Inhibition Mechanism of Transverse Cracking in Ceramic Abradable Coatings with Whisker‐Doped “Mud” Layers

Novel 4-phase ceramic abradable seal coatings have been prepared by plasma spray. The as-prepared coatings were characterized by SEM, EDS and XRD. The results showed that the 4-phase ceramic abradable coating exhibited porous structures and had good adhesion with the bond coat. The ZrO2 and Al2O3 in the coating existed in the form of tetragonal-ZrO2 and α-Al2O3, respectively. The result of thermal shock test revealed that compared with the 3-phase abradable coating, the thermal shock life of the 4-phase abradable coating had increased tenfold to 1900~2000 cycles from 100~200 cycles. The reason is owing to the reduction of oxygen partial pressure and thermal stresses at the top/bond coat interface due to the addition of the fourth nanoAl2O3 phase.

Double notched specimens of nickel base superalloy N18 have been subjected to creep fatigue loading at 650°C. Finite element analysis using the elastoviscoplastic constitutive equation of N18, exhibited a strong stress relaxation at the notch root. By introducing the computed viscoplastic stress gradient into the calculation of the stress intensity factor (SIF), crack growth rates measured on notched specimens are much closer to those measured on smooth specimens. INTRODUCTION With the need of improvement of aircraft gas turbine performances, security components, such as turbine and compressor discs, should be subjected to increasing stresses and temperatures. They now tend to be fabricated with powder metallurgy superalloys which have excellent mechanical properties under these conditions. However the presence of inclusions introduced in the melt process (Denda et al (1)) can considerably affect their fatigue life. Therefore disc failure due to crack initiation on inclusions at firtree fixtures of turbine blades is a key problem for engine designers (McClung (2)). Short crack propagation is studied, in double notched specimens of N18, subjected to a specific loading schedule corresponding to service conditions. EXPERIMENTAL PROCEDURE Material The alloy tested, N18 is an advanced Nickel base y strengthened disc alloy. It was developped by SNECMA as a PM alloy for service up to 700°C. Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 438 Localized Damage Currently N18 is being used for compressors and turbine discs of the M88 engine for the new Rafale fighter. The chemical analysis of the material tested is reported in table 1. Further informations about this alloy can be found in the work of Guedou et al (3) and Wlodek et al (4). TABLE 1Chemical composition ofNIS (At %) C Co Ni Cr Mo Al Ti B Hf Zr 0,015 15,7 base 11,5 5,5 4,35 4,35 0,015 0,45 0,03 Creep fatigue testing. Double edge notched specimens (see figure 1) were machined into a turbine disc and were subjected to trapezoidal cycles (10-300-10): 10 s loading, 300s dwell time, 10 s unloading with Ra=0 at 650°C in air corresponding to service conditions. Cracks initiate on artificial U-shape defects which are nearly 100 p,m deep. The crack length is obtained through the potential technique. The potential is measured around each notch during the whole crack propagation, the uncracked side being used as a reference to detect crack initiation earlier. The calibrating curve was obtained by marking the fracture surface, using a change in crack path with temperature and cycling mode. Under usual testing conditions the crack path is intergranular while in marking conditions (400°C, pure fatigue IHz) the crack path becomes transgranular (see figure 2). Crack initiation can be detected as soon as cracks are 120|imdeep. EXPERIMENTAL RESULTS Experiments have been conducted with nominal loads of 700, 800 and 900MPa. Crack growth rates were plotted as a function of the SIF, using a general expression of the SIF for semi-elliptical cracks growing from a notch given by Kujawski (5). If we compare these results with those obtained by G.Hochstetter (6) on the same material under the same testing conditions applied on CT specimens, crack growth rates in notched specimens are much higher than those in CT specimens. FINITE ELEMENT ANALYSIS Finite element computations, using the elastoviscoplastic constitutive equations of N18, have been conducted on Zebulon, the Finite Elements software developped by Ecole des Mines de Paris. The meshed structure has been subjected up to 300 cycles (10-300-10). Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 Localized Damage 439 The notch root is highly plastified at the first loading, and within a few cycles the tensile stresses and strains become steady and the stress-strain curve hysteresis loop is insignificant (7) As a matter of fact the center of the sample remaining elastic, the plastic deformation of the notch root is limited, thus creep deformation during dwell time becomes quickly negligible. Looking at the local peak tensile stresses, figure 4, for a nominal load of 500MPa, almost no stress redistribution occurs, the stress profile could have been calculated in elasticity. However for higher loads, the stresses near the surface are much lower than in elasticity while at the center of the specimen they are sligthly higher. Then when the structure is unloaded, see figure 5, the notch root go through compression. The stress ratio can be very negative behind the surface (-0.7 for a nominal load of lOOOMPa) but quickly becomes positive inside the structure. The viscoplastic steady stress profiles, Aa=amax, at peak nominal stress for each nominal load were calculated and fitted by five degree polynomials, that were used in the calculation of the SIF (7) derived from the solution of RJ.Hand (8). The experimental results are then in better agreement with those obtained on CT specimens, (see figure 6) though for short cracks, i.e. cracks under the influence of the notch, crack growth rates are still higher than for long cracks in CT specimens DISCUSSION As the stress relaxation is very strong at the notch root (see figure 4), the SIF predicted in viscoplasticity for short cracks are lower than those predicted in elasticity. For long cracks, SIF are higher, the stresses being sligthly higher near the center of the sample than in elasticity. Then for short cracks the da/dN versus AK curve is shifted to the left whereas for longer cracks it is shifted to the right (see figure 3). So the slope of the curve being lower in viscoplasticity than in elasticity, the results are in better agreement with those obtained on CT specimens (see figure 3). Nevertheless short crack growth rates are still higher than for long cracks in CT specimens. This can be explained by a change in the opening threshold. In the SIF calculation proposed in (7) the crack was supposed to open as soon as the local stress became positive throughout the whole crack propagation. But this opening threshold changes with the loading conditions and the crack length. As the stress ratio is highly negative at the notch root and slightly positive at the center of the sample (see figure 5), stress profiles Aa=Aaeff should be used instead of Aa=amax in the calculation of the SIF, to be able to predict crack propagation rate in a notched sample from crack propagation law obtained on CT specimens.. Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533 440 Localized Damage CONCLUSIONS (1) Creep fatigue tests were performed at 650°C, with a dwell time of 300 sec, on the Nickel base superalloy N18, for nominal loads of 700, 800 and 900 MPa. (2) The Finite Element analysis, conducted using the elastoviscoplastic constitutive equations of N18, exhibit a strong stress redistribution at the notch root (3) Using solution for KI which takes into account the cyclic stress redistribution at the notch root the results are in better agreement with those obtained by G.Hochstetter on CT specimens. (4) Opening thresholds should be taken into account in the determination of the crack propagation law. ACKNOWLEDGEMENTS SNECMA is gratefully acknowledged for fruitful discussions and financial support. REFERENCES 1. Denda, T., Bretz, P.L. and Tine, J.K., ' Inclusion size effect on the fatigue Crack propagation mechanism and fracture Mechanics of a Superalloy', Met Trans A, Vol 23A, Feb 1992, pp. 519-526. 2. McClung, R.C., ' a Simple Model for Fatigue Crack Growth near Stress Concentrators', Trans of ASME, Vol 113, Nov 1991, pp. 542-548. 3. Guedou, JY., Lautridou, JC. and Honnorat, Y., ' N18, PM Superalloy for Discs : development and applications', in Superalloy 92 (Ed S.D. Antolovitch, R.W. Stusrud, R.A. macKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom), pp. 267 to 216proceedings of the Minerals & Materials Society., Cincinnati, 1992. 4. Wlodek, ST., Kelly, M. and Alden, D. ,' The Structure of N18', in Superalloy 92 (Ed S.D. Antolovitch, R.W. Stusrud, R.A. macKay, D.L. Anton, T. Khan, R.D. Kissinger, D.L. Klarstrom), pp. 267 to 276, Proceedings of the Minerals & Materials Society., Cincinnati, 1992. 5. Kujawski, D. , ' Estimations of Stress Intensity Factors for Small Cracks at Notches', Fatigue Fract. Engng Mater. Struct., Vol. 14, No. 10, pp. 953-965, 1991 6. Hochstetter, G., ' Propagation des Fissures a haute temperature dans le superalliage pour disque de turbomachine N18, interaction entre la nature des solicitations mecaniques et les effets d'oxydation.' Thesis of Ecole Nationale Superieure des Mines de Paris, 14 January 1994. 7. Pommier, S., Rongvaux, J.M., Prioul, C., Frangois, D., ' Stress intensity factor for creep fatigue cracks growing from a notch', in Structural Integrity Tenth Conference on Fracture (Ed ESIS) Berlin, FRG, 20-23 Sept. 1994, to be published. 8. Hand, R.J., ' Stress Intensity Factors for penny and half penny shaped Cracks subjected to a stress Gradient', Int. J. of Fract., Vol 57, 1992, pp. 237-247. Transactions on Engineering Sciences vol 6, © 1994 WIT Press, www.witpress.com, ISSN 1743-3533

Objectives of this study are to examine influence of the surface crackles fractal characteristics of CRTSII track slab on its propagation velocity and stress intensity factor (SIF) and to improve the service performance of track slab. In this paper, fractal dimension D of transverse crack on the surface of CRTSII track slab was calculated by image processing, digital image technology, and box‐counting method. Based on the fractal structure of transverse crack on track slab surface, a fractal transverse crack propagation model was established. The impact of fractal effect on the surface crack propagation velocity and SIF of CRTSII track slab was analyzed quantitatively. The fractal dimension D of transverse crack is 1.0652 on the surface of CRTSII track slab. The actual measured crack propagation velocity V0 on the surface of the track slab can be corrected as the true crack propagation velocity V. When the V0/Cr = 0.899 < 1, the SIF K′(L(1.0652, t), V) of transverse dynamic fractal crack propagation on the surface of track slab has been equal to 0, and the V0 is always lower than Rayleigh wave velocity. In addition, the true attenuation rate of SIF for CRTSII track slab surface transverse dynamic crack propagation is faster than the actual measured value.

Utilising a series of mechanically over‐excavated cavities along borehole is a novel technique for enhancing the permeability of soft coal seams and, consequently, gas drainage. The evolution of cracks induced by a wide range of pressure‐relief around an over‐excavated hole is intrinsically complex. In this study, the mechanical behaviour and crack evolution of the specimens containing an over‐excavated hole under uniaxial compression loading were studied by experimental and 3D numerical simulation. The results indicated that the peak strength and elastic modulus of the specimens gradually decrease with increasing cavity diameter and length, which is also verified by the numerical simulation. The inclusion of cylindrical cavities in over‐excavated holes results in reduced crack initiation stress and a greater degradation of peak stress and elastic modulus, despite having an equivalent volume to the ellipsoidal cavity. This is likely attributed to the difference in stress concentration between the cylindrical and ellipsoidal cavities. The crack propagation process can be classified into four stages based on the acoustic emission (AE) event counts, initial crack compaction, stable crack propagation, unstable crack propagation and post‐peak failure stage. The two AE indices, rise angle and average frequency value, demonstrated that the failure is dominated by tensile crack and gradually transformed to shear crack. Stress redistribution is essential in the initiation and propagation of cracks. Tensile stress concentration leads to cracks forming at the top and bottom of the hole, which propagate in the direction of loading. Compressive stress concentration results in shear cracks forming at the left and right sides of the hole, which propagate diagonally. The failure pattern of the specimen is ultimately determined by a combination of tensile and mixed crack propagation. The experimental and numerical results contribute to a deeper understanding of the crack evolution mechanism of coal seams with over‐excavated holes.

Abradable/abrasive sealing systems are currently used in gas turbines to reduce the blade tip gas leakage and consequently improve the turbine efficiency. The coatings selection is directly related to the section in which they are used. Seal systems for hot gas paths are primarily required to withstand high temperature. The abradable coating should be easily removed by the tip blade without causing significant blade wear, whereas the blades should have sufficient cutting capabilities. Durability properties, such as erosion resistance, are also required. Owing to their temperature capabilities, porous ceramic coatings are successfully used as abradable coatings. Although they are characterized by good abradability properties, their resistance to environmental attacks, such as solid particle erosion, is limited by the porous microstructure which negatively affects their service life. It is apparent that durability and abradability are the main targets to be simultaneously achieved for ensuring longer service life and improved efficiency. The present work is aimed at developing new abradable/abrasive coatings pairs able to ensure both the durability performances of the coatings and good abradability properties. Three ceramic abradable coatings with DVC and porous microstructure have been studied. The down-selection process has been carried out by considering the microstructure, the hardness, the tensile adhesion strength, the erosion resistance, and the furnace cycle test resistance. A composite coating made by NiCoCrAlY matrix containing abrasive grits applied by electrolytic process was selected as abrasive material system. The abrasive grits (patent application in process by GE Oil&Gas) consists of a mixture of ceramic particles. These grits ensure both short-term cutting capability and thermal stability, assuring the clearance maintenance over time. The abradability of the seal system was assessed by a properly designed test, namely Rub Rig test, which simulates the blade incursion in the abradable coating. Surface patterns on abradable coating were also considered to further enhance the abradability. Engine tests are foreseen for assessing the service behavior of this seal system.

Stress redistribution around clusters of broken fibres in a composite

Abradable coatings are employed in modern gas turbine engines to minimize clearance and reduce over-tip leakage by allowing blade tips to cut into the coating. While such clearance control coatings have been used in the compressor modules of jet engines and stationary gas turbines for many years, their use in the High Pressure (HP) section of turbine modules is relatively new. Because of the high temperatures encountered there, ceramic materials must be used to provide a durable seal. In this paper the performance of two novel ceramic abradable seal coatings is reviewed and compared to a baseline system. Emphasis is placed on the resistance to thermal shock and erosion. The abradability of coatings by hard tipped blades is reviewed using a wear map consisting of five distinctly different rub test conditions. Among the coatings considered, an APS sprayed dysprosia stabilized zirconia ceramic abradable offers a potentially superior combination of thermal shock resistance, abradability and erosion resistance for high temperature turbine seal applications.

Interfacial stress and crack propagation experimental study in mini-LED chip debonding

An analytical approach to calculate stress concentration factors of machined surfaces

The damage process of transverse cracks in cross-ply and quasiisotropic toughened CFRP laminates under static loading is studied. To investigate specimen configuration effect on width-direction propagation of transverse cracks, coupon specimens with ranges of specimen width and transverse ply thickness are tested. Detailed observations of transverse cracks indicate that there are consecutive three-stage processes of edge crack initiation, edge crack increase without inward propagation, and crack propagation across the width in quasiisotropic laminates, whereas simultaneous transverse crack propagation takes place across the width in conjunction with new edge crack formation in cross-ply laminates. The detailed process of transverse cracking is investigated using three-dimensional FEA in order to clarify the difference of damage process between quasiisotropic and cross-ply laminates. It is shown that damage mode transition from edge cracking to widthwise propagation can be characterized with energy release rates near free-edges and inner regions. Finally, an average propagation model based on the conventional two-dimensional models is presented and verified with experimental results and FEA. Useful information about experimental results of transverse crack propagation and the characterization of edge cracking and inward propagation isprovided.

ASTM third symposium on composite materials: fatigue and fracture: Orlando, FL, USA, 6–7 November 1989

Fatigue growth of matrix cracks in the transverse direction of CFRP laminates

Only a few types of commercially available high temperature ceramic abradable coatings are presented on the market and most of them consist of partially stabilized yttria zirconia with polymer porosity former agent and/or hBN solid lubricant. The basic demand placed on abradable coatings include balance between hardness and erosion resistance. The contribution focuses on the description of microstructure, phase composition and hardness of alternative atmospheric plasma sprayed ceramic abradable coatings deposited from four different experimental powder mixtures: (i) commercial yttria-zirconia + 5 wt. % of experimental BaF2/CaF2, (ii) commercial yttria stabilized zirconia + 10 wt. % of experimental BaF2/CaF2, (iii) R&D powder SrxTiOy and (iv) R&D powder SrxTiOy + 5 wt. % of polyester. The abradable coating systems were of ~ 150 μm thick CoNiCrAlY bond coat and of ~ 800-1000 μm thick ceramic top coat. The microstructure and phase composition of all atmospheric plasma sprayed coating systems were evaluated by the means of scanning electron microscopy and X-ray diffraction techniques. To estimate coatings basic parameters the Rockwell hardness HR15Y was measured.

This study investigates the failure mechanisms of spur gears under bending fatigue loading, focusing on crack initiation, propagation paths, and stress field distribution. Finite element simulations are conducted to obtain linear elastic stress–strain data at the tooth root, which are then incorporated into the Smith‐Watson‐Topper (SWT) multi‐axial fatigue model to predict the crack initiation location. Subsequently, a moving contact load model with time‐dependent magnitude and direction is developed through the secondary implementation of an Abaqus DLOAD user subroutine using Fortran programming. During crack propagation analysis, variations in stress intensity factors (SIF), crack propagation paths, and stress field distribution are examined, whereas the NASGRO model is applied to estimate fatigue life under different operating conditions. The results indicate that the predicted fatigue life under moving contact load closely matches experimental values. During crack propagation, merging cracks lead to interactions and overlapping of stress fields, which ultimately reach a new equilibrium after merging. Furthermore, compared to loading at the highest position of single‐tooth contact (HPSTC), the dynamic effect of the moving contact load effectively alleviates stress concentration and reduces the crack propagation rate.

Contact-induced radial cracking in ceramic coatings on compliant substrates was analyzed recently. Radial cracks initiate at the coating/substrate interface beneath the contact where maximum flexural tension occurs, and an analytical expression for the onset of radial cracking in monolayer coatings was formulated on the basis of the classical solution for flexing plates on elastic foundation. In the present study, the analytical expression was derived for the case of ceramic bilayer coatings on compliant substrates, which have significant applications in the structure of dental crowns. It was found that the analytical solution for bilayer-coating/substrate systems can be obtained from that of monolayer-coating/substrate systems by replacing the neutral surface position and the flexural rigidity of monolayer coating with those of bilayer coating. The predicted critical loads for initiating radial cracking were found to be in good agreement with existing measurements and finite element results for glass/alumina, glass/glass-ceramic, and glass/Y2O3-stabilized ZrO2polycrystal bilayers on polycarbonate substrates. Limitations of the present analysis are discussed.