Elastic-plastic Stages Research Articles

High-performance bamboo-wood composite materials based on the natural structure and original form of bamboo: Fracture behavior and mechanical characterization

Reducing the destruction of bamboo's original structure to produce high-utilization and high-performance engineered bamboo products has long been a goal in the industry. This study developed a new natural arc-shaped laminated bamboo-wood lumber (NALBWL) based on the natural structure and original form of bamboo, using equal arc-shaped bamboo split (EASB) and poplar veneer. A multi-scale analysis of the bending performance and crack propagation dynamics of EASB and NALBWL was conducted. EASB's modulus of elasticity (MOE) and rupture (MOR) were 1.06 and 1.17 times higher than those of air-dried bamboo, respectively, exhibiting excellent mechanical properties. Additionally, its toughness in the full and elastic-plastic stages were 1.24 and 1.38 times higher than those of air-dried bamboo. NALBWL's MOE and MOR were 12.73 GPa and 174.41 MPa, fully meeting structural material requirements and even surpassing most engineered bamboo products. Moreover, NALBWL's radial bending load values were 2.29 and 2.24 times higher than that of EASB and air-dried bamboo, respectively, and it also demonstrated excellent water heat and drying resistance in impregnation and peeling performance. Real-time crack propagation analysis indicated that NALBWL's superior mechanical properties benefited from the respective advantages of bamboo and poplar veneer. This work showcased the application of "natural form-inspired design" in developing bamboo-wood composite materials, and the conclusion provided a reference for the sustainable development of engineered bamboo products and the study of biomass material fracture mechanisms.

Crashworthiness of AAG joints with a foam aluminum layer gusset plate

The gusset joint is the most widely used joint system in an aluminum alloy single-layer reticulated shell structure; it plays a vital role in the spatial grid structure. In this paper, the effects of different thicknesses of aluminum alloy gusset plates on the dynamic response and failure modes of aluminum alloy gusset (AAG) joints were studied by impact tests of three specimens. The test results show that the failure modes of AAG joints are a slight depression of the gusset plate, the local buckling of members, the local distortional buckling of members, and tearing of the lower flange and a web of members. The AAG joint inclines along the direction of the failed member and collapses. The finite element model of the AAG joint was established using ANSYS software. The stress, displacement, and deformation modes in the test and numerical simulation results were compared and analyzed, and the rationality of the numerical simulation results was verified. Based on the failure mode of the specimens, a design method for improving the impact resistance of AAG joints with aluminum foam layer gusset plate (AFLGP) is proposed. The numerical simulation results show that an AAG joint with an AFLGP effectively reduces the maximum peak value of impact loads through the deformation of the aluminum foam. In the elastic and elastic-plastic stages, AAG joint with AFLGP had better impact resistance, and the energy absorbed by its rods was 25.44% ∼ 30.36% lower than that of AAG joints alone. Finally, the influence of the thickness of the AFLGP on the impact force and energy absorption of the AAG joint was studied. The aluminum foam gusset layer plate is approximately twice as thick as the aluminum alloy gusset plate had the best protection effect on the AAG joint.

Investigation on the bearing capacity evolution of building pile foundation during permafrost degradation

The warming and melting of permafrost due to climate warming pose a considerable threat to the integrity of the Pan Arctic building, thus jeopardizing sustainable development. The increase in ambient temperature in permafrost areas will cause deterioration in the bearing capacity of building pile foundations. Considering the continuous deepening of the active layer (za), the present paper used small-scale physical modeling to investigate the potential variation of bearing capacity and load transfer mechanism of pile foundations under the scenario of continuous degradation of permafrost. The ultimate bearing capacity of a single pile and the undrained shear strength of the ground under different za are estimated by cone penetration tests. In the static load test of single piles, the axial load-settlement, axial force of pile shaft, and earth pressure at the pile tip are measured. The results show that the rise in ground temperature and the deepening of the za shorten the elastic and elastic-plastic stages of the load-displacement curve, resulting in a gradual decline in the bearing capacity of a single pile. The pile-soil interface temperature is always higher than the adjacent ground temperature at the same depth. Adfreezing force of the pile-soil interface decreases due to the increase in ground temperature and water content. With the deepening of za, the peak point of the shaft resistance decreases from −30 cm to −60 cm under the ultimate state. Meanwhile, with more axial load transfer along the pile shaft to the pile tip, the share ratio of pile tip resistance to ultimate stress gradually increases. In addition, the temperature rise of frozen soil at the pile tip accelerates the settling rate of the pile, which eventually causes the pile foundation failure.

DETECTION OF HAZARDOUS STATES AND FAULT CONTROL
 IN STRETCHED STEEL ELEMENTS BY THERMAL METHOD

The article discusses the areas of effective application, the features of heat generation and the main possibilities
 of the passive method of thermal non-destructive testing of steel elements and products. The basic requirements
 for passive thermal control equipment are formulated. The mechanism of heat generation in steel is described and
 the structure of the formula for changing its temperature during deformation is substantiated. It is shown that
 the diagnostics of steel elements in the passive mode should be carried out with the functioning of heat sources in them,
 arising as a result of force effects on structures. The features of determining the stress state in steel elements by
 the passive thermal method are considered.
 
 It is shown that the passive method of thermal non-destructive testing of steel structures, loaded with
 continuously increasing or decreasing forces in time, under the influence of which internal deformation heat sources
 and surface temperature fields function in potentially dangerous zones of elements, makes it possible to control the
 operability in elastic, elastic-plastic stages of steel operation, the origin and development of destruction. The power
 of thermal deformation sources, the places of their occurrence and the kinetics of development depend on the conditions
 of force loading, external temperature influences, material and structural shape of the elements.

Flexural strengthening of damaged steel beams with prestressed CFRP plates using a novel prestressing system

This study developed a novel prestressing system of the carbon fiber reinforced polymer (CFRP) plate to strengthen damaged steel beams, which reduced the operation space at the tensioning end, thereby extending the application range of prestressing system. Flexural tests were conducted to verify this system and investigate the effects of the adhesive layer, prestressing level, and CFRP plate area on the strengthening efficiency, and subsequently, a parametric analysis was carried out using finite element modeling to obtain more influencing rules of the unbonded CFRP plate-strengthened steel beams. The results indicated that satisfactory strengthening efficiency could be achieved by the novel prestressing system. The adhesive layer significantly affected the strain distributions of the steel–CFRP composite section, whereas it had a very slight effect on the flexural behavior of the steel beams, and the bonded and unbonded CFRP plates had a similar strengthening efficiency. Nonprestressed CFRP plate exhibited limited improvement in the flexural capacity and stiffness of the damaged steel beams under the normal service state; in contrast, the prestressed CFRP significantly increased the flexural behavior of the beams in the elastic and elastic–plastic stages owing to the high strength utilization of the CFRP plate. In addition, as the prestressing level increased, strengthening efficiency increased. Solely increasing the area or elastic modulus of the CFRP plate improved the strengthening efficiency slightly, whereas applying a prestress simultaneously clearly increases the strengthening efficiency. The strengthening efficiency increases with artificial notch depth.

Elastic–Plastic Stage of Transverse Deformations in the Compressed Zone of Real Reinforced Concrete Beams

The relationship between transverse and longitudinal deformations (Poisson ratio) in the compressed zone of bending reinforced concrete elements is still not properly considered in designs. This factor is important as it is related to the formation of transverse cracks in the compressed zone of the elements. The main aim of the present research was an experimental investigation of the Poisson ratio in the compressed zone of real single- and two-layer reinforced concrete beams as well as a comparison with the theoretical model proposed in this study. The elastic and elastic–plastic stages were analyzed using experimental data for real beams with a span of 8 m. It was shown that the term “Poisson coefficient” had a limited validity for elastic longitudinal deformations up to 0.5‰. After reaching this limit, the suitable term was “Poisson ratio”. The obtained results allowed a more accurate prediction of transverse deformations and the corresponding cracks, defining a new limit state for bending reinforced concrete elements. Therefore, the authors recommend that the obtained results are considered in order to increase the design accuracy of reinforced concrete bending elements. The outcomes of this study and the proposed theoretical concept should be included in modern design provisions.

Pseudo-Dynamic Test on Composite Frame with Steel-Reinforced Recycled Concrete Columns and Steel Beams

Pseudo-dynamic tests were conducted on a 2/5-scaled, three-story, two-span frame with steel-reinforced recycled concrete (SRRC) columns and steel beams. The El-Centro earthquake waves, Taft earthquake waves, and Lanzhou artificial earthquake waves were considered as the main loads to study the seismic behavior. The failure modes, displacement and acceleration time history curves, hysteretic characteristics, energy dissipation, stiffness degradation, and inter-story drift capacity of the composite frame were analyzed. Results showed that the composite frame did not show plastic deformation during the whole test process, and the steel beams and columns basically did not yield. Under the action of three seismic waves, the displacement response, acceleration response, and restoring force response of the composite frame were increased with seismic intensity, while the hysteretic curves and energy dissipation were different as the seismic wave changed. The seismic response of the composite frame was greatly affected by the spectral characteristics of the loading ground motion and the hysteretic energy mainly consisted of recoverable elastic deformation energy. Under the action of Taft wave with the input peak acceleration of 400 gal (rare earthquake), the stiffness degradation of composite frame was the largest, which reduced to 47% of the initial stiffness. This indicated that the composite frame had good energy dissipation performance in the elastic and elastic-plastic stages, and still possessed good rigidity after a rare earthquake, fully achieving the design purpose of “no collapse in major earthquake”.

Metoda procjene kolapsa čeličnog okvira temeljena na faktoru dinamičkog povećanja

To study the laws of evolution of dynamic increase factors (DIFs), relationship between the DIFs and progressive collapse resisting capacity of a structure with web cleat joints under the column removal scenario, a refined finite element model is established based on an existing quasi-static test. The laws of evolution of DIFs are studied for different loading type, beam height, and joint rotation stiffness. It was observed that the displacement-based dynamic increase factor (DIFu) increases with an increase in the load level in elastic and elastic-plastic stages, and it decreases with an increase in the load level in the plastic stage. With an increase in structural damage, DIFu reaches the maximum value when the frequency ratio is equal to 1 and decreases with a further increase in frequency and damping ratios. The load-based dynamic increase factor (DIFp) is negatively corelated to the “arch compression effect” of the beam. Finally, a method based on DIFu is proposed for judging the collapse of structures with web cleat joints. In addition, an improvement is provided for the design the web cleat joint. The improvement is proved to be reasonable and effective with the help of collapse evaluation method proposed in this study.

Study on the bond performance of bamboo-steel interface after cyclic loading

To study the bond performance of bamboo-steel composite members after cyclic loading, 16 bamboo-steel specimens with pure bonding interface and reinforced bonding interface, were designed to be conducted push-out test after cyclic loading. The types of interfaces and the displacement levels of cyclic loading were set as the basic experimental parameters. The failure modes, ultimate bearing capacity, bonding stress and slippage were investigated. Furthermore, a new theoretical model to calculate the shear stress and slippage at interface was proposed and verified. The results showed that specimens with reinforced bonding interface had ductile failure mode, higher bearing capacity and bonding stress compared to specimens with pure bonding interface. The interface had little damage when the specimens subjected to cyclic load in elastic stage, the bearing capacity had no significant changes. While in the elastic–plastic stages, the bearing capacity of specimens dropped significantly as the displacement levels increased, Specimens with reinforced bonding interface had better damage resistance and smaller load degradation.

The axial compressive behavior of stone-lightweight aggregate concrete-filled steel tubular stub columns

To study the axial compressive mechanical properties of stone-lightweight aggregate concrete-filled steel tubular (SLACFST) stub columns, five circular stone-lightweight aggregate concrete-filled steel tubular (C-SLACFST) and square stone-lightweight aggregate concrete-filled steel tubular (S-SLACFST) stub columns were made by taking the volume replacement ratio of natural gravel aggregate and ceramsite in lightweight aggregate concrete as a variation parameter. After observing their mechanical process and failure modes, the axial load–displacement curves of the specimens were obtained. The influence of gravel replacement ratio on the energy dissipation and stiffness of the specimens was analyzed, and a simplified model for predicting their stiffness degradation was proposed. The plastic damage model was then used to verify those specimens with a finite element, and this model was subsequently adopted in the whole process analysis of these specimens and in finding out the loading relationship between the steel tube and core stone-lightweight aggregate concrete (SLAC). The effects of varying parameters on the compressive behavior of the specimens were investigated, and the extent to which these varying parameters are related to the ultimate bearing capacity of SLACFST was assessed via a grey correlation analysis. Results show that the rigidity of the calculated load–displacement curves in the elastic and elastic–plastic stages is greater than that of the test curves, but the ultimate bearing capacity and peak displacements agree with the test values within an acceptable error. Moreover, the simulated failure modes of the control specimens are the same as the test failure modes. Before reaching the ultimate bearing capacity, the core SLAC made a significant contribution to the bearing capacity of the specimen due to the constraint of steel tube. The negligible influence of gravel replacement ratio on the ultimate bearing capacity of SLACFST specimen. In addition, the nature gravel can inhibit the floating of LAC to a certain extent, so as to achieve the purpose of using SLAC to make SLACFST, reducing the self-weight of the structure while having higher bearing capacity.

Static and dynamic size optimization of nonlinear continuous structure based on fully stressed design

In order to realize the fully stressed criterion, that is, the stress of the nonlinear engineering structure is uniform along the height direction under the action of wind load or ground motion, the section size of the continuous structure is optimized based on mechanical analysis. The engineering structure is simplified as a continuous cantilever with variable cross-section, and its material constitution is nonlinear. The wind load and ground motion are represented by three kinds of horizontal static loads: uniform load, inverted triangle load, and inertia force related load. In order to meet the needs of different projects, hollow circular section and box section are designed, respectively. By establishing different expressions of section moment, the optimal section size distribution is solved. Then the optimal stiffness distribution of nonlinear structure is proposed. The correctness of the theory is verified by the finite element method. The results are suitable for the elastic and elastic-plastic stages of the structure, and are effective for both static loads and dynamic actions. The optimal section size distribution and structural shape are different under different loads. Finally, a practical design example is shown.

Theoretical Assessment Of The Bearing Capacity Of A Steel Beam Reinforced With Carbon Fiber

To reinforce steel beams, modern carbon fiber materials that are attached to the beam from the stretched side using a special two-component adhesive can be effectively used. Compared to traditional methods of cross-section increase by means of additional steel elements attached by welding or on bolts, reinforcing with carbon fiber plastic has a number of advantages. Carbon plastic has a much smaller weight than a steel element that has the same load-bearing capacity; the adhesive layer prevents corrosion of the reinforced element; work on strengthening is carried out in a short time and with less labor expenditures etc. The article presents the results of theoretical studies of beams reinforced with carbon fiber lamellas. For the theoretical assessment of the strength of a reinforced steel beam, deformations and stresses in the section at the elastic, elastic-plastic and plastic stages of steel work are considered. It is established that a significant effect of carbon fiber reinforcement is observed even when the stresses in it are determined by the strength of the adhesive joint. Formulas, taking into account the strength of the adhesive layer determined experimentally, for determining the strength of a reinforced beam are obtained. A theoretical model of the operation of bent steel elements is constructed and theoretical dependencies are obtained that can be used for calculations of steel beams reinforced with carbon plastic.

НАПРЯЖЕННОЕ СОСТОЯНИЕ ПЛОСКИХ ОБРАЗЦОВ С НАПЫЛЕННЫМСЛОЕМ ПРИ ИСПЫТАНИЯХ НА ИЗГИБ

The subject of study is the analysis of the stress state during bending, both in the elastic and elastic-plastic stages, of a flat sample with a sprayed layer having different mechanical properties (elastic modulus and yield strength) than the substrate material. The goal is to select options for coatings of small (soft layer), neutral and large (hard layer) stiffness concerning the base metal, which has a lower yield strength, with different ratios of their thicknesses (0.2, 0.4 and 0.6). Tasks: to analyze the distribution of normal stresses, to determine the most loaded points on the surface of the sprayed layer and on the lower surface of the substrate, to determine the conditions and causes of the appearance of cracks in the sprayed layer and plastic deformations in the substrate. The methods used are computer simulations and analytical calculations. The following results were obtained. The nature of the distribution of normal stresses substantially depends on the ratio of the thicknesses of the sprayed layer and the substrate and does not change with their proportional increase or decrease. The most loaded points in the section where the normal stresses reach maximum values are the points on the surface of the sprayed layer and on the bottom surface of the substrate. The level of maximum normal stresses both in the sprayed layer and the substrate significantly exceeds (by 2 orders of magnitude) the tangents. That is, they are the main cause of the appearance of cracks in the sprayed layer and plastic deformations in the substrate. At the same time, the results of calculations using analytical formulas based on the laws of solid mechanics and computer simulations are the same, which confirms the adequacy of the modeling. The level of stresses at a constant thickness of the substrate is affected by both the relative thickness of the sprayed layer and its rigidity. The maximum stresses in the sprayed layer decrease with an increase in its relative thickness and increase with an increase in its relative rigidity. Соnclusion. The results obtained by various methods of theoretical studies allow us to determine the directions of improvement of equipment, technological methods of selecting modes, active materials for producing sprayed layers with high quality for assemblies and parts of various configurations and different materials of products and sprayed with the material. The findings of the work can be used in practice in the development of technological maps of the processes of deposition. The scientific novelty of the obtained results consists of the following: methods for studying the stress state of flat samples with a sprayed layer during bending tests have been improved; for the first time, the results of the influence of a set of characteristics of the deposition conditions on the strength characteristics of the sprayed layer on the substrate were obtained.

In-situ tensile testing of ZrCu-based metallic glass composites

ZrCu-based bulk metallic glass composites (BMGCs) are well known for their plastic deformability, superior to traditional metallic glasses (MGs), which is attributed to a unique dual-phases structure, namely, the glassy matrix and unstable B2 phase. In the present study, in-situ tensile testing is used to trace the deformation process of a ZrCu-based BMGC. Three deformation stages of the BMGC, i.e., the elastic-elastic stage, the elastic-plastic stage, and the plastic-plastic stage are identified. In the elastic-elastic and elastic-plastic stages, the yield strength and elastic limit are major influenced by the volume fraction of the B2 crystals. In the plastic-plastic stage, the B2 phase stimulates the formation of multiple shear bands and deflects the direction of shear bands by disturbing the stress field in front of the crack tip. The deformation-induced martensitic transformation of the metastable B2 phase contributes to the plasticity and work hardening of the composite. This study highlights the formation and propagation of multiple shear bands and reveals the interactions of shear bands with structural heterogeneities in situ. Especially, the blocking of shear bands by crystals and the martensitic transformation of the B2 phase are critical for the mechanistic deformation process and illustrate the function of the B2 phase in the present BMGCs.

Plate reinforced square hollow section X-joints subjected to in-plane moment

The static test of 13 square hollow section (SHS) X-joints with different β and different types of plate reinforcement under in-plane moment in brace was carried out. Experimental test schemes, failure modes of specimens, moment-vertical displacement curves, moment-deformation of the chord, and strain strength distribution curves were presented. The effect of β and plate reinforcement types on in-plane flexural property of SHS X-joints was studied. Results show that punching shear of chord face disappears, brace material fracture appears and concave and convex deformation of chord decrease when either collar plates or doubler plates were welded on chord face. Moment-vertical displacement curves of all specimens have obvious elastic, elastic-plastic and plastic stages. As β increases, the in-plane flexural ultimate capacity and initial stiffness of joints of the same plate reinforcement type increase, but ductility of joints decreases. With the same β, the in-plane flexural initial stiffness and ultimate capacity of doubler plate reinforced joints, collar plate reinforced joints, and unreinforced joints decrease progressively. Thickness of reinforcement plate has no obvious effect on in-plane flexural initial stiffness and ultimate capacity of joints. As thickness of reinforcement plate increases, the ductility of reinforced X-joints decreases. The concave and convex deformation of every specimen has good symmetry; as β increases, the yield and ultimate deformation of chord decrease.

Optimization of Stepped Elastic Plastic Plates

A method of analysis and optimization of stepped plates made of elastic plastic materials is developed. The stress-strain of the plate is defined for the initial elastic and subsequent elastic plastic stages of deformation. Necessary optimality conditions are derived with the aid of variational methods of the theory of optimal control. This results in a differential-algebraic system of equations. The latter is solved numerically. The effectivity of the design established is assessed in the cases of one-and multi-stepped plates assuming the material obeys the Tsai-Wu or von Mises yield condition.

A Comparative Analysis on the Longitudinal Compression Characteristics of Juvenile and Mature Northeast Chinese Ash (Fraxinus mandshurica Rupr.) Subjected to Alkaline Treatment

Longitudinal compression can help wood form some folds on the wood cell walls after a suitable softening procedure. These folds can enhance the one- and multi-dimensional bending performances of wood. The longitudinal compression properties of alkali-treated juvenile and mature northeast Chinese ash (Fraxinus mandshurica Rupr.) were analyzed. Elastic and elastic-plastic stages were inferred from the longitudinal compression curves. Scanning electron microscopy images showed that some folds formed in the wood cell wall vessels and fibers. X-ray microdensitometer test results showed a decrease in the fluctuation of the wood cell wall density of the specimens. The swelling and degradation or extraction of hemicellulose, lignin, and extractive occurred after alkaline treatment. Slippage between cellulose chains and curving within a cellulose chain were inferred during longitudinal compression. Juvenile wood specimens had higher modulus of elasticity and larger variability than mature wood after alkaline treatment. This finding can be attributed to the higher extent of degradation or extraction of hemicelluloses, lignin, and extractive, as well as the smaller microfibril angle and the similar cellulose crystallinity of treated juvenile wood compared to those of mature wood specimens after alkaline treatment.