Laser Shock peening is a surface treatment that is well known to impart beneficial residual stresses within metals and alloys . Laser shock peening is usually common using a pulse laser, in the nanosecond (ns) regime, however, the use of both pico and femtosecond lasers are also being reported for demonstrating fundamental results . Laser shock peening is known to enhance the properties of metallic materials in general and in doing so, helps longevity, performance and safety of parts operating in various industrial sectors. Within the railway sector, there is an increasing need for reducing cracked and broken rails. This can pose a huge threat to public safety and is a big challenge for the railway sector in general. Poor structural integrity and functional properties of material as well as rail fatigue and crack propagation of rail heads are responsible for an increase in the number of defects that lead to breakage and failure of rails. The rail breaks cost a fortune to replace and maintenance costs are also high, which have the potential for financially straining the railway sector. One way this can be reduced is by applying laser shock peening to not just new but affected rail structures in order to provide effective and long-term solutions while improving the structural integrity of rail tracks. As such this study focuses on the feasibility of applying laser shock peening to rail track material by inducing beneficial effects into the material to solve some of the aforementioned problems.
Figure 1 shows a finite element stress model on rail track under loading in (a) and the deformation of rail track under loading in (b). The main idea of a finite element stress analysis is to determine and locate the critical stress zones and deformation so that laser shock peening can be targeted to critical area where the deformation and stress is maximum. From stress shown in Figure 1(a), it was observed that the web and fillets have maximum stress, whereas the head of the rail has maximum deformation. It is therefore indicative that high stresses at the web section will be present, because the area is small and the force acting per unit area therefore becomes high, so the rail breakage or crack probability in the web section of rail will also be high. Accordingly, web fillets and heads are the sections that can be treated prior to cracks forming or post-crack forming by laser shock peening to prevent failure and further elongate the functional life of rail track.
2. Experimentation and Methodology
A finite element model was developed using structural analysis of ANYSIS 18.1 workbench by importing a 3-D CAD model of the rail track from CATIA V5 to understand the stresses. As a preliminary study, a cutting of a rail track material X80PSL2 steel was then used to undertake laser shock peening surface treatment. Laser shock peening was conducted using an Nd:YAG pulse laser (7000 series, Litron, Rugby UK). A focal length of 250mm was used to maintain a spot size of 2mm. The total peened area on each sample was 30mm by 30mm (see Figure 2). Various conditions were applied using ablative and without ablative coating as shown in Tables 1 and Table 2.
Flowing water was used for the experiments with a thickness layer of about 1mm to 2 mm. The sample was moved by motorised stage such that the laser beam always followeds a Zig-Zag pattern from bottom right to the top left of the sample. The ablative layer was applied between each pass for samples treated with multiple layers.
After laser shock peening, each sample was used to measure the hardness value by the Vickers hardness test method. The sample was indented with a load of 1Kg, applied for 10 to 20 seconds, and an average hardness value was taken from 5 measurements from each condition. Residual stresses were measure using an incremental hole drilling technique . This was conducted by adhering a wired strain gauge with an amplifier, over the top of the laser- peened and untreated regions and drilling into the strain gauge at incremental steps to a depth of about 2mm. The strain wasis then tabulated into residual stress, aided by the software of the apparatus.
3. Results and Discussion
Figure 3 shows the hardness of all samples treated with laser shock peening along with the untreated surface. It is generally seen that hardness did not change drastically and on occasion (Sample F), showed reduction by 30% on the surface, indicating that the surface may have become tensile. The average untreated hardness was measured to be 244 HV, whilst the only closest value to this was that of Sample C, measuring 252HV which was only a 4 % increase. Having said that, it is not necessary that hardness should always increase on the surface. Often, the shock-waves would travel down some distance into the sub-surface layer and may result in generating compressive stress into the sub-surface, but not reflect in the same manner at the top-near surface layer. This is where the hardness was measured in our case, so strengthening could have taken place in the sub-surface layer of the steel herein, but is not measuring the same at the top. Cross-sectioning of the samples could yield a clear picture.
With time and resource constraints, residual stresses were therefore measured on selected samples that were laser shock-treated as well as the untreated samples. Thus, two samples were selected for residual stress analysis: Sample C and Sample F. Sample F was selected as it measured the lowest surface hardness of 186 HV, whilst Sample C was selected as it comprised the highest surface hardness.
3.2 Residual stress
Residual stress was measured first for the untreated surface. The maximum compressive residual stress obtained was 106 MPa at 224µm depth. The untreated sample showed residual stress of around 11 MPa and began to reduce to a highest of -64MPa at a depth of 640µm (see Figure 4). The untreated material curve then went up in tension within the bulk at a depth of about 1mm, which was residual from its original manufacturing process.
The maximum compressive residual stress obtained in sample C was -238MPa at 16µm depth. This was the sample that showed the highest hardness on the surface. Thereafter, the compression began to reduce as the shockwaves dissipated further within the sub-surface at around 160µm depth and the residual stress values remained close to the 0 MPa line to a depth of 1mm. Compared to the untreated sample, it was evident that the laser shock peening on the X80PSL2 rail-track steel induced compressive stress using the ablative tape.
Sample F comprised laser shock peening without coating with 1 layer. The energy applied to this sample was somewhat the same as that of Sample C except that 90% overlap was used compared to Sample C with 75% spot overlap; however, the surface hardness as previously shown was significantly low. The residual stress at the surface was measured to be 14 MPa around the surface region (16µm), but then the curve began to further dip in compression to a maximum of -637 MPa at a depth of 224 µm. The surface and sub-surface beyond this point still remained in compression to -151MPa at a depth beyond 1mm.
This goes to show that laser shock peening without coating significantly improved the residual stress induction within the X80PSL2 rail track steel. The overlap of 90% obviously had contributed to this as it allowed the shock waves to penetrate deeper within the metal. Visually, it was evident that the surface was however, somewhat roughened, but this is expected as the ablative tape protected the surface in Sample C, whereas Sample F (LSPwC) suffered more roughening.
A finite element model was developed to understand the distribution of stresses and deformation acting upon a rail track. The results yielded that head and web of the rail appeared more prone to any surface defects or cracks, so laser shock peening should be targeted on these specific areas of rail in future studies. Various laser shock peening surface treatments were applied to a X80PSL2 rail-road steel. Hardness did not significantly change on the surface however, with selected samples; the residual stresses measured demonstrated that laser shock peening induced compressive stress within the material using both selected parameters. The sample treated with conventional laser shock peening with an ablative layer and a 75% overlap rendered a maximum compressive stress of -238MPa at 16µm, whilst the sample with 90% overlap but laser shock peened without coating, rendered a maximum compression of -673MPa at a depth of 224µm and remained in compression beyond 1mm depth.
Further investigations should focus on microstructural evaluation as well as cross-sectional hardness and full characterization to understand the effects of the laser material interaction of X80PSL2 rail track steel. It was clearly evident from this study that laser shock peening induced beneficial stresses. It also has the potential to go a long way in extending service life and avoiding maintenance of rail tracks; additionally, it could also contribute to improving the safety factor.
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