Fatigue Crack Growth In Specimens Research Articles

Paradox of Shorter Residual Fatigue Life due to Omission of Low‐Amplitude Cycles and Its Significance for Testing

ABSTRACTThe work investigates nonvalidity of the common presumption that the nondamaging cycles do not influence residual fatigue life. Paradoxically, application of the full loading spectrum (more cycles) resulted in approximately 2.3 times longer life of the fatigue crack growth specimens than application of the spectrum with 33% of the smallest amplitudes omitted. Unlike in humid air (controlled relative humidity of 50% at 23°C), the effect disappeared in a dry‐air chamber (relative humidity <10% at 23°C), where both fatigue lives were short. The mechanism responsible for these effects was identified as the oxide‐induced crack closure, an extrinsic mechanism unrelated to material damage. Oxide debris developed on fracture surfaces was observed by light and scanning electron microscopy, whereas crack closure was measured during the experiments. The presented counterintuitive behavior in humid air may result in wrong assessment or prediction of components residual fatigue lives, which can be nonconservative in some scenarios.

Numerical Analysis of Single Edge Notched Tension Specimen With Fatigue Crack Parameter of Conventional Specimen Using Linear Elastic Fracture Mechanics

This paper describes the numerical analysis of planar crack growth in high strength steel API 5L X70 whose crack growth parameter is adopted from experimental compact tension (CT) specimen in previous literature. Apart from the fact that conventional fatigue crack growth specimen has bogus geometry constraints, the Single Edge Notched Tension (SENT) better replicate the crack-tip constraint conditions experienced in structures. Linear elastic fracture mechanics (LEFM) crack orientation is modelled with the finite element method in SENT model considering its geometry functions a/W ratio to determine its crack growth rate based on constant amplitude load. The virtual crack closure technique tool in MSC Marc/Mentat software with adaptive and global remeshing is applied to assess high cycle fatigue crack propagation using the SENT model. The crack growth pattern for the 3-dimensional simulation characteristics is similar with that of the CT Specimen experimental procedure. Furthermore, the results of the crack propagation and the cycle count demonstrated good agreement with bearable discrepancy with maximum percentage difference of about 14.1 % for the HAZ and 6.4% for the weld and parent metal compared to the experimental resultsfrom literature.

Fatigue Crack Growth Behavior of TA29 Titanium Alloy at Different Temperatures

TA29 titanium alloy forging was heated by duplex annealing. The fatigue crack growth behaviour of the alloy at room temperature, 400 °C, 500 °C and 600 °C was studied. The relationship between the fatigue crack growth rate (Da/dN) and the stress intensity factor amplitude (△K) of TA29 alloy at different temperatures was revealed by Paris formula. The fatigue fracture morphology of fatigue crack growth specimens was analysed by scanning electron microscopy (SEM). The results showed that the TA29 titanium exhibited good crack growth resistance at room temperature. With the increase of test temperature, the C value of the Paris formula increased, the m value decreased, and the fatigue crack growth rate increased. From the fatigue fracture morphology of the specimens, it was found that the fractures of fatigue crack growth specimens at different temperatures exhibited typical pre-splitting zone, steady-state expansion zone and rapid expansion zone. As the temperature increased, the range of the pre-cracking zone was larger, the range of the steady-state extension zone and the rapid expansion zone were smaller. At room temperature, there was no obvious fatigue strips in the steady-state expansion zone, and the transient fracture zone exhibited the characteristics of cleavage fracture, while the steady-state expansion zone at high temperature showed obvious fatigue strips and secondary cracks, and the transient fracture zone was a typical ductile fracture mechanism.

Effect of thickness on Fatigue Crack Growth of Aluminium alloys

The effect of thickness on fatigue crack growth has been investigated for aluminium alloys such as a high-strength aluminium alloy (A7075-T6), a medium strength alloy (A6063-T6), and A2024-T351. The fatigue tests were conducted using Instron 8801 fatigue testing machine on all those materials with thicknesses of 12.7, 15.8, and 20 mm. All specimens were undergoing finishing process on the surface. These tests were run under constant amplitude load for each different thickness and materials. Fatigue crack growth specimen made of A2024-T351 and A6063-T6 aluminum alloys with thickness of 12.7 mm were higher than specimen made of A7075-T6 aluminum alloy with thickness of 20 and 12.7 mm and A6063-T6 aluminum alloy with thickness of 15.88 mm. It is concluded that fatigue crack growth decreases significantly with increasing thickness, and the crack initiates very fast on surface material.

Influence of the Notch Radius on Fatigue Crack Propagation in Beam Specimens of 2017A-T4 Alloy

We present experimental results on the fatigue crack growth in specimens with rectangular cross sections made of 2017A-T4 aluminum alloy under bending. The specimens were weakened by sharp and blunt one-sided notches (for the notch root ρ = 0.2; 5; 10 and 22.5 mm). The tests were performed on a MZGS-100 fatigue test bench in the low- and high-cycle fatigue modes by imposing a constant value of the nominal load ratio R = – 1; 0 and a moment amplitude M a = 15.84 N·m. The results of fatigue tests were then analyzed in terms of the parameter ΔK including the influence of the notch.

Fatigue Crack Growth in Specimens with Rectangular Section under Torsion and Bending with Torsion

The paper presents the test results on the fatigue crack growth under torsion and proportional bending with torsion in the aluminium alloy EN AW-2017A-T4. The tests were performed at the fatigue test stand MZGS-100 at the constant amplitude of moment MB = MT = MBT = 15.84 N⋅m and stress ratio R = -1. Plane specimens with stress concentrators in form of the external one-sided blunt notch were tested. The results of experiments were described on graphs of the fatigue crack length „a” versus numbers of cycles N. A non-uniform fatigue cracks growth on both lateral surfaces of specimens was observed during experimental tests.

EFFECT OF THE POST WELD HEAT TREATMENTS ON THE FATIGUE CRACK GROWTH BEHAVIOR IN FRICTION STIR WELDING OF A HEATTREATABLE ALUMINUM ALLOY

The effect of the post weld heat treatments (PWHTs) on the fatigue crack growth (FCG) behavior in the welded zone of AA6063T5 fabricated by the friction stir process was investigated. The FCG specimens are machined in which the loading axis is put perpendicular to the welding line and the initial notches are introduced in the welded zone. The experimental results showed the FCG rates are sensitive to the PWHT solutions. The FCG resistance in the welded zone could be fully restored to that of base metal by using PWHT. While the PWHT solution solely restores the precipitates dissolved and/or coarsened during welding process has a minor effect on the FCG rates, the PWHT solution remarkably recrystallizes the grain microstructure has a significant effect here.

In-situ scanning electron microscopy studies of small fatigue crack growth in ultrasonic consolidation bonded aluminum 2024 laminated structure

This work used in-situ scanning electron microscopy method to investigate the small fatigue crack behavior of the aluminum alloy 2024 laminated structure produced by ultrasonic consolidation process. The influence of local microstructure on the crack propagation of the laminates after different heat treatments (T4 and T62) has been compared and discussed. It was found that both the precipitations and the interfaces of the layers could retard crack growth and alter the growth direction, and the effect of interfaces was much stronger. Due to the higher ratio of the toughness to bond strength, the barrier effect of the interfaces on impeding the fatigue crack growth in specimens after the T4 heat treatment was more evident.

Analysis of the Fatigue Crack Propagation Life-Span Based on Linear Elastic Fracture Mechanics

Abstract. The fatigue crack propagation life-span of the engineering structure is studied. Linear elastic fracture mechanics is applied to analyze the life-span of fatigue crack growth of specimen, which is under constant amplitude load. The software of Fatigue is used to calculate the life-span of a center crack plate steel specimen. The result show that the calculated values of the life-span are basically well with the experimental data.

Fatigue and fracture behaviour of friction stir welded aluminium–lithium 2195

Abstract Aluminium–lithium (Al–Li) alloys offer attractive properties for lightweight aerospace structures, due to their low density, high strength and fatigue crack growth resistance. Although there are many advantages with Al–Li alloys, limitations remain while using conventional joining techniques. Friction stir welding is a well-established solid-state joining process that is expected to reduce many of the concerns about Al–Li welding. The work presented in this paper involves the characterisation of the fatigue performance of the AA2195-T8X at room temperature. SN and crack growth tests of base material and friction stir welded 5 mm thick specimens were performed. During crack growth tests, three different R ratios (minimum remote stress/maximum remote stress), 0.1, 0.5 and 0.8, were used per each three different material conditions: base material, heat affected zone (HAZ), and weldment. M(T) specimens containing notches at the centre of the weld, at the HAZ and at the base material, were tested. The fatigue crack growth specimens were left with an un-cracked ligament for final evaluation of fracture toughness. Novel results are presented for fatigue crack growth and toughness on T–L orientation. The results for SN fatigue behaviour, fatigue crack growth and toughness of the studied alloy and its friction stir weldments present high values when compared with data found in the literature.

Numerical predictions and experimental measurements of residual stresses in fatigue crack growth specimens

Controlling macro residual stress fields in a material while preserving a desired microstructure is often a challenging proposition. Processing techniques which induce or reduce residual stresses often also alter microstructural characteristics of the material through thermo-mechanical processes. A novel mechanical technique able to generate controlled residual stresses was developed. The method is based on a pin compression approach, and was used to produce well-controlled magnitudes and distributions of residual stresses in rectangular coupons and compact tension specimens typically used in fatigue crack growth testing. Residual stresses created through this method were first computationally modeled with finite element analysis, and then experimentally reproduced with various levels of pin compression. The magnitudes and distributions of residual stresses in experimental specimens were independently assessed with fracture mechanics methods and good correspondence was found between residual stresses produced using the pin compression and processing techniques. Fatigue crack growth data generated from specimens with low residual stresses, high residual stresses resulting from processing, and high residual stresses introduced through the new pin compression technique were compared and validated. The developed method is proposed to facilitate the acquisition and analysis of fatigue crack growth data generated in residual stresses, validate residual stress corrective models, and verify fatigue crack growth simulations and life predictions in the presence of residual stresses.

Exploiting bulk residual stresses to improve fatigue crack growth performance of structures

Bulk residual stress, normally viewed as a nuisance, requires additional safety margin and reduces structural performance. However, by carefully considering residual stress during the design and manufacturing processes, it should be possible to remove the uncertainty associated with residual stress and optimize the residual stress in a part for maximum performance. This analysis, design, and test program clearly demonstrates the potential for such improvement and a method to achieve it. An aluminum–lithium extrusion was selected, and its residual stress and crack growth properties were measured. A design study for center-cracked fatigue crack growth specimens was conducted with the specimen excise location within the extrusion cross-section as the design variable. The stress intensity factors due to residual stress were predicted with finite element analysis and incorporated into crack growth predictions by superposition. For validation, several specimens were produced and tested. Test results confirmed the analytical predictions, and the longest crack growth life was 3.7 times the shortest.

Growth of naturally initiated fatigue cracks in ferritic gas turbine rotor steels

Smooth test specimens of 12% CrNiMoV and 1% CrMoV steel have been subjected to strain-controlled low-cycle fatigue loading. Several specimens were instrumented with potential drop probes. As fatigue cracks grew deeper than 0.2–0.3 mm, they caused a steadily increasing potential drop over the length of the specimen. These LCF tests were terminated after a certain fraction of the nominal life. A stepwise increasing high-cycle fatigue load was subsequently used to propagate the crack to fracture. The contour of the low-cycle fatigue crack could be identified on the fracture surface. In agreement with a model suggested by Tomkins, its growth rate was found to be proportional to the crack depth and to be a power function of the plastic strain amplitude. For an assumed initial crack depth of 50 μm, this relationship generally provided conservative life predictions deviating from the observed number of cycles by less than a factor of 5. A second model assumed the crack growth rate to be proportional to a power of the cyclic J integral. Using standard Paris type crack growth data, this model also tended to give conservative predictions, in particular for the longer lives. The deviation from the observed number of cycles was generally less than a factor of 3. The high-cycle fatigue tests permitted the fatigue limit of the cracked specimens to be determined. The threshold stress intensity range obtained from these tests consistently fell below that from tests with standard fatigue crack growth specimens.

Determining the potential drop calibration of a fatigue crack growth specimen subject to limited experimental observations : McKeighan, P.C.; Smith, D.J. Journal of Testing Evaluation, Vol. 22, No. 4, pp. 291–301 (Jul. 1994)

Determining the potential drop calibration of a fatigue crack growth specimen subject to limited experimental observations : McKeighan, P.C.; Smith, D.J. Journal of Testing Evaluation, Vol. 22, No. 4, pp. 291–301 (Jul. 1994)

Load shedding effect on constant-ΔK testing

This paper investigates the effect of load shedding steps on constant-ΔK testing. ASTM fatigue crack growth specimens have been used in constant-ΔK tests for many fatigue investigations. In the tests, load levels need to be gradually stepped down to keep the stress intensity factor constant. This work points out that care should be taken in choosing the length of the step in order to control ΔK variation. Formulas for ΔK as well as crack growth rate variability are derived. The experimental application of these techniques is also presented.

The influence of crystal orientation on the high temperature fatigue crack growth of a Ni-based single crystal superalloy

Compact-type (CT) fatigue crack growth (FCG) specimens have been prepared from single crystals of SRR99. The macroscopic crack planes {hkl} and propagation directions 〈UVW〉 were (110)[1̄10], (110)[1̄11], (110)[001], (111)[11̄0] and (001)[010]. FCG tests were conducted at 650 and 850°C in air at 10 and 1 Hz at an R ratio of 0.1. At 650°C, (110) and (111) specimens showed a frequency-dependent FCG rate, not apparent for (001)[010], due to increased cuboidal cross-slip and reduced reversibility at lower frequency. Slip homogenization at 850°C prevented this effect. (001)[010] demonstrated a lower Paris regime FCG rate at 850°C than at 650°C due to crack bifurcation at the higher temperature. Conversely, a higher FCG rate occurred for (110) and (111) specimens at 850°C, as crack tortuosity decreased as it changed from propagation along the γγ′ interface at 650°C to precipitate cutting at 850°C. An orientation-dependent FCG rate and fracture path was found. At 650°C orientations able to accommodate crack tip shear on primary slip planes near-parallel with the crack front provided the best FCG resistance. At 850°C the best FCG response was found for the same orientations that now developed large scale ridges as a result of out-of-plane cuboidal slip.

Integrable K-solutions for common fatigue crack growth specimens

Alternative stress intensity factor solutions are developed for several common fatigue crack propagation specimens. Unlike the K-solutions presently employed, the K-solutions developed herein allow direct integration of the Paris fatigue crack growth rate power law while retaining the same degree of accuracy.

Determining the potential drop calibration of a fatigue crack growth specimen subject to limited experimental observations : 55787 McKeighan, P.C.; Smith, D.J. Journal of Testing Evaluation, Vol. 22, No. 4, pp. 291–301 (Jul. 1994)

Determining the potential drop calibration of a fatigue crack growth specimen subject to limited experimental observations : 55787 McKeighan, P.C.; Smith, D.J. Journal of Testing Evaluation, Vol. 22, No. 4, pp. 291–301 (Jul. 1994)

Life extension technology for steam pipe systems—I. Development of material properties

Material properties of A106B low-carbon steels were developed for life prediction analyses of steam pipes operated at elevated temperatures but in the sub-creep temperature range. Tensile, fracture toughness, fatigue crack growth rate and low-cycle fatigue properties were obtained on the piping steel at 24°C (75°F) and 288°C (550°F). The latter temperature corresponded to the highest operating temperature of nuclear plant steam piping. Increasing the test temperature from 24°C (75°F) to 288°C (550°F) decreased the yield strength and fracture toughness of the steel. Fatigue crack propagation rate properties at 24°C (75°F) and 288°C (550°F) were found to be comparable. In the low-cycle fatigue tests, below a strain amplitude level of approximately 0.5%, cyclic softening was observed, while at higher strain levels, cyclic hardening was present. Based on the results of tensile and incremental-step fatigue testing, the strain-life curve was predicted. The predicted strain-life curve was found to be in agreement with the experimental result. The fracture surfaces of fracture toughness specimens showed ductile fracture, while striations were observed on those of fatigue crack growth specimens. Fatigue striations were also observed on the fracture surfaces of low-cycle fatigue specimens. Fatigue initiation was associated with inclusions. It was shown that plastic straining in A106B steel could be detected by acoustic emission and by monitoring the eddy current response. These nondestructive evaluation techniques exhibit possibilities for in-situ monitoring of fatigue deformation. While the development of material properties for the life prediction assessment of steam pipes is included in Part I of this paper, the establishment of a quantitative life prediction methodology and inspection criteria is contained in Part II. The developed life prediction methodology quantifies the effects of operating parameters on the remaining life of steam pipes using the material properties obtained in Part I.

Fatigue crack growth under random loading

An experimental technique is developed whereby a personal computer is used to control a hydraulic servo-pulser testing machine and generate random loads for loading a fracture specimen. Thus giving a control over random load generation and hence random load history. This allows for the generation of load histories following different distribution families and parameters and subjecting fatigue crack growth specimen to them. Different steels are used to study fatigue crack growth behaviour under random loading. These are AISI 1018, AISI 4340 and stainless steel 17/7 PH. Test results for these steels are reported here. The effect of frequency on steel AISI 1018 under random loads is also studied and presented. The phenomenon of fatigue crack growth retardation and acceleration are clearly exhibited in this study. Plot of fatigue crack length vs number of cycles is observed to be random in path as cyclic loading progresses. Fatigue life is also found to be random. These phenomena occur for loads following a fixed distributed family and parameters.