INVESTIGATION ON FATIGUE BEHAVIOR AND FATIGUE CRACK GROWTH OF SPRING STEEL. PART I. WÖHLER CURVE AND FRACTURE SURFACES

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1 Journal Lyubov of Chemical Nikolova, Technology Rozina Yordanova, and Metallurgy, Donka 49, Angelova 1, 2014, INVESTIGATION ON FATIGUE BEHAVIOR AND FATIGUE CRACK GROWTH OF SPRING STEEL. PART I. WÖHLER CURVE AND FRACTURE SURFACES Lyubov Nikolova, Rozina Yordanova, Donka Angelova Department of Physical Metallurgy and Thermal Equipment University of Chemical Technology and Metallurgy 8 Kl. Ohridski, 1756 Sofia, Bulgaria r.yordanova@uctm.edu Received 30 July 2013 Accepted 15 November 2013 ABSTRACT Fatigue behavior of spring steel at different stress ranges was investigated. The specimens were machined in hour-glass shape with polished surface and were subjected to symmetric cyclic rotating-bending fatigue in air and room temperature to fracture. Fatigue life time data was obtained and Wöhler curve of the investigated steel at different conditions is plotted. A mathematical description of the obtained results is presented. The fatigue test results are filled out with images and analysis of the fracture surface made with scanning electron microscope. A replication technique is used for short fatiguecrack growth monitoring for two specimens. The experimental data are presented in plots Crack lengths, a cycles, N. Keywords: rotating-bending fatigue, Wöhler curve, fractography, crack growth; crack path. INTRODUCTION The materials applied for springs are extended to metallic and nonmetallic types, in addition, among the metals there are many types, such as spring steel, stainless steel, nickel alloy, etc. Their required properties vary accordingly. However, whatever the applications, it is certain that a high stress during cyclic loading and prolonged reliability should be required. The fatigue strength is affected by many factors, such as material, shape of final components, stress and atmosphere. Generally fatigue failure occurs by propagation of subcritical cracks ranged from several microns to a few thousand microns [1]. EXPERIMENTAL Material and specimens. For this study spring steel was used. The chemical composition of the steel contains 0,819 % mass Carbon i.e hypereutectoid steel. The specimens were machined in hour-glass shape as shown on Fig. 1. Testing and equipment. 12 hourglass shaped specimens were subjected to symmetric cyclic rotatingbending fatigue (RBF) to fracture at different stress ranges (Δσ=800, 1000, 1200, 1400 and 1500 MPa) and at the following testing conditions: R=-1, f=11 Hz, air environment and room temperature. Tests were performed on a table model Fatigue Rotating Bending Machine, FATROBEM-2004, designed and assembled in Fracture and Fatigue Laboratory in UCTM Sofia (Fig. 2). Scanning electron microscope (SEM) was used for microstructure and fatigue fracture surface observations. Crack Growth Monitoring Method. A replication technique was used for short fatigue-crack growth monitoring. This method consists of making a replica (stamp) of the specimen s surface and subsequent monitoring of crack length and its propagation. Replicas are taken into two mutually opposite surfaces of the specimen, which ensures full coverage of the cylindrical surface of the working area of the specimen. At the beginning the making of replicas and their examination is in a certain number of cycles until the discovering of the crack, after that the interval is reduced significantly, which will ensure better monitoring of growth crack. To obtain a 23

2 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 RESULTS AND DISCUSSION All 12 specimens were tested under Rotating- Bending Fatigue to fracture. The Wöhler curve was obtained by the well-known standard procedure and presented in Fig. 3. Data used for plotting the Wöhler curve is presented in Table 1. Mathematical description of the experimental data was done and according to it, the analytical form of the curve was found as shown on Fig. 3 and equation (1). Fig. 1. Geometry of the specimens. Fig. 2. Presentation of fatigue machine electric engine 1, driving belt 2, ball-bearing unit 3, testing box 4, specimen 5, device for loading 6, counter 7, device for circulation and aeration of corrosion agent 8. replica it is used synthetic acetate film type Agar scientific 0.35 mm thick. After the experiment all replicas with cracks are subjected to closer monitoring and then it is measured the crack lengths at the corresponding number of cycles. Measurement of cracks is done using a microscope by a scale located in the eyepiece [2, 3]. Δσ= 6171,8 N f -0,1465 (1) The coefficients in equation (1) were calculated using the experimental data and applying the method of least squares. On Fig. 3 are also shown fractographic images of some of the tested specimens showing specific areas from the fractured surfaces as crack initiation zone, crack propagation zone and zone of final fracture. Specimens tested at stress range Δσ=1500 MPa (specimens 10 and 12) show several crack initiation zones which corresponds to the shorter lifetime of these specimens [4, 5]. Table 2 presents the number of cycles to failure and the stress ranges of the tested specimens for better understanding of Fig. 3. In Fig. 4 some crack paths on one of the fractured surfaces of specimen 7 are presented. By performing closer observation and analysis of the shown fractured surface a few crack initiation zones were found and 3 of them are chosen and indicated by black dash-line ovals; the magnified images of the indicated 3 zones are shown separately in the black boxes in the same figure. It can be observed that in the earlier stage of the fatigue process the three cracks originated from different spots and propagated independently from each other. In the further stages of fatigue development the cracks interacted and Table 1. Data number of cycles to failure, N f - stress range, Δσ. Specimen Δσ, MPa N f, cycles Specimen Δσ, MPa N f, cycles

3 Lyubov Nikolova, Rozina Yordanova, Donka Angelova Fig. 3. Wöhler curve and fracture surfaces corresponding to the experimental stress range points. 25

4 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 Fig. 4. Fractographic analyses of specimen 7. this interaction caused exhaustion of local plasticity and crack merging. This process gradually involved other cracks and finally a was formed leading to the final fracture of the specimen. That interaction of the cracks on the fractured surface can be also indirectly conformed by the results from replica monitoring of the specimen surface fatigue crack appearance and growth. In this work the replicas from 2 experiments (speci- Table 2. Number of cycles and stress ranges of specimens shown on Fig. 3. Image Specimen Δσ, N f, Fractographic characterization MPa cycles Image Crack initiation zone Image Crack initiation zone/crack propagation zone Image Zone of final failure Image Crack initiation zone/crack propagation zone Image Zone of final failure Image Microcracks zone of final failure Image Fracture surface Image MnS inclusion at crack initiation zone Image Crack initiation zone Image Microcracks zone of final failure Image Crack initiation zone Image Crack initiation zone Image Crack propagation zone/zone of final failure Image Crack initiation zone 26

5 Lyubov Nikolova, Rozina Yordanova, Donka Angelova Table 3. Main s crack length for specimen 7. Specimen 7 a, [μm] N, [cycles] а, [μm] N, [cycles] a, [μm] N, [cycles Table 4. Main s crack length for specimen 9. Specimen 9 a, [μm] N, [cycles] а, [μm] N, [cycles] a, [μm] N, [cycles] a, [μm] N, [cycles] mens 7 and 9) were monitored using a metallographic microscope at magnification 250x and the cracks lengths were measured. Data obtained from monitoring the replicas and the measured cracks length (Tables 3 and 4) are plotted as Crack length, a -Numbers of cycles, N and shown in Figs. 5 and 6 for specimens 7 and 9, respectively. In both cases the initiates first. Fig. 7 presents the comparison between the crack growth in the two specimens. It is seen that the of the specimen tested at higher stress range initiates in an earlier stage in comparison with the initiation stages of the all the other cracks; the and all the other cracks on the surface of the other specimen show smaller difference at the number of cycles to initiation and a kind of competition between the cracks for turning into the main specimen crack. Crack length a, [μm] Fig. 5. Plots Crack length, a Numbers of cycles, N for specimen 7. Crack length a, [μm] CONCLUSIONS Δσ=1200 MPa Number of cycles N, [cycles] Fig. 6. Plots Crack length, a Numbers of cycles, N for specimen 9. Crack length a, [μm] Δσ=1400 MPa Number of cycles N, [cycles] Δσ=1400 MPa Δσ=1200 MPa Number of cycles N, [cycles] Fig. 7. Comparison of plots Crack length, a Numbers of cycles, N for specimens 7 and 9 tested under RBF. Fatigue behavior of spring steel was investigated under symmetric cyclic rotating-bending fatigue at room temperature and in-air conditions. The Wöhler curve of the steel was obtained by conducting 12 experiments using the well-known standard 27

6 Journal of Chemical Technology and Metallurgy, 49, 1, 2014 procedure. The curve is illustrated by the fractographic images of some of the tested specimens, which show specific areas from the fractured surfaces. Specimens tested at higher stress range show several crack initiation zones (specimens 7, 10 and 12) which corresponds to the smaller crack propagation zones and the shorter lifetime of these specimens. Mathematical processing of the experimental data was done and accordingly the analytical form of the curve was found. By performing deeper analyses on the fracture surface of specimen 7 a few crack initiation zones were found. It is observed that in the earlier stage of the fatigue process the three cracks originated from different spots and propagated independently from each other. In the further stages of fatigue development the cracks interacted and this interaction caused exhaustion of local plasticity and crack merging. This process gradually involved other cracks and finally a was formed leading to the final fracture of the specimen. That interaction of the cracks on the fractured surface was also indirectly conformed by the results from replica monitoring of the specimen surface fatigue crack appearance and growth. Replication technique was used for crack growth monitoring. Data obtained from measuring cracks lengths on replicas for two specimens are presented together with the corresponding numbers of cycles in plots Crack length, a -Numbers of cycles, N. In both cases the initiates first. From the graphic illustrating the comparison between both those testes it can be concluded that the main surface crack in the specimen tested at higher stress range initiates in an earlier stage than the other cracks and is well separated from them. At the same time the in the other specimen propagates at condition of interaction with all the other cracks at no big difference between their initiation stages. Acknowledgements This study was supported by University of Chemical Technology and Metallurgy (UCTM), Sofia, Bulgaria, and the Scientific Research Center at UCTM. REFERENCES 1. S. Suresh, Fatigue of Materials, Cambridge Univ. Press, Cambridge, UK, K.J. Miller, Metal Fatigue Past, Current and Future, Proc. Inst. Mech. Engrs, London, R. Yordanova, Modeling of fracture process in lowcarbon 09Mn2 steel on the bases of short fatigue crack growth experiments. Comparative analyses on the fatigue behavior of other steels, PhD Thesis, University of Chemical Technology and Metallurgy, Sofia, Bulgaria, 2003, (in Bulgarian). 4. L. Nikolova, R.Yordanova, Sv. Yankova, Wöhler curve and fatigue characteristics of spring steel, Engineering Sciences, L, 2, 2013, L. Nikolova, R. Yordanova, Z. Todorova, D. Angelova, Z. Naydenova, B. Yordanov, Influence of the structure of steels on the fatigue process, Scientific Proceedings, year XX, number 1 (133),p 74-78, june 2012, (in Bulgarian). 28