Damage Evolution Behavior Research Articles

Thermal Fatigue Damage Evaluation of EB-PVD TBC Specimens - Its Potential and Problems

Thermal fatigue damage evolution behavior in thermal barrier coatings (TBCs) was studied, by employing the originally designed two dimensional ring-shape TBC specimen. The TBC specimen consisted of Ni-based superalloy IN738LC substrate, bond coat, and 8 wt.% Y2O3-stabilized ZrO2 (YSZ) top coat. The top coat was fabricated by electron-beam physical vapor deposition (EB-PVD) method with 250 micron-meters in thickness. Three kinds of MCrAlY bond coat alloys were specified as an experimental variable. Through the work, special attention was paid not only to the failure life of TBC specimen, but also to the underlying failure mechanisms. Some problems have been also pointed out, on feeding back these experimental findings to engineering applications.

3D-Simulation of ductile failure in two-phase structural steel with heterogeneous microstructure

The effort of this study is to develop a simulation method to predict the effect of micro-structural morphology in two-phase steel, Ferrite–Pearlite steel, on structural performance in terms of ductile failure resistance. This is based on a clarification of a damage mechanism to control the ductile cracking with focusing on the heterogeneity in strength of microstructure. The large number of micro-voids nucleation at lower strength side near two-phase boundary associated with the localization of stress/strain is found to control ductile cracking. According to this experimental result, we develop meso-scale 3D-model to reproduce micro-structural morphology of practical two-phase steel of interest for analyzing stress/strain localization behaviors associated with heterogeneity of microstructure. Moreover, damage evolution model including plastic potential to nucleate micro-void is proposed, so that ductile crack nucleation and subsequent growth and linking could be simulated. The ductile cracking behavior for two-phase structural steel is simulated by means of the developed meso-scale 3D micro-structural FE-model together with the damage evolution model. The effect of applied triaxial stress state on critical macro-strain for ductile cracking as well as damage evolution behavior, in which the critical strain could be decreased with increasing stress triaxiality, is numerically predicted.

A fatigue damage model for composite materials

AbstractComposite materials using polymer resins as matrices possess viscoelastic properties such that the fatigue behavior of the composite could be changed by different stress levels, stress ratio, stress frequency, or temperature. Based on a physical phenomenon of damage growth, this work develops a fatigue damage model including the nonlinear effects of stress ratio and stress frequency on the damage processes for carbon/epoxy composites. A damage index is defined and used to confirm the damage evolution behavior, and a series of fatigue tests of unidirectional specimens under monotonous loading and two‐stress level loading are conducted to test the proposed fatigue damage model. The results reveal that the proposed model could reasonably predict the fatigue life of composite materials under complicated loading conditions and it also includes the sequence effect of cyclic block loadings. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers

Damage Analysis of Glass-to-Metal Diffusion Welded Joints

The glass-to-metal seals are usually used in the solar thermal power (STP) and electronic devices. However, the requirement of mechanical properties in the STP is much higher than that of electronic devices, because the glass-to-metal joints used in the STP need to have anti-fatigue performance in adition to higher static tensile strength. Under the repeated fluctuating loads, damage and failures of glass-to-metal seals in the STP often lead to serious consequences. Therefore, analysis of damage evolution and fracture behavior of glass-to-metal diffusion welded joints was performed in this paper. Firstly, the finite element (FE) model of glass-to-metal welded joints was established in accordance with the STP welded structures. And damage simulation was carried out by the FE software ABAQUS. Also, the work illustrates the modeling of damage in terms of traction versus separation to simulate crack propagation and introduces the use of traction-separation law as a damage initiation and evolution criteria. The microgram of damage distribution in the glass side near the interface could be characterized by Scanning Electron Microscope (SEM), which was compared with predictions obtained by finite element method (FEM) analysis. As result, the damage criteria on the lap joints in conjunction with FM analysis were used to optimize the glass-to-metal diffusion welding technology. The above results provide the basis of design against damage and reliable estimation of glass-to-metal seals.

Fatigue Damage Model for Numerical Assessment of Fatigue Characteristics

The aim of this study is the development of a numerical technique applicable for the fatigue assessment based on the damage mechanics approach. The generalized elasto-visco-plastic constitutive equation, which can consider the internal damage evolution behavior, is developed in order to numerically evaluate the material fatigue responses. Explicit information of the relationships between the mechanical properties and material constants, which are required for the mechanical constitutive and damage evolution equations, are derived. The performance of the developed technique has been verified using the S-N relationship assessment for STS304 stainless steel.

Damage evolution behavior of CFRP laminates under post-impact fatigue with water absorption environment

In this study, the damage evolution behavior was evaluated considering the effect of the textile structure and water absorption. Damage observation was conducted by the integration of non-destructive and direct observation methods. Candidate textile reinforcements were T300-3k plain woven fabric (PW) and T700S-12k multi-axial knitted fabric (MA). The effect of water absorption on the performances of compression after impact (CAI) and PIF were small in PW CFRP laminates. Conversely, PIF properties of water-absorbed MA CFRP laminates drastically decreased than that of dry ones. CAI strength was not affected by water absorption. PIF performance of dry MA CFRP was fairly higher than that of the others. From the precise observation, some evidences of interfacial deterioration caused by water absorption were confirmed in both PW and MA CFRP laminates.

연속체 손상역학을 이용한 수치 피로시험 기법

Once assessment of material failure characteristics is captured precisely in a unified way, it can bedirectly incorporated into the structural failure assessment under various loading environments, based on the theoretical backgrounds so called Local Approach to Fracture. The aim of this study is to develop a numerical fatigue test method by continuum damage mechanics applicable for the assessment of structural integrity throughout crack initiation and structural failure based on the Local Approach to Fracture. The generalized elasto-visco-plastic constitutive equation, which can consider the internal damage evolution behavior, is developed and employed in the 3-D FEA code in order to numerically evaluate the material and/or structural responses. Explicit information of the relationships between the mechanical properties and material constants, which are required for the mechanical constitutive and damage evolution equations for each material, are implemented in numerical fatigue test method. The material constants selected from constitutive equations are used directly in the failure assessment of material and/or structures. The performance of the developed system has been evaluated with assessing the S-N diagram of stainless steel materials.

EXPERIMENTAL OBSERVATIONS OF THE FAILURE MECHANISMS IN A BULK METALLIC GLASS COMPOSITE

AbstractThe deformation and damage evolution behavior of a Ni-based bulk metallic glass composite reinforced with elongated brass phase is studied under cylindrical indentation. The estimated fracture toughness values based on the energy dissipation and the in-situ observation during the loading reveal the details of the damage evolution and toughening mechanisms in this composite system. The results indicate that the enhanced toughness of the BMG composite is plausibly an outcome of crack bridging mechanisms by the ductile brass phase, rather than a diffused array of nucleated shear bands in the hard BMG and arrest by the ductile reinforcing phase.

Influence of Phase Stability on Radiation Damage Properties: Plutonium-Gallium Alloys

AbstractModeling cascade and fission damage evolution of actinide materials of all kinds is essential for understanding their aging characteristics. As an example of how exotic some of the damage evolution behavior can be, plutonium-gallium (Pu-Ga) alloys in the δ-phase (fcc lattices) are explored. Aging emanates from the wide variety of spontaneous decay and fission products that, in the case of the Pu, are such species as helium (He) and uranium, among others, as well as interstitials, and vacancies. To aid in our understanding, the modified embedded atom method (MEAM) formalism is applied to the Pu-Ga-He system. The behavior of defects in the fcc (δ) phase of Pu-based materials is strongly influenced by the metastability of this phase. The influence of this metastability on minimum displacement threshold energy, point defect characteristics and He bubbles is delineated. The roles of short-range ordering and transformations of voids into stacking fault tetrahedra in the aging process are also examined.

Damage Mechanics Implementation of Material Design Simulator for Layered Ceramic Composites

The objective of this study is to establish a numerical technique applicable for the assessment of damage characteristics of laminate ceramic. A numerical simulator, which can be used for the optimal material design, was developed based on the damage mechanics approach. To evaluate the internal damage evolution behavior within composite material, generalized damage evolution equation was developed and implemented in way of finite element method. By virtue of an automatic data interface between the commercial FEA results and damage calculation, a simple and cost-effective damage evaluation system is constructed. It was found that the developed system can be used for the evaluation of the material performance of composite.

Microstructure dependent viscoplastic damage modelling of tin–lead solder

A theoretical framework describing the nonlinear mechanical behaviour of multiphase solders is introduced, along with its numerical implementation. Next to viscoplasticity, damage has been included in the model using a gradient-enhanced nonlocal damage description. Microstructural data are accounted for in the model by means of a phase field. In here, the methodology followed is generic and can be readily extended to numerous multi-phase materials. The model is applied to the eutectic Sn–Pb system whose mechanical properties are known to be heavily dependent on the underlying microstructure. The results indicate that coarser microstructures exhibit a more pronounced localisation behaviour compared to finer ones leading to higher local values for the stresses and viscoplastic strains. Furthermore, different damage evolution behaviour is observed for the investigated microstructures, with a coarser microstructure leading to earlier microstructural failure.

Detection of Micro-Damage Evolution of Air Plasma-Sprayed Y<SUB>2</SUB>O<SUB>3</SUB>-ZrO<SUB>2</SUB> Thermal Barrier Coating through TGO Stress Measurement

The micro damage evolution behavior in air plasma-sprayed 8 mass% Y2O3 stabilized ZrO2 thermal barrier coating (APS-TBC) after isothermal heat exposure at 1150 � C up to 200 h was observed. Residual stress of thermally-grown-oxide (TGO) layer was measured through the TBC layer using Cr 3þ luminescence spectroscopy and the measured stress levels were compared with micro damage evolution behavior stored in the TBC system. With heat exposure time of longer than 10 h, microfracture behaviors were noted in the TBC layer. In-plane residual stress of the TGO layer increased for up to 50 h of heat exposure time, and thereafter, diminished with further increase. These behaviors are strongly correlated with the thickness and the residual stress of the TGO layer. This decrease in the TGO stress well contrasts with the evolution of the micro fracture behavior, which is also related to the thickness and undulation of the formed TGO layer. The change of TGO stress, � tgo, depending on the average thickness of the TGO layer, �

Fracture Behavior and Water Migration in Heterogeneous and Porous Rocks

Based on the heterogeneous and porous characteristics of rock materials, a flow-stressdamage (FSD) model, implemented with the Rock Failure Process Analysis code (RFPA2D), is used to investigate the behavior of fluid flow and damage evolution, and their coupling action in rock sample that are subjected to both hydraulic and uniaxial compressive loading. A highly heterogeneous sample, containing grains, grain boundaries and weak zones, is employed in the numerical simulation. The simulation results provide a deep insight in the physical essence of the evolutionary nature of fracture phenomena as well as the fluid flow in heterogeneous materials, especially when they are highly stressed. The simulation result suggests that the nature of fluid flow and strength character in rocks strongly depends upon the heterogeneity of the rocks.

A Numerical Investigation of the Progressive Failure of Jointed Rock Slope Subjected to Transient Seepage

Disastrous rock slope failures have been posing a hazard to people’s lives and causing enormous economic losses worldwide. Numerical simulation of rock slope failure can lead to improve the degree of understand of such phenomenon so as to predict and avoid the occurrence of these disastrous events. In order to simulate the global behaviors of rock slope failure under the high seepage pressure and the local behaviors of the occurrence of hydraulic fracture in the pre-existing rock joints effectively, a powerful finite element tools F-RFPA2D, is adopted. The simulation takes into account of the growth of existing fractures and the initiation of new fractures under various of hydraulic pressure in different heterogeneities medium. The behavior of fluid flow and damage evolution, and their coupling action are studied in small specimens that are subjected to both hydraulic and biaxial compressive loadings. The influence of the ratio (the initial horizontal stress to the initial vertical stress) and the distance between the two existing cracks on the fracture propagation behaviors are investigated. Moreover, based on the fundamental study of hydraulic fracture, the progressive failure of rock slope under the influence of the increase in hydraulic pressure was also studied in the paper.

Development of Damage Assessment Method for Steel Structures Using Material-Structure Coupling Analysis

Once assessment of material failure characteristics is captured precisely in a unified way, it can be directly incorporated to the structural failure assessment under various loading environments, based on the theoretical backgrounds so called Local Approach to Fracture. The aim of this study is the development of an expert system applicable for the assessment of structural integrity throughout crack initiation and structural failure based on the Local Approach to Fracture. The generalized elasto-visco-plastic constitutive equation, which can consider the internal damage evolution behavior, was developed and employed in the 3-D FEA code in order to numerically evaluate the material and/or structural responses. Explicit information of the relationships between the mechanical properties and material constants, which are required for the mechanical constitutive and damage evolution equations for each material, was implemented in an automatic system using genetic algorithm based on an inference system. The material constants selected from genetic search and constitutive equations are used directly in the failure assessment of material and/or structures. The performance of the developed system has been evaluated for the S-N relationship assessment of several materials as well as the crack initiation assessment of various weldments in steel structures.

A Numerical and Experimental Study of Cavitation in a Hot Tensile Axisymmetric Testpiece

During axisymmetric hot tensile testing, necking normally takes place due to the thermal gradient and the accumulation of microdamage. This paper introduces an integrated technique to predict the damage and necking evolution behaviour. Firstly, a set of multiaxial mechanism-based unified viscoplastic-damage constitutive equations is presented. This equation set, which models the evolution of grain boundary (intragranular) and plasticity-induced (intergranular) damage, is determined for a free-cutting steel tested over a range of temperatures and strain rates on a Gleeble thermomechanical simulator. This model has been implemented using the CREEP subroutine of the commercial finite element (FE) solver ABAQUS. Numerical procedures to simulate axisymmetric hot tensile deformation are developed with consideration of the thermal gradient along the axis of the tensile testpiece. FE simulations are carried out to reproduce the necking phenomenon and the evolution of plasticity-induced and grain boundary damage. The simulated results have been validated with experimental tensile test results. The effects of necking and its associated stress state on flow stress and ductility are investigated. The flow stress and ductility data obtained from a Gleeble material simulator under various hot deformation conditions have also been numerically studied.

Predicting material strength, damage, and fracture The synergy between experiment and modeling

The Taylor cylinder impact test, the plane-strain tensile test, and explosively driven hemisphere test represent readily conducted experiments that probe the deformation, damage evolution, and fracture behavior of materials. Because these tests are very sensitive to large gradients of stress, strain, strain rate, and shock loading, we are using them to evaluate and validate the correctness of our mechanical models that are implemented and destined to be implemented into large-scale 3-D simulation codes.

High Aspect Ratio Microstructures in LiNbO3 Produced by Ion Beam Enhanced Etching

AbstractThis work presents data on damage evolution, volume expansion and etching behavior of LiNbO3 irradiated with Ar+-ions as a function of irradiation and etching conditions. Single crystals of x- and z-cut LiNbO3 were irradiated at room temperature and 15 K using Ar-ions with energies between 60 and 600 keV and ion fluences between 5 × 1012 and 1 × 1015 cm-2. The damage formation investigated with RBS channeling analysis depends on the crystal cut as well as on the irradiation temperature. Irradiation of z-cut material at 300 K causes complete amorphization at 0.4 dpa (displacements per target atom). In contrast 0.27 dpa are sufficient to amorphize the x-cut LiNbO3. Irradiation at 15 K reduces the number of displacements per atom necessary for amorphization to 0.18 dpa. To study the etching behavior ∼500 nm thick amorphous layers were generated via multiple irradiations with Ar+-ions. Etching was performed in HF-solution of different concentration and at different temperatures. The influence of the irradiation and etching conditions on damage formation and etching of LiNbO3 is discussed. In conclusion, negligible etching of the perfect crystal, high etching rates and high contrast of Ion Beam Enhanced Etching (IBEE) allow the realization of high aspect ratio microstructures in LiNbO3.

Tensile damage evolution behavior in plasma-sprayed thermal barrier coating system

Cracking and fracture behavior in a plasma-sprayed thermal barrier coating system were evaluated using a sandwiched specimen under uni-axial tensile stress combined with an in situ microscope observation. It was found that initial crack initiation and transverse crack propagation occurred in the top coat layer. The number of transverse cracks increased rapidly and then saturated with an increase in tensile strain. After the saturation of the multiple cracks, the cracks passed through the interface between the top coat and bond coat, and propagated in the bond coat. Finally, decohesion occurred at the bond coat/substrate interface when the transverse cracks reached the interface. A model was proposed to describe the fracture processes.

Influence of SiC and Al2O3 particulate reinforcements and heat treatments on mechanical properties and damage evolution of al-2618 metal matrix composites

This investigation is mainly aimed to study the influence of SiC and Al2O3 particles on the mechanical properties and damage evolution behaviors of an aluminum alloy Al-2618. Heat treatments for the composites are also studied to optimize their mechanical properties. The results of tensile tests show that SiC particulate reinforcement has advantages over Al2O3 reinforcement in both strength and ductility for the composites. T4 treatment is suggested for the composites rather than conventional peak-aging treatment (T6). T4 heat treatment with an additional of 0.6% pre-strain can result in same UTS and a 0.2% proof stress for the composites as high as T6 treatment but the final elongation under T4 treatment is larger than that under T6 treatment by more than 100%. Based on observation of damage evolution behaviors of the reinforcing particles, a theory that strength of the composites is mainly decided by the balance between reinforcing particles sharing load and making strain discontinuity in the matrix is proposed to interpret the test results. Their tolerance for large local strain at the interface, their high K1c and their low thermal expansion make SiC particles sharing much load and the better reinforcement over Al2O3 particles in respect to both strength and ductility of the composites.