Fatigue Crack Growth Rate Properties Research Articles

Fatigue crack growth rate and mechanical properties of one-step double-side friction stir welded AA6061-T6

Friction Stir Welding (FSW) is commonly used in transportation industries but faces challenges when joining thick plates. To address this, double-side FSW (DFSW) has been introduced, involving two steps: FSW on one side, flipping the plate, and then FSW on the opposite side. However, this process is time-consuming. To overcome this, a one-step DFSW technique was developed, where two tools simultaneously operate on both sides. This study aimed to investigate the impact of tool rotation speed on the fatigue crack growth rate of AA6061-T6 in one-step DFSW. One-step DFSW was performed at different upper and lower tool rotational speeds: 1500/900 rpm, 1500/1200 rpm, and 1500/1500 rpm. Tensile and fatigue crack growth tests were conducted on the joined specimens. The microstructure of the welded joint and the surface fractures resulting from the tests were analyzed using a scanning electron microscope (SEM). Results showed that the specimen welded at 1500/1500 rpm exhibited the lowest fatigue crack growth rate. The Paris constants (C and n) were determined as 4.348E-08 and 4.16, respectively.

Hydrogen-related degradation of fracture properties and altered fracture behavior of Cr–Mo steel used in hydrogen stationary vessels

This study comprehensively analyzes the effect of high-pressure hydrogen gas on the elastic-plastic fracture toughness, fatigue crack growth rate (FCGR) properties, and the corresponding fracture behavior of Cr–Mo steel in hydrogen stationary vessels. The fracture toughness value under 99 MPa hydrogen pressure significantly reduced by approximately one-third of the value tested in ambient air while FCGRs tested under hydrogen environments at a specified ΔK exhibited an approximate two-order magnitude increase compared to those observed in ambient air. The hydrogen-assisted fracture mechanism, encompassing crack initiation, propagation, and crack arrested, was meticulously examined. The distinct variations in fracture modes with changes in the ΔK range can be attributed to the synergistic interaction of HELP and HEDE mechanisms. Leak before break (LBB) behavior was not satisfied for the hydrogen vessel due to a significant reduction in the fracture toughness of materials when exposed to 99 MPa H2.

A study on XFEM simulation of tensile, fracture toughness, and fatigue crack growth behavior of Al 2024 alloy through fatigue crack growth rate models using genetic algorithm

AbstractThis study aims to numerically validate and simulate the fatigue crack growth rate (FCGR), tensile, and fracture toughness properties of T3 and T351 tempered aerospace grade Al 2024 alloy. The extended finite element method (XFEM) coupled with a genetic algorithm (GA) was used to predict FCGR behavior in the framework of an in‐house developed MATLAB code. The numerical simulation was performed using three different FCGR models, i.e., Paris, Forman, and Nasgro. The FCGR prediction ability of simulation models was improved and compared further using GA. The Nasgro model with genetically optimized parameters shows the highest FCGR prediction accuracy compared to other FCGR models. The critical stress intensity factor and von Mises stress distribution under static loading were also predicted for C(T) specimen. A similar XFEM approach was used for analyzing the tensile behavior using a center cracked specimen at the gage section of a dog bone‐shaped specimen.

Effects of laser shock peening on fatigue crack growth rate and fracture properties of AA2524 aluminum alloy

Effects of laser shock peening on fatigue crack growth rate and fracture properties of AA2524 aluminum alloy

Fracture toughness and fatigue crack growth rate properties of AM repaired Ti–6Al–4V by Direct Energy Deposition

The fracture toughness (KIC) and fatigue crack growth (FCGR) properties of Ti–6Al–4V “repaired” via Direct Energy Deposition (DED) additive manufacturing process were investigated. The KIC and FCGR tests were performed in conformance to the ASTM standard E1820 and E647. Direct current potential drop (DCPD) was used to monitor the crack growth and Digital Image Correlation (DIC) was used to compute the mouth opening displacement. The properties were measured for AM repaired samples along the bond-line present between half AM and half stock Ti–6Al–4V and compared with fully built wrought annealed stock Ti–6Al –4V. The assessment of the crack growth path and the fracture surface were performed using optical and scanning electron microscopy to understand the failure mechanism in detail. The measured properties are discussed in relation to the effects of microstructure.

The Effect of the Welding Direction on Fatigue Crack Propagation Rate of Welded Shell Kiln

In this paper, the fatigue crack growth rate (FCGR) properties of welded kiln shell at various positions relative to the weld has been proposed. This is achieved by combining an experimental and three-dimensional finite element model are used to demonstrate the results of fatigue crack growth rate curves in the kiln welded shell. Experimental marked crack fronts are used to determine the da/dN at local regions and provide the profile information for FE modeling. Stress Intensity Factor (SIF) at local regions is determined by FE analysis and the driving force ΔK of FCGR can be derived. Furthermore, from the mixture method, material constants of Paris law (c and m) on a welded-parts and non-welded part is measured. When crack grow across the weld-line, material constants of Paris law vary according to the effect of the welded part. In the result, the constant of Paris can be used to predict a fatigue crack growth of rotary kiln structure in finite element analysis.

A Comparative Study of Fracture Toughness at Cryogenic Temperature of Austenitic Stainless Steel Welds

The ITER magnet system is based on the “cable-in-conduit” conductor (CICC) concept, which consists of stainless steel jackets filled with superconducting strands. The jackets provide high strength, limited fatigue crack growth rate and fracture toughness properties to counteract the high stress imposed by, among others, electromagnetic loads at cryogenic temperature. Austenitic nitrogen-strengthened stainless steels have been chosen as base material for the jackets of the central solenoid and the toroidal field system, for which an extensive set of cryogenic mechanical property data are readily available. However, little is published for their welded joints, and their specific performance when considering different combinations of parent and filler metals. Moreover, the impact of post-weld heat treatments that are required for Nb3Sn formation is not extensively treated. Welds are frequently responsible for cracks initiated and propagated by fatigue during service, causing structural failure. It becomes thus essential to select the most suitable combination of parent and filler material and to assess their performance in terms of strength and crack propagation at operation conditions. An extensive test campaign has been conducted at 7 K comparing tungsten inert gas (TIG) welds using two fillers adapted to cryogenic service, EN 1.4453 and JK2LB, applied to two different base metals, AISI 316L and 316LN. A large set of fracture toughness data are presented, and the detrimental effect on fracture toughness of post-weld heat treatments (unavoidable for some of the components) is demonstrated. In this study, austenitic stainless steel TIG welds with various filler metals have undergone a comprehensive fracture mechanics characterization at 7 K. These results are directly exploitable and contribute to the cryogenic fracture mechanics properties database of the ITER magnet system. Additionally, a correlation between the impact in fracture toughness and microstructure resulting from the above treatment is provided.

Fracture toughness and fatigue crack growth rate properties in wire + arc additive manufactured Ti‐6Al‐4V

AbstractThis paper presents an experimental investigation of the fracture and fatigue crack growth properties of Ti‐6Al‐4V produced by the Wire + Arc Additive Manufacture (WAAM®) process. First, fracture toughness was measured for two different orientations with respect to the build direction; the effect of wire oxygen content and build strategy were also evaluated in the light of microstructure examination. Second, fatigue crack growth rates were measured for fully additive manufactured samples, as well as for samples containing an interface between WAAM® and wrought materials. The latter category covers five different scenarios of crack location and orientation with respect to the interface. Fatigue crack growth rates are compared with that of the wrought or WAAM® alone conditions. Crack growth trajectory of these tests is discussed in relation to the microstructure characteristics.

Crack propagation and fracture toughness of Ti6Al4V alloy produced by selective laser melting

The fracture toughness (K1c) and fatigue crack growth rate (FCGR) properties of selective laser melted (SLM) specimens produced from grade 5 Ti6Al4V powder metal has been investigated. Three specimen orientations relative to the build direction as well as two different post-build heat treatments were considered. Specimens and test procedures were designed in accordance with ASTM E399 and ASTM E647 standard. The results show that there is a strong influence of post-build processing (heat treated versus ‘as built’) as well as specimen orientation on the dynamic behaviour of SLM produced Ti6Al4V. The greatest improvement in properties after heat treatment was demonstrated when the fracture plane is perpendicular to the SLM build direction. This behaviour is attributed to the higher anticipated influence of tensile residual stress for this orientation. The transformation of the initial rapidly solidified microstructure during heat treatment has a smaller beneficial effect on improving mechanical properties.

Improved test method and analytical modelling for fatigue crack growth in coarse‐grain titanium alloy with rough fatigue surfaces

ABSTRACTFatigue crack growth rate properties are typically determined by experimental methods in accordance with ASTM Standard E647. These traditional methods use standard notched specimens that are precracked under cyclic tensile loads before the main test. The data that are produced using this approach have been demonstrated elsewhere to be potentially adversely affected by the test method, particularly in the threshold region where load reduction (LR) methods are also required. Coarse‐grained materials that exhibit rough and tortuous fatigue surfaces have been observed to be strongly affected by the tensile precracking and LR, in part because the anomalies caused by crack closure and roughness‐induced closure become more important. The focus of the work reported in this paper was to further develop methods to determine more accurate fatigue crack growth rate properties from threshold through to fracture for coarse‐grained, β‐annealed, titanium alloy Ti‐6Al‐4V extra low interstitial thick plate material. A particular emphasis was put upon the threshold and near threshold region, which is of strong importance in the overall fatigue life of components. New approaches that differ from the ASTM Standard included compression precracking, LR starting from a lower load level and continuing the test beyond rates where crack growth would otherwise be considered below threshold. For the threshold regime, two LR methods were also investigated: the ASTM method and a method where the load is reduced with crack growth such that the crack mouth opening displacement is held constant, in an attempt to avoid remote closure. Constant amplitude fatigue crack growth rate data were produced from threshold to fracture for the titanium alloy at a variety of stress ratios. Spike overload tests were also conducted These data were then used to develop an improved analytical model to predict crack growth under spectrum loading and the predictions were found to correlate well with test results.

마찰교반용접된 7075-T651 알루미늄 합금 용접부의 피로균열전파율의 공간적 불규칙성 (LT 방향의 시험편에 대하여)

본 연구의 목적은 마찰교반용접된 7075-T651 알루미늄 합금 용접부에 대한 피로균열전파율의 공간적 불규칙성을 고찰하기 위한 것이다. 본 연구에서는 이전의 피로균열전파 실험 데이터를 활용하였다. 피로균열전파율의 공간적 불규칙성을 명확히 이해하기 위하여, 피로균열전파 실험은 일정 응력확대계수범위 제어하에서 수행되었다. 재질, 즉 마찰교반용접된 7075-T651 알루미늄 합금 용접부의 용접재와 모재에 대하여 피로균열전파율의 공간적 불규칙성을 조사하기 위하여 실험 데이터가 해석되었다. 결론적으로 용접재의 변동성이 모재의 변동성보다 높게 나타났으며, Weibull 통계 해석에 의하여 변동성을 평가할 수 있음을 알았다.

Identification of fatigue crack growth mechanisms in IN100 superalloy as a function of temperature and frequency

ABSTRACTA study is undertaken to investigate the fatigue crack growth rate properties of polycrystalline IN100 through the identification of crack growth mechanisms as a function of temperature, frequency and ΔK. An additional goal is to determine the stress free activation energy of IN100. Constant amplitude, load controlled tests are performed at room temperature (22 °C), 316 °C, 482 °C and 649 °C under two different loading frequencies of 20 and 0.33 Hz. These specimens are then analysed via scanning electron microscopy (SEM) to determine failure mechanisms. SEM shows that, as temperature increased from room temperature to 649 °C, the fracture mechanism transitions from transgranular to intergranular. The fracture mechanism is shown to transition from intergranular to transgranular at elevated temperatures as da/dN increases as a result of growing ΔK. Scanning electron microscopy shows that, as frequency decreases from 20 to 0.33 Hz at 649 °C, the fracture mechanism transitions from transgranular to intergranular.

Microstructural Effects on the Mechanical Properties of ATI 718Plus® Alloy

Four microstructural variants of ATI 718Plus® alloy (718Plus) have been investigated to elucidate the effects of grain size, precipitate size, morphology, and phase fraction (δ and γ′) on mechanical properties such as low cycle fatigue (LCF) life, fatigue crack growth rate (FCGR) properties, and dwell FCGR behavior at both 649°C and 704°C under 100 s dwell and nondwell conditions. Similar tests have also been performed on Waspaloy in two comparative microstructural conditions. LCF test results demonstrate that all four microstructural conditions of 718Plus have superior life compared with Waspaloy under all investigated test conditions. FCGR results show that, at both test temperatures, all microstructural conditions of 718Plus and Waspaloy exhibit identical behavior in the steady-state regime, except that 718Plus exhibits a much higher threshold stress intensity (ΔKTH). However, the dwell FCGR results show that Waspaloy displays better steady-state crack growth resistance under dwell conditions. However, with a thermal exposed precipitate microstructure, 718Plus shows considerable improvement in this response.

Improved test method for very low fatigue-crack-growth-rate data

Currently, in North America, the threshold crack-growth regime is experimentally defined by using ASTM Standard E647, which has been shown in many cases to exhibit anomalies due to the load-reduction (LR) test method. The test method has been shown to induce remote closure, which prematurely slows down crack growth and produces an abnormally high threshold. In this paper, the fatigue-crack growth rate properties in the threshold and near-threshold regimes for a titanium alloy, Ti-6Al-4V (STOA), are determined by using the LR test method and an improved test method. The improved method uses ‘compression–compression’ precracking, as developed by Pippan, Topper and others, to provide fatigue-crack-growth rate data under constant-amplitude loading in the near-threshold regime, without load-history effects. Tests were conducted over a wide range in stress ratios (R = 0.1–0.7) on compact C(T) specimens for three different widths (25, 51 and 76 mm). The slitting method was used on 51 mm C(T) specimens to confirm that the material did not contain significant levels of residual stresses from forming and/or machining. A crack-mouth-opening-displacement gage was used to monitor crack growth. Data from the ASTM LR method gave near-threshold values that were found to be dependent upon the specimen width. However, data from the compression precracking constant amplitude (CPCA) loading method gave near-threshold data independent of specimen width. A crack-closure analysis was performed for both the LR and CPCA data, to correlate data at the various stress ratios. The CPCA data correlated well with the effective stress-intensity factor range against rate relation, whereas the LR data exhibited significant threshold fanning with both stress ratio and specimen width.

Fatigue Crack Growth Behavior of Metastable Austenitic Stainless Steel in Cryogenic High Magnetic Field Environments

This article studies the fatigue crack growth in a metastable austenitic stainless steel in cryogenic high magnetic field environments. Fatigue crack growth tests were performed with the compact tension (CT) specimens at liquid helium temperature (4 K) in magnetic fields of 0 and 6 T, and the crack growth rate data were expressed in terms of the J-integral range during fatigue loading. The J-integral range values were evaluated using an elastic-plastic finite element analysis. The measurement of martensite phase in the test specimens and the fractographic examination were also carried out. The high magnetic field effect on the fatigue crack growth rate properties at 4 K is discussed in detail.

Predicting fracture and fatigue crack growth properties using tensile properties

The safe-life assessment of components requires information such as the plane stress ( K c ), plane strain ( K Ic ), part-through fracture toughness ( K Ie ), and the fatigue crack growth rate properties. A proposed parametric/theoretical approach, based on an extended Griffith theory is used to derive fracture toughness properties and generate fatigue crack growth rate data for a range of alloys. The simplicity of the concept is based on the use of basic, and in most cases available, uniaxial stress–strain material properties data to derive material fracture toughness values. However since the methodology is in part based on an empirical relationship a wide ranging validation with actual data is required. This paper uses steel, aluminum and titanium based alloys from a pedigree database to quantify material properties sensitivity to the predictions for K Ic and K c and the subsequent estimation of Δ K th threshold and the Paris constants, C and n values. A sensitivity analysis using experimental scatter bounds show the range of d a/d N predictions can be achieved. It is found K Ic /Δ K th ratios designated as α has a range of 5–25 irrespective of tensile ductility, ε f , and is insensitive to it. The value of Δ K th for all the alloys considered was found to be proportional to the final elongation, ε f , and an empirical relationship describing Δ K th as a function of ε f was established. Furthermore it is suggested that, with the knowledge of appropriate tensile properties and the estimated range of K Ic /Δ K th ratios for the different alloys applying this method could be an appropriate tool that can be used to conservatively predict fracture and fatigue in similar alloy categories. Thus helping to reduce costs and optimize the number of experimental tests needed for alloy characterizations.

Fatigue‐crack growth properties of thin‐walled superelastic austenitic Nitinol tube for endovascular stents

Over the past 10 years, the supereleastic nickel-titanium alloy Nitinol has found widespread application in the manufacture of small-scale biomedical devices, such as self-expanding endovascular stents. Although conventional stress/strain-life (S/N) analyses are invariably used as the primary method for design against fatigue loading and for predicting safe lifetimes, fracture mechanics-based methodologies provide a vital means of assessing the quantitative effect of defects on such lifetimes. Unfortunately, fracture mechanics studies on fatigue in Nitinol are scarce, and most results do not pertain to the (thin-walled tube) product forms that are typically used in the manufacture of endovascular stents. In the current work, we document the basic fatigue-crack growth properties of flattened thin-walled ( approximately 400 microm thick) Nitinol tubing in a 37 degrees C air environment. Crack-growth behavior is characterized over a wide range of growth rates ( approximately 6 orders of magnitude) and load ratios, that is, as a function of the alternating and maximum stress intensities, at 50 Hz. Limited experiments at both 5 and 50 Hz were also performed in 37 degrees C air and simulated body fluid to determine whether the cyclic frequency affects the fatigue behavior. Fatigue-crack growth-rate properties in such thin-walled Nitinol tube are found to be quite distinct from limited published data on other (mainly bulk) product forms of Nitinol, for example, bar and strip, both in terms of the relative fatigue thresholds and the variation in steady-state growth rates.

FATIGUE CRACK GROWTH UNDER MIXED‐MODE I AND II LOADING

Abstract— Mixed‐mode fatigue crack growth has been studied using four point bend specimens under asymmetric loads. A detailed finite element analysis provides the stress intensity factors for curved cracks under different mixed‐mode load conditions. Both fatigue crack growth direction and crack growth rate are studied. The maximum tangential stress and the minimum strain energy density criteria were found to provide satisfactory predictions of the crack growth directions. An effective stress intensity factor was used to correlate the fatigue crack growth rates successfully. It is found that the use of mode I fatigue crack growth rate properties results in a conservative crack growth rate prediction for mixed‐mode load conditions.

Damage tolerance of wrought alloy 718 Ni- Fe-base superalloy

The influence of a modified heat treatment (MHT) and the standard heat treatment (SHT) on the damage tolerance of alloy 718 turbine disk material has been studied over a range of temperatures— from room temperature to 650 °. The influence of these heat treatments on creep, low-cycle fatigue (LCF), notch sensitivity, cyclic stability, and fatigue crack growth rate (FCGR) properties has been studied. The microstructure developed through the MHT sequence is shown to be damage tolerant over the temperature range studied. Shot peening leads to a marked improvement in the LCF crack initiation life of the MHT material relative to the SHT material at 650 °. Serrated grain boundaries formed through controlled precipitation of grain-boundary 5 phase are beneficial to elevated- temperature FCGRs. The S-phase precipitates formed at an angle to the grain boundaries do not make the material notch sensitive.

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.