damages at different depths were simulated by quasi-static indentation method, and macro–micro characterization was performed. • Influences of foreign object damage on fatigue behavior of actual tubular specimens were first studied. • Fatigue failure mechanism of tubular specimens damaged by foreign objects was analyzed. • Fatigue notch factor and damage tolerance method were used to predict the fatigue strength of specimens damaged by foreign objects. • Fatigue properties of specimens with damage defects were analyzed based on K-T diagram. 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. [ABSTRACT FROM AUTHOR]

In this study, foreign object damage on aircraft high pressure compressor blades made of Inconel718 nickel-based superalloy was experimentally simulated at the working temperatures of the blades. Steel balls (as a foreign objects) with velocities of 292, 208, 133 m/s were impacted on the surface of Inconel718 flat specimens at various temperatures (400, 475, 550, and 625℃). In order to characterize induced craters, impact sites were studied by scanning electron microscopy to identify mechanical features such as microcracks and notches. Then, the specimens were 3D scanned to measure the depth of each crater. It was found that variation of stress concentration factors and induced damages (including microcracks and notches, loss of material, and shear bands) could occur as a result of steel ball impact with velocities of 292 and 133 m/s at temperatures of 625 and 400℃, respectively. [ABSTRACT FROM AUTHOR]

In this paper, a new method is proposed to calculate the fatigue life and defect tolerance for a 30CrMnSiA steel specimen with foreign object impact damage and scratch damage. First, the processes of foreign object impact and scratch are simulated. Then, the Lemaitre’s plasticity damage model is adopted to calculate the initial damage field arising from the impact process and scratch process. Second, the Chaudonneret’s damage model is applied to calculate the fatigue damage for the specimen with a defect under multiaxial fatigue loading. Then, the FE implementation of the damage mechanics model is presented in the platform of ANSYS, in which the coupling effect between the stress and damage fields is considered. Finally, this proposed method is applied to fatigue life and defect tolerance calculation for the 30CrMnSiA steel specimen with an impact defect and a scratch defect. The experiments were conducted to evaluate the approach mentioned above. [ABSTRACT FROM AUTHOR]

During their service, titanium alloys are likely to suffer from the foreign object damage (FOD), resulting in a decrease in their fatigue strength. Laser shock peening (LSP) has been proved to effectively increase the damage tolerance of military engine components by introducing a magnitude compressive residual stress in the near-surface layer of alloys. In this paper, smooth specimens of a TC4 titanium alloy were used and treated by LSP and subsequently exposed to FOD, which was simulated by firing a steel sphere with a nominal velocity of 300 m/s, at 90° with the leading edge of the LSP-treated region using a light gas gun. All impacted specimens were then subjected to fatigue loading. The results showed that LSP could effectively improve the fatigue strength of the damaged specimens. The effect of LSP on the fatigue strength was assessed through fracture observations, microhardness tests and residual stress analyses. The residual stresses due to the plastic deformation caused by LSP and the FOD impact, which were found to play a crucial role on the fatigue strength, were determined using the commercial software ABAQUS. [ABSTRACT FROM AUTHOR]