Damage Accumulation Behavior Research Articles

Investigation of creep damage accumulation under variable loading conditions in nickel-based single crystal superalloys

Nickel-based single crystal (SX) turbine blades typically experience varying load conditions during operation. It is essential to consider the effects of changing stress and temperature when evaluating creep damage. The commonly used time-fraction method, based on the linear damage rule (LDR), has been demonstrated to be inconsistent with experimental results under certain loading conditions. There is a lack of research on damage accumulation under variable loading conditions specifically for SX superalloy. This paper conducted two-stage creep experiments for SX superalloy DD6 at 980 °C with stress levels of 230 MPa and 320 MPa, and at 1050 °C with stress levels of 135 MPa and 230 MPa. The load for the experiment was set based on the typical operating conditions of the SX turbine blades. The evolution of the microstructure under variable loading was observed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The result of creep experiments revealed that the damage accumulation behavior of the SX superalloy does not conform to the LDR. Furthermore, in two-stage loading tests, the creep damage accumulation behavior differs from the classical sequence effect of fatigue loading. SEM and TEM analyses suggested that this phenomenon might be associated with the varying degradation rates of microstructures in SX superalloy.

Applicability evaluation of damage evolution models in simulating repeated low-velocity impact behavior of composite laminates

This study aims to provide a valuable reference for selecting appropriate damage evolution models (DEMs) when simulating the behavior of repeated low-velocity impacts (LVIs). Four DEMs are systematically evaluated in simulating mechanical response, dissipated energy, damage evolution, and accumulation under three repeated impacts. 3D Puck failure criteria and various DEMs combined with element characteristic lengths associated with physical fracture modes are used to predict repeated LVI damage behavior. Delamination behavior is simulated by zero-thickness cohesive elements. Two damage indexes are proposed to quantify and compare the damage distribution and accumulation. The post-peak softening and LVI behavior predicted by four DEMs are compared and verified through representative volume elements simulation and LVI experiments. The findings reveal that DEMs have varying degrees of influence on the mechanical response, damage evolution, damage distribution, and accumulation under repeated impacts. The exponential models proposed by P. Maimı et al. and Matzenmiller et al. are more suitable to simulate mechanical response or damage accumulation behavior under repeated impacts than the linear damage evolution model due to their non-linear post-peak softening characteristics. In contrast, Linde model shows brittle behavior due to the rapid damage accumulation during repeated impacts, rendering it unsuitable for repeated impact simulation.

Application of ductile fracture model for the prediction of low cycle fatigue in structural steel

The complexity of the loading conditions makes it challenging to accurately predict the damage evolution and resulting ductile fracture in materials and structures. This paper presents an application method to predict fracture initiation for low cycle fatigue of 4-point bending in structural steel SS275. The swift isotropic hardening law was used in the numerical simulation to account for large strain deformation. The damage indicator framework with the Hosford-Coulomb (HC) model was adopted to characterize the damage accumulation and fracture behavior. Uniaxial tension tests on specimens with various fracture modes were conducted to determine the material properties required for calibration of the plasticity model and fracture model param1eters. Non-linear finite element analyses were performed to identify fracture model parameters based on the loading path of fracture initiation. The presented HC model was applied to an extremely low fatigue test (less than 100 cycles) and demonstrated good accuracy in predicting fracture initiation for complex loading scenarios.

Fatigue damage evolution and failure of pre‐corroded aluminum alloy 7075‐T651 under air and corrosion environments

AbstractThis paper presents characterization for fatigue failure analysis of precorroded AA7075‐T651 under air and exfoliation corrosion (EXCO) environments, with respect to damage accumulation, macrocrack and microcrack behavior via 3D digital image correlation (DIC), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). It is shown that that corrosion pits induce fatigue cracking with different nucleation times, and locations. It is furthermore shown that while the corrosive environment accelerates crack formation, it significantly hinders fatigue crack propagation at low stress amplitudes due to crack closure and tip passivation. Five dominant fatigue microcrack initiation types (pit bottom, corrosion tunnel, corrosion micropit, corrosion jut‐in, and chapped feature) and relevant mechanisms were identified, related to corrosion pit macro–micro morphology, material microdefect, local material grain structure, and loading condition.

Evaluation of tensile fatigue behavior of carbon fiber reinforced polymer modified by single‐wall carbon nanotubes

AbstractThe fatigue behavior of epoxy‐based carbon fiber reinforced composite with additional modification by single‐wall carbon nanotubes (SWCNT) has been investigated. Neat carbon fiber reinforced polymer and four types of specimens with different SWCNT weight content were tested: 0.1%, 0.2%, 0.3%, and 0.5 wt%. To evaluate damage accumulation and fatigue behavior of open‐hole specimens, digital image correlation was implemented. The strain fields were obtained at predefined lifetimes and the stiffness evolution during cyclic tension‐tension loading was analyzed. Composites were studied using scanning electron microscopy to reveal peculiarities of structure, damage, and fracture. Achieved improvement of fatigue properties was discussed in terms of enhanced interlaminar strength and matrix stiffness. The addition of 0.2% of CNT increases the ultimate tensile strength by 7.3%, failure strain by 7.5%, while the elastic modulus is nearly unaffected. The fatigue durability is increased by 3.4 times.

Prediction of transverse crack multiplication of CFRP cross-ply laminates under tension-tension fatigue load

This study aims to predict the transverse crack accumulation behavior of CFRP cross-ply laminates for evaluation of their remaining life. The tension-tension fatigue tests of CFRP cross-ply laminates were used to investigate the relationship between transverse crack accumulation behavior and the number of cycles of the fatigue tests. We also studied the damage accumulation behavior, including delamination of cross-ply laminates under fatigue loading, and it was indicated that the damage accumulation behavior depends on the maximum load of the fatigue tests. We proposed a method that can predict transverse crack accumulation behavior using a power-law between transverse crack density growth rate and the energy release rate associated with single transverse crack formation. As a result, the proposed method can estimate the applied number of cycles on CFRP cross-ply laminates.

Failure evaluation of neutron-irradiated SiC/SiC composites by underwater acoustic emission

Understanding the statistical properties of mechanical properties of non-irradiated and neutron-irradiated SiC/SiC composites is essential for component design. This study aims to evaluate the detailed damage accumulation behavior of composites focusing on two fracture parameters: proportional limit stress (PLS) and acoustic emission (AE) onset stress. The developmental underwater AE technique, which is benefit in non-contact in-situ failure monitoring method during mechanical testing and in handle of the irradiated material, was first applied to evaluate damage accumulation behavior. Two types of chemical vapor infiltration SiC/SiC composites were used: one reinforced with Hi-Nicalon Type-S SiC fiber and one reinforced with Tyranno-SA3 SiC fiber in the form of satin-woven 2D fabrics with pyrolytic carbon interface. Neutron irradiation in the High Flux Isotope Reactor at Oak Ridge National Laboratory reached a fluence of 30 dpa at a temperature of 620–670°C. Four-point flexural tests were conducted to evaluate post-irradiation strength. Weibull statistics did not suggest marked degradation of composite strength. Detailed failure behavior evaluated by AE demonstrated no irradiation-induced change of the AE onset stress (i.e., crack initiation equivalent stress). Failure probability analysis suggests that increasing the reliability of composites (i.e., the Weibull modulus rather than strength itself) is essential to expanding the design margin and benefiting from a probabilistic design approach.

Damage analysis of a SiCf/SiC ceramic matrix composite under stepwise fatigue loading with acoustic emission

Continuous fiber-reinforced ceramic matrix composites (CMCs) exhibit different damage mechanisms at multiple scales under cyclic loading. In this paper, the tension-tension fatigue behavior of a plain woven SiCf/SiC CMC was investigated, and damage accumulation and evolution process were studied in detail via acoustic emission (AE) method. With the increase of cycles, the material exhibits obvious hysteresis behavior affected by interfacial slip and wear mechanisms. Most of the fibers with radial fracture characteristic have relatively high strength, showing excellent toughening property. In the stepwise cyclic loading process, the Kaiser effect of AE determines the initiation of AE activities at each initial loading moment, which shows obvious nonlinear damage accumulation behavior of the material. High-energy events are related to significant matrix cracking and fiber fracture, and the evolution process of material damage initiation and propagation is monitored in real time.

Effect of heat treatment on mechanical properties of carbon-fiber-reinforced themoplastic

In this study, the bending properties and damage accumulation behavior of carbon-fiber-reinforced thermoplastics (CFRTP) were investigated to understand the effect of heat treatment on crystallization. Further, three-point bending tests using the acoustic emission technique were performed. Results indicated that a short period of heat treatment increased the CFRTP bending strength; however, for an excessively long heat treatment (30 h), the CFRTP bending strength decreased. To understand cause for the increase and decrement in the bending strength via heat treatment, surface observations using a laser microscope and thermal property measurement using differential scanning calorimetry were performed. Results showed that the 30-h heat-treated specimens exhibited fiber-matrix debonding before bending tests because of matrix shrinkage caused by crystallization and thermal degradation. Fourier transform infrared spectroscopy analysis revealed that the cause of deterioration was oxidation, and oxidation cleaved the molecular chains and embrittled the matrix. Consequently, we could control the bending properties by crystallization, and the relationship between crystallization and strength agreed with the formulated constitutive equation. However, we must address the thermal degradation when the materials are subjected to heat treatment.

Wind Turbine Lifetime Control Using Structural Health Monitoring and Prognosis

In wind energy operation and maintenance costs significantly contribute to the overall cost. This paper proposes a novel adaptive lifetime control approach for wind turbines to reduce operation and maintenance costs. The approach is based on a cascade structure with the outer loop utilizing structural health monitoring and prognosis techniques to determine suitable controller parameters and reference values of the inner loop. The trade-off between power production and load reduction is balanced to achieve predefined service lifetime using the knowledge of current system state-of-health and predicted future damage accumulation behavior. Unscheduled downtime is avoided by guaranteeing the predefined lifetime, hence reducing the maintenance cost. Simulation results using a reference wind turbine model show that the proposed control strategy can regulate the lifetime or the accumulated damage to the desired value with a reasonable sacrifice in harvested power.

A Modified Model for Nonlinear Fatigue Damage Accumulation of Turbine Disc Considering the Load Interaction Effect

Fatigue damage accumulation theory is one of the core contents in structure fatigue strength design and life prediction. Among them, the nonlinear damage model can overcome the shortcomings of the linear damage model, which takes the loading sequence effect into account. Besides, the loading interaction cannot be ignored for its profound influence in damage accumulation behavior. In the paper, some commonly-used methods of the linear and nonlinear fatigue damage accumulation theory are investigated. In particular, a modified nonlinear fatigue damage accumulation model which considers the effects of loading sequences as well as loading interactions on fatigue life is developed, and a load interaction parameter is obtained by analyzing damage models which assumes that the load logarithm ratio between adjacent stress levels can characterize this phenomenon. Finally, the modified model is employed to predict the fatigue life of high pressure turbine disc. Moreover, comparison is made between the experimental data as well as the predicted lives using the Miner’s rule, the Ye’s model, and the modified model.

Fatigue damage behavior in carbon fiber polymer composites under biaxial loading

An investigation into the damage accumulation and propagation behavior in carbon fiber reinforced polymer (CFRP) composites under complex in-phase biaxial fatigue loading has been conducted. The goal is to capture early stage damage and obtain an improved understanding of damage propagation and associated degradation in material properties. Both cross ply and quasi isotropic laminate configurations have been studied and the tests were conducted under constant amplitude in-phase biaxial loading. An optimization technique was used to design the cruciform specimens for each stacking sequence. To understand the propagation of damage from the micro-to the macroscale, the fractured surfaces were analyzed, during various stages of fatigue, using electron microscope assisted fractography and a high-resolution camera. Material property degradation was determined by measuring the change in specimen stiffness to analyze the progression of fatigue damage and is correlated to the micro- and macroscale damage mechanisms and the biaxial fatigue loading parameters. The results provide insight into the initiation and propagation of damage mechanisms in CFRP composites which is essential to understanding the fatigue behavior of composite materials under complex multiaxial loadings.

A New Cumulative Fatigue Damage Rule Based on Dynamic Residual S-N Curve and Material Memory Concept

This paper introduces a new phenomenological cumulative damage rule to predict damage and fatigue life under variable amplitude loading. The rule combines a residual S-N curve approach and a material memory concept to describe the damage accumulation behavior. The residual S-N curve slope is regarded as a variable with respect to the loading history. The change in slope is then used as a damage measure and quantified by a material memory degeneration parameter. This model improves the traditional linear damage rule by taking the load-level dependence and loading sequence effect into account, which still preserves its superiority. A series of non-uniform fatigue loading protocols are used to demonstrate the effectiveness of the proposed model. The prediction results using the proposed model are more accurate than those using three popular damage models. Moreover, several common characteristics and fundamental properties of the chosen fatigue models are extracted and discussed.

Cracking evolution behaviors of lightweight materials based on in situ synchrotron X-ray tomography: A review

Damage accumulation and failure behaviors are crucial concerns during the design and service of a critical component, leading researchers and engineers to thoroughly identifying the crack evolution. Third-generation synchrotron radiation X-ray computed microtomography can be used to detect the inner damage evolution of a large-density material or component. This paper provides a brief review of studying the crack initiation and propagation inside lightweight materials with advanced synchrotron three-dimensional (3D) X-ray imaging, such as aluminum materials. Various damage modes under both static and dynamic loading are elucidated for pure aluminum, aluminum alloy matrix, aluminum alloy metal matrix composite, and aluminum alloy welded joint. For aluminum alloy matrix, metallurgical defects (porosity, void, inclusion, precipitate, etc.) or artificial defects (notch, scratch, pit, etc.) strongly affect the crack initiation and propagation. For aluminum alloy metal matrix composites, the fracture occurs either from the particle debonding or voids at the particle/matrix interface, and the void evolution is closely related with fatigued cycles. For the hybrid laser welded aluminum alloy, fatigue cracks usually initiate from gas pores located at the surface or sub-surface and gradually propagate to a quarter ellipse or a typical semi-ellipse profile.

A probabilistic analyzed method for concrete fatigue life

In this manuscript, a novel analyzed method is proposed for stochastic analysis of concrete fatigue life. Starting from the material randomness and the typical fatigue damage accumulation behavior, a newly developed stochastic fatigue damage model (SFDM) is introduced to calculate the fatigue life. Then, a sensitivity analysis towards SFDM is carried out, based on which, the random model parameters representing the concrete fatigue mechanisms are verified. Based on the collected test data of different material strength levels, the probabilistic distributions of the random model parameters are identified, and these are used for the subsequent probabilistic analysis of concrete fatigue life. To implement the probabilistic analysis, a probability density evolution equation is developed by employing the probability density evolution method (PDEM). Through solving this equation, the probability density functions (PDF) of random concrete fatigue life and the corresponding mean and variance as well as their evolution with different loading levels are obtained.

Investigation of Cumulative Fatigue Damage Through Sequential Low Cycle Fatigue and High Cycle Fatigue Cycling at High Temperature for a Type 316LN Stainless Steel: Life-Prediction Techniques and Associated Mechanisms

Cumulative fatigue damage under sequential low cycle fatigue (LCF) and high cycle fatigue (HCF) cycling was investigated at 923 K (650 °C) by conducting HCF tests on specimens subjected to prior LCF cycling at various strain amplitudes. Remnant HCF lives were found to decrease drastically with increase in prior fatigue exposure as a result of strong LCF–HCF interactions. The rate of decrease in remnant lives varied as a function of the applied strain amplitude. A threshold damage in terms of prior LCF life-fraction was found, below which no significant LCF–HCF interaction takes place. Similarly, a critical damage during the LCF pre-cycling marking the highest degree of LCF–HCF interaction was identified which was found to depend on the applied strain amplitude. In view of the non-linear damage accumulation behavior, Miner’s linear damage rule proved to be highly non-conservative. Manson’s damage curve approach, suitably modified, was found to be a better alternative for predicting the remnant HCF life. The single constant (β) employed in the model, which reflects the damage accumulation of the material under two/multi-level loading conditions is derived from the regression analysis of the experimental results and validated further.

Macrodamage Accumulation Model for a Human Femur.

The objective of this study was to more fully understand the mechanical behavior of bone tissue that is important to find an alternative material to be used as an implant and to develop an accurate model to predict the fracture of the bone. Predicting and preventing bone failure is an important area in orthopaedics. In this paper, the macrodamage accumulation models in the bone tissue have been investigated. Phenomenological models for bone damage have been discussed in detail. In addition, 3D finite element model of the femur prepared from imaging data with both cortical and trabecular structures is delineated using MIMICS and ANSYS® and simulated as a composite structure. The damage accumulation occurring during cyclic loading was analyzed for fatigue scenario. We found that the damage accumulates sooner in the multiaxial than in the uniaxial loading condition for the same number of cycles, and the failure starts in the cortical bone. The damage accumulation behavior seems to follow a three-stage growth: a primary phase, a secondary phase of damage growth marked by linear damage growth, and a tertiary phase that leads to failure. Finally, the stiffness of the composite bone comprising the cortical and trabecular bone was significantly different as expected.

Quantifying grain boundary damage tolerance with atomistic simulations

Grain boundaries play a pivotal role in defect evolution and accommodation within materials. Irradiated metals have been observed to form defect denuded zones in the vicinity of grain boundaries. This is especially apparent in nanocrystalline metals, which have an increased grain boundary concentration, as compared to their polycrystalline counterparts. Importantly, the effect of individual grain boundaries on microstructural damage tolerance is related to the character or structural state of the grain boundary. In this work, the damage accommodation behavior of a variety of copper grain boundaries is studied using atomistic simulations. Damage accumulation behavior is found to reach a saturation point where both the free volume and energy of a grain boundary fluctuate within an elliptical manifold, which varies in size for different boundary characters. Analysis of the grain boundaries shows that extrinsic damage accommodation occurs due to localized atomic shuffling accompanied by free volume rearrangement within the boundary. Continuous damage accumulation leads to altered atomic structural states that oscillate around a mean non-equilibrium state, that is energetically metastable. Our results suggest that variation of grain boundary behavior, both from equilibrium and under saturation, is directly related to grain boundary equilibrium energy and some boundaries have a greater propensity to continually accommodate damage, as compared to others.

저주기 피로부하에서 F82H 강 TIG 용접 접합부의 피로손상거동

Reduced activation ferritic/martensitic steels are recognized as the primary candidate structural materials for fusion blanket systems. Welding is an inevitable for breeding blanket for pressure tightness and radioisotope confinement. Especially, TIG welding was chosen for sealing because it has the largest gap allowance compared to the other welding methods, and its properties are controllable by feed wire and welding conditions. In this study, the low cycle fatigue test using two-type gage such as extensometer and strain gage was applied to the TIG welded joint of F82H steel, for evaluating fatigue damage accumulation behavior of the HAZs. As the result, the over-tempered HAZ have shown a higher fatigue damage accumulation compared with other materials at all the testing conditions.

Texture, Second-Phase Particles, and the Anisotropy of Deformation Behavior in TTMP AZ61

The deformation, damage accumulation, and fracture behavior of thixomolded and thermomechanically processed AZ61 were studied by room temperature uniaxial straining. The material was examined in as-molded, as-rolled, and rolled + annealed conditions. The yield strength and work hardening behavior are controlled primarily by grain size and crystallographic texture, which reaches a maximum of only 4.8 multiples of random density in the as-rolled material. The low degree of texture results in a low degree of plastic anisotropy (Lankford r value), suggesting good formability properties. In addition, it is shown that although the β-phase particles begin to crack as early as 4 pct elongation, their presence does not lead to a decrease in ductility, as β-phase cracks do not coalesce to initiate fracture.