Research Article | Open Access
Fatigue Crack Behavior of Stainless Steel 304 by the Addition of Carbon Nanotubes
Fatigue is the main source of almost half of whole mechanical failures. This research investigated the effect on cyclic fatigue behavior of stainless steel 304 (SS304) when including carbon nanotubes (CNTs) at the crack tip. The cyclic fatigue tests were conducted on compact tension (CT) specimens to establish the relationship between crack growth and the number of cycles (a-N). It is found that the incorporation of a small amount of CNTs increased the fatigue life of the SS304/metal. Micrographs showed that the enhancement in fatigue life is caused by CNTs dense arrangement around the crack tip, entangled with each other, and finer grain size. Smooth bonding at the interface of the CNTs and SS304 grains is also observed.
Fatigue is progressive and localized structural damage that occurs when a material is subjected to cyclic loading and is one of the primary causes for catastrophic failure in structural materials. Enhancement of fatigue life is desired for improving utility of the materials.
Researchers have studied the consequences of fractal phenomena of metal material. They described that the affected fatigue life is divided into the following three factors:  (i) material and structure-related factors, such as (a) type of dislocation slip, (b) grain size, (c) dislocation density and distribution, (d) texture, and (e) residual stresses; (ii) loading factors: (a) stress ratio, (b) mean stress, (c) frequency, (d) temperature, and (e) environment; and (iii) designing factors: (a) specimen shapes and dimensions (b) surface conditions, like finishing and hardening or softening treatment of the surface layer.
Cracks can be arrested and blunt by the following techniques: (a) applying compressive residual stress on the crack tip [2–6], (b) grain refinements [3, 4, 6, 7], (c) changing the shape of the crack tip [2, 8, 9], and (d) reorientation of the crack-growth direction [2, 10, 11]. These approaches have been achieved by using various methodologies and techniques, that is, (a) laser shock processing (LSP), a cold-worked process, and shot peening (SP); (b) severe plastic deformation (SPD); (c) welding and electromagnetic heat; and (d) pinhole, dimples, drilling stop holes, and inserting pins. These techniques have some application limitations such as a specimen size, arrangement of instruments, and residual stress after treatment like welding. In this work, these limitations have been considered, thereby yielding a seamless application with no defects after treatment.
Because of the outstanding mechanical and physical properties and large specific surface area of carbon nanotubes (CNTs), CNT-based composites are expected to show significantly improved mechanical performance, when compared with the neat matrix material . Recently, CNTs have been used to arrest the crack propagation of polymer composites [13–21]. The crack-bridging and pullout causes the fatigue crack growth suppression in composites and has been reported as the main mechanism of crack arresting [17–21]. It has been also observed that changing the physical dimension of CNTs and improving the dispersion could enhance fatigue life significantly.
Stainless steel 304 (Cr18Ni8) is one of the most widely used and oldest stainless steels. It was originally called 18-8, which stood for its chromium and nickel contents . It is mainly used in the nuclear industry and in the marine environment because it is slightly higher in strength and wear resistance than the type 316. Typical applications include tanks and containers for a large variety of liquids and solids.
Ren et al.  reported the effect laser shock processing (LSP) on 00Cr12 alloy’s mechanical properties at high temperatures. The results suggested that LSP has a significant effect on enhancing fatigue by applying the compressive residual stress. At high temperatures, the plasticity of alloys enhanced and formed the stable dislocations. It is relatively effective to enhance the mechanical properties. In another work by Ren et al., LSP was used to enhance the fatigue behavior of the structure . They observed LSP at different laser intensities which had an obvious influence on fatigue crack initiation and the growth of the hole crack.
Zhang et al.  described an order of magnitude diminution in the fatigue crack growth rates for an epoxy system with the addition of a small amount of CNTs. They observed that the crack suppression was made by pullout CNTs that bridge the crack interface. This group of researchers also studied the effect of CNT dimensions on fatigue crack growth suppression in epoxy composites in another study . They discovered that fatigue crack growth rates can be significantly reduced by reducing the diameter and increasing the length of the CNTs and improving the nanotube dispersion.
In this work, we enhanced the fatigue life of stainless steel 304 (SS304) by the addition of small amounts of CNTs on localized structure. To the best of our knowledge, there are no studies reporting the effect of CNTs on the fatigue life of metals (steel). Fatigue tests were conducted using compact tension (CT) specimens with and without multiwalled carbon nanotubes (MWCNTs). The fracture surfaces of the treated and untreated SS304 were examined by using scanning electron microscopy. It observed that the addition of CNTs has significant effect on fatigue crack growth of SS304.
In this work, the SS304 is used in plate form with a chemical composition in weight of 18% Cr, 10.5% Ni, 0.08% C, 1.0% Si, 2.0% Mn, 0.045% P, 0.03% S and balance Fe as received from Kunshan DKC Co., Ltd. Other mechanical properties of the as-received steel plate at room temperature were: ultimate tensile strength of 520 N/mm2, yield strength of 205 N/mm2 and 40% elongation, respectively. The chemical compositions and important parameters of as received MWCNTs from Xuzhou Jiechuang New Material Technology Co., Ltd. are listed in Table 1.
2.2. Sample Preparation
The specimens for the CT fatigue test were prepared from an annealed sheet of SS304 in accordance with ASTM E647-12 standards. A notch was made by using a computer numerical control (CNC) cutting machine. A CNC drilling machine was used to drill two holes in the specimen. A precrack needed to be created in the specimens. A crack was made by using a Mitsubishi high performance wire EDM systems. The crack had a length of 4 mm and a width of 0.012 mm. The dimensions of the specimen are shown in Figure 1.
CO2 laser was used to diffuse the MWCNTs on the substrate material’s crack tip. The laser beam was focused perpendicularly on the sample surface using a planoconvex lens ( mm) with the spot diameter of 2 mm and an exposure time of ~1 seconds. This configuration gives a laser power of 1000 W at the sample surface. The expected temperature of 1000°C on the sample surface was considerably less than the melting point 1399°C of SS304. The laser parameters are tabulated in Table 2. The specimens were treated on both sides with the same conditions, as presented in Figure 2.
2.3. Measurement and Methods
Fatigue tests were conducted at room temperature on a servohydraulic test machine (MTS Model 810). CT specimens were prepared for fatigue test. The specimens were fatigue-tested under cyclic load and the stress ratio, defined as the ratio of minimum stress to maximum stress, was set to . The shape of the loading waveform was sinusoidal and the frequency used was 20 Hz. The testing conditions used in the experiment are presented in Table 3. The number of cycles to failure (N) and crack growth (a) were recorded for each specimen. Fracture surfaces of the fatigue tested samples were polished and etched then observed using scanning electron microscope (SEM). The Energy Dispersive X-Ray Spectroscopy (EDS) defines the chemical composition after addition of the MWCNTs.
3. Results and Discussion
3.1. Fatigue Crack Growth
The crack propagation behavior of the MWCNTs for treated and untreated specimens at same load condition shows in Figure 3. For treated MWCNTs specimens, cracks grow slower than untreated specimens. The small addition of MWCNTs shows a significantly larger improvement in fatigue life. The fatigue test showed the median fatigue life of the treated specimen improved 40% over the untreated. These are significant improvements in the fatigue life of SS304 and represent an opportunity to utilize SS304 in high-cycle fatigue applications by adding small fractions of CNTs. It has been reported that CNTs can suppress the failure in material . The filler scale also has effects on crack propagation. The filler in nanosize may encounter crack propagation more than the filler in microsize. However, SEM characterization confirms fatigue enhancement mechanisms, as will be discussed later.
The larger effect that the addition of CNTs has on fatigue life over neat SS304 may be explained by first considering the observed failure mechanisms in specimens subjected to cyclic loading. The neat SS304 had significant and extensive damage after a few cycles at cyclic stress amplitudes. With continued cycling, cracking began to propagate rapidly. The treated CNT specimen had limited damage at same stress levels. With continued cycling, cracking propagated slowly.
The effectiveness of treated CNTs specimens at the same cyclic stress level relative to neat SS304 can be explained by the fatigue failure mechanism described above. Damage propagation occurs, with untreated SS304 at a high stress level and at a rapid rate. Under these conditions, there are no obstacles or, in this case, no CNTs in the path of the damage that could slow damage propagation. However, the damage propagation of treated SS304 at the same stress level is slower and at a widely spaced crack front. This can be slowed relatively effectively since a larger fraction of the strain energy must be dissipated in overcoming the obstacles. Thus, the addition of CNTs is more likely to be effective in improving fatigue life.
3.2. Microstructure Morphology
Figure 4 shows the SEM micrographs of the fracture surfaces of the two specimens after polishing and etching with acid. The MWCNTs containing a specimen show MWCNTs entangled with each other. It also shows random distribution of MWCNTs (see in Figure 4(b)). The entanglement of MWCNTs and high dens dispersion ensure the high strength and resist the crack growth.
The small amount of the MWCNTs was added at the SS304 crack tip of SS304 by using a laser as a heating medium. The crack tip is chilled by conduction through the metal around, after laser treatment. The metal at the crack tip is refined in the process of crystallization which creates a smooth bonding with MWCNTs. The interface of the grain boundary around the crack tip was enhanced by refining the metal structure. The alloy plasticity is enhanced at high temperatures, making dislocation slip easier . It increases the interface resistance of the crack extension and work of the crack extension as well to prevent the crack propagation .
The addition of the MWCNTs also results in change in metal fatigue properties. The distribution of MWCNTs across the whole cross-sectional area is uniform as shown in Figure 5. The dispersions of the MWCNTs beside the crack is more than the other areas are shown by an arrow. The substrate interface shows the strong and smooth bonding of MWCNTs with the substrate. It observed that the bottom part of MWCNTs grains diffused and formed a bond. The high dispersion and fine grains of MWCNT is a corroboration to enhance the fatigue life of the component.
Figure 6 presents the high dense dispersion of the CNTs on the application surface. The MWCNTs shaped like half tubes are shown by arrows in Figure 6. The composition of the treated area was measured by EDX yielding a 51.73% C—22.03% O—0.29% Si—1.1% Mn—3.05% Cr—19.32% Fe and 2.48% Ni. This indicates that the CNTs were oxidized and solidified in half-tube shape as shown in Figure 6(c). It can also be seen in the Figure 6 that CNTs entangled each other and made stable dislocations. The arrows point to the nanotubes reshaped in half-tube structures. The high dispersion of the CNTs, high dislocation density and stable dislocations, and fine grain boundaries are the main reasons that the SS304 resisted crack growth and had enhanced fatigue life [20, 21, 23].
Suppression of the crack propagation in SS304 by the addition of MWCNT was investigated. It is observed that the addition of small amounts of MWCNTs significantly improves the fatigue performance of SS304. We report an increase in fatigue life of SS304 in the high cycle. Surface structures of the treated area showed that the enhancement in fatigue life is caused by the CNTs dense arrangement around the crack tip. Entanglement of MWCNTs with each other makes the dislocation stable and the grain size finer. The MWCNTs have smooth bonding at the interface. This work demonstrated the potential of CNTs in improving the fatigue life of SS304.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank the financial support of the National Natural Science Foundation of China (Grant no. 51075050) and Chinese Government Scholarship Council. They would also like to thank Professor CunShan Wang from the School of Material, Professor XingLin Guo, Professor MingFa Ren from the School of Engineering Mechanics, and Associate Professor Asif Iqbal from the School of Mechanical Engineering for help and guidance in the experiment.
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Copyright © 2014 Rizwanulhaque Syed and Wei Jiang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.