Subsurface Cracks Research Articles

Mechanism of subsurface microstructural fatigue crack initiation during high and very-high cycle fatigue of advanced bainitic steels

Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.

Geohazards explained 10

On the eve of the new year, 2021, a single landslide claimed 70 souls in Ask, a village in Norway. This tragic event highlighted, once again, the need to understand whether research efforts to map landslide susceptible areas could help save lives and if these identified landslide‐prone regions change with time. A landslide is a downslope gravitational mass wasting of earth materials. Hence, a classification model could estimate the likelihood of a landslide occurring under certain terrain conditions studying the landslide predisposing factors, such as hillslope inclination and land cover, of old landslides. Projecting these likelihoods in a landscape would be a landslide susceptibility map, which highlights areas that could potentially generate a landslide without any implication of an occurrence time. However, landslide predisposing factors change over time, resetting those susceptibility estimates—they are not static as traditionally assumed by most models. These changes could be evident, such as artificial alterations in land cover, or disguised, such as accumulated damage on hillslopes in the form of subsurface cracks due to a large earthquake. In times referred to as legacy effects, those latter hidden effects could be assessed by studying the spatial distribution of those landslides triggered by the same event. This perspective lists several potential biases of the time‐invariant landslide susceptibility approach and offers hints to overcome these challenges using a more dynamic model that evolves.

Morphology of subsurface cracks in glass-ceramics induced by Vickers indentation observed by synchrotron X-ray multiscale tomography

The characterization of subsurface cracks induced by indentation is a challenge for understanding contact damage, impact, wear, erosion, and abrasion of brittle materials, because the crack pattern observable on the surface is only a part of the total crack system. Here we applied synchrotron X-ray multiscale tomography to observe the morphology of subsurface cracks produced by Vickers indentation in a novel CaO–Al2O3–SiO2 glass-ceramic with plate-like crystals forming a house-of-cards microstructure. It revealed a diverse type of crack systems around the semispherical microcrack zone beneath the indent, including a new mode II inclined lateral crack driven by the maximum shear stress. Tomography images provided knowledge on how the heterogeneous microstructure affected the toughening processes such as crack deflection, crack bridging, and microcracking.

Analysis and countermeasures of early delamination of the nitrided tappet insterts in the valve train of a high speed and heavy load diesel engine

Early delamination of the nitrided tappet inserts in the valve train of a high speed and heavy duty diesel engine occurred and thus the failure mechanism of early delamination was investigated with the method of numerical simulation and experiment at first. Additionally, the countermeasure was put forward and identified using a valve train bench test rig. Flying off of the tappet in valve train took place, causing repeated impact forces between the interfaces of the tappet and cam. Early delamination of the compound layer (CL) was formed once the surface crack connected with the subsurface crack. Removal of the CL of the nitrided tappet inserts was verified as an effective countermeasure for avoiding the early delamination as well as protecting the matching part, which suggests the hard and brittle CL of the nitrided part should be removed under a heavy load condition especially accompanying with repeated impact forces.

Finite element analysis of laser-generated Rayleigh wave for sizing subsurface crack in frequency domain

In order to realize the quantitative detection of subsurface cracks, finite element method (FEM) was used to study the interaction of laser-generated Rayleigh wave and subsurface crack with different buried depths and lengths in this paper. The transmitted and reflected Rayleigh waves of subsurface cracks were analyzed in frequency domain, it is found that the center frequency of transmission wave decreases with the increase of crack length when the buried depth of crack remains unchanged, and increases with the increase of crack depth when the crack length remains unchanged. The center frequency of reflected Rayleigh wave is independent of crack length, and has a quadratic function relation with the buried depth of subsurface crack. The relationships, between the ratio of buried depth to incident Rayleigh wavelength and the ratio of reflected Rayleigh wave frequency to incident Rayleigh wave frequency, and the ratio of crack length to incident Rayleigh wavelength and the ratio of transmitted Rayleigh wave frequency to incident Rayleigh wave frequency, were obtained respectively based on the frequency analysis. The relationships were verified by the simulation signals, it is found that the maximum relative error of the buried depth of the subsurface cracks determined by the proposed relationship is 5.939%, and the maximum relative error of the length of the subsurface cracks is 12.149%. The research results show that the proposed method is promising in quantitative characterization of the buried depth and length of subsurface cracks in metallic materials.

Experimental study on the mechanism of strain rate on grinding damage of zirconia ceramics

To study the mechanism of strain rate on the grinding damage of zirconia ceramics (ZrO2), a ZrO2 surface grinding experiment was conducted to analyse the effect of strain rate on the grinding force, surface morphology and subsurface cracks. As the strain rate increased, the grinding force decreased significantly; the grinding scratches gradually became continuous and clear, and the grinding surface gradually became smooth. Increasing the strain rate can greatly reduce the length of subsurface cracks; however, the length of subsurface cracks drops slowly and tends to stabilise when the strain rate reaches a certain value. The direction of subsurface cracks is related to the internal defects of the material, and there is a trend of approaching the surrounding cracks and defects. When the internal defects were stimulated by the impact stress wave, the cracks started to grow and expand from the tip, and the expanding direction remained at approximately 125° in the direction of the defect length. The results show that an increase in the grinding strain rate can effectively improve the grinding quality of zirconia ceramics, which has guiding significance for practical production.

High temperature fatigue behavior of a near-alpha titanium alloy

High-cycle and very-high-cycle fatigue at 450 °C in near-α titanium with bi-modal microstructure are investigated. Stress-life duality appears with one data group spanning from 105 to 107 and the other from 107 to 109 cycles, characterized by surface and subsurface cracking, respectively. Misfit strain induced by the high misorientation of a particular primary α-grain relative to its surroundings, termed as local texture, promotes the subsurface fatigue-crack initiation. Fatigue strength increases up to 100 MPa owing to cyclic pre-strain at 450 °C, but the duality remains. Strain hardening by activation of prismatic <a> slip is responsible for the improved fatigue strength.

Relationship between subsurface stress and wear particle size in sliding contacts during running-in

The results of an experiment set using a pin-on-disk rig on samplers made of ST37 disk and bearing steel pin are reported to characterize the wear rate of the specimen, weight loss, friction coefficient behavior, and wear particle size during running-in. Using the equations of subsurface stress for spherical contact, the location and value of maximum subsurface stress in the running-in period in three dimensions is predicted. The results indicate that there exists a relation between the size of the detached particles and the location at which the maximum subsurface shear stress occurs. Based on wear particles’ SEM images, there is a direct relationship between the size of the wear particles with the applied load, applied speed, and the initial surface roughness. It is shown that increasing the load and speed results in a rise in the friction coefficient, which affects the magnitude and location of the maximum subsurface shear stress. The latter is responsible for the nucleation and propagation of subsurface cracks.

Non-destructive Inspection of Subsurface Cracks in Carbon-fiber and Glass-fiber Reinforced Polymeric Materials Using Digital Shearography Method

Non-destructive Inspection of Subsurface Cracks in Carbon-fiber and Glass-fiber Reinforced Polymeric Materials Using Digital Shearography Method

Distribution Characteristics of Sub-Surface Cracks in Fused Quartz Ground with Different Worn Wheels.

The crack distribution characteristics in grinding damage have a significant impact on subsequent polishing efficiency and part strength. Grinding tests were carried out on fused quartz using grinding wheels with different wear states. The results showed that the cracks produced by sharp abrasive grits were mainly near chevron cracks and had no preferred direction. However, the blunt abrasive grits produced near partial cone cracks had a preferred direction. At a depth of 96.7 μm from the surface, the amount of cracks in the range of 50°~90° with an inclination angle between the crack and the cutting direction could reach 88.9%. The statistical results showed that the depth and total length of cracks produced by sharp grits were larger than those produced by blunt grits (the maximum crack depth difference was about 40 μm). Therefore, it was concluded that sharp abrasive grits were not conducive to damage control. The findings of this research enhance our understanding of the formation mechanism of grinding damage.

Experiment and theoretical prediction for subsurface microcracks and damage depth of multi-crystalline silicon wafer in diamond wire sawing

Diamond wire saw slicing is an important process of multi-crystalline silicon (mc-Si) solar cell substrate processing. In wire sawing of silicon crystal, the brittleness removal of materials will lead to subsurface microcrack damage, reduce the fracture strength, increase the breakage probability of the as-sawn wafer and affect the subsequent surface etching texture of wafer. In this paper, a mathematical model calculating subsurface damage depth (SSD) of mc-Si is established based on the analysis of subsurface crack system in diamond wire sawing process. The correctness and rationality of the model are verified by experiments. Then, the model is used to predict the variation trend of the microcracks number (CN) and subsurface damage depth under different process parameters and saw wire parameters. The results show that the SSD decreases and CN increases with the increase of wire speed and decrease of feed speed; With the increase of abrasive density on wire surface, the SSD decreases and CN increase. The change of abrasive average size has little effect on the SSD and CN; a smaller abrasive size range is beneficial to reduce the SSD and increase CN.

Investigation of surface quality, microstructure, deformation mechanism, and fatigue performance of additively manufactured 304L stainless steel using grinding

Ground 304L stainless steel (SS) made by laser powder bed fusion (LPBF) process was comprehensively investigated from the aspects of surface integrity, residual stress, microstructure, and mechanical properties. The experimental results indicated that grinding substantially improved the surface quality characteristics and significantly increased the fatigue life by more than two times. The removal of LoF (Lack of fusion) defects and keyholes near the as-built surface plays a critical role in improving fatigue performance due to the enhancement of fatigue crack initiation resistance. In addition, the ground subsurface cracks and newly formed lath-like grains were discovered in ground LPBF 304L SS, and their formation mechanisms were discussed with the aid of electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). This work brings new insights into the underlying principles of improving the mechanical properties of LPBF metallic materials through high-efficiency grinding.

Research on crack monitoring technology of flexible eddy current array sensor based on TMR sensors

The TMR (Tunnel MagnetoResistance) sensors are used to replace the original induction coils, and the TMR sensors are arrayed along the x-axis direction, which quantitatively monitor crack propagation through the response sequence of TMR sensors. Firstly, a current dipole model is proposed to characterize the influence of disturbed magnetic induction intensity at the test point, and the method of quantitative monitoring of subsurface crack is proposed. Then, the finite element model of the sensor is established to analyze the disturbed effect of crack propagation on eddy current with different excitation frequencies. Finally, the experiments on the surface and subsurface crack monitoring with different depths are carried out. The experimental results show that the crack propagation can be monitored quantitatively through the response sequence of the TMR sensors. Also, the effect of subsurface crack monitoring can be improved by increasing the longitudinal distance between the test point and the crack (LDTC).

A peridynamic frictional contact model for contact fatigue crack initiation and propagation

Cyclic contact between components can cause cracks in the contact area. Peridynamics can overcome the difficulty of solving cracks and other discontinuities in classical continuum mechanics. A new peridynamic sliding frictional contact model is proposed and verified by a two-dimensional point contact sliding simulation. Then, a two-dimensional plane strain elastoplastic peridynamic model of a single micro-contact asperity is established, and the fatigue crack initiation and propagation in the micro-contact area during fretting wear is analyzed. The results show that in fretting wear, crack initiation originates from the area of maximum shear stress below the surface. The subsurface crack initially propagates in the direction parallel to the surface, and then gradually propagates to the surface. Finally, the crack grows to the free surface and forms wear debris. The contact pressure and the coefficient of friction in the contact surface can affect the crack initiation depth and crack propagation path, thereby affecting the size of wear debris. The crack propagation path is not sensitive to the change of the coefficient of friction of the crack surface. This study helps to understand the mechanism of fretting wear and provides a reference for predicting fretting wear.

Damage behavior of heavy-haul rail steels used from the mild conditions to harsh conditions

Systematic experimental investigations were conducted to study the damage behavior of rail steels with eutectoid and hypereutectoid microstructure under different axle load and curve radius conditions. The wear resistance and resulting failure mechanism, as well as subsurface crack development were evaluated by wheel/rail rolling contact test rig, which can realize a broad range of working conditions from mild condition to harsh condition. Results demonstrate that the wear resistance and rolling contact fatigue (RCF) damage depend on not only the working conditions employed, but also the microstructures. With the increase of axle load and reduction of curve radius, the wear rate increases for both rail steels. Damage mechanisms for the two rail materials are mainly surface fatigue cracks and delamination, which become much serious from the mild condition to harsh condition. The hypereutectoid rail possesses a better wear resistance than eutectoid rail in general. Wear regime map suggests that the hypereutectoid rail did not have a catastrophic wear region, but can be observed in eutectoid rail. The subsurface analysis shows that the working condition has a significant impact on surface crack development depending on specific microstructure types. The hypereutectoid rail is more suitable for harsh working conditions than eutectoid rail.

Antagonistic effect of electrochemical corrosion on the mechanical wear of Monel 400 alloy in seawater

The tribocorrosion behavior as well as the corrosion-wear interaction of Monel 400 alloy in seawater were investigated under varied loads. An abnormal antagonistic effect of corrosion on wear was found, which was highly related to the peculiar corrosion products in the shape of nanospheres only generated under the tribocorrosion condition due to the selective corrosion of Ni-rich components. A boundary lubrication layer, consisting of the nanospheres squeezed and spread by friction stress, leaded to the reduction of friction coefficient. Moreover, the material loss rate was greatly alleviated because the friction-induced surface delamination and sub-surface crack propagation were prevented.

Quality and flexural strength of laser-cut glass: classical top-down ablation versus water-assisted and bottom-up machining.

The growing applicability of glass materials drives the development of novel processing methods, which usually lack comprehensive comparison to conventional or state-of-art ones. That is especially delicate for assessing the flexural strength of glass, which is highly dependent on many factors. This paper compares the traditional top-down laser ablation methods in the air to those assisted with a flowing water film using picosecond pulses. Furthermore, the bottom-up cutting method using picosecond and nanosecond pulses is investigated as well. The cutting quality, sidewall roughness, subsurface damage and the four-point bending strength of 1 mm-thick soda-lime glass are evaluated. The flexural strength of top-down cut samples is highly reduced due to heat accumulation-induced cracks, strictly orientated along the sidewall. The subsurface crack propagation can be reduced using water-assisted processing, leading to the highest flexural strength among investigated techniques. Although bottom-up cut samples have lower flexural strength than water-assisted, bottom-up technology allows us to achieve higher cutting speed, taper-less sidewalls, and better quality on the rear side surface and is preferable for thick glass processing.

Influence of the compound layer on the rolling contact fatigue properties of nitrided medium-carbon steel at the elevated temperature

Influence of the compound layer (CL) on the rolling contact fatigue (RCF) properties of plasma nitrided steel was experimentally investigated at the elevated temperature. The results show that with the removal of its CL, the rated life (L10), the median life (L50) and characteristic life (L63.2) of the nitrided steel increased by 157.9%, 57.9%, and 54.0%, respectively. Spalling and delamination are two main RCF failure modes of the nitrided steel independent on the CL. Spalling was caused by the mutual collection of two ring-type cracks or a ring-type crack and the main crack perpendicular to the rolling direction, while delamination was associated with the convergence of the ring-type crack and the subsurface main crack which was induced by not only the maximum shear stress but also the vein-like structure. Early spalling of CL made more severe stress concentration, which accelerated the initiation and propagation of the ring-type crack and then significantly reduced the RCF life of the nitrided steel. Consequently, a brittle CL is suggested to be removed to improve the service life of the industrial parts made of the nitrided steel and operating under the alternating stress.

Estimation of crack growth on rails under the combined influence of thermo-mechanical loading due to the sliding of wheel

Development and growth of surface and subsurface cracks on rail profile is one of the major reasons for deterioration of rails near the halting zones. Rail tracks are subjected to combination of thermal loads during the sliding motion of the wheel while braking and structural load due to the weight of the vehicle. In this paper, an FEA model is been developed to study and analyze the effect of coupled thermo-mechanical load during braking on contact stress and contact area at the rail-wheel interface. Both elastic and elasto-plastic FEA models are used and compared with analytical Hertzian approach to accurately predict rail-wheel contact parameters. A synthetic loading spectrum is proposed to simulate the combination of thermal and thermo-mechanical loading cycles on the rail during braking of vehicle. The loading cycle is used to predict the growth of previously present flaws using Modified Paris Law and Generalized Frost-Dugdale approach. It has been found that elasto-plastic FEA is inclined to predict lesser contact stresses than the elastic FEA or Hertzian model. Furthermore, shape of the contact patch for elasto-plastic FEA is observed to be unsymmetrical in the rolling direction unlike the elliptical contact area in elastic FEA approach. Macro scale initial cracks having length more than 2 mm are also realized to grow faster than their smaller counterparts under the combined effect of thermal and mechanical loads.

Hydrogen-Induced Delayed Fracture in a 1.5 GPa Dual-Phase Steel Via Coalescence of Surface and Sub-Surface Cracks

Hydrogen-Induced Delayed Fracture in a 1.5 GPa Dual-Phase Steel Via Coalescence of Surface and Sub-Surface Cracks