Crack Propagation Time Research Articles

Crack Propagation in Freshwater and Saline Ice

AbstractCrack propagation in columnar saline and freshwater ice has been investigated with high-speed photography, acoustic emission detection and the resistance method. High-speed photography was found to be a single reliable technique. The resistance method proved effective for freshwater ice samples, but not for saline ice samples due to the presence of conductive fluid inclusions. Acoustic emissions pinpointed the moment of crack initiation, but did not correspond to the crack propagation time. Crack velocity has been characterized over a temperature range of -5°C to -30 °C for freshwater and saline ice. Freshwater ice exhibited an overall average velocity of 198 m/s, and did not vary with temperature. Crack velocity in saline ice demonstrated temperature dependence, increasing from an average of 86 m/s in the -5°C to -20°C range, to 131 m/s at -30°C. The crack velocity was also shown to have a general dependence on fracture toughness K' of the material, however, the microstructural variation between samples is also shown to influence significantly the crack behavior in both saline and freshwater ice. Nonuniform crack tip advance and crack reorientation were observed as crack slowing mechanisms in freshwater ice, while in saline ice fracture crack tip blunting on voids greatly reduced average crack velocities.

An experimental cum numerical technique to determine dynamic interlaminar fracture toughness

A method combining experimental and finite element analysis is developed to determine interlaminar dynamic fracture toughness. An interlaminar crack is propagated at very high speed in a double cantilever beam (DCB) specimen made of two steel strips which are bonded together by epoxy with a precrack of about 40 mm length. The face of the front cantilever is bonded to a large solid block and a special fixture is designed to apply impact load to the rear cantilever through a load bar. In the load bar, a compressive square shaped elastic stress pulse is generated by impacting it with a striker bar which is accelerated in an air gun. The rear cantilever is screwed to the load bar; when the incident compressive pulse reaches the specimen, a part of the energy is reflected into the load bar and the rest of it passes to the specimen. By monitoring the incident and the reflected pulses in the load bar through strain gauges, deflection of cantilever-end is determined. The crack velocity is determined by three strain gauges of 0.2 mm gauge length bonded to the side face of the rear cantilever. Further, the first strain gauge, bonded very close to the tip of the precrack, and the crack velocity determine the initiation time of crack propagation. The experimental results are used as input data in a finite element (FE) code to calculate J-integral by the gradual release of nodal forces to model the propagation of the interlaminar crack. The initiation fracture toughness and propagation fracture toughness are evaluated for an interlaminar crack propagating with a velocity in the range of 850 to 1785 m/s. The initiation toughness and propagation toughness were found to vary between 90–200 J/m 2 and 2–13 J/m 2, respectively.

Fatigue and fatigue crack propagation in AlSi7Mg cast alloys under in-service loading conditions

Abstract In this study, the influence of microstructure and microporosity on fatigue crack initiation and propagation in AlSi7Mg cast alloy has been investigated at very high numbers of cycles (10 8 –10 9 ). Two alloys with similar chemical composition were used; one had finer microstructure, somewhat high microporosity and better static strength properties. The experiments were performed both at constant cyclic loads and with load sequences that simulated random in-service loading conditions of automobiles. The results show that casting pores predominantly influence the number of cycles to crack initiation. For in-service loading conditions, the detrimental effect of microporosity on crack formation cannot be adequately predicted on the basis of linear damage calculation, because of premature crack initiation during high loads of the random sequence. Comparison of the number of cycles to fracture as determined with smooth cylindrical specimens and with pre-cracked flat specimens shows the relatively high portion of the crack propagation time. The good fatigue crack growth properties of both AlSi7Mg alloys in the threshold regime (which are even better for the coarser alloy) are explained by roughness-induced crack closure.

Spalling Life Model With Relaxed Distribution Constraints, for Rough Hertz Line Contacts

A Hertz contact spalling fatigue life model is constructed without the constraints of a two-parameter Weibull life distribution. The model is based on the time of crack propagation following a simple Paris law, through the Hertzian alternating shear stress field, from populations of surface defects with depth (severity) distributed according to the Greenwood-Williamson asperity model, to the depth of the maximum alternating shear stress. The life distribution shows moderate deviations from the Weibull form, with the dispersion strongly dependent on the contact parameters. The 90 percent survival quantile is obtained as a function of: Hertz pressure, EHD film ratio, asperity traction coefficient, asperity height, slope and spectrum width, fatigue limit stress and stress concentration factor. This model provides insights into the interrelationship of life with its parameters, not available from models adhering to the power law constraints required to produce a Weibull life distribution. No unbridgeable contradictions emerge between the present formulation and published engineering models.

Fracture mechanics of blunt cracks in a ductile steel

The propagation of blunt notches in stainless steel has been studied experimentally and analysed using generalized fracture mechanics (GFM), which takes account of inelastic and non-linear deformation. According to this theory, the critical apparent energy release rate, which is equivalent toJc, is given byJc=k1 (ɛ0)cW0c for an edge crack of lengthc in a thin sheet (plane stress), wherek1 (ɛ0) is a dimensionless function of strain, ɛ0, andW0c is the input energy density remote from the crack at the time of crack propagation. The validity of this equation was demonstrated for blunt cracks and the functionk1 (ɛ0) evaluated. The value ofJc was measured for blunt cracks of different lengths and tip diameters, and also for different crack extensions.Jc was found to be independent of crack length for the smallest tip radius, but became systematically length-dependent as the radius increased. However, the dependence ofJc on crack length, tip radius and crack extension can be expressed by a single empirical function, as is suggested by GFM. The propagation of cracks from blunt notches in ductile materials can, therefore, be handled by fracture mechanics methods.

Simplified Contact Fatigue Life Prediction Model—Part I: Review of Published Models

Part I of this two-part paper reviews eleven published spalling fatigue life models, to identify consensus parameters and relationships and to compare model strengths and limitations, as a basis for the construction of a simplified, readily calculated engineering model encompassing the major life variables, based on explicit assumptions and derivations. Part II of the paper presents the new model. It uses the applied alternating shear stress field to define the critical stress as a function of depth, and computes life as crack propagation time through this field. The model defines surface defects as critical defects. Material fatigue susceptibility, fatigue limit stress and the defect severity distribution are the main endurance parameters. In addition to Hertz pressure, life-modifying variables are: interface traction, surface microgeometry and EHD film. It is shown that the new model is capable of representing the influence of all major consensus parameters on life, by relationships that fall within the bounds of previously published behavior.

Simplified Contact Fatigue Life Prediction Model—Part II: New Model

Part I of this two-part paper reviews eleven published models as a basis for the construction of a simplified, readily calculated engineering model encompassing the major life variables, based on explicit assumptions and derivations. Part II of the paper presents the new model. It uses the applied alternating shear stress field to define the critical stress as a function of depth, and computes life as crack propagation time through this field. The model defines surface defects as critical defects. Material fatigue susceptibility, fatigue limit stress and the defect severity distribution are the main endurance parameters. In addition to Hertz pressure, life-modifying variables are: interface traction, surface microgeometry and EHD film. The influence of the variables in the new model on life distribution and on load/life law are compared to the reviewed models and it is shown that the new model is capable of representing the influence of all major consensus parameters on life, by relationships that fall within the bounds of previously published behavior.

Inclusion size effect on the fatigue crack propagation mechanism and fracture mechanics of a superalloy

Low cycle fatigue life of nickel-base superalloys is enhanced as a consequence of inclusion reduction in the melt process; however, the functional dependencies between fatigue characteristics and inclusions have not been well investigated. In this study, the propagation mechanism of the fatigue crack initiated from inclusions is examined in fine-grained IN718, which is a representative turbine disc material for jet engines. There is a faceted-striated crack transition on the fracture surfaces. This faceted-striated transition also appears in theda/dN vs crack length curves. It is observed that the faceted crack propagation time can be more than 50 pct of total lifetime in the low cycle fatigue test. The significance of inclusion size effect is explained on the premise that the faceted fatigue crack propagation time scales with the inclusion size, which is taken as the initial crack length. A predictive protocol for determining inclusion size effect is given.

Effects of phosphorus segregated at grain boundaries on stress corrosion crack initiation and propagation of rotor steel in boiling 40 wt.% NaOH solution

The effects of phosphorus segregated at grain boundaries on intergranular corrosion have been investigated by an electrochemical method. The stress corrosion crack initiation and propagation of rotor steels was studied as a function of applied stress in boiling 40 wt.% NaOH solution at an applied passive potential of −530 mV SHE using the electrical potential method. Phosphorus segregation leads to intergranular corrosion susceptibility. Intergranular attack by anodic dissolution is responsible for the crack initiation, irrespective of the applied stress and phosphorus doping. The decrease in stress corrosion crack propagation time due to phosphorus segregated at grain boundaries is enhanced with increasing applied stress and is presumably related to crack branching. Fringe patterns on the intergranular facets of the fracture surface are found to be associated with martensite lath boundaries and are inferred to be caused by strain-induced dissolution.

Effect of CaCl<sub>2</sub> Concentration on the Probability Distribution of Stress Corrosion Cracking Failure Time of Type 304 Stainless Steel

In order to compare the failure time distribution of a laboratory accelerated test with the field failure time distribution of stress corrosion cracking of Type 304 stainless steel, the effect of CaCl2 concentration on the probability distribution of the failure time Tf, crack initiation time Ti and crack propagation time Tp for smooth plate specimens with or without crevice under a constant condition of 373K and 200MPa has been analyzed. Two parameter Weibull distribution fitted approximately to all the distributions obtained. The apparent shape parameter mi' for Ti of both type of the specimens with or without crevice was found unity, indicating that a chance failure mode was operating. On the other hand, mf' and mp' for Tf and Tp show a maximum value of 4 or 6 at an intermediate concentration range of 30 or 40mass% CaCl2, corresponding to a wearout failure mode, and decrease to unity with a decrease of CaCl2 concentration. This fact means that SCC fracture and propagation at a low concentration range take place by a change failure mode. Since the same chance failure mode had been reported for the field failure time distributions of Type 304 and 316 stainless steel heat exchanger tubes, an approximate acceleration factor of the laboratory test conducted under the condition of 25 mass% CaCl2, 373K, 200MPa with crevice was estimated to be ca. 104 by comparing the mean failure time of both distributions of the field and the accelerated test.

Life Assessment of an HP-IP Rotor Under Creep Service Conditions

A CrMoV high and intermediate-pressure (HP-IP) rotor exhibited in-service cracking in the blade groove walls. Cracks initiated from a notch and propagated under creep conditions to a depth of 0.255 in. (6.5 mm). Since the unit was base loaded, the contribution of cyclic loading (startups and shutdowns) was not significant. The total life of the groove walls is predicted by applying a life prediction approach that accounts for both crack initiation and propagation. Crack initiation life is based on short-term creep tests extrapolated to long times using a time-temperature parameter. Crack propagation time is estimated using a newly developed time-dependent fracture mechanics concept. A parameter Ct is used to express the crack growth rate under creep loading conditions. Pertinent material properties such as creep deformation and creep crack growth rate were obtained by testing the material from the subject rotor. The results obtained by this analysis show that about 50 percent of the estimated life was spent in crack propagation. Predicted service life time compares very favorably with the actual operating life of the rotor.

Ａｌ−Ｚｎ−Ｍｇ合金の定電位応力腐食割れのワイブル分布を用いた評価

Effects of controlled potential on fracture time, tf, crack-initiation time, ti and crack-propagation time, tc in the stress corrosion cracking (SCC) of an Al-6.13 wt%Zn-2.10wt%Mg alloy were investigated by the Weibull's distribution method. Specimens were solutlon treated at 460°C for 1h, quenched into ice-water and aged at 120°C for 24h. The SCC test was performed at 33°C in 3.3%NaCl+0.2%H2O2 aqueous solution of pH 5.2 under controlled potential. The constant stress of 60% of the yield stress was applied. All the shape parameters (m) were larger than 1 indicating the wearing-out failure. The shape parameters for tf and ti decreased under the nobler potential than the rest one, while the shape parameters for tf, ti and tc increased under the lower potential than the rest. The life time, η for tf and ti decreased under the nobler potential than the rest and increased under the lower potential than the rest. The life time for tc was shorter than the half for ti. The major part of the life time for tf was that for ti. This implies that the life time for tf depends on that for ti. The probability density function f(t) which corresponded to the actual failure was determined from the values of m and μ.

Evaluation of the damping of stresses in a specimen with an edge crack being failed by bending

1. The low-frequency components of the field of internal stresses have the determining action on crack propagation as the result of the longer damping of them and the high amplitude of the oscillations. 2. The source of the low-frequency components of the field of internal stresses is the external loads, in the absence of which crack propagation is impossible. 3. Significant bending of the crack growth trajectory in failure of a rod by three-point bending is caused by more rapid damping of the external load applied to the short end of the rod in comparison with the total time of crack propagation and with the time of relaxation of the external load applied to the long end.

Interaction entre le fluage et la corrosion electrochimique en milieu de sels fondus: Cas de l'acier inoxydable AISI 304L dans NaCl-CaCl 2 fondu à 570 °C

Creep tests at a constant load have been performed in molten NaCl-CaCl 2 at 570 °C. A tensile apparatus was set up to determine the electrochemical parameters (the potential vs. an Ag⋎Ag + reference electrode, and the current density) during mechanical tests. The fractured area, observed by means of a scanning electron microscope, showed intergranular stress corrosion cracking; when the temperature was kept at 570 °C, precipitation of M 23C 6 induced crack propagation. The crack incubation time t i and crack propagation time t p were determined from creep rate curves ≐ = f(t). t i and t p were functions of the applied stress. The crack propagation rate constant was slightly dependent on the applied stress. The test piece potential varied during mechanical testing. Anodic dissolution at the bottom of cracks seemed to play an important part in the corrosion cracking process.

３０４ステンレス鋼の応力腐食割れ破断寿命の確率分布に及ぼすＣａＣｌ２濃度の影響

In order to compare the failure time distribution of a laboratory accelerated test with the field failure time distribution of stress corrosion cracking of Type 304 stainless steel, the effect of CaCl2 concentration on the probability distribution of the failure time Tf, crack initiation time Ti and crack propagation time Tp for smooth plate specimens with or without crevice under a constant condition of 373K and 200MPa has been analyzed. Two parameter Weibull distribution fitted approximately to all the distributions obtained. The apparent shape parameter mi' for Ti of both type of the specimens with or without crevice was found unity, indicating that a chance failure mode was operating. On the other hand, mf' and mp' for Tf and Tp show a maximum value of 4 or 6 at an intermediate concentration range of 30 or 40mass% CaCl2, corresponding to a wearout failure mode, and decrease to unity with a decrease of CaCl2 concentration. This fact means that SCC fracture and propagation at a low concentration range take place by a change failure mode. Since the same chance failure mode had been reported for the field failure time distributions of Type 304 and 316 stainless steel heat exchanger tubes, an approximate acceleration factor of the laboratory test conducted under the condition of 25 mass% CaCl2, 373K, 200MPa with crevice was estimated to be ca. 104 by comparing the mean failure time of both distributions of the field and the accelerated test.

クリープき裂伝ぱに及ぼす繰返し応力変動の影響

In order to investigate the effect of cyclic stress change on creep crack propagation, deformation tests on smooth specimens and crack propagation tests on notched specimens of 0.16% carbon steel were carried out at 673K (400°C) under a time-dependent fatigue condition.The crack propagation rate in time-dependent fatigue was much faster than that in static creep. It was owing to the acceleration of creep strain rate in the vicinity of crack tip because the dynamic recovery took place in the compression period. In these tests, the effect of the transition from small scale creep to large scale creep was small. The crack propagation rate had fairly good correlation with the experimentally evaluated creep J-integral range, Delta;Jc, which was equivalent to the analytically evaluated creep J-integral range based on the deformation property in time-dependent fatigue.In detail, however, the crack propagation rate in time-dependent fatigue was rather slower than that in static creep for an equal Delta;Jc value. It was concluded to be attributable to the existence of the incubation time of crack propagation during the tensile stress hold time in time-dependent fatigue. The equivalent creep J-integral range, (Delta;Jc)eq, which was the effective component of Delta;Jc for the creep crack propagation rate under a fatigue condition could be evaluated by the following equation;(Delta;Jc)eq=Delta;Jc/γ, γ=1+C11-R/C2+τH, where C1 and C2 are constants, R is the stress ratio, τH is the nondimensional tensile stress hold time.

Effects of Compressive Overloads on Fatigue Crack Growth

A phenomenological examination of acceleration in the rate of fatigue crack growth on H-6AL-4V beta annealed and 7050-T73 plate materials produced by compress!ve overloads was conducted. The effects of various magnitudes of compressive overloads and the sequence of tension-compression overload cycle with applied load ratio R= -1.0 on fatigue crack growth were investigated. The effects of truncation of compressive loads in a realistic flight-bv-flight spectrum of the C-141 aircraft were also examined. Specimens used in the test had center cracks and the through-the-thickness cracks emanating from one side of a A in. hole. When the overload cycle was changed from R = 0 to R = -1.0, with the constant amplitude loads at R = 0 and peak values of overload held constant, fatigue crack growth rate was increased by 5 to 40%. The rate increases with the increase of the overload ratio. The influence of sequence of tension-compression overload cycle with R = -1.0 on crack growth was less than 10%. The fatigue crack propagation times were shortened less than 15% when the compressive overloads were applied in blocked constant amplitude fatigue tests. Increasing the magnitude of compressive overload did not change the crack growth rate significantly. If the compressive loads in a realistic flight-by-flight spectrum of the C-141 aircraft are truncated, the predicted fatigue crack propagation time will be unconservative by as much as 50%.

繰返し荷重下におけるステンレス鋼およびその溶接継手の応力腐食割れに関する研究(第1報)

In the first stage of the research on stress corrosion cracking under dynamic load, we tried to study the phenomenon under pulsating trapezoid load (Fig.4).The tested material is AISI 304 stainless steel and the corrosion medium is a boiling solution of 42 percent MgCl2 (B.P.: 154°C). The apparatus used in this report is a constant tensile loading type and we can change the load time and the no-load time, arbitrarily.The results obtained from this experiment are summarized as follows:1) The damage due to stress corrosion cracking under pulsating trapezoid load is not equal to the accumulated value of static damage. This damage is not only influenced by the dynamic effect due to the pulsating load but also by the length of no-load time.2) We can presume the propagation time under the following assumption: the total sum of the damages due to 1) load time, 2) no-load time and 3) dynamic effect is equal to the damage under static load.3) In the case of the ratio of load time to no-load time being constant, the crack propagation time is prolonged by the prolongation of the loading period.

オーステナイト系不銹鋼の応力腐食割れに関する研究(第7報)

We investigated the effects of cold working on the stress corrosion cracking of austenitic stainless steels in the boiling solution of 42% MgCl2.In the case of an 18-8 stainless steel which is transformed partially into quasi-martensite by cold working, both induction period and crack propagation time are changed by cold working. Especially, the crack propagation time is very much lengthened by heavy cold working.But in the case of other stainless steels, both induction period and crack propagation time are hardly changed by cold working.As a result of the quasi-martensite obstructing the advance of crack, the crack propagation time is lengthened. And this effect is generally called "keying effect".The electro-chemical potential of test piece indicated at the crack-initiation time is a little changed by cold working. When the test piece is cold worked lightly such as 7% or 15%, it indicates a slightly lower potential than the original test piece. But when the test piece is cold worked heavily such as 30% or 45%, it indicates a slightly higher potential than the original one.The slip bands occurring at the time of cold working are never attacked during the stress corrosion test, but the slip bands occurring in the MgCl2 solution by the applied test load are gradually attacked during the stress corrosion test. When a slip band occurs, its fault plane has not any protective film, If this unprotected fault plane appears in the MgCl2 solution, it is attacked. But if it appears in the atmosphere, its area is suddenly oxidized and goes into passive state.The higher the applied stress is, the more of cracks appear on the surface of tested specimen. When the test piece is cold worked, this relation is shifted toward upward stress followed by the rising of yield point.

Fatigue of bitumen and bituminous mixes

This paper gives details of an investigation into the fundamental fatigue properties of bitumen and bituminous mixes. Fatigue tests carried out under constant bending stress, at varying temperatures between −13·5°C and +25°C, show that the material exhibits fatigue properties over wide ranges of stress and that for a particular temperature and speed of loading the log stress-log number of cycles to failure relationship is linear between 104 and 108 cycles. The life under constant stress is highly dependent on the temperature of the test, a low temperature giving a longer life at a particular stress; it is also dependent to some extent on the speed of loading, but taking into account the stiffness of the material which depends on temperature, speed of loading, rheological characteristics and composition of the mix, it has been found that when the logarithm of the strain is plotted against the logarithm of the number of cycles to failure all experimental results at different speeds and temperatures for one mix lie with a certain amount of scatter about a straight line. It appears therefore that the factor affecting the fatigue life is one of strain rather than stress, and the effects of temperature and speed can be accounted for by their effect on the stiffness of the specimen. This has been confirmed by tests under constant torsional strain at different temperatures between −20°C and +40°C, but at the higher temperatures under this type of loading the fatigue life includes a considerable crack-propagation time. Similar results have been obtained from mixes containing different amounts of aggregate, but as the quantity of aggregate in the mix is reduced so the life for a given strain increases, suggesting that the criterion of failure may be one of tensile strain in the bitumen present in the mix. Some tests have also been carried out on pure bitumen specimens at different temperatures.Careful examination of the fatigue cracks and failure surfaces shows that in nearly all cases failure originates on the principal tensile plane. The effects of such factors as surface conditions, void content and rest periods have also been studied.