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•The location-controlled cracks were obtained with parallel-hatching scan strategy.•The LAGB generation and cracking are two competing ways to release stress.•The nanopore weakening ...effect on GB strengths was calculated to be nearly 70%.•Geometric effects on crack and microstructure were revealed in multi-wall samples.
Cracking is a critical issue in the additive manufacturing of pure tungsten (W). To eliminate crack formation, it is imperative to gain an in-depth understanding of the underlying mechanisms behind this process. In this study, we systematically investigated the crack behaviors of single tracks, thin walls, and cubes fabricated using powder bed fusion–laser beam (PBF–LB) technology with nonrotational parallel-hatching scanning. The energy framework was employed to elucidate the mechanism of crack formation. The longitudinal cracks appearing in the microstructures of single tracks and the through cracks existing in thin walls and cubes were characterized. Notably, periodic through cracks extended upward across the sample, appearing at every single hatch in unidirectional samples and at every other hatch in bidirectional samples. The horizontal, longitudinal, and transverse cross sections of cubes were studied to clarify the correlation between through crack arrangement and solidification microstructure. Based on a comprehensive analysis of grain boundaries, we proposed a deformation-cracking competition mechanism in PBF–LB tungsten. Geometric effects in the crack and microstructure were also revealed. This study could provide valuable insights into the formation of cracks in PBF–LB tungsten and serve as a foundation for future investigations aimed at eliminating cracks.
A particular emphasis is placed on the thermal aged oxide/oxide ceramic matrix composites under cyclic thermal shocks, which exhibit hierarchical internal structures spanning multiple volume/scales. ...The present work focuses on establishing thermo-mechanical loading conditions-structure-property linkages for the thermal aged oxide/oxide ceramic matrix composites under cyclic thermal shocks. Firstly, the matrix micro-cracks and delamination were identified by SEM observation, and the compressive mechanical behaviors were studied after different cyclic thermal shock numbers. The thermal aging process resulted in apparent changes in thermal shock resistance of materials. Considering the material deterioration induced by the thermal aging process, aging-related factor was introduced to predict the compress behaviors of the thermal aged oxide/oxide ceramic matrix composites under cyclic thermal shocks. The correlation between porosity and strain energy release rate was also constructed, which provided an excellent agreement with the experiments.
•Cyclic thermal shock test are conducted on thermal aged CMCs for different aging conditions.•Variations of the hierarchical porosity in the matrix characterize the collective behavior of micro-structural evolution.•The thermal aged CMCs under cyclic thermal shocks show a rapid decrease in elastic modulus compared to the original CMCs.•Correlation between porosity and energy release rate is proposed for the thermal aged CMCs under cyclic thermal shocks.
•Evolution of the micro-porosity in the matrix can provide a more precise description of the overall thermal aging damage process.•The decrease of compressive strength after aging is the macroscopic ...reflection of the microstructure variation.•A damage model is used to describe the thermal aging-induced damage evolution in CMCs based on the micro-porosity increment.•The phenomenological damage model predicts a simple linear relation between the composites damage and the micro-porosity increment of the CMCs.
Long-term thermal aging is a typical factor affecting the thermo-mechanical fatigue life for hot-end components in the gas turbine. The present work focuses on the development of thermal aging-induced damage in 2-D woven oxide/oxide ceramic matrix composites from micro-mechanism and macroscopic mechanical performance. The porosity evolution and mechanical performance after long-term thermal aging were characterized through mercury intrusion measurements and uniaxial compressive tests, respectively. The results show that the decrease of micro-porosity directly reflects the irreversible evolution of material microstructure in the thermal aging process, and the decrease of compressive strength after aging is the macroscopic reflection of the microstructure variation. The porosity increment of matrix was thus used to characterize the thermal aging-induced damage, establishing a unique analysis model between the increment of micro-porosity under thermal aging and the corresponding degradation of material compressive strength. The experimental results are in good agreement with the established model.
•Developed multiscale models of an 8HS woven composite based on realistic geometry.•The crimp zone is the weakness place of composite as obvious stress concentrations.•Matrix cracking & fiber ...fracture are failure mechanisms of CMCs for axis tension.•The failure mechanisms for transverse tension are delamination & matrix cracking.
8-harness satin (8HS) woven composites display complex architecture and a certain feature of mutually orthogonal reinforcement, which arise particular interest for aerospace structural groups. In this study, based on realistic geometry parameters and material property, multiscale finite element models of an 8HS woven composite are established. The effective properties of fiber bundles can be obtained by the micro-scale model and then used in the meso-scale model. The meso-scale model is established to predict the global failure behavior of 8HS woven composites under tensile load using the periodic boundary conditions. The predict results are compared to experiments from tensile tests and microstructure analysis on oxide/oxide 8HS woven ceramic matrix composites. The good correlation of the comparison shows that the multiscale modeling approaches can accurately predict the mechanical properties and failure mechanism of 8HS woven composites under plane tensile load when considering the particular material features.
Using random variables to describe uncertain parameters in structural systems, its initial strength and the evolution process of the strength degradation is regarded as the Gamma process. In this ...article, we propose a new method on reliability sensitivity numerical analysis of mechanical structure based on Gamma processes. Then, we use the fourth moment method based on frequency curve of Pearson to solve the problem of reliability calculation with random parameters of arbitrary distributions. Formulas for calculating the reliability sensitivity with respect to the mean and the variance of the random variables are derived. The reliability analysis of the welded box girders of crane is taken as an example to verify the proposed method. The results show that the method can effectively solve the problem of the reliability sensitivity of structural systems with strength degradation.
•Developed a new continuum thermo-mechanical fatigue damage model for ox/ox-CMCs.•The model can predict damage evolution under coupled cyclic thermal shocks & mechanical loading.•The damage consists ...of the thermal stress-related damage and the mechanical fatigue damage.•Validated the fatigue damage model based on detail experiments.
In the present work, the tension-compression fatigue behavior of the 2-D woven oxide/oxide-ceramic-matrix composites is investigated. Experimental investigations reveal different fatigue failure mechanism for the original and the cyclic thermal shocked ox/ox-CMCs, and both materials undergo cyclic softening in the whole loading history. Based on the decrease of the elastic modulus, a new fatigue damage model is proposed to characterize the fatigue damage evolution and predict the fatigue life of both the original and the cyclic thermal shocked ox/ox-CMCs. The total damage is separated into the thermal stress-related damage from thermal shocks and the fatigue damage from applied mechanical loads. The good agreement between the proposed model and experimental data indicates that the damage model has the potential to describe complex thermomechanical damage of the ox/ox-CMCs under coupled cyclic thermal shocks and mechanical loading.
Oxide/oxide-ceramic matrix composites (ox/ox-CMCs) are excellent candidates for material applications in high-temperature and oxidizing environments, which exhibit complex couplings between the ...thermal and mechanical fields. Uniaxial tensile tests are conducted for 0°/90° and ± 45° fiber orientation of the thermal shocked ox/ox-CMCs. The study reveals that the anisotropy properties of the thermal shocked ox/ox-CMCs result in different degrees of thermomechanical damage, which are reflected in the degeneration of the modulus. Based on the detailed experiments and analysis, a cyclic thermal shock-induced thermomechanical damage model of the 2-D woven ox/ox-CMCs is proposed, incorporating two tensile scalar damage parameters, and one shear scalar damage parameter. The model will contribute to predicting the thermomechanical damage evolution throughout the composite structure under thermal transient conditions. In addition, the model was validated with numerical simulation to demonstrate the damage model performance, which is relatively crucial in damage tolerance analysis in the aeronautical industry.
•The cyclic thermal shocked induced thermo-mechanical damage in ox/ox-CMCs was investigated systematically.•Proposed a macroscopic thermo-mechanical damage evolution model of ox/ox-CMCs under cyclic thermal shock.•The model can predict the thermomechanical damage evolution of the composite's mechanical throughout the structure.•The internal mechanism of thermomechanical damage in ox/ox-CMCs through strain energy was rational physical interpreted.
The oxide/oxide ceramic-matrix composites exhibits superior mechanical properties and complex nonlinear behavior in a high-temperature environment. This paper developed a constitutive model utilising ...an elastoplastic modeling framework to represent the nonlinear mechanical response of the thermal shocked ox/ox-CMCs under off-axis tensile loading accurately. The model captures main features such as the effects of the thermal shock temperatures on material response. The main interest of the proposed model is the ability to predict the nonlinear mechanical response for thermal shocked ox/ox-CMCs, which is of importance for industrial applications. The validity of the constitutive model is verified by analysing the mechanical behavior of an open-hole specimen and comparing the simulation result with the experimental data.
•Monte Carlo simulation employed to deal with VHCF data sparsity.•A machine learning model with ease of implementation proposed for VHCF life prediction.•The model demonstrated good accuracy using a ...shallow neural network structure.
Few machine learning (ML) models were applied for very-high-cycle fatigue (VHCF) analysis and these methods encounter limitations in data sparsity and overfitting. The present work aims to overcome data sparsity and propose an easy-to-use and nonredundant ML model for VHCF analysis. Monte Carlo simulation (MCs) is run to enlarge dataset size and a ML method is proposed to investigate the synergic influence of defect size, depth, location and build orientation on Ti-6Al-4V. The coefficient factor that indicates the percentage variation between the predicted and experimental fatigue lives can reach up to 0.98, meaning that the model demonstrates good prediction accuracy.
•Established a quantified relation between thermal shock and damage in the CMC.•Introduced a damage evolution model for cyclic thermal shock for the CMC material.•Illustrated correlations between ...energy dissipation and material damage under cyclic thermal shock conditions.•Proposed a continuum damage model for thermomechanical fatigue of the CMC material.
Thermal shock is a typical loading case for high temperature components in turbines. The ceramic matrix composites become attractive for their high temperature behavior. However, their low heat conductivity makes thermal shock damage a typical damage mechanism for the turbine application. In the present paper, microstructural evolutions of the oxide/oxide ceramic matrix composite under thermal shock are studied experimentally. The mechanical behavior is related to the thermal shock temperature and cycles. The thermomechanical damage is characterized by degradation of the macroscopic elastic modulus. A thermal shock damage model is introduced based on the continuum damage mechanics principle, which can describe monotonic as well as cyclic thermal shock damage processes.
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