Surface Crack Nucleation Research Articles

Effect of PtAl coating on the HCF property of a coated third-generation single crystal superalloy with sheet specimens at 900 ℃

High-cycle fatigue (HCF) is one of the primary failure modes for turbine blades in aero-engines. Therefore, comprehending the effect of coating on the HCF property of single crystal (SX) superalloys is vital for the safe service of turbine blades. In order to mimic the degradation of the fatigue property for the turbine blades during service, this study investigated the effect of PtAl coating on the HCF property of a third-generation Ni-based SX superalloy using sheet specimens at 900 ℃ and different maximum stresses (σmax). The results indicated that both coating and σmax affected the fatigue properties and crack initiation process of SX superalloy. A transition in fatigue crack initiation site from the internal micropores to the surface cracks for both uncoated and coated alloys has been observed as the σmax decreased. However, the coating significantly increased the σmax required for the transition of the crack initiation site. Meanwhile, the HCF property of the SX superalloy was reduced with the deposition of PtAl coating at lower σmax, and the debit in HCF life was more pronounced with decreasing the σmax. The coating facilitated the rapid nucleation of surface cracks and promoted the transition of the fatigue crack initiation site under higher σmax. On the other hand, since the HCF life is controlled by the crack growth rate, grain boundaries in the interdiffusion zone (IDZ) and secondary reaction zone (SRZ) accelerated the growth of the surface crack under lower σmax, resulting in a shorter crack initiation stage and lower fatigue life compared to the uncoated alloy. This study will be helpful in accurately predicting the HCF life of the coated Ni-based SX superalloys.

Microtexture effects on fatigue and dwell-fatigue lifetimes of Ti-6Al-4V

The influence of microtexture on the fatigue and dwell-fatigue response of Ti-6Al-4V was investigated considering two regions of a part showing different degrees of microtexture. A high degree of microtexture resulted in a significant reduction in fatigue lifetime. No difference in crack initiation mechanism was observed, as main cracks formed along basal twist grain boundaries (BTGB) connected to the specimen surface. However, a key contribution of faster small crack growth in the strongly microtextured region is suggested. A 120 s dwell time resulted in a dwell-fatigue lifetime debit exceeding a factor of 10 in this region while it remained below a factor of 3 in the weakly microtextured region. In the dwell-sensitive region, internal nucleation of the main crack occurred at a BTGB and long range facetted growth was enabled by the adjacent microtextured region with a dominant [0001] orientation. In contrast, surface crack nucleation typical of fatigue failures dominated in the dwell-insensitive region. The limited growth of internal cracks suggests that the lack of path for rapid crack growth has a pivotal effect for the low dwell sensitivity. The competition between fatigue and dwell-fatigue damage highlights the importance of considering both in modeling approaches to obtain accurate lifetime predictions.

Resistance of Heterogeneous Metal Compositions to Fracture under Dynamic and Cyclic Loads

This paper presents the results of experimental data analysis, which indicate an increased resistance of heterogeneous multilayer clad composites to dynamic loading destruction compared with homogeneous materials. The reason for this is the crack retardation caused by lamination at the boundary of the layers. The destruction of heterogeneous compact composite samples by cyclic off-center stretching also occurs with crack retardation, with the fractogram clearly demonstrating the transverse tightening of the sample section. We argue that crack nucleation plays a decisive role in the process of dynamic destruction of heterogeneous composites obtained by both multilayer cladding and explosion welding. This study presents generalized calculated data confirming the influence of the sign and magnitude of residual stresses (the appearance of a stress discontinuity) on the conditions of fatigue surface crack nucleation and propagation. Unlike homogeneous materials obtained by casting, forging (rolling), or cladding, which are characterized by a linear dependence of the crack propagation velocity on the dynamic stress intensity coefficient, for multilayer composites consisting of strong and viscous layers, a sharp crack deceleration is observed. This is due to the transition of the crack boundary between the strong and viscous layers. This paper presents studies of the corresponding properties of adjacent layers on the integral characteristics of the deposited composite.

Fracture of single crystal silicon caused by nonlinear evolution of surface acoustic waves

High-amplitude surface acoustic waves (SAWs) undergo nonlinear sharpening during propagation due to the elastic nonlinearity of the material, which leads to material fracture. The degree of nonlinear evolution and material strength are usually characterized by two parameters: the second-order nonlinearity parameter and the critical fracture stress. To study the influence of processing defects, residual stress and temperature on these two parameters, we carry out molecular dynamics modeling of nonlinear propagation of SAWs on non-ideal surfaces. The Stillinger and Weber (SW) and the Lee-modified second nearest-neighbor modified embedded-atom-method (2NN MEAM) potentials are used to study the SAW nonlinear evolution and surface crack nucleation in silicon 1111¯1¯2 geometry, respectively. The results show that processing defects have the greatest influence on the SAW evolution and material strength. For a defined level of defects, the size of initial SAW wavelength directly affects the degree of attenuation of harmonic waves and stress concentration, and then determines the measured nonlinear parameter and critical stress. Temperature can affect the magnitude of the two characterization parameters through thermal dissipation and thermal fluctuations. In the presence of residual stresses only, the relative degree of variation of these two parameters is weak. This study may provide a valuable reference for the establishment of nonlinear SAW atomic-level theory and material characterization experiments.

Analysis of subsurface layer formation on a pearlitic rail under heavy haul conditions: Spalling characterization

The depth of rolling contact fatigue (RCF) defects in railways plays a significant role in the rail lifetime, affecting rail maintenance procedures due to the amount of material to be removed from the railhead. Therefore, a precise characterization of the damaged rail is needed to define the optimal material removal. In the present study, an ex-service and damaged rail that contained defects due to RCF was investigated in detail. The microstructural characterization was conducted on particular sites using optical microscopy (OM), scanning electron microscopy (SEM), SEM with focused ion beam (SEM-FIB), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). Three layers were identified at the rail cross section: a white etching layer (WEL) was the topmost layer and comprised martensite; a transitional layer (TL) with martensite and retained austenite was identified below the WEL; followed by a severely plastically deformed layer with refined grains and a large number of high-angle boundaries (HABs) and elongated pearlite colonies. In this last region, {111} and {110} were common crystallographic orientations. A gradual decrease in the hardness was observed along these layers. The WEL achieved a hardness value of 12 GPa, while the TL had an average value of 8 GPa, and measurements on the plastically deformed layer indicated a hardness of 6 GPa. The observed microstructural transformation suggests that thermal loading led to a thermally produced WEL (TP-WEL). The brittle nature of the WEL and the surface shear deformation and high levels of stress promoted the nucleation of surface cracks and propagation of them through the WEL. The presence of the different metallurgical phases in the TL promoted a change of crack orientation, leading to material detachment.

A new statistical software for the estimation of P-S-N curves in presence of defects: statistical models and experimental validation

It is well-known in the literature that fatigue failures originate from the weakest element within the component loaded volume. In particular, for metallic materials the fatigue crack typically nucleates from the most critical surface defect in the High-Cycle-Fatigue (HCF) region; whereas, it generally nucleates from the most critical internal defect in the Very-High-Cycle-Fatigue (VHCF) region, at stress amplitudes below the so-called ‘transition stress amplitude’. Therefore, regardless of the fatigue region, the P-S-N curves must necessarily take into account the random distribution of the critical defect size. However, in the literature, there are few statistical models that can model the dependency between the fatigue life and the defect size or that can take into account the presence of different failure mechanisms (e.g., surface crack nucleation in HCF and internal crack nucleation in VHCF). In the present paper, a new software for the estimation of the P-S-N curves is proposed. The statistical P-S-N models recently proposed by the authors and implemented in the software are described in the paper. The procedure for the parameter estimation is also explained in detail and the software is finally validated with experimental datasets collected by the authors or available in the literature.

A Small-Scale Experimental Study of the Damage Due to Intermittent Shoe Braking on the Tread of High-Speed Train Wheels

Recent interest in emergency shoe braking in high-speed trains stimulated research on the damage due to sliding contact between brake blocks and wheel treads. Bi-disc tests, with rolling–sliding contact between specimens extracted from wheels and brake blocks, were previously performed in constant working conditions to reproduce the temperature of the wheel during braking. However, braking is often a discontinuous operation, obtained by intermittent contact between brakes and tread, to prevent excessive heating. In this article, braking is simulated by discontinuous tests, periodically interrupted to cool down the specimens. Three wheel steels (HYPERLOS, SANDLOS, and CLASS B) were tested against the same cast iron brake material. Measurements of the friction coefficient, contact surface temperature, and weight variation were performed. Moreover, optical microscopy observations and hardness tests were done on the cross section of the wheel specimens. Material transfer from the brake to the wheel disc with the formation of a discontinuous third-body layer was observed. This layer, when worn away, likely contributes to surface crack nucleation. In addition, the debris of both materials enhances abrasive wear. If compared with the continuous tests, the intermittent tests showed higher friction coefficient fluctuations and in some cases significantly different weight variation. Among the steels, the weight loss and subsurface hardening of HYPERLOS was higher in the discontinuous tests than in the longest continuous tests. CLASS B showed similar behavior in both tests. An effect of the temperature cycles on the friction coefficient and the wear behavior was hypothesized.

Effect of shoe braking on wear and fatigue damage of various railway wheel steels for high speed applications

The effect of shoe braking on three railway wheel steels was studied by means of bi-disc tests, in order to evaluate the applicability of shoe braking as an emergency stop braking system in high-speed passenger trains. Disc-shaped specimens, extracted from real wheels, were initially paired with discs extracted from cast iron brake blocks, in condition of rolling and sliding such to reach the typical temperature of the surface of a wheel rim during stop braking. Subsequently, the wheel discs were coupled with rail steel discs in rolling and sliding dry contact. Some of them were cut and analysed; other ones were subsequently subjected to rolling and sliding contact with rail specimens in wet condition, with various test duration. Measurements of the coefficient of friction and of the weight loss were carried out during the tests; at the end of the tests, hardness measurements and microstructure observation were made on the disc cross-sections. The main damage phenomena observed in the specimens subjected to wheel-brake contact and dry wheel-rail contact were ratcheting, wear and nucleation of surface cracks. Traces of cast iron, previously transferred from the brake specimens to the wheel ones, were detected. The main damage phenomenon observed in the specimens subjected to wheel-brake, dry wheel-rail and wet wheel-rail contact was rolling contact fatigue, developing by the propagation of surface cracks due to the pressurization of the water entrapped inside the cracks. All these phenomena were found to be similar to those observed in wheel specimens subjected to wheel-rail contact only. The effect of the cast iron traces on surface crack propagation was evaluated by means of finite element simulations and it was found to be limited. Therefore, the application of shoe braking as emergency stop braking system to high speed trains did not show any relevant contraindications.

A Methodology for Predicting Surface Crack Nucleation in Additively Manufactured Metallic Components

The as-built surfaces of metallic parts made by additive manufacturing (AM) processes often contain notch-like features that serve as stress concentration sites where fatigue crack nucleation may occur. These notch-like features, which result from the surface roughness inherent to AM processes, should be considered in assessing the fatigue performance of AM parts. In this investigation, a methodology for predicting surface crack nucleation in AM materials has been developed by considering (1) the stress concentration factors (kt) associated with the surface micro-notches, (2) the cycles to surface crack nucleation, and (3) the threshold stress ranges for the growth or non-growth of the microcracks. The combination of the micro-notch and low-cycle fatigue (LCF) life model, named micro-LCF model, shows that there exists a minimum surface roughness profile, defined in terms of a ratio of valley depth (Rvi) to valley spacing (λi) of 0.01, below which fatigue life debit does not occur. Furthermore, fatigue crack nucleation and growth dominates at Rvi/λi 0.18 and kt > 1.5. A surface roughness with an Rvi/λi ratio = 1 and kt = 3 is also identified as the worst-case condition where fatigue life is entirely controlled by crack growth. This methodology has been applied to a nickel-based superalloy AM 718Plus to confirm the various ways in which surface roughness affects surface crack nucleation and growth. In addition, potential fatigue life enhancement that can be achieved by machining portions, if not all, of the as-built surfaces is assessed.

Study of the damage induced by thermomechanical load in ER7 tread braked railway wheels

This work aims to better understand the complex damage phenomena taking place at the wheel/brake block interface due to the thermomechanical load. An experimental procedure, articulated in three series of tests carried out with a bi-disc machine, was designed in order to experimentally simulate in controlled laboratory conditions the thermomechanical history of the real wheel during stop braking. The first series of tests was performed on ER7 wheel steel discs paired with cast iron shoe material discs, setting the sliding speed and the contact load in such a way to generate the heat flux needed to reproduce the typical tread temperature of a real wheel in stop braking. The second series was carried out by repeating the tests in the conditions of the first series and subsequently subjecting the tested wheel specimens to rolling/sliding contact with discs of 350HT rail steel. The third series was carried out by repeating the two phases of the second series and subsequently adding water to the contact interface of the wheel-rail specimens. Measurements of friction coefficient, surface temperature and weight changes were carried out during the tests. At the end, cross-sections of the specimens were observed with an optical microscope. The hardness along the depth was measured. It was observed that during the braking phase parts of the wheel specimen surface are coated by a discontinuous layer of cast iron that is transferred from the brake block specimens. During the braking phase and the subsequent phase of dry contact with the rail specimen, the transferred material is removed, promoting the nucleation of surface cracks; in addition, surface cracks are generated also by ratcheting due to high friction. During the subsequent wet contact phase, these cracks propagate in the wheel disc due to the pressurization of the fluid entrapped inside the cracks. The propagation of surface cracks in wet contact was assessed by a fracture mechanics approach, including the Finite Element simulation of a surface crack with entrapped fluid. The stress intensity factor range during a load pass was calculated and compared with the propagation threshold of the ER7 steel, determining this way the critical depth of surface cracks. This study is a step towards a damage tolerant approach for the designing and maintaining tread-braked wheels.

Effect of Prior Creep Strain on High Cycle Fatigue Life of TI 834

Room-temperature fatigue tests were conducted on Ti 834 with prior creep strains accumulated under constant load at 550 °C and 600 °C, respectively. Microstructural and fractographic examinations on specimens with prior creep strain > 3% revealed the failure process consisting of multiple surface crack nucleation and internal void generation by creep, followed by fatigue crack propagation in coalescence with the internally distributed damage, leading to the final fracture. The amount of prior creep damage increased with creep strain. The fatigue life of Ti 834 was significantly reduced by prior creep straining. The behavior is rationalized with the integrated creep-fatigue theory.

Description of the Kinetics of Short Surface Fatigue Crack Growth Using the Parameters of Fatigue Curves

To describe the kinetics of short surface crack nucleation and growth in metals and alloys, an approach has been developed, which is based on deformation curves in the low-cycle fatigue region. In view of the known Ramberg–Osgood stress–strain relationship, the deformation fatigue curves have been replaced by traditional fatigue curves. Analysis of known experimental data shows that the kinetics of short surface fatigue crack initiation and growth can be described by the parameters of fatigue curves for both the low- and high-cycle fatigue region, which makes such results more informative.

Effect of Wear on Surface Crack Propagation in Rail–Wheel Wet Contact

The interaction between wear and rolling contact fatigue in wet contact was investigated by means of experiments on a two-disk test bench. Specimens of various railway wheel steels were coupled with specimens of the same rail steel, subjected to rolling and sliding wet contact with varying sliding/rolling ratio. Some tests with a dry rolling–sliding contact phase preceding wet contact were carried out as well (dry–wet tests). The pressurization of the fluid entrapped in the surface cracks was the cause of a rapid and severe damage in the dry–wet tests, due to the nucleation of surface cracks by ratcheting in the dry phase, which subsequently propagate in the wet phase. In the wet tests, the fluid pressurization effect was much mitigated due to the absence of initial surface cracks; the specimens subjected to higher sliding/rolling ratio showed the best performance against rolling contact fatigue, due to the effect of wear in reducing the length of surface cracks. A model for assessing the interaction between wear and rolling contact fatigue was proposed, based on the correction of the classical Paris law for crack propagation by taking into account the effect of wear on crack length reduction. The application of the model to the experimental tests allowed finding a correlation between the model predictions and the occurrence of fatigue failure.

A model for predicting plastic strain and surface cracks at steady-state wear and ratcheting regime

Unidirectional plastic strain accumulation (ratcheting) is one of the main causes of surface crack nucleation in rails and wheels in dry condition. It is related to frictional forces which develop at the contact interface due to sliding, especially in curve and in braking. Surface cracks generated by ratcheting can subsequently lead to severe damage when environmental fluid contaminants (such as rain or snow) are added at the contact interface, due to the complex solid-fluid interaction, which can enhance crack growth.In dry condition, wear and ratcheting can reach an equilibrium, such that the strain field and the crack depth are stationary. Understanding such steady-state deformation regime can be a key factor for predicting the expected crack depth and scheduling a correct maintenance programme.In this paper a model for predicting the strain field at steady-state regime is proposed. Such model, based on an integral equation, allows predicting the strain field and crack morphology at high cycle number with no need of iterative numerical simulation. The potentiality of the model was proven both in characterizing the cyclic plasticity behaviour of materials and in predicting the maximum expected crack depth in full scale railway wheels.

Damage phenomena in lubricated rolling-sliding wear of a gas carburised 0.85%Mo low-alloyed sintered steel: theoretical analysis and experimental verification

ABSTRACTThe resistance to surface and subsurface damage during lubricated rolling-sliding wear of a carburised low-alloy sintered steel and the effect of shot peening were investigated. The formation of both contact fatigue cracks and of brittle tensile cracks may be predicted by a theoretical model that was experimentally validated. Carburising is effective in increasing the resistance to contact fatigue, but pores in a hard and brittle matrix may act as pre-existing cracks. Shot peening increases the contact fatigue resistance since compressive residual stresses oppose the nucleation of surface cracks.

Tribomechanical analysis of interlayer shear and surface crack nucleation processes in compacted disperse materials

The effect of the internal (interparticle) and external friction on the limiting state generation has been analyzed in terms of the shear-induced failure criterion of the surface layers in the compacted disperse materials. The proposed design scheme based on the Mohr-Coulomb combined diagram has made it possible to develop an original method and derive analytic dependences for determining the normal and tangential stresses in the interlayer shear sites along with the orientation angles of these sites. It has been shown that, with increasing coefficient of external friction, the pressure in the interlayer shear sites reduces until tensile stresses are generated. This is accompanied by the nucleation of the crack at a right angle close to the side surface perpendicular of the formed item.

Surface contact fatigue failure of a case hardened pinion shaft

An investigation was made to determine the causes of surface contact fatigue failure of a case hardened driver pinion located in the intermediate shaft of a reducer gearbox used in a sugar and alcohol mill. The examination of the component revealed the presence of a cemented layer substantially thicker than that generally specified for pinions devised for this application. This, associated with the massive presence of brittle threadlike carbon-rich cementite phase (Fe3C) in prior austenite grain boundaries of the pinion teeth, favored surface crack nucleation and propagation during cyclic loading, leading to spallation of the contact surface with the counterpart gear, which impaired the system's operation. Poor carburization practice was discovered as the root cause of the mechanical failure, thus demanding the implementation of a new manufacturing route to avoid problems in similar load-bearing rotating components.

Creep corrosion cracking of Sn–3.0Ag and Sn–0.5Cu solder alloys in NaCl solution

The surface crack nucleation of Sn–3.0Ag and Sn–0.5Cu solder alloys has been examined by performing sustained tensile-loading tests in 0.9 mass% NaCl solution at room temperature. For Sn–3.0Ag alloy, many cracks nucleate and propagate on the side surface of the specimen, similarly to Sn–3.0Ag–0.5Cu alloy reported previously. For Sn–0.5Cu alloy, such cracks are not observed, and ordinary creep deformation occurs in the solution. The effect of sustained applied stress, i.e., creep, on the dissolution of ions is smaller for Sn–0.5Cu alloy than for Sn–3.0Ag alloy. The present results suggest that there are differences in the susceptibility to cracking under applied stress in a solution, i.e., creep corrosion cracking, among lead-free solder alloys.

Surface crack nucleation and propagation in electrodeposited nanocrystalline Ni-P alloy during high cycle fatigue

The morphology of specimen surface after fatigue fracture was evaluated in connection with grain orientation distribution and grain boundary microstructure to reveal a mechanism of fatigue fracture in nanocrystalline materials. The electrodeposited and sharply {001} textured Ni -2.0 mass% P alloy with the average grain size of ca. 45 nm and high fractions of low-angle and Σ3 boundaries showed 2 times higher fatigue limit than electrodeposited microcrystalline Ni polycrystal. The surface features of fatigued specimen were classified into two different types of morphologies characterized as brittle fracture at the central area and as ductile fracture at the surrounding area.

Microstructural influence on small fatigue cracks in a ferritic–martensitic steel

Microstructure effects on fatigue crack initiation and propagation in ferritic–martensitic dual phase steel were investigated. Slip bands were formed in ferrite grains after several thousand cycles with ensuing crack initiation due to dislocation pile-up. Subsurface observations using a focused ion beam (FIB) and crystallographic analyses using electron backscatter diffraction (EBSD) measurements showed that crack initiation occurred as a result of the activation of a slip system having a high Schmid factor. Surface crack nucleation occurred quite frequently at ferrite/martensite and ferrite/ferrite boundaries, with crack propagation in the ferrite grains. This initiation mode can be attributed to the mismatch stresses at ferrite/martensite phase boundaries and at high angle grain boundaries.