Fatigue Crack Growth Tests Research Articles

Microstructure and mechanical properties of crack - free Ni - based GH3536 superalloy fabricated by laser solid forming

Laser solid forming (LSF) was used to fabricate dense and crack-free specimens from solution-strengthened nickel-based superalloy GH3536 in this study. Tensile, and fatigue crack growth (FCG) testing was carried out in both horizontal and vertical build orientations with the condition at 25 °C and 500 °C. Three regions can be detected in the specimen: the additive manufacturing zone (AM), the heat affected zone (HAZ) and the substrate. M23C6 and M6C carbides were found in the specimens, only a small amount of hole defects was observed in the samples, and no cracks were found. The tensile test results show that the specimens have obvious anisotropy, and the horizontal specimens displayed higher strength (up to 855.3 MPa ultimate tensile strength at 25 °C). It is worth noting that there is no obvious anisotropy in the FCG rates of the specimens in different directions, and only when the ΔK is about 45–60 MPa m1/2, the FCG rates of the vertical specimens are higher. The increase in temperature will significantly reduce the tensile performance and increase the FCG rate of the specimens.

Couple effects of temperature and fatigue, creep-fatigue interaction and thermo-mechanical loading conditions on crack growth rate of nickel-based alloy

The ambient and high-temperature fatigue crack growth behaviors in C(T) and SENT specimens of Ni-based superalloy for turbine disk application were studied in a wide interval of temperatures 25–750°C using a combination a electro- and servohydraulic test systems and fractographic investigations. The fatigue, creep-fatigue interaction and thermo-mechanical in-phase fatigue (TMF IP) crack growth tests are performed under isothermal and dynamic waveforms loading conditions. The interpretation of the experimental results is given in terms of the traditional stress intensity factors and C-integral as well as new normalized cyclic fracture diagrams. It is found that there are definite temperature-sensitive regions separate for harmonic fatigue and creep-fatigue interaction loading conditions in which the crack growth rate of Ni-based alloy increases sharply. Scanning electron microscopy in longitudinal sections containing cracks revealed the mechanisms responsible for fatigue crack initiation and growth. The couple effect of temperature ranging and isothermal and dynamic waveforms loading conditions on fatigue life was discussed.

Creep, fatigue, and creep-fatigue crack growth behaviours of P92 steel at 600 °C

High-temperature components in power generation plant are exposed to creep, fatigue, and creep-fatigue environment during service. The components are usually under multiaxial state of stress condition. Understanding how the material behaves under these loading and environment is essential in order to sustain and keep the plant safe. The present paper aims to investigate the creep rupture and crack growth behaviours of P92 steel at 600 °C. For creep rupture test, notched bars with two different notch radii were prepared and tested under creep condition between 250 and 3500 hours at 600 °C, while the C-shaped specimen was prepared for fatigue and creep-fatigue crack growth tests. The material tested under creep condition showed notch strengthening effect where the life of notched bar specimen increased compared to smooth specimens when the net stress against creep time data was plotted. The effect was more significant as the notch radius decreased. It was also observed that the rupture life of all specimens was von-Mises stress controlled. Based on the fatigue test, it was found that the frequency in a range of 0.1 Hz–10 Hz was insignificantly affecting the crack growth rate. Under creep-fatigue, however, the material showed frequency-dependent behaviour. Observation on the fracture surface revealed that the ductile dimple associated with plasticity was dominant for all creep specimens. In addition, frequency independent specimen was associated with the transgranular fracture, thus flat appearance was evident, while fracture surface of frequency dependent specimen was roughly associated with intergranular fracture.

Comparison of wrought and additively manufactured IN718 concerning crack growth threshold and fatigue crack growth behaviour

Rotary components in turbine machinery such as gas turbine blades and discs or turbocharger wheels need to be designed so that high cycle fatigue (HCF) loads are well below the fatigue endurance limit to ensure safe operation. This requires an accurate measurement of the fatigue endurance limit and the related intrinsic fatigue crack growth (FCG) threshold. Furthermore, knowing the FCG threshold is crucial in order to assess the criticality of initial defects (e.g. cavities, carbide nests, etc.). In this study, the influence of the manufacturing route on the FCG threshold and the FCG behaviour at 650 °C in air of the nickel based superalloy IN718 is investigated. For this, a per Laser Powder Bed Fusion (LPBF) process manufactured version is compared to a conventionally wrought IN718 material state, both in the short and long crack regime. To measure crack growth increments of about 1 μm, an alternative pre-cracking and threshold test procedure is proposed for high temperature testing. To assess the behaviour of both material states in the short crack growth regime, cyclic R-curves have been generated taking the influence of two different R ratios into account. A further comparison of the FCG behaviour of the LPBF and the wrought material state is made on the basis of da/dN-ΔKI plots, with a focus on the Paris regime created from both classical FCG tests as well as continued threshold tests. The results observed are discussed in relation to the initial characterization of the two material states, which includes tensile tests at room temperature and at 650 °C as well as microstructural investigations.

Modelling of the transition from mode I to crystallographic crack growth in a single crystal gas turbine blade alloy under service-like loading conditions

In fatigue life prediction of single crystal gas turbine blades, the risk of rapid crystallographic crack growth along the close-packed planes poses a large uncertainty. A criterion is proposed to predict the transition from mode I to crystallographic crack growth, which is necessary for reliable prediction of the number of cycles from crack initiation to the onset of crystallographic crack growth. The proposed criterion is calibrated against tests performed under a wide range of conditions representative for a gas turbine blade, including isothermal fatigue crack growth tests and thermomechanical fatigue crack growth tests, some including hold times and pre-test aging.

The difference in fatigue crack growth induced by internal and external hydrogen in selective laser melted 304L stainless steel

The mechanical properties of selective-laser-melted (SLM) 304L steel under hydrogen charging and hydrogen environment were investigated by tensile and fatigue crack growth (FCG) tests. SLM samples showed higher mechanical properties and weaker hydrogen embrittlement compared with wrought samples, and the FCG rates of SLM 304L tested in a hydrogen environment were higher than hydrogen-charged samples. More obvious cleavage platforms and lamellar structures appeared and cracking tended to the multi-directional feature in the hydrogen gas environment. External hydrogen promotes slip planarity, deformation twinning, and plastic localization near the crack tip, and then causes the acceleration of FCG in SLM 304L steel.

Transition mechanism of cycle- to time-dependent acceleration of fatigue crack-growth in 0.4 %C Cr-Mo steel in a pressurized gaseous hydrogen environment

Fatigue crack-growth (FCG) tests were conducted in 90-MPa-hydrogen gas on three martensitic steels with tensile strengths of 811, 921 and 1025 MPa. Increased strength levels resulted in augmented, hydrogen-induced FCG acceleration. In the highest-strength material, the FCG rate per cycle was dependent on test frequency, i.e., the crack-growth distance was proportional to load duration. Several observations and analyses revealed that such time-dependent FCG was due to stress-driven cracking along hierarchical martensite boundaries, stemming from the hydrogen-induced degradation of their cohesive strengths as a result of competition between mechanically-determined crack-tip stress (driving stress) and statistically-distributed boundary strength (resistance stress).

Measurement-driven, model-based estimation of residual stress and its effects on fatigue crack growth. Part 2: Fatigue crack growth testing and modeling

This paper assesses the accuracy of fatigue crack growth (FCG) predictions for high-strength aluminum samples containing residual stress (RS) and complex two-dimensional cracks subjected to constant amplitude load. FCG predictions use linear-elastic, multi-point fracture mechanics. A first prediction includes RS estimated by the model described in Part 1; a second prediction includes RS measured by the contour method. FCG test data show a significant influence of RS. Ignoring the RS results in a +60% error in predicted FCG life (non-conservative). Including RS improves predictions of crack growth significantly (errors better than +26% (estimated RS) and −14% (measured RS)).

Effects of orientation on the fatigue crack growth behaviors of the ZK60 magnesium alloy in air and PBS

Strong anisotropic corrosion and mechanical properties caused by specimen orientations greatly limit the applications of wrought magnesium alloys. To investigate the influences of specimen orientation, the corrosion tests and (corrosion) fatigue crack growth tests were conducted. The rolled and transverse surfaces of the materials show distinct corrosion rate differences in the stable corrosion stage, but the truth is the opposite for the initial stage of corrosion. In air, specimen orientations have a significant influence on the plastic deformation mechanisms near the crack tip, which results in different fatigue fracture surfaces and cracking paths. Compared with R-T specimens, N-T specimens show a slower fatigue crack growth (FCG) rate in air, which can be attributed to crack closure effects and deformation twinning near the crack tip. The corrosion environment will not significantly change the main plastic deformation mechanisms for the same type of specimen. However, the FCG rate in phosphate buffer saline (PBS) is one order of magnitude higher than that in air, which is caused by the combined effects of hydrogen-induced cracking and anodic dissolution. Owing to the similar corrosion rates at crack tips, the specimens with different orientations display close FCG rates in PBS.

Development and implementation of a simultaneous fatigue crack growth test setup for polymeric hybrid laminates

Delamination is a common and highly relevant in-service failure mode of laminated structures. Cyclic mechanical loads superimposed with environmental factors, such as hot-humid air, are most critical. To evaluate the cyclic debonding resistance of polymeric hybrid laminates, fracture mechanics approaches based on double cantilever beam (DCB) specimens are well established. However, conventional fatigue test setups allow for measurement of just one specimen which is rather time consuming. The main objective of this paper was to develop and implement a novel fatigue test setup for simultaneous characterization of multiple DCB specimens. To validate the simultaneous fatigue testing system, steel/epoxy laminates of different number of steel plies and epoxy adhesive layers with or without filler were characterized using the novel fatigue test setup and a conventional electrodynamic test machine. Fatigue crack growth data was fitted by a form of the Hartman-Schijve crack growth equation. To derive a statistically significant threshold value a fitting procedure was employed. A good agreement of the crack growth kinetics data at ambient conditions was obtained confirming the high quality and reliability of the novel simultaneous fatigue test setup. Moreover, reliable data were gathered at the beginning of the displacement–controlled experiments in the instable crack propagation regime, as the displacement amplitude was reached from the first cycle onward. The novel test methodology is of special relevance for development and screening of adhesive formulations.

Fatigue growth rate of inclined surface cracks in aluminum and titanium alloys

In this paper the fatigue crack growth tests were carried out on surface-crack tension (SCT) specimens, made of 7050 and Ti6Al4V alloys, with initial semi-elliptical surface cracks. Pure Mode I conditions were realized on SCT specimens with crack plane located orthogonal to the loading direction, while Mixed-mode conditions were observed on SCT specimens with inclined crack. Optical microscope measurements and the crack mouth opening displacement (CMOD) method were respectively used to monitor crack length and calculate crack depth. Current crack shape during the tests was highlighted by alternation of loading spectrum with baseline load block and a marker load block. The stress strain field along the crack front of semi-elliptical cracks in the SCT specimens was assessed by Finite Element Method (FEM) analysis. The stress intensity factors (SIFs) were calculated along crack fronts and equivalent elastic SIF formulation was used for crack growth rate assessment under mixed mode conditions. As a result, the fracture resistance parameters of aluminum and titanium alloys were obtained for two crack propagation directions under Mode I and Mixed-mode loading. The benefits of using the computational and experimental results of SCT specimen for the assessment of the surface crack growth rate in aluminum and titanium alloys under Mixed-mode loading conditions were stated.

Mechanical performance improvement of Al–Cu–Mg using various thermomechanical treatments

A thermomechanical treatment (TMT) geared to stimulate solute atom clustering was devised to achieve a comprehensive balance of engineering properties in an Al–Cu–Mg alloy. The effect of the TMT on the microstructure and properties of an AA2524 like Al–Cu–Mg alloy were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and atom probe tomography (APT) along with tensile testing and fatigue crack growth testing. Our goal was to exceed threshold levels for fatigue crack growth (FCG) resistance, ductility and strength, using the T3 condition as a benchmark. During the TMT, dislocations and vacancies were introduced by solution treatment, quenching and pre-ageing, followed by asymmetric rolling. This enables short-circuit diffusive pathways that facilitate the formation of Mg–Cu co-clusters that effectively regulate the subsequent precipitation and influence mechanical properties. The contribution to critically resolved shear stress (CRSS) of these clusters of TMT alloys was significantly higher than the T3 state. It is proposed that Goss-grain orientations and shear textures introduced by the asymmetric rolling are also partly responsible for the enhancement of FCG resistance. The potential for solute clusters to contribute to fatigue properties is discussed in terms of the cyclic deformation shearing them, thus liberating their solute which can then bind with vacancies that would otherwise condense at the crack tip.

Effect of heat treatment on fatigue crack growth performance of AlSi10Mg aluminum alloy submitted to LPBF

Aluminum alloys are used in several industries, namely in the aeronautic and aerospace industries due to its good corrosion resistance and good mechanical strength/weight ratio. The optimization of fatigue crack growth resistance in AlSi10Mg aluminum alloy is a vital issue for safety-relevant components, which are designed to work for a large number of loading cycles before periodic inspections. An unclear subject is the benefit effect of heat treatments in the fatigue crack growth of AlSi10Mg aluminum alloy submitted to LPBF. Therefore, the current work purposes to analyze the effect of a heat treatment at low temperature on fatigue crack growth of AlSi10Mg aluminum alloy submitted to LPBF. Fatigue crack growth tests in Mode I are performed in compact tension (CT) specimens at two different conditions: as-build and heat treated at low temperature. The possible befits effect is analyzed, as well as the effect of the average stress on the fatigue crack growth. The crack closure parameter is studied in order to analyze the different fatigue crack growth behaviors, as well as the microstructure, hardness, and fractography. From this work, the main achievement is the microstructural transformations associated with the standard heat treatments (T6 and stress relived at 300 °C) are unfavorable to fatigue strength in the AlSi10Mg aluminum alloy submitted to LPBF, while the non-standard stress relief heat treatment, performed at a lower temperature, cause a benefic effect in the fatigue crack growth resistance.

Fatigue fracture tests on Al-Li 2198-T851 specimens under mixed-mode conditions

Aluminum-lithium alloys deliver numerous benefits for applications in which weight represents a key concern, thanks to their reduced mass density and enhanced mechanical properties compared to more traditional aluminum alloys. From the first generation of these alloys, significant anisotropy was observed in mechanical properties that was responsible of unpredictable failures during manufacturing. This paper reports an experimental campaign on the fracture anisotropy for the third generation aluminum-lithium alloy Al-Li 2198-T851. Particularly, fatigue crack-growth tests were carried out on the Middle Tension M(T) specimens with an initial central notch having different angles (30°, 45° and 60°) with the normal to the loading axis, in such a way to highlight the potential anisotropic characteristics of the material. Two types of specimen were manufactured: longitudinally (L) and transversally (T) with respect to the rolling direction of the sheets. crack gauges and strain gauges were glued on specimens to measure the crack-growth rates and central alignment of load during tests.A significant difference of the crack-growth rates was observed for the two types of specimen, while cracks propagated perpendicularly to the loading axis irrespective of the angle of the central notch. Additionally, larger scatter of paths was observed for (L) specimens, while (T) specimens presented crack paths almost perpendicular to the loading axis, suggesting that cracks tended to propagate between the grains of the material.

Methodology to evaluate stress corrosion cracking in ethanol environments, applied to circumferential welds on API 5 L steel pipelines

This work presents an experimental methodology developed to perform fatigue crack growth (FCG) and slow strain rate (SSR) tests in ethanol environments aiming to evaluate stress corrosion cracking (SCC) susceptibility of circumferential welds on steel pipelines. FCG and SSR specimens were machined from a welded pipe and the notches were properly designed to promote crack propagation in the different regions of the weld. Tests were carried out keeping the crack-tip region fully immersed in an ethanol solution, which was fueled by a circulation system to ensure replenishment and aeration throughout the test. When applied to a welded API X70 steel pipe, this experimental methodology proved to be an efficient and simple method to achieve relevant and important informations on environmentally assisted crack growth and SCC susceptibility. The method developed here is inserted in the aspects as follows:•Perform tests in slow strain rate and cyclic bend loading in circulating ethanol, to promote the fracture in the different regions of a circumferential weld joint of a steel pipe.•Investigate sensitivity to stress corrosion cracking (SCC) of these different weld regions in ethanolic environment.•This method presents constructive details of a suitable apparatus which the experiments can be easily replicated.

Mixed mode surface crack growth in aluminium alloys under complex stress state

In this study fatigue crack growth tests were carried out on hollow cylindrical specimens, made of D16T and B95AT aluminum alloys (analogue of 2024 and 7075 aluminum), with initial external semi-elliptical surface cracks and undergoing complex stress state. The crack growth behavior of aforementioned materials was studied under cyclic axial tension, pure torsion and combined tension+torsion fatigue loading. Optical microscope measurements and the crack mouth opening displacement (CMOD) method were respectively used to monitor crack length and calculate crack depth. The experimental crack front positions were highlighted by using a beach mark procedure during the tests. The stress strain field along the crack front of semi-elliptical cracks in the cylindrical hollow specimens was assessed by Finite Element Method (FEM) analysis. The stress intensity factors (SIFs) were calculated by the 3D M-integral approach and the distributions of equivalent elastic SIFs along the crack front were used for crack growth rate assessment under mixed mode conditions. As a result, the fracture resistance parameters of aluminum alloys under complex stress state were presented. The simulation of cracks propagation turned out to be consistent with experimental results.

3D FEA based surrogate modeling in fatigue crack growth life assessment

Runtime efficient models designed for damage tolerant life assessment are desired in Structural Health Management and Digital Twin development. While FEM is commonly used in the industry to assess health of a nominal structure design while in service, in probabilistic assessments, reduced order models are preferred due to lower runtime compared to the deterministic models but at the cost of solution accuracy. Readily available machine learning algorithms coupled with deterministic 3D simulations for modeling fatigue crack growth provide a feasible path to reach a better runtime-accuracy compromise. In this study, a fatigue crack growth testing procedure along with measurement data are used for validation purposes and for laying out details of the modeling process. Accuracy and solution runtime of the 3D FEA based surrogate models are assessed to demonstrate the efficiency of the method.

Automated crack length measurement for mixed mode fatigue cracks using digital image correlation

It is well known that mode II leads to a kinking and mode III to a twisting of a fatigue crack. On the surfaces of an SEN specimen, an overlapping of these two crack loading modes occurs. Superimposed with mode I, this results in very complex fatigue crack paths and, in some cases, multiple crack branches. Thus, established methods for mode I crack growth measurement cannot be utilized. Therefore, the digital image correlation (DIC) technique is used to determine the crack length of mixed mode cracks on the surface of a specimen by means of the maximum principal strain. By Gehri et al. (2020) a concept is described to obtain the crack path by morphological thinning of the area on the surface, where the maximum principal strain exceeds a threshold value. In this paper, an extension of the provided MATLAB script is proposed that includes two criteria for measuring the crack length versus the number of load cycles (a-N-curve). For the first criterion, the crack opening is used to determine the current crack tip position and to obtain the crack length as a function of the number of load cycles. In the second criterion, a threshold value of the maximum principal strain is used for determining the number of cycles at the detected crack length. The new evaluation routine with both criteria is initially calibrated on mode I experiments using the direct current potential drop method. Moreover, mixed mode experiments under tension-torsion loading are conducted, to validate the method and compare the two proposed criteria. The experimental results in terms of the a-N-curves obtained with the marker load technique are in good agreement with the results using the presented method. Thus, the extended tool provides an evaluation routine to determine a-N-data for mixed mode fatigue crack growth tests independently from the crack orientation and possible branching.

Evaluation of Fatigue Crack Propagation Rate at the Interface between Si Die and Underfill in Real Semiconductor Package Structures

In this study, fatigue crack growth rate at Si/UF interface was evaluated by four-point bending fatigue crack growth tests. At the bottom of the Si die, the fatigue crack growth behavior was similar to that of the UF bulk, in which the transition from the threshold to steady-state crack growth was observed. The power exponent of the Paris law in the steady-state region was similar to that of the UF bulk, and the fatigue crack growth rate was different between the side and bottom of the Si die. These results suggest that the fatigue crack growth behavior at the interface strongly depends on the surface roughness of the Si die and the manufacturing process of underfilling.

Macrostructures and mechanical properties in laser melting deposited titanium alloy plate at different thickness positions

Mechanical properties, including tensile properties and fatigue crack growth behaviors, and their relationships with macrostructures at different thickness positions in plates of laser melting deposited Ti–5Al–5Mo–5V–1Cr–1Fe, are experimentally investigated in this paper. It is observed that there display obvious differences in macrostructure features between overlap zones and non-overlap zones along thickness direction, which leads to anisotropies and heterogeneities of tensile properties including yield stress, tensile strength and total elongation. In fatigue crack growth tests with thin compact tension specimens at different thickness positions, crack paths are deviated from mode I with varying angles and fatigue crack growth rates display obvious fluctuations, which are mainly caused by the macrostructure features in overlap zone. In the end, evaluation methods are discussed for fatigue crack growth behaviors from the perspective of mechanics-materials interactive mechanisms, which helps strive for the future processing-material-structure-property-evaluation paradigm for additively manufactured metal structures.