Slow Crack Growth Rate Research Articles

Hydrogen-induced sustained load cracking in single crystal Fe-3wt.%Si

Sustained load slow crack growth rate in Fe-3wt%Si single crystals was determined to be about 5 × 10 −8 m/s at room temperature in 1 atm gaseous hydrogen. Extensive plastic deformation was observed to accompany crack growth. During crack growth, slip line upsets of about 10 μ m spacing were generated behind the advancing crack tip but the crack advance step size was on the order of 1 μ m. Further investigation to clarify the mechanism is in progress.

Effect of thermal history on the initiation of slow crack growth in linear polyethylene

Increasing the cooling rate from the melt has been found to increase the rate of slow crack growth at 42°C. For various histories of cooling rate, annealing and specimen geometries, the initiation rate increased when the density decreased. The effect of residual stresses on slow crack growth was separated from the effect of morphology as reflected by density. It is concluded that the variation in yield point is a factor that affects the rate of slow crack growth. The disentanglement rate of fibrils is also important, but it is primarily controlled by molecular weight and branching.

The analysis of instrumented impact tests using a mass-spring model

The Charpy impact test is modelled as a mass on a spring which is loaded via a contact stiffness. The strain energy release rate G is evaluated for this model for the case of slow crack growth rate initiation. The solution involves the kinetic energy and is shown to be harmonic in nature. Correction factors are then deduced for tests in which Gc is to be evaluated via load, energy and displacement and these are applied to a wide range of experimental data. It is concluded that the displacement measurement method gives the least dynamic error and that loads are, by a large margin, the worst.

Progress for on-line acoustic emission monitoring of cracks in reactor systems

This paper reviews accomplishments and planned tasks for the NRC-sponsored research program concerned with “Acoustic Emission/Flaw Relationships for Inservice Monitoring of Nuclear Reactor Pressure Boundaries”. The objective of the acoustic emission (AE) monitoring program is to develop and validate the use of AE methods for continuous surveillance of reactor pressure boundaries to detect flaw growth. Topics discussed include testing AE monitoring on reactors, refinement of an AE signal identification relationship, study of slow crack growth rate effects on AE generation, and activity to produce an ASTM standard for AE monitoring and to gain ASME code acceptance of AE monitoring.

Application of hydrogen embrittlement models to the crack growth behavior of fusion reactor materials

Hydrogen induced crack growth rates of a 12 Cr-1 Mo ferritic stainless steel (HT-9) have been estimated for three sources of hydrogen: the plasma, nuclear reactions and aqueous corrosion. Estimates of crack growth rates were derived using hydrogen embrittlement models which describe the temperature and hydrogen activity dependence of cracking. A crack growth rate of 10−3 cm/s at a reactor operating temperature of 400°C was obtained for a steady-state hydrogen concentration of 0.5 appm resulting from (n, p) reactions, while a much slower crack growth rate was predicted for the same steady-state hydrogen concentration with an alternate model. These calculations have shown the need for further research to assess the effect of temperature on crack growth. Other sources of hydrogen give very slow hydrogen induced crack growth rates at reactor operating temperatures while significant hydrogen induced crack growth rates are possible at lower temperatures. For instance, hydrogen from an aqueous corrosion reaction could produce a crack growth rate of 10−7 cm/s at 25°C which could be significant during extended downtime. Also, a non-equilibrium hydrogen uptake from the plasma could occur from surface reaction controlled effects, and a crack growth rate of 10−1 cm/s was estimated for this condition at a temperature of 75°C. Based on this analysis, hydrogen induced crack growth is not considered significant for HT-9 during reactor operation but may be a problem during extended downtime if the temperature decreases to a value less than 100°C.

The relationship of the initiation stage to the rate of slow crack growth in linear polyethylene

The rate of notch opening was observed in single-edge notched tension specimens of linear high-density polyethylene under plane strain conditions as a function of applied stress and notch depth. The initial rate of notch opening was 22K 4.3 μm min−1 (where units ofK are MN m−3/2). The initial rate of notch opening was constant until the opening at its root was 25 to 30μm for all stresses and notch depths. The accelerating part of the notch opening against time curve and the time to failure could be predicted from the initial rate of notch opening. It is concluded that the same mechanism governs the initiation stage and the subsequent crack growth rate.

Influence of grain size on the mechanical behaviour of some high strength materials

Combining in an additive or synergetic manner the most potent strengthening mechanisms available in an alloy is the art of the metallurgist. The various models proposed in the literature in order to interpret the Hall-Petch relation are critically reviewed by comparison with experimental data. The pile-up models and the work hardening theories must include the inner structure of the grain in the case of alloys hardened by a second phase. Similarly, the properties and structure of the grain boundaries are influenced by impurities or the presence of particles. Ultra-fine grain sizes can provide ductility to high strength materials when surface preparation eliminates microcracks. In steady-state creep equations, introducing the influence of grain size in complex alloys by incorporating the Hall-Petch stress as one component of the internal stress helps in rationalizing the existence of an optimal grain size where creep resistance is maximized. Slower crack growth rates can be obtained by controlling the grain boundary structure as well as grain size. Fatigue tests at room temperature clearly point out the interest of small grain sizes for reducing crack initiation, usually associated, however, with lower propagation threshold and somewhat faster growth rates.

衝撃過大荷重による疲労き裂進展の遅延現象について

As one of the on-going studies to investigate the fatigue crack growth characteristics in impact fatigue, the fatigue crack growth behavior resulting from the single application of impact over-load was investigated by using Dual-Phase steel with ultimate tensile strength of about 800MPa as an experimental material. Discussions were made on the retardation of crack growth rate, crack opening behavior after over-loading and fractographical aspect in comparison with the results obtained through the application of single static over-load with peak stress hold time of 3min.The major conclusions obtained are summarized as follows:(1) Retardation of crack growth rate after impact over-loading was not so distinct when compared with the case after static over-loading.(2) The values of crack opening ratio after impact over-loading were higher than after static over-loading.(3) Fractographical observations showed that abrasions were caused on the fractured surfaces after impact over-loading, but not so after static over-loading. The following speculation on the crack tip plastic deformed zone was needed to explain the peculiar crack growth behavior after im- pact over-loading as mentioned above. In comparison with the condition of the plastic zone constructed by static over-load, the size of the zone constructed by impact over-load is smaller, but the strength of this small plastic zone is higher, in other words, the higher density of slip bands is attained under impact over-load.

Creep crack growth behavior of type 316L steel

The creep crack growth behavior of a 316L type stainless steel was studied at 600°C and 650°C using widely different specimen geometries. A strong emphasis was laid on crack initiation, early crack growth, and slow crack growth rates ( a). The applicability of four load parameters, the stress intensity factor, the nominal stress, the reference stress, and the contour integral C * , was investigated. Finite element calculations were made to compare the experimental load line displacement rates with the calculated values. It is shown that it is possible to correlate time to initiation ( T i ) with C * . The obtained experimental relation is not in full agreement with a theoretical expression derived from models for creeping solids. The reasons for this discrepancy are discussed. Furthermore, it is observed that there is no unique correlation between a and any of the four investigated load parameters, especially at low crack growth rates ( a0 −3mm/h). At large crack growth rates ( a≳10 −2 mm/h) the apparent correlation between a and C * is essentially due to the fact that the displacement rate is controlled by crack growth and not by the overall creep behavior of the specimens. A simple model based upon the data on crack initiation and the creep ductility exhaustion concept is shown to give results in reasonable agreement with the experiments.

Crack initiation in cyclic compression and its applications

The characteristics and applications of crack initiation under far-field cyclic compressive loads are examined in notched specimens of a lower strength steel. The fatigue cracks, initiated at the notch root due to residual tensile stresses, grow at a progressively decreasing velocity before arresting completely. Simple correlations are explored between the total fatigue crack growth distance and the size of an effective damage zone estimated from the initial crack growth rate. Some important applications of crack initiation in cyclic compression in both long and short fatigue crack problems are demonstrated. Experimental results are presented to show that crack initiation in cyclic compression offers some interesting possibilities for obtaining accelerated estimates of slow fatigue crack growth rates, while minimizing some of the inherent uncertainties arising from the artifacts of conventional test techniques. Possible applications of this method are also discussed with respect to the study of physically-short flaws and of the evaluation of the progressive development closure during fatigue.

The extension of cracks at high temperature by growth and coalescence of voids

The problem of creep crack growth is studied analytically in this paper. The analysis assumes a sufficiently slow crack growth rate so that elastic effects can be neglected and hence the near tip stress field is well approximated by the C*(HRR) field. The crack is assumed to advance continuously due to the growth and coalescence of voids directly ahead of the crack tip. An exact solution for the entire growth historya(a), 0 ⩽a < ∞, is obtained as a function of crack extensiona andC* for a wide variety of void growth mechanisms. The solution shows that, for a finite nucleation zone with sizeR, the steady state crack extension rateass =a(a → ∞) is attained exponentially fast with characteristic distanceR. Another consequence of the solution is that is proportional toC*. An important result is that, for crack extensiona less thanR, the transient crack growth ratea(a) can be approximated by a closed form asymptotic series.

An Assessment of Strain-Aging Effect on Weldable Offshore Structural Steel

Summary During the fabrication of offshore structures, the steel plate condition changes from the as-received normalized state to a strain-aged state. This paper assesses the effect of 5% prestrain and aging at 250C [482F] for 1 hour on the mechanical properties of normalized low-alloy BS 4360–50D steel plates obtained from two different suppliers. For both plates, strain aging led to an increase in yield strength, ultimate strength, Charpy and crack opening displacement (COD) transition temperatures, and fatigue resistance. The results of fracture toughness tests show that the degree of sensitivity to strain aging may depend on the type of test used; therefore, a test that more closely simulates offshore loading conditions, such as a COD test, is most appropriate. The paper also shows that the impact of strain aging on the integrity of offshore structures is minimized by the competing effects of lower fracture toughness and slower crack-growth rates. Introduction The selection of steel for offshore structures is governed by several factors, including strength, notch toughness, and fatigue strength. These properties are, in general, established on the basis of the as-received, normalized steel condition. Since offshore structures are generally fabricated by forming steel plates into tubular members and connecting these members by welding, a change in the properties would be expected as a result of strain aging. The strain occurs during forming, and aging is accelerated by the temperature rise during welding. Strain aging manifests itself mainly by an increase in yield strength on aging after straining. Strain aging may also change other properties, such as ultimate tensile strength, ductility, ductile/brittle transition temperature, fatigue strength, and electrical and magnetic properties. Nevertheless, the increase in yield stress on unloading and aging is the most universal indication for the occurrence of strain aging. Therefore, strain aging can be detected simply by the tensile test (Fig. 1). The increase in yield stress after aging, is often used as a measure of the strain-aging susceptibility. This change in properties is attributed to the pinning of the free dislocations that are formed during straining by the carbon and nitrogen atoms that are dissolved in ferrite. It is now universally accepted that strain aging occurs in the presence of interstitial elements such as carbon, nitrogen, and boron. Since virtually all offshore structural steels contain carbon and nitrogen and do not contain boron, it is the first two elements that are of greatest interest. It is therefore suggested that the effect of strain aging can be restricted by lowering the concentration of carbon and nitrogen atoms that are dissolved interstitially in the ferrite. The effect of free nitrogen can be reduced adding nitride-forming elements, such as aluminum. To bind the free carbon atoms, elements that form stable carbonitrides and carbides, such as niobium, vanadium, molybdenum, or titanium, are added. Available evidence suggests that a nitrogen content of 0.0001% produces detectable aging, while 0.001 to 0.002% produces severe aging. Increases above this level probably have little further effect. This suggests that the uncombined nitrogen must be reduced to about 0.0001% or less to eliminate strain aging. Wilson and Russell suggested that in moderately deformed steels, the segregation of about 0.0005 to 0.002 wt% carbon or nitrogen solute (depending on the exact dislocation density) is sufficient to complete dislocation locking. Since the solubility of nitrogen in ferrite is about 100 times that of carbon, it is always suggested that strain aging at low temperatures (less than 100C [212F]) is attributed to nitrogen. At high aging temperature &amp;gt;100C [212F]), the solubility of carbon is increased, and strain aging is produced even when nitrogen has been completely removed from solution. Aging at higher temperatures &amp;gt;350C [662F]) enables substitutional solutes, such as phosphorus, niobium, nickel, silicon, and titanium, to cause strain aging. Strain aging is known to affect toughness adversely. Baird recently has shown that the increase in impact transition temperature after 10% prestrain and aging at 250C [482F] for 30 minutes is not very sensitive to composition, prior heat treatment, or type of impact specimen. The overall increase (caused by both straining and aging) lies in the range of 40 to 70C [104 to 158F] for nearly all steels. Recognizing the potential effect of strain aging on fracture toughness of offshore structural steel, some technical standards restrict the use of Charpy V-notch impact test results from the parent plate to the conditions where the forming strain does not exceed 5% and the material temperature does not exceed 650C [1,202F]. Otherwise, Charpy tests shall be done with specimens machined from fabricated plates. The present study is aimed at establishing the effect of strain aging on one class of steel plates, BS 4360–50D, which is used extensively in the fabrication of offshore structures. JPT P. 141^

Temperature effects on fatigue thresholds and structure sensitive crack growth in a nickel-base superalloy

Fatigue thresholds and slow crack growth rates have been measured in a powder formed nickel-base superalloy from room temperature to 600°C. Two grain sizes were investigated: 5–12 μm and 50 μm. It is shown that the threshold increases with grain size, and the difference is most pronounced at room temperature. Although crack growth rates increase with temperature in both microstructures, the threshold is only temperature dependent in the material with the larger grain size. It is also only in the latter that the room temperature threshold falls when the load ratio is increased from 0.1 to 0.5. At 600°C the higher load ratio causes a 20% reduction in the threshold irrespective of grain size. The results are discussed in terms of surface roughness and oxide-induced crack closure, the former being critically related to the type of crystallographic crack growth, which is in turn shown to be both temperature and stress intensity dependent.

Fracture surface features of polyethylene pipe samples

AbstractSlow crack growth behavior of medium‐density polyethylene pipe samples has been investigated by applying standard and nonstandard fracture mechanics test methods. Slow crack growth rate varied by several orders of magnitude among the PE2306 materials from different manufacturers. Slow crack generated surfaces exhibit fibrous textures of varying degrees of coarseness when examined under the scanning electron microscope. When the measured fiber heights, δ/2, of these samples were plotted against the applied stress intensity factor, KI, they showed a simple power law relationship (of exponent 2). Assuming that the pulled‐out fibers transmit almost uniform tensile stresses across the crazed zone at the crack front, the Dugdale‐Barenblatt model was introduced to predict the fiber height. Although the model needs further modification, it provides a general power law relationship between the fiber height and the applied stress intensity factor.

On the influence of fatigue underloads on cyclic crack growth at low stress intensities

The influence of variable-amplitude loading in the form of block underload cycling is examined for (plane strain) near-threshold fatigue crack propagation in a bainitic 2.25Cr-1Mo pressure vessel steel (ASTM A542 class 3), tested in ambient temperature moist air at low load ratios ( R = 0.05). It is shown that the application of fatigue underloads with stress intensity ranges below the threshold ΔK 0 can result in significant transient retardations in initial growth rates when cycling is subsequently resumed at baseline levels above the threshold. The magnitudes of the retardation are found to be dependent on the baseline ΔK B and underload ΔK u stress intensities. Using electron spectroscopy for chemical analysis and scanning Auger spectroscopy, the observations are shown to be consistent with concepts of oxide-induced crack closure, which arises from extra corrosion debris (Fe 2O 3) generated within the near-threshold crack during the underload cycling period. Such oxide deposits, which are of the order of pulsating crack-tip-opening displacements ΔCTOD, can lead to increased closure loads and reduced effective stress intensity ranges, thereby retarding subsequent initial growth rates at baseline levels. It is shown that sub-threshold fatigue underloads have negligible effects on baseline near-threshold growth rates when the underload ΔCTOD appears to be smaller than the existing (baseline) excess oxide thickness. In this instance the crack remains effectively closed during underload cycling, thus restricting further oxide growth. The relevance of these observations to current procedures for the determination of threshold values and slow crack growth rates at low stress intensities is discussed in detail.

Cyclic Plasticity and Fatigue of Structural Components

An analytical approach for low-cycle fatigue prediction is presented. The approach combines a cyclic plasticity model with the finite element method and a damage accumulation criterion for ductile metals. The cyclic plasticity model is based on the concept of the combination of several yield surfaces. The surfaces are related to the material uniaxial stress-strain curve idealized by piecewise linear segments. The damage criterion is based on the Coffin-Manson formulae modified for the mean stress variation effect. It is extended to the multiaxial varying stress-strain field and applied for both the crack initiation and the crack growth processes. The stable slow crack growth rate is approximated by the damage accumulation gradient computed from the cracked finite element models. This procedure requires fatigue testing data of only smooth specimens under constant strain amplitudes. The present approach is illustrated by numerical examples of an aircraft wing stiffened panel subjected to compression, which causes material yielding and residual tension.

Acoustic emission: A means of measuring crack growth at elevated temperatures

Acoustic-emission (AE) technique has been applied to detect and continuously monitor subcritical crack growth in Zr-25% Nb. a commercial alloy for reactor-core components. The tests conducted in the dimensions of time, temperature, stress and local environment registered a significant difference in AE aclivity. The time-domain analysis of acoustic emissions suggested the crack growth to be intermittent Further investigations to interpret the AE response to cracking identified tracture mechanisms operative during slow crack growth. The analytical parameters of crack growth were then correlated with AE data which produced some confidence in the use of AE techniques to quanhtatively measure slow crack growth and crack growth rates during tracture. The results are presented to show a good fit between estimated and observed crack lengths and velocities. Finally, the viewpoints reached from AE studies were compared with tractographic observations and were found in agreement, providing a great deal of faith in the use of AE techniques.

Crack closure and crack propagation rates in 7050 aluminum

The effects of crack closure on fatigue crack growth rate in 7050 Al were studied with the aid of small-foil strain gages glued at positions near to the crack plane. The crack closure stress was determined from the change in the slope of the local strain-nominal stress curves. The crack closure stress was found to increase slightly with crack length under a cyclic loading total amplitude of Δσ = 88 MPa and R ratio R = 0.95. The crack closure stress σ c was found to increase moderately with increasing R-ratio ( R = σ min σ max ) for th1e same cyclic stress level of 88 MPa. Above a critical R-ratio value of 0.25 the crack closure stress is smaller than the minimum applied stress. The modified crack growth law, which states that crack growth rate ( da dN ) is proportional to the nth power of the effective stress intensity factor ( ΔK eff = K max- K in), cannot account for the dependence in growth rate on the R-ratio. For cyclic loadings with R-ratios less than 0.25 the da dN vs ΔK eff curves did not emerge as expected from the modified law. For cyclic loadings with R-ratios larger than 0.25 the crack growth rate still increased with R-ratio even though the value of ΔK eff remained the same. The retardation of crack growth rate that occurred after tensile overloads was accompanied by an increase in crack closure stress. A log-log plot of da dN vs ΔK eff showed that the modified crack growth law was reasonably obeyed. But it was found that after a single overload the minimum value of ΔK eff generally was reached only after the minimum value da dN had occurred. Cyclic compressive stresses which decreased the retardation effect of tensile overloads were found to depress the crack closure stress to a low value. A large compressive spike also depressed the crack closure stress and increased the crack growth rate after a tensile overload.

INFLUENCE OF VARIOUS PARAMETERS ON THE DETERMINATION OF THE FATIGUE CRACK ARREST THRESHOLD Members of the Fatigue Commission of the French Metallurgical Society*

Abstract —This report presents the results of a round robin study on the fatigue crack arrest threshold (δKth) of 2618 aluminium alloy and AISI 316 steel.The main purpose of this work was to develop a method for the determination of δKth, and to examine the influence of various test parameters on this threshold.Among the parameters considered, only the load ratio (R) and the environment (vacuum) appear to have a significant influence on very slow fatigue crack growth rates (FCGR). Moreover, while the results obtained with the 316 steel show great scatter, the importance of the adopted procedure is pointed out.

ＰＭＭＡ材のき裂成長速度と破壊じん性

The effect of temperature on the relation between fracture toughness and slow crack growth rate was investigated for PMMA in the temperature range -30°C to 80°C. Fracture toughness was measured by Gurney's sector method.The results obtained are summarized as follows;(1) A scaling law rather than an Arrhenius type equation holds between the crack growth rate and the fracture toughness. The exponent of the scaling law is nearly constant over the entire temperature range tested.(2) The fracture surfaces associated with slow crack growth are classified into three typical types, i. e., mist surface with few radial lines, mist surface with many radial lines and mirror surface. These typical surfaces are closely related to the crack growth rate and testing temperature.