In ̄uence of foreign-object damage on crack initiation and early crack growth during high-cycle fatigue of Ti±6Al±4V (original) (raw)
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Engineering Fracture Mechanics, 2000
The objective of this work is to provide a rationale approach to de®ne the limiting conditions for high-cycle fatigue (HCF) in the presence of foreign-object damage (FOD). This study focused on the role of simulated FOD in aecting the initiation and early growth of small surface fatigue cracks in a Ti±6Al±4V alloy, processed for typical turbine blade applications. Using high-velocity (200±300 m/s) impacts of 3.2 mm diameter steel spheres on the¯at surface of fatigue test specimens to simulate FOD, it is found that the resistance to HCF is markedly reduced due to earlier crack initiation. Premature crack initiation and subsequent near-threshold crack growth is primarily aected by the stress concentration associated with the FOD indentation and the presence of small microcracks in the damaged zone (seen only at the higher impact velocities). Furthermore, the eect of residual stresses and microstructural damage from FOD-induced plastic deformation at the indent sites are assessed in terms of fatigue strength degradation. 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 DK $ 1 MPa p m, i.e., a factor of roughly two less than the``worst-case'' threshold stress-intensity range in Ti±6Al±4V for a crack of a size large compared to microstructural dimensions (à`c ontinuum-sized'' crack). Correspondingly, for FOD-initiated failures, where the critical condition for HCF must be de®ned in the presence of microstructurally small cracks, the Kitagawa±Takahashi diagram, with the limiting conditions of the stress-concentration corrected 10 7 -cycle fatigue limit and the``worst-case'' DK TH fatigue threshold, is proposed as a basis for design against FOD-induced HCF failures. Ó
International Journal of Fatigue, 2001
The role of residual stresses in the high cycle fatigue (HCF) strength of Ti-6Al-4V subjected to foreign object damage (FOD) was evaluated on simulated airfoil and rectangular geometries. Both real and simulated impacts were conducted using spherical projectiles launched at 300 m/s and quasi-static chisel indentation, respectively. The spheres used were 1 mm diameter glass beads while the quasi-static indentor had a radius of 1 mm. The airfoil specimens had leading edge (LE) radii of either 0.13 or 0.38 mm and were indented at 30°to the airfoil LE. The rectangular plates were 1.25 mm thick and were indented quasi-statically at 0°. All specimens subjected to FOD were subsequently tested in uniaxial HCF at a frequency of 350 Hz using a step loading procedure to determine the fatigue limit corresponding to 10 7 cycles. Before the HCF testing, half of the specimens were stress relief annealed to remove residual stresses. Results indicate that stress relief generally improves the fatigue limit stress, indicating that tensile residual stresses are present after both quasi-static or dynamic indentation. For the dynamic impacts, the craters formed have less influence on the fatigue strengths than that predicted from conventional notch fatigue analysis. Published by Elsevier Science Ltd.
Role of foreign-object damage on thresholds for high-cycle fatigue in Ti-6Al-4V
Metallurgical and Materials Transactions A, 2000
The increasing incidence of military aircraft engine failures that can be traced to high-cycle fatigue (HCF) has prompted a reassessment of the design methodologies for HCF-critical components, such as turbine blades and disks. Because of the high-frequency vibratory loading involved, damagetolerant design methodologies based on a threshold for no crack growth offer a preferred approach. As impact damage from ingested debris is a prime source of HCF-related failures, the current study is focused on the role of such foreign-object damage (FOD) in influencing fatigue crack-growth thresholds and early crack growth of both large and small cracks in a fan blade alloy, Ti-6Al-4V. FOD, which was simulated by the high-velocity (200 to 300 m/s) impact of steel spheres on a flat surface, was found to reduce markedly the fatigue strength, primarily due to earlier crack initiation. This is discussed in terms of four salient factors: (1) the stress concentration associated with the FOD indentation, (2) the presence of small microcracks in the damaged zone, (3) the localized presence of tensile residual hoop stresses at the base and rim of the indent sites, and (4) microstructural damage from FOD-induced plastic deformation. It was found that no crack growth occurred from FOD impact sites in this alloy at ⌬K values below ϳ2.9 MPaΊm, i.e., over 50 pct higher than the "closure-free", worst-case threshold value of ⌬K TH ϭ 1.9 MPaΊm, defined for large cracks in bimodal Ti-6Al-4V alloys at the highest possible load ratio. It is, therefore, concluded that such worst-case, large fatigue crack thresholds can, thus, be used as a practical lower-bound to FOD-initiated cracking in this alloy.
On the application of the Kitagawa–Takahashi diagram to foreign-object damage and high-cycle fatigue
Engineering Fracture Mechanics, 2002
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 1 and 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. Ó
Thresholds for high-cycle fatigue in a turbine engine Ti–6Al–4V alloy
International Journal of Fatigue, 1999
The characterization of critical levels of microstructural damage that can lead to fatigue-crack propagation under high-cycle fatigue loading conditions is a major concern for the aircraft industry with respect to the structural integrity of turbine engine components. The extremely high cyclic frequencies characteristic of in-flight loading spectra necessitate that a damage-tolerant design approach be based on a crack-propagation threshold, ⌬K TH . The present study identifies a practical lower-bound large-crack threshold under high-cycle fatigue conditions in a Ti-6Al-4V blade alloy (with ෂ60% primary α in a matrix of lamellar α+β). Lower-bound thresholds are measured by modifying standard large-crack propagation tests to simulate small-crack behavior. These techniques include high load-ratio testing under both constant-R and constant-K max conditions, performed at cyclic loading frequencies up to 1 kHz and R-ratios up to 0.92. The results of these tests are compared to the near-threshold behavior of naturallyinitiated small cracks, and to the crack initiation and early growth behavior of small cracks emanating from sites of simulated foreign object damage.
The resistance to impact damage and subsequent fatigue response of two titanium alloys
Materials Science and Engineering: A, 2002
Two high performance titanium alloys, Ti-6Al-4V in a mill annealed form and Ti-25V-15Cr-2Al, have been assessed for their resistance to foreign object damage (FOD) and subsequent response to fatigue loading. Laboratory specimens, designed to simulate the cross sectional geometry of gas turbine aerofoils, were subjected to particle impacts of varying energy at edge locations in an attempt to represent severe particle ingestion during typical service. The form of impact damage is fully characterised in each alloy and related to the energy and specific location of impact. Compared to pristine specimens, the fatigue strength of damaged specimens is significantly reduced in both materials. Analytical techniques to account for the influence of FOD on fatigue strength are explored.
Foreign-object damage and high-cycle fatigue of Ti–6Al–4V
Materials Science and Engineering: A, 2001
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 ⌬Kෂ1 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.
On Fatigue Behavior of Small Cracks Induced by Foreign-Object Damage in TI-6AL-4V
A threshold curve method previously developed was applied to analyze the high-cycle fatigue threshold of small cracks induced by foreign-object damage (FOD) in Ti-6Al-4V alloy for which experimental results can be obtained from recent publications. Experimental results found in the literature showed that the small cracks induced by FOD in Ti-6Al-4V alloys could propagate at ∆K as small as 1 MPa m 1/2 , i.e., well below the "worst-case" ∆K thR threshold of 1.9 MPa m 1/2 for long cracks in this alloy (load ratio R=0.95). According to the results obtained in the present work the threshold for propagation of small cracks (≥20µm) can be as small as 0.8 MPa m 1/2 considering a stress ratio R = 0.8, for which the threshold for long cracks is about 2.3 MPa m 1/2 . Further, the minimum stress ratio R at which the analyzed cracks can propagate seems to be about 0.6, for which the threshold for large cracks is about 2.7 MPa m 1/2 . It is also shown that for microstructurally small cr...
Fatigue & Fracture of Engineering Materials and Structures, 2008
A B S T R A C T Foreign object damage (FOD) has been identified as one of the main life limiting factors for aeroengine blades, with the leading edge of aerofoils particularly susceptible. In this work, a generic edge 'aerofoil' geometry was utilized in a study of early fatigue crack growth behaviour due to FOD under low cycle fatigue (LCF), high cycle fatigue (HCF) and combined LCF and HCF loading conditions. Residual stresses due to FOD were analyzed using the finite element method. The longitudinal residual stress component along the crack path was introduced as a nodal temperature distribution, and used in the correction of the stress intensity factor range. The crack growth was monitored using a nanodirect current potential drop (DCPD) system and crack growth rates were correlated with the corrected stress intensity factor considering the residual stresses. The results were discussed with regard to the role of residual stresses in the characterization of fatigue crack growth. Small crack growth behaviour in FODed specimens was revealed only after the residual stresses were taken into account in the calculation of the stress intensity factor, a feature common to LCF, HCF and combined LCF + HCF loading conditions.
Foreign-object damage and high-cycle fatigue: role of microstructure in Ti–6Al–4V
International Journal of Fatigue, 2001
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 ⌬Kෂ1 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.