16:00   Fatigue Crack Growth and Life Prediction Methods II
Chair: Hans Ansell
16:00
30 mins 		FATIGUE LIFE PREDICTION OF SMALL NOTCHED TI-6AL-4V BASED ON THE THEORY OF CRITICAL DISTANCE
Yoichi Yamashita, Masaharu Shinozaki, Hiroshi Kuroki, Yusuke Ueda
Abstract: The high cycle fatigue (HCF) of titanium alloy turbine engine components remains a principal cause of failures in aircraft engines. HCF can result from vibration, forced response, unsteady aerodynamic loads, or other fluctuating loads. Gas turbine engine rotor blades and stator vanes are subject to all types of loads and are particularly vulnerable to HCF failures. Rotating blades in gas turbine engine are often subjected to impact from debris ingested into the engine during takeoff and landing which is called foreign object damage (FOD). Such debris may result in nicks, dents and scratches in the leading or trailing edges of airfoils which in turn, reduces the fatigue strength of the material. Therefore, FOD-induced HCF is one of the significant themes in fatigue problems of aero-engine component. In these view points, notched fatigue problem is important to investigate the effects of Foreign-object damage (FOD) on fatigue strength of Ti-6Al-4V alloy. This study investigated the estimating method for notched fatigue strength of aero-engine fan blade and compressor blade of forged Ti-6Al-4V alloy. In the experimental study, very small notched round bar tension-compression fatigue tests have been conducted with forged Ti-6Al-4V alloy to investigate the effect of notch radius and notch depth on fatigue strength of the material. Notch radius is varied from 0.05 to 0.2mm and notch depth is varied as 0.1, 0.3 and 0.5mm. And stress ratio ( = σmin / σmax ) effect on fatigue strength also investigated to taking into account of residual stress effect on FOD-induced HCF. The fatigue tests shows that the larger notch radius gives the increase of fatigue strength and the larger notch depth gives the decrease of fatigue strength. And in analytical study, the method for prediction of notched Ti-6Al-4V specimens using the theory of the critical distance (TCD) has been investigated. The TCD applied to fatigue problems assumes that fatigue damage depends on the stress field distribution in the vicinity of the stress concentrator. In other words, the TCD assumes that fatigue damage can correctly be estimated only if the entire stress field damaging the fatigue process zone is taken into account. Critical distance stress is defined as the average stress in the material characteristic length, L, where critical distance is defined as the distance of L from the notch-tip. And material characteristic length , L is calculated as L = F (1/π)(ΔKth / Δσ0)^2 where ΔKth is the range of the threshold value of the stress intensity factor and Δσ0 is the plain fatigue limit. The formulations to calculate the critical distance stress were constructed for notched round bar tensile specimen based on the equations of notch-tip elastic stress distribution and equilibrium of axial force applied to specimen. Generally, it is considered that fatigue life is composed of fatigue crack initiation life and fatigue crack growth life. In this viewpoint, the fatigue crack initiation life of the notched specimen can be determined by the round bar tension-compression fatigue test data and calculation life of fatigue crack growth. Fatigue crack growth life is calculated based on the stress intensity factor taking into account of the stress concentration in the vicinity of the notch-tip and Paris’ type crack growth property of Ti-6Al-4V material used. It has been found that there exists the good correlation between critical distance stress and crack initiation life of notched round bar tensile specimen. Therefore, fatigue life of Ti-6Al-4V material with various notch radius and notch depth can be predicted from the relationships between the critical distance stress and fatigue crack initiation life. And further study of predicting FOD-induced HCF life were conducted with above mentioned fatigue life prediction method based on critical distance approach and residual stress effects on notched fatigue life of Ti-6Al-4V material. Fatigue life design curve for FOD-induced HCF life are proposed taking into account of the follwing factors; (1) FOD depth (notch depth) (2) Residual stress (3) Notch radius.
16:30
30 mins 		A CASE STUDY OF MULTIPLE-SITE FATIGUE CRACK GROWTH IN THE F-18 HORNET BULK-HEAD
Borje Anderson, Anders Blom
Abstract: In the F-18 Hornet a pre-IVD etching technique is believed to produce surface crack-like flaws (pre-IVD etch pits) from which multiple small surface cracks might grow and subsequently link-up as a result of flight manoeuvre induced loading. The early stages of the growth of these crack-like flaws are of interest to allow proactive maintenance action planning. The research goal of the present study was to develop and validate a tool for reliable analysis of fatigue growth following initiation of several small surface flaws that grow and coalesce when subjected to service loading. Both analytical and experimental methods were applied in a case study of an isolated detail of an F-18 Hornet bulkhead structure selected for this purpose. In the analytical part of the study, the bulkhead detail was assumed to contain randomly distributed crack-like flaws resulting from the OEM/IVD process. A computational framework was developed with which stress intensity factors for multiple three-dimensional quarter- and half-elliptical cracks of aspect ratio 0.1
17:00
30 mins 		EXPERIMENTAL AND THEORETICAL COMPARISON OF SOME MULTIAXIAL FATIGUE DESIGN CRITERIA IN THE CONTEXT OF LIFE ASSESSMENT OF ROTATING PARTS IN TURBOENGINES
Vincent Bonnand, Jean-Louis Chaboche, Hacene Cherouali, Philippe Gomez, Pascale Kanoute, Didier Pacou, Pascal Paulmier, Elisabeth Ostoja-Kuczynski, Francois Vogel
Abstract: Rotating parts in turboengines, like turbine or compressor discs, are subjected to severe and complex cyclic loads. T With the objective to assess current design methodologies, an important program has been conducted on two classes of classical disc materials, a titanium alloy and a nickel-based alloy, that involved four main partners : Snecma, Turboméca, Onera and CEAT. In addition to standard LCF strain controlled fatigue tests (with various strain ratio) and HCF tests (under stress control), the experimental program includes a significant number of complex multiaxial tests, on tubular specimens and on specially designed cruciform specimens. Several fatigue life prediction methods have been studied, involving various multiaxial criteria, that are compared with experimental results. It is demonstrated that any of the classical multiaxial fatigue rules is able to corrrectly predict both equibiaxial and shear fatigue conditions.