Macroscopic Crack Growth Rate Research Articles

Estimation of Fatigue Crack Growth Rate in Heat-Resistant Steel by Processing of Digital Images of Fracture Surfaces

The micro- and macroscopic fatigue crack growth (FCG) rates of a wide class of structural materials were analyzed and it was concluded that both rates coincide either during high-temperature tests or at high stress intensity factor (SIF) values. Their coincidence requires a high level of cyclic deformation of the metal along the entire crack front as a necessary condition for the formation of fatigue striations (FS). Based on the analysis of digital fractographic images of the fatigue fracture surfaces, a method for the quantitative assessment of the spacing of FS has been developed. The method includes the detection of FS by binarization of the image based on the principle of local minima, rotation of the highlighted fragments of the image using the Hough transform, and the calculation of the distances between continuous lines. The method was tested on 34KhN3M steel in the initial state and after long-term operation (~3 × 105 h) in the rotor disk of a steam turbine at a thermal power plant (TPP). Good agreement was confirmed between FCG rates (both macro and microscopic, determined manually or using digital imaging techniques) at high SIF ranges and their noticeable discrepancy at low SIF ranges. Possible reasons for the discrepancy between the micro- and macroscopic FCG rates at low values of the SIF are analyzed. It has also been noted that FS is easier to detect on the fracture surface of degraded steel. Hydrogen embrittlement of steel during operation promotes secondary cracking along the FS, making them easier to detect and quantify. It is shown that the invariable value of the microscopic FCG rate at a low SIF range in the operated steel is lower than observable for the steel in the initial state. Secondary cracking of the operated steel may have contributed to the formation of a typical FS pattern along the entire crack front at a lower FCG rate than in unoperated steel.

Minimum fatigue striation spacing and its stress amplitude dependence in a commercially pure titanium

AbstractWe report the fatigue striation spacing of commercially pure titanium with respect to stress amplitude. Rotating bending fatigue tests were performed at 150, 175, 200 and 227 MPa in which samples did not fail at 150 MPa. No fatigue striation up to a certain crack length is observed for the studied conditions. For each condition, a minimum striation spacing value is found that decreases from 0.45 to 0.36 to 0.24 μm with increasing stress from 175 to 200 to 227 MPa, respectively. The minimum striation spacing remains constant up to a certain crack length for each condition. Thereafter, it starts increasing with crack length. It is quite understandable that the constant striation spacing over a certain crack length may not represent the macroscopic fatigue crack growth rate. The fatigue crack growth mechanism during the constant striation spacing region has been discussed in detail.

Grain size effects on NiTi shape memory alloy fatigue crack growth

Abstract

Fractograpic Observations and Effect of Stress Ratio on Fatigue Striations Spacing in Aluminium Alloy 2024 T351

In this paper, effect of stress ratio was investigated on macro and micro of fatigue crack growth rates (FCGRs) of Al-alloy 2024 T351. Microscopic fatigue crack growth rate present a change of slope with respect to the macroscopic fatigue crack growth rate. In addition, an increasing in stress ratio increases the fatigue life. Fractographic examination showed the presence of striations in Paris domain and secondary cracks at grain boundaries.

Fatigue Crack Striation Spacing for Welded and Base Material CT Steel Specimens

Scanning electron microscopy (SEM) observation of the fracture surface of CT specimens was carried out at CEMUP in the context of a FEUP R&D programme for the characterization of fatigue crack growth (FCG) of base material and welded steel CT specimens, carried out under constant load range, reported in a companion paper. The inclusion of specimens of welded material in the FCG characterization programme justified the need for relatively large size specimens. The thickness of the specimens (B=32mm) was approximately identical to the thickness of the structural application of interest, since with thinner specimens the possible effects of residual stress fields would be at least partially lost. The CT specimens were designed according to ASTM E647 standard. The FEI QUANTA 400 FEG / EDAX Pegasus X4M available at CEMUP was used for the SEM observations, carried out in the fatigue fracture surfaces of base and welded (HAZ) steel. Both in base material as well as in welded specimens, it was found that there are regions of the fracture surfaces where no striations are observed, mixed with other regions (patches) with striations. Some regions are featureless (ropey appearance). It was not possible to find a preferential orientation of the striation spacing, that might be directly related with the macroscopic direction of crack propagation; on the contrary, the regions presenting striations seem to have a mainly random orientation. This fact contributes to the absence of correlation between the distance between striations (s) measured using SEM, and the macroscopic fatigue crack growth rate da/dN. Only for substantial values of the crack length/specimen width (a/W) ratio a reasonable agreement between s and da/dN is found. The relationship s/(da/dN) presents values that decrease up to approximately one, as a/W increases. For low values of a/W, s is one or two orders of magnitude greater than da/dN. In the specimens analyzed, fatigue crack growth rates below approximately 2 to 3 x10‑7 m/cycle are associated with approximately constant s values. This border is over the transition between regions I and II of the typical sigmoidal log-log plot of the da/dN vs. DK (I corresponding to the near threshold regime and II to the Paris law regime), and this fact should be accounted for in failure analysis involving SEM of fracture surfaces. Finally, the distance between striations s proved to be insensitive to the presence of important residual stresses in the case of the welded specimens.

Micromechanically derived lowest threshold stress intensity factor for macroscopic stress-corrosion cracking in unidirectional GFRP composites

Observation by scanning electron microscopy of a typical fracture surface in glass fiber-reinforced polymer (GFRP) composites shows that the fracture surface of each fiber is characterized by a mirror region surrounded by a hackle region. This indicates a two-stage fracture process: (1) a ‘‘slow’’ (meaning time-dependent) fracture of a portion of the glass fiber is followed by (2) a ‘‘fast’’ unstable fracture across the remainder of the glass fiber. On the premise that the time-dependent failure of GFRP composites in acid environments is controlled by the initiation and slow growth of a crack from a pre-existing inherent surface flaw in a glass fiber, a micromechanical model of stress-corrosion crack growth (sometimes called the Sekine–Miyanaga–Beaumont model) was constructed [1–3]. Using this model, an equation was derived for the macroscopic crack growth rate as a function of the apparent crack tip stress intensity factor for the mode I. If tougher and more ductile matrices are used, there exists a lowest threshold value of the stress intensity factor. By assuming that the polymeric fibrils of matrix or ligaments stretched between the crack surfaces behave according to an ideal cohesive force model, i.e., the Dugdale model [4], the lowest threshold stress intensity factor was given in an explicit form [3]. However, the examination in the case of a more general cohesive force model, i.e., the Barenblatt model [5], has been left until now, although this examination will give a physically definite description of the lowest threshold stress intensity factor. We begin this examination with a brief review of our previous article [3]. In bulk glass, the stable crack growth rate due to stresscorrosion cracking da/dt can be given by

A Progressive Damage Model for the Prediction of Fatigue Crack Growth in Bonded Joints

A procedure to predict fatigue crack growth in bonded joints is developed in this work within the framework of cohesive zone modelling and finite element analysis. The idea is to link the fatigue damage rate in the cohesive elements to the macroscopic crack growth rate through a damage homogenisation criterion. In this way, the experimental crack growth rate is related directly to damage evolution in the cohesive zone, that is, no additional parameters have to be tuned beside the quasi-static cohesive zone parameters. The procedure was implemented in the ABAQUS finite element software using the USDFLD subroutine. In order to evaluate the crack growth rate automatically, the strain energy release rate (crack driving force) was calculated and updated automatically within the subroutine. Mode I and Mode II loading cases have been dealt with and the implementation for Mixed Mode I/II is under way.

Stress-Corrosion Cracking in Unidirectional GFRP Composites

A micromechanical theory of macroscopic stress-corrosion cracking in unidirectional glass fiber-reinforced polymer composites is proposed. It is based on the premise that under tensile loading, the time-dependent failure of the composites is controlled by the initiation and growth of a crack from a pre-existing inherent surface flaw in a glass fiber. A physical model is constructed and an equation is derived for the macroscopic crack growth rate as a function of the apparent crack tip stress intensity factor for mode I. Emphasis is placed on the significance of the size of inherent surface flaw and the existence of matrix crack bridging in the crack wake. There exists a threshold value of the stress intensity factor below which matrix cracking does not occur. For the limiting case, where the glass fiber is free of inherent surface flaws and matrix crack bridging is negligible, the relationship between the macroscopic crack growth rate and the apparent crack tip stress intensity factor is given by a simple power law to the power of two.

A Comparison of Macroscopic Fracture Surface and Crack Growth Rate of Ti-6Al-4V

A comparison between crack growth rate (da/dN) vs. effective stress intensity range factor (ΔKeff) curve behavior and microscopic and macroscopic fracture surface of commercial Ti-6Al-4V alloy are presented. Three different regimes are correlated with characteristics measured on the fracture surfaces. Three regions can be observed in which part I is rough and darker than others parts known as pre-transition, part II is smooth and light known as transition region and part III is a little darker than part II known as post-transition region. In the present investigation the correlation of fatigue crack growth rate for Ti-6Al-4V and microstructure of fracture surface has been presented.

Fractographic reconstitution of fatigue crack history – Part I

ABSTRACTQuantitative fractography of components which have failed under cyclic loading can offer very valuable information usually unattainable by any other means – in particular, a detailed description of fatigue crack growth (FCG) in space and in time. The reliable characterization of crack propagation provides data important in engineering applications. The main source of information on the relationship between fractographic findings and macroscopic crack growth rate (CGR) is striation spacing. This quantitative microfractographic characteristic was the basis for the methods described in the paper. An application of the fractographic reconstitution of FCG is illustrated by an example of an aircraft wing spar fatigued during a full‐scale test.

On the relation between micro- and macroscopic fatigue crack growth rates in aluminum alloy AMS 7475-T7351

This paper discusses the relationship between striation spacing, i.e., the microscopic crack propagation rate, as measured in postmortem fractographic inspection of fatigue fractured surfaces, and the macroscopic crack propagation rate, i.e., da/dN, as monitored during fatigue crack growth tests. Compact tensile specimens C(T) in prevalent plane-strain conditions were extracted in LT orientation from the center of a 2-in. thick rolled plate of a SAE-AMS 7475-T7351 Al alloy. Testpieces were fatigue tested according to ASTM-E647 standard, at room temperature in a servo-hydraulic closed-loop MTS testing machine operating with the unloading elastic compliance technique. da/dN-ΔK data points were collected in the Paris’ law validity region, with crack growth rates typically ranging from 0.18 to 2.02 µm/cycle. Topographical survey was conducted on the test specimen fracture surfaces in a scanning electronic microscope in order to determine striation spacing created during the fatigue test. Macro- and micro-crack growth rates were compared and good correlation have been obtained for the data within the range of ΔK assessed in the study. Results of crack growth rates have been quantitatively evaluated in terms of fatigue life estimation.

Micro-mechanical theory of macroscopic stress-corrosion cracking in unidirectional GFRP

A micro-mechanical theory of macroscopic stress-corrosion cracking in a unidirectional glass fibre-reinforced polymer composite is proposed. It is based on the premise that under tensile loading, the time-dependent failure of the composite is controlled by the initiation and growth of a crack from a pre-existing inherent surface flaw in a glass fibre. A physical model is constructed and an equation is derived for the macroscopic crack growth rate as a function of the apparent crack tip stress intensity factor for mode I. Emphasis is placed on the significance of the size of inherent surface flaw and the existence of matrix crack bridging in the crack wake. There exists a threshold value of the stress intensity factor below which matrix cracking does not occur. For the limiting case, where the glass fibre is free of inherent surface flaws and matrix crack bridging is negligible, the relationship between the macroscopic crack growth rate and the apparent crack tip stress intensity factor is given by a simple power law to the power of two.

알루미늄 합금의 랜덤하중 하에서 발생한 피로파면 해석 방법

Striation is a typical pattern observed on the fatigue fracture surface and the striation spacing is known to correspond to a macroscopic fatigue crack growth rate. Many models for the prediction for the formation of such striation have been proposed. However, these theories and methods can not be applied under random loading spectrum. In this study, the fatigue tests were carried out on aluminum alloy under random loading spectrum. The fatigue fracture surfaces were observed in the scanning electron microscope (SEM) and great quantities of SEM micrographs were synthesized and saved in computer system. The space and morphology of several large-scale striations, which are expected to form at the relatively greater load range in loading block, were observed. The crack length for each loading block was decided in consideration of regularity and repetition of those striations. It showed that the applicability of fractographic methods on the fatigue fracture surface under random loading spectrum.

Microscopic and macroscopic growth rates of fatigue cracks in steels and alloys under the influence of crack closure

On the basis of the experimental data, we show that the difference between the macroscopic and microscopic fatigue crack growth rates in the second section of the kinetic diagram of fatigue fracture is caused by the effect of crack closure within the limits of its existence. We establish the relationships between the macroscopic and microscopic fatigue crack growth rates and the structure of the material in the second section of the diagram for various values of the load ratio with regard for the effect of crack closure and propose a procedure of examination of the fracture processes in structural materials based on the analysis of microscopic and macroscopic fatigue crack growth rates.

The role of reinforcement particles during fatigue cracking of a micral-20tm-reinforced 6061 alloy

This paper reports a study on the role of reinforcement particles during fatigue crack propagation in a metal matrix composite comprising 6061 aluminium reinforced with spherical polycrystalline oxide particles. The key observation relates to how A f , the area fraction of reinforcement particles appearing on the fatigue fracture surface, varies with the applied stress intensity factor, K. It has been possible to separate the effects of AK, ΔK eff and K max on A f , and to show that, in this particular composite, A f is related to K 2 max . This means that no direct correspondence exists between A f and the fatigue crack growth rate. It was also observed that A f was higher under plane-strain conditions than under plane-stress conditions. It appears that the interaction of the crack-tip stress field with particles does not significantly alter the macroscopic crack growth rate even though the crack may be deflected away from the particles; crack steering may be a more physically meaningful concept than crack-tip repulsion or attraction.

The Initiation of Corrosion Fatigue Cracking from Pitting in 3.5Ni-Cr-Mo-V Low-Pressure Rotor Steel.

Corrosion and Corrosion fatigue properties of 3.5 Ni-Cr-Mo-V steam turbine low-pressure rotor steel were investigated. Immersion tests were performed on 3.5Ni-Cr-Mo-V steel by dipping the specimen into deionzed water and 10 ppm NaCl solution. Observation of SEM and X-ray cartography showed that MnS inclusions were partly dissolved and corrosion pits were initiated from the edge of the dissolved MnS. Long-term corrosion fatigue tests were done in aqueous environment up to 5×108 stress cycles. The S-N curve was consisted of two different curves and there appeared substantial reduction in fatigue strength over 5×107 stress cycles. The fatigue crack initiation and growth from pits can be considered as the main factor of reduction of fatigue strength. Assuming the corrosion pit as a semi-elliptic surface crack, the critical stress intensity factor range ΔΚth for corrosion fatigue crack initiation was calculated. The average value of ΔΚth was around 1.50 MPam1 / 2, about one half of the value obtained from conventional macroscopic fatigue crack growth rate tests.

Fatigue behavior of Alloy 600 in NaCl solution at room temperature

Fatigue crack growth (FCG) rates of mill annealed Alloy 600 in NaCl solution at 25°C were studied by a fracture mechanics test method. Compact tension (CT) specimens were tested under load control with a sinusoidal wave form, in accordance with ASTM specification E-647-83, to investigate the effects of environment, load frequency ( f), and load ratio ( R = P min P max ) on FCG rates. The FCG rates of Alloy 600, R = 0.1, f = 1 Hz, were quite similar in air, distilled water, and NaCl (0.6 M, 0.1 M, and 0.001 M) solution at room temperature. An environmental enhancement effect on the FCG rate of Alloy 600 was not significant in the NaCl solution. Variations of the load frequency (0.03–3 Hz) influenced little on the FCG rates of Alloy 600 in air and 0.1 M NaCl solution which increased with increasing the load ratio at any given value of stress intensity factor range ΔK. The load ratio effects were successfully accounted for by applying the Walker model. Compared with the corrosion effects, the test results showed that the pure mechanical fatigue dominated on the FCG rates of Alloy 600 in chloride solution at room temperature. The SEM fractographs showed that significant striations and transgranular fracture mode were observed on tested specimens, and striation spacing measurements were found to agree with macroscopic crack growth rate.

破壊力学 Ａｌ‐１５Ｚｎ粉末冶金合金における表面微小疲労き裂の発生および進展挙動

Reversed plane bending fatigue tests were conducted on extruded Al-15Zn P/M (Powder Metallurgy) aluminum alloy using two types of specimens with different notch dimensions. The initiation and growth of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail.A large difference in crack initiation strength was observed between the extruded (L-C orientation) and transverse directions (C-C orientation). It was found that the strength of shallow notched specimen, when evaluated by true stress at the notch root, was lower than that of deep notched one. If the stress intensity range of inclusion at which the fatigue crack initiated was calculated, the difference in fatigue strength depending on specimen orientation and notch size could be eliminated. And also, a prediction method of fatigue strength in smooth specimen was proposed on the basis of the extreme value statistics. The macroscopic fatigue crack growth rate could be expressed by the Paris equation in terms of the maximum stress intensity factor irrespective of stress level and the notch size, and the growth rate was found faster in C-C orientation specimen than that in L-C orientation specimen.

Ａｌ‐２５Ｓｉ粉末鍛造アルミニウム合金における表面微小疲労き裂の発生および進展挙動

Reversed plane bending fatigue tests were carried out on two kinds of forged Al-25Si P/M (Powder Metallurgy) aluminum alloys having different sizes and distributions of Si particles. The initiation and growth behaviors of small surface fatigue cracks were continuously monitored by the replica technique and investigated in detail.It was found that the crack initiation strength of the material with fine Si particles was higher than that with coarse Si particles. Although little difference in strength was observed between the forged and transverse directions in the former material, some difference was found depending on the specimen orientations in the latter material. When the stress intensity range was calculated by considering the difference of the projected area of Si particles to the plane perpendicular to the loading axis, the difference in fatigue strength among the specimens oriented differently could be explained.The macroscopic crack growth rate, da/dn, could be expressed by the Paris equation in terms of the maximum stress intensity factor, Kmax, irrespective of stress level and specimen orientation. The fatigue crack growth rate was found faster in the coarse grained material than that in the fine grained material, and as the crack propagated along the Si particles, more deflective behavior of crack growth was observed in the material with fine Si particles, which was thought to result in the increase in crack growth resistance.

Effect of mean stress on hydrogen assisted fatique crack propagation in duplex stainless steel

Hydrogen assisted subcritical cleavage of the ferrite matrix occurs during fatigue of a duplex stainless steel in gaseous hydrogen. The ferrite fails by a cyclic cleavage mechanism and fatigue crack growth rates are independent of frequency between 0.1 and 5 Hz. Macroscopic crack growth rates are controlled by the fraction of ferrite grains cleaving along the crack front, which can be related to the maximum stress intensity, K max. A superposition model is developed to predict simultaneously the effects of stress intensity range ( ΔK) and K ratio ( K min/ K max). The effect of K max is rationalised by a local cleavage criterion which requires a critical tensile stress, normal to the {001} cleavage plane, acting over a critical distance within an embrittled zone at the crack tip.