Theoretical Stress Analysis of Gas Turbine Blade Made From Different Alloys

Blades may be considered to be the heart of turbine without blade there would be no power and the slightest fault in blade would mean a reduction in efficiency and costly repairs. The centrifugal force is one of the problems faced by the designer of blades especially at the first stages. The designer aims at reducing the stresses with in the allowed limit. The ANSYS 15 software was used as far as it is the most effective in analyzing the different numerous cases of stresses, the blades with limited root in all direction (X,Y,Z) were taken into consideration . The centrifugal forces were applied on the rotor blades at running speed of 6000 r.p.m., The finite element models of the blade were constructed using D3-10noded Tetrahedron elements shape, SOLID 187, mesh of the entire blade 23406 Node,136575element. The average of normal stress, Von misses, Maximum principle stress, Minimum principle stress were calculated according to ANSYS 15 program, these stresses are as the result of the effect of centrifugal force for all planes along the blades and then values of stresses were compared to the curves for each alloy.The current research concluded that the Titanium alloy is the best alloy used in terms of reducing stresses due to centrifugal force, that is because density of Titanium alloy used is less than that of other used alloys, leading a reduction in centrifugal forcess that are directly proportional to mass.


INTRODUCTION
Shattering of turbine blades are one of the most critical troubles in power generating industry. Rotor blades are the very generally used machine parts used in high-technics applications and in general are used as a lot of arrangement mechanical element. These blades are subjected to different loads like centrifugal loading. Centrifugal forces is the main component participate in to the stress produced in the blades [1].
Centrifugal loads are formed by high rotational speeds at (6000 r.p.m).The formed Centrifugal force depend upon the size of the rotor and the rotational speed of the rotor. As it is well known, that the centrifugal force Fc can be defined as : Fc = mrw², where, (m) is the mass (kg), (r) is the radius of rotation and w² is the angular speed. Gas turbine blades alloys are designed mainly to protect the structural material against loads, corrosion, oxidation, erosion and high temperature environments.
The main reason of shut down in turbo machine is the shatter of rotor blade. The failure of the rotor blade may guide to serious consequences both physically and economically. Hence, the suitable design of the turbo machine blade plays a necessary role in the suitable functioning of the turbo machine [5].

Al-Rafidain Engineering Journal (AREJ)
Vol.24, No1, October 2019, pp. 10-18 In the current study, the first stage rotor blade of a two-stage gas turbine has been analyzed for structural static using ANSYS 15.0Software which is powerful Finite Element Software. In the process of getting mechanical stresses in the rotor blade has been evaluated using four different alloys; namely: Inconel 718, Inconel 625, MarM200, Titanium alloys .And the dimensions of the blade for the current study shown in Table.1.The blade was cut into twenty one sections along the length of the Airfoil with ANSYS software as shown in Fig.1The finite element models of the blade were constructed using D3-10-noded Tetrahedron elements shape, SOLID 187, mesh of the entire blade 23406 Node, 136575 element as shown in Fig. 2. And all results take from the option of the average of stresses in ANSYS software as shown in Fig.3.    Naga Bhushana Rao et.al [9] had done research on turbine blade used in marine applications. The blade was observed for structural analysis at elevated temperatures and under the action of large centrifugal force, the material used was nickel based super alloy, it was investigated that high stresses and strains were observed near to the root of the turbine blade and upper surface along the blade root, maximum temperature is observed at the blade tip and minimum at the root of the blade.

MATERIAL AND MATERIAL PROPERTIES
Four alloys have been used in the current study:

3-1-INCONEL 718&INCONEL 625 alloys
It is a high strength, heat resistant superalloy (HRSA) that is used extensively by the aerospace industry for the hot parts of gas turbine engines such as, turbine disks, blade .[2]

3-2-Mar M-200 alloy
The material of the blade is taken as MAR M-200 isconsider superNickel alloy. It is one of the proper material for blades of Gas turbineon account of its altitude yield strength,

3-3-Titanium Alloy
Accelerates formation of chromia at metal/oxide interface; decrease thermal expansion coefficient in Ni base alloys. These titanium alloys are foundamentlly used for represent materials for hard tissues. [3].

RESULTS AND DISCUSSIONS
Following Fig.(4,to7) show the results of stress analysis in INCONEL625 alloy,we notice that the concentration of stresses rate is in the back of the base of the airfoil,which gradually decreases along the airfoil near the top of the blade, because the centirfugal force is higher at the base and lower at the top of the blade due to the decrease in the mass of blade whenever the direction from the base to the top of the blade and  The following Fig. (8 to 11) show the stress analysis for INCONEL718 alloy,the result obtaained can be discussed as an ANSYS program where the value of the largest stress at the base of the airfoil and the lowest at the top of the blade is explained by the fact the high centrifugal force at the base of the airfoil and less as we movement to the top of the blade, and       The following Fig. (12-to 15) show the resultof MarM-200 alloy blade,the highest value of the stresses is at the base of the airfoil,the reason is that the centrifugal force are high at the base of airfoil and decrease as we move up at the top of the blade this is due to the gradual reduction of the blade mass along the airfoil, and Table(       The following Fig.(16-to19) result of TITANUM alloy blade,it was observed that the highest value of the stresses at the base of the airfoil and gradually decrease as the hight reaches the top of the blade to reach the least value of the stresses where the stresses are located at the back of the base of the airfoil, and Al-Rafidain Engineering Journal (AREJ) Vol.24, No1, October 2019, pp. 10-18 Table 6: Result of average stress analysis on blade of gas turbine made of TITANIUM alloy by ANSYS program in all sections along the blade.
The following Fig.(21,22,23,24) comparison for all alloys used in current research.The concentration of the stresses is at the base of the airfoil for all the alloys used in this study.and the resulting stresses, such as normal stress,maximum&minimum principle stress and von misses stresswhich were analyzed by the ANSYS software are lower in Titanium alloy than other alloys because the density of Titannium is less than the density of other alloys.   Inconel 625 Inconel 718 Al-Rafidain Engineering Journal (AREJ) Vol.24, No1, October 2019, pp. 10-18

CONCLUSION
The results achieveddue to the centrifugal forces on the blade is debated below. 1-It was conclusion that the stresses produced by the centrifugal forces (normal stress, maximum and minimum principle stress, von misses stress) were concentrated at the base airfoil from the back. 2-It was observed that stresses along the blade be higher value when back to the root and less at the top of the blade as shown in Fig.(21,22,23,24). 3-The centrifugal force depends on the mass where all less than the mass of the blade less centrifugal force and thus reduce the stresses resulting from this force , which was observed in the Titanium alloy of less density compared with the other alloys . 4-The tensile strength(normal stress) of the centrifugal force on the blade made of the Titanium alloy density is less than the tensile stress caused by the other alloys because the Titanium alloy density is lower than that of the other alloys thiscauses to reduce centrifugal force then causes to reduce stresses as shown in Fig.(21). 5-Maximum principle stress of the blade made of Titanium alloy has less than the maximum principle stress of the blade made of alloy s Inconel625,Inconel718,MarM-200) because the Titanium alloy density is lower than that of the other alloys so causes to reduce centrifugal force then causes to reduce stresses as shown in Fig.(22) & Fig.(4,8,12,16). 6-That the negative value of the minimum principle stress of all alloys used indicate that the blade is subjected to compression stress in some blade parts of all alloys used as shown in Fig.(23) . 7-It was noted that the highest stress equivalent to the blade made of Titanium alloy is 26089Mpa, and the equivalent stress of the blade made of Inconel625 is 48271Mpa, and the highest equivalent stress of blade made of Inconel718 is 47631Mpa, and the equivalent stress on the blade made of Mar M200 is 48995MPa this shows that the best on the safe matter of the high stresses is the titanium alloy this is due to the fact that Titanuim alloy density is less than the density of the alloys used as shown in Fig.(6,11,15,20) . 8-It is conculosion thatthe Titanium alloy is the best alloy used in terms leading to reduce stresses due to centrifugal force, because density of Titanium alloy used is less than of other used alloys,so,leading a reduction in centrifugal forcess which are directly propotional to mass,there by reducing the stresses of these forces to the Titanium alloy that is density less than of the alloys used in this study. 9-It was noted that the results of stresses for the INCONEL718,INCONEL625 and MarM-200 alloys are closer, they are nicle base alloys and they have the nearestdensity