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Sung R. Choi - One of the best experts on this subject based on the ideXlab platform.

  • characterization and simulation of Foreign Object Damage in curved and flat sic sic ceramic matrix composites
    Ceramics International, 2019
    Co-Authors: Michael J Presby, Rabih Mansour, K Manigandan, Gregory N Morscher, Frank Abdi, Cody Godines, Amirhossein Eftekharian, Sung R. Choi
    Abstract:

    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.

  • Foreign Object Damage by spherical steel projectiles in an n720 alumina oxide oxide ceramic matrix composite
    Journal of the American Ceramic Society, 2014
    Co-Authors: Sung R. Choi, Calvin D Faucett, Donald J Alexander
    Abstract:

    Foreign Object-Damage (FOD) phenomena of an N720/alumina oxide/oxide ceramic matrix composite (CMC), impacted by 1.59-mm spherical chrome steel projectiles up to Mach 1, were assessed at ambient temperature at a normal incidence angle in both partial and full supports. The impact Damage was in the form of craters, matrix/fiber tow breakage, compaction of the material, delamination and cone cracks, and their occurrence and degree depended on both impact velocity and type of target supports. The partial support resulted in significant Damage with increasing impact velocity, accompanying substantial strength degradation. The presence of tensile stress and presumably stress wave interaction at the backside of a target could have been responsible for greater impact Damage in partial support. Although the CMC targets impacted at 340 m/s were on the verge of being penetrated, the targets still survived catastrophic failure retaining about 68% of the as-received strength, indicative of relatively superior FOD resistance as compared to monolithic ceramic counterparts. A quasi-static analysis of impact force prediction was made based on the energy balance principle and was validated indirectly using the experimental data on frontal impact Damage size.

  • Combined Effects of CMAS and FOD in Ceramic Matrix Composites
    Volume 1: Aircraft Engine; Ceramics; Coal Biomass and Alternative Fuels; Controls Diagnostics and Instrumentation, 2012
    Co-Authors: Sung R. Choi, D. Calvin Faucett
    Abstract:

    The combined effects of CMAS (calcium magnesium aluminosilicate) and FOD (Foreign Object Damage) were determined in three different ceramic matrix composites (CMCs), two melt-infiltrated (MI) SiC/SiCs and one oxide/oxide. Foreign Object Damage was introduced at ambient temperature in CMC test targets using 1.6 mm steel ball projectiles at an impact velocity of 340 m/s utilizing a ballistic impact gun. One type of target support, partial support, was used. The impact-Damaged test coupons were then CMAS-exposed at 1200°C for 10 hrs in air. Additional tests were also performed by exposing impact-Damaged test coupons to a mixture of CMAS and salt (sodium sulfate) at 1200°C for 10 hrs in air. The combined effects of FOD and CMAS or CMAS/salt were quantified by determining the residual strengths of test coupons after exposures. Also, six different sands ingested into aeroengines were analyzed in their chemical compositions. The melt CMAS solid, melted at 1300°C and solidified, was assessed in its mechanical properties of microhardness and fracture toughness.

  • effects of target size on Foreign Object Damage in gas turbine grade silicon nitrides by steel ball projectiles
    Journal of Engineering for Gas Turbines and Power-transactions of The Asme, 2012
    Co-Authors: Sung R. Choi, Zsolt Racz
    Abstract:

    Foreign Object Damage (FOD) phenomena of two gas-turbine grade silicon nitrides (AS800 and SN282) were assessed at ambient temperature applying impact velocities from 20 to 300 m/s using 1.59-mm diameter hardened steel ball projectiles. Targets in a flexural configuration with two different sizes (thicknesses) of 1 and 2 mm were ballistic-impacted under a fully supported condition. The severity of impact Damage, as well as the degree of post-impact strength degradation, increased with increasing impact velocity, increased with decreasing target size, and was greater in SN282 than in AS800 silicon nitride. The critical impact velocity where targets fractured catastrophically decreased with decreasing target size and was lower in SN282 than in AS800. Overall, FOD by steel projectiles was significantly less than that by silicon-nitride ceramic counterparts, due to much decreased Hertzian contact stresses. A correlation of backside cracking velocity versus target size was made based on a simplified elastic foundation analysis.

  • effects of target size on Foreign Object Damage in gas turbine grade silicon nitrides by steel ball projectiles
    Volume 1: Aircraft Engine; Ceramics; Coal Biomass and Alternative Fuels; Wind Turbine Technology, 2011
    Co-Authors: Sung R. Choi, Zsolt Racz
    Abstract:

    Foreign Object Damage (FOD) phenomena of two gas-turbine grade silicon nitrides (AS800 and SN282) were assessed at ambient temperature applying impact velocities from 20 to 300 m/s using 1.59-mm diameter hardened steel ball projectiles. Targets in a flexural configuration with two different sizes (thicknesses) of 1 and 2 mm were ballistic-impacted under a fully supported condition. The severity of impact Damage, as well as the degree of post-impact strength degradation, increased with increasing impact velocity, increased with decreasing target size, and was greater in SN282 than in AS800 silicon nitride. The critical impact velocity where targets fractured catastrophically decreased with decreasing target size and was lower in SN282 than in AS800. Overall, FOD by steel projectiles was significantly less than that by silicon-nitride ceramic counterparts, due to much decreased Hertzian contact stresses. A correlation of backside cracking velocity versus target size was made based on a simplified elastic foundation analysis.Copyright © 2011 by ASME

Ramakrishna T Bhatt - One of the best experts on this subject based on the ideXlab platform.

  • Foreign Object Damage of two gas turbine grade silicon nitrides at ambient temperature
    26th Annual Conference on Composites Advanced Ceramics Materials and Structures: A: Ceramic Engineering and Science Proceedings Volume 23 Issue 3, 2008
    Co-Authors: Sung R. Choi, Michael J Pereira, Lesley A Janosik, Ramakrishna T Bhatt
    Abstract:

    Foreign Object Damage (FOD) behavior of two commercially available gas-turbine grade silicon nitrides (AS800 and SN282) was determined at ambient temperature through extensive strength testing of flexure test specimens impacted by steel-ball projectiles with a diameter of 1.59 mm in a velocity range from 220 to 440 m/s. AS800 silicon nitride exhibited a greater FOD resistance over SN282, primarily due to its greater value of fracture toughness (K IC ). This key factor (K IC ) affecting FOD resistance was further evidenced by the indentation strength response of both materials as well as by a quasi-static phenomenological impact model. The Damage generated by projectile impact seemed to be in the forms of well- or ill-developed cone cracks and of lateral cracks.

  • effect of projectile materials on Foreign Object Damage of a gas turbine grade silicon nitride
    ASME Turbo Expo 2005: Power for Land Sea and Air, 2005
    Co-Authors: Sung R. Choi, Ramakrishna T Bhatt, Zsolt Racz, David N Brewer, John P Gyekenyesi
    Abstract:

    Foreign Object Damage (FOD) behavior of AS800 silicon nitride was determined using four different projectile materials at ambient temperature. The target test specimens rigidly supported were impacted at their centers by spherical projectiles with a diameter of 1.59 mm. Four different types of projectiles were used including hardened steel balls, annealed steel balls, silicon nitride balls, and brass balls. Post-impact strength of each target specimen impacted was determined as a function of impact velocity to better understand the severity of local impact Damage. The critical impact velocity where target specimens fail upon impact was highest with brass balls, lowest with ceramic ball, and intermediate with annealed and hardened steel balls. Degree of strength degradation upon impact followed the same order as in the critical impact velocity with respect to projectile materials. For steel balls, hardened projectiles yielded more significant impact Damage than annealed counterparts. The most important material parameter affecting FOD was identified as hardness of projectiles and was correlated in terms of critical impact velocity, impact deformation, and impact load.Copyright © 2005 by ASME

  • Foreign Object Damage in disks of gas-turbine-grade silicon nitrides by steel ball projectiles at ambient temperature
    Journal of Materials Science, 2004
    Co-Authors: Sung R. Choi, Lesley A Janosik, J. M. Pereira, Ramakrishna T Bhatt
    Abstract:

    Foreign Object Damage (FOD) behavior of two commercial gas-turbine-grade silicon nitrides, AS800, SN282, was determined at ambient temperature through postimpact strength testing of target disks impacted by steel ball projectiles with a diameter of 1.59 mm in a velocity range from 115 to 440 m/s. AS800 silicon nitride exhibited a greater FOD resistance than SN282, primarily due to its greater value of fracture toughness ( K _Ic). The critical impact velocity V _c for which the corresponding postimpact strength was the lowest was V _c ≈ 440, 300 m/s for AS800, SN282, respectively. A unique lower strength regime was typified for both silicon nitrides depending on impact velocity, was attributed to significant radial cracking. The Damage generated by projectile impact was typically in the form of ring, radial, cone cracks with their severity, combination being dependent on impact velocity. Unlike the thick (3 mm) flexure bar target specimens used in previous studies, the thin (2 mm) disk target specimens exhibited a unique back-side radial cracking on the reverse side just beneath the impact sites at, above impact velocities of 160 m/s for SN282, 220 m/s for AS800.

  • Foreign Object Damage in flexure bars of two gas turbine grade silicon nitrides
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
    Co-Authors: Sung R. Choi, Michael J Pereira, Lesley A Janosik, Ramakrishna T Bhatt
    Abstract:

    Abstract Foreign Object Damage (FOD) behavior of two commercial gas-turbine grade silicon nitrides, AS800 and SN282, was determined at ambient temperature through strength testing of flexure test specimens impacted by steel ball projectiles with a diameter of 1.59 mm in a velocity range from 220 to 440 m/s. AS800 silicon nitride exhibited a greater FOD resistance than SN282, primarily due to its greater value of fracture toughness ( K Ic ). The use of an additional equiaxed, fine-grained silicon nitride (NC132) showed that fracture toughness was a key material parameter affecting FOD resistance. The observed Damages generated by projectile impact were typically in the forms of well- or ill-developed ring and cone cracks with little presence of radial cracks.

  • Foreign Object Damage behavior of a sic sic composite at ambient and elevated temperatures
    ASME Turbo Expo 2004: Power for Land Sea and Air, 2004
    Co-Authors: Sung R. Choi, Michael J Pereira, Ramakrishna T Bhatt, John P Gyekenyesi
    Abstract:

    Foreign Object Damage (FOD) behavior of a gas-turbine grade SiC/SiC ceramic matrix composite (CMC) was determined at 25 and 1316°C, employing impact velocities from 115 to 440 m/s by 1.59-mm diameter steel-ball projectiles. Two different types of specimen support were used at each temperature: fully supported and partially supported. For a given temperature, the degree of post-impact strength degradation increased with increasing impact velocity, and was greater in a partially supported configuration than in a fully supported one. The elevated-temperature FOD resistance of the composite, particularly under partially supported loading at higher impact velocities ≥ 350 m/s, was significantly less than the ambient-temperature counterpart, attributed to a weakening effect of the composite. For fully supported loading, frontal contact stress played a major role in generating composite Damage; whereas, for partially supported loading, both frontal contact and backside bending stresses were combined sources of Damage generation. The SiC/SiC composite was able to survive higher energy impacts without complete structural failure but suffered more strength affecting Damage from low energy impacts than AS800 and SN282 silicon nitrides.Copyright © 2004 by ASME

P J Withers - One of the best experts on this subject based on the ideXlab platform.

  • residual stresses due to Foreign Object Damage in laser shock peened aerofoils simulation and measurement
    Mechanics of Materials, 2015
    Co-Authors: Bing Lin, Jie Tong, M Preuss, S Zabeen, P J Withers
    Abstract:

    Foreign Object Damage (FOD) to the leading edge of aerofoils has been identified as one of the main life-limiting factors for aeroengine compressor blades. Laser-shock peening (LSP) has been proposed as a means of increasing the material’s resistance to such impact Damage. In this work, a three-dimensional finite element (FE) model has been developed to simulate the residual stresses due to head on (0°) and 45° impacts by a cuboidal projectile on aerofoil specimens treated with LSP. The Johnson–Cook (JC) material model was employed to describe the strain rate-dependent material behaviour; whilst the Johnson–Cook dynamic failure model was considered in 45° FOD simulation, where significant loss of material occurred. The strain rate sensitivity of the model at selected high strain rates was assessed against the data from the literature. The numerical results from the simulation of head-on impact were compared with the measurements by depth-resolved synchrotron X-ray diffraction on the mid-plane. The models were then used to predict the 3D residual stress distributions due to 0° and 45° FOD impacts, and the results were compared with the strain maps obtained from high-energy synchrotron X-ray diffraction. Good to excellent correlations between the simulations and the measurements have been found.

  • evolution of a laser shock peened residual stress field locally with Foreign Object Damage and subsequent fatigue crack growth
    Acta Materialia, 2015
    Co-Authors: S Zabeen, M Preuss, P J Withers
    Abstract:

    Abstract Foreign Object Damage (FOD) can seriously shorten the fatigue lives of components. On the other hand, laser shock peening improves fatigue life by introducing deep compressive residual stress into components. In this paper we examine how the non-uniform steep residual stress profile arising from FOD of laser peened aerofoil leading edges varies as a function of fatigue crack growth under high cycle fatigue and mixed high and low cycle fatigue conditions. The ballistic FOD impacts were introduced by impacting a cube edge head-on (at an angle of 0°) to the leading edge. The residual stress distributions have been mapped by synchrotron X-ray diffraction prior to cracking and subsequent to short (∼1 mm) and long (up to 6 mm) crack growth. The results suggest that the local residual stress field is highly stable even to the growth of relatively long cracks.

  • residual stresses caused by head on and 45 Foreign Object Damage for a laser shock peened ti 6al 4v alloy aerofoil
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2013
    Co-Authors: S Zabeen, M Preuss, P J Withers
    Abstract:

    Abstract This paper investigates the effect on the residual stresses of Foreign Object Damage (FOD) to a previously laser shock peened (LSP) leading edge (LE). FOD was introduced onto the LE of the aerofoil-shaped specimen through ballistic impacts of a cube edge at angles of 0° and 45° to the leading edge. The residual stress distribution was mapped around the FOD notch by synchrotron X-ray diffraction. The results suggest that for both impact angles, the FOD event superimposed a significant additional residual stress on top of the pre-existing stress associated with the LSP process. In particular, the compressive stress was found to be largest directly beneath the notch and the tensile region, seen previously for unpeened aerofoils beneath the compressive zone, was absent due to the pre-existing peening stress field. This may help to explain the improved fatigue strength observed previously. It is shown that the FOD notch created by 45° impact was asymmetric in shape and smaller in depth compared to that created at 0°. The residual stresses were somewhat larger for the 0° impact condition than for 45° partly due to the larger notch depth introduced in the former case.

  • synchrotron strain mapping of the residual strain distribution around Foreign Object Damage in laser shock peened ti 6al 4v alloy
    Materials Science Forum, 2010
    Co-Authors: S Zabeen, Jie Tong, S. Spanrad, M Preuss, P J Withers, J Schofield
    Abstract:

    The current study investigates the effect of Foreign Object Damage (FOD) on the pre-existing compressive residual stress field associated with laser shock peening (LSP) and its evolution upon combined LCF/HCF cycling. FOD was introduced onto an aerofoil-shaped specimen that had been previously LSP treated through ballistic impacts at angles of 0° and 45° to the leading edge. It is shown that the FOD notch created by 45° impact was asymmetric in shape and smaller in depth compared to that created at 0° impact. Significant through thickness compression was introduced parallel to the leading edge as a result of the LSP process. The residual strain distribution was mapped around the FOD notch by synchrotron X-ray radiation. The results show predominantly compressive stresses ahead of the notch, being greater for the 0 compared to 45 impact. No significant stress relaxation was observed after a combined (1000 HCF cycles superimposed on 1 LCF cycle) cycle.

  • residual stress analysis around Foreign Object Damage using synchrotron diffraction
    Materials Science Forum, 2006
    Co-Authors: Philipp Frankel, J. Byrne, Jian Ding, M Preuss, P J Withers
    Abstract:

    The current study compares the residual strain around Foreign Object Damage (FOD), measured using synchrotron diffraction, to the strain predicted by a plastic model with power-law dependence. It is shown that the measured strains are significantly lower than those predicted by the model. This may be explained in part, by the inability of the model to account for Damage mechanisms such as micro-cracking and shear band formation.

R O Ritchie - One of the best experts on this subject based on the ideXlab platform.

  • mechanical relaxation of localized residual stresses associated with Foreign Object Damage
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2003
    Co-Authors: Brad L Boyce, J. O. Peters, Xi Chen, J W Hutchinson, R O Ritchie
    Abstract:

    Abstract Foreign-Object Damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue (HCF) life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the Damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations. However, recent experimental studies have shown the unexpected propensity for impact-induced fatigue crack formation at locations of compressive residual stress in the vicinity of the impact site. To address this issue, in situ and ex situ spatially-resolved X-ray diffraction and numerical modeling are utilized to show that the initial residual stress state can be strongly relaxed during the fatigue loading process. The magnitude and rate of relaxation is strongly dependent on the applied loads. For a Ti–6Al–4V turbine blade alloy, little relaxation was observed for an applied maximum stress of 325 MPa (0.35 σ y , where σ y is the yield stress), and cracks tended to form in subsurface zones of tensile residual stress away from the Damage sites. In contrast, at an applied maximum stress of 500 MPa (0.54 σ y ), equal to the smooth-bar 10 7 -cycle endurance strength, cracks tended to form at the Damage sites in zones of high stress concentration that had initially been in strong compression, but had relaxed during the fatigue loading.

  • on the application of the kitagawa takahashi diagram to Foreign Object Damage and high cycle fatigue
    Engineering Fracture Mechanics, 2002
    Co-Authors: J. O. Peters, Brad L Boyce, Xi Chen, J W Hutchinson, J M Mcnaney, R O Ritchie
    Abstract:

    The role of Foreign-Object Damage (FOD) and its effect on high-cycle fatigue (HGF) failures in a turbine engine Ti– 6Al–4V alloy is examined in the context of the use of the Kitagawa–Takahashi diagram to describe the limiting conditions for such failures. Experimentally, FOD is simulated by firing 1and 3.2 mm diameter steel spheres onto the flat specimen surface of tensile fatigue specimens at velocities of 200 and 300 m/s. Such Damage was found to markedly reduce the fatigue strength of the alloy, primarily due to four factors: stress concentration, microcrack formation, impact-induced plasticity and tensile residual stresses associated with the impact Damage. Two groups of fatigue failures could be identified. The first group initiated directly at the impact site, and can be readily described through the use of a fatigue-crack growth threshold concept. Specifically, a Kitagawa–Takahashi approach is presented where the limiting threshold conditions are defined by the stress-concentration corrected smooth-bar fatigue limit (at microstructurally small crack sizes) and a ‘‘worst-case’’ fatigue-crack growth threshold (at larger crack sizes). The second group of failures was caused by fatigue cracks that initiated at locations far from the impact site in regions of high tensile residual stresses, the magnitude of which was computed numerically and measured experimentally using synchrotron X-ray diffraction. Specifically, these failures could be rationalized due to the superposition of the residual stresses on the farfield applied mean stress, leading to a locally elevated load ratio (ratio of minimum to maximum loads). The effects of residual stress, stress concentration, and microstructurally small cracks are combined in a modified Kitagawa– Takahashi approach to provide a mechanistic basis for evaluating the detrimental effect of FOD on HCF failures in Ti– 6Al–4V blade alloys. 2002 Elsevier Science Ltd. All rights reserved.

  • mechanical relaxation of localized residual stresses associated with Foreign Object Damage
    Lawrence Berkeley National Laboratory, 2002
    Co-Authors: Brad L Boyce, J. O. Peters, Xi Chen, J W Hutchinson, R O Ritchie
    Abstract:

    Foreign-Object Damage associated with the ingestion of debris into aircraft turbine engines can lead to a marked degradation in the high-cycle fatigue life of turbine components. This degradation is generally considered to be associated with the premature initiation of fatigue cracks at or near the Damage sites; this is suspected to be due to, at least in part, the impact-induced residual stress state, which can be strongly tensile in these locations.

  • Foreign Object Damage and high cycle fatigue of ti 6al 4v
    Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2001
    Co-Authors: J. O. Peters, R O Ritchie
    Abstract:

    Abstract Recent high-cycle fatigue (HCF) related failures of gas-turbine jet engines have prompted a re-examination of the design methodologies for HCF-critical components, such as titanium alloy turbine blades. As Foreign-Object Damage (FOD) from ingested debris is a key concern for HCF-related failures of such blades, the current study is focused on the role of simulated high velocity FOD in affecting the initiation and early growth of small surface fatigue cracks in a Ti–6Al–4V alloy, processed for typical blade applications. It is found that resistance to HCF is markedly reduced, primarily due to earlier fatigue crack initiation. The mechanistic effect of FOD on such premature fatigue crack initiation and the subsequent crack growth is discussed in terms of four prominent factors: (i) the presence of small microcracks in the Damaged zone; (ii) the stress concentration associated with the FOD indentation; (iii) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites; and (iv) microstructural Damage from FOD-induced plastic deformation. In view of the in-service conditions, i.e., small crack sizes, high frequency (>1 kHz) vibratory loading and (depending on the blade span location) high mean stress levels, a Damage-tolerant design approach, based on the concept of a threshold for no fatigue-crack growth, appears to offer a preferred solution. It is shown that FOD-initiated cracks that are of a size comparable with microstructural dimensions can propagate at applied stress-intensity ranges on the order of ΔK∼1 MPa√m.

  • Foreign Object Damage and high cycle fatigue role of microstructure in ti 6al 4v
    International Journal of Fatigue, 2001
    Co-Authors: J. O. Peters, R O Ritchie
    Abstract:

    The Objective of this study was to evaluate the influence of microstructure on the susceptibility to high-cycle fatigue (HCF) failure in Ti–6Al–4V following Foreign-Object Damage (FOD), specifically by comparing a fine-grained bi-modal microstructure with a coarse grained lamellar microstructure. FOD was simulated by high-velocity impacts of steel spheres on a flat surface. This caused a marked reduction in the smooth-bar fatigue strength in both microstructures, primarily because of the premature initiation of fatigue cracking resulting from the stress concentration associated with Damage site and FOD-induced microcracking. The FODinitiated microcracks were found to be of a size comparable with microstructural dimensions, and on subsequent fatigue loading were seen to propagate at applied stress-intensity levels below K1 MPa m 1/2 , i.e. a factor of roughly two less than the ‘worstcase’ threshold stress-intensity range in Ti–6Al–4V for a crack of large size compared to microstructural dimensions (a ‘continuumsized’ crack). A rational approach against HCF failures from such microcracks is proposed for the fine-grained bi-modal microstructure based on the Kitagawa–Takahashi diagram. For the bi-modal microstructure, the Kitagawa–Takahashi diagram provides a basis for describing the threshold conditions for FOD-induced HCF failures, in terms of the stress concentration corrected smooth-bar fatigue limit for small crack sizes and the worst-case threshold for larger continuum-sized cracks. However, this approach was found to be less applicable to the coarse grained lamellar microstructure, primarily because of low small-crack growth resistance relative to its higher smooth-bar fatigue limit.  2001 Elsevier Science Ltd. All rights reserved.

Theodore Nicholas - One of the best experts on this subject based on the ideXlab platform.

  • high cycle fatigue a mechanics of materials perspective
    2006
    Co-Authors: Theodore Nicholas
    Abstract:

    Part I Introduction and Background. 1. Introduction. 2. Characterizing Fatigue Limits. 3. Accelerated Test Techniques. Part II Effects of Damage on HCF Strength. 4. HCF/LCF Interactions. 5. Notch Fatigue. 6. Fretting fatigue. 7. Foreign Object Damage. Part III Applications. 8. HCF Design Considerations. Appendicex

  • chapter 7 Foreign Object Damage
    High Cycle Fatigue#R##N#A Mechanics of Materials Perspective, 2006
    Co-Authors: Theodore Nicholas
    Abstract:

    Foreign Object Damage (FOD) involves the Damage caused by Objects ingested into turbine engines. These events typically take place upon takeoff and landing. Ingested Objects, which involve a range of soft and hard materials of all sizes and shapes, cause Damage when they strike rotating blades, static vanes, or other parts of the engine where they cause a reduction in strength of the component. Such events are difficult to prevent although methods, such as the use of screens is unsuccessfully attempted at times in the past. A greater concern arises when the component that is Damaged is also undergoing some type of forced or resonant vibration that can lead to high-cycle fatigue (HCF) in the Damaged area. The problem of FOD is somewhat related to fatigue of notches, but is much more complicated because it involves additional aspects. The severity of a notch, from a geometric point of view, is characterized by a stress concentration factor, k t , which, under FOD, can vary significantly in service. Different types of Damages are illustrated in this chapter involving a wide variation in geometry and corresponding values of k t . Robust engineering methods are needed to predict the influence of these geometric variables on HCF threshold stresses.

  • An Assessment of Laboratory Techniques for Simulating Foreign Object Damage on a Leading Edge Geometry
    2003
    Co-Authors: S.r. Thompson, John J. Ruschau, Theodore Nicholas
    Abstract:

    Abstract : Foreign Object Damage (FOD) from particles ingested into jet engines can have a detrimental effect on the fatigue strength of fan and compressor airfoils. The Damage caused by these particles often is in the form of a geometric discontinuity like a notch. However the presence of residual stresses and substructural Damage in regions adjacent to the notch prohibit the use of simple notch analyses. In this investigation, three different (quasi-static, pendulum, and ballistic) techniques of imparting Damage are studied with respect to the Damage they create and the resultant high cycle fatigue (HCF) strength of a titanium simulated airfoil geometry. The ballistic technique is used as a baseline as it most closely simulates an Object being ingested into an aircraft engine. In this case, steel spheres having diameters ranging from 0.5 to 2.0 mm are used as the impacting Objects at velocities over a range from 40 to 520 m/s. For the quasi-static and pendulum cases, hardened steel indentors with radii similar to those used for the ballistic impacting were used. Step loading tests in tension at a frequency of 350 Hz are used to establish the fatigue limit stress corresponding to 10(exp 7) cycles. The role of residual stresses is identified through the use of samples subjected to stress relief annealing after impact. Simple notch analysis is used to estimate the effect of the geometry of the notch. Comparisons between as-impacted and stress relieved specimens were used to help identify the affect of residual stresses.

  • high cycle fatigue behavior of ti 6al 4v with simulated Foreign Object Damage
    Mechanics of Materials, 2001
    Co-Authors: Shankar Mall, Joseph L Hamrick, Theodore Nicholas
    Abstract:

    Abstract This study investigated the high cycle fatigue (HCF) behavior of a titanium alloy, Ti–6Al–4V, after being subjected to simulated Foreign Object Damage (FOD). For this purpose, rectangular specimens were Damaged to various depths by steel indenters of different diameters under quasi-static loading to simulate Damage found in turbine engine airfoils during service. The fatigue strength of these specimens for 10 7 cycles was measured. Finite element analysis was used to compute the deformation and stress state created by the simulated FOD and to explain the relationship between Damage and fatigue strength. The examination of Damage mechanisms on the fracture surface near FOD showed a region of either macro bands (in the X-shape) of intense plasticity (i.e. intensely deformed material) having strain levels of 15–20%, shear bands, or shear stress-induced cracks depending upon the indentation depth and indenter diameter. The depth from the specimen edge up to the center of these X-shaped macro bands on the fracture surface had a simple empirical relationship with the reduction in fatigue strength. The residual stresses, introduced from the FOD, caused the effective stress ratio or mean stress in the vicinity of FOD to be different than that applied to the specimen. These modified stress states provided the explanation for reduction in the fatigue strength of the material with FOD.

  • influence of Foreign Object Damage fod on the fatigue life of simulated ti 6al 4v airfoils
    International Journal of Impact Engineering, 2001
    Co-Authors: John J. Ruschau, Theodore Nicholas, S.r. Thompson
    Abstract:

    Abstract The fatigue behavior of Ti-6Al-4 V samples that have undergone Foreign Object Damage (FOD) was investigated. A unique test specimen configuration that replicates the leading edge of a typical fan blade was ballistically shot with 1.0 mm diameter glass spheres to simulate actual FOD. Samples were shot at various incident angles to investigate the influence of impact angle on fatigue strength. Two stress ratios were examined: R=0.1 and 0.5. Attempts were made to correlate the measured Damage (macro-/microscopic) to the debit in fatigue strength. The FOD impact sites of fatigue samples were examined prior to testing via a scanning electron microscope (SEM); post-test examinations of the fatigue initiation sites were also performed on selected test samples. Subsurface or internal Damage resulting from the FOD was also investigated from precision micro-sectioning of selected samples. Off-angle impacts were found to be more detrimental than head-on (0°) impacts. Fatigue strength losses, some as high as 50%, showed little or no correlation with leading edge thickness or depth of notch.