Effects Of Foreign Object Damage Research Articles

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

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

Effect of foreign object damage on high-cycle fatigue strength of titanium alloy for aero-engine blade

To address the fatigue behavior of damaged aero-engine blade, in the current paper, the research on high cycle fatigue (HCF) strength of TC11 titanium alloy after subjected to foreign object damage (FOD) were presented. First of all, FOD simulation tests were carried out on simulated flat blade samples using gas cannon test system. The macro and micro impact damage of samples were examined in detail. The FOD test results shows that the microscopic characteristics of damage area include smooth area, micro cracks, micro notches, shear bands and loss of material (LOM). Then, HCF tests were carried out on the damaged flat blade samples using HCF fatigue testing machine through stepping method. The undamaged standard test specimens were also tested for HCF limit as baseline. The fatigue results show that the damage depth L2 related to impact velocity has a great influence on fatigue limit. The greater the impact velocity, the lower the fatigue limit. Microscopic observation on fatigue fracture was conducted by metallographic microscope and found that FOD would change the microstructure of material in damage area, made the phase to be slender and in a certain arrangement direction.

Effect of Foreign Object Damage at Different Impact Parts for TC4 Titanium Alloy Blades

The effect of foreign object damage at different impact parts of TC4 Titanium Alloy blades is studied in this paper and impact damage from common foreign objects at different parts of the inlet side of the blade has been simulated in the process of aircraft skidding using the ANSYS/LS-DYNA software based on the Kinematic hardening plastic constitutive model. The results show that the damage depth of the sandstone increases linearly with the increase of relative kinetic energy when the same foreign object strikes different parts, but there is a critical relative kinetic energy of 2.54J and 2.50J for the steel ball and screw, respectively. When the relative kinetic energy is less than the critical value, the damage depth increases continuously with the relative kinetic energy, and when the relative kinetic energy is greater than the critical value, the damage depth decreases continuously with the relative kinetic energy.

Synchrotron Strain Mapping of the Residual Strain Distribution around Foreign Object Damage in Laser Shock Peened Ti-6AL-4V Alloy

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.

Fatigue Life and Short Crack Behavior in Ti-6Al-4V Alloy; Interactions of Foreign Object Damage, Stress, and Temperature

High-cycle fatigue (HCF) failures associated with foreign object damage (FOD) in turbine engines of military aircrafts have been of major concern for the aeronautic industry in recent years. The present work is focused on characterizing the effects of FOD on crack initiation and small crack growth of a Ti-6Al-4V alloy at ambient and also elevated temperatures. Results show that the preferred crack initiation site depends on applied stress and temperature as maximum fractions of cracks emanating from the simulated damage site, and naturally initiated cracks are observed at 25 °C under the maximum stress of 700 MPa and at 300 °C under the maximum stress of 300 MPa. The fatigue crack growth rate is influenced by increasing temperature, and the FCG rate at 300 °C is higher than that at room temperature under the same ΔK, whereas this effect for FOD-site initiated cracks is not so remarkable. This observation seems to be due to the effect of stress relaxation at 300 °C. Results also indicate that fatigue crack initiation life (Ni) and fatigue life (Nf) are expressed by three-parameter Weibull distribution function.

Effects of Mechanical Surface Treatment on Fatigue Failure in Ti-6Al-4V: Role of Residual Stresses and Foreign-Object Damage

Mechanical surface treatments can play a major role in the enhancement of fatigue life in metallic materials. Deep rolling and laser shock peering processes, for example, can induce several favorable effects, including deep cases of compressive residual stresses, work hardening and low surface roughness. Since the fatigue life of smooth specimens is invariably dominated by crack-initiation processes, the presence of high near-surface dislocation densities can improve fatigue lifetimes substantially, even if residual (macro) stresses are not stable during fatigue. Conversely, in engineering applications such as turbine engine airfoils, the surfaces are very often notched or damaged, for instance by foreign-object impacts, which severely reduces the crack-initiation phase. In such cases, the fatigue life is dominated by crack propagation and a stable compressive residual stress state is crucial for enhancing fatigue life. In the present work the effect of mechanical surface treatments on the fatigue behavior and residual stress state of smooth and on foreign-object damaged specimens of the Ti-alloy Ti-6Al-4V is investigated. The results show that deep rolling can substantially improve the fatigue life of smooth specimens at elevated temperature (450°C) due to a higher resistance against fatigue crack initiation. For laser shock peened and foreign-object damaged specimens, information on the residual stress state in and around the foreign-object impacts yields a deeper insight into the nature of local fatigue damage and microcrack propagation. The results indicate that laser shock peening process parameters have to be very carefully chosen in order to diminish the effects of foreign object damage.

Effect of foreign object damage on the fatigue strength of an xd™ γ-TiAl alloy

High cycle fatigue has become the leading cause of failure in gas turbine engine blades in recent years [1]. The majority of these failures were associated with surface damage such as foreign object damage (FOD). Therefore, the ability of a material to resist impact damage and the subsequent fatigue propagation of any cracks that may result has become a crucial design consideration. Substantial research has been conducted on the effects of simulated FOD in conventional turbine blade materials where the formation of cracks at the impact site was not a common occurrence [2,3,4]. However, impacts of more brittle materials, such as �-TiAl, would likely result in the formation of cracks. Steif et al [5] have examined the formation of cracks associated with impacts in several TiAl-based alloys. The effects of ballistic impacts on the fatigue behavior of a Ti-48Al-2Nb-2Cr �-TiAl alloy were examined by Lerch et al [6]. Fatigue strength was correlated with a somewhat arbitrary crack length with some success; however, no correlation with the long-crack fatigue threshold was made. The use of a fatigue threshold limit may be necessary for design of turbine blades given the limited fatigue crack growth resistance exhibited by �-TiAl. The present study investigates the reduction in elevated temperature fatigue strength of a Ti-46.8Al­ 2.1Nb-1.1Mn-0.1Si-1.4B alloy caused by simulated foreign object damage. This work is part of a larger effort to support the transition of �-TiAl to industrially relevant gas turbine engine applications.