STUDY THE INFLUENCE OF REVERSED BENDING FATIGUE TEST ON THE MECHANICAL PROPERTIES OF CARBON STEEL ALLOYS

Transcription

1 International Journal of Mechanical Engineering and Technology (IJMET) Volume 9, Issue 9, September 2018, pp , Article ID: IJMET_09_09_027 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed STUDY THE INFLUENCE OF REVERSED BENDING FATIGUE TEST ON THE MECHANICAL PROPERTIES OF CARBON STEEL ALLOYS Qasim Bader Lecturer in College of Engineering, University of Babylon, Iraq ABSTRACT This paper study the influence of reversed bending fatigue tests of a cylindrical specimens made of Carbon Steel alloys with different content of carbon on the mechanical parameters: yield strength, ultimate strength, percentage of area reduction and the percentage of elongation. The fatigue experiments were carried out at room temperature, applying a fully reversed cyclic load with the frequency of 50 Hz and mean stress equal to zero (R= 1), on a cantilever reversedbending fatigue testing machine; the stress ratio was kept constant throughout the experiment. A stress life approach based on SN curves (Stress Cycle curves) is employed. Different stress amplitudes used to the specimens on whom fatigue test was carry out in such a method that no specimen reached to its fracture cycle with two percentages 50% &75% from the total cycles of failure of the original test. Then, mechanical tests were done on the specimens and the parameters were determined for each specimen. The results obtained were compared with the data estimated from the specimen on whom no fatigue test was run and the tensile and hardness test was carried out on it directly. Morphological observation on fracture surface of specimens was done using scanning electron microscope (SEM) to study the fracture nature Results show that, tensile property is more affected with fatigue test in comparison to the other mechanical parameters and this applicable with some references and for a large number of steels, there is a direct correlation between tensile strength and fatigue strength; highertensilestrength steels have higher endurance limits. Key words: Reversed Bending, Fatigue Test, Stress life approach, Mechanical Properties, FEA. Cite this Article: Qasim Bader, Study the Influence of Reversed Bending Fatigue Test on the Mechanical Properties of Carbon Steel Alloys, International Journal of Mechanical Engineering and Technology 9(8), 2018, pp editor@iaeme.com

2 Qasim Bader 1. INTRODUCTION Fatigue is the progressive, localized, and permanent structural change that occurs in a material subjected to repeated or fluctuating strains at nominal stresses that have maximum values less than (and often much less than) the tensile strength of the material [1].It has been estimated that fatigue contributes to approximately 90% of all mechanical service failures. Fatigue is a problem that can affect any part or component that moves.the Fatigue is an important parameter to be considered in the behavior of mechanical components subjected to constant and variable amplitude loading. Mechanical, metallurgical and environmental variables can influence the fatigue resistance of a structural component. Fatigue is the process of a cumulative damage in a benign environment that is caused by repeated fluctuating loads and, in the presence of a stress concentrators like notches and fillets. In cyclic loading, the effect of the notch or the fillet is usually less than predicted by the use of the theoretical factors. The difference depends upon the stress gradient in the region of the stress concentration and on the hardness of the material [2]. Fatigue is the fracture of a material when subjected to repeated (cyclic) small stresses below the plastic limit. This causes tiny small cracks to be generated within its structure. These tiny cracks do not cause failure immediately. With each application of stress, the cracks grow until the material breaks Mechanical, metallurgical and environmental variables can influence the fatigue resistance of a structural component [3].There is three basic factors necessary to cause fatigue: (1) a maximum tensile stress of sufficiently high value, (2) a large enough variation or fluctuation in the applied stress, and (3) a sufficiently large number of cycles of the applied stress [4].The relations between fatigue strength and other mechanical properties especially the tensile strength of metallic materials are reviewed after analyzing the numerous fatigue data available,the qualitative relations between fatigue strength and hardness,strength (tensile strength and yield strength) and toughness (static and impact toughness ) are established by [5]. Rotating bending fatigue (RBF) tests yield generally higher endurance limit results than uniaxial fatigue tests on the same materials. Which test method is more applicable to part design is not clear. While the uniaxial tests are more flexible in stress ratio and yield more conservative endurance limits, R.B.F. tests often closely simulate the real life operating conditions of rotating parts [6,11]. 2. EXPIERMENTAL WORK The experimental work included assessment of fatigue life specifications by using stress life approach for three carbon steel alloys supplied from the local market. The experimental procedure consists of four parts, the first one deals with the selection of materials used and the specimens preparation, the second part deals with different mechanical tests such as tensile test, hardness test,impact test and roughness test and the third one includes details of reversed bending fatigue test for three types of specimens (Original,0.5 of the total cycles of original fatigue test and 0.75 of the total cycles of original fatigue test), and finally the details of Microscopic inspection for the fractured specimens Material Selection In this work, three materials (low, medium and high carbon steel alloys) treated commercially, were used in this investigation, Those types of steel alloys have a wide application in industry, the results obtained were within the specification limits and as shown in table (1) [7] editor@iaeme.com

3 Study the Influence of Reversed Bending Fatigue Test on the Mechanical Properties of Carbon Steel Alloys Elem. C Si Mn P S Cr Mo Ni Al Cu V Fe LCS Bal. Table 1 Chemical composition analysis of Carbon Steel Alloys Measured MCS Bal. Standard HCS LCS MCS Bal HCS Mechanical Tests The amount of energy absorbed by the material when subjected to sudden force measured by clamping a specimen of known dimensions firmly in position and breaking it with a swinging pendulum is known impact strength.charpy and Izod impact tests have been done on three carbon steel alloys use standard specimens. Table 4 shows the summary of experimental calculations performed to calculate the fracture toughness of the materials used in the study. It is noted from the figure (1), the impact strength increase with decrease of carbon content; perhaps the reason for this is due to the decrease in carbon weight, leading to the increased resistance of material to crack propagation which blocks the transmission of the crack through the surface. Tensile test was conducted on original specimens using the microcomputer controlled electronic universal testing machine as shown in figure (2) and an average value of three readings for each test has been taken to satisfy an additional accuracy. Hardness is the resistance of the material to permanent indentation or penetration or scratching, various hardness tests have been done and the average value of three readings was recorded; for the specimens subjected to modify fatigue tests with two percentages 0.5 & 0.75 of the total number of cycles to failure. On the other hand another tensile testing machine type WP 300 have been used to suit the configuration of specimens used in fatigue test; the results of basic mechanical properties of carbon steel alloys are given in table 2 and table 3 respectively; for more details about the Tensile and Hardness tests procedure; see [8]. Surface roughness and surface integrity resulting from manufacturing processes are both important considerations in fatigue design. Generally the presence of stress concentrations originating from the surface topography effects on the Fatigue damage on the surface of a component [9]. Once the manufacturing process of the specimens was done, then the surface roughness was measured by using a portable Surface Roughness tester type ( SADT) as shown in figure (3) and in order to reduce human errors during the measurement, the reading was taken for three times at different points and for all specimens, then, the average and total surface roughness, Ra and Rt are calculated and summarized in table editor@iaeme.com

4 Qasim Bader Table 2 Results of Tensile test for of Carbon Steel Alloys Material Property Value Tensile Strength σ u (MPa) 470 Yield Strength σ Low carbon steel y (MPa) 350 Elongation [%] 26 (AISI 1020) Reduction of Area [%] 55 Modula's of Elasticity (GPa) 202 Medium carbon steel (AISI 1037) High Carbon Steel (AISI 1078) Tensile Strength σ u (MPa) 575 Yield Strength σ y (MPa) 480 Elongation [%] 18 Reduction of Area [%] 35 Modula's of Elasticity (GPa) 206 Tensile Strength σ u (MPa) 675 Yield Strength σ y (MPa) 510 Elongation [%] 15 Reduction of Area [%] 30 Modula's of Elasticity (GPa) 200 Table 3 Results of Hardness test for of Carbon Steel Alloys Material Property Value Brinell Hardness (HB) 135 Low carbon steel Vickers Hardness (HV) 142 Rockwell Hardness (HRB) 74 Brinell Hardness 172 Medium Carbon Steel Vickers Hardness (HV) 180 Rockwell Hardness (HRB) 85 Brinell Hardness (HB) 200 High Carbon Steel Vickers Hardness ( HV) 220 Rockwell Hardness (HRB) 95 Material Type Table 4 Summary of experimental impact resistance test Absorbed Energy (J) Impact Strength (kj/m2) Fracture Toughness (MPa. m) Low Carbon Steel Medium Carbon Steel High Carbon Steel Table 5 Results of Surface Roughness Material R a [μm] R t [μm] max. Low carbon steel Medium carbon steel High carbon steel Table 6 Fatigue limit for carbon steel Alloys Material Type SN Equation Fatigue limit (MPa) Low Carbon Steel σ f = N f Medium Carbon Steel σ f = N f High Carbon Steel σ f = N f (σ f /σ u ) editor@iaeme.com

5 Study the Influence of Reversed Bending Fatigue Test on the Mechanical Properties of Carbon Steel Alloys Figure 1 Relations between the amounts of C in Carbon Steel alloys and the Impact toughness Figure 2 Tensile Test used for original specimens Figure 3 Experimental roughness Test editor@iaeme.com

6 Qasim Bader 2.3. Fatigue Test In the revolving fatigue testing machine, a rotating sample which is clamped on one side is loaded with a concentrated force. The load is applied at one end of the sample and with the help of a motor, rotation about its own axis is achieved. Due to this rotation, a load reversal condition is achieved at two opposite sides on the circumference of the specimen. A triangular bending moment is developed in the specimen. Following a certain number of load cycles, the sample will rupture as a result of material fatigue A cantilever bending fixture was designed to test the steel specimens based on the critical (i.e. failure) location. Cantilever bending was used in order to minimize the magnitude of the applied loads necessary to achieve the desired nominal stresses. Stresses at which the material fails below the load cycle limit are termed fatigue limit.sn curves are plotted by using software of Fatigue instrument presented in PC which is connected directly to fatigue machine as shown in figure (4), for more details about the fatigue test specimens geometry and procedure, see [10]. Figure 4 Experimental Fatigue Test 3. RESULTS Reversed bending fatigue tests on three types of specimens made of three selected alloys of carbon steel have been conducted to investigate changes on the mechanical properties (Yield strength, Ultimate strength, Hardness, Percentage of area Reduction and the Percentage of elongation). Different fatigue loads are applied to the specimens on which fatigue test was run in such a way that no specimen reached to its fracture cycle with two percentages 50% & 75 % of the total cycles to failure obtained from the original Fatigue test; Fig.(5) show SN curve for three alloys of carbon steel based on different stress amplitude. Results found were generated in figures (6 to 10) which are represent the effect of fatigue test on different mechanical properties based on two percentages (50 &75) % of the total number of cycles to failure obtained from conduct original tests on high carbon steel alloy. From the results it is obtained that the Ultimate Tensile strength, Hardness and the Yield strength of the material increased, the percent of reduction in area and elongation are decreased for all three selected materials. Also from results it is found that all selected materials during this test neither subjected to cyclically strain hardening or softening because in almost tests the ratio of Tensile Strength to Yield strength is between 1.2 and 1.4 [12].The process of achieve test fracture for the different fatigue specimens has been done to check the nature of fracture editor@iaeme.com

7 Yield Strength/ Original Yield Strength Alternating Stress MPa Study the Influence of Reversed Bending Fatigue Test on the Mechanical Properties of Carbon Steel Alloys Fracture surfaces of failed specimens have been analyzed using Optical Microscope (OM) and Scanning Electron Microscope Zeiss type (EVO 50).Samples for microstructure examination were ground using different grades of wet silicon carbide papers (260, 500, 800, 1200 and 2000), then the samples were polished using two type of alumina (0.5 micron and 0.3 micron). Distilled water and alcohol were used to clean the samples in succession. Etching was carried out with naital (2 % HNO3) in alcohol followed by washing them with water and alcohol. Figure (11) shows SEM micrograph of different region of fracture surface of AISI Numerical and Experimental Fatigue life equations are estimated in table (7). The results indicate that there is acceptable error between experimental and numerical methods y = x HCS:Exp. MCS:Exp. LCS:Exp y = x y = x Cycles to Failure 1.2 Figure 5 SN curve for various carbon steel alloys 75 % of Fracture Cycles of Original Fatigue Test 50 % of Fracture Cycles of Original Fatigue Test Number of Cycles Figure 6 Comparison Fatigue effect on Yield Strength for HCS editor@iaeme.com

8 % Reduction in Area\%Reduction in Original Area % Elongation / % Original Elongation Tensile Strength/ Original Tensile Strength Qasim Bader % of Fracture Cycles of Original Fatigue Test 50 % of Fracture Cycles of Original Fatigue Test Number of Cycles Figure 7 Fatigue Effect on percentage of Tensile Strength for HCS % of Fracture Cycles of Original Fatigue Test 50 % of Fracture Cycles of Original Fatigue Test Number of Cycles Figure 8 Fatigue effect on percentage of elongation for HCS % of Fracture Cycles of Original Fatigue Test 50 % of Fracture Cycles of Original Fatigue Test Number of Cycles Figure 9 Fatigue effect on reduction in area for HCS editor@iaeme.com

9 Brinell Hardness/ Original Brinell Hadness Study the Influence of Reversed Bending Fatigue Test on the Mechanical Properties of Carbon Steel Alloys % of Fracture Cycles of Original Fatigue Test 50 % of Fracture Cycles of Original Fatigue Test Number of Cycles Figure 10 Fatigue effect on Brinell hardness for HCS Figure 11 SEM micrograph of different region of fracture surface of AISI editor@iaeme.com

10 Qasim Bader Mat. LCS MCS HCS Table 7 Exp. and Num. fatigue limit for different carbon steel alloys SN Equation Fatigue limit (MPa) Exp. Num. Exp. Num. σl = Nf σl = Nf σl = Nf σl = Nf σl = Nf σl = Nf CONCLUSIONS From the results obtained it is concluded that, tensile property is more affected with fatigue test in comparison to the other mechanical parameters and for a large number of steels, there is a direct correlation between tensile strength and fatigue strength; highertensilestrength steels have higher endurance limits. Fatigue ratio which is representing ratio of endurance limit to tensile strength remains same regardless the weight of carbon presented in material. REFERENCES [1] ASM Handbook,"Properties and Selection: Irons, Steels, and HighPerformance Alloys", ASM Handbook Committee, Volume 1, p , [2] Juli A. Bannantine, Jess J. Comer, James, L. Handrok, Fundamentals of Metal Fatigue Analysis, prentice hall, Englewood Cliffs, New Jersey 07632,1989. [3] ASM Handbook, Fatigue Failures, Failure Analysis and Prevention, Vol. 11, ASM International, [4] Jaap Schijve, "Fatigue of Structures and Materials ", Springer, Second Edition, [5] J.C PANGS, et.al," Relations between fatigue strength and other mechanical properties of metallic materials", Shenyang National Laboratory for Material Science, Chines Academy of Science, [6] Robert C. O'Brien," Impact and Fatigue Characterization of Selected Ferrous P/M Materials", Hoeganaes Corporation, Riverton, New Jersey 08077, [7] John E. Bringas," Handbook of Comparative world Steel Standards", ASTM data series; DS67B, 3rd edition, [8] Qasim Bader & Emad K. Njim, Effect of V notch shape on Fatigue Life in Steel beam made of Mild Steel AISI 1020, International Journal of Mechanical and Production Engineering Research and Development (IJMPERD),ISSN(P): ; ISSN(E): ,Vol. 4, Issue 4, [9] N.A. Alang, et.al, Effect of Surface Roughness on Fatigue Life of Notched Carbon Steel, International Journal of Engineering & Technology IJETIJENS Vol.: 11 No: 01, [10] QASIM BADER & EMAD K. NJIM "Effect of V notch shape on Fatigue Life in Steel beam made of Mild Steel AISI 1020 ", International Journal of Mechanical and Production Engineering Research and Development (IJMPERD),ISSN(P): ; ISSN(E): ,Vol. 4, Issue 4, [11] Kadhim Naief Kadhim and Ahmed H. ( Experimental Study Of Magnetization Effect On [12] Ground Water Properties).Jordan Journal of Civil Engineering, Volume 12, No. 2, 2018 [13] Khalil Farhangdoost, Ehsan Homaei," Influence of rotating bending fatigue test on the mechanical parameters of standard specimens", Material Since and Engineering, editor@iaeme.com