Fatigue Behavior Of Specimens Research Articles

Comparative study on tensile and high cycle fatigue behaviour of 316L(N) SS hardfaced with Ni-Cr-B-Si alloy by GTA and laser cladding processes

In sodium-cooled fast reactors (SFR), 316L(N) SS grid plate is hardfaced with Ni-Cr-B-Si alloy to achieve higher wear resistance. Tensile and fatigue forces are acting at the interface between substrate and deposit due to different thermal expansion coefficients of those two materials, which can cause cracking of deposit and fracture during operation. Thus, it is very important to consider appropriate hardfacing method which can provide higher tensile and fatigue strength to avoid cracking/debonding at the interface. To find a solution to this problem, two hardfacing techniques, namely Gas Tungsten Arc (GTA) and Laser cladding (LC), are taken into consideration. Hardfaced specimens are prepared using each process on which tensile and high cycle fatigue tests are conducted. From the experimental testing, stress-strain and S-N curves are generated to predict the tensile and fatigue behaviour of specimens. Fractographic studies are conducted at fractured surfaces to understand the fatigue crack nucleation and propagation characteristics. The experimental results for both processes are compared. Tensile and fatigue strength of LC specimens are ∼11 % and ∼17 % less than those of GTA specimens due to its higher brittleness. Thus, GTA process is recommended as the efficient hardfacing process for grid plate of SFR.

Investigating post-processing impact on fatigue performance of LPBF Ti6Al4V with heat treatment, high pressure heat treatment, and dry electropolishing strategies

This study investigates the impact of post-processing techniques on the fatigue behaviour of specimens in Ti6Al4V made by laser powder bed fusion (LPBF) for relevant applications, such as motorsport. Despite the LPBF advantages in terms of complex design and weight reduction, challenges such as reduced fatigue performance persist. Therefore, the application of post-processing treatments can play a critical role. In addition to traditional sandblasting (SB), three post-processing conditions were investigated to enhance mechanical properties under quasi-static and fatigue loading conditions: heat treatment (HT) and the innovative High Pressure Heat Treatment (HPHT) along with dry electropolishing (EP) treatment. The results indicate that both the HT and HPHT treatments produced a macrostructure with equiaxial grains composed of fine α + β lamellae. Mechanical analyses demonstrated that the HPHT-SB-EP condition exhibits properties comparable to those of forged Ti6Al4V used in the automotive field, with a good balance between strength and ductility. The EP treatment effectively reduced surface roughness and inherent surface irregularities caused by the LPBF process. The best fatigue results were achieved with the HPHT-SB-EP condition, despite data scattering. An increase in fatigue resistance was observed by using SB at a higher pressure.

Fatigue limit prediction of cracked and notched specimens related to grain size

Fatigue behavior of specimens is influenced by crack/notch geometry and material properties. Here, a unified fatigue limit model is proposed to describe notch geometry and grain size (G) influences by analyzing fatigue measurements on specimens from 11 kinds of metals (G = 6 to 120 μm) and containing defects (crack, U-notch, sharp and blunt V-notches). Based on Atzori-Lazzarin diagram, fatigue limit of notched specimens (crack is special notch) is linked to notch depth, root radius and opening angle, and the intersections in the diagram are related to G. Additionally, inevitable fatigue-limit scatter is quantified by incorporating normal distribution theory. Results show that proposed model with 96% reliability can predict fatigue limits of different-metal specimens with various defects.

Fatigue behaviour of notched laser powder bed fusion AlSi10Mg after thermal and mechanical surface post-processing

Laser powder bed fusion (LPBF) as an additive manufacturing technology offers high potential to fabricate parts with complex geometries layer-by-layer. However, these parts have inhomogeneous microstructure and very poor surface quality in their as-built condition. The presence of high surface irregularities especially in the downskin surfaces is a challenging issue that can directly influence their mechanical performance especially under fatigue loading conditions. Hence, applying post-treatments to modulate these imperfections can play a critical role. In this study, the individual and hybrid effects of different post-treatments including T6 heat treatment and shot peening on microstructure, mechanical properties and fatigue behaviour of LPBF V-notched AlSi10Mg specimens were investigated. Two different shot peening processes were applied on both as-built and heat treated specimens using steel and ceramic shots with different Almen intensity, shot diameter and shot hardness. The specimens were comprehensively characterized in terms of microstructural features, surface morphology and surface roughness. Mechanical properties including microhardness and residual stresses were measured and fatigue behaviour of the specimens was determined using a stair-case method; fracture surfaces were also critically analyzed. The results of the analysis performed both on the smooth section and the notched section (including notch root, up and down skin areas) indicated the importance of the choice of shot peening parameters with respect to the target geometry and its material properties. In this case, the shot peening treatment with smaller media and lower intensity was more efficient in terms of surface modification and homogenization especially in the downskin surfaces leading to higher fatigue strength. The significant finding of this study is that by pairing the heat treatment and shot peening, the effect of the presence of the notch can be masked obtaining almost the same fatigue strength for the notched specimens as the un-notched counterparts.

Thermal Stability of Residual Stresses in Differently Deep Rolled Surface Layers of Steel SAE 1045

The stability of compressive residual stresses generated by deep rolling plays a decisive role on the fatigue behavior of specimens and components, respectively. In this regard, deep rolling at elevated temperature has proven to be very effective in stabilizing residual stresses when fatigue analysis is conducted at ambient temperature. However, since residual stresses can be affected not only by plastic deformation but also when thermal energy is provided, it is necessary to analyze the influence of temperature and time on the relaxation behavior of residual stresses at elevated temperature. To evaluate the effect of deep rolling at elevated temperatures on stability limits under thermal as well as combined thermo-mechanical loads, the present work introduces and discusses the results of investigations on the thermal stability of residual stresses in differently deep rolled material conditions of the steel SAE 1045.

Performance properties of WMA modified binders and asphalt mixtures containing PPA/SBR polymer blends

The performance properties of asphalt binders and mixtures can be enhanced utilizing several modifiers, including; Poly Phosphoric Acid (PPA), Warm Mix Asphalt (WMA) modifiers, and Styrene–Butadiene Rubber (SBR). The current study evaluated the effect of PPA and WMA contents on rheological behavior of SBR modified binders and SMA mixtures. The modified binders were subjected to rotational viscosity, Dynamic Shear Rheometer, and Bending Beam Rheometer, Multiple Stress Creep Recovery (MSCR), and Linear Amplitude Sweep (LAS) tests. The SBR/PPA and SBR/PPA/WMA modified mixtures were subjected to Indirect tensile strength (ITS), dynamic creep, resilient modulus ( Mr), wheel track, and four-point beam fatigue (FPB) tests. To analyze the data, two-factor analysis of variance (ANOVA) was investigated. Based on the results of the MSCR test at both stress levels, modification of base bitumen by SBR, PPA and WMA additives causes an enhance in permanent deformation performance of original bitumen. LAS test results indicated that, utilization of SBR and WMA additives improves the fatigue life of bitumen. Also, by addition of PPA, the fatigue life of SBR modified binders increases. whereas, the fatigue lives are higher than original binders ones. Based on results, utilization of SBR and PPA enhances the Mr, rutting properties, ITS, FE, and fatigue behavior of specimens. By increasing the PPA percentage, the rutting and fatigue behavior enhances. Whereas, it causes a decreases in Mr and ITS of modified mixtures. Among warm additives, sasobit has better effect on enhancing the performance of binders and mixtures.

The effect of mode II fatigue crack growth rate on the fractographic features of CFRP composite laminates: An acoustic emission and scanning electron microscopy analysis

The present study is focused on the characterization of the fatigue damage features in carbon/epoxy laminates under mode-II loading conditions. To this aim, a sinusoidal cyclic load was applied to the End-Notched Flexural (ENF) specimens and the fatigue behavior of specimens was investigated. Scanning Electron Microscope (SEM) was used to identify the damage features on the fracture surface, i.e. fiber imprints, cusps, roller cusps, and striations. It was found that the fatigue damage features, such as cusps and striations, completely depended on the fatigue crack growth rate, da/dN. In addition, a linear relationship between the fatigue striation space and the strain energy release rate range (ΔGs) and the hysteresis loop area was established. The Acoustic Emission (AE) method was also employed to characterize the damage features. The obtained results showed that higher AE energy indicates larger and rougher cusps and striation features.

Influence of welded joint microstructures on fatigue behaviour of specimens with a notch in the heat affected zone

In this paper, the results of determining the Paris law coefficients, C and m, are presented, based on the experimental data obtained by three-point bending testing of 8 welded joint specimens. The specimens were taken from a welded plate, with P460NL1 used as the parent material and VAC 65 used as filler material. The specimens had a 1.4 mm notch in the heat affected zone, either in the root or the weld face side. After the fatigue crack growth tests performed using the Fractomat 7609/213 (RUMUL), the obtained crack growth rate vs. number of cycles (da/dN vs. N) diagrams were used to calculate the coefficients. The values of these coefficients were then compared to each other, in order to determine the cause of noticeable differences, which were assumed to be a result of different microstructures in different specimens, caused by the increased temperature measured in the specimens near the end of the welded joint. Since the specimens used in the experiment had differences in terms of notch position and temperature, they were divided into four groups. Analysis of the results involved the comparison of obtained a-N curves with the images of microstructures, as seen on the broken specimens, in order to determine whether the changes in the curve's slope correspond to the changes in microstructures along the specimens' cross-sections. The experimental results confirmed this assumption, and it was determined that the crack growth rate was the highest in the weld metal, whereas the lowest rates (highest resistance to fatigue crack growth) were observed in the parent material, with the heat affected zone being between these two value ranges.

Powder Recycling Effects on the Tensile and Fatigue Behavior of Additively Manufactured Ti-6Al-4V Parts

Additive manufacturing technology has enabled industries to generate functional parts with an increased level of complexity via layer-by-layer fabrication. In laser-powder bed fusion (L-PBF), powder is often recycled due to its high cost. However, there is no comprehensive study on how powder recycling affects its rheological properties, as well as the mechanical and fatigue behavior of the manufactured part. This study compares powder characteristics and mechanical performance of as-built and machined specimens fabricated from new and heavily used Ti-6Al-4V powder. Powder characteristics include particle size distribution and morphology, flowability, apparent density, compressibility, thermal conductivity, oxygen concentration, and more. Results indicate that particle size distribution becomes narrower and flowability increases with recycling. Not a significant effect of recycling was observed on the monotonic tensile and fatigue behavior of specimens in the as-built surface condition. However, machined specimens fabricated from used powder demonstrated longer fatigue lives in the high cycle regime.

Residual Fatigue Properties of Asphalt Pavement after Long-Term Field Service

Asphalt pavement is widely used for expressways due to its advantages of flexibility, low cost, and easy maintenance. However, pavement failures, including cracking, raveling, and potholes, will appear after long-term service. This research evaluated the residual fatigue properties of asphalt pavement after long-term field service. Fatigue behavior of specimens with different pavement failure types, traffic load, service time, and layers were collected and characterized. Results indicate that after long-term field service, surface layer has a longer fatigue life under small stress levels, but shorter fatigue life under large stress levels. Longer service time results in greater sensitivity to loading stress, while heavier traffic results in shorter fatigue life. Surface and underneath layers present very close fatigue trend lines in some areas, indicating that the fatigue behavior of asphalt mixture in surface and underneath layers are aged to the same extent after eight to ten years of field service.

Importance of Residual Stresses and Surface Roughness regarding Fatigue of Titanium Forgings

Abstract This paper presents the results of a long-term research program aimed at developing qualitative and quantitative design guidelines for the use of mechanical surface treatments designed to improve the fatigue life of structural components. High cycle fatigue tests were performed on planar four-point bending specimens derived from Ti-6Al-4V pancake forgings with a mill-annealed microstructure. The high cycle fatigue behavior of specimens with different surface conditions (as-forged and machined) in both an unpeened and a shot peened state was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layers were carried out. The as-forged surface state exhibits a stress distribution with significant compressive stresses near the surface, resulting in equilibrium tensile stresses in the depth. When the tensile stresses were exposed by machining a bordering surface, a distinct decrease in the fatigue strength was observed. In such cases, a shot peening treatment was shown to improve the fatigue strength. Square edges lead to a decrease in the fatigue strength, which could be aggravated by shot peening.

Four Point Bending Fatigue Tests of Forged Ti 6Al 4V

Abstract The paper derives from a long-term research program that is aimed at developing qualitative and quantitative design guidelines to influence mechanical surface treatments in order to improve the fatigue life of structural components. A four point bending test rig was developed using finite element analysis. High cycle fatigue tests were performed on plane specimens taken from Ti 6Al 4V forgings with mill-annealed or bimodal microstructure. The high cycle fatigue behaviour of specimens with two different surface conditions (as-forged and machined) was compared. In order to assess the fatigue failure mechanisms, detailed investigations of the surface layer were carried out. Residual stresses were shown to play an important role in fatigue.

Biomimetic coupling effect of non-smooth mechanical property and microstructural features on thermal fatigue behavior of medium carbon steel

Some kinds of particular functions possessed by natural organisms are often formed by coupling up the multiple typical features on their body surfaces. Inspired by the coupling phenomenon in biological system, the medium carbon steel specimens with the coupling effect of non-smooth mechanical property and microstructural features were fabricated by laser processing. Thermal fatigue behavior of specimens with biomimetic coupling surface was investigated and compared. The results confirmed that such a biomimetic method has the beneficial effect on improving the thermal fatigue property of medium carbon steel specimens. The related mechanisms behind the biomimetic coupling effect for explaining the enhanced thermal fatigue resistance were discussed preliminarily.