Spall Initiation Research Articles

Analysis of the spalling process of rock mass around a deep underground ramp based on numerical modeling and in-situ observation

A comprehensive analysis method based on numerical simulation, displacement monitoring and digital borehole photography was proposed and employed to analyze the spalling characteristics and damage range. A continuous–discontinuous numerical model was set up, and which was calibrated based on field displacement monitoring data from Hongtoushan copper mine, and a developed continuous-discontinuous element method (CDEM) coupling the finite element method and the discrete element method was used to investigate the spalling initiation and propagation in the deep underground ramp under periodic stoping conditions, which was verified by the digital borehole photography. The results showed the initiation, propagation of ramp spalling associated with continuous periodic mining-induced stress, and parallels to the vertical stress direction. Severely spalling failure occurs at the left wall of the ramp, and the spalling depth is about 0.6 m. The spalling strain analysis also indicates that large-scale spalling failure occurs in the left wall, and which was verified by the digital borehole photography observation. The numerical simulation and field observation have good consistency. The comprehensive analysis method provide more realistic results, which helps to understand spalling failure mechanisms in deep hard rock mining and provide references for rock support.

Laboratory Study of the Fracturing Process in Marble and Plaster Hollow Plates Subjected to Uniaxial Compression by Combined Acoustic Emission and Digital Image Correlation Techniques

Hollow plates of plaster and calcitic and dolomitic marbles, extracted from greek quarries, with a single pre-existing cylindrical hole of various diameters are subjected in uniaxial compression. The influence of the material and the hole’s diameter on the fracturing process are studied and compared to the literature. Full-field digital image correlation technique and acoustic emission analysis are employed for the monitoring of the fracturing process, the initiation of the main phenomena, their interaction, and the failure mode of the physical models. New precise techniques on the determination of the true time of the primary fracture and spalling initiation by digital image correlation are implemented and presented. The study highlights a new behavioural type of such physical models, with different fracture patterns than those presented in the literature. A comparison between the required applied axial stress for the primary fracture and spalling initiation of the studied materials and the previously published results is presented, showing an exponential relation to the hole’s diameter.

Slip-Rolling Behavior of Ductile and Austempered Ductile Iron Containing Niobium or Chromium

Abstract The use of high niobium alloyed cast iron alloys is a relatively new approach in which the niobium addition intends to improve the properties of the material by the precipitation of hard niobium carbides during solidification. Steels can be replaced by ductile cast iron in some rolling applications, such as gears and cams, in order to reduce material costs. The aim of this work is to evaluate ductile iron alloyed with 1 weight percent (wt.%) niobium for the as cast specimens and with 1.8 wt.% and 2.4 wt.% niobium for the austempered specimens under lubricated slip-rolling tests using mixed/boundary conditions in an Amsler-type machine. Austempered ductile iron (ADI) alloyed with 1 wt.% chromium, or Carbidic ADI, was tested for comparison. For the as cast conditions, the niobium addition resulted in an increase of wear resistance owing to the low contact pressure of these tests. However, for the austempered specimens, the best performance was found for unalloyed ADI. The main factor acting in the initiation and propagation of cracks in ductile iron is the presence of the graphite nodules. The coarse carbides also contributed to the initiation of cracks and spalling of the material.

An elasto-plastic-damage model for initiation and propagation of spalling in rolling bearings

This study develops an elasto-plastic-damage model to investigate the spalling initiation and propagation behavior of bearings under rolling contact fatigue loading. The intrinsic coupling relationship between the fatigue damage and the elasto-plastic behavior of materials is considered by specifying the damage-related Helmholtz free energy and dissipative potential function in the thermodynamic framework. The damage evolution law is derived based on the plastic strain and the non-proportional cyclic stress. The developed model is implemented into finite element analysis for rolling contact of cylindrical bearings to evaluate the evolutions of the stress, strain and fatigue damage. The initiation and propagation processes of spalling in bearings are simulated. The predicted results of the spalling lifetime are compared with the experimental data from the literature. The damage-induced plastic deformation causes the shear stress-strain curve to possess hysteresis loops. It is observed that the fatigue damage competes with the plastic deformation to influence the shear stress that plays a major driving role in spalling. Moreover, the effects of the contact pressure and the coefficient of friction on the spalling process are studied.

Prognostics Models for Railroad Tapered Roller Bearings with Spall Defects on Inner or Outer Rings

Rolling contact fatigue (RCF) is one of the major causes of failure in railroad bearings used in freight service. Subsurface inclusions resulting from impurities in the steel used to fabricate the bearings initiate subsurface fatigue cracks, which propagate upwards and cause spalling of the rolling surfaces. These spalls start small and propagate as continued operation induces additional crack formation and spalling. Studies have shown that the bearing temperature is not a good indicator of spall initiation. In many instances, the temperature of the bearing increases markedly only when the spall has spread across major portions of the raceway. In contrast, vibration signatures can be used to accurately detect spall initiation within a bearing and can track spall deterioration. No monitoring technique can indicate the growth rate of a spall or determine residual useful life. Hence, the main objective of this study is to develop reliable prognostic models for spall growth within railroad bearings that are based on actual service life testing rather than theoretical simulations. The data used to devise the models presented here were acquired from laboratory and field testing that started in 2010. Growth models are provided for spalls initiating on the bearing inner (cone) and outer (cup) rings. Coupling these prognostic models with a previously developed vibration-based bearing condition monitoring algorithm will provide the rail industry with an efficient tool that can be used to plan proactive maintenance schedules that will mitigate unnecessary and costly train stoppages and delays and will prevent catastrophic derailments.

Numerical Study of the Fracturing Process in Marble and Plaster Hollow Plate Specimens Subjected to Uniaxial Compression

Physical models of plaster, calcitic and dolomitic marbles from greek quarries with a single pre-existing cylindrical hole of various diameters, subjected in uniaxial compression are simulated numerically with a bonded particle model by employing the two- and the three-dimensional versions of the Particle Flow Code. The linear parallel bond model is employed for the plaster’s simulation, and respectively the flat-joint model for the marbles. The calibration procedure and results are presented, as well as the simulation results of the hollow specimens. The micro-cracking, the fracturing process and the sequence of their appearance during the numerical tests of the hollow plates are in good agreement with the laboratory tested physical models. Each fracture pattern is similar to the one of the physical models. Also, the regions of micro-cracking in the numerical models is quite similar to the regions of intense deformation observed from digital image correlation analysis on the respective physical models. The followed methodology for the determination of the phenomena’s onset is presented as well. A comparison between the peak strength and the required applied axial stress for the primary fracture and spalling initiation of the numerical and the physical models is presented along with the accompanied scale effect. Additional numerical investigation is performed, quantifying the stress distribution along and normal to the primary fracture’s path during the numerical test. The current study reveals the potential of the bonded particle model to reliably simulate laboratory and field structures, at least for the studied materials.

A Coupled Multibody Finite Element Model for Investigating Effects of Surface Defects on Rolling Contact Fatigue

A coupled multibody elastic–plastic finite element (FE) model was developed to investigate the effects of surface defects, such as dents on rolling contact fatigue (RCF). The coupled Voronoi FE model was used to determine the contact pressure acting over the surface defect, internal stresses, damage, etc. In order to determine the shape of a dent and material pile up during the over rolling process, a rigid indenter was pressed against an elastic plastic semi-infinite domain. Continuum damage mechanics (CDM) was used to account for material degradation during RCF. Using CDM, spall initiation and propagation in a line contact was modeled and investigated. A parametric study using the model was performed to examine the effects of dent sharpness, pile up ratio, and applied load on the spall formation and fatigue life. The spall patterns were found to be consistent with experimental observations from the open literature. Moreover, the results demonstrated that the dent shape and sharpness had a significant effect on pressure and thus fatigue life. Higher dent sharpness ratios significantly reduced the fatigue life.

Behavior of ring-restrained high-performance concrete under extreme heating and development of screening test

This study evaluated the behavior of restrained high-performance concrete in response to the extreme heating associated with fire. This was done by estimating the thermal stress based on the strain induced in the restraining steel ring and the vapor pressure in the restrained concrete under the conditions corresponding to the RABT 30 rapid heating curve. The thermal stress was calculated based on the thin-wall cylinder model theory. A spalling failure model based on a tensile strain failure model was proposed. The results indicated that the model was suitable for determining the spalling initiation point as well as the spalling depth.

Understanding the factors controlling rolling contact fatigue damage in VIM-VAR M50 steel

Sub-surface initiated spalling remains a key factor in determining the ultimate life of properly maintained bearings. In its early stages, spalling is manifested by the development of cracks and accompanying microstructure alterations, so-called butterflies, around the microstructure inhomogeneities. Base upon a unique three-dimensional microscopic characterisation of a large population of butterflies in VIM-VAR M50 samples that underwent rolling contact fatigue under different experimental conditions, the key factors determining butterfly nucleation and growth has been identified. The work identifies the conditions for crack nucleation and growth, and quantitatively relates them to microstructure. The model encompasses the sub-surface stress field and the microstructural parameters of the material leading to crack growth. Outputs of numerical evaluation of the model show good agreement with experimental data concerning number density, depth distribution and size distribution of butterflies across the wide range of fatigue test conditions.

Spall Propagation Characteristics of Refurbished VIM–VAR AISI M50 Angular Contact Bearings

During times of restricted supply, bearing refurbishment offers an attractive avenue to maintain operational readiness. However, bearing operation after fatigue spall initiation on refurbished bearings has not been extensively studied. In this study, spall propagation characteristics were compared between new and refurbished vacuum induction-melted, vacuum arc remelted AISI M50 208-size angular contact bearings. A control group of new AISI M50 bearings was evaluated for spall propagation characteristics as a baseline. Another group of AISI M50 bearings were subjected to an accumulated 11.5 billion stress cycles at a maximum Hertzian contact stress of 1.93 GPa and a temperature of 127 °C followed by Level II refurbishment. The refurbished bearings were evaluated for spall propagation characteristics and compared to the baseline bearings. Spalls were initiated via seeded Rockwell C hardness indents and propagated at a maximum Hertzian contact stress of 2.65 and 2.41 GPa, respectively, on both groups of bearings. The propagation rates of the bearings were measured in real time using an oil debris monitor. Pre- and post-tested bearings were examined for changes in microstructure, residual stress and retained austenite as a function of depth in the circumferential direction. No statistically significant difference in spall propagation characteristics was observed between new and refurbished bearings at the operating conditions and accumulated stress cycles studied here.

Influence of tunnel wall roughness and localized stress concentrations on the initiation of brittle spalling

Brittle failure of massive hard rocks occurs when the stress near the excavation reaches the in situ strength. After reviewing published papers, we found that two approaches, one using UCS field ci ≈ 0.45 UCSlab and the other using UCS field cd ≈ 0.8 UCSlab, have been used to estimate the in situ strength of massive rock, where UCSfield and UCSlab are the field and laboratory uniaxial compressive strengths, respectively, and the superscripts denote UCSfield defined by the crack damage stress or crack initiation stress. In this study, a continuum model considering the “as built profile” was constructed to simulate brittle rock failure in two orthogonal tunnels in the 420 level of Atomic Energy of Canada Limited’s (AECL’s) Underground Research Laboratory. Results obtained from the simulation with in situ strength at UCS field cd seem to capture actual rock conditions better than that with the in situ strength at UCS field ci . Opening wall roughness has a great influence on the brittle failure of massive hard rocks. It was found that compared with the smooth boundary condition, stress can concentrate around the rough boundary and the induced tensile stress driven by compressive stress loading can promote rock spalling failure. If the opening roughness is overlooked, the field uniaxial compressive strength of UCSfield will be underestimated.

Use of 3D Finite-Element Models for Predicting Intermediate Damage Limit States in RC Bridge Columns

AbstractIn this paper, three-dimensional (3D) continuum-based finite-element (FE) simulations are implemented for estimating intermediate damage limit states in flexure-dominated ductile reinforced concrete (RC) bridge columns. Results from the 3D FE simulations were compared and validated against the experimental data from four large-scale tests. Statistical error measures and test analysis correlation metrics were utilized to quantitatively evaluate the accuracy of the models. The validated models were then applied to determine the intermediate damage limit states based on the simulation results. Onset of yielding was associated with the tensile strains on the extreme reinforcement, and compressive strains on the outer concrete surface elements were used to identify the initiation and significant growth of spalling of the cover concrete. The results show that the 3D FE simulations were efficient in predicting intermediate damage limit states in a consistent manner with the experimental observations extr...

On the dynamic tensile strength of Zirconium

Despite its fundamental nature, the process of dynamic tensile failure (spall) is poorly understood. Spall initiation via cracks, voids, etc, before subsequent coalesce, is known to be highly microstructure-dependant. In particular, the availability of slip planes and other methods of plastic deformation controls the onset (or lack thereof) of spall. While studies have been undertaken into the spall response of BCC and FCC materials, less attention has paid to the spall response of highly anisotropic HCP materials. Here the dynamic behaviour of zirconium is investigated via plate-impact experiments, with the aim of building on an ongoing in-house body of work investigating these highly complex materials. In particular, in this paper the effect of impact stress on spall in a commercially sourced Zr rod is considered, with apparent strain-rate softening highlighted.

Three-Dimensional Finite Element Elastic–Plastic Model for Subsurface Initiated Spalling in Rolling Contacts

In this investigation, a three-dimensional (3D) finite element (FE) model was developed to study subsurface initiated spalling observed in rolling line contact of tribo components such as bearings. An elastic–kinematic hardening–plastic material model is employed to capture the material behavior of bearing steel and is coupled with the continuum damage mechanics (CDM) approach to capture the material degradation due to fatigue. The fatigue damage model employs both stress and accumulated plastic strain based damage evolution laws for fatigue failure initiation and propagation. Failure is modeled by mesh partitioning along unstructured, nonplanar, intergranular paths of the microstructure topology represented by randomly generated Voronoi tessellations. The elastic–plastic model coupled with CDM was used to predict both ratcheting behavior and fatigue damage in heavily loaded contacts. Fatigue damage induced due to the accumulated plastic strains around broken intergranular joints drive the majority of the crack propagation stage, resulting in a lower percentage of life spent in propagation. The 3D FE model was used to determine fatigue life at different contact pressures ranging from 2 to 4.5 GPa for 33 different randomly generated microstructure topology models. The effect of change in contact pressure due to subsurface damage and plastic strain accumulation was also captured by explicitly modeling the rolling contact geometry and the results were compared to those generated assuming a Hertzian pressure profile. The spall shape, fatigue lives, and their dispersion characterized by Weibull slopes obtained from the model correlate well with the previously published experimental results.

An experimental investigation into the micro-mechanics of spall initiation and propagation in Ti–6Al–4V during shock loading

Titanium alloys have long been considered candidate materials for use in monolithic armor systems due to their excellent specific mechanical properties and relative ease of manufacture. However, high production costs combined with a propensity to fail due to adiabatic shear band formation and spallation have limited the adoption of titanium alloys in armor systems. In this paper, the spall behavior of the workhorse titanium alloy, Ti–6Al–4V, is investigated via a series of plate impact experiments. Back-scatter electron (BSE) microscopy and electron back-scatter diffraction (EBSD) are carried out on recovered specimens to investigate the nature of spall initiation and propagation. It is found that spall initiation occurs by the nucleation of voids at the grain boundaries between plastically hard/soft grains of the dominant hcp α phase. Further, it is shown that spall then propagates via the growth/coalescence of the nucleated voids into facets, followed by the coalescence of the facets through highly localized ductile bridging. A number of suggestions are made to improve the spall resistance of titanium alloys based on these observations.

Hypervelocity Impact Response of Ti-6Al-4 V and Commercially Pure Titanium

Titanium alloy, Ti-6Al-4V, and commercially pure (CP) Titanium will be used to protect the Solar Probe Plus (SPP) spacecraft against hypervelocity impacts by solar dust particles. The results of six hypervelocity impact (HVI) tests performed on Ti-6Al-4V and CP monolithic samples (3 each) are evaluated in terms of cratering and spall damage, and compared with crater depth and spall initiation predictions using the Ballistic Limit Equation (BLE) for Titanium shields developed at NASA Johnson Space Center and hydrocode computations. In the tests, 2017-T4 aluminum spheres with a diameter of 2.35mm were used to impact the shields at an impact velocity of about 7km/s. In general, the measured and predicted values of crater depth are found to be in good agreement with each other, regardless of the type of Titanium alloy. In terms of spall damage, two of the three Ti-6Al-4V samples exhibit a noticeable incipient spall and one sample shows a very faint incipient spall, whereas only one of the CP samples shows a faint incipient spall. Polished cross- sections of the samples also indicate sub-crater damage modes such as microcracking, shear localization and large scale grain deformation, and possibly recrystallization. Differences observed in the macroscopic and microscopic damage phenomenologies exhibited by the two types of Titanium alloys are presented and discussed in terms of their implications on the BLE for shielding design calculations.

Numerical modeling of sub-surface initiated spalling in rolling contacts

A 3D finite element model was developed to investigate the influence of microstructure topology on the stochastic nature of rolling contact fatigue. Grains of the material microstructure are modeled with random Voronoi tessellations. Continuum damage mechanics and mesh partitioning are implemented to capture the initiation and propagation phases of fatigue damage that lead to spalling. Simulated fatigue spalling is shown to progress similarly to experimental observations of rolling contact fatigue. The fatigue lives obtained with the model exhibit scatter on par with empirical measures and are fit well by 2 and 3-parameter Weibull distributions.

Novel method for rolling element bearing health assessment—A tachometer-less synchronously averaged envelope feature extraction technique

The assessment and diagnosis of bearing health using vibration data has been a research topic of interest for many years and includes developments in an assortment of signal processing methods and classification algorithms. This paper investigates detecting bearing degradation at different levels of damage, in that estimating the bearing health at the various stages of degradation is important for predicting failure as well as making maintenance decisions. The proposed technique does not require a measure of the rotational shaft speed or bearing cage speed, which makes it very suitable in certain applications in which it is very difficult or not cost effective to measure the rotational speed. To effectively estimate the bearing health state, a novel tachometer-less synchronously averaged envelope (TLSAE) signal processing and feature extraction technique for rolling element bearing is proposed. The Tachometer-Less Synchronous Averaged Envelope (TLSAE) method consists of first using a narrow band pass filter around a calculated bearing fault frequency of interest and using the derivative of the phase of the Hilbert Transform of this narrow band signal to generate a synthesized tachometer signal that is representative of the impact due to a bearing defect. This synthesized tachometer signal is combined with the high frequency envelope method to perform synchronous averaging on the envelope signal, resulting in a defect synchronous envelope spectrum in which the frequency content is in terms of the fault frequency orders. The proposed method is further compared and evaluated with other existing methods, in particular to the traditional Fourier Transform technique, the bearing envelope analysis technique, and the empirical mode decomposition signal processing methods on the basis of whether each method provides an enhanced level of indication that can determine the health of rolling element bearings. Data from a bearing test-rig is used to facilitate the comparison and evaluation of the signal processing methods. Vibration data was collected from the test-rig for bearings with different levels of degradation. The calculated vibration features from the tachometer-less synchronously averaged envelope (TLSAE) technique are compared to the other feature extraction techniques; with the synchronous average method providing a set of bearing vibration features that can distinguish all three levels of damage on the outer race of the rolling element bearing. Future work looks to further investigate this proposed technique for data collected during a run to failure test in order to consider its merits for early detection of incipient bearing damage and whether it provides a consistent monotonic trend from spall initiation to bearing failure.

Effects of plasticity on subsurface initiated spalling in rolling contact fatigue

In this investigation an elastic–plastic Voronoi finite element (EPVFE) model for rolling contact fatigue of Hertzian line contact was developed. The EPVFE model coupled with damage mechanics approach is used to investigate rolling contact fatigue (RCF) of tribo-components. Mises based plasticity model with kinematic hardening was employed to incorporate the effects of material plasticity. The model considers both; stress and accumulated plastic strain based damage laws in the constitutive damage modeling. The results indicate that once a fatigue crack initiates within the domain, the fatigue damage induced due to the accumulated plastic strains around the crack tip drives the majority of the crack propagation stage. The results from this investigation reveal that depending on the contact pressure the crack propagation stage can constitute 15–40% of the total life. The spall shape, fatigue lives and Weibull slopes obtained from the EPVFE model correlate well with the experimental results. The developed model was also used to investigate the effects of initial material imperfections such as inclusions and elastic modulus inhomogeneity on the fatigue lives. The results indicate that material flaws within the domain lead to a significant decrease in fatigue lives and increase in life scatter.

凍結融解作用を受ける岩石の破壊プロセスと耐久性の評価

Durability evaluation of rocks subjected to freezing-and-thawing is important for stability assessment of a rock fill dam as well as rock slopes, especially in cold weather regions. In this study, freezing-and-thawing tests up to 300 cycles were carried out on twelve kinds of rock samples in order to characterize fracture propagation by freezing-and-thawing and evaluate the durability, and fracture propagation was observed. The fracturing process was characterized by macroscopic observation by digital camera and changes in physical properties. Fracture initiation and propagation in the specimen were also observed by using X-ray CT. It was found that the fracturing process depends on the rock type. For rocks which possess preferred orientation of pre-existing fractures and anisotropy of tensile strength, only one fracture propagated dominantly and split the rock sample in half. On the other hand, for isotropic rock, propagation of several fractures occurred and rock samples were split into several parts. Additionally, spalling continuously progressed once the loss of weight exceeded 5% for isotropic rocks because fractures had developed throughout the specimen. This result indicates that freezing-and-thawing cycles at 5% weight loss is assumed to be the spalling initiation. Therefore, we can conclude that the number of freezing-and-thawing cycles to the spalling initiation is the important indicator of the durability evaluation. It was also pointed out that both supply capacity of water to the crack tip and crack growth resistance affect the durability, because the tensile strength divided by the percentage of water absorption is in good correlation with the number of cycles to the spalling initiation. It is concluded that the durability can be easily evaluated by both tensile strength and the percentage of water absorption.