Non-propagating Fatigue Crack Research Articles

Conditions for the Transition from Nonlocalized to Localized Damage in Metals and Alloys. Part 1. Crack Sizes at Fatigue Limit

The paper addresses special features of nonlocalized and localized fatigue damage in metals and alloys. The sizes of maximum fatigue cracks at a fatigue limit in metals and alloys of various grain sizes and strengths are analyzed. The authors discuss the linear and nonlinear fracture mechanics criteria that relate the fatigue crack sizes and the fatigue limits. The experimental values of the fatigue crack size are compared to those calculated by different criteria. Special features of initiation of nonpropagating fatigue cracks in the presence of stress concentrators are studied.

Analysis on the Behavior of Nonpropagating Fatigue Cracks under Steep Stress Gradients

A mechanism for the formation of nonpropagating fatigue cracks ahead of a notch root is presented. The stress gradients near the elongated notch root along with the propagation of short crack and the resulting nonpropagating crack lengthsanpcare estimated. The local stress which is higher than the unnotched material fatigue limitS0initiates the crack from a notch root and local steep stress gradient as a very important element leads to the nonpropagating crack. The value ofanpcdepends on the material properties, and specimen geometry as well as applied loading. The characteristic size of the short cracka0which depends on the material properties associates with the fatigue stress concentration factorKf. The estimated values ofanpcare in fairly good agreement with the experimental values available.

Mechanical Strength Properties of Wind-Lens Blades

For the safety and reliability of small size Wind-lens turbine, several experiments of mechanical properties of blade's material, Dicyclopentadiene was conducted. Vickers hardness was Hv =13.8 independent of load times and load magnitudes. Fatigue strength of the specimen contained some scale voids were quite low compared to the plain specimen contained no voids. Fatigue strength of notched specimens was lower than that of plain specimen. Non-propagating fatigue crack was not observed on this material. Fatigue crack growth property was obtained by three point bending fatigue test. The relationship between fatigue crack growth rate and the stress intensity factor range, ΔK, was obtained. Charpy impact absorption energy of Dicyclopentadiene was three times as that of ABS resin.

Small Fatigue Crack Growth Behavior from Artificial Notch with Focused Ion Beam in Annealed 0.45% Carbon Steel

The aim of this study is firstly to investigate the applicability of a sharp notch with Focused Ion Beam (FIB) as a crack for fatigue limit evaluation. Secondly we investigate a condition in which artificial defects (drilled hole, FIB notch) can be used as a crack for fatigue limit evaluation. To achieve the aim, the growth behaviors and the non-propagating crack sizes of small fatigue cracks initiated from a FIB notch and a drilled hole are carefully compared with those of an annealed fatigue crack which imitates an ideally sharp crack. The results show that a FIB notch can be used as a crack for fatigue limit evaluation under some conditions. The results also show that the condition which controls the applicability of an artificial defect as an ideal crack for fatigue limit evaluation is strongly dependent on the relation between (i) the length of a non-propagating fatigue crack and (ii) the crack length when the small fatigue crack growth behaviors from an artificial defect and an ideal pre-crack become almost the same. It is found that the length of (ii) can be obtained by the analyses using the number of cycles from a certain crack length to failure.

OS0501 FIB加工による人工欠陥を有するS45C焼鈍し材の微小疲労き裂進展挙動(OS5-1き裂進展,OS-5 材料の疲労挙動と損傷評価1)

A defect with Focused Ion Beam (FIB) can be used as a crack for fatigue limit evaluation under some conditions. The condition which controls the applicability of an artificial defect as an ideal crack for fatigue limit evaluation is strongly dependent on the relation between (i) the length of a non-propagating fatigue crack and (ii) the crack length when the small fatigue crack growth behaviors from an artificial defect and an ideal pre-crack become almost the same. It is found that the length of (ii) can be obtained by the analyses using the number of cycles from a certain crack length to failure.

Fatigue damage of low amplitude cycles in low carbon steel

The influences of low load cycles on fatigue damage in 0.15% C steel (C15E, No. 1.1141) are investigated in the very high cycle fatigue regime using ultrasonic fatigue testing equipment. Constant amplitude (CA) endurance limits at limiting lifetime of 109 cycles are determined in cyclic tension–compression and cyclic torsion tests. Non-propagating fatigue cracks are found in specimens subjected to cyclic torsion loading at the endurance limit. The endurance limit is considered as maximum stress amplitude where possibly initiated fatigue cracks do not propagate to failure. Two-step variable amplitude (VA) tension–compression endurance tests are performed with repeat sequences consisting of high stress amplitudes above the endurance limit and far greater number of cycles below. The measured lifetimes are compared with linear damage accumulation calculations (Miner calculations). If the high stress amplitude is more than approximately 13% above the CA endurance limit, detrimental influences of low load cycles and failures at low damage sums are found. If the high stress is less than 13% above the CA endurance limit, numerous low load cycles cause prolonged fatigue lifetimes and specimens can sustain large damage sums without failure. Two-step VA fatigue crack growth investigations show that load cycles below the threshold stress intensity accelerate crack growth, if the high stress intensity is 18% or more above the CA threshold stress intensity. In repeat sequences with high stress intensities 14% above threshold stress intensity, low load cycles decelerated and stopped fatigue crack growth. Low load cycles can reduce or prolong fatigue lifetimes of low carbon steel and one reason is the accelerated or retarded fatigue crack growth due to numerous low amplitudes, and the maximum load amplitude of a VA load sequence determines whether detrimental or beneficial effects prevail.

The fatigue behavior of I-phase containing as-cast Mg–Zn–Y–Zr alloy

The fatigue behavior of as-cast Mg–12%Zn–1.2%Y–0.4%Zr alloy has been investigated. The S– N curve showed that the fatigue strength at 10 7 cycles was 45 MPa. Scanning electron microscopy observations on the surfaces of the failed and unfailed specimens (after up to 1 × 10 7 cycles) suggested that the slip bands could act as preferential sites for non-propagating fatigue crack initiation, and the I-phase could effectively retard fatigue crack propagation (FCP). The macro fracture morphology clearly indicated that the overall fracture surface was composed of three regions, i.e. a fatigue crack initiation region (Region 1), a steady crack propagation region (Region 2) and a tearing region (Region 3). High-magnification fractographs showed that only porosities can act as the crack initiation sites for all specimens. Moreover, for specimens with fatigue lifetimes lower than 2 × 10 5 cycles, the cracks mostly initiated at the subsurface or surface of the specimen. However, when the fatigue lifetime was equal to or higher than 2 × 10 5 cycles, the fatigue crack initiation sites transferred to the interior of the specimen. The maximum stress intensity factors corresponding to the transition sites between Regions 1, 2 and 3 were 2 and 4.2 MPa m 1/2, respectively. When the maximum stress intensity factor K max was lower than 4.2 MPa m 1/2, in the steady crack propagation region, due to the retarding effect of I-phase/α-Mg matrix interfaces, the fatigue cracks tended to pass the I-phase/α-Mg matrix eutectic pockets directly and propagated through the grain cells, resulting in the formation of many flat facets on the fracture surface. However, when the maximum stress intensity factor was higher than 4.2 MPa m 1/2, in the sudden failure region, the rigid bonding of I-phase/α-Mg matrix interfaces was destroyed and the cracks preferentially propagated along the interfaces, which resulted in the fracture surface being almost completely composed of cracked I-phase/α-Mg matrix eutectic pockets. Based on microstructural observation and the fracture characteristics of the two regions, it is suggested that with an increase in crack tip driving force, the FCP mode changes from transgranular propagation to intergranular propagation.

Short crack threshold estimates to predict notch sensitivity factors in fatigue

The notch sensitivity factor q can be associated with the presence of non-propagating fatigue cracks at the notch root. Such cracks are present when the nominal stress range Δ σ n is between Δ σ 0/ K t and Δ σ 0/ K f, where Δ σ 0 is the fatigue limit, K t is the geometric and K f is the fatigue stress concentration factors of the notch. Therefore, in principle it is possible to obtain expressions for q if the propagation behavior of small cracks emanating from notches is known. Several expressions have been proposed to model the dependency between the threshold value Δ K th of the stress intensity range and the crack size a for very small cracks. Most of these expressions are based on length parameters, estimated from Δ K th and Δ σ 0, resulting in a modified stress intensity range able to reproduce most of the behavior shown in the Kitagawa–Takahashi plot. Peterson or Topper-like expressions are then calibrated to q based on these crack propagation estimates. However, such q calibration is found to be extremely sensitive to the choice of Δ K th( a) estimate. In this work, a generalization version of El Haddad–Topper–Smith’s equation is used to evaluate the behavior of cracks emanating from circular holes and semi-elliptical notches. For several combinations of notch dimensions, the smallest stress range necessary to both initiate and propagate a crack is calculated, resulting in expressions for K f and therefore for q. It is found that the q estimates obtained from this generalization, besides providing a sound physical basis for the notch sensitivity concept, better correlate with experimental data from the literature.

Endurance limit and threshold stress intensity of die cast magnesium and aluminium alloys at elevated temperatures

High cycle fatigue properties of the high-pressure die-cast magnesium alloys AZ91 hp, AS21 hp and AE42 hp and of the aluminium alloy AlSi9Cu3 are investigated at elevated temperatures. Fatigue tests are performed at ultrasonic cyclic frequency and load ratio R=−1. Compared with ambient air environment, the S–N curves determined in warm air of 125 °C (magnesium alloys) and 150 °C (aluminium alloy) are shifted towards lower cyclic stresses. The mean endurance limits at 10 9 cycles of 41 MPa (AZ91 hp), 27 MPa (AS21 hp), 35 MPa (AE42 hp) and 61 MPa (AlSi9Cu3) are 70–90% of the respective stresses found in ambient air, and fracture mechanics tests delivered threshold stress intensities at elevated temperature of about 80% of the respective values at 20 °C. Fatigue cracks initiate, nearly, exclusively at casting porosity and fracture surfaces appear similar at low and elevated temperatures. Fatigue cracks may form at voids at stresses below the endurance limit, and non-propagating fatigue cracks of considerable length (greater 1 mm) are found in runnout specimens. Considering porosity as an initial crack, critical stress intensity, K crit is evaluated, which quantifies the sensitivity of a material to defects. Materials can sustain cyclic stresses without fatigue failure, if the cyclic stress intensity determined for their most damaging porosity is lower than K crit.

On application of the french lines for the prediction of the number of cycles to failure

The application of the French lines to the prediction of high-cycle fatigue life according to the time of initiation of a macrocrack is justified. The French line is regarded as a line separating the regions of propagating and nonpropagating fatigue cracks. A method for the evaluation of the French line parameters in the region of low-cycle loading via the parameters of the Coffin-Manson diagram is proposed.

The mechanisms of energy dissipation in the non-propagating fatigue cracks in metallic materials

In a result of experimental investigations of titanium alloy VT-18U and alloy steel 15H2N specimens with an edge fatigue crack of mode I at bending vibrations it was revealed that the energy dissipation in a non-propagating crack is caused mainly by the elasto-plastic area around the crack tip and practically is not related to the friction between the crack surfaces. The level of energy dissipation in a crack is determined uniquely as a function of the cyclic (nominal) stress intensity factor range and is not effected by the initial level of damping of an intact specimen. This function can be used for the prediction of damping change caused by the crack initiation and growth and for the diagnostics of damage based on the prediction of damping change.

Application of Roller-Working on Improving the Fatigue Strength of Notched Austenitic Stainless Steel

About 90% of steel structure™s failure was induced from the notched parts due to the stress concentration. Therefore it was very important to research how to improve the fatigue property of notched parts when the stainless steel was used as structural material. Rotating bending fatigue test was preformed to investigate the effect of different notch-machining process (Conventional lathe-machining, Combination of lathe-machining and roller-working) on fatigue strength of the typical austenitic stainless steel SUS304. Fatigue limit of roller-worked specimen was improved to 285% compared to that of lathe-machined specimen. It was found that, work hardening effect induced in roller-working process played an important role to improve the fatigue strength. In addition, the residual compressive stress and the fibriform structure in the specimen surface-layer induced by roller-working were also effective to improve the fatigue strength. Non-propagating fatigue crack was observed in the notch surface of the roller-worked SUS304 for the first time.

The mechanisms of energy dissipation in the non-propagating fatigue cracks in metallic materials

In a result of experimental investigations of titanium alloy VT-18U and alloy steel 15H2N specimens with an edge fatigue crack of mode I at bending vibrations it was revealed that the energy dissipation in a non-propagating crack is caused mainly by the elasto-plastic area around the crack tip and practically is not related to the friction between the crack surfaces. The level of energy dissipation in a crack is determined uniquely as a function of the cyclic (nominal) stress intensity factor range and is not effected by the initial level of damping of an intact specimen. This function can be used for the prediction of damping change caused by the crack initiation and growth and for the diagnostics of damage based on the prediction of damping change.

Non-propagating fatigue crack behaviour in Al 2024-T351 : Plumtree, A. and Varvani-Farahani, A. Mater. Sci. Forum (1996) 217–222 (3), 1377–1382 Conference: Aluminium Alloys—Their Physical and Mechanical Properties, ICAA5, Part 3, Grenoble, France (1–5 July 1996)

Non-propagating fatigue crack behaviour in Al 2024-T351 : Plumtree, A. and Varvani-Farahani, A. Mater. Sci. Forum (1996) 217–222 (3), 1377–1382 Conference: Aluminium Alloys—Their Physical and Mechanical Properties, ICAA5, Part 3, Grenoble, France (1–5 July 1996)

Contribution of the cyclic loading portion below the opening load to fatigue crack growth

A simple test procedure involving stress ratio changes at the fatigue threshold is proposed to reveal the role of the lower portion of the loading cycle below K op in the fatigue crack growth behaviour. It is observed for both Al 2024-T3 and Al 7475-T761 alloys that the initially non-propagating fatigue crack at the fatigue threshold resumes growth upon diminishing K min while keeping K th,max constant. These experimental findings can be interpreted by means of a modified crack closure concept in which the contribution of the lower portion of the loading cycle below K op (including also a part of compressive loading if R < 0) to the variation in the stress state experienced by the fatigue crack tip is taken into account.

The influence of strenght and stress ratio on short-crack thresholds and non-propagating fatigue cracks: Lal, D.N. Fatigue Fract. Eng. Mater. Struct. (Apr. 1993) 16 (4), 419–428

The influence of strenght and stress ratio on short-crack thresholds and non-propagating fatigue cracks: Lal, D.N. Fatigue Fract. Eng. Mater. Struct. (Apr. 1993) 16 (4), 419–428

Relation between fatigue threshold and fatigue limit for surface-rolled specimens

After cylinder notch fatigue specimens of 40 CrNiMo steel were rolled, their fatigue limit increased by 41%. The rolled specimens did not fracture, even though they had been loaded for 107 cycles under fatigue limit stress, but a non-propagating fatigue crack was generated. Thus the value of the fatigue limit depends on the fatigue threshold value ΔKth of the metal of the rolled layer. Plastic deformation increased ΔKth in these experiments. It can be inferred that ΔKth of the rolled layer increases from the occurrence of plastic deformation and microvoids on the layer. Calculation of the effect of residual stress in the crack wake on the stress intensity factor ΔK indicates that residual compression stress decreases ΔK by 21.5 MPa √M. It was calculated that rolling induced both the length of the non-propagating crack and the increase of fatigue limit. The calculated values are in accord with experiment. Analysis and calculations indicate that the non-propagating crack is generated on the rolled layer. Thus the fatigue limit is improved because rolling produces residual compression stress in the layer (which decreases the stress intensity factor), and increases ΔKth of the layer.

Estimating the viability of a tension bolt in a gas turbine

An estimate has been made of the crack growth period in an M200 x 4 mm coupling bolt made of 26KhN3M2FA steel in a gas turbine operating at 293–523 K under fewcycle loading arising from thermal expansion of the disks together with the dimensions of nonpropagating fatigue cracks under many-cycle loading due to vibration. The critical ring crack depth in a 26KhN3M2FA steel bolt is 26 mm at a stress of 320 MPa. The crack develops to a critical size from a depth of 10 mm during 30,000 start-stop cycles.

Correlation of residual stresses and fatigue limit with torsion under conditions of stress concentration Report 1. Substantiation of strength and longevity criteria

Results are given for a calculation and experimental study of the effect of residual stresses on fatigue limit with torsion for specimens with circular notches of semicircular and rectangular shape made of steels 40, 30KhGSA, EI961, and aluminum alloy V95. The possibility is demonstrated of a criterion for mean integral residual stresses through the thickness of a surface layer equal in depth to a nonpropagating fatigue crack in order to estimate the influence of residual compressive stresses on fatigue limit in the case of torsion with stress concentration.

Defect-tolerant fatigue designs for automative components fabricated from high strength low alloy structural steel: Dhar, S. Eng. Fract. Mech. 1989 32, (6), 1009–1016

A simple test procedure involving stress ratio changes at the fatigue threshold is proposed to reveal the role of the lower portion of the loading cycle below Kop in the fatigue crack growth behaviour. It is observed for both Al 2024-T3 and Al 7475-T761 alloys that the initially non-propagating fatigue crack at the fatigue threshold resumes growth upon diminishing Kmin while keeping Kth,max constant. These experimental findings can be interpreted by means of a modified crack closure concept in which the contribution of the lower portion of the loading cycle below Kop (including also a part of compressive loading if R < 0) to the variation in the stress state experienced by the fatigue crack tip is taken into account.