The micromechanism of fatigue damage in an interstitial-free (IF) steel sheet has been studied using fully reversed stress amplitudes (Deltasigma/2). The stress-life (S-N) curve of the steel sheet has been generated, together with a series of interrupted fatigue tests at each of the chosen Deltasigma/2, to study the progress of fatigue damage in terms of the initiation, growth, and coalescence of the fatigue cracks on the surfaces of the sheet specimens using scanning electron microscopy. The steel sheet possesses a higher endurance limit (0.98 times its yield strength (YS)), as compared to conventional low-carbon steel sheets. This is attributed to (1) the occurrence of nonpropagating microcracks initiating primarily at the inclusions below the endurance limit and (2) a significant delay in the spread of plastic deformation, until Deltasigma/2 is close to YS. Above the endurance limit, widespread plastic deformation through slip bands promotes the formation of fatigue cracks at the ferrite grain boundaries and occasionally within a ferrite grain body, as well as at inclusions. Fatigue failure is preceded by the significant growth of grain-boundary cracks over and above those at inclusions and the ferrite grain body. A series of grain-boundary cracks link up to form mesocracks, one of which grows to cause the final failure. The predominance of grain-boundary cracks in the process of fatigue failure is attributed to the lesser cohesive strength of the grain boundaries caused by the depletion of interstitials. DOI: 10.1007/s11661-008-9537-y

(c) The Minerals, Metals & Materials Society and ASM International 2008

(ProQuest: ... denotes formulae omitted.)

I. INTRODUCTION

INTERSTITIAL-FREE (IF) steel sheets are extensively used to meet the severe cold-forming requirements of automotive industries. Detailed reports[1,2] exist that are related to the processing routes and properties, especially formability, etc., on these steel sheets. These sheets are primarily used in automobile structural components, which experience cyclic loading in service; hence, knowledge related to the fatigue properties of these materials is important. The number of research reports on the fatigue behavior of IF steel sheets, however, is limited. Earlier works indicate that efforts are primarily aimed at the estimation of fatigue life,[3] with insignificant emphasis on the understanding of the micromechanism of fatigue damage, in terms of the initiation and growth of fatigue cracks at various microstructural locations. Microstructures play a decisive role in the process of fatigue damage, as the latter is primarily governed by the local stress conditions at and around its various features.[4] Hence, an understanding of the micromechanism of fatigue in an extremely clean ultra-low-carbon ferritic microstructure, in which grain boundaries are depleted of interstitials (such as C, N, etc.), presents an interesting problem from the viewpoint of material science. Studies of this nature can sequentially elucidate the preferred location for the initiation of microcracks, the nature of the growth of these microcracks, and, thus, the weak links in the microstructure that provide the low-energy path in which the cracks prefer to grow. The IF steels are important commercial materials and a systematic study of the micromechanism can provide greater insight into both the alloy design and the consequent engineering applications in steels with a similar microstructure. This report is centered on addressing this issue.

The micromechanism of fatigue damage in structural components involves several sequential stages, e.g., (1) the initiation of microcracks, (2) the growth and coalescence of microcracks to form a dominant macrocrack, and (3) the propagation of the macrocrack to cause complete failure. In addition, fatigue damage is commonly associated with the formation of slip bands on the specimen surface, which are known to govern the mechanism of crack initiation and growth.[4] In a flawfree, homogeneous material, a significant fraction of the total lifetime is spent before the first detectable microcrack appears.[5,6] The latter is defined as a discontinuity, all the dimensions of which are small in comparison to the characteristic microstructural dimensions, e.g., grain size, etc.[7] Earlier experimental observations using optical and electron microscopy suggest that, in homogeneous materials, such cracks generally initiate at free surfaces.[5] Therefore, the preferred and most widely used technique for studying fatigue crack initiation is the promotion of the natural initiation of microstructurally small surface cracks in smooth specimens.[7] This method ensures the unrestrained initiation of the microcrack at a location determined entirely by the crack itself. Following initiation, the cracks do not grow equally; thus, it is important to know which of the initiated cracks grow faster than the others and can be considered to dominate the process of fatigue failure.

The design of automotive components fabricated from IF steel sheets (e.g., roof panels, door panels, etc.) is commonly stress based. Hence, from the point of view of the application, it is useful to study the process of fatigue damage with respect to applied stress. During some preliminary studies, it emerged that the nature of microcrack initiation in steel sheets is predominantly governed by the imposed stress amplitude and that its location is significantly different for stress levels above and below the endurance limit. Hence, the primary interest in the present investigation is to reveal the micromechanism of fatigue damage on the surfaces of an IF steel sheet, both above and below the endurance limit. In this study, first, stress-controlled fatigue tests have been carried out using smooth specimens, to estimate the fatigue life at various stress amplitudes. Next, a series of tests were carried out that were interrupted at various stages of fatigue life for a given stress amplitude. The surfaces of these specimens have been studied using scanning electron microscopy, in order to detect the initiation of the microcracks, examine their growth, and study their coalescence behavior as it applies to the formation of macrocracks. The results generated by the study illustrate the progression of fatigue damage at various levels of applied stress and with an increasing number of loading cycles. The prevailing mechanism of the initiation and growth of fatigue cracks and the effect of these cracks on fatigue life are discussed.

II. EXPERIMENTAL DETAILS

The experiments carried out to achieve the goals of this investigation are the following: (1) characterization of the investigated steel sheet in terms of cleanliness, microstructure, and tensile properties; (2) fatigue testing of the sheet specimens; and (3) examination of specimen surfaces after a controlled amount of fatigue damage. The relevant details of the experimental procedures are presented in this section.

A. Characterization of the Investigated Steel

The material selected for this investigation is a 1.25-mm-thick industrially processed Ti-stabilized IF steel sheet. The as- received steel sheet is known to be cold rolled from an initial thickness of 3.2 mm, batch annealed at 680 [degrees]C to 700 [degrees]C, and temper rolled. The sample was obtained as rectangular pieces approximately 960 x 600 mm in dimension, marked with rolling directions, supplied courtesy of Tata Steel, Ltd. (Jamshedpur, India). The composition of the investigated steel is shown in Table I. The standard metallographic technique of grinding and polishing (up to a 0.25-[mu]m diamond finish) was employed, to obtain specimens for the characterization of nonmetallic inclusions in terms of their volume fraction, average size, and chemical nature. The volume fraction of the nonmetallic inclusions in the investigated steel was determined using the Japanese standard JIS G0555.[8] The inclusion size was estimated using a field-emission gun-scanning electron microscope (FEG-SEM) and the annotation toolbar available in the software associated with the microscope. Energy-dispersive X-ray (EDX) analyses were done at the center of the observed inclusions, using the SEM to ascertain their chemical nature. Specimens for examining the microstructure on the surface and at the transverse sections of the sheet were polished and chemically etched using Marshall's reagent.191 The average grain size was determined using the lineal intercept method, as suggested in the ASTM Standard E-1 1296,[10] using an image analyzer that considered 100 fields at a magnification of 200 times. The microhardness of the steel was estimated using a Vickers indenter at a 245.2-mN load for an indentation duration of 30 seconds. The tensile properties of the investigated steel sheet were determined using 6-mm-wide specimens that had a 25-mm gage length and that were fabricated in line with the ASTM Standard E8M-04.[1,] The longitudinal axes of the specimens corresponded to the rolling direction of the sheet. Tensile tests were carried out with the help of a 100-kN-capacity screw-driven tensile-testing machine at a crosshead speed of 5 mm/min, which corresponds to a nominal strain rate of 3.33 x 10^sup -3^ s^sup -1^. B. Fatigue Test

The specimens for fatigue tests were fabricated from the selected steel sheet; the longitudinal axes were kept parallel to the rolling direction of the sheet. A configuration typical of a specimen is shown in Figure 1. The fatigue tests were carried out in laboratory air at room temperature ([asymptotically =]300 K), using a 100-kN servohydraulic fatigue testing machine at R = -1, at a frequency of 10 Hz, using sinusoidal waveform. During these tests, care was taken to ensure that (1) the specimen was properly aligned in the grips and (2) there was no bending or buckling of the sheet specimen during specimen fixing and its testing; a commercial antibuckling fixture was used. Two different types of stresscontrolled fatigue tests were carried out to (1) generate the stress-life (S-N) curve and (2) study the initiation and growth of fatigue cracks on the surfaces of the sheet specimens intermittently at various stages of fatigue life, at each of the selected stress amplitudes. The applied stress amplitude for the tests was selected between 0.5 and 1.3 times the yield strength (YS) of the steel. The rationale for carrying out stress-controlled fatigue tests above the YS is that, in the case of IF steel sheets, it is neither possible to initiate a sufficient amount of fatigue damage nor to achieve a systematic understanding of the progression of fatigue damage below the YS in an optimum time span.

The surfaces of the specimens were polished, using a standard metallographic technique with up to a 0.25micron diamond paste, prior to their testing, for the generation of the S-N curve. The stress-controlled fatigue tests were carried out to record the number of cycles to failure (N^sub f^) at a particular value of the applied stress amplitude. The S-N curve was constructed by plotting the applied stress amplitude (...) against the corresponding number of cycles to failure (N^sub f^) at that Deltasigma/2. In this investigation, failure is defined as the complete fracture of the specimens. For the interrupted fatigue tests, the surfaces of the specimens were polished and etched with Marshall's reagent, to reveal the ferrite grain boundaries prior to fatigue testing. In these experiments, tests were interrupted at various stages of fatigue life, e.g., at 1, 2, 5, 10, 20, 50, 70, 90, and 95 pet of the average N^sub f^ for each of the chosen Deltasigma/2. The surface of a specimen at the varied interruptions was examined using an SEM.

C. Scanning Electron Microscopy Study

The primary aim of the scanning electron microscopy study was to detect the microcracks and examine the nature of their growth (both in number and in size) and their coalescence behavior at various stages of fatigue ufe, for all the selected stress amplitudes (Deltasigma/2). Accordingly, the surfaces of the specimens, after the interrupted fatigue tests, were studied using an SEM, to characterize the fatigue cracks in terms of their (1) location in the microstructure, (2) number density, and (3) size. For estimating the number density of microcracks in a particular specimen, a surface area approximately 30 x 40 mm^sup 2^ in the center of the specimen was studied, using the FEG-SEM at a constant magnification of 2000 times. The total number of fields examined (including those that both did and did not exhibit cracks), the field size (in [mu]m^sup 2^), and the number and locations of microcracks were recorded. The number density was then calculated as the average number of microcracks per unit area (mm^sup 2^) recorded at different microstructural locations. For estimating the average crack size, at least 30 representative microcracks initiating at different microstructural locations in each specimen were taken into consideration. The maximum dimensions of the observed cracks were measured using the measurement toolbar available with the FEG-SEM software. The data generated on the number density and average size of the fatigue cracks were plotted against the number of imposed loading cycles for a particular value of Deltasigma/2. This was done to examine the nature of the initiation and growth of cracks at various microstructural locations. Representative photographs were recorded during the scanning electron microscopy examinations, in order to understand the mechanisms of the initiation, subsequent growth, and coalescence of the microcracks. In addition to these studies, fractographic examinations were also carried out on the failed specimens; in these examinations, a SEM was used to determine the predominant mode of fatigue fracture in the steel sheet.

III. RESULTS

A. Investigated Material

The chemical composition of the investigated IF steel sheet is shown in Table I. The volume fraction of inclusions in the sheet is 0.04 pct. A histogram showing the size distribution of the inclusions is shown in Figure 2. The mean and the maximum sizes of the observed inclusions estimated from 100 readings are 2.5 and 7.0 [mu]m, respectively; the majority (> 80 pct) of the inclusions, however, are found to be below 3.3 [mu]m in size. A few typical inclusions and their microanalyses are shown in Figure 3. The microanalyses indicate that the inclusions are rich in Fe, Ti, Al, Mg, S, N, and oxygen. The most frequently occurring inclusions are titanium nitride that is 1 to 3 [mu]m in size and shaped like a rhomboid (Figure 3(a)). In addition to TiN, mixed nitrides (Figure 3(b)), globular oxides (Figure 3(c)), a mixture of oxides and sulfides (Figure 3(d)), silicates (Figure 3(e)), and sulfides (Figure 3(0) are also found. A representative microstructure of the steel is shown in Figure 4. The microstructure exhibits equiaxed ferrite on the surface as well as at the cross sections; at both locations, the equiaxed ferrite displays similar values of grain size (12.5 and 13.5 [mu]m, respectively) but marginally varying values of microhardness (110 and 86.7 HV, respectively). The estimated values of the tensile properties are shown in Table II. The steel sheet does not exhibit yield point phenomenon.

B. S-N Curve

The estimated S-N curve (for the investigated IF steel sheet) is shown in Figure 5. It is observed that, as the applied stress amplitude is increased (from 0.5 to 1.3 YS), the number of loading cycles required to cause failure decreases continuously. The curve exhibits a knee at 0.98 YS (-140 MPa), below which there is no failure until 7 x 10^sup 6^ cycles. The endurance limit has been defined here as the maximum stress at which no failure occurs until 7 x 10^sup 6^ cycles. The estimated fatigue life of each specimen, the average value at each stress level, and their associated coefficient of variation (CEV) are presented in Table III, for easy reference. Although a smooth curve could be drawn through the data points in Figure 5, the results in Table III indicate that there is considerable scatter associated with the data on the number of cycles to failure (N^sub f^), for specimens tested at identical stress amplitudes. The magnitude of scatter (i.e., the CEV) tends to decrease with an increase in the stress amplitude.

C. Fatigue-Crack Initiation

The interrupted fatigue tests were conducted at each of the selected stress amplitudes, to study the progression of fatigue damage on the surface of the sheet specimens. The initiation of microcracks is considered to be the first stage in the process of fatigue damage. Representative specimens were subjected to interrupted tests for examinations under a SEM (1) to detect the formation of the microcracks and (2) to determine the number of cycles required for their initiation at a given stress amplitude. There are three preferential locations for the initiation of microcracks in IF steel sheets, depending upon the applied stress amplitudes and the number of loading cycles. These locations are (1) nonmetallic inclusions, (2) the grain boundary, and (3) the grain body. Some typical microcracks initiated at these locations are shown in Figures 6 through 8.