Fatigue Crack Nucleation Life Research Articles

Prediction of low-cycle fatigue properties of additive manufactured IN718 by crystal plasticity modelling incorporating effects from crystallographic orientations and defects

ABSTRACT In this work, wire arc additive manufactured IN718 materials with different textures and defect characteristics were obtained via two deposition strategies: parallel mode (P_mode) and oscillation mode (O_mode). The influences of these microstructure variables on the low-cycle fatigue (LCF) properties were systematically investigated by experimental and crystal plasticity finite element (CPFE) frameworks. Results revealed that the weak texture of the O_mode sample may be detrimental to LCF due to strain incompatibility between adjacent grains. However, large-sized pore defects in the P_mode samples significantly increased the dispersity of LCF life. The sensitivity of fatigue crack initiation to critical defect size and location was further accurately captured based on the stored energy density criterion, and the fatigue crack nucleation life was also quantified. Finally, the LCF limit was calculated based on the Coffin–Manson equation, and the life prediction intervals of WAAMed-IN718 alloys with different deposition strategies were unified.

An Overview of Self-Heating Phenomena and Theory Related to Damping and Fatigue of Metals

This paper presents an overview of the self-heating phenomena and the continuum thermodynamics framework related to the damping and fatigue of metals. The self-heating process under cyclic loading generally undergoes three phases: Phase I with gradually increasing temperature to a stabilized or steady-state in Phase II, followed by Phase III with an accelerated temperature increase until the test sample ruptures. Although energy dissipation and heat generation are all captured by the first law of thermodynamics, the functional form of the heat source(s) with entropy change is not formulated for engineering materials. Experimentally, infrared (IR) thermographic techniques can measure the surface temperature variation during constant-amplitude fatigue testing. The observed relationship between the stabilization temperature or temperature increase rate and the applied stress amplitude is often used to infer the fatigue endurance limit, above which point heat generation from “damage” leads to acceleration of self-heating. The IR thermographic fatigue testing offers a rapid alternative method to assess the material’s fatigue strength. But, the full physical interpretation of the phenomena remains a challenge. On the other hand, the Tanaka-Mura–Wu model is introduced to describe fatigue crack nucleation via accumulation of dislocation dipole pile-up, which provides a class-A prediction (forecast before even happening) for fatigue crack nucleation life in terms of the material’s elastic modulus, Burgers vector, surface energy, and the loading parameter such as cyclic stress/strain range. Then, the release of dislocation dipole pile-up energy to form new crack surfaces is brought into the energy argument. With the inclusion of crack formation energy in the first law of thermodynamics, a unified framework of deformation, damping, fatigue, and self-heating may be established for structural design.

Developing parametrically upscaled constitutive and crack nucleation models for the [formula omitted] Ti64 alloy

This paper develops Parametrically Upscaled Constitutive Model (PUCM) and the Parametrically Upscaled Crack Nucleation Model (PUCNM) for a commercially used α/β-phase Ti64 alloy. These thermodynamically consistent macroscopic constitutive models bridge spatial scales through the explicit representation of Representative Aggregated Microstructural Parameters (RAMPs). The PUCNM is an indicator of the probability of crack nucleation in the local underlying microstructure. A symbolic regression-based machine learning method operates on data-sets generated by image-based micromechanical crystal plasticity simulations to derive constitutive coefficients as functions of the RAMPs. The PUCM/PUCNMs development (both calibration and validation) uses data from microstructural characterization as well as mechanical tests, including constant strain-rate, creep, and strain-controlled dwell tests for the Ti64 alloy. Parametric study is conducted with the experimentally-validated PUCM and PUCNM to investigate the effect of RAMPs on dwell fatigue crack nucleation life. Finally, the combined PUCM/PUCNM tool is used for examining the impact of microstructure on fatigue crack nucleation in an engine blade model under simulated operating conditions. The results from these examples clearly exhibit the promise of the PUCM/PUCNM models in predicting fatigue crack nucleation in the microstructure of real structural applications, as well as demonstrating the microstructural influence.

Low-Cycle Fatigue Life of Continuously Cyclic-Hardening Material

Abstract A general fatigue life equation is derived by modifying the Tanaka-Mura-Wu dislocation pile-up model for variable strain-amplitude fatigue processes, where the fatigue crack nucleation life is expressed in terms of the root-mean-square of plastic strain range. Low-cycle fatigue tests were conducted on an austenitic stainless steel. At 400 °C and 600 °C, the material exhibits continuously cyclic-hardening behavior. The root-mean-square of plastic strain ranges is evaluated from the experimental data for each test condition at strain rates ranging from 0.0002/s to 0.02/s. The variable-amplitude Tanaka-Mura-Wu model is found to be in good agreement with the low-cycle fatigue (LCF) data, which effectively proves Miner's rule on the stored plastic strain energy basis.

Fatigue Behavior of Additively Manufactured IN718 with Columnar Grains

Fatigue failure of polycrystalline materials is often dominated by crack initiation processes, which are strongly influenced by salient features existing in the microstructure. Herein, a crystal plasticity framework based on the finite element method is used to quantitatively assess the mechanistic drivers existing in columnar IN718 polycrystals for fatigue crack nucleation. To that end, microstructurally differing polycrystalline samples are subjected to strain‐controlled cyclic loading at ambient conditions to elucidate the microstructural mechanisms of fatigue damage. 2D grain structures obtained from the cellular automata (CA) simulations of the additive manufacturing of IN718 are utilized to construct representative 3D microstructures for use in computational analyses. A criterion related to the stored energy density (SED) is used to predict the scatter in fatigue crack nucleation life. This criterion presents a unique and consistent approach toward predicting fatigue crack initiation at the microstructural scale. It is demonstrated that SED is necessary and sufficient to drive crack nucleation in low cycle fatigue. The results show that grain boundaries are the preferred crack initiation sites.

Numerical tools to investigate mechanical and fatigue properties of additively manufactured MS1-H13 hybrid steels

Additive manufacturing (AM) has been recently used to deposit metal powder on top of conventional metals. Of particular interest is hybrid additively manufactured steels which were found to be a suitable solution to benefit from features of each metal at different spots of a mechanical component. Due to its superior mechanical characteristics, maraging steel (MS1) has recently attracted tremendous attention for additive manufacturing applications mainly in aerospace, tool and die, and marine industries or to be 3D printed on top of other metals as a hybrid product using different techniques such as Direct Metal Laser Sintering (DMLS). In this paper a predictive finite element (FE) model and a combined analytical-numerical framework were developed to evaluate the mechanical performance of hybrid additively manufactured components and facilitate the prediction of hardness and fatigue life of these parts. The proposed tools were employed in two scopes: First to simulate the indentation hardness test of hybrid DMLS-MS1-H13 steels; and second to calculate fatigue crack nucleation life of maraging steel including defects (i.e. welding residual stresses). Parameters such as local and global displacements, changes in Young’s modulus, and hardness, high cycle fatigue life, welding temperature distribution, and residual stress were investigated. The hardness experiments were carried out to improve the reported data found in similar studies, which were used as the main resource to validate the proposed numerical framework. The capabilities of the presented frameworks enable this work to target existing ambiguities in additively manufactured mechanical components.

On Tanaka‐Mura's fatigue crack nucleation model and validation

AbstractTanaka‐Mura's fatigue crack nucleation model is revisited. A dimensional problem is found in their cyclic plastic strain formulation obtained by integration of the displacement function, and hence, it is rederived based on the true strain definition. Using the corrected strain formulation, a new fatigue crack nucleation life expression is obtained, and for the first time, low cycle fatigue lives of several metals and alloys are predicted without resorting to experiments. With such theoretical life as the baseline, other factors such as surface roughness, environment, and even high‐temperature damage mechanisms can be delineated in further studies.

Microstructure-sensitive fatigue crack nucleation in a polycrystalline Ni superalloy

Large-grained polycrystalline Ni alloy RS5 has been tested in fatigue. Morphology and texture have been characterised using EBSD and utilised to construct representative 3D finite element crystal plasticity models. A stored energy criterion has been used to predict scatter in fatigue crack nucleation life and the results compared with experimental findings. Good quantitative prediction of experimental fatigue lives is obtained. The observed progressive increase in scatter with decreasing strain range is captured. The stored energies for fatigue crack nucleation determined for Ni alloy RS5 and ferritic steel and were found to be 13,300J/m2 and 580J/m2 respectively, showing very good consistency with the corresponding Griffith fracture energies of 48,700J/m2 for Ni alloy and 1900J/m2 for ferritic steel.Local microstructural variations are shown to influence corresponding grain-level stress–strain response. At the microstructural level, purely elastic, reversed plastic and ratcheting behaviour are all observed. In addition, plastic and elastic shakedown are also found to occur which depend upon features of the microstructure and the nature of the applied loading. These phenomena all influence fatigue crack nucleation.

Microstructure-sensitive prediction and experimental validation of fretting fatigue

This paper is concerned with the development of a micro-mechanical methodology for prediction of fretting fatigue crack nucleation life and short crack propagation. The methodology employs critical accumulated plastic slip as a fatigue indicator parameter for microstructure-sensitive crack nucleation. Crystal plasticity unit-cell models are employed for calibration of constitutive and crack nucleation parameters and a crystal plasticity frictional contact model of the fretting test rig is developed to predict the micro-scale surface fretting damage, crack nucleation and estimated micro-crack propagation. The methodology is validated against fretting fatigue tests using a fretting bridge arrangement for 316L stainless steel.

3D analysis for pit evolution and pit-to-crack transition during corrosion fatigue

This paper presents a deterministic model to predict the pit evolving morphology and crack initiation life of corrosion fatigue. Based on the semi-ellipsoidal pit assumption, the thermodynamic potential including elastic energy, surface energy and electrochemical energy of the cyclically stressed solid with an evolving pit is established, from which specific parameters that control the pit evolution are introduced and their influence on the pit evolution are evaluated. The critical pit size for crack nucleation is obtained from stress intensity factor criterion and the crack nucleation life is evaluated by Faraday’s law. Meanwhile, this paper presents a numerical example to verify the proposed model and investigate the influence of cyclic load on the corrosion fatigue crack nucleation life. The corrosion pit appears approximately as a hemisphere in its early formation, and it gradually transits from semicircle to ellipsoid. The strain energy accelerates the morphology evolution of the pit, while the surface energy decelerates it. The higher the stress amplitude is, the smaller the critical pit size is and the shorter the crack initiation life is.

Corrosion and fatigue behavior of new Al alloys

The aircraft industry is constantly looking for improved materials which would offer benefits in terms of performance, weight and cost savings. The aim of this work was to evaluate the fatigue life of two new Al alloys: AA2198-T851 and AA7081-T73511 that are promising substitute of the base alloys AA 2524 T3 and AA7050 T745. It was considered the effect of a saline environment on the fatigue crack nucleation life of preconditioned specimens. The fatigue tests were conducted under air and pre-soaked conditions, with R = 0.1, 30Hz frequency and sinusoidal wave shape. Comparing with the base alloy, the AA2198 T851 presented an increased tensile strength, while the AA7081-T73511 alloy presented slightly decreased tensile strength. The corrosion test has showed that all the Al alloys studied presented similar corrosion resistance at saline environment. From the fatigue test it was possible to see the deleterious effect of the saline environment on the fatigue life of all alloys studied with pits formation intensively on the specimens’ surface, acting as stress concentrators reducing the fatigue life. The fracture surface evaluation has shown that, after soaking, fatigue failure took place from many nucleation sites.

SIMS analysis of low content hydrogen in commercially pure titanium

Due to their excellent specific strength and environmental resistance, titanium and its alloys have been widely used as structural components. However, titanium is very active, so that not only oxygen and nitrogen, but also hydrogen may be readily introduced into the material during manufacturing. Titanium usually contains a specific amount of hydrogen, and it exists in the titanium as a hydride or solid solution dependent on the hydrogen content. The hydrogen solid solubility limit of pure titanium (a-phase titanium) seems to be higher than 10 ppm [1], although some values are reported. That is to say, if the hydrogen content of pure titanium is less than 10 ppm, all the hydrogen is thought to be a solid solution in the titanium. On the other hand, when the hydrogen content is more than the solid solubility limit, hydrides would appear. As the hydride is brittle, it weakens the strength of the titanium. Therefore, the hydrogen content in commercially pure titanium (CP-Ti) is within 150 ppm [2]. The present authors carried out a fatigue strength evaluation of titanium samples using the hydrogen removed sample (hydrogen content is 2.7 ppm) and normal sample (34 ppm) [3, 4]. On the surface of the 2.7 ppm one, the hydride did not exist, while on the 34 ppm one, the hydride was present. It was found that the fatigue crack nucleation life of the 34 ppm one is greater than that of the 2.7 ppm one, and the fatigue crack propagation life of the 34 ppm one is the same as the 2.7 ppm one. This specific observation of the crack propagation indicated that the material behavior near the grain boundary is important for the crack propagation [3, 4]. It is then important to understand the actual state of hydrogen near the grain boundary for the CP-Ti applications. There are some methods to detect hydrogen in materials, and each method has some merits and demerits. The hydrogen thermal desorption analysis method (TDS and TDA) can be used for the average hydrogen content and energy state analysis [5]. As already mentioned, hydrogen in the CP-Ti exists as a solid solution or hydride, and the hydrogen localized around the grain boundary should be evaluated. The localized hydrogen can then be visualized by the following method.

A probabilistic methodology for fatigue life prediction

This paper describes efforts in the probabilistic modelling of fatigue. The methodology proposed deals with crack nucleation from surface defects within a fracture mechanics framework. The methodology provides a quantitative understanding of the effects of bulk defects and their size distribution on the variation of fatigue life. It also considers the contributions of the variations of fatigue crack nucleation life and of crack growth resistance to the variation of fatigue life. A PM Ni-based superalloy Udimet 720 was used to evaluate the probabilistic methodology by an analytical solution and a Monte Carlo simulation. The model predictions were found to be in good agreement with the experimental results. However, it is found that Monte Carlo simulation provides a closer agreement with the experimental life distribution than the analytical solution. This is because the Monte Carlo simulation removes some untrue assumptions which have to be assumed to obtain the analytical solution.

A RELIABILITY‐BASED MODEL TO PREDICT SCATTER IN FATIGUE CRACK NUCLEATION LIFE

This paper investigates the scatter inherent in the early stages of fatigue life. A probabilistic fatigue model is proposed which relates the microstructural heterogeneity to the scatter in crack nucleation life. The crack nucleation life is defined as the number of cycles necessary to develop a crack with a length equal to the grain size. The model assumes homogeneity at the level of the grain size. A fracture mechanics‐based microstructural model is used to describe the response of the grains. The primitive random variables which drive crack nucleation are identified and recent developments recorded in the literature are used to describe their statistical characteristics. First order reliability methods are used to predict the statistical distribution of fatigue crack nucleation life. Comparisons are made with trends in experimental observations.