Crack Incubation Research Articles

Fracture propagation characteristics of pre-cracked red clay in roadbed filling under three-point bending

Red clay was a special roadbed filling soil that was easy to expand and shrink because of the characteristics of high liquid limit and high porosity, leading to the appearance of a large number of cracks. In order to understand the cracking mechanism and nonlinear fracture failure process of red clay, the fracture toughness and crack sensitivity of red clay with different saturation, compaction and gravel content were analyzed by three-point bending cracking experiment and Image processing technology, and then the cohesive crack model was established to describe the fracture toughness. The results showed that the cracking process of red clay could be divided into three stages: crack incubation period, crack initiation and propagation period and visible cracks appearance stage. There was a conic function relationship between fracture toughness and saturation and a positive correlation between fracture toughness and compactness. The influence of gravel content on the final fracture path of red clay was the most significant, while the effects of saturation and compaction could be ignored. In addition, the fracture toughness converted from the cohesive crack model was basically as the same as the measured. This work provided a basis for theoretical analysis and numerical calculation for the fracture toughness of red clay under three-point bending.

Environment-Assisted Crack Initiation in Aluminum Alloys Studied by Local Probe Techniques

Environment-assisted crack initiation in aluminum alloys is strongly related to the physical and chemical reactivity of intermetallic particles (IMPs) whatever their location, i.e., at grain boundaries or in the matrix. Therefore, this paper first focuses on the most recent contribution of microscale local probe techniques to the study of processes occurring at the coarse IMPs that need to be taken into account in the initiation stage of environment-assisted cracking (EAC). A critical review of microscale electrochemistry, chemistry, and analysis of the influence of stress on IMPs leading to the microscale analysis of crack incubation is presented. Moreover, the contribution of hydrogen to the initiation of EAC remains a widely debated issue. Therefore, the second part of this paper more specifically reviews and summarizes the contribution of some specific local probe techniques to a better understanding of the contribution of hydrogen enrichment to environment-assisted crack initiation. The remaining challenges in future studies needed to fully elucidate hydrogen-assisted cracking mechanisms at the microscale are discussed.

Stress corrosion cracking behavior of 316 L SS in HF vapor environment based on corrosion degradation as a precursor

The stress corrosion susceptibility and stress corrosion cracking (SCC) behavior of 316 L SS in HF vapor was investigated through slow strain rate test (SSRT) and constant strain test. Results show that the SCC susceptibility of 316 L reaches almost 77% in HF vapor at 50 °C. The preferential reaction between F- and (Fe, Cr) elements results in granular corrosion product growing on surface and dealloying region forming near surface, which are important precursors for crack incubation. The {111}< 110 > slip system is prone to be activated under tensile loading, which tears granular corrosion product and dealloying region, causing transgranular SCC.

Development of a microstructural cohesive zone model for intergranular hydrogen environmentally assisted cracking

During the initial stages of hydrogen environmentally assisted cracking (HEAC), including crack incubation, initiation and microstructurally short cracking, the geometrical configuration of the microstructure greatly influences the crack growth behaviour. Therefore, there is a big incentive to generate a model which can replicate intergranular HEAC at a microstructural scale. This report provides a general framework to implement a microstructural intergranular HEAC model by using a cohesive zone approach in Abaqus. The parameters of the phenomenological model were fitted by using in-situ synchrotron tomography observations of crack initiation and propagation during HEAC of AA7449-T7651. After fitting the parameters, the real HEAC behaviour of the aluminium alloy 7449-T7651 has been replicated accurately. Several characteristic HEAC features were achieved, including crack segmentation, preferential cracking along grain boundaries with a high resolved normal stress and cracks slowing down at grain boundary triple junctions. Comparisons with experimental observations show the suitability of this approach for the prognosis of crack initiation and propagation at a microstructural scale under HEAC conditions.

Image-based multiscale modeling with spatially varying microstructures from experiments: Demonstration with additively manufactured metal in fatigue and fracture

This manuscript presents a novel modeling framework to predict mechanical performance in material with spatially varying microstructure. The framework combines an efficient process model for AM, a database of experimental 3D images of defects in AM metal, and a microstructure-based multiscale modeling method that leverages recent advances in reduced order modeling. Thus the examples presented will explore heterogeneous and processing dependent dispersion of voids in additively manufactured (AM) metals. The method presented here allows for parametric studies with repeated instantiations of different possible configurations of microstructures (images of defects) throughout the simulated part. Two demonstrations of the method are provided using a database of synchrotron x-ray computed tomography images of porosity collected at the Advanced Photon Source for Inconel 718 built with Laser Engineered Net Shaping®: one case is high cycle fatigue crack incubation and the other is fracture initiation. In both, we show that the model can capture the effects on performance of variability within and between builds. Although not all variability is captured or quantified, the method shows promise for application in AM metals because of its unique ability to mechanistically connect part-scale performance with individual microstructures and the distribution of these microstructures throughout the part.

Universal Material Constants for MultiStage Fatigue (MSF) Modeling of the Process–Structure–Property (PSP) Relations of A000, 2000, 5000, and 7000 Series Aluminum Alloys

A MultiStage Fatigue (MSF) model that admits different hierarchical microstructural features and their stereological information is used to predict the fatigue behavior of 17 different processed aluminum alloys: A000 series (A319, A356, A357, and A380), 2000 series (2024, 2055, 2099, 2198, 2297), 5000 series (5052, 5456), and 7000 series (7050, 7055, 7065, 7075, 7085, 7175). A single set of MSF model constants was validated for all of the aforementioned aluminum alloys, wherein the variation in fatigue life has been captured according to distinct microstructural features (pore size, pore nearest neighbor distance, porosity, particle size, grain size, crystallographic orientation, and misorientation) that differ arising from their native material processing method (casting, rolling, or extrusion). The MSF model’s total number of cycles distinguishes two different regimes: crack incubation (Inc) and Microstructurally Small Crack (long cracks are not considered herein). The previous MSF model in the literature had been associated with the pore size, pore nearest neighbor distance, porosity, particle size, and grain size, but a new contribution in this work is the contribution of the grain orientation and misorientation angles. We show that the MSF model now has the necessary and sufficient equations to predict the Process–Structure–Property relationships for aluminum alloys, allowing for expansion of fatigue prediction even beyond the alloys studied herein.

Microscopic defects formed during crack incubation, initiation and propagation processes causing hydrogen-related fracture of dual-phase steels

Factors promoting hydrogen embrittlement of dual-phase (DP) steels fractured in the elastic region have been investigated from the viewpoint of the relationship between hydrogen states present in the vicinity of a fracture surface and hydrogen embrittlement susceptibility. Hydrogen embrittlement susceptibility was evaluated using slow strain rate testing (SSRT) and constant load testing (CLT). The hydrogen states present in the vicinity of the fracture surface were analyzed using thermal desorption analysis (TDA). The results indicated that fracture strength significantly decreased only at higher hydrogen contents as the crosshead speed was reduced. Additionally, the hydrogen desorption profiles showed that the hydrogen content corresponding to each temperature peak markedly increased with decreasing crosshead speed at higher hydrogen content. Only specimens fractured at a lower crosshead speed at higher hydrogen contents showed both a lower-temperature peak and also a higher-temperature peak. Specimens fractured by CLT also showed a higher-temperature peak, which was detected before fracturing at the applied load for 20 h. This indicates that microscopic defects might have formed and grown to cause damage even in the elastic region. In addition, it was confirmed that microscopic defect formation changed depending on the hydrogen content, crosshead speed and loading time, and the states of defect formation significantly affected hydrogen embrittlement susceptibility.

Porosity and liquation cracking of dissimilar Nd:YAG laser welding of SUS304 stainless steel to T2 copper

In this study, we present experimental laser welding SUS304 stainless steel to T2 copper. The forming mechanism of fusion zone polygonal porosity and heat affected zone (HAZ) liquation cracking were systemically investigated and possible solutions to the welding process were also proposed. We deemed that the generation of HAZ liquation cracking mainly underwent three stages: crack incubation, crack initiation and crack growth. Formation of HAZ liquation cracking was closely related to precipitation of Cu-Fe compounds at grain boundaries and grain boundary liquation. The occurrence of porosity was determined by the keyhole instability correlated with fluid flow, keyhole free surface evolutions and composition segregation in a welding process. The susceptibility to HAZ liquation cracking can be effectively lowered by controlling the heat input during laser welding. In addition, the porosity in the fusion zone can be eliminated by reasonably adjusting laser deflection angle in the experiments, which greatly improved the microstructure and mechanical properties of welded joints.

Relevance of factor of safety based on number of cycles in the prediction of fatigue crack initiation as per A16 sigma-d approach

The focus of this paper is to bring out the conservatism present in the fast reactor design standard RCC MRx A16 in predicting fatigue crack initiation for a component that has a crack-like defect. Full-scale pipe bend test results were used for comparing the fatigue life predicted based on the RCC MRx A16 approach. A representative pipe bend of size 570 mm outer diameter and 15 mm thickness has been used for the purpose. The selected pipe bend is made up of austenitic stainless steel (SS 316 LN) and is provided with a surface notch 22.5 mm long and 2.1 mm in depth to simulate the potential flaw, which could be present in the piping system. Cyclic testing has been carried out to determine fatigue life. These tests were conducted in two stages. During the first stage, the finite-sized notch was sharpened into a crack-like defect allowing it to develop into a natural crack (85,150 cycles). In the second stage, the testing was continued to estimate the number of cycles required to advance the crack by > 50 μm and thereby compare prediction capability of the sigma-d approach given in the RCC MRx A16. The number of cycles required for crack incubation and advancement was obtained by periodic measurement of the measured readings obtained from the crack depth gauge. A comparison of the number of cycles was predicted by the RCC MRx A16 method, and the experimental results show that the A16 prediction was very conservative. Useful suggestions were proposed in this paper for improving the fatigue crack initiation prediction capability.

X-Ray Computer Tomography Study of Degradation of the Zircaloy-2 Tubes Oxidized at High Temperatures

ABSTRACT The investigations of high-temperature oxidation of zirconium alloys, applied for fuel pellets in nuclear power plants, are usually limited to oxidation kinetics, phase transformations and microstructural characterization. The purpose of this research was to characterize the degradation phenomena occurring within oxide layer and at the interface oxide/metal, on internal and external Zircaloy-2 tube surfaces, below and over crystalline transformation temperature of zirconium oxides. The commercial tubes were oxidized at 1273 K and 1373 K in calm air for 30 min and then examined with a technique novel for such purpose, namely a high-resolution X-ray computer tomography. The light microscopy was used to examine the cross-surfaces. The obtained results show that the form and intensity of oxide damage is significant and it is in a complicated way related to oxidation temperature and on whether external or internal tube surface is studied. The found oxide layer damage forms include surface cracks, the detachment of oxide layers, the appearance of voids, and nodular corrosion. The oxidation effects and damage appearance are discussed taking into account the processes such as formation of oxides, their phase transformation, stress-enhanced formation and propagation of cracks, diffusion of vacancies, formation of nitrides, diffusion of hydrogen into interface oxide-metal, incubation of cracks on second phase precipitates are taken into account to explain the observed phenomena.

Data-Driven Mechanistic Modeling of Influence of Microstructure on High-Cycle Fatigue Life of Nickel Titanium

A data-driven mechanistic modeling technique is applied to a system representative of a broken-up inclusion (“stringer”) within drawn nickel-titanium wire or tube, e.g., as used for arterial stents. The approach uses a decomposition of the problem into a training stage and a prediction stage. It is applied to compute the fatigue crack incubation life of a microstructure of interest under high-cycle fatigue. A parametric study of a matrix–inclusion–void microstructure is conducted. The results indicate that, within the range studied, a larger void between halves of the inclusion increases fatigue life, while larger inclusion diameter reduces fatigue life.

Crack incubation in shot peened AA7050 and mechanism for fatigue enhancement

AbstractShot peening is a dynamic cold‐working process involving the impingement of peening media onto a substrate surface. Shot peening is commonly used as a surface treatment technique within the aerospace industry during manufacturing to improve fatigue performance of structural components. The compressive residual stress induced during shot peening results in fatigue crack growth retardation, improving the performance of shot‐peened components. However, shot peening is a compromise between the benefit of inducing a compressive residual stress and causing detrimental surface damage. Because of the relatively soft nature of AA7050‐T7451, shot peening can result in cracking of the constituent precipitate particles, creating an initial damage state. The aim of this paper is to understand the balance and fundamentals of these competing phenomena through a comparative study throughout the fatigue lifecycle of baseline versus shot‐peened AA7050‐T7451. Microstructure and surface topology characterization and comparison of the baseline and shot‐peened AA7050‐T7451 has been performed using scanning electron microscopy, electron backscatter diffraction, energy dispersive spectroscopy, and optical profilometry techniques. A residual stress analysis through interrupted fatigue of the baseline and shot‐peened AA7050‐T7451 was completed using a combination of X‐ray diffraction and nanoindentation. The fatigue life performance of the baseline versus shot‐peened material has been evaluated, including crack initiation and propagation. Subsurface particles crack upon shot peening but did not incubate into the matrix during fatigue loading, presumably due to the compressive residual stress field. In the baseline samples, the particles were initially intact, but upon fatigue loading, crack nucleation was observed in the particles, and these cracks incubated into the matrix. In damage tolerant analysis, an initial defect size is needed for lifetime assessment, which is often difficult to determine, leading to overly conservative evaluations. This work provides a critical assessment of the mechanism for shot peening enhancement for fatigue performance and quantifies how incubation of a short crack is inhibited from an initially cracked particle into the matrix within a residual stress field.

Investigation of fatigue crack incubation and growth in cast MAR-M247 subjected to low cycle fatigue at room temperature

MC carbide particles (with Hafnium and/or Tantalum as constituent metallic element, M) were observed to crack extensively in a cast polycrystalline nickel-base superalloy, MAR-M247, when subjected to low-cycle fatigue loading at room temperature. High resolution secondary electron images taken on the surface of a double edge notch test specimen revealed that approximately half the carbide particles cracked in the highly-strained notch section of the specimen. These images further illustrated that the average surface area of cracked particles was approximately three times that of the uncracked particles. Additional analysis illustrated that the cracks within a large number of particles aligned nearly perpendicular to the loading direction. However, high aspect ratio particles (with aspect ratio $${>}3$$ ) were prone to incubate cracks aligned along its major axis, independent of the loading direction. Additionally, forward-scattered imaging often showed a high density of slip bands interaction with most of the particles which cracked. The life limiting crack growth in MAR-M247 was observed to be crystallographic in nature, as the crack grew along slip bands as measured by high-resolution electron backscatter diffraction, even after spanning many grains. Statistically representative microstructure models of MAR-M247 were generated and used in the crystal plasticity finite element simulations. As expected, there was a significant variation in the computed stress state among constituent carbide particles. The stress state of the carbide particles was found to be heavily influenced by the stress in surrounding grains and the orientation of the major axis of the particles with respect to applied load direction. For particles that intersect the free-surface, stress was found to be highly concentrated at the free surface and a positive correlation between the magnitude of free-surface area and the maximum principal stress was found. Additionally, high stress concentrations were observed in regions where carbide particles intersect grain boundaries.

Intragranular cracking as a critical barrier for high-voltage usage of layer-structured cathode for lithium-ion batteries

LiNi1/3Mn1/3Co1/3O2-layered cathode is often fabricated in the form of secondary particles, consisting of densely packed primary particles. This offers advantages for high energy density and alleviation of cathode side reactions/corrosions, but introduces drawbacks such as intergranular cracking. Here, we report unexpected observations on the nucleation and growth of intragranular cracks in a commercial LiNi1/3Mn1/3Co1/3O2 cathode by using advanced scanning transmission electron microscopy. We find the formation of the intragranular cracks is directly associated with high-voltage cycling, an electrochemically driven and diffusion-controlled process. The intragranular cracks are noticed to be characteristically initiated from the grain interior, a consequence of a dislocation-based crack incubation mechanism. This observation is in sharp contrast with general theoretical models, predicting the initiation of intragranular cracks from grain boundaries or particle surfaces. Our study emphasizes that maintaining structural stability is the key step towards high-voltage operation of layered-cathode materials.

Cyclic behavior and modeling of small fatigue cracks of a polycarbonate polymer

The fatigue behavior of a polycarbonate (PC) thermoplastic material was experimentally investigated and modeled using a MultiStage Fatigue (MSF) model that evaluates fatigue crack incubation, Microstructurally Small Crack (MSC) growth, and Long Crack (LC) growth. A set of fully reversed strain controlled tests were conducted, and an analysis of the fracture surfaces was performed using Scanning Electron Microscopy (SEM) in order to quantify the structure-property relationships for the MSF model. Fractography of the microstructure revealed that incompletely melted PC pellets were present in the polymer material that nucleated the cracks along with crazes generated on the surface. Crack lengths and fatigue crack growth rates for the MSC regime were measured from striation observations on the fracture surfaces. Discontinuous crack growth (DCG) cycles between fatigue striations, for the current iteration of the model, are accounted for by the MSF crack incubation regime. Finally, the microstructure sensitive MSF model was implemented using the observed fatigue crack growth measurements. In addition, a Monte Carlo (MC) Simple Random Sampling (SRS) routine was implemented to quantify the model uncertainty for crack growth.

A 3-D model for quantification of fatigue weak-link density and strength distribution in an A713 cast aluminum alloy

A quantitative model which took into account the 3-D effects of pores on the local stress/strain fields was developed to quantify the fatigue weak-link density and strength distribution in an A713 Al alloy. In the model, a digital pore structure was first constructed using single-sized pores (15μm in diameter) that had a total volume fraction same as that of the pores measured experimentally in the alloy. In the surface randomly selected by cross-sectioning through the simulated pore structure, the stress and strain fields around each pore in the surface were quantified using a 3-D finite element model under an applied cyclic stress, and the fatigue crack incubation life at the pore was estimated with a micro-scale Manson-Coffin equation. The quantified rate of fatigue crack initiation at these pores was found to be a Weibull function of the applied stress, which was consistent with the experimental result measured in the alloy. The density and strength distribution of fatigue weak-links could then be derived and used to evaluate the fatigue crack initiation properties of the alloy. By matching to the experimentally measured fatigue weak-links, the minimum critical pore size for fatigue crack initiation was determined to be 11μm in diameter at the cyclic maximum stress of 100% yield strength of the alloy.

Trans-scale analysis of composite overwrapped pressure vessel at cryogenic temperature

Composite over-wrapped pressure vessels (COPVs) are ideal structures for the gaseous helium vessels that is located within the liquid hydrogen (LH2) fuel tank. Their thermo-mechanical properties at cryogenic temperatures are critical to ensure the safety and reliability requirements. Most of the current analyses of thermal deformation only focus on the mismatch of thermal expansion coefficients between metal liner and composite winding layer, as the winding layers were usually assumed to be homogeneous material. However, the mismatch deformation between the reinforcing phase and the matrix phase is also significant to the micro-crack incubation, which is critical to the failure of structure. In this paper, a computational model of COPVs is developed based on the micro-structure of COPV. Finite element analysis is performed on a representative volume element based on the detailed structure of fiber/matrix to determine the effective elastic constants and thermal expansion coefficients at micro-scale. This micromechanical model gives higher prediction quality and more details about the thermo-mechanical deformation of COPV at cryogenic temperature. These microscopic details are critical to the future design of COPV structure with consideration of crack incubation.

Creep Stress Relaxation and Crack Incubation in the Presence of Structural Elastic Follow-Up

This paper explores the behavior of steel components operating at high temperature subjected to combinations of applied and residual stresses. Two series of experiments are summarized that used Type 316H stainless steel samples tested at 550 °C. First, creep stress relaxation under displacement control with elastic follow-up is explored. Experimental results are compared with theoretical predictions and show that accounting for stress history during relaxation is necessary. A second set of experiments explores how combinations of applied and residual stress influence creep crack incubation, again with elastic follow-up. Incubation times with elastic follow-up close to conventional displacement control are longer than constant load.

Small fatigue crack initiation and growth mechanisms of nickel-based superalloy GH4169 at 650 °C in air

The aim of this paper is to identify the fatigue crack initiation and small crack growth mechanisms of GH4169 at 650°C in air. Results show that fatigue cracks tend to initiate at oxidized inclusions. The crack incubation period takes up more than 90% of the fatigue lifetime. The δ phase can improve the crack initiation life and the total fatigue lifetime. The similarity between small crack and long crack growth data at relatively high ΔK can be attributed to the similarity of propagation mode. Two three-stage models are proposed to describe the cracks initiation and propagation processes.

Microstructure-sensitive small fatigue crack growth assessment: Effect of strain ratio, multiaxial strain state, and geometric discontinuities

Fatigue crack initiation in the high cycle fatigue regime is strongly influenced by microstructural features. Research efforts have usually focused on predicting fatigue resistance against crack incubation without considering the early fatigue crack growth after encountering the first grain boundary. However, a significant fraction of the variability of the total fatigue life can be attributed to growth of small cracks as they encounter the first few grain boundaries, rather than crack formation within the first grain. This paper builds on the framework previously developed by the authors to assess microstructure-sensitive small fatigue crack formation and early growth under complex loading conditions. The scheme employs finite element simulations that explicitly render grains and crystallographic directions along with simulation of microstructurally small fatigue crack growth from grain to grain. The methodology employs a crystal plasticity algorithm in ABAQUS that was previously calibrated to study fatigue crack initiation in RR1000 Ni-base superalloy. This work present simulations with non-zero applied mean strains and geometric discontinuities that were not previously considered for calibration. Results exhibit trends similar to those found in experiments for multiple metallic materials, conveying a consistent physical description of fatigue damage phenomena.