Fatigue Crack Propagation Threshold Research Articles

Effect of low temperature on fatigue crack propagation behavior of QP980 steel and laser-welded joint

This paper aims to investigate the fatigue crack propagation behavior of quenching and partitioning 980 steel (as base metal) and its welded joint at low-temperature environments. The fatigue crack propagation tests are carried out at 25°C, −40°C and −80°C under R=0.1, 0.3 and 0.5. The results show that the fatigue crack propagation threshold value increases and fatigue crack propagation rate decrease of base metal, whereas welded joint behaved opposite results as the temperature decreases. The fatigue ductility to brittle transition temperature of base metal is lower than that of welded joint. Compared with the welded joint, the base metal has higher fatigue resistance.

Effect of compression precracking on the near threshold fatigue crack propagation in an AISI 316L stainless steel

This work addresses an alternative test procedure based on fatigue precracking under compression to mitigate the potential effects of crack closure on near-threshold fatigue data and, thus, to obtain more consistent and reliable ΔKth-values. The method relies on performing a compression precracking, in which both maximum and minimum loads are compressive, thereby resulting in a tensile residual stress field at the crack tip. Fatigue crack growth (FCG) tests conducted on an AISI 316L stainless steel provide the experimental data upon which the near-threshold fatigue crack growth rate data for different test techniques can be determined. The experimental program includes FCG tests conducted on conventional C(T) specimens with crack size to specimen width ratio a/W=0.2, single edge notched SE(B) specimens under three-point bending with a/W=0.2 and two different span sizes, S, characterized by S/W=4 and 8. Overall, the present investigation shows that combining the compression precracking technique with the utilization of non-standard bend specimens having increased span improves evaluation of the near-threshold fatigue crack growth rate data for the tested AISI 316L steel, thereby providing more accurate ΔKth-values.

Toward developing Ti alloys with high fatigue crack growth resistance by additive manufacturing

Fatigue crack growth behaviors were investigated by three-point bending tests for TA19 alloy fabricated by laser metal deposition and four kinds of heat-treated samples. The crack growth resistance of the TA19 samples in the near-threshold regime and Paris regime was evaluated through the experimental characterization and theoretical analysis of the interaction between fatigue crack and α/β phase interface, columnar prior-β grain boundary and colony boundary. The results show that in the near-threshold regime, the fatigue crack propagation threshold and resistance increase with the increase of widths of lamellar αp phases and colonies, and the decrease of the number of α laths with an angle (φ) relative to the applied stress direction ranging from 75° to 90°. In the Paris regime, the fatigue cracking path can be deflected at colony boundaries or columnar prior-β grain boundaries. The larger the deflection angle, the more tortuous the cracking path and the lower the fatigue crack growth rate. The angle (γ) of the columnar prior-β grain growth direction relative to the build direction affects not only φ of different α variants, but also the fatigue cracking path deflection angle (θij) at columnar prior-β grain boundaries. An optimal combination of γ = 0°-15°-0°-15° for several adjacent columnar prior-β grains is derived from the theoretical analysis, and that can effectively avoid φ being in the range from 75° to 90° and make θij as large as possible. Such findings provide a guide for the selection of scanning strategies and process parameters to additively manufacture Ti alloys with high fatigue damage tolerance.

Recent developments in the determination of fatigue crack propagation thresholds

The impact of crack closure and environmental effects on the experimental determination of the fatigue crack propagation threshold is a major problem for the assessment of cyclically loaded components, especially at low stress ratios R. In this work, the influence of the experimental procedure and air humidity on da/dN−ΔK data at different R is discussed. Unlike the results at R=−1, the threshold values obtained at R≈0.8, i.e. under negligible crack closure levels, show a very small scatter band regardless of the variation of the test parameters and environmental conditions.

A theoretical model for predicting the temperature-dependent fatigue crack propagation threshold of nickel-based superalloys

The fatigue crack propagation threshold is an essential predictor for the safety of crack-containing components during their expected life. In this work, the fatigue crack propagation of a material is considered from an energy perspective and a critical energy density related to fatigue propagation was presented. A temperature-dependent crack fatigue propagation criterion was proposed. The temperature-dependent fatigue crack propagation threshold model for superalloys was developed. The model reveals the quantitative relationship between fatigue crack propagation threshold, Young's modulus, and temperature. The model predictions were in good agreement with all the available experimental data for eleven superalloys. A convenient and simple optional method for predicting the fatigue crack propagation threshold of superalloys at elevated temperatures was achieved. In addition, the limitations and applicability of the model are discussed. The results of the research are of potential application to aero-engine structural integrity studies.

Estimation of the Kitagawa-Takahashi diagram by cyclic R curve analysis

The Kitagawa-Takahashi (KT) diagram is a proven concept for describing the fatigue limit in presence of a defect or crack. It can be determined empirically with great experimental effort. It can also be estimated by means of the El Haddad relationship if the endurance limit and the long fatigue crack propagation threshold are available in reasonable accuracy. A third option is the determination using the cyclic R-curve, which describes the dependency of the fatigue crack propagation threshold on the crack growth at the short crack propagation stage. This can be experimentally determined using a closure-free initial pre-crack. It can then be applied to the determination of crack arrest for a given applied load and a given defect or crack size. Compared to the other two methods mentioned above, this option has considerable advantages: It can be applied to any component and any stress ratio. It allows the treatment of multiple cracks and provides estimations of the S-N curve in the finite life regime as well as at the endurance limit. Compared to the empirical determination of the KT diagram, the experimental effort is significantly lower and compared to the El Haddad approach it avoids problems such as the use of non-conservative long fatigue crack propagation thresholds (when the conventional load reduction method is applied to materials prone to corrosion) and the mathematical predetermination of the curve shape. The work introduces the method and provides a critical discussion as well as quantitative comparison between the different methods.

Determination of fatigue crack propagation thresholds using small-scale specimens

The damage tolerance approach is widely used in the design and estimation of inspection intervals of safety-relevant metallic components subject to fatigue loading. The approach relies on the knowledge of the fatigue crack propagation characteristics, wherein a relevant role is played by the fatigue crack propagation threshold. Nevertheless, the use of material data determined by testing on conventional specimens is not straightforward in the case of thin-walled components such as turbine blades or additively manufactured parts, in which the local variation of material properties in highly stressed regions must be considered. In these cases, the possibility of investigating the fatigue crack propagation properties on a limited portion of material is crucial. For this purpose, a new test procedure has been developed for small-scale specimens which allows the determination of the intrinsic fatigue crack propagation threshold and the near-threshold regime. The validity and limitations of the method are demonstrated on the high strength steel S960QL, along with a comparison with data determined by testing on conventional geometries.

Determination of fatigue crack growth in the near-threshold regime using small-scale specimens

A complete description of the experimental procedure for characterizing the intrinsic fatigue crack propagation threshold (ΔKth,eff) as well as the fatigue crack growth rate (FCGR) in the near-threshold regime using small-scale specimens is presented. A comparative study is carried out on the high strength steel S960QL considering different single edge notch bend (SENB) specimen geometries. On one hand, the reference dimensions of 6 mm thickness (B) and 19 mm width (W) are analysed and referred to as conventional specimens. On the other hand, small-scale specimens with B = 3 mm and W = 4 and 6 mm are also considered. Several loading configurations (3-, 4- and 8-point bending) are used and a load ratio R = 0.8 is applied to avoid crack closure effects. The direct current potential drop (DCPD) technique is used to monitor the crack length. The reduced dimensions of the small-scale specimens imply the necessity of implementing a modified testing procedure compared to the recommendations of current standards for the generation of FCGR data. The results show a good agreement between tests conducted on different specimen sizes, which opens new perspectives in the use of small-scale specimens for characterizing the fatigue crack growth properties in metallic materials. Recommendations and limitations of the procedure are provided and discussed.

Effect of long-period stacking ordered structure on very high cycle fatigue properties of Mg-Gd-Y-Zn-Zr alloys

Magnesium alloys with a long-period stacking ordered (LPSO) structure usually possess excellent static strength, but their fatigue behaviors are poorly understood. This work presents the effect of the LPSO structure on the crack behaviors of Mg alloys in a very high cycle fatigue (VHCF) regime. The LPSO lamellas lead to a facet-like cracking process along the basal planes at the crack initiation site and strongly prohibit the early crack propagation by deflecting the growth direction. The stress intensity factor at the periphery of the faceted area is much higher than the conventional LPSO-free Mg alloys, contributing higher fatigue crack propagation threshold of LPSO-containing Mg alloys. Microstructure observation at the facets reveals a layer of ultrafine grains at the fracture surface due to the cyclic contact of the crack surface, which supports the numerous cyclic pressing model describing the VHCF crack initiation behavior.

Progress in the measurement of the cyclic R-curve and its application to fatigue assessment

The cyclic R-curve describes the resistance of a material to fatigue crack propagation from the short to the long crack regimes and it is therefore an essential ingredient in any fracture mechanics-based fatigue assessment procedure. This work presents different testing procedures employed in the experimental determination of the cyclic R-curve, especially focusing on the comparison with long fatigue crack propagation thresholds obtained by means of the compression precracking load reduction (CPLR) procedure. The tests were performed on the EA4T steel considering different stress ratios. The results show a good reproducibility of the cyclic R-curves at every stress ratio and for any testing procedure. In addition, the cyclic R-curves were used in a fracture mechanics-based assessment to predict the fatigue limits of specimens containing micro-notches.

Determination of fatigue crack propagation thresholds for steel in presence of environmental effects

The experimental determination of the resistance to fatigue crack propagation in steel, particularly in the so-called near-threshold regime, is a crucial issue for safety-relevant components which are designed to operate for a large number of loading cycles before undergoing periodic inspections. This work summarizes the most relevant results of extensive experimental campaigns conducted at BAM and IPM over the last years, which have been devoted to the determination of fatigue crack propagation characteristics on medium and high strength steels. The influence of environment, oxide-induced crack closure and test methodologies in the near-threshold regime are discussed.

Achieving exceptionally tensile properties and damage tolerance of 5083 aluminum alloy by friction stir processing assisted by ultrasonic and liquid nitrogen field

The emergence of gradient structures has realized the possibility of improving the strength and plasticity of metals simultaneously. This study analyses the effects of introducing ultrasonic and liquid nitrogen during conventional stirring and friction treatment. The results show that the mechanical properties and damage tolerance of 5083 aluminum (Al) alloy can be substantially improved by the simultaneous application of two external fields (called UNFSP specimens). The average yield strength and tensile strength of the UNFSP specimens were 58.2% and 27.5% higher than those of the as-received 5083 Al alloy without reduction in its elongation, respectively. In addition, compared with the as-received 5083 Al alloy, the threshold of fatigue crack propagation and the fracture threshold of the UNFSP specimens also increased by 62.9% and 25.7%, respectively. The main reason for these improvements was the contribution of the gradient structure with high density of geometrically necessary dislocations, thicker fine grains layer, and seriously refined Fe/Mn-containing particles.

Influence of Cu precipitates and C content on the defect tolerance of steels

Understanding the effects of microstructural defects, which can have a high impact on service life, is of great importance in the design of cyclically loaded components. For this, the size of the defect as well as the material’s ability to counteract microstructural defects have to be considered. In this study, the defect tolerance of differently heat-treated Cu alloyed steels with two different C contents (0.005 and 0.21 wt% C) has been investigated by testing specimens with artificial defects. From the results of fatigue experiments, defect tolerance is assessed based on different approaches: (i) a comparison of stress intensities, derived from Murakami’s √area approach, (ii) comparing fatigue lifetime for a given defect size and stress amplitude, (iii) the reduction of fatigue strength by a defect for a given defect size, and (iv) using the Kitagawa-Takahashi diagram, enabling the calculation of the critical defect size √area0 as well as fatigue crack propagation threshold ΔKth,calc. The results show that for the 0.005 wt% C steel an increase in aging time is accompanied by an increase in defect tolerance, which correlates with a higher cyclic hardening potential induced by the formation of Cu precipitates. However, the results show that strength and ductility also influence defect tolerance. By using the short-time procedure PhyBaLCHT, which is based on cyclic indentation tests, all these factors influencing defect tolerance can be determined efficiently. The 0.21 wt% C steel does not reveal an improved defect tolerance which is presumably caused by the dominating pearlite phase.

Numerical calibration of the direct current potential drop (DCPD) method in fracture mechanics fatigue tests

Fatigue strength and fracture mechanics concepts are linked together by the so-called cyclic R-curve, i.e. the crack size dependence of the fatigue crack propagation threshold. The cyclic R-curve requires to identify experimentally the extension of a propagating crack in the mechanically short crack regime. One of the most used techniques adopted in experimental tests is the direct current potential drop (DCPD) method, according to which the electrical resistance of the tested specimen increases due to crack propagation and the resulting electric potential change is used to derive the crack length thanks to proper calibration curves. In this work, the DCPD technique was applied in fracture mechanics fatigue tests of carbon steel bars weakened by a single-edge, semi-elliptical pre-crack. First, it was assumed that the semi-elliptical pre-crack propagates under fatigue axial loading in such a way to keep an iso-stress intensity factor (SIF) KI crack front. Accordingly, the corresponding crack pattern was derived by means of 3D structural FE analyses using the Peak Stress Method (PSM). Afterwards, the DCPD calibration curves were derived through 3D electrical FE analyses. The effects of the locations of the current and of the potential probes were investigated. Finally, a three-probe dual channel DCPD technique was applied to compensate any temperature variation of the tested specimen.

A unified mean stress correction model for fatigue thresholds prediction of metals

Two fatigue characteristic thresholds, i.e. fatigue limit Δσe and fatigue crack propagation threshold ΔKth, have significant mean stress sensitivity. In the work, a new mean stress correction model for the two fatigue thresholds is proposed based on the cyclic strain energy density concept. Extensive experimental data in the literature is unified evenly by the proposed normalized fatigue threshold-R curves. Besides, a comparison of the proposed model with the existing models is carried out, which demonstrates that the present work can achieve more accurate predictions with the advantage of being of easy use and of being independent on any material constant.

Effect of laser peening with different power densities on vibration fatigue resistance of hydrogenated TC4 titanium alloy

Laser peening (LP) treatment with different power densities was used to improve the vibration fatigue resistance of hydrogenated TC4 titanium alloy. Compressive residual stress (CRS), micro-hardness and micro-structure of hydrogenated LPed specimens were firstly measured. Vibration fatigue test was then carried out to compare the fatigue life and fractures of LPed and non-LPed hydrogenated specimens. It was found that LP-induced high level CRS suppressed the permeation of hydrogen atoms through material surface, which may reduce the hydrogen embrittlement sensitivity. The CRS also prohibited the fatigue crack initiation and propagation by increasing the fatigue limit and crack propagation threshold of hydrogenated specimen. Grain morphologies of α phase and β phase were refined after LP. Beneficial crystal defects inside the α phase and β phase tangled more free hydrogen atoms, which was believed to be crucial factors to inhibit the generation of initial hydrogen-induced crack. Fracture morphologies further confirmed that LP changed the location of fatigue crack initiation (FCI) and slowed down the fatigue crack growth (FCG) rate of hydrogenated specimens. Finally, a strengthening mechanism of LP on the vibration fatigue resistance of hydrogenated specimen was proposed.

Influence of Welded Pores on Very Long-Life Fatigue Failure of the Electron Beam Welding Joint of TC17 Titanium Alloy.

The electron beam welding process is widely used in the connection among titanium alloy material parts of aero-engines. Its mechanical properties need to meet the requirements of long life and high reliability. In this paper, the static strength and the fatigue failure behavior of the electron beam weldments of TC17 titanium alloy were investigated experimentally under low amplitude high frequency (20 kHz), and the mechanical response and failure mechanism under different external loading conditions were analyzed. In summary, the samples were found to have anisotropic microstructure. The tensile strength of the PWHT of TC17 EBW joint was ~4.5% lower than that of the base metal. Meanwhile, compared with the base metal, the fatigue strength was reduced by 45.5% at 109 cycles of fatigue life. The fracture analysis showed that the fatigue failure of the welded joint of TC17 alloy was caused by the welded pores and the fatigue cracks initiated from the welded pores. A fine granular area (FGA) was observed around the crack initiation region. The existence of pores caused the stress intensity factor of the fine granular area (KFGA) to be inversely proportional to the fatigue life. The KFGA calculation formula was modified and the fatigue crack propagation threshold of the welded joint of TC17 alloy was calculated (3.62 MPa·m1/2). Moreover, the influences of the effective size and the relative depth of the pores on the very long fatigue life of the electron beam welded joint of TC17 titanium alloy were discussed.

Effects of the Electron Beam Welding Process on the Microstructure, Tensile, Fatigue and Fracture Properties of Nickel Alloy Nimonic 80A

The purpose of this study was to evaluate rotary bending high-cycle fatigue properties and crack growth of Nimonic 80A-based metal and electron beam-welded joints. All the tests were performed at room temperature. Fracture surfaces under high-cycle fatigue and fatigue crack growth were observed by scanning electron microscopy. Microstructure, hardness and tensile properties were also evaluated in order to understand the effects on the fatigue results obtained. It was found that the tensile properties, hardness and high-cycle fatigue properties of the welded joint are lower than the base metal. The fracture surface of the high-cycle fatigue shows that fatigue crack initiated from the surface under the high stress amplitude and from the subsurface under the low stress amplitude. The effect of the welding process on the statistical fatigue data was studied with a special focus on probabilistic life prediction and probabilistic lifetime limits. The fatigue crack growth rate versus stress intensity factor range data were obtained from the fatigue crack growth tests. From the results, it was evident that the fatigue crack growth rates of the welded are higher than the base metal. The mechanisms and fracture modes of fatigue crack growth of welded specimens were found to be related to the stress intensity factor range ΔK. In addition, the effective fatigue crack propagation thresholds and mismatch of welded joints were described and discussed.

The cyclic R-curve – Determination, problems, limitations and application

The so-called cyclic R curve, i.e. the crack size dependence of the fatigue crack propagation threshold in the physically short crack regime, is a key parameter for bringing together fatigue strength and fracture mechanics concepts. Its adequate determination is of paramount importance. However, notwithstanding this relevance, no test guideline is available by now and only very few institutions have spent research effort on cyclic R curves so far. The aim of the present paper is to give an overview on the state-of-the-art. Besides an introduction into the basic principles, the discussion will concentrate on the experimental determination on the one hand and questions of its application on the other hand.

Fatigue crack propagation for Q345qD bridge steel and its butt welds at low temperatures

AbstractSteel bridges fabricated with Q345qD steels face critical challenges when operating in cold regions with a low ambient temperature. This study aims to investigate, via an experimental program, the low‐temperature fatigue crack propagation behavior of Q345qD bridge steel base material and its butt welds. The testing program comprises a series of Charpy impact tests and fatigue crack propagation tests at the room temperature, −20°C and −60°C. The experimental results demonstrate a reduced crack propagation rate in the base material, but an increasing crack propagation rate in the butt welds, with a decreasing ambient temperature. The base material also shows enhanced fatigue crack propagation thresholds with the decreasing temperature. The ductile‐to‐brittle transition temperature for fatigue is lower than that for fracture in the base material while the weld metal exhibits an opposite trend. Generally, the butt welds present higher resistance against fatigue crack propagation and larger Charpy toughness values than do the base material at all tested temperatures. The Paris‐law parameters measured at the room temperature for the base material leads to a conservative assessment of the crack propagation life for a welded joint under a low ambient temperature.