Incipient Crack Research Articles

Retrieved Highly Crosslinked UHMWPE Acetabular Liners Have Similar Wear Damage as Conventional UHMWPE

Highly crosslinked UHMWPE is associated with increased wear resistance in hip simulator and clinical studies. Laboratory and case studies, however, have described rim fracture in crosslinked acetabular liners. Controversy exists, therefore, on the relative merits of crosslinked liners over conventional liners in terms of wear performance versus resistance to fatigue cracking. We asked whether crosslinked liners would show less surface damage than conventional liners but would be more susceptible to fatigue damage. We examined 36 conventional UHMWPE and 39 crosslinked UHMWPE retrieved implants with similar patient demographics and identical design for evidence of wear damage, including articular surface damage, impingement, screw-hole creep, and rim cracks. We observed no difference in wear damage scores for the two liners. Conventional liners more frequently impinged but were more often elevated with smaller head sizes. We observed creep in approximately 70% of both types of liners. Incipient rim cracks were found in five crosslinked liners, and one liner had a rim fracture. Only one conventional liner had an incipient rim crack. Contrary to our expectation, damage was similar between crosslinked and conventional UHMWPE liners. Moreover, the 15% occurrence (six of 39) of incipient or complete fractures in crosslinked liners as compared with a 3% occurrence (one of 36) in conventional liners may have implications for the long-term performance of crosslinked liners. Longer-term studies will be necessary to establish the fate of rim cracks and thus the overall clinical fatigue performance of crosslinked liners.

Identifying technology for structural damage based on the impedance analysis of piezoelectric sensor

PZT piezoelectric ceramic was used as sensing element to fabricate piezoelectric sensor. The fabricated PZT piezoelectric sensor was embedded into and affixed to the structure to detect the structural damage, respectively. The structural crack damage was investigated using the impedance spectra of the sensor. The results show that the electric impedance of the PZT piezoelectric sensor with both different arrangements can reflect the variations of the structural crack damage in the testing frequency ranges. In the frequency range of 20–70 kHz, both the impedance value and the resonance frequency of the embedded PZT piezoelectric sensor can show the incipient crack damage and the increase of crack depth clearly. The impedance spectra of the affixed and embedded PZT piezoelectric sensors show the similar variation regularity with increasing the structural crack damage degree. Furthermore, the impedance spectra variation of the PZT piezoelectric sensor with different arrangements in the thickness resonance frequency range is more obvious than that in the planar resonance frequency range. A scalar damage metric is presented based on the impedance spectra of the PZT piezoelectric sensor around the resonance frequency, and the cracks in different positions of the structure were also analyzed using the damage metric. The variation of structural crack damage can be observed effectively and obviously using this damage metric.

On the topological derivative due to kink of a crack with non-penetration. Anti-plane model

A topological derivative is defined, which is caused by kinking of a crack, thus, representing the topological change. Using variational methods, the anti-plane model of a solid subject to a non-penetration condition imposed at the kinked crack is considered. The objective function of the potential energy is expanded with respect to the diminishing branch of the incipient crack. The respective sensitivity analysis is provided by a Saint-Venant principle and a local decomposition of the solution of the variational problem in the Fourier series.

Simulation of Outside-in Cracking in Boiling Water Reactor Fuel Cladding Tubes under Power Ramp

Abstract In power ramp tests, some high burn-up boiling water reactor (BWR) fuels failed by outside-in cracking. After detailed post irradiation examinations, it was deduced that fracture of radially oriented hydrides precipitated at the outer surface had resulted in the formation of incipient radial cracks that propagated towards the inner surface. Since crack propagation is a key process that dominates the outside-in failure, experimental data of outside-in cracking are obviously required. The objective of the present study is to evaluate the threshold conditions and the velocity of the outside-in cracking during a power ramp to assess the integrity of BWR fuels during a power ramp. In the experiments, by using unirradiated Zry-2 fuel cladding tubes, the outside-in cracking was examined both with and without the thermal gradient. Without the radial thermal gradient, there was no radial delayed hydride cracking in all experimental conditions examined. On the other hand, with the radial thermal gradient, the velocity of the outside-in cracking was significantly higher than that expected from the data obtained under the isothermal condition in our previous study. The threshold depth of the incipient crack was found to be ∼0.1 mm at the hoop stress of 300 MPa. It was suggested that the outside-in cracking during the power ramp is strongly dependent on the distribution of dissolved hydrogen as a result of the thermal diffusion.

Corrosion in cracked and uncracked concrete – influence of crack width, concrete quality and crack reopening

Cracks influence the durability of reinforced concrete (RC) structures in aggressive environments by accelerating the ingress of corrosive agents to the embedded steel. This study investigated the influence of crack width (0·7 mm, 0·4 mm and incipient cracks), binder type (100% CEM I (ordinary Portland cement – OPC) and 50/50 OPC/Corex slag blend), water-to-binder (w/b) ratio (0·40 and 0·55) and crack reopening on chloride-induced corrosion in RC specimens with constant cover of 40 mm. Over the study period (31 weeks), corrosion rates varied from passive values of 0·01 μA/cm2 to active values of 1·50 μA/cm2. Results indicate that corrosion rate is sensitive to crack width, concrete quality (i.e. binder type and w/b ratio) and crack reopening. For a given binder type and w/b ratio, corrosion rate increased with increasing crack width; however, this increase was smaller in Corex slag specimens (increases of 40% or more) than in OPC specimens (increases 210% or more). Corex slag specimens generally had lower corrosion rates compared to OPC specimens. For both OPC and Corex slag specimens, corrosion rate decreased with decreasing w/b ratio. Crack reopening increased the corrosion rate in the cracked specimens. This increase in corrosion rate was higher if the specimen was actively corroding prior to the crack reopening.

Suitability of Various Measurement Techniques for Assessing Corrosion in Cracked Concrete

A critical evaluation of results from three corrosion assessment methods used in an experimental study to investigate the influence of cracks on the rate of chloride-induced corrosion is presented in this article. The objective was to determine the suitability and reliability of the corrosion assessment methods for application in cracked concrete. Beam specimens (100 x 100 x 500 mm [3.9 x 3.9 x 19.7 in.]) with crack widths of 0.7 and 0.4 mm (0.028 and 0.016 in.), as well as incipient cracks, were made using two binder types (100% CEM I ordinary portland cement [OPC]) and 50/50 OPC/ slag blend), two water-binder ratios (w/b) (0.40 and 0.55) and a constant cover of 40 mm (1.6 in.) to steel reinforcing. The specimens were subjected to repeated cycles of 3-day wetting with 5% NaCl, and 4-day air drying for 32 weeks. Half-cell potential (HCP), resistivity (Wenner probe), and corrosion rate (coulostatic) measurements were taken at the end of each 3-day wetting period. Chloride conductivity tests were also performed on companion cube specimens at 28 and 90 days. No significant differences in resistivity values were noted between cracked and uncracked specimens, despite the fact that the corrosion rate increased with increasing crack width. The insensitivity of resistivity measurements to the presence of cracks was attributed to the fact that measurements were taken after every 3-day wetting period, when the specimens would have been saturated with salt solution. The chloride conductivity and resistivity measurements gave a similar indication of the corrosion rates. HCP and corrosion rate measurements showed a good correlation, and they gave a clear indication of corrosion in cracked reinforced concrete (RC) specimens, although for practical applications, HCP measurements need to be complemented with other corrosion assessment techniques. For the resistivity assessment technique—in addition to concrete quality and magnitude of resistivity—correction factors are required to relate the resistivity of uncracked concrete to corrosion characteristics of cracked concrete. A methodology for the application of correction factors is proposed.

Planning Rail Grinding Using Crack Growth Predictions

Rail grinding is a widely used rail industry technique for maintaining the quality and safety of railway track. Rail grinding can be used to restore the profile of the rails, remove surface plastic damage and remove or reduce in size very early stage surface breaking rolling contact fatigue cracks. This paper focuses on the use of rail grinding to extend the safe life of a rail that would otherwise be limited by rolling contact fatigue (RCF) defects. The optimum grinding strategy for a variety of conditions has been investigated using a model that predicts the growth of surface breaking cracks in rails loaded by rail vehicle wheel contacts, taking into account natural wear of the rail and rail grinding. It was found that a grinding strategy which removes incipient cracks in a specific period of time without being over-conservative and producing excessive material loss must take into account all traffic using the line, not just the traffic perceived to be most damaging. A mixed traffic case is modelled, revealing the potential for interaction between different vehicle types in determining maintenance requirements.

Stress Intensity Factor at the Tip of Cladding Incipient Crack in RIA-Simulating Experiments for High-Burnup PWR Fuels

RIA-simulating experiments for high-burnup PWR fuels have been performed in the NSRR, and the stress intensity factor K I at the tip of cladding incipient crack has been evaluated in order to investigate its validity as a PCMI failure threshold under RIA conditions. An incipient crack depth was determined by observation of metallographs. The maximum hydride-rim thickness in the cladding of the test fuel rod was regarded as the incipient crack depth in each test case. Hoop stress in the cladding periphery during the pulse power transient was calculated by the RANNS code. K I was calculated based on crack depth and hoop stress. According to the RANNS calculation, PCMI failure cases can be divided into two groups: failure in the elastic phase and failure in the plastic phase. In the former case, elastic deformation was predominant around the incipient crack at failure time. K I is available onlyin this case. In the latter, plastic deformation was predominant around the incipient crack at failure time. Failure in the elastic phase never occurred when K I was less than 17 MPam1/2. For failure in the plastic phase, the plastic hoop strain of the cladding periphery at failure time clearly showed a tendency to decrease with incipient crack depth. The combination of K I, for failure in theelastic phase, and plastic hoop strain at failure, for failure in the plastic phase, can be an effective index of PCMI failure under RIA conditions.

Direct observation of plasticity and quantitative hardness measurements in single crystal cyclotrimethylene trinitramine by nanoindentation

Investigation of deformation beginning with elasticity and continuing through the elastic–plastic transition to incipient cracking has been conducted for (210), (021) and (001) oriented single crystals of the explosive cyclotrimethylene trinitramine, commonly known as “RDX”. Nanoindentation was performed with a conical tip over a range of loads. The resulting load–depth data exhibited distinct, reproducible, orientation-dependent load excursions demonstrating elastic–plastic transitions. Indent impressions were imaged by atomic force microscopy revealing deformation features consistent with slip on six planes. Impressions on the (210) and (001) planes showed deformation pile-up features associated with the zone axes of slip planes. Slip traces were evident on the (210) plane indicating slip on four planes and suggesting cross-slip. Height data, for impressions formed by progressively increasing loads, indicated one additional slip system consistent with (010) slip. All of the orientations exhibited cracking thresholds at very low loads. The reduced elastic moduli were anisotropic and the hardness values were isotropic indicating limited plasticity. Maximum shear stresses estimated from a Hertzian model, at load excursions, were within 1/15 to 1/10 of published shear moduli, indicating deformation initiated near the theoretical yield strength, presumably by homogeneous nucleation of dislocations. The material strength parameters and deformation pathways inferred from this work are compared to previous microhardness investigations in which the ambiguity of results can be attributed to the effects of cracking and simultaneous slip on multiple systems. A mechanistic explanation for the hindered plasticity, and cracking, observed for RDX is offered in terms of compatibility conditions.

The effect of rubber contact on the fretting fatigue strength of railway wheel tire

The cause of the ICE train derailment, which occurred in 1998 at Eschede, was fatigue failure originating on the inside of the wheel tire. Rubber-sprung resilient wheels were used for the trailer cars. The wheel tire is mounted on the wheel disc. Thirty-four rubber pads were arranged between the wheel disc and the wheel tire. It was postulated that fretting fatigue between the rubber block and the inner side of the tire might have an influence on the initiation of the incipient crack. In order to clarify the influence of the rubber contact on the fatigue strength of the tire, fretting fatigue experiments under rubber contact conditions were performed. During the fundamental fretting fatigue test using bridge pads and small size carbon steel specimens, no typical fretting damage such as fretting wear and minute cracks were observed due to contact of the rubber. Stress conditions of the rubber-sprung wheel under vertical and lateral wheel loads were evaluated by a three-dimensional elastic stress analysis. Since the rubber is a super-elastic material, the Mooney–Rivlin model was used in the FEM calculation. It was found that the wheel tire is subjected to a cyclic stress during one revolution of the wheel and the maximum stress occurred at the center of the inner surface of the tire where the fatigue crack initiated. Fatigue strength of the wheel tire was determined by the rotating bending fatigue testing of specimens taken from the tire. It was found that the tire with an 862 mm diameter at a wheel load of 80 kN had a safety factor more than 3.5 from a fatigue limit diagram with a failure probability of 0.01. To confirm the fretting damage under the rubber contact and the result of the fatigue strength evaluation, fatigue tests of a full size wheel were made. After 20 million cycles at the wheel load of 280 kN, which was just below the endurance limit estimated by the endurance limit diagram, no fretting damage and no fatigue cracks were observed. The wheel was, however, fractured at 1.56 million cycles under the maximum load of 308 kN, which was just above the endurance limit. The estimation of the safety factor of 3.5 estimated from the endurance diagram was confirmed by the full size fatigue testing. It was concluded that there was no effect of fretting due to the rubber contact on the fatigue strength of the rubber-sprung single-ring railway wheel.

A two-parameter analysis of S– N fatigue life using Δ σ and σmax

The effect of the load ratio, R, or the mean-stress on fatigue life has been recognized for more than a hundred years. In considering the mean-stress effects in the stress-life ( S– N) approach, research efforts have been mostly concentrated in establishing correlating functions in terms of the flow stress or yield stress or the ultimate tensile stress, etc., by taking, say, R = −1 test results as a reference. Very little effort has been made towards understanding the role of stress range Δ σ and the maximum stress σ max, (or σ mean) in the fatigue crack nucleation and propagation and also how to relate this to both the stress-life and the fracture-mechanics descriptions. In this paper we first examine crack nucleation based on the stress-life approach using a two-parameter requirement in terms of Δ σ and σ max, and then connect it to crack propagation using the Kitagawa diagram as the incipient crack grows to become a long crack. Since stress-life data include both nucleation and propagation, the connection of the safe-life approach to the fracture-mechanics analysis is pertinent. Comparison of the present analysis with experimental data taken from the literature demonstrates that a two-parameter approach in terms of Δ σ and σ max forms a basis for the S– N analysis.

Stress Intensity Factor at the Tip of Cladding Incipient Crack in RIA-Simulating Experiments for High-Burnup PWR Fuels

RIA-simulating experiments for high-burnup PWR fuels have been performed in the NSRR, and the stress intensity factor K I at the tip of cladding incipient crack has been evaluated in order to investigate its validity as a PCMI failure threshold under RIA conditions. An incipient crack depth was determined by observation of metallographs. The maximum hydride-rim thickness in the cladding of the test fuel rod was regarded as the incipient crack depth in each test case. Hoop stress in the cladding periphery during the pulse power transient was calculated by the RANNS code. K I was calculated based on crack depth and hoop stress. According to the RANNS calculation, PCMI failure cases can be divided into two groups: failure in the elastic phase and failure in the plastic phase. In the former case, elastic deformation was predominant around the incipient crack at failure time. K I is available onlyin this case. In the latter, plastic deformation was predominant around the incipient crack at failure time. Failure in the elastic phase never occurred when K I was less than 17 MPam1/2. For failure in the plastic phase, the plastic hoop strain of the cladding periphery at failure time clearly showed a tendency to decrease with incipient crack depth. The combination of K I, for failure in theelastic phase, and plastic hoop strain at failure, for failure in the plastic phase, can be an effective index of PCMI failure under RIA conditions.

Method of the shortcut estimation of rails' reliability by the fatigue strength criterion

The diagram of damage of structural steels under bending and contact loading has been constructed. The criterion of the critical state of steels is proposed, which corresponds to the formation of incipient surface cracks in the rolling friction zone. The methods of shortcut estimation of the damaged state of railway rails, which provide prediction of their reliability and life under service conditions, are proposed. A set of experimental studies is made with objectives to justify the proposed diagram, criterion and methods.

Measurement of residual stresses in rails by neutron diffraction

Rolling contact fatigue damage is accumulated in rails during the repeated passage of trains over the rails, and an incipient crack may initiate due to the rolling contact fatigue. Rail failures can occur due to crack growth in the rails. In order to prevent such rail failures, the estimation of the behavior of internal rail cracks is required on the basis of an exact engineering analysis model as well as a method to detect rail defects before the crack grows to critical size. It is expected that the residual stress distribution in a rail has a significant influence on the growth rate of the crack. The purposes of this study are to measure the residual stress distribution in rails utilizing the neutron strain measurement. The test samples were taken from rails used in a service line in Japan for about six years (222 million gross tons). The neutron measurement was conducted using the Residual Stress Analyzer (RESA) at the Japan Atomic Energy Agency (JAEA). This report presents the results of the neutron strain measurements on a longitudinal-cut slice of a rail.

Crack propagation by differential insolation on desert surface clasts

In the southwest U.S., cracks in alluvial fan surface clasts have a preferred orientation independent of rock fabric and shape. In this paper, we show that differential insolation of incipient cracks of random orientations predicts a distribution of crack orientations consistent with field observations. In this model, crack growth by hydration and/or thermal weathering is primarily a function of local water content at the crack tip. Crack tips that experience minimal solar insolation maintain a greater average moisture and, hence, weather more rapidly than cracks that experience greater solar insolation. To show this, we used a numerical radiative transfer code to quantify the solar insolation of rectangular cracks at 35° N. latitude with a range of depths and orientations. The amount of solar energy reaching the bottom of each crack was calculated at 5-min intervals over the day for several days of the year to determine hourly, daily, seasonal, and annual energy deposition as a function of crack depth and orientation. By assuming that only crack orientations that effectively shield their interiors and minimize their water loss are able to grow, the pattern of cracks produced by the model is consistent with field observations. The annual average insolation, which controls water retention, is associated with the two primary modes of crack orientation. The effect of daily recharge by summer rains of the North American monsoon system is consistent with the observed deviations from these primary modes. Model results suggest that both the annual average insolation and the daily pattern of rainfall is recorded in the preferred crack orientations of surface clasts in the southwest U.S.

Non-singular term effect on the fracture quantities of a crack in a piezoelectric medium

The static problem of a crack in a piezoelectric plate subjected to biaxial loading at infinity is analyzed. The aim of this paper is to estimate the influence of non-singular terms originated by the load biaxiality on the stress fields and on the elastic and electric displacements in the vicinity of the crack tip. An analytical method for seeking the electro-elastic solution is proposed. The novel procedure involves a transformation of similarity induced by the fundamental matrix that enables to express the equations governing the problem in terms of complex potentials. The application of the boundary conditions leads then to the formulation of Hilbert problems whose solutions allow to obtain the generalized stress and displacement components. Numerical results and graphs are presented and discussed for various loading conditions. The non-singular solution is compared to the asymptotic one, generally considered in the literature when analyzing fracture problems. In particular, it is shown that the direction of incipient crack extension, sought through the maximum circumferential stress criterion, can be seen to deviate from the crack axis as the collinear load increases, although geometry and applied load are symmetric.

Dynamic Dislocation-Defect Analysis and SAXS Study of Nanovoid Formation in Aluminum Alloys

Crystalline defects other than the essential dislocations are produced by dislocation intersections resulting in debris, which can transform into loops, point defects, and∕or nanovoids. The stress concentrations ahead of slip clusters promote void formation leading to incipient cracks. To evaluate the progression of these processes during deformation, dynamic dislocation-defect analysis was applied to nominally pure aluminum, Al–Mg, and Al–Cu alloys. In the case of nanovoid formation, small angle X-ray scattering (SAXS) was used to quantitatively assess if the void size and its volume fraction can be determined to directly correlate with the measured thermodynamic response values. The SAXS signal from the nanovoids in nominally pure aluminum is distinctly measurable. On the other hand, thermomechanical processing of even nominally pure aluminum results in the formation of nanoprecipitates, which requires future calibration.

Experimental investigations of spontaneous bimaterial interfacial fractures

Experimental investigations of spontaneous bimaterial interfacial fractures

Failure Mechanism and Mechanical Characteristics of Rockmass Containing Close Intermittent Joints under Plane Stress Condition

Biaxial compression model tests on rockmass containing intermittent joints under plane stress condition were carried out in this study, where the laser speckle photo technique was used for measuring the displacement field. The results obtained have indicated that the laser speckle method was effective and the measuring value was about 14% less than the measuring value of dial gauges. Based on the displacement field and the principal stress field which were measured by laser speckle photo technique, dial gauges and strain gauges, the angle of wing incipient crack, the evolution of stress field at the joint tip and the path of crack extension were deeply studied. The deformation and failure characteristics, including the fracture mechanism of rockmass, were also analyzed in this study.

Incipient Crack Identification in Beam Structures Using the Slope of the Anti-Resonance Curve

Crack damage brings a serious threat to the safety of mechanical and civil structures, and the problem of incipient damage identification of structures has been paying attention as a puzzle by many researchers in recent years. To seek for an alternative solution of the problem, a method for incipient crack localization using the slope of the anti-resonance curve is proposed in this paper. The method makes use of the driving-point mechanical impedance characteristics of cracked beams stimulated by harmonic force. To characterize the local discontinuity due to the presence of crack, a simplified rotational spring model is presented to model the crack. Subsequently, the proposed method is verified by a numerical example of cracked beam under simple support or cantilever boundary conditions.