Quasi-static Loading Conditions Research Articles

Rock crack initiation triggered by energy digestion

The critical value of rock failure is determined by irreversible deformation (inelastic deformation, damage, and other internal dissipation) processes and external conditions before rock failure. Nevertheless, a thorough explanation of the mechanism causing cracks in rock material has not yet been provided. The strain energy theory is applied in this work to assess the initiation of rock cracks and investigate the relationship between energy digestion and rock strength. Firstly, the uniaxial compression test was conducted on sandstone samples under quasi-static loading conditions and the results of energy evolution, non-linear cumulative digestion, and stored ultimate energy were obtained. Then, a novel algorithm for assessing the initiation of rock cracks has been put forth. The concept of energy digestion index (EDI), which is the ratio of digested energy over the external loading energy, has been developed to characterize the energy absorption capacity of rock material. The result shows a relationship between the maximum growth rate of energy digestion and the increasing rate of variable elasticity modulus and crack initiation. The mechanical characteristics and peak strength of the rock material are negatively correlated with the EDI. By monitoring the digested energy status, an evaluation of the residual strength is introduced based on the relationships, which will initiate further research into in-situ monitoring and failure prediction.

Quasi-static and dynamic shear-out resistance in a steel fin-plate connection: An experimental and numerical investigation

This study presents an experimental and numerical investigation into the structural collapse of a fin-plate connection due to column loss, by subjecting the joint to quasi-static and dynamic loading conditions. The joint connection was designed using short segments of HEB220 and IPE160 sections for the column and beam, respectively, with the aim to promote bearing failure as the predominant failure mode. The experimental results revealed the same collapse modes induced by shear-out failure of the beam web under both low and high loading rates. Despite the increased resistance of the joint at high loading rates, high deflections and low energy dissipation were observed. The experimental results were complemented by numerical findings from a validated finite element model in Abaqus/Explicit. Even though a good agreement between experiments and numerical simulations was achieved, the numerical model overestimated the displacement at failure due to the limitations of the customary material model. The findings of this study demonstrate the deficiencies of fin-plate connections in a sudden column loss scenario.

Viscoelastic Model of Fiber Concrete under Dynamic Loading Considering the Effect of Initial Stresses

Statement of the problem. The objective of the study was to identify the peculiarities of the stress-strain state of fiber concrete under two-stage static-dynamic loading and to develop a variant of a nonlinear viscoelastic model of concrete reinforced with steel fiber. Results. The paper presents the results of experimental tests of fiber-concrete prisms under quasi-static, dynamic and static-dynamic loading conditions. On the basis of the analysis of experimental data it is established that at dynamic loading there is an upward shift of the boundary of macrocrack formation. A nonlinear viscoelastic model of fiber concrete is proposed and compared with experimental data, taking into account the effect of the initial stress level caused by the service load. The parametric study performed using this model shows that as the loading rate increases, a more intensive dynamic hardening of concrete is observed. Conclusions. The proposed viscoelastic model of fiber concrete allows to estimate the ultimate strength and corresponding strain under static-dynamic mode of loading due to an accidental action caused by the collapse of one of the load-bearing members of the structural system of the building.

Effects of strain rate on dynamic deformation behavior and microstructure evolution of Fe-Mn-Cr-N austenitic stainless steel

In this work, the effects of strain rate on dynamic hot deformation behavior and microstructure evolution of Fe-Mn-Cr-N steel were studied under quasi-static and dynamic loading conditions using a Gleeble-3800 thermal-mechanical simulator and Split-Hopkinson pressure bar (SHPB) system. The stress-strain curves show that the yield and flow stresses are both significantly dependent on strain rate, also the strain rate sensitivity increases from 0.050 at a low strain rate (0.001–0.1 s−1) to 1.507 at a high strain rate (4500–5500 s−1). At a low strain rate (0.001–0.1 s−1), the microstructure evolution is dominated by dislocation slip and dynamic recovery (DRV). When strain rates are increased to 2500–3500 s−1, dislocation slip and mechanical twining together coordinate plastic deformation. When the strain rate was 4500–5500 s−1, the dynamic recrystallization (DRX) based on twinning formed in the deformation bands, and its fraction increased with the increasing strain rate due to adiabatic temperature rise (ΔT). The high micro-hardness value was obtained mainly because of the strain rate hardening (2500–3500 s−1), but the decrease at the strain rate of 4500–5500 s−1 is primarily attributed to recrystallization softening after deformation. The modified Johnson-Cook (J-C) model considering adiabatic temperature rise agrees with experimental results. It can be used to predict the plastic deformation behavior of Fe-Mn-Cr-N steel under high strain rate loading.

Numerical-experimental analysis of laser-welded small energy absorbers with direct impact Hopkinson method

This research paper presents a comprehensive numerical-experimental analysis of the phenomena observed during the compression of small energy absorbers (EAs) with dimensions 23 × 23 mm and a thickness of 0.6 mm. This study uses the direct impact Hopkinson (DIH) method. The locus is the detailed prediction of the strength of laser-welded joints in terms of energy absorption under both quasi-static and dynamic loading conditions. Furthermore, the paper explores the potential utility of the described methodology for modeling and predicting the failure mechanisms of small energy absorbers. This investigation delves into key factors that affect the distribution of cracks in deformed energy absorbers. These include: the detail of the reproduction of the absorber geometry, the impact of finite element formulations and modeling method (2D/3D elements) and the complexity of the material model for propagated cracks. Two material models are considered, the simplified Johnson–Cook model and the tabulated Johnson-Cook with Hockett–Shelby extrapolation model, which incorporate linear damage dependent on the stress triaxiality and Lode angle. The study also highlights the advantages and disadvantages of the DIH test method. Some features cannot be effectively observed through experimental testing alone. The experimental procedure involved initial testing and calibration of the material models for both the parent material and the welded joints at three different strain rates. Subsequently, the validated energy absorber models are subjected to static and dynamic crushing. The results are subsequently compared with the virtual results obtained using the gravity hammer method, which is a more readily accessible testing approach. Remarkably, a very high correlation between the numerical simulation and experimental data is observed, providing a comprehensive understanding of the research problem related to laser-welded absorbers.

Influence of strain rate on the work hardening, strain induced martensite formation, strain partitioning, and variant selection in a medium-Mn steel

In this study, we investigated the effect of strain rate on the work hardening, strain-induced martensite transformation, strain partitioning, and crystallographic variant selection in a medium-Mn steel. Uniaxial compression tests were performed under quasi-static and dynamic loading conditions at the strain rates of 10−3, 1 and 103s−1. Besides, to determine the contribution of temperature rise due to adiabatic heating at a higher strain rate, compression tests were carried out at 409 K (estimated temperature rise) under quasi-static conditions. The work hardening rate was higher at high strain rates and it rapidly decreased at all strain rates up to a true plastic strain of ∼0.05. Beyond the true plastic strain of ∼0.05, the work hardening rate increased at a low strain rate up to a true plastic strain of ∼0.15 while at a high strain rate, it remained relatively constant and gradually decreased beyond a true plastic strain of ∼0.15. From the analysis of microstructures obtained from electron backscatter diffraction, it was observed that the strain-induced transformation of retained austenite to α′-martensite is significantly impeded at higher strain rates and in the sample tested at 409 K at low strain rates. The higher stability of retained austenite is attributed to the adiabatic heating which decreases the chemical driving force for retained austenite to martensite phase transformation and increases the stacking fault energy. Analysis of the misorientation axis and angle evolution revealed that while the Kurdjumov–Sachs relationship was followed under quasi-static conditions, it deviated at higher strain rates. Analysis of crystallographic variants related to strain-induced martensitic transformation at a true plastic strain of ∼0.15 revealed that the samples deformed at a lower strain rate did not show any noticeable variant selection. However, in samples deformed at a higher strain rate, a significant variant selection was observed. It is reasoned that the martensitic transformation nucleates initially on favorably oriented habit planes of retained austenite grains at all strain rates and other variants get activated with an increase in strain. As the transformation is impeded beyond a critical strain at a higher strain rate, the variants activated at lower strains remains predominant.

Investigating post-processing impact on fatigue performance of LPBF Ti6Al4V with heat treatment, high pressure heat treatment, and dry electropolishing strategies

This study investigates the impact of post-processing techniques on the fatigue behaviour of specimens in Ti6Al4V made by laser powder bed fusion (LPBF) for relevant applications, such as motorsport. Despite the LPBF advantages in terms of complex design and weight reduction, challenges such as reduced fatigue performance persist. Therefore, the application of post-processing treatments can play a critical role. In addition to traditional sandblasting (SB), three post-processing conditions were investigated to enhance mechanical properties under quasi-static and fatigue loading conditions: heat treatment (HT) and the innovative High Pressure Heat Treatment (HPHT) along with dry electropolishing (EP) treatment. The results indicate that both the HT and HPHT treatments produced a macrostructure with equiaxial grains composed of fine α + β lamellae. Mechanical analyses demonstrated that the HPHT-SB-EP condition exhibits properties comparable to those of forged Ti6Al4V used in the automotive field, with a good balance between strength and ductility. The EP treatment effectively reduced surface roughness and inherent surface irregularities caused by the LPBF process. The best fatigue results were achieved with the HPHT-SB-EP condition, despite data scattering. An increase in fatigue resistance was observed by using SB at a higher pressure.

Experimental Study on the Self-Initiated Static-Dynamic State Transition Mechanism of Strain Rockburst

During the entire processes of rockburst catastrophe, the self-initiated static-dynamic state transition behaviour is a turning point in the development of strain rockburst from static deformation to dynamic failure. However, its dynamic process and triggering mechanism have not yet received sufficient attention and accurate revelation. This study focuses on the scientific issue, by designing experiment we have successfully achieved the physical simulation of rockburst under quasi-static displacement loading conditions. Then, the static-dynamic state transition process was studied on a millisecond time scale, and rockburst triggering mechanism was investigated from mechanical perspectives. Through experiment, the self-initiated elastic rebound dynamic behaviour of the surrounding rock is discovered and identified as the key to triggering rockburst. Mechanically, this behaviour directly causes an impact load on the burst rock, which can be considered the direct cause of rockburst.

Advanced tensile fracture analysis of alumina ceramics: Integrating hybrid finite-discrete element modeling with experimental insights

This study offers novel insights into the Mode I tensile response of an alumina ceramic through the use of computational modeling and the flattened Brazilian disk (FBD) experiments. A modified hybrid finite-discrete element method (HFDEM) is developed, integrating a coupled damage and friction cohesive model and a microscopic stochastic fracture model with a Weibull strength distribution by Monte Carlo simulation. The model is used to simulate direct tensile failure processes under quasi-static loading conditions, providing qualitative and quantitative predictions of direct tensile failure processes of an alumina ceramic. Concurrently, quasi-static flattened Brazilian disk tests (indirect tensile tests) are performed on a standard MTS machine coupled with a high-speed camera. The modified HFDEM model is also applied to reproduce the FBD experiments, and our simulated tensile strength is consistent with the experimental results. The results of the modified HFDEM model show three kinds of phenomena (i.e., “underestimation”, “reasonable estimation”, and “overestimation” of the indirect tensile strength) and four different types of associated fracture and fragment patterns of FBD testing. The integration of simulation and experimental results reveal relationships between fracture patterns, fragment geometry, tensile strength, and indirect tensile strength. The fracture and fragmentation patterns derived from our modified HFDEM model can be utilized to analyze the “tensile strength” measured in BD testing. Overall, this research offers important insights and direction for future Brazilian disk experiments and tensile strength assessments, enhancing our comprehension of ceramic failure mechanisms under tensile loadings.

Study of conduction welded C-struts for a thermoplastic composite fuselage

In this research, conduction welded C-struts, part of a thermoplastic composite fuselage designed and manufactured in the framework of the Clean Sky 2 STUNNING project, are investigated. Five specimens made of two C-section profiles are manufactured and welded using conduction welding in three different configurations with variations in the direction and distance of the two welded joints. Preliminary numerical analysis using the virtual crack closure technique are conducted to obtain an initial evaluation of the specimens behavior, in preparation of the tests. Experiments are performed under quasi-static loading conditions to measure the strength of the welds. Comparisons with the preliminary numerical analyses show a good agreement in terms of the predicted maximum load, while a clear difference is observed in the initial stiffness, due to the compliance of the support structure. The numerical model is updated, leading to results that closely match the experimental behavior. For all the analyzed specimens, the separation occurs suddenly and no signs of propagation are observed. Experimental and numerical data show no relevant difference in the joint strength among the different conduction welding configurations.

Numerical studies of the energy absorption capacities and deformation mechanisms of 2D cellular topologies

This paper investigates the energy absorption capacities of selected cellular topologies under quasi-static loading conditions. Twenty topologies with nearly identical relative densities belonging to 4 groups were examined: honeycomb, re-entrant, bioinspired and chiral. The topologies were modeled using an experimentally validated numerical ABSplus model and subsequently subjected to in-plane uniaxial compression tests. The findings revealed the topologies with the most favorable energy absorption parameters and the main deformation mechanisms. The topologies were classified by mechanism, and a parametric study of basic material properties, namely modulus of elasticity, yield stress, and ductility, was performed for a representative topology from each mechanism. The results indicated that the honeycomb group topologies were characterized by the largest average absorbed energy, and yield stress was found to have the greatest impact on energy absorption efficiency regardless of the main deformation mechanism.

Enhancing crashworthiness characteristics of modified hexagonal honeycomb structural panels through stretching and bending of ribs

The effects of stretching and bending of concave, convex, and horizontal ribs on the energy absorption characteristics of modified hexagonal honeycomb structural panels are investigated under in-plane quasi-static compression loading conditions. In this regard, five hierarchical modified structural panel configurations were fabricated using polylactic acid material (PLA) via a fused filament 3D printer, ensuring uniform wall thickness in the 1.5–2 mm range. The panels included the regular hexagonal panel (RHP), regular hexagonal panel with concave rib (RHCP), regular hexagonal panel with convex rib (RHXP), regular hexagonal panel with concave and horizontal ribs (RHCRP), and regular hexagonal panel with convex and horizontal ribs (RHXRP). In-plane quasi-static compressive loading tests were conducted to analyze crush resistance characteristics, and buckling modes of the modified honeycomb panels were examined through experimental and finite element analysis procedures. The result indicates that the specific energy absorption capacity (SEA) of RHXRP is increasing significantly compared to the SEA capacity of other categories of the structures. The changes in failure modes and increased crush energy absorption characteristics of modified RHP with the introduction of concave, convex, and horizontal ribs are elaborated.

Investigation of the mechanical properties and propagation of cracks in prosthetic teeth composed of porcelain and dental ceramics using cohesive meshing under quasi-static loading conditions

With the significant progress in dental science and the reliance of dental researchers on understanding the mechanical properties and behaviors of teeth and materials used for tooth replacement, there is an increasing demand for study and research in this sector. Composite samples, dental porcelains, and materials used in 3D printers have gained significance as substitutes for human teeth. Therefore, it is crucial to conduct research and analyze these materials' physical and mechanical characteristics compared to human teeth. Tooth damage and fracture may arise from various causes, such as falls, sports-related incidents, or traffic accidents. Tooth fractures may vary in severity, ranging from mild fractures that include chipping of the tooth's surface layers, known as enamel and dentin, to more serious ones, such as diagonal, vertical, or horizontal fractures extending into the root of the tooth.

Synergy of Spall Strength and Toughness in Nanograined Metals.

Under shock loading, the spall strength of nanocrystals exhibits intricate grain-size effects due to the presence of abundant grain boundary and dislocation activities. However, the influence of size on spall toughness and void evolution has been largely overlooked. This study employs molecular dynamics simulations to investigate the damage accumulation characteristics of nanocrystalline aluminum across various grain sizes. Unlike the trade-off observed in quasi-static loading conditions, our study reveals a consistency in which grain size governs both nanovoid nucleation and coalescence, yielding a novel spall strength-toughness synergy. These insights highlight grain sizes that are particularly susceptible to spall fracture, offering a crucial understanding of nanocrystal failure mechanisms in extreme environments.

Performance analysis of fiber-reinforced polypropylene composite laminates under quasi-static and super-sonic shock loading conditions for impact application

In the present research, glass and basalt fiber-reinforced polypropylene composites’ behaviour in quasi-static and dynamic conditions is studied. Composites were fabricated by vacuum assisted Compression molding method. Composites failure under quasi-static tension and compressive conditions was studied along with its failure behaviour under low-velocity impact and super-sonic shock loading under dynamic conditions. The study results showed that basalt fiber-reinforced polypropylene (Basalt/PP) composite’s tensile and compressive strength is higher than glass fiber-reinforced polypropylene (Glass/PP). The Basalt/PP showed no penetration against low velocity impact (LVI) with negligible deformations till 50 J. However, the Glass/PP perforated at 50 J with various failure patterns occurring at back side. The fiber-matrix interface adhesion plays an important role in super-sonic shock loading by absorbing shock wave energy due to ductile nature of polypropylene and the two composites absorbed energy via matrix and fibers failure, no brittle failure of laminates occurred under shock loading.

Progressive collapse behaviour of earthquake-damaged interior precast concrete joints with headed bars and plastic hinge relocation

This study investigated progressive collapse behaviour of earthquake-damaged interior precast concrete (PC) joints with headed bars. The main objective was to quantify the impact of slight and moderate earthquake damage on their residual collapse resistance. The research began with a numerical investigation to understand the interaction between a 2D frame’s global response to seismic loading and its local response during a threat-independent progressive collapse event. Additionally, a separate set of numerical studies examined interior PC joints under cyclic loading to determine maximum drift ratio demands, yielding 2.5% for slightly and 3.5% for moderately damaged joints. Subsequently, an experimental programme under quasi-static loading conditions was conducted on five interior PC joints under cyclic loading, followed by progressive collapse resistance tests. Test results indicated that damaged joints exhibited flexural and catenary action during progressive collapse, but compressive arch action could not be mobilised due to residual cracks from the initial cyclic loading phase. Comparing damaged joints to fully intact ones from a companion study revealed deterioration in stiffness and deformation capacity during a subsequent progressive collapse, with degree of severity proportional to the extent of damage sustained in the cyclic loading phase. Relative to the intact joints without being subjected to cyclic loading, moderately damaged specimens exhibited a degradation of 63% in stiffness and 38% in deformation capacity, compared to slightly damaged specimens, which showed reductions of 38% in stiffness and 28% in deformation capacity. In contrast, strain hardening of steel bars in both slightly and moderately damaged specimens led to slightly greater collapse resistance than the intact joints. Additionally, failure mode shifted from ductile fracture to premature pull-through failure of headed bars in moderately damaged specimens. In this regard, in post-earthquake progressive collapse assessment, it is recommended to use maximum crack width thresholds of 0.2 to 1.0 mm for slight damage and 1.0 to 2.0 mm for moderate damage.

Investigation of circular aluminium tube expansion by rigid die as energy absorbers using digital image correlation technology

This paper investigates the expansion of circular aluminium tubes using a rigid die under quasi-static loading conditions through experiments and finite element simulations. This study primarily focuses on the expansion of thin-walled tubes with a thickness ranging from 3.95 to 5 mm, and the finite element model is in good agreement with the experimental results. Additionally, the study emphasizes the impact of the coefficient of friction, energy dissipation properties and the diameter-to-thickness ratio on the energy expenditure associated with the expansion process of aluminium tubes using a rigid die. This study use Digital Image Correlation (DIC) technology to evaluate pictures obtained during experimental procedures, enabling accurate quantification of displacements, stresses, and rotations of major planes. The findings indicate that the energy absorption resulting from friction plays a significant role, with the coefficient of friction serving as a important parameter throughout the expansion process. The energy absorbed by the friction between tube-die contact surfaces was significantly higher than that absorbed by the plastic bending of the aluminium tube. For specimen AT-5.0-1, the energy absorbed by friction and bending is 3352 and 570 J, respectively. Result shows that a notable increase in the ultimate load, rising from 46.03 to 61.54 kN, is required for expanding the aluminium tube with a thickness of 3.95 to 5.0 mm. The local strain patterns seen on the surface of the tube offer useful insights, therefore enhancing researchers’ ability to examine particular points on the specimen’s surface without the need for fixed reference points prior to conducting experiments.

Experimental investigation of 3D printed bio-inspired Xylotus lattice structure for energy absorption under quasi-static axial loading conditions

The demand for robust, lightweight polymer components in various industries has prompted researchers to turn to nature's structures for inspiration. Leveraging Polymer Additive Manufacturing (PAM), specifically Fused Filament Fabrication (FFF), complex bio-inspired lattice polymer structures have been successfully realized. This experimental study focuses on the development of novel 3D printed bioinspired Xylotus lattice structure with elements inspired by xylem and lotus. The primary goal was to evaluate the deformation behaviour and energy absorption characteristics of 3D printed Xylotus lattice structure under quasi-static compressive loading and compare the results with existing research. The hybrid (Xylotus) structure of xylem and lotus exhibited a sequential failure pattern, starting with axial cracks and followed by buckling. Furthermore, an analysis of energy absorption showed that the xylem lattice outperformed the lotus lattice, thanks to its robust tubular elements. Notably, the Xylotus lattice displayed the highest energy absorption capabilities, capitalizing on features from both lotus and xylem. The energy absorption of the Xylotus lattice structure surpassed that of the xylem and lotus structures by 13% and 29.2%, respectively. The xylotus lattice structure exhibited 38% higher energy absorption compared to the existing research. Moreover, the specific energy absorption of the Xylotus lattice structure outperformed the lattice structures reported in the existing research by 37%. This study offers valuable insights into the structural behaviour, energy absorption, and specific energy absorption of bio-inspired lattice Xylotus structure. The findings contribute significantly to the design of resilient, lightweight components, supporting the advancement of bio-inspired structures for diverse applications in various industries.

Effect of heat treatment on mechanical compression properties of C250 maraging steel fabricated by directed energy deposition

The mechanical compression properties of C250 maraging steel fabricated by directed energy deposition (DED), in the as-deposited and post-fabrication heat-treated conditions, are investigated under quasi-static loading conditions. The microstructure of the material is studied using analytical transmission electron microscopy and a comparison is made between the DED fabricated and conventionally produced wrought materials. The results reveal that the DED fabricated material, in the as-deposited condition, exhibits lower yield and ultimate compressive strengths compared to the commercially heat-treated wrought alloy, due to the absence of strengthening precipitates and the presence of a retained austenite phase. However, post-fabrication thermal treatment significantly improves the compressive strengths of the as-processed material to levels comparable to those of the conventionally produced wrought material. This is attributed to the formation of nano-sized Ni3Mo strengthening precipitates, a reduction in the amount of austenite phase, and the refinement of prior-austenite grains during the thermal treatment. Moreover, the DED fabricated material, in the heat-treated condition, with the presence of an austenite phase shows higher ductility than the commercially heat-treated wrought material.

Biaxial behaviour of concrete and its failure mechanics under quasi-static and dynamic loading: A numerical study

In the present study, the biaxial behaviour of concrete and its mechanics under quasi-static and dynamic loading conditions are analysed using the meso-model of concrete. A three-phase cubic meso-model is developed, which consists of mortar, interfacial transition zone, and aggregate. A finite element split Hopkinson pressure bar is developed for the biaxial dynamic loading. The mechanics of the crack pattern development in concrete under uniaxial and biaxial loading are investigated. The failure process and its mechanics for each loading condition are also explained in four steps. Similar cracks are obtained for the quasi-static and dynamic biaxial loading; however, the cause of failure is found to be contrasting.