Reduction In Crack Growth Rate Research Articles

Molybdate inhibition of environmental fatigue crack propagation in Al–Zn–Mg–Cu

MoO 4 2 - effectively inhibits environmentally assisted fatigue crack propagation in 7075-T651 stressed during full immersion in low-chloride solution, as understood by hydrogen environment embrittlement and film rupture where MoO 4 2 - -enhanced passivity reduces H production and uptake due to reduced crack hydrolysis, buffered pH, and a diffusion-barrier film. Inhibition is governed by the balance between crack tip strain rate and repassivation kinetics which establish the stability of the passive film. Inhibition is promoted by reduced loading frequency, reduced stress intensity range, increased crack tip MoO 4 2 - concentration, and potentials at or anodic to free corrosion. The inhibiting effect of MoO 4 2 - parallels that of CrO 4 2 - , but molybdate effectiveness is shifted to a lower frequency regime suggesting the Al x Mo y O z passive film is less stable against crack tip deformation. For high R loading at sufficiently low frequencies MoO 4 2 - fully inhibits EFCP, quantified by reduced crack growth rate to that typical of ultra-high vacuum, reduction in crack surface facets typical of hydrogen embrittlement, and crack arrest. Chromate did not produce such complete inhibition. Methods exist to incorporate molybdate or Mo in self-healing coating systems, but the complex effects of mechanical and electrochemical variables must be understood for reliable-quantitative fatigue performance enhancement.

Assessing the effects of riveting induced residual stresses on fatigue crack behaviour in lap joints by means of fractography

The interference fit provided by solid rivets induces a residual stress field beneficial to the fatigue performance of riveted lap joints. Attributing benefits in crack growth nucleation and growth behaviour within this residual stress field is non-trivial. Once fatigue cracks become visible on the surface of a joint they have already grown beyond the region of beneficial residual stress. In order to circumvent this problem, fractographic techniques were employed to evaluate postmortem the influence of the rivet squeeze force and resulting residual stress field on crack behaviour. Results demonstrated that within the range of rivet squeeze forces studied, a 3-fold reduction in crack growth rate is achievable at high rivet squeeze forces, representing a marked improvement in damage tolerance of the joint.

Dwell time effect on fatigue crack growth of RR1000 superalloy

The requirement for improved understanding of the behaviour of turbine disc alloys at elevated temperatures has led to an increased interest in the contribution of time dependant mechanisms to high temperature fatigue crack growth. A study has been conducted on a new powder alloy to investigate the contribution of such mechanisms when the applied waveform is varied in terms of hold periods and the influence of limited thermal exposure is included. Variable waveform tests performed in air at 725°C have indicated that the addition of a hold time at maximum load in a fatigue cycle tends to increase the crack growth rate per cycle in the as heat treated material. Crack growth in thermally exposed material is retarded by up to a 10 s hold time and then accelerated as the hold time increases further. Rapid near crack tip stress relaxation induced by γ′ coarsening is proposed to have a beneficial effect on the severity of this type of damage which causes the crack growth rate reduction for short hold times.

On the local variation of the crack driving force in a double mismatched weld

A material inhomogeneity in the direction of crack extension causes a difference between the near-tip crack driving force, J tip, and the nominally applied far-field crack driving force, J far. This difference can be quantified by a material inhomogeneity term, C inh, which is evaluated by a post-processing procedure to a conventional finite element stress analysis. The magnitude of the material inhomogeneity term is evaluated for cracks in an inhomogeneous welded joint made of a high-strength low-alloy steel. Both a crack proceeding from the under-matched (UM) to the over-matched (OM) and from the OM to the UM weld metal are treated. The effects of the inhomogeneity of the different material parameters (modulus of elasticity, yield strength, and strain hardening exponent) on C inh and J tip are systematically studied. The results demonstrate that the material inhomogeneity term is primarily influenced by the inhomogeneity of the yield strength. A crack growing towards an OM/UM interface experiences an accelerated crack growth rate or a pop-in, an UM/OM interface leads to a reduced crack growth rate or a crack arrest. The application of global assessment methods of the mismatch effect which are included in the Engineering Treatment Model (ETM) or in the Structural Integrity Assessment Procedures for European Industry (SINTAP) is discussed.

Impact and Fatigue Crack Growth Behaviour of a Yield Strength Graded Al-Zn-Mg Alloy

Impact properties and fatigue crack growth behavior of a newly developed Al-Zn-Mg functionally graded material (FGM) is studied. The gradient in terms of hardness (about 85 to 130 VHN) and yield strength (about 260 to 360 MPa) is established along the width of the Charpy-impact and Compact Tension specimens. In both these test specimens two types of FGMs, depending on the direction of crack propagation, are produced. In the first case, FGM-I, the notched surface is made harder and the hardness value decreases towards the other surface in the direction of crack growth. In the second case, FGM-II, the hardness gradient is reversed and the crack propagation takes place from the softer side to the harder side. The impact and fatigue crack growth experiments are also carried out for the homogeneous materials with different hardness values and the results obtained from the FGMs are compared with them. The results of impact tests show that FGM-II absorbs about 20% greater energy than the highest energy absorbed by any of the isothermally aged homogeneous materials. Fatigue crack growth studies for the FGM-I show a reduction in crack growth rate as the crack propagates towards softer (ductile) side and finally crack arrest is observed. On the other hand, for the case of FGM-II, where the crack propagates towards the harder side, the crack growth rate first increases and then decreases. However, in this case the crack follows a tortuous path and it starts bending upwards after about 20 mm crack length, which might be responsible for higher energy absorption in impact tests.

A study of crack growth retardation due to artificially induced crack surface contact

The contact of the cracked surfaces during a part of a loading cycle generally results in a reduced crack growth rate. A critical experiment was designed to evaluate the influence of the crack surface contact on crack growth. A round compact specimen made of 1070 steel with a round hole at the wake of the fatigue crack was designed. Two mating wedges were inserted into the hole of the specimen while the external load was kept at its maximum in a loading cycle. In this way, the wedges and the hole in the specimen were in firm contact during the entire loading cycle in the subsequent loading. Experiments showed that the addition of the wedges resulted in a reduction of crack growth rate in the subsequent constant amplitude loading. However, crack growth did not arrest. With the increase in the subsequent loading cycles, crack growth rate increased. The traditional crack closure concept cannot explain the experimental phenomenon because the effective stress intensity factor range was zero after the insertion of the wedges. The detailed stress–strain responses of the material near the crack tip were analyzed by using the finite element method with the implementation of a robust cyclic plasticity theory. A multiaxial fatigue criterion was used to determine the fatigue damage based upon the detailed stresses and strains. The crack growth was simulated and the predicted results were in good agreement with the experimental observations. It was confirmed that the stresses and strains near the crack tip governed cracking behavior. Crack surface contact reduced the crack tip cyclic plasticity and the result was the observed retardation in crack growth.

Multiple cracking under thermal fatigue

Tests were performed on austenitic stainless steels. In order to investigate thermal fatigue resistance of quasi-structural specimens, test facilities enforcing temperature variations similar to those found under the operative conditions have been developed. Multiple cracking networks similar to those detected during in-service inspections have been reproduced. Experiments and simulations deal with crack initiation and crack growth. In the case of multiple cracking simulation, a Skelton's modelling has been used. A shielding effect between cracks is evidenced. It leads to a dramatic reduction of crack growth rate. Such effect is also crucial for the crack network stability under additional loading.

Effect of interface layer on fatigue crack growth in an aluminium laminate

ABSTRACTIn order to study the effect of an interface layer on fatigue crack growth, a thin pure Al layer was sandwiched between two LY12 plates using explosive bonding. Experiment shows that as a fatigue crack approaches the interface a remote plastic zone appears in the soft Al layer. Energy dissipation in the interface leads to significant deceleration of crack growth rate. FEM analysis shows that crack arrest is associated with load and distance between the crack tip and the interface. The size of the plastic zone can be calculated and used to predict the reduction in crack growth rate. The predictions agree well with the experimental results.

Influence of LCF overloads on combined HCF/LCF crack growth

Fatigue crack growth rates have been studied in forged Ti–6Al–4V aero-engine disc material under the conjoint action of LCF and HCF cycles at room temperature. An overload has been introduced into the LCF cycle component of the test sequence, which has been based on a ratio of HCF to LCF cycles of 1000:1. Systematic increases in the overload, applied prior to the commencement of the HCF cycles, clearly demonstrated a diminution in the contribution of the HCF cycles to crack growth rates. Accompanying the reduction in crack growth rates is an increase in the stress intensity range at which the HCF cycles begin to contribute to the crack growth rate. For three HCF cycle stress ratios ( R=0.7, 0.8 and 0.9), the severity of the overload required to negate the effect of the HCF cycles has been determined. Additionally, threshold values for the same stress ratios have been found and used to predict the onset of HCF cycle contribution to crack growth, and the effect of multiple vs. single overloads has been investigated. Modelling to predict crack growth rates and the onset of minor cycle damage has been undertaken.

The effect of texture variation on delayed hydride cracking behavior of Zr–2.5%Nb plate

In order to investigate the effect of texture variation on the delayed hydride cracking behavior in Zr–2.5%Nb plates, crack growth rate and K IH tests have been carried out at temperature ranges varying from 415 to 506 K after texture modification by rolling. The texture variation of plates was achieved by direct-rolling and cross-rolling. Texture was measured through the determination of inverse pole figures, from which the basal pole components were calculated. The results have shown that the texture of a plate in which the basal poles are concentrated in the transverse direction can be changed significantly by cross-rolling. The crack growth rate increases exponentially with the basal pole component in the direction normal to the cracking plane. The increase in stress relieving temperature on cold worked material reduces crack growth rate. K IH decreases linearly with the basal pole component, and a behavior of which could be explained by the uniformly dispersed aggregate composite theory.

Effects of Dendrite Cell Size and Eutectic Si Particle Morphology on Fatigue Crack Growth in Cast and HIPed AC4CH Alloys

Fatigue crack growth tests were performed for structurally controlled AC4CH cast alloys. Elimination of casting defects from the casts was achieved by means of post-casting HIP treatment. Compliance change measurement was made during the fatigue crack growth test to investigate crack closure. The level of crack closure was comparable among the alloys with different microstructure. Sheroidizing and refining of the eutectic Si particles are essential in terms of reduced propensity of particle fracture, which results in prolonged stable crack growth region of the da/dN-ΔK relationship and improved fatigue crack growth resistance at the high ΔK. At low ΔK, however, the fatigue crack growth rate is controlled by general matrix strength. The higher proof stress due to the irregular shaped Si particles and fine dendrite cell size results in the reduced crack opening displacement of each fatigue cycle or the reduced crack growth rate.

The effect of dielectric coatings on igscc in sensitized type 304 ss in high temperature dilute sodium sulfate solution

Zirconium dioxide(ZrO 2) coatings were applied to sensitized Type 304SS C(T) crack growth rate specimens and to small coupons for corrosion potential measurements via electrophoretic deposition. The interfacial resistance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS). Electrochemical corrosion potentials (ECPs) of un-coated and coated samples were measured against an external pressure balanced reference electrode (EPBRE) in dilute Na 2SO 4 (0.0005 m) solution at 288°C, and it was found that the ECP of the coated samples were consistently 200 to 400 mV more negative than that of uncoated samples. A coated C(T) specimen and an identical, uncoated specimen were exposed simultaneously under load in a refreshed autoclave, in order to measure the crack growth rates. The results from the long-term experiments (400 hours) showed that the surface coatings reduced the IGSCC crack growth rate (CGR) by a factor of more than 10. The reduction in crack growth rate and the negative shift in the ECP were found to scale with interfacial resistance as predicted by the Coupled Environment Fracture Model and the Mixed Potential Model, respectively, assuming that the exchange current density for oxygen reduction scales inversely with the interfacial resistance.

Fatigue propagation behaviour of polystyrene/polyethylene blends

Fatigue crack propagation (FCP) of injection-moulded polystyrene (PS) and 95/5, 85/15 and 70/30 PS/high-density polyethylene (HDPE) blends at loading frequencies of 2 and 20 Hz was studied. The FCP results showed that increasing the HDPE content caused a progressive reduction of the fatigue crack growth rates, especially when a styrene/ethylene– butylene/styrene (SEBS) terpolymer was added as a compatibilizer. Increasing the loading frequency also led to a fatigue crack growth rate reduction. Moreover, the fatigue crack growth rates were lower at a given cyclic stress intensity factor range, ΔK, when the crack propagated normal, instead of parallel, to the melt-flow direction during injection moulding. Fractographic observations indicated that discontinuous growth bands (DGBs), associated with the fracture of crazes in the plastic zone, were present through most or all of the fracture surfaces of the PS/HDPE specimens. In the presence of sufficient HDPE, these DGBs were formed by the initiation, growth and coalescence of large dimples initiated at HDPE particles ahead of the microscopic crack front, similar to a multiple crazing effect. The loading frequency effect on the FCP behaviour of these blends is attributed to a time-dependent deformation process. It is concluded that the FCP behaviour of these blends is strongly affected by the loading direction with respect to the matrix and minor phase orientation, by the presence of a compatibilizer, by the composition of the blend and by the testing conditions. © Chapman & Hall.

Crack propagation under cyclic hydraulic pressure loading

Mode I propagation rates for small cracks in a hardened AISI 52100 bearing steel under cyclic hydraulic pressure loading have been measured. Stress intensities were generated by applying high pressure fluid at the mouth of the crack without external far-field loads. Effects of crack size, cyclic pressure range, ambient fluid viscosity, and frequency have been observed. Some test conditions produced d a/d N values comparable to baseline growth rate data collected in air under uniaxial cyclic tension while others resulted in significantly reduced or no crack propagation. Reduced crack growth rates are probably due to a lack of full fluid penetration into the crack or incomplete expulsion of fluid from between the crack faces both of which reduce Δ K eff. Test data are compared to predictions from the characteristic penetration time model of Hsia and Xu. Qualitative agreement is observed, but some simplifications in the model, for example, constant viscosity, would require modification before qualitative comparison with experimental data can be made.

Effects of beryllium on fatigue crack propagation of A357 alloys containing iron

Abstract An investigation of iron-bearing compounds, silicon particles and Mg2Si precipitates on fatigue crack growth behaviour in A357 alloys has been performed. The effect of Be on increasing ΔKth is more apparent in alloys with higher Fe content than in alloys with medium and lower Fe content. The fatigue life of the higher and medium Fe-content Be-containing alloys will be improved much as well as that of in the lower Fe content of alloys. Major fatigue cracks induced by cracking of iron-bearing compounds cracking were found in the medium and higher Fe-content alloys. In the lower Fe-content alloys, irregular silicon particles appear to be the main source of fatigue crack nucleation. In region I of low fatigue crack growth rates, the higher threshold stress intensity is attributed to the greater mechanical strength and to enhanced roughness-induced crack closure and deflection from more tortuous crack paths. The superior fatigue crack propagation resistance at higher growth rates of regions II and III is attributed to greater fracture toughness and to reduced crack growth rates in the Be-containing alloys regardless of Fe levels. The fracture surface morphologies of near-threshold crack growth contain stepped patterns and cleavage-like facets. With increasing crack growth rate, the fracture surface roughness increases and striations are formed in region II. At higher fatigue crack growth rates, the formation of a dimple surface and cleavage plane replaces striation formation.

The effect of infiltration induced crack closure on crack growth retardation

It is now well known that extra materials on the flanks of a fatigue crack will cause premature crack closure and a reduction in crack growth rate. These extra materials may be residual plastic wake, corrosion/oxidation products, phase transformation and fluid. Micro-roughness of the fatigue fracture surface is also capable of bringing about premature crack closure. The phenomenon of crack growth retardation following a single tensile overload has been attributed to some of these crack closure mechanisms. Considerable amount of research has been carried out on the mechanisms and effects of naturally occurring crack closure on fatigue crack growth. By contrast, relatively little attention has been given to artificially induced crack closure. Recently, it has been noted that artificially created crack closure may lead to the development of practical fatigue crack repair methods. This work presents the preliminary results on the effects of using alumina powder reinforced epoxy resin infiltration to induce crack closure.

THREE‐DIMENSIONAL CONSTRAINT EFFECTS ON STRESS INTENSITY DISTRIBUTIONS IN PLATE GEOMETRIES WITH THROUGH‐THICKNESS CRACKS

AbstractThree‐dimensional finite element analyses of through‐thickness cracks in plate geometries are presented. It is shown that the local constraint variation towards the crack tip causes an increase in stress intensity which is neglected in analyses which assume a constant stress state throughout the geometry. This means that stress intensity factors for these types of crack geometries are a factor (1 — v2)‐0.5 higher than generally reported in stress intensity handbooks.Analyses on crack tunnelling situations show that if plane stress dominates the global behaviour, an almost constant stress intensity across the thickness is reached for a relative tunnelling depth of a 2.5% of the plate thickness. For geometries which are not predominantly in plane stress this value will be somewhat larger. Crack front tunnelling does not influence the mean stress intensity across the thickness.Shear lips are shown to reduce the mean equivalent stress intensity (the mode I stress intensity which has the same energy release rate as the actual combined mode I, II and III loading) by a factor equal to the square root of the ratio between plate thickness and projected length of the crack front. This may explain the reduction in crack growth rate caused by shear lips during fatigue crack growth experiments. The reduction of the mode I stress intensity factor is considerably larger, which may cause a further reduction of fatigue crack growth rate for crack growth mechanisms that depend primarily on the mode I crack tip loading.Analyses on CNT and SENB specimens show that the conclusions reached on infinite plate models also hold for real structures. However for an SENB specimen with an uncracked ligament equal to the plate thickness, the overall constraint is larger than that of a pure plane stress situation, and the effect of stress intensity increases due to the constraint transition is less pronounced.

Fatigue behavior in the potentiostatic passive corrosion regime of the iron-base superalloy A-286

Surface crack initiation and propagation behavior of the iron-base superalloy A-286 were tested using smooth hourglass specimens under passive corrosion conditions. In the very underaged (VUA) condition, cracks were initiated at slip steps (stage I) and later propagated in a stage II mode, resulting in a minimum or dip in crack growth rate. For the highly overaged condition (HOA), cracks initiated at inclusions, and only stage II was observed; therefore, anomalies in crack growth behavior were not observed. A planar slip distribution, observed in VUA, was associated with a reduced crack growth rate. In general, the VUA microstructure was superior to HOA, as it exhibited an impressive combination of excellent mechanical properties, decreased susceptibility to corrosion, good resistance against crack initiation, and low crack growth rate, under corrosion fatigue conditions. Possible reasons for such behavior are discussed.

The effect of crack closure on the grain size dependence of fatigue crack growth threshold

Recent studies (I-7) clearly indicate a strong dependence of fatigue threshold parameter, A K on grain size in several alloy systems. Attempts to explain these observations on the basis of crat~tortuosity (1,8), fracture surface roughness (5,9) and crack closure (6) appear to present a fairly clear picture of the mechanisms that cause a reduction in crack growth rates at threshold. In general, it has been shown that coarse grained microstructures exhibit higher fatigue threshold in low carbon steels (1,5) aluminium alloys (7) and titanium alloys (6). In spite of these observations, there exists (10-1#) considerable uncertainity about the manner in which the AK~L depends on prior austenitic grain size in quenched and tempered steels. Studies in quenched and tempered steels demonstrating both an increase (3,12,14) as well as a decrease (11,12) in AKth with an increase in prior austenitic grain size can be sought to illustrate this point. Occasionally , the absence of any sensitivity of AKth to the variations in prior austenitJc grain size has also been reported (11,13). While a few investigators (5-7) comfortably rationalised the grain size effects on AK~L on the basis of crack closure by a comparison in terms of the closure-free component of the thresho~Ifc~, AK -f such an approach has yet to be extended to high strength steels, An attempt has been made in t~et ,pthrg sent study to explai. n the effect of pri, or austeniti.c grain size on &Kth on the basis of crack closure measurements in a high strength steel.

The Effect of Fast-Neutron Irradiation on the Fatigue-Crack Growth Behavior of Several Austenitic Stainless Steels and Weldments

Fatigue-crack growth specimens from a number of austenitic stainless steels and weldments (annealed Types 304 and 316, 20% cold-worked Type 316, SA-351 Grade CF8, Type 304/308 shielded-metal-arc (SMA) weldments, and Type 316/IN-82/IN-718 gas-tungsten-arc weldments) were irradiated in Experimental Breeder Reactor-II. Postirradiation crack growth testing showed little or no effect of irradiation on the crack growth behavior of annealed Type 304, cold-worked Type 316, and Type 304/308 SMA weldments. On the other hand, irradiation produced a minor reduction in crack growth rates in annealed Type 316 and minor increases in crack growth rates in SA-351 Grade CF8 castings and Type 316/IN-82/IN-718 GTA weldments over certain ranges of ..cap alpha..K.