Fatigue Crack Growth Threshold Research Articles

Nodule influence on fatigue crack growth thresholds in nodular cast iron

The influence of microstructure i.e. nodules (voids) on the fatigue threshold value in nodular cast iron is numerically investigated. It is qualitatively shown that the fatigue threshold value is unaffected by the individual nodules and that the threshold value is proportional to the bulk Young's modulus. Furthermore it is shown that nodules (voids) do not influence the crack tip conditions (stress-intensity factor and crack tip opening displacement) if the voids do not lie very close to the tip for physically long cracks. It is concluded that linear elastic fracture mechanics is valid for these types of materials and no influence of the nodules need be taken into count for fatigue calculations when dealing with long cracks.

Fatigue crack growth threshold conditions at notches. Part II: generalization and application to experimental results

The theoretical foundation of a micromechanical model that accounts for the fatigue crack growth threshold conditions at notches was described in Part I of this study. Strictly speaking, the proposed formulation is restricted to the analysis of a component with an elliptical notch under antiplane stress. In this section of the study, the expressions derived in Part I are generalized for application to axial stress states and non‐elliptical notch geometries. The procedure is validated by comparing the model’s predictions with reported experimental results.

Fatigue crack growth threshold conditions at notches. Part I: theory

A micromechanical description of the fatigue crack growth process at notches is presented. Crack interaction with the plastic slip barriers of the material (e.g. grain boundaries) and the influence of the notch stress gradient are intrinsically taken into account in the model. Both the notch fatigue crack initiation limit and the limit for propagation up to failure (i.e. the conventional notch fatigue limit) are clearly identified and calculated. The formation of non‐propagating cracks is also explained.

833 熱応力下疲労き裂停留現象に関する実験的研究

Near threshold fatigue crack growth (fcg) rate was measured under a stress intensity range (△K)-normalized crack length (a/W) relationship, equivalent to that for a circumferential crack in a thick-walled finite-length cylinder (which is suddenly cooled inside). The tests were conducted fundamentally in accordance with ASTM E-647. The objective of the experiment was to verify the validity of the thermo-elastic analysis for the crack by the authors. Tentative results showed that the threshold intensity range for the thermal-stress equivalent △K-a/W relationship was close to that obtained from that by standard ASTM K-decreasing near threshold fcg test.

Mixed-mode fatigue-crack growth thresholds in Ti-6Al-4V at high frequency

Multiaxial loading conditions exist at fatigue-critical locations within turbine engine components, particularly in association with fretting fatigue in the blade dovetail/disk contact section. For fatigue-crack growth in such situations, the resultant crack-driving force is a combination of the influence of a mode I (tensile opening) stress-intensity range, {Delta}K{sub I}, as well as mode II (in-plane shear) and/or mode III (anti-plane shear) stress-intensity ranges, {Delta}K{sub II} and {Delta}K{sub III}, respectively. For the case of the high-cycle fatigue of turbine-engine alloys, it is critical to quantify such behavior, as the extremely high cyclic loading frequencies ({approximately}1--2 kHz) and correspondingly short times to failure may necessitate a design approached based on the fatigue-crack growth threshold. Moreover, knowledge of such thresholds is required for accurate prediction of fretting fatigue failures. Accordingly, this paper presents the mixed-mode fatigue crack growth thresholds for mode I + II loading (phase angles from 0{degree} to 82{degree}) in a Ti-6Al-4V blade alloy. These results indicate that when fatigue-crack growth in this alloy is characterized in terms of the crack-driving force {Delta}G, which incorporates both the applied tensile and shear loading, the mode 1 fatigue-crack growth threshold is a lower bound (worst case) with respect to mixed-mode (I + II)more » crack-growth behavior.« less

Explanation of an apparent abnormality in fatigue crack growth rate curves in titanium alloys

A surprising phenomenon is investigated where titanium alloys exhibit no threshold fatigue crack growth value if K max in the K max-constant testing procedure exceeds a certain value. The crack growth rate increases with decreasing Δ K up to final fracture. The phenomenon was found repeatedly for Ti–6Al–2Sn–4Zr–6Mo above K max=21 MPa√m (equal to 72% of K IC), and its causes were investigated. The same crack growth rates as in the K max-constant test were reproduced by two independent experimental procedures, the so-called “jump” test and sustained K cracking experiments along with a calculation. It is demonstrated that the observed phenomenon is not a special crack growth feature or a new phenomenon, but simply caused by time-dependent crack growth, which is known to exist in titanium alloys or steels. Fractographic work revealed that intergranular crack growth along α and transformed β grain boundaries increases with decreasing Δ K and increasing K max value, accompanied by creep deformation in the transformed β grains. The conditions for time-dependent cracking are believed to be a sufficiently high stress and strain field in the crack tip region, along with hydrogen-assisted cracking.

Hydrogen-assisted fatigue crack growth in beta-annealed Ti-6Al-4V

In the present study, the effect of internal hydrogen on beta-annealed Ti-6Al-4V subjected to a very small amplitude fatigue loading (a cyclic amplitude equal to 5% of the maximum load or a stress ratio, R, of 0.90) was determined. The fatigue crack growth rates and fatigue crack growth threshold stress intensities were determined and compared for beta-annealed Ti-6Al-4V with hydrogen contents of 40 ppm, 370 ppm, and 1000 ppm. The observed secondary cracking and unique fatigue crack growth kinetics in 1000 ppm hydrogen-charged Ti-6Al-4V are discussed in relation to stress-assisted hydride information in the crack tip region.

Fatigue crack growth thresholds—the influence of Young's modulus and fracture surface roughness

In the present investigation it is shown that the effective fatigue threshold is uniquely correlated to the Young's modulus for a wide range of metallic and composite materials (Δ K th,eff=1.64·10 −2· E). It is also demonstrated that the crack closure level K cl increases with increased roughness of the fracture surface H̄. K cl and H̄ are quantitatively related via the equation K cl =1.2· H ̄ 1/3 for steels with widely different mechanical properties and grain sizes (120 MPa< R p <1100 MPa, 1 μm< λ<100 μm). This relation can be extended to materials other than steels (e.g. aluminium and WC-Co alloys) by normalising against Young's modulus. The roughness value H̄ represents the standard deviation of height of the fracture surface and is shown to be simply related to the length and angle distributions of the linear length elements constituting the fracture profile.

Load history dependence of fatigue crack growth thresholds for a Ti-alloy

Fatigue crack growth thresholds for Ti-alloy IMI 834 were determined at K max values equal to 6, 10 and 25 MPa m , using two different load patterns, both including an abrupt change in Δ K. Increasing Δ K applied to the initially dormant crack resulted in higher fatigue crack growth thresholds as compared to the situation when an initially growing crack was arrested by decreasing Δ K values. The discrepancy can be attributed to the crack tip shielding associated with residual stresses in front of the crack rather than with fracture surface contact behind the crack tip. Practical merits of the two experimental techniques are discussed.

CONSIDERING ENVIRONMENTAL CONDITIONS IN THE DESIGN OF BONDED STRUCTURES: A FRACTURE MECHANICS APPROACH

The continued airworthiness of ageing aircraft and long‐term durability of new airframes depends, in part, on the integrity of adhesive bonds used for repairs and joining structural components. Additionally, the advent of composite materials and advanced repair techniques incorporating composites has increased the number of adhesively bonded joints specified for use in aerospace structures. Traditionally, adhesive bonds have been analysed and designed using a dependable and rigorous stress‐based approach. However, the need to address the effect of bondline flaws and to understand the fatigue characteristics of bonded joints has led to the adoption of a discipline already common in the design of metallic components—fracture mechanics. To understand the durability of bonded structures, however, it is further necessary to examine the effect of environmental exposure on the performance of the adhesive bondline. Familiarity with the stress‐based and fracture mechanics analytical approaches, as well as an understanding of environmentally induced trends in bond performance is paramount to quality design. This paper will briefly discuss the attributes of the two main forms of bonded joint analysis, and will broadly outline a design approach that uses fracture mechanics and accounts for environmental effects. Experiments discussed in this paper were performed specifically to use fracture mechanics in assessing the environmental effects on a toughened epoxy adhesive. Results indicate that the Mode I fracture toughness and fatigue crack growth threshold of this adhesive are significantly reduced upon exposure to a high temperature, high humidity aircraft service environment. These results will be used to illustrate the philosophical arguments supporting the design approach.

Cyclic fatigue crack growth behaviour in β-(Si-Al-O-N) at ambient and elevated temperatures

Four-point bending fatigue tests on a hot-pressed sintered Sm–β-(Si–Al–O–N) ceramic were conducted at room temperature, 900 °C and 1000 °C in air under different load ratios and cyclic frequencies. The growth of indentation cracks was measured during the fatigue tests. The results indicate that the cyclic fatigue crack growth threshold is lower and crack growth rates are higher, for given values of Kmax, at 1000 °C than those at room temperature. The cyclic fatigue crack growth behaviour at 900 °C is similar to that at room temperature. It was found that the crack growth retardation due to cyclic fatigue loading is much more pronounced at higher frequencies. An increase in cyclic frequency from 1 to 10 Hz cause a reduction of up to two orders of magnitude in crack propagation rates. High-temperature cyclic fatigue crack growth rates increased and threshold stress intensity factor ranges decreased with increasing load ratio. Possible mechanisms for cyclic crack growth are discussed.

Behavior of Fatigue Crack Growth Threshold of SUS304 Steel in Low .DELTA.K Region at an Elevated Temperature by the Increase of Kmax.

Fatigue crack growth tests using center-cracked [M (T)] specimens and single-edge-cracked (SEC) specimens of SUS304 steel were conducted to examine the behavior of fatigue crack growth in the near-threshold regime at 623 K. Crack closure was eliminated by keeping Kmax constant in ΔK decreasing test in order to obtain easily the threshold value expressed in terms of effective stress intensity factor range, ΔKett. By increasing the value of Kmax and/or ΔK for a crack in a threshold condition, the fatigue crack growth behavior was investigated even in the region below the threshold value obtained by using the conventional ΔK-decreasing test. When the value of Kmax was increased in the conventional threshold condition, crack growth was observed in the lower ΔK region. Crack growth was further observed repeatedly after successive increase in Kmax in the newly established lower region below the conventional threshold value. In the region below a certain value of ΔK, however, crack no longer grew even after the change of Kmax. Discussion was made on the characteristics of threshold.

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C

Fatigue and creep fatigue crack growth behaviour of alloy 800 at 550°C have been studied to analyse defect assessment in a steam generator. Different grades of alloy 800 have been investigated to reproduce the in service conditions. Fatigue crack growth (FCG) tests were conducted on CT20 and tubular specimens, then on welded tubes. Furthermore the influence of hold times on fatigue crack growth behaviour was studied.The results obtained on material simulating the weld heat affected zone are in agreement with the tests conducted on welded tubes. Fatigue crack growth characteristics of aged and cold-worked aged material seem to be slightly improved in comparison with base material. Finally a hold time of one minute increases strongly the FCG threshold value determined in pure fatigue but has a negligible influence on crack growth rates.

Influence of Density and Sintering Temperature on Fatigue Crack Growth and Fracture Toughness of FL4405 High Strength PM Steel

Density and sintering temperature effects were investigated with FL4405 low alloy PM high strength steel. Room temperature, plane strain fracture toughness KIc, and constant amplitude fatigue crack growth with (min/max) load ratios of 0·05 and 0·5 and macroand microfractography were evaluated. Two density levels, 7·0 and 7·4 g cm−3, and two sintering temperatures, 1120 and 1315°C, were investigated. Increasing the density and sintering temperature had no influence on stress intensity threshold and near threshold fatigue crack growth resistance where transcrystalline/transparticle cleavage existed. At higher fatigue crack growth rates, where ductile dimples existed, both higher density and higher sintering temperature enhanced fatigue crack growth resistance. The KIc and Kmax values at fatigue crack growth fracture increased at the higher density and increased further at the higher sintering temperature. All final fracture regions were brittle at the macroscale while containing ductile dimples at the micro scale. No crack closure was found in fatigue crack growth tests and no load ratio effects were present at low fatigue crack growth rates. Significant load ratio effects were present at high fatigue crack growth rates. PM/0770

Fatigue crack growth resistance in SiC particulate and whisker reinforced P/M 2124 aluminum matrix composites

Fatigue crack growth is examined in P/M 2124 aluminum alloys reinforced with SiC particles (SiC p) and whiskers (SiC w) over a wide spectrum of growth rates from 10 −12 to 10 −4 m per cycle. Effects of aging treatment, orientation of crack growth direction with respect to the rolling direction, mean stress (or stress ratio), and reinforcement volume percent on the fatigue crack growth threshold are investigated in terms of crack tip shielding mechanisms. Comparison to fatigue crack growth in the unreinforced alloy indicates that crack growth resistance in the composites is superior to the monolithic alloy at low stress intensity ranges, Δ K. Specifically, at the lower growth rates the superior crack growth resistance of the composites is due to the formation of tortuous crack paths and a consequently enhanced roughness induce crack closure. Fatigue response near threshold is found to be relatively insensitive to changes in aging treatment with observed variations reflecting the associated changes in plastic behavior. In whisker reinforced composites, the orientation of the crack with respect to the rolling direction had a significant effect on fatigue crack growth rate while in the particulate reinforced composites orientational effects were not significant. Increasing the volume fraction resulted in higher crack growth resistance at low growth rates in the particulate reinforced materials. At high stress ratios, resistance in both whisker and particulate reinforced composites was lowered, and the measured fatigue crack propagation threshold, Δ K th, was found to be independent of the reinforcement morphology and volume percent.

Fatigue crack growth threshold: implications, determination and data evaluation

In standard test procedures the determination of fatigue crack growth threshold values is part of da/dN–ΔK-curve determinations, and with one specimen only one threshold value can be determined. Thus, the complete characterization of the threshold behaviour of a certain material, for example, by ΔKth–R-curves, is extremely time consuming, specimen consuming and expensive. Based on experimental experience, a procedure is described which allows a complete ΔKth–R-curve determination with a single specimen. The threshold behaviour of a material can be described by four constants, which can easily be determined. A very simple analytical description of the threshold behaviour of materials is given, by which even quite complicated appearing ΔKth-R-curves can be represented.

Application of positron annihilation lineshape analysis to fatigue damage and thermal embrittlement for nuclear plant materials: Uchida, M., Ohta, Y., Nakamura, N. and Yoshida, K. 13th International Conference on NDE in the Nuclear and Pressure Vessel industries, Kyoto, Japan (22–25 May 1995) 349–353

Fatigue crack growth (FCG) threshold has been determined on Ti6A14V, Al 7475-T7351, quenched and tempered steel 24CrMo4 as well as on the Ni-base alloys Inconel 617 and Nicrofer 5219 Nb. The main objective of the studies was to prove the transferability of the experimentally determined FCG threshold to service type fatigue loading. For FCG threshold testing, the following requirements must be fulfilled: 1.(A) a FCG threshold has to be entirely above Kop;2.(B) a FCG threshold has to exist and has to be measured over the fatigue loading range Kmax, i.e. K∗max< Kmax<KIc;3.(C) a FCG threshold has to be the upper bound amplitude of cycles at a given Kmax, the action of which does not produce FCG no matter how these threshold amplitudes are applied:4.(D) a FCG threshold must be a material parameter useful for material characterization and FCG design methodology under all fatigue loading conditions.It was shown that all the experimental procedures, i.e. type I, III and IV tests, furnished ΔK T values for Al 7475-T7351, Ti6A14V and the quenched and tempered steel 42CrMo4. The results suggest that this might be true for all Al alloys, Ti alloys and steels. Therefore, any one test method can be used to determine ΔKT for these materials. The ASTM E 647 recommended procedure for threshold testing furnishes ΔKT values for these materials as well, if the R -ratio is equal to or higher than Rc.For the Ni-base alloys (Inconel 617 and Nicrofer 5219 Nb) only type III and IV tests can be used to determine a FCG threshold ΔKT. This is due to the different behaviour of Ni-base alloys with respect to Kop.

Fatigue of in situ Reinforced Ti–8.5Al–1B–1Si

The effect of temperature on the fatigue and fracture properties of an in situ reinforced super α alloy Ti–8.5Al–1B–1Si (wt. %) was investigated. At room temperature the as-extruded composite has a strength of 631 MPa with limited ductility. On increasing the temperature to 700 °C only a marginal drop in strength to 610 Mpa was observed along with a significant improvement in ductility to 5.9%. Low-cycle fatigue results indicate a marginal decrease in fatigue life as temperature is increased from room temperature to 700 °C. Fatigue crack growth studies in the as-extruded microstructure indicate a strong influence of R-ratio on both the threshold for fatigue crack growth and crack growth rates in the Paris regimes. At elevated temperatures, the resistance to fatigue crack growth increases with temperature below approximately 500 °C. At 600 °C, however, there is an increase in the near threshold crack growth rate due to embrittlement effects. At higher δK values , the resistance to fatigue crack growth at elevated temperatures is always better than that at room temperature. This improvement is attributed to the increase in the inherent resistance

Crack Closure Effects on Fatigue Crack Growth Thresholds and Remaining Life in an HSLA Steel

The effects of crack closure on the near-threshold corrosion fatigue crack growth behavior of Mil S-24645 HSLA steel and its weld metal have been investigated in air, ASTM seawater at the free corrosion potential, and ASTM seawater at −0.8V and −1.0V (SCE) using frequencies of 10, 2, and 0.2 Hz, and a stress ratio, R = 0.1. Remaining life, in the presence and absence of crack closure, has been estimatedas a function of applied stress range for a structure containing a 3-mm-deep surface semi-elliptical flaw.

Fatigue crack growth threshold and short crack growth in austenitic materials: Huthmann, H., Livesey, VB. and Robert, G. Int. J. Pressure Vessels Piping (1996) 65, 231–239

Fatigue crack growth threshold and short crack growth in austenitic materials: Huthmann, H., Livesey, VB. and Robert, G. Int. J. Pressure Vessels Piping (1996) 65, 231–239