Failure Mechanisms Of Structures Research Articles

Molecular dynamics simulation of structural and mechanical features of a Polymer-bonded explosive interface under tensile deformation

Polymer bonded explosives (PBX) are kind of particulate-reinforced composite materials in which interface interaction is of great significance to its structural and mechanical features. In this work, effect of temperature and strain rate on the microstructure, mechanical properties and fracture damage mechanism of TATB-F2314 are studied using molecular dynamics simulations. The TATB layers at the TATB-F2314 interface are deformed, leading to a rough and undulate surface that facilitates the formation hydrogen bonds between TATB and F2314. Intermixing phase is characterized for the first time at the TATB-F2314 interface. The interfacial structures and mechanical properties of TATB-F2314 depend strongly on temperature and strain rate. F2314 experiences a ductile-to-brittle transition at its glass transition temperature, which exerts great influence on the structural evolution and failure mechanism of TATB-F2314. The fracture mainly appears on F2314 under a quasi-static or low strain rate tension but transfers to TATB layers near to the interfacial intermixing phase at a high strain rate. Our simulations reveal the effect of temperature and strain rate on the microstructure, mechanical behavior and fracture damage mechanism of TATB-F2314 interface, which is useful for the design, preparation and safe use of PBX.

Influencing Factors for the Instability and Collapse Mode of the Goaf Structure in a Gypsum Mine

Large-area goafs in a gypsum mine tend to collapse after 10 or more years, but the influencing factors are still unclear, and the effects of multiple factors have not been comprehensively considered. In this study, the failure mechanism and collapse mode of the room-pillar goaf structure were analyzed, and the uniaxial compressive strength tests of the pillars under different conditions were carried out in a laboratory. The influences of water, temperature, and time on the strength of the gypsum rock were considered. These three factors weakened the gypsum rock in different degrees. After 120 days of immersion, water had the greatest effect with a strength-weakening rate of 52.61%. After 20 temperature cycles, changes in temperature had little effect with a strength-weakening rate of 12.60%. After 25 years of aging, the strength-weakening rate of time was 25.13%. These results show how different factors affect the instability and collapse of the goaf structure, which are of great significance for predicting and preventing this from happening.

Simplicity versus accuracy trade-off in estimating seismic fragility of existing reinforced concrete buildings

This paper investigates the trade-off between simplicity (modelling effort and computational time) and result accuracy in seismic fragility analysis of reinforced concrete (RC) frames. For many applications, simplified methods focusing on “archetype” structural models are often the state-of-practice. These simplified approaches may provide a rapid-yet-accurate estimation of seismic fragility, requiring a relatively small amount of input data and computational resources. However, such approaches often fail to capture specific structural deficiencies and/or failure mechanisms that might significantly affect the final assessment outcomes (e.g. shear failure in beam-column joints, in-plane and out-of-plane failure of infill walls, among others). To overcome these shortcomings, the alternative response analysis methods considered in this paper are all characterised by a mechanics-based approach and the explicit consideration of record-to-record variability in modelling seismic input/demands. Specifically, this paper compares three different seismic response analysis approaches, each characterised by a different refinement: 1) low refinement - non-linear static analysis (either analytical SLaMA or pushover analysis), coupled with the capacity spectrum method; 2) medium refinement - non-linear time-history analysis of equivalent single degree of freedom (SDoF) systems calibrated based on either the SLaMA-based or the pushover-based force-displacement curves; 3) high refinement - non-linear time-history analysis of multi-degree of freedom (MDoF) numerical models. In all cases, fragility curves are derived through a cloud-based approach employing unscaled real (i.e. recorded) ground motions. 14 four- or eight-storey RC frames showing different plastic mechanisms and distribution of the infills are analysed using each method. The results show that non-linear time-history analysis of equivalent SDoF systems is not substantially superior with respect to a non-linear static analysis coupled with the capacity spectrum method. The estimated median fragility (for different damage states) of the simplified methods generally falls within ±20% (generally as an under-estimation) of the corresponding estimates from the MDoF non-linear time-history analysis, with slightly-higher errors for the uniformly-infilled frames. In this latter cases, such error range increases up to ±32%. The fragility dispersion is generally over-estimated up to 30%. Although such bias levels are generally non-negligible, their rigorous characterisation can potentially guide an analyst to select/use a specific fragility derivation approach, depending on their needs and context, or to calibrate appropriate correction factors for the more simplified methods.

Nonlinear Simulation and Vulnerability Analysis of Masonry Structures Impacted by Flash Floods

The vulnerability of buildings impacted by flash floods is often assessed by empirical approaches. However, the damage-developing mechanisms of buildings remain unclear. This study presented a nonlinear numerical analysis for the interaction between flash floods and masonry structures. A new Structured ALE (S-ALE) solver in an explicit finite element platform LS-DYNA was applied. Thirty different scenarios, including two different FE models, three different water depths, and five different flow velocities, were studied. Nonlinear structural responses and failure mechanisms of masonry structures were analyzed and compared. Results showed that due to the consideration of wall damage, the time evolution of the impact force on the entire building was distributed in a multipeak pattern. Under the impact of flash floods, the building was in a complex bending-shear state, and the overturning moment was the principal reason for the building damage. The critical role played by the structural measure was reaffirmed. Moreover, the physical vulnerability was quantified through a macroscopic damage index, the lateral drift ratio of the ground floor. It can be concluded that the physical vulnerability depends on both the local structural strength (local view) and the structural resistance hierarchies (global view).

Application and Effect of Loading Rates on Coal Sample Failure

In order to evaluate the coal pillar stability in recovery of residual room pillars under different mining rates, this paper studies the influence of loading rate on the mechanical properties of the coal body. The uniaxial compression tests of coal samples in Yangcheng area at different loading rates were carried out with the MTS815 electrohydraulic servo rock mechanics test system. The stress-strain curves and the evolution characteristics of AE signals were analyzed. At same time, the mechanism of damage and failure of specimens are also discussed. The results show the following. (1) With the increase in loading rate, the ultimate stress and ultimate strain of specimens decrease first and then increase. (2) Loading rate has a significant effect on the stability adjustment of specimens. With the decrease in loading rate, the earlier the stress adjustment is, the larger the adjustment range is, and the failure mode changes from shear failure to tensile failure. (3) In addition, when the loading rate increases, the AE evolves from continuous dense to discrete catastrophe, which indicates that the failure of the sample at a larger loading rate is sudden, which is not conducive to the maintenance of the stability of the coal pillar. (4) Finally, the failure mechanism of the specimen structure under different loading rates is obtained, and the improvement measures for the effect of mining velocity of working face on the stability of coal pillar are put forward. The results reveal the loading rate effect of mechanical properties of coal and provide a reference for controlling the stability of the residual coal pillar.

Flexural strength test and meso-mechanical evolution behavior of cement concrete based on discrete element method

Each damage of cement mixture is closely related to the heterogeneity and discontinuity of its internal materials. It is very important to analyze the mechanical evolution behavior of cement mixture with the theory of meso-mechanics to reveal the failure mechanism of cement mixture structure. In this paper, the microscopic parameters of the discrete element model are obtained in the compressive tests of concrete mixture cubes. The three-dimensional discrete element model of cement mixture was established by using these microscopic parameters. Under the action of a certain rate load, the generation and propagation process, stress distribution, transfer and displacement field evolution behavior of the meso-fracture between the cement mixture particles are analyzed. The results show that the error between the simulated discrete element flexural strength test and the actual standard test (flexural strength, micro-strain at the bottom of the specimen) is within 10%, which indicates that the discrete element method can well simulate the discontinuous and heterogeneous materials. When the external load acts, the horizontal and vertical displacement of the cementitious material is greater than that of the coarse aggregate, and the displacement angle of the coarse aggregate is greater than that of the cementitious material. When the horizontal tensile stress between the particles exceeds the bond strength of the material, the bond between the particles will break. There are few micro-cracks at the initial stage. In late stage, the micro-cracks penetrate each other and form larger cracks. Micro-cracks usually occur at the weak bonding position between coarse aggregate and cementitious material. The upper particle changes from compressive stress to tensile stress gradually with the duration of load. The bottom particles are in the non-uniform distribution of tensile stress and compressive stress, and the stress values between the particles are also in the non-uniform distribution.

Recrystallization-mediated crack initiation in tungsten under simultaneous high-flux hydrogen plasma loads and high-cycle transient heating

Tungsten and tungsten-based alloys are the leading material choices for the divertor plasma facing components (PFCs) in future fusion reactors. Recrystallization may occur when they undergo high heat loads, drastically modifying the predesigned grain structures and the associated desired mechanical properties. However, the influence of recrystallization on the thermal fatigue behavior of tungsten PFCs still remains unclear. In this study, ITER-grade tungsten was simultaneously exposed to a high-flux hydrogen plasma (∼5 × 1024 m−2 s−1) and high-cycle (104–105) transient heat loads in the linear plasma device Magnum-PSI. By correlating the surface temperature distribution, obtained by analyzing temperature-, wavelength-, and surface-dependent emissivity, and the surface modifications of the plasma exposed specimens, the crack initiation heat flux factor threshold was found to be ∼2 MW m−2 s0.5 (equivalently, ∼0.07 MJ m−2 for a 1 ms pulse). Based on electron backscatter diffraction analyses of cross-sections near the crack initiation sites, faster recrystallization kinetics near the surface compared to literature was observed and the surface cracks preferentially initiated at high angle grains boundaries (HAGBs). Upon recrystallization, the yield strength decreases which entails increasing cyclic plastic strains. The HAGBs fraction is increased, which constrains the transfer of plastic strains at grain boundaries. The recrystallization decreases the dislocation density, which promotes heterogeneous deformation. All these mechanisms explain the reduced crack initiation threshold of recrystallized tungsten compared to its as-received counterpart. The results provide new insights into the structural failure mechanisms in tungsten PFCs exposed to extreme fusion plasmas.

Seismic response and failure mechanism of underground frame structures based on dynamic centrifuge tests

AbstractDuring the 1995 Kobe earthquake in Japan, the Daikai subway station suffered total collapse, which made the research on the seismic failure mechanism of underground frame structures become a hot topic. The author has carried out a lot of research on earthquake failure mechanism of underground frame structures based on numerical method. To validate and further study the seismic failure response and mechanism of underground frame structures, a series of comparative dynamic centrifuge tests were carried out. The research conclusions are as follows. The vertical earthquake action significantly increased the axial pressure on the columns of an underground frame structure, reducing their horizontal deformation capacity. The columns subjected to high axial compression were prone to brittle damage under a relatively strong horizontal earthquake, which led to the overall collapse of an underground frame structure. The areas near the intersection of the ceiling slab and the sidewalls were the weak part of the underground frame structure and were vulnerable to tensile cracks under earthquake. The columns' top and bottom were the fragile parts and were prone to concrete cracking or even shear failure under an earthquake. If the columns could remain intact and effectively provided vertical support under a strong earthquake, it was beneficial to reduce the risk of the overall collapse of an underground frame structure.

Influence of exterior infill walls on the performance of RC frames under tsunami loads: Case study of school buildings in Sri Lanka

This paper assesses the structural performance of RC frame buildings subjected to tsunami-induced loads, accounting for the influence of exterior masonry infill walls on the overall structural performance. Both the in-plane and out-of-plane contributions of masonry infill walls are considered in the analysis. To illustrate the importance of accounting for exterior infill walls in the response of structures to tsunami, two case study buildings are considered and modelled in 3D. The first case study is a typical two-storey school building in Sri Lanka, and the second is a modified version of this design configuration proposed in Sri Lanka after the 2004 Indian Ocean Tsunami to provide more redundancy against scour. Through these case studies, the effect of the non-uniform distribution of infill walls in the building and their failure (or “breakaway”) on building performance is considered. The building performance is characterized by a number of response parameters (i.e., first yielding, development of two hinges, and shear failure in ground floor columns). The paper shows that the in-plane behaviour of exterior infill walls increases the flexural capacity and lateral stiffness of the structure, as would be expected. However, it also shows that an assumption of non-breakaway infill walls consistently leads to premature structural failure mechanisms, associated with the concentration of drag forces on seaward columns only. The results demonstrate that a good estimation of the location and occurrence of shear failure in structural elements can only be achieved by explicitly considering the out-of-plane behaviour and failure of exterior infill walls during an incremental tsunami load analysis. Finally, the Froude number assumed for the analysis is seen to strongly affect the performance of both structural and non-structural components, highlighting the importance of choosing realistic tsunami properties to perform a reliable capacity assessment.

Full-scale experimental and numerical investigation on the ductility, plastic redistribution, and redundancy of deteriorated concrete bridges

Due to structural degradation, the performance of concrete bridges may degrade with time and result in catastrophic consequences. A novel approach is developed for evaluating the time-variant reliability of multi-girder concrete bridges considering the effects of the load-carrying mechanism and redundancy. By considering three failure modes at both the component and system levels, a new performance indicator is proposed for quantitatively evaluating the load elastic distribution and plastic redistribution among multiple girders. The adverse effects of material deterioration on the structural capacity, ductility, redundancy, load-carrying capacity, and failure mechanism are also investigated and incorporated into the analytical procedure, in which an incremental nonlinear finite element analysis of a 3D fiber beam element is used. Furthermore, the results associated with full-scale destructive tests of two in situ deteriorated bridges, a reinforced concrete (RC) and a prestressed RC (PRC) T-girder bridge, are adopted to evaluate the accuracy of the proposed approach. The feasibility and satisfactory performance of the proposed framework are evaluated using these two real-world bridges. The results demonstrate that the load-carrying mechanism and redundancy significantly affect the structural ultimate load-carrying capacity and time-variant reliability of deteriorating structures.

A displacement/damage controlled seismic design method for MRFs with concrete-filled steel tubular columns and composite beams

A displacement/damage controlled (DDC) seismic design method for composite (steel/concrete) frames, consisting of circular concrete-filled steel tube (CFT) columns and composite beams (steel beams connected with concrete floor slabs) is developed in this study. The proposed seismic design method controls displacement and damage in a direct way for all seismic performance levels including the one near collapse. Through empirical expressions this method can estimate the inter-storey drift ratio (IDR) of a designed structure and evaluate the damage index (DI) of critical members for a given seismic intensity. A reduced number of design iterations is achieved while the computationally demanding non-linear time-history analysis can be avoided. The necessary empirical expressions of the design method are derived by means of statistical and sensitivity analysis of a large response databank consists of IDR and DI that cover all the way from elastic behavior to final global dynamic instability. This response databank is created by performing extensive parametric incremental dynamic analyses of many composite framed structures of the kind considered here under many seismic motions and different soil types. Design examples reveal that the DDC design method successfully estimates the targeted IDR for the desired seismic performance level as well as controls the DI in critical beam-to-column joints in order to avoid a soft-storey failure mechanism or partial loss of structure. Compared to all steel framed structures, the composite frames considered here exhibit a better seismic performance with beams and columns exhibiting a lower DI. The low-damage performance of composite frames is mainly emphasized as the number of storeys increases, while both the IDR and DI tend to fall within lower performance levels than those of the corresponding all steel frames for the same seismic intensity.

Dynamic test and numerical simulation on avoiding the weak-story failure mechanism in structures using LSFDs

The structural damage or collapse caused by weak-story failure mechanisms poses a great threat to the safety of human life and property under strong earthquakes. Many researchers have attempted to transform this unexpected failure mechanism into the desired overall failure mechanism by installing various energy dissipation devices on unsafe structures. This paper introduced a lattice-shaped friction device (LSFD), which is a friction device with hardening postyielding stiffness, into a steel frame with a weak-story failure mechanism. Then, shaking table tests of a three types of two-story steel frames—a frame with LSFDs, a frame with traditional friction brace dampers (FBDs), and a bare frame—were carried out. The seismic responses of the hardening postyielding stiffness of the LSFD on the weak-story failure mechanism of the frame were emphatically studied. The results showed that there was little difference in the seismic responses between the two damped structures under moderate and weak earthquakes. The distribution of maximum story drift for the structure with LSFDs was more uniform, which effectively suppressed the weak-story failure under strong earthquakes, whereas the structure with FBDs had serious deformation concentrations. The numerical simulation results of the structure with LSFDs in the shaking table test showed that the simplified model results were basically consistent with the experimental results. Hence, this model could be used to analyze the seismic performance of damped structures with LSFDs.

Research on Change Laws of Pore Water Pressure of Soft Clay in the Large-scale Wave Flume

The study on the interaction between wave structure and soft clay foundation is carried out in the large scale wave flume trough of the Academy of communications and transportation under the construction of the research on the failure mechanism and design parameters of the port hydraulic structure under the harsh hydrology condition. The remolding technology of the soft clay foundation of the Yangtze River mouth is studied. The model test scale of is 1:5. In the test, the strength of the original soil can be reduced to the similarity of the test phenomenon. The soil strength is reduced by increasing the water content of the original soil, and a series of tests are made on the characteristics of the original soil. The strength of soil increases with the increase of static time. The early growth is fast and the later growth is slow. In the experiment, the soil pressure and pore water pressure sensor were arranged to observe the change rule of soil pressure and pore water pressure under the action of wave. By arranging water pressure sensor and laser displacement sensor, the force and settlement of the structure of the semicircle caisson were observed. In this paper, the variation rule of earth pressure and pore water pressure under different wave heights and longtime waves are studied. The pore pressure and soil pressure change with the wave action, the influence of pore pressure increases with the increase of wave height. Maximum pore pressure occurs at a certain depth of soil. The change in pore pressure is irregular. After a certain value is reached, the soil is softened and destroyed.

Simulation of energy evolution during deformation failure process of the layer-intact structure

To research the failure mechanism of the layer-crack structure of surrounding rock in deep mine roadway, the combination (Layer-Intact structure) of shallow layer-crack coal and deep complete coal was taken as the research object, and the mechanical behavior and energy evolution law of the Layer-Intact structure under uniaxial and biaxial compression were studied by numerical simulation with particle flow code (PFC2D). The results show that: (1) In the Layer-Intact structure, layer-crack specimen is destroyed prior to intact specimen; The degree of fragmentation increases with the increase of confining stress, and decreases with the increase of fissure number; (2) The confining stress and the fissure number have a significant impact on the basic mechanical parameters of the Layer-Intact structure. The peak stress of the structure increases first and then decreases with the increase in the confining stress, and decreases with the increase in the fissure number; (3) When fissure number is constant, the energy ratio of layer-crack specimen (strain energy stored in the layer-crack specimen to the the whole specimen) increases first and then decreases with the increase in confining stress, while the energy ratio of intact specimen (strain energy stored in the intact specimen to the whole specimen) decreases first and then increases. When the confining stress is constant, with the increase in the fissure number, energy storage capacity of the Layer-Intact structure is reduced, the energy ratio of layer-crack specimen decreases, while the energy ratio of intact specimen increases. The research can provide some reference for revealing the energy release for dynamic instability of layer-crack structure.

Damage Quantification of Frame-Shear Wall Structure with Metal Rubber Dampers under Seismic Load

The seismic load can bring serious damages to the coupling beams and frame beams in the frame-shear wall structure. Such damages can hardly be repaired, causing functional loss to the structure. One of the best ways to consume seismic energy input, rationalize the damage mode of structure, and prevent serious damages to the main components is to install metal rubber (MR) dampers materials in the frame-shear wall structure. Based on Park and Ang’s damage model, this paper presents pertinent formulas of damage indexes, and uses them to quantify the failure mode. Then, the correspondence was established between damage level of the target structure, reasonable damage mode, and damage index. Finally, two ABAQUS models of an 18-floor frame-shear wall structure were constructed: an uncontrolled structure (traditional frame-shear wall structure) and a controlled structure (frame-shear wall structure with MR dampers). Through comparative analysis, the characteristic parameters of the damper were determined for the reasonable damage mode, and the failure mechanism of the target structure was identified accurately under seismic load.

Experimental study on soft PSD material of dual pulse solid rocket motor

In order to study the failure reason of the soft PSD in the dual pulse solid rocket motor (SRM), the deformation process of the intermediate section of the second pulse combustion chamber was simplified to the two-dimensional plane strain state, and the calculation method of the circumferential strain of the soft PSD was obtained. The influencing factors of the circumferential strain of the soft PSD were studied. The main factors affecting the circumferential strain of the soft PSD are the gap between the soft PSD and the propellant grain, and the circumferential strain on the inner surface of the propellant grain. The calculation method could be used to initially estimate the circumferential strain of the soft PSD, and then predict the rationality and feasibility of the design scheme. The apparent morphology and area change rate of EPDM materials of PSD under different strains were studied by DIC tensile test. The variation of the porosity of the EPDM material with the increase of strain was obtained by micro-CT. By comparing the SEM results of the fracture and the slit of the tensile test piece, the failure mode of the EPDM material of PSDs was determined, and the failure mechanism of the PSD structure was obtained. The conclusions obtained in this paper can provide a useful reference for the design of the PSD in dual pulse SRM.

Compressive Properties of 316L Stainless Steel Topology‐Optimized Lattice Structures Fabricated by Selective Laser Melting

Some lattice structures with different relative densities (0.15–0.5) are designed by topology optimization and fabricated by selective laser melting. The temperature history of lattice structures is monitored experimentally to reveal the forming reason of the deviation of relative densities. The effect of relative densities on compressive properties of lattice structures, including mechanical properties, failure mechanism, and energy absorption capabilities, are systematically investigated. The results show that compressive properties of lattice structures vary dramatically with the changing of relative densities. Due to the existence of surface‐shaped struts, the compressive strength of topology‐optimized lattice structures has superior performance comparing to most of the other structures with rod‐shaped struts. The failure mechanism of lattice structures can be changed from shear‐ to bending‐dominated when increasing relative densities as a result of decreasing the thickness of struts. A systematic investigation of the fracture generation of struts is conducted through both deformation analysis and the finite element method. The fitting curves are established successfully to predict the mechanical properties of designed lattice structures. The aforementioned results verify the feasibility of designing high‐performance cellular structures via topology optimization. Moreover, herein a comprehensive solution to tailor desired properties for satisfying the requirements of functional components is provided.

Numerical Analysis of Geotechnical Problems of Historic Masonry Structures

Geotechnical modeling of complex soil problems requires an advanced 3D discretization and a sophisticated soil models. The aim of the present study is to accurately analysis the geotechnical and induced structural failure mechanisms observed in the field and experimental studies. In order to provide a contribution to quantify the stress and deformation analysis and settlement of soft clay subsoil, a geotechnical analytical model is presented in details. The study presents an attempt of a PLAXIS 3D FE analysis model to simulate the subsoil problems, deformation and stress analysis of the multi-layer masonry structures of Abu Serga church in Cairo, Egypt which is loaded in plane. Advanced soil plastic modeling has been used throughout the different phases of the numerical analysis. The numerical and experimental analysis allowed us to define the pathology of this unique architectural heritage and to estimate the subsoil deformation as well as the stress–strain analysis of its superstructure, also to define the ultimate load that they can survive under their present conditions. The results are discussed with respect to the stress–strain and volumetric behavior of the soil. Consequently it can be concluded that modeling the behavior of subsoil soft clay underneath Abu Serga church by the Numerical analysis is suitable for understanding of the studied behavior of this kind of problematic soil.

Multi-echelon support method to limit asymmetry instability in different lithology roadways under high ground stress

This paper investigated the effect of a multi-echelon support on the asymmetry structural instability of different lithologykilometre-deep roadways. An engineering background investigation was carried out to determine asymmetry instability in different lithology kilometre-deep roadways using acoustic emission signals of in situ rock and 3D in situ ground stress. The mechanical coupling-bearing structure and multi-echelon support method of the surrounding rock were proposed based on the analysis of the secondary stress distribution and strength degradation of the surrounding rock. A numerical simulation and scale-model experiments were conducted to analyse the asymmetry structural failure mechanism and the effect of a multi-echelon support. The results indicated that the high ground stress created a serious impact on the stability of the coupling-bearing strata, an increase in horizontal ground stress created obvious extensions of the weak bearing stratum and the transfer of the main and key bearing strata. The larger asymmetric deformation phenomena of surrounding rock was often accompanied by wedge-shaped roof caving, serious floor heave and spalling, which indicated the decline of the bearing capacity of the surrounding rock. The redesigned multi-echelon support was shown to reduce the scope of a weak bearing stratum and limit the transfer of the main and key bearing strata, preventing the roadway asymmetry structural instability to a certain degree and relieving plastic flow in the surrounding rock.

A detailed analysis on the microstructure and compressive properties of selective laser melted Ti6Al4V lattice structures

In this paper, Ti6Al4V octahedron lattice structures with top cap and bottom base were fabricated through selective laser melting (SLM). The structural integrity, microstructural evolution, compressive properties and failure mechanism of as-fabricated (AF) and vacuum annealing treated (VAT) samples were studied through both experiments and numerical modeling. The X-ray computed tomography analysis revealed that vacuum annealing had an insignificant effect on internal pore elimination and porosity reduction. For both the AF and VAT samples, larger pores exhibited more irregular shape than smaller pores. The microstructural analysis suggested that vacuum annealing was able to transform acicular α/α` martensites into uniformly distributed lamellar α + β phases and also to cause the formation of nano-particle precipitation and dislocation. The compressive test indicted that the lattice structure with confined top and bottom had much better compressive properties than those without. Also, vacuum annealing significantly improved the compressive strength by 26% due to the synergistic effect of residual stress relief, nano-particle precipitation and dislocation strengthening. The ductility of the VAT sample was also improved as compared to the AF sample, which was mainly attributed to the formation of α + β phases.