Experimental Investigation on Fatigue Curve Parameters of 6061 T6 ...

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 3 (2016) pp 2108-2111 © Research India Publications. http://www.ripublication.com

Experimental Investigation on Fatigue Curve Parameters of 6061 T6 Aluminium Alloy 1

J.Selvakumar1 Associate Professor, Department of Mechanical Engineering, Paavaai Group of Institutions, Namakkal, Tamilnadu, India.

M.D.Mohangift2 Associate Professor, Department of Mechanical Engineering, Panimalar Engineering College, Chennai, Tamilnadu, India.

2

Dr.S.Johnalexis3 Professor, Department of Automobile Engineering, Kumaraguru College of technology, Coimbatore, Tamilnadu, India. 3

Abstract The Aluminium 6061-T6 alloy has wide application in industries due to its high performance and mechanical behavior. Normally Non-ferrous alloys will not exhibit exact endurance limit due to manufacturing defects. To overcome the inaccuracy in life estimation so many techniques adopted. The EIFS method, AMWD, FESL are used to find the fatigue life. The fatigue curve parameters are described by Gatts equation, hyperbolic function, exponential function and power function. In this work the fatigue test results are correlated to endurance limit. Two stress level and three stress level analyses are carried out and error % in Gatts equation and hyperbolic function are compared.

critical intensity factor HAZ delays the crack growth which leads to plastic zone formed near the crack tip. It is achieved by micro structural transformation [2, 3]. The fatigue curve parameters are described by Gatts equation, hyperbolic function, exponential function and power function [7, 8]. In this work the fatigue test results are correlated to endurance limit. Two stress level and three stress level analyses are carried out and error % in Gatts equation and hyperbolic function are compared In composites the fatigue lives affected by various metallurgical properties like reinforcement size, types, orientation and matrix alloy used. For different stress ratios the macrofractographs show same pattern with different lives [4, 9].

Keywords: Endurance limit, Gatts equation, Hyperbolic Function, Stress Combination.

Experimental Procedure The specimens are prepared to standard size. To overcome the manufacturing defects and machining defects specimens were polished. Due to polishing the flaws at surface levels removed and stress concentration induced by surface quality also avoided. The standard specimens were tested at different stress levels in INSTRON fatigue test machine at the frequency of 15Hz. The stress ratio is taken as-1 for achieving complete stress reversal. Fatigue causes brittle like failure even in normal ductile material. The endurance limit of standard specimen is 140MPa. In our work the specimens tested at 5 different stress levels likely 320MPa, 290 MPa, 240 MPa, 200 MPa and 170 MPa. The Results of Fatigue Tests of Specimens are tabulated

Introduction The model of stable crack growth is developed by linear elastic fracture mechanics. Non ferrous metals will show wide range of variation in fatigue life. It is caused due to manufacturing defects and lower elasticity of metals. The initial fatigue quality of any metal depends on initial equivalent flaw size. Creep fatigue life prediction model constructed by using Applied Mechanical Work Density method (AMWD) [1]. The Gatts equation describes the life of ferrous and non ferrous metals with minimal error. The plastic deformation starts at low stress level it causes residual stress in the specimen which helps to increase the fatigue life dominently. The interrelationship between stress range, mean stress and stress amplitude make the life prediction as complicated one and it forcing the work to do more tests and analysis. The life analysis is combined with probability theory and plastic failure of materials. The 6061-T6 Aluminium alloy has used in varies fields due to its high performance in terms of corrosion resistant and surface treatment. The material behavior is changed when welded with dissimilar materials. The HAZ plays very important role along with critical intensity factor. Below the

Table 1: Fatigue test results of Al6061-T6 alloy specimens Stress level number 1 2 3 4 5

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Stress (MPa) 320 290 240 200 170

Life N Cycles 1930 19640 58450 309700 1139500

International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 3 (2016) pp 2108-2111 © Research India Publications. http://www.ripublication.com Each stress level has its own K value corresponding to (1-C). The value of (1-C) is recommended as 0. 5 by R. R. Gatts corresponding K value calculated and tabulated in table 3

Gatts Equation Gatts equation is

Table 3: K value for Al6061-T6 alloy at (1-C) =0. 5

Where N-number of cycles to failure -maximum cyclic stress MPa  R-endurance limit MPa K, (1-C) co-efficient of Gatts equation This equation gives the relation between endurance limit and fatigue life corresponding to applied stress with the help of two coefficients K and (1-C)(5).

Stress level number i 1 2 3 4 5

Determination of Gatts Coefficient by 2 stress level analysis The coefficients K and (1-C) are calculated by equating the Gatts equation at 2 stress level such way that 12.

Stress MPa

Life N cycles

K*10-8

320 290 240 200 170

1930 19640 58450 309700 1139500

-35. 982 -1. 170 2. 851 2. 152 1. 892

In the above table the value of K varies from-35. 982X10-8 to 2. 851X10-8. At higher stress level the value of K is lower compared to lower stress levels. The value of K varies with respect (1-C). The value of (1-C) is 0. 5 which was suggested by R. R. Gatts, but most of the cases the value is not matching. so we take the value of (1-C) from 0. 3to 0. 7 and corresponding K values are tabulated TABLE 4: Take the Gatts Coefficient K value of Al6061-T6 at (1-c) from 0. 3 to 0. 7.

Let’s equate the equations 2 and 3 to each other and write the equation for the coefficient (1-C) at the given stress combination.

Stress level number i 1 2 3 4 5

By using the above expression the values of (1-C) is calculated for 10 combination of stress. The value of K is calculated by using (1-C) value in equation 2or 3. The calculation accuracy can be verified by the equality of K1=K2

Stress Life K*10-8 MPa cycles 1-C= 0. 1-C= 0. 1-C= 0. 1-C= 0. 1-C= 0. 3 4 5 6 7 320 1930 -251. 87 290 19640 -24. 58 240 58450 -6. 65 200 309700 0 170 1139500 1. 20

-116. 94 -9. 94 -0. 07 1. 34 1. 63

-35. 98 17. 99 -1. 17 4. 68 2. 85 5. 22 2. 15 2. 69 1. 89 2. 06

56. 54 8. 86 6. 92 3. 07 2. 19

Table 2: Coefficients of Gatts Equation (1-C) and K Values at 2 Stress Level Analysis Stress combination number j

Stress level Number

1 2 3 4 5 6 7 8 9 10

1, 2 1, 3 1, 4 1, 5 2, 3 2, 4 2, 5 3, 4 3, 5 4, 5

Coefficients of Gatts equation 1-c K*10-8 0. 568 3. 063 0. 571 4. 639 0. 567 2. 536 0. 566 2. 013 0. 619 5. 600 0. 558 2. 489 0. 549 1. 985 0. 470 1. 948 0. 466 1. 817 0. 447 1. 770

Figure 1: Variation of K with respect to 1-C at different stress levels for Al6061-T6 alloy. It is evident from fig-1, the value of K increases with increase in (1-C) irrespective of applied stress. The value of K is negative when the applied stress is high and (1-C) is value is low. The high variation of K is obtained at high stress level. The variation range is directly vary with respect to applied stress. For any (1-C) value the value of K increases when the applied stress decreases. The large variation in K value is obtained at low (1-C) value.

From the table-2 it is observed that the value of (1-C) vary from 0. 619 to 0. 447 and most of the values very closer to 0. 55at the same time the value of K varies from 5.600X10-8 to 1. 770X10-8 and most of the values closer to 2X10-8.

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International Journal of Applied Engineering Research ISSN 0973-4562 Volume 11, Number 3 (2016) pp 2108-2111 © Research India Publications. http://www.ripublication.com The coefficient β and calculated value of fatigue limit σR is determined by the expressions β= (7)

Determination of Endurance Limit By 3 Stress Level Analysis To determine the endurance limit and Gatts coefficients we take 3 stress levels and its corresponding life. The stress levels 1, 2, 3 are taken such a way that 123 by satisfying the above condition we make 10 Stress combination.

σR

Table 6: Co-efficent of hyperbolic function , calculated endurance limit in 2 stress level analysis with error %

Table 5: Coefficients of Gatts Equation (1-C), K and calculated endurance limit Values at 3 Stress Level Analysis with error % for Al6061-T6 alloy.

Stress Stress calculated %error  combination level endurance limit number j number 1 1, 2 2159. 717 283. 07 102. 19 2 1, 3 5275. 246 229. 80 64. 14 3 1, 4 7226. 992 196. 42 40. 30 4 1, 5 8855. 906 168. 57 20. 41 5 2, 3 39232. 21 164. 15 17. 25 6 2, 4 33164. 89 183. 61 31. 15 7 2, 5 39381. 16 163. 67 16. 91 8 3, 4 173460. 7 161. 43 15. 71 9 3, 5 39488. 24 163. 65 16. 89 10 4, 5 89973. 98 155. 55 11. 10 From the table-6 it was observed that the value of is not limited to 1000 as suggested by B. S. Shul’ginov it varies from 2159. 717 to 89973. 98. The error %is very high compared to Gatts equation.

Gatts equation Error % coefficients 1-c K*10-8 0. 5720 0. 5211 0. 5321 0. 4517 0. 5132 0. 5365 0. 6015 0. 5847 0. 5327 0. 4581

0. 122 4. 145 3. 860 12. 962 7. 368 3. 481 19. 515 9. 650 3. 439 2. 217

(8)

Where and are arbitrarily chosen stresses such that > , and N1 and N2 are the respective lives to be determined from the test results. By using the above expression the values of  and endurance limit are tabulated for 10 combinations of stress and the error % also tabulated.

Let us equate the right-hand members of the equations 2, 3, 5 to one another and find the calculated endurance limit, K, (1C) are tabulated with error % for 10 possible stress combinations

Stress Stress calculated combination level endurance number j number i limit 1 1, 2, 3 154. 07 2 1, 2, 4 155. 42 3 1, 2, 5 151. 83 4 1, 3, 4 180. 47 5 1, 3, 5 159. 52 6 1, 4, 5 150. 25 7 2, 3, 4 185. 45 8 2, 3, 5 161. 66 9 2, 4, 5 150. 09 10 3, 4, 5 143. 75

=

10. 05 11. 01 8. 45 28. 90 13. 94 7. 32 32. 46 15. 47 7. 21 2. 68

The calculated value of endurance limit by using Gatts equation is closer to experimental value but the value of (1-C) has no consistency it varying from 0. 6015to 0. 4518 with an average of 0. 5304.

Conclusions In this work fatigue life analysis were performed for Al 6061T6 Aluminium alloy by using Gatts equation and hyperbolic function. Two and three stress level analysis were utilized for describe the fatigue curve. The following conclusions are from the present work. 1. The fatigue curve parameters of Gatts equation (1-C) and K has better agreement with experimental values when it was found from three stress level analysis 2. It is experimentally observed that the reduction in stress led to proliferation at high stress level. 3. The Gatts equation gives the minimum error in description of fatigue curve compared to hyperbolic function

Hyperbolic Function The S-N curve described by hyperbolic function requires ultimate stress of the material. In hyperbolic function, describing S − N curves we have to demonstrate the parameter σu used in σ −N equation. Generally the fatigue life does not correspond to the ultimate strength of the material (6). In the following hyperbolic relationship between life N and maximum cycle stress σu is proposed for describing a fatigue curve. N= (6)

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Where N-Number of cycles to failure -Maximum cyclic stress MPa σu-Ultimate stress MPa -Endurance limit MPa β is the coefficient

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