Rational design of artificial micro-structured metamaterials with advanced mechanical and physical properties that are not accessible in nature materials is challenging and important. In our paper, ...mechanical designs of 2D and 3D chiral mechanical metamaterials are reviewed, and their mechanical behaviors and deformation mechanisms can be investigated through equilibrium principle, strain energy analysis, micropolar elasticity and homogenization theories. Afterwards, multifunctional properties of chiral mechanical metamaterials are elaborated, such as: vibration attenuation, impact energy absorption and negative coefficient of thermal expansion (CTE). Finally, several successful industrial applications of chiral mechanical metamaterials are demonstrated, such as: morphing airfoil smart deployable antenna and reconfigurable structures, auxetic stent, chiral flexible electronics and phase transforming metastructures, etc. Finally, perspectives and challenges on chiral mechanical metamaterials are discussed.
Rational design of artificial micro-structured metamaterials with advanced mechanical and physical properties that are not accessible in nature materials is challenging and important. In the past several years, making use of the node rotation and ligament bending deformation features of chiral elements, various types of 2D and 3D chiral mechanical metamaterials are designed and proposed for industrial application. In our paper, mechanical designs of 2D and 3D chiral mechanical metamaterials are reviewed, and their mechanical behaviors and deformation mechanisms can be investigated through force and momentum equilibrium principle, strain energy analysis, micropolar elasticity and homogenization theories. Afterwards, multifunctional properties of chiral mechanical metamaterials are elaborated, such as: vibration attenuation and bandgap features, impact energy absorption and negative coefficient of thermal expansion (CTE). Finally, several successful industrial applications of chiral mechanical metamaterials are demonstrated, such as: morphing airfoil with chiral core configuration, shape memorial smart deployable antenna and reconfigurable structures, auxetic stent for biomedical application, chiral flexible electronics and phase transforming metastructures with shape switching abilities, etc. Display omitted
•Design of chiral mechanical metamaterials are reviewed, theoretical models are elaborated for exploring the deformation mechanisms.•Multifunctional mechanical benefits and limitations of chiral mechanical metamaterials are reviewed and discussed.•Industrial applications of chiral mechanical metamaterials are reviewed, perspectives and challenges are discussed.
•A modified generalized local model considering multiaxial stress and stress gradient is developed.•A novel probabilistic framework for notch fatigue analysis considering size effects is ...established.•Probabilistic fatigue life prediction of TA19 notched specimens with different scales are conducted.
Structural integrity assessments with discontinuities are critical for ensuring operational life and reliability of engineering components. In this work, through combining with the generalized local model, a probabilistic framework is proposed for fatigue life assessment of notched components under size effects, in which the Smith–Watson–Topper damage parameter is utilized to characterize the multiaxial stress state at the notch tip. In particular, an effective stress concept is introduced to characterize the inhomogeneous stress distribution within the notch region. Finally, experimental data of TA19 notched specimens with different scales are utilized for model validation and comparison, results show that the proposed framework yields acceptable correlations of predicted fatigue lives with experimental ones.
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•Combined critical plane-critical distance approach for notch fatigue analysis under multiaxial loadings is proposed.•Eight life assessment procedures with different coupling sequence of critical ...plane and TCD concepts are explored.•Procedures using the CPA before the TCD own better predictions than that after the TCD.•Procedures regarding the critical distance as a fatigue lifetime related function provides better correlations than others.
Combinations of geometric discontinuities and multiaxial loads appear commonly in engineering components, which raise both stress gradient and multiaxial stress states near the notch root. This work studies the combined critical plane approach with the theory of critical distance for fatigue analysis of notched components under multiaxial loadings. Taking the Fatemi-Socie model for instance, different coupling sequence of critical plane and critical distance concepts is discussed. In particular, the influence of employing the point method and the line method of the theory of critical distance on predicting performance as well as the rationality of regarding the critical distance as a material constant or as a function related to fatigue life are also investigated. Accordingly, 8 life assessment procedures are summarized and the optimal procedure is determined and verified by experimental data of Al 7050-T7451 and GH4169 alloys. Results show that the majority of the predicted points fall within the ±2 scatter band according to experimental results and the procedures which employ the theory of critical distance after using the critical plane approach and regarding the critical distance as a function related to fatigue life provide better accuracy on fatigue life prediction than others.
The mechanical properties of artery stent are of key importance to the mechanical integrity and biomechanical performance reliability of stent-plaque-artery system. In this paper, making use of ...auxetic deformation features of chiral structures and mechanical benefits of structural hierarchy, two types of innovative chiral stents with auxetic properties are proposed: (a) anti-tetrachiral stent with circular and elliptical nodes; (b) hierarchical anti-tetrachiral stents with circular and elliptical nodes. Firstly, the in-plane mechanical properties of anti-tetrachiral structures are investigated theoretically, and uniaxial tensile experiments are performed for verification; Secondly, design procedures of anti-tetrachiral stent and hierarchical anti-tetrachiral stent with circular and elliptical nodes are elaborated. Effects of stent geometrical parameters on the tensile mechanical behaviors of these stents are studied with finite element analysis (FEA). It is found that the mechanical behaviors of anti-tetrachiral stent can be tailored through adjusting the levels of hierarchical structures and unit cell design parameters. Finally, the deformation of anti-tetrachiral and hierarchical anti-tetrachiral stents during stenting process are investigated with FEA. It is found that the proposed anti-tetrachiral and hierarchical anti-tetrachiral stents exhibit remarkable radial expanding abilities while maintaining axial stability, thus show promising performances for practical clinical applications.
•A single crack propagation model was established to predict the crack propagation life which is in good agreement within in situ X-ray CT imaging of the crack front.•A synergistic multiple fatigue ...crack growth (smFCG) model was developed based on multiple defects measured a priori by X-ray CT to depict the competitive cracking effect.•Compared with the single crack model, the smFCG model predicts a shorter propagation life (by 5–10%) when multiple defects are involved.
Defects introduced during additive manufacturing currently control fatigue resistance and lead to a large scatter in lifetime, with pancake shaped lack of fusion (LOF) defects being particularly potent. In this study the fatigue crack propagation life of selective laser-melted (SLM) AlSi10Mg alloy is considered in cases where single cracks and multiple cracks can initiate from LOF defects under high cycle fatigue (HCF). Firstly, the aspect ratios of initially long fatigue cracks were determined for critical LOF defects obtained from X-ray CT renderings using the critical defect regularization method, and the response surface method used to obtain the stress intensity factor of the crack front quickly and continuously. Then a single crack propagation model considering the evolution of the crack aspect ratio established to predict the crack propagation life which is in good agreement within in situ X-ray CT imaging of the crack front when a single crack is dominant. The crack propagation phase was predicted to represent 35–60% of the total fatigue life representing a larger fraction at high stress amplitudes. Multiple cracks were found to initiate cracks at the larger stress amplitudes. In cases where multiple cracks arise this is non-conservative and so a synergistic multiple fatigue crack growth (smFCG) model was developed based on multiple defects measured a priori by X-ray CT to depict the competitive cracking effect. Compared with the single crack model, the smFCG model predicts a shorter propagation life (by 5–10%) when multiple defects are involved since it considers all the initial defects within the crack initiation region. Given the propensity of large numbers of defects in AM material this approach may be more appropriate in many cases.
•Deterministic and probabilistic methods are used to analyze a reactor pressure vessel.•Assuming shallower cracks can be more conservative than assuming deeper ones.•Master Curve methods are ...implemented in FAVOR for fracture toughness analysis.•Master Curve method is more realistic in modeling fracture toughness.•Warm prestressing effect decreases failure probability significantly.
Both deterministic and probabilistic methods are used to analyze a reference reactor pressure vessel (RPV) subjected to pressurized thermal shocks (PTSs). The FAVOR code was applied to calculate the probabilities for crack initiation and failure of a RPV subjected to two PTS transients, by considering different crack types, sizes and orientations. The Master Curve methods are implemented in the FAVOR code for a more realistic consideration of fracture toughness of the irradiated RPV.
The analysis shows that a postulated underclad crack is the most conservative crack assumption. Assuming shallower cracks can be more conservative than deeper ones due to the fact that both KI and KIC at the crack tip increase with crack depth. Considering the warm prestressing effect (WPS) reduces the failure probability by more than two orders of magnitude.
In this analysis, the FAVOR model for the calculation of fracture toughness is more conservative than the Master Curve method. But the Master Curve method is more realistic than the FAVOR model and thus its application is recommended.
•Linear elastic analysis leads to a more conservative result.•Both deep and surface points of the crack tip should be considered in the analysis.•During the MLOCA transient, constraint loss occurs ...for the shallow crack.•Safety margin of the RPV is increased based on the K–T approach.
The integrity analysis of a reactor pressure vessel subjected to pressurized thermal shocks is performed. Linear elastic analysis leads to a more conservative result than the elastic–plastic analysis if the warm prestressing effect is not considered. The stress intensity factor for the deepest point of a surface crack front is not always larger than that for a surface point, indicating that both the deepest and surface points of the crack front should be considered. The safety margin of the reactor pressure vessel is larger based on the K–T approach than that only based on a K approach.
In the lifetime prediction and extension of a nuclear power plant, a reactor pressure vessel (RPV) has to demonstrate the exclusion of brittle fracture. This paper aims to apply fracture mechanics to ...analyse the non‐uniform cooling effect in case of a loss‐of‐coolant accident on the RPV integrity.
A comprehensive framework coupling reactor system, fluid dynamics, fracture mechanics, and probabilistic analyses for the RPVs integrity analysis is proposed. The safety margin of the allowed RTNDT is increased by more than 16°C if a probabilistic method is applied. Considering the non‐uniform plume cooling effect increases KI more than 30%, increases the failure frequency by more than 1 order of magnitude, and increases the crack tip constraint due to the resulting higher stress. Thus, in order to be more realistic and not to be nonconservative, 3D computational fluid dynamics may be required to provide input for the fracture mechanics analysis of the RPV.
Highlights
Fluid dynamics and fracture mechanics methods are applied to integrity analysis of a RPV.
Non‐uniform cooling effect increases the crack tip constraint and KI.
Non‐uniform cooling effect increases the fracture frequency by 2 orders of magnitude.
Probabilistic fracture mechanics method predicts a less‐conservative result
•Subsurface microstructural fatigue crack initiation is a leading mode of HCF/VHCF failure of bainitic steels.•Multiple scenarios of micro-mechanisms are responsible for subsurface microstructural ...fatigue crack initiation.•Formation of fine granular layer depends on scenarios of micro-mechanisms of HCF/VHCF failure.•Retained austenite determines the development of localized cyclic plastic deformation in bainitic multiphase structure.
Advanced bainitic steels with the multiphase structure of bainitic ferrite, retained austenite and martensite exhibit distinctive fatigue crack initiation behavior during high cycle fatigue/very high cycle fatigue (HCF/VHCF) regimes. The subsurface microstructural fatigue crack initiation, referred to as “non-inclusion induced crack initiation, NIICI”, is a leading mode of failure of bainitic steels within the HCF/VHCF regimes. In this regard, there is currently a missing gap in the knowledge with respect to the cyclic response of multiphase structure during VHCF failure and the underlying mechanisms of fatigue crack initiation during VHCF. To address this aspect, we have developed a novel approach that explicitly identifies the knowledge gap through an examination of subsurface crack initiation and interaction with the local microstructure. This was accomplished by uniquely combining electron microscopy, three-dimensional confocal microscopy, focused ion beam, and transmission Kikuchi diffraction. Interestingly, the study indicated that there are multiple micro-mechanisms responsible for the NIICI failure of bainitic steels, including two scenarios of transgranular-crack-assisted NIICI and two scenarios of intergranular-crack-assisted NIICI, which resulted in the different distribution of fine grains in the crack initiation area. The fine grains were formed through fragmentation of bainitic ferrite lath caused by localized plastic deformation or via local continuous dynamic recrystallization because of repeated interaction between slip bands and prior austenite grain boundaries. The formation of fine grains assisted the advancement of small cracks. Another important aspect discussed is the role of retained austenite (RA) during cyclic loading, on crack initiation and propagation in terms of the morphology, distribution and stability of RA, which determined the development of localized cyclic plastic deformation in multiphase structure.
Gradient structure (GS) is commonly designed and processed in engineering materials to improve mechanical properties especially fatigue performance by taking advantage of the strengthened surface. ...However, whether the very-high-cycle fatigue (VHCF) property can be improved by GS is questioning due to the different crack initiation mechanisms between low-, high-cycle and VHCF. In this paper, GS of a Ti-6Al-4V alloy is generated by pre-torsion and characterized by electron backscatter diffraction. Then the VHCF behavior of the GS specimen is studied. The fractography and synchrotron radiation X-ray microtomography presented detailed characteristics of the internal crack initiation region in VHCF of the titanium alloy with GS. The results indicated that, in contrast to the low- and high-cycle regimes, the VHCF strength is reduced for the specimens with GS. Thus, the GS induced by pre-torsion cannot enhance the VHCF strength of the titanium alloy. This implies that VHCF test (property) is an important consideration for the microstructural designed materials. The graphical abstract is available in Supplementary information.
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