Softened Layer Research Articles

Research on the influence of rock fracture toughness of layered formations on the hydraulic fracture propagation at the initial stage

Deep underground rocks exhibit significant layered heterogeneity due to geological evolution and sedimentation. Rock fracture toughness, as one of the important indicators of hydraulic crack propagation, also exhibits heterogeneous distribution. In order to investigate the influence of non-uniform fracture toughness of layered rocks on hydraulic crack propagation, this paper establishes a planar three-dimensional hydraulic crack propagation model. The model is numerically solved using the 3D displacement discontinuity method (3D-DDM) and the finite difference method. The calculation results indicate that when the distribution of the fracture toughness of layered rocks changes from uniform to non-uniform, the fracture morphology develops from a standard circular crack to an elliptical crack. When the difference of the rock fracture toughness between adjacent rock layers and the middle rock layer (pay zone) is large enough, the fracture morphology will develop towards a rectangular shape. In addition, when the fracture toughness of rock layers is non-uniformly distributed, the hydraulic crack not only rapidly expand in the softening layer (rock layer with lower fracture toughness), but also slowly propagate in the strong layer (rock layer with higher fracture toughness). However, the propagation speed in the softening layer is much faster than that in the strong layer. The results indicate that the heterogeneity of rock fracture toughness has an important impact on the morphology, propagation speed, and direction of hydraulic fractures.

Temperature field simulation and optimization of processing parameters for laser-assisted machining of 3Y-TZP ceramics

To obtain low surface roughness in the laser-assisted machining of 3Y-TZP ceramics, a laser-assisted heating model of 3Y-TZP ceramics was established. The temperature field in the cutting zone was simulated using ABAQUS software to analyze the effects of laser power, rotational speed, and warm-up time on softening layer depth (distance between the workpiece surface and lower boundary of softening layer) and thickness (distance between the upper and lower boundaries of softening layer). Furthermore, the accuracy of the simulation results was verified through experiments. The Plackett-Burman design, Steepest ascent experiment, and Box-Behnken design method were used sequentially to optimize process parameters, and the surface quality, chip morphology, and tool wear of 3Y-TZP ceramic with laser-assisted cutting and traditional cutting were compared under the best process parameters. The results show that the rotational speed and laser power are the main factors influencing the softening layer depth and thickness, and the suitable warm-up time is 15 s; the most influential factor on the surface roughness is the cutting depth, and the optimized process parameter combinations for the best surface roughness are as follows: laser power of 90.3 W, rotational speed of 400 r/min, cutting depth of 0.21 mm, and feed speed of 4.4 mm/min; with laser-assisted, the surface roughness value of the workpiece is reduced by 54.12 %, the amount of tool wear is reduced by 56.5 %, the chip morphology is transformed from powder to strip with a length of more than 50 μm, and the primary material removal mode is changed from brittle removal to plastic removal.

Effects of peening duration on surface and wear properties of aircraft graded AA2017 alloy

This investigation addresses the effects of peening duration during SP on surface properties of AA2017 alloy. SP performed with different durations (0, 70, 140 and 210 s) and imparts the modification in the surface region. Alloy strength and structural parameters like crystallite size (D), dislocation density (ρ) microstrain (ε) and lattice constant (a) are increased with SP duration till 140 s. After 210 s, these properties declined due to the effect of softening caused by rise in surface temperature through over peening duration, and exhibited dynamic recovery. Severe plastic deformation developed by SP, created rough surface in all SPed samples and shows responsible for low wettability, high contact angles and low surface energy in comparison with AR sample. The SP140 exhibited maximum hardness (234 ± 7.1 HV) and maximum residual compressive stress (RCS) (−51 MPa) as compared with that of AR sample shown 70 ± 2.1 HV and 20 MPa respectively. Also, the cross-sectional measurements over a thickness of 200 μm confirmed their improvement. These results favoured for SP140 sample to yield lower wear rate about 0.776 mm3/ KN-m compared to AR sample (1.046 mm3/ KN-m). Further increase in shot peening duration to 210 s induces softened layer by rise in surface temperature and presence of Cu as major alloying element in AA2017 exhibits superior thermal conductivity and resulted in detrimental effect on wear resistance. The combination of several factors such as structural parameters, RCS and hardness in the surface region of SP140 sample are responsible to develop the wear resistance of AA2017 alloy.

Formation mechanisms of affected layers induced by grinding hardened AISI 52100 steel

To establish a systematic cognition of formation mechanisms of affected layers in hardened AISI 52100 steel, the experiments were conducted to measure and characterize the microstructure and properties of the affected layers subject to a varying grinding depth and thermal and mechanical effects at different-levels. It is found that the affected layers can be formed mechanically or thermally depending on whether the mechanically-induced effect predominates the thermally-induced effect, and the 25 μm is exactly the critical grinding depth of the above two effects. From the novel perspective of thermo-mechanical decoupling, when the grinding depth exceeds 25 μm, grinding temperature is higher than the nominal austenization temperature (Ac1) of the bulk material, the thermally-induced effect becomes dominate, the thickness of the white layer changes abruptly, and there is an obvious dark layer underneath. When the grinding depth is less than 25 μm, grinding temperature is lower than Ac1, the mechanically-induced effect becomes dominate, the white layer is extremely thin and no dark layer can be observed. The thermally-induced affected layers are formed mainly by a rapid austenite transformation, accompanied by the fragmentation and dissolution of carbide particles, and the white layer is formed on the ground surface. Meanwhile, the subsurface undergoes tempering subjected to high temperature; this leads to the decomposition of austenite and precipitation of a large amount of cementite. With a continuous dynamic recrystallization, a dark softening layer is formed under the white layer. The mechanical-induced affected layers are formed by fragment and refinement of nano- and micro-structures subject to severe plastic deformation and dynamic recovery.

Unraveling size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti by non-local crystal plasticity modeling

Titanium alloy is prominent in manufacturing of high-performance miniature devices, while size-affected plastic heterogeneity and asymmetry in micro-scaled deformation are not well understood due to its complex deformation mechanism. How the interplay among dislocation slip, deformation twinning, and strain gradient affects the size-dependent plastic heterogeneity and asymmetry is an intractable and non-eluded issue that needs to be unraveled. In this research, a non-local crystal plasticity model (NLCPM) considering discrete twins was established and implemented into a finite element framework. The proposed NLCPM successfully represented the size-dependent asymmetric mechanical response, primary and secondary twinning, and texture evolution. The size-dependent fracture behavior, plastic heterogeneity, and T-C asymmetry during micro-scaled deformation of commercial pure titanium (CP-Ti) were explored by coupling micro-tension/compression test and full-field simulation. Results showed that T-C asymmetry in flow stress was enhanced in the coarse-grained and small-diameter samples. For the sample with D≥0.6 mm, the ductility of CP-Ti was improved with the increase of grain size as there are more prevailing twins and the concentrating stress could be relieved by the abundant slip rather than cracking. Grain-level strain is more homogeneous in compression when d was increased from 60 to 110 μm since more non-prismatic slips were activated that accommodated strain along various directions. Meanwhile, plastic strain was allocated within discrete twin bands and thus alleviated strain concentration. Furthermore, the hardening effect of twin boundaries cancelled out the softening effect near the free surface, resulting in a narrower softening layer in the coarse-grained samples compared to its grain size. With the decrease of the sample size, the proportion of the softening layer increased, while the volume fraction of twinning decreased, leading to a reduction in macroscopic stress and fracture strain. This work enhances the in-depth understanding of size-affected plastic heterogeneity and asymmetry during micro-scaled deformation of CP-Ti and supports the forming of high-quality micro components under complex deformation path.

Oxidation of cellulose molecules toward delignified oxidated hot-pressed wood with improved mechanical properties

Wooden building materials have advantages in terms of biodegradability, non-toxicity, pollution-free and recycling. Currently, applications of natural wood are extremely limited because of low density, low strength and toughness. Therefore, we reported an effective modification strategy with nano-scale cellulose nanofibrils design to prepare a synergistically enhanced cellulosic material. Via three steps: i) the secondary alcohol hydroxyl groups in C2, C3 position were cut; ii) oxidize the hydroxyl group at C2, C3 position to achieve dialdehyde cellulose; and iii) oxidized again to obtain dicarboxylic cellulose. Subsequently, thanks to the regulation of the average moisture content, the moisture content in the wood surface and subsurface increased in a short time. The wood softening layer contributes to the hotpressing treatment of the wood. The mechanical properties and dimensionality have been greatly improved. The obtained delignified oxidated hot-pressed wood with 0.55 mmol/g carboxyl group content demonstrates excellent strength of 328.8 ± 7.43 MPa and Young's modulus of 8.1 ± 0.14 GPa, which is twice than that of natural wood. Delignified oxidated hot-pressed wood also shows exceptional toughness of 8.3 ± 0.28 MJ/m3. Other than that, the shore hardness indicates 0.55 mmol/g carboxylic group, which could increase the hardness at the wood surface hardness to 72.5 ± 4.29°.

Simulation analysis and optimization of laser preheating SiC ceramics based on RSM

In this paper, the regression prediction models of surface reference temperature and softening layer depth are obtained through preheating simulation and experimental verification based on the surface response methodology (RSM), and the simulation predictions of surface reference temperature and softening layer depth are optimized. Firstly, a three-dimensional transient heat transfer model is established for turning SiC ceramics as the research object, and the concept of surface reference temperature is proposed. The single factor simulation and experiment are carried out by changing the laser power, and the simulation value of surface reference temperature and the experimental value are compared to verify the effectiveness of the heat transfer model. Then, the material removal mechanism is explained according to the material fracture theory, and the concept of softening layer depth is proposed. The multivariate prediction models are established to obtain the surface reference temperature and the softening layer depth based on the RSM by selecting three factors: laser power, spot diameter and rotational speed. Finally, the desirability function analysis (DFA) is used to compare the measured value of surface reference temperature with the predicted value, which proves that the regression prediction model is effective. The range of laser power for single-factor experiment of surface reference temperature is 175 W–235 W, and the error of the results is between 7.57% and 10.82%. The Box-Behnken experimental design is adopted with the following factors and levels: laser power (200 W–230 W), spot diameter (1 mm–1.4 mm) and rotational speed (1080r/miñ2160r/min); the regression models are analyzed by residual analysis, ANOVA, 3D surfaces, and contour plots to prove the reliability. Based on the DFA method to select the level of each factor: laser power of 205 W and 220 W, spot diameter of 1 mm, and rotational speed of 1620r/min, the errors between the optimal and experimental values of surface reference temperature are 9.82% and 7.72% respectively, which not only achieve the optimization of the desirability function of the heat transfer model, but also prove that the regression prediction model has good agreement with the experimental values. The regression prediction model of softening layer depth establishes the functional relationship between softening layer depth and different factors in the study range and provides direct reference for the selection of cutting depth in laser assisted machining (LAM) SiC ceramics.

Processability improvement of laser-assisted micro-machining 95W-3.5Ni-1.5Fe alloy based on surface and subsurface characteristics

Laser-assisted micro-machining proves advantageous in improving the machinability of hard-brittle materials, but its effectiveness is easily weakened due to the uncertainty and complexity of microstructural transformation characteristics. In this research, a comprehensive investigation of the sustainable production mechanisms of tungsten heavy alloys was presented through recognizing the surface morphologies, subsurface microstructures, as well as micro-tool wear features, supported by the critical outcomes from single laser scanning (SLS) and laser-assisted micro-milling (LAMM) experimental tests. A laser thermal affected zone, which is composed mainly of surface oxide layer, near-surface softening layer, and subsurface metamorphic layer, was produced in laser heating. The relationship model between surface energy density and the thickness of laser thermal affected zone was determined. On this basis, a microstructure-based selection approach of the LAMM conditions, such as laser power and the depth of cut, was adopted to achieve a high-performance surface. Results indicated that a nanometer-level roughness surface, Sa = 57.03 nm, with no apparent residual thermal affected layer and newly generated subsurface damage layer were identified while suppressing flank wear. It is confirmed that the adaptability between the thickness of laser thermal affected layer and the depth of cut impacts LAMM sustainable production and justifies its application prospect.

Influence of ultrasonic agitation on the abrasive wear characteristics of Al-Cu/ 2 vol% Grp composite

In the present study, the abrasive wear behavior of Al–4.4 wt% Cu composite reinforced with 2 vol% graphite particle (Grp) has been investigated. In the preparation of composite, Ultrasonic Treatment (UT) is provided in the composite melt for the uniform distribution of reinforcement particles. Two bond abrasive wear tests are conducted for composites treated with ultrasound and without UT and base alloy. The results of abrasive wear studies indicate that at 5 and 10 Newton (N) loads, the composite with UT has a higher coefficient of friction (COF) and wear resistance than that of the base alloy (Al–4.4 wt% Cu). Whereas, at 15 and 20 N load, the value of COF and wear resistance is lower for the composite. Two abrasive wear mechanisms micro-plowing and micro-cutting have been observed during the wear tests of base alloy and composites. The analysis of worn-out sample surfaces at higher load reveals that softened material layer due to localized elevation in temperature between two contact surfaces during wearing acts as a tribolayer in base alloy while in composites both softened material layer and graphite layer have worked together as tribolayer.

Plasticity improvement and radiation hardening reduction of Y doped V-4Cr-4Ti alloy

In current work, V-4Cr-4Ti and V-4Cr-4Ti-2Y alloys are prepared using mechanical alloying and spark plasma sintering. Sequential-beam irradiation of 30 keV H+ and 50 keV He+ ions, are carried out at 450 ℃ to the peak damage of 0.18 dpa. The influence of Y on the microstructure, mechanical properties, radiation induced defect and radiation-hardening are investigated using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), mechanical test, and nano-indentation test. Two kinds of Y-rich particles precipitated in Y added alloy, one is Y2O3 phase with a mean size of 60 nm, the other is YVO4 with a diameter of ∼10 nm, but no carbide of Y is observed. The results show that Y can significantly improve plasticity of V base alloy, reduce the size and number density of radiation induced dislocation loops, limit radiation hardening. Two different fitting process with different initial values and boundary conditions are used to analysis radiation hardening, the fitting results indicate a slight softening of matrix near damage layer which named softened layer. This softened layer can affect nanoindentation test results especially for shallow irradiation. A plausible explanation for the formation of this softened layer could be the oxygen diffuses from unirradiated region to damage layer.

Mechanical characterization of the enamel microstructure treated with non alcoholic drinks

In previous studies, it has been shown that the microstructure of prismatic dental enamel presents differences between the external and internal zone. Radial enamel is found in the outer third of enamel and has higher microhardness values than enamel with Hunter- Schreger Bands (HSB) that occupies the inner 2/3. Our aim was to evaluate the mechanical behavior of radial enamel and HSB due the action of a non-alcoholic beverage in vitro. Longitudinal sections of dental crowns were embedded in polymer, worn and polished with sandpapers of decreasing granulation. The samples were immersed in a flavoured natural water for 12 minutes. Nanohardness tests (Triboindenter Hysitron) were performed on the radial and HSB enamel before and after the exposure to the drink. Hardness determinations "H", reduced modulus "Er" and contact depth "hc" were obtained. The percentage of reduction of hardness was determined. The values found in healthy radial enamel were H: 5.48±0.23 GPa; Er: 86.97±8.11 GPa; hc: 149.73±4.25 nm and in HSB H: 4.24±0.43 GPa; Er: 75.24±7.09 GPa; hc: 176.36±11.29 nm. After exposure to beverage, it was found in the radial enamel H: 2.22±0.31 GPa; Er: 58.73±10.79 GPa; hc: 270.29±21.22 nm, and in HSB H: 1.54±0.42 GPa; Er: 48.11±6.54 GPa; hc: 350.10±63.33 nm. After the drink action, the values of hardness of the radial enamel and HSB decreased and the trend observed in healthy enamel remained, where the highest values corresponded to the radial enamel. The percentage reduction of H in the radial enamel was 59.48% and in the HSB enamel it was 63.67%. The contact depth increased by about 50%.The decrease in hardness is related to the mineral loss produced by the acids contained in the drink. We conclude that the action of the non-alcoholic beverage produces a decrease in the mechanical properties in both the radial enamel and the HSB. The lower values in the reduced module Er indicate the formation of a superficial softened layer, the enamel with HSB being more vulnerable.

Rainfall-induced landslide in loess area, Northwest China: a case study of the Changhe landslide on September 14, 2019, in Gansu Province

A large-scale loess landslide occurred in Xiaozhuang Village, Changjiahe Town, Tongwei County, Gansu, at approximately 11:00 am on September 14, 2019. A volume of 774 × 104 m3 of a historical sliding mass was reactivated by rainfall and slipped down along a weak interface. The sliding mass accumulated in the valley of the Kushui River, blocking the channel for a distance of ~ 1000 m and destroying several roads and the Yangpo Bridge. This landslide is a typical case of rainfall-induced revival of a historical seismic landslide in the loess region of Northwest China. On the basis of site investigation, Google satellite image, unmanned aerial vehicle photography, and numerical simulation, the deformation and failure characteristics of this landslide are described in detail, and the possible inducing factors and failure mechanism are preliminarily assessed. The Tongwei earthquake in 1718 and the Haiyuan earthquake in 1920 caused the slope to slide multiple times in history. Due to continuous rainfall, the existing softened layer (previous sliding surface) was highly saturated due to the perched water on the top of the relatively impervious mudstone and formed the critical weak zone. Therefore, previous sliding mass in the lower and middle slope was reactivated along the softened layer on September 14, 2019. We suggest that the perched water mainly came from the dominant seepage channels such as sinkholes and underground rivers. In addition, sinkholes on the previous sliding body are the key factors controlling the boundary of the landslide. The movement of the landslide lasted for 12 h, and the main landslide lasted for only 60 s, with a runout distance of 60 m and an average velocity of 5 m/h. Study of the formation conditions and failure mechanism of the Changhe landslide is of great significance for the early identification and risk prevention of this kind of landslide in the loess region of Northwest China.

Failure analysis of tapered roller bearing inner rings used in heavy truck

Hoop brittle fracture took place on several tapered roller bearing inner rings during assembling with the intermediate shaft. All the cracking took place on the laser marking area on the small head end of inner ring representative of product identification. Detailed metallurgical analysis revealed that two kinds of metallurgical defects mainly occurred on the small head end of cracked inner rings: (1) several of deep and narrow near-surface micro-grooves resulting from laser marking were distributed dispersively on the small head end of inner ring, (2) the thermal softening resulting from grinding burn occurred on the small head end of inner ring, causing decrease in surface hardness as well as development of tensile stress. The crack origins were all located on the overlapping zone of the laser marking area and grinding burn area. Cracks initiated from the micro-grooves and propagated toward the inner along the axial under the action of assembly stress. The presence of the near-surface micro-grooves as sharp notches on the laser-marked small head end was mainly responsible for the fracture failure of inner rings. The occurrence of the thermal softening layer due to grinding burn on the small end of inner ring, consequently leading to decreasing in the surface hardness and resultant low tensile strength, promoted the crack initiation and propagation.

Microstructure and mechanical properties of double-side friction stir welded 6082Al ultra-thick plates

In the present work, 80 mm thick 6082Al alloy plates were successfully double-side welded by friction stir welding (FSW). The relationship between the microstructures and mechanical properties was built for the double-side FSW butt joint with more attention paid to the local characteristic zones. It was shown that a phenomenon of microstructural inhomogeneity existed in the nugget zone (NZ) through the thickness direction. The grain size presented an obvious gradient distribution from the top to the bottom for each single-pass weld, and the microhardness values decreased from both surfaces to the middle of the NZ. The lowest hardness zone (LHZ) exhibited a “hyperbolical”-shaped distribution extending to the middle of the NZ. Similar tensile properties were obtained in the three sliced specimens of the FSW joint, and the joint coefficient reached about 70% which achieved the same level as the conventional FSW Al alloy joints. Finite element modeling proved that the “hyperbolical”-shaped heat affected zone (HAZ) was beneficial to resisting the strain concentration in the middle layer specimen which helped to increase the tensile strength. Based on the analysis of the hardness contour map, tensile property and microstructural evolution of the joints, an Isothermal Softening Layer (ISL) model was proposed and established, which may have a helpful guidance for the optimization on the FSW of ultra-thick Al alloy plates.

Influence of in-service thermal softening on wear and plastic deformation in remanufactured hot forging dies

The need for economic and greener production methods necessitates remanufacturing of used hot forging dies. However, it has been reported from industries that the remanufactured forging dies show higher wear rate, and plastic deformation as compared to new dies. In this work wear and plastic deformation behaviour in new, and remanufactured hot forging dies at different stages of forging cycles are experimentally studied. Die geometry was digitised using three dimensional optical scanner to analyse the wear depth, and plastic deformation. Compared to new dies, significant wear and plastic deformation was found in remanufactured dies. Since in-service microstructural changes are expected to influence wear and plastic deformation behaviour, microhardness tests were conducted on die specimen to measure the variations in hardness from surface to core regions of die. This study reveals that a thermally softened layer grow on the die surface with each forging cycle and in subsequent cycles, this layer gets removed by in-service wear and plastic deformation. In addition, the core hardness in new and remanufactured dies was found to decrease with increasing hot forging test duration. Consequently, remanufactured dies exhibits a lower core hardness, as compared to new dies, which eventually results in higher wear loss and plastic deformation.

Fabrication, characterization and comparative analysis of mechanical properties of micro features generated by reverse micro EDM

Moderate to high aspect ratio micro features (projected) finds wide application in MEMS components, fuel cells for micro reactors, sensors and actuators, micro optical components etc. Reverse micro electro discharge machining (RMEDM) is one of the extensively used process for fabricating such micro features. The present authors have already developed a technique for generating different shapes and sizes of micro features by suitable modification of tool in RMEDM. Mechanical properties like hardness and elastic modulus of these micro features play a significant role in determining their suitability for various applications. Therefore, the objective of this paper is (a) to generate projected micro features corresponding to tapered blind hole depths of 0.1 mm and 0.3 mm using RMEDM and (b) to determine the effect of tapered blind hole depth on the hardness and elastic modulus of the projected micro features. Hardness and elastic modulus were measured using nano indentation technique. Results indicate that starting from the periphery, both hardness as well as elastic modulus first increase (presence of recast layer) and then decrease (presence of heat affected zone) for projected micro feature corresponding to 0.1 mm hole depth before attaining the properties of the parent material. Due to high amount of debris agglomeration in case of 0.3 mm hole depth, high amount of abnormal discharges occur which do not provide sufficient time for the formation of thermally softened layer as was in the case of micro feature corresponding to 0.1 mm hole depth. Hardness, therefore, is always high starting from recast layer to the HAZ before finally attaining the hardness of the parent material. Debris agglomeration did not have much influence on the elastic modulus and the variation of elastic modulus of the micro feature corresponding to 0.3 mm hole depth remains fairly uniform a compared to the parent material.

Effects of Cutting Speed on Surface Integrity and Fatigue Performance of Hard Machined Surfaces

The study investigates the effects of cutting speed on the surface integrity and fatigue performance of hard machined surfaces. The results demonstrated that a higher cutting speed induces more compressive residual stresses and results in an increasingly softened layer. Crack initiation and crack propagation lives were extended by up to 192% and 168%, respectively, relative to increases in the cutting speed. Consequently, the fatigue life was extended by up to 174% with increases in the cutting speed. The fatigue tests demonstrated that the cutting speed significantly influences fatigue life and that the effect further increases with decreases in the level of loading.

Effect of Coating Removal by Induction Heating on the Microstructure and Properties of Deck Steel

Deck steel with anti-skid coating is treated by induction heating for coating removal. The microstructure of cross-section before and after coating removal is observed by metallographic microscope and scanning electron microscope. Z-direction micro-hardness of cross-section before and after coating removal is tested by micro-hardness tester. Results show that the softening layer of about 3 mm, which the average micro-hardness of the layer is 244 HV, is formed on the surface of the sample. The micro-hardness decreases by 3.5% compared with the untreated sample. Coating removal by induction heating will not have an impact on the metallographic structure. But carbide particles will be precipitated at the grain boundary. At last, the influence of induction heating on the performance of deck steel is briefly analyzed.

Influence of rake angle on surface integrity and fatigue performance of machined surfaces

This study investigates the influence of the rake angle on the surface integrity and fatigue performance of hard machined surfaces. The results demonstrate that a higher rake angle induces more compressive residual stresses and a more softened layer. Changing the rake angle was found to increase the crack initiation life up to 104% and crack propagation life up to 256%. Consequently, changing the rake angle increased the fatigue life up to 224%. The fatigue tests demonstrated that the rake angle has a significant influence on the fatigue life and that the effect is further increased if the loading is reduced.

Influence of feed rate on surface integrity and fatigue performance of machined surfaces

This study investigates the influence of the feed rate on the surface integrity and fatigue performance of machined surfaces. The results demonstrate that a higher feed rate increases crack initiation life and crack propagation life. A higher feed rate induces more compressive residual stresses and a more softened layer. The feed rate influences crack initiation life up to 45% and crack propagation life up to 149%. Consequently, the feed rate affects fatigue life up to 132%. The fatigue tests substantiate that the feed rate influences fatigue life significantly and that the effect increases significantly if the loading is reduced.