L-shaped Specimens Research Articles

Shear behavior of 3DPM-NM specimens with different interfacial locking designs

Since 3D printed concrete has a promising application in engineering, the combination of 3D printing concrete and other building materials has become a hot research topic.This paper investigates the shear properties of 3DPM-NM specimens which use 3D printed mortar (3DPM) as the precast part and normal mortar(NM) as the cast-in-place part. Four different types of interfacial locking design were analyzed in this paper, including L-shaped, C-shaped, S-shaped and K-shaped. The shear damage law of the specimens was analyzed using digital image correlation (DIC) technology. The results show that L-shaped specimens are basically damaged along the shear surface during the loading process. Most of the initial cracks in the C-shaped, S-shaped and K-shaped specimens appeared along the interfacial bonding surface during the loading process. Then, the cracks in the C, S and K-shaped specimens continued to develop along the interfacial bonding surface or spilt through the specimens directly. The results show the shear properties of the C-shaped specimens are similar to those of L-shaped specimens. And the mechanical properties of S-shaped and K-shaped specimens are better than those of L-shaped specimens which increased by 96% (S-shaped), 78% (K-shaped) respectively. The mean slip is increased by 10% in S-shaped specimens and 28% in K-shaped specimens compared with that of L-shaped specimens. According to the AASHTO GSCB INTERIM-2003 specification, the modified formula for the shear bearing capacity of 3DPM-NM specimens is proposed in this study. The calculated resultsare in good agreement with the test results, which theoretically provide guidance for the application of 3DPM, lay the foundation for its feasibility as a structural body of a building.

Finite Element Analysis of Axial Compression Behavior of L-Shaped Concrete-Filled Steel Tubular Columns with Different Combinations

L-shaped concrete-filled steel tubular (CFST) columns, a kind of structural member appropriate for high-rise buildings, not only avoid the defect of conventional square columns protruding from the wall but also have the green and low-carbon properties of steel structures appropriate for fabricated construction. To learn more about their axial compression behavior, refined 3D finite element models were established using the general finite element software ABAQUS. The reliability of the models was subsequently verified based on failure tests and load–displacement relation tests on eight L-shaped specimens. The axial compression mechanism of L-shaped CFST columns was investigated using the verified finite element models. Further systematic parameter analysis was carried out to investigate the influence of parameters such as steel strength, concrete strength, length ratio of long limb to short limb, the angle between the two limbs, and combination methods on the axial compression behavior of L-shaped CFST columns. The results demonstrate that the angle between the two limbs has a significant impact on the stress distribution of concrete and steel pipes. The corner effect increases as the angle between the two limbs decreases. The combination of F-type specimens can better exert the constraint effect of steel pipes on concrete, while the triangular cavity of unequal-limb specimens and specimens with an included angle of 60° cannot effectively trigger the interaction between steel pipes and concrete. The initial stiffness of L-shaped CFST columns increases with an increase in concrete strength and a decrease in limb length ratio, which is not sensitive to changes in steel strength and the included angle. The peak bearing capacity of the specimens increases with increases in steel strength and concrete strength and a decrease in the limb length ratio. Compared to C-type and Z-type specimens, the initial stiffness of F-type specimens is slightly higher, and the peak bearing capacity is significantly increased.

Support Structures Optimisation for High-Quality Metal Additive Manufacturing with Laser Powder Bed Fusion: A Numerical Simulation Study.

This study focuses on Metal Additive Manufacturing (AM), an emerging method known for its ability to create lightweight components and intricate designs. However, Laser Powder Bed Fusion (LPBF), a prominent AM technique, faces a major challenge due to the development of high residual stress, resulting in flawed parts and printing failures. The study's goal was to assess the thermal behaviour of different support structures and optimised designs to reduce the support volume and residual stress while ensuring high-quality prints. To explore this, L-shaped specimens were printed using block-type support structures through an LPBF machine. This process was subsequently validated through numerical simulations, which were in alignment with experimental observations. In addition to block-type support structures, line, contour, and cone supports were examined numerically to identify the optimal solutions that minimise the support volume and residual stress while maintaining high-quality prints. The optimisation approach was based on the Design of Experiments (DOE) methodology and multi-objective optimisation. The findings revealed that block supports exhibited excellent thermal behaviour. High-density supports outperformed low-density alternatives in temperature distribution, while cone-type supports were more susceptible to warping. These insights provide valuable guidance for improving the metal AM and LPBF processes, enabling their broader use in industries like aerospace, medical, defence, and automotive.

Lateral load response of L-shaped steel–concrete composite shear walls using multi-partition steel tube

Behavior of L-shaped multi-partition steel–concrete composite shear walls (MP-SC-CSWs) subjected to the cyclic lateral load is experimentally and numerically evaluated. Such composite shear walls possess proper capacity, stiffness, and ductility, as the steel tube strengthened by inner steel plates considerably confines the infill concrete. Four large-scale L-shaped MP-SC-CSW specimens were designed, fabricated, and tested. The L-shaped specimens were exposed to lateral hysteretic loading and axial compression loading. The experimental parameters were axial load ratios and height-to-depth ratios (wall aspect ratios). The experimental results indicated that the L-shaped MP-SC-CSWs had high lateral capacities, energy dissipations, and good ductility. Based on the experimental observations, specimens failed in flexure mode. Increasing the axial load ratio enhances the capacity to dissipate energy but decreases the ductility. The tested specimens could meet the drift requirement of the specification even when subjected to a high design axial load ratio of 0.76. Additionally, the wall aspect ratios had little influence on the ductility. 3D nonlinear finite element models of tested specimens were established and verified with test results to conduct further numerical studies. A simplified method based on the plastic stress distribution was developed and confirmed by the experimental and finite element analysis to calculate the flexural capacities of MP-SC-CSWs.

BEARING FAILURE MODE AND STIFFNESS ANALYSIS OF L-SHAPED PARTS CONNECTED BY BEECH AND SELF-TAPPING SCREW COMPOSITE DOWELS

This study examined the performance of beech and self-tapping screw composite dowel applied to non-extruded L-shaped member joints and carries out theoretical calculation and analysis of stiffness. The test results indicated that eleven specimens showed three failure modes. Itwas found that the composite dowel had different degrees of bending phenomenon. Thefailure modes were also proved by the finite element analysis. Since the spruce-pine-fir was in the middle of the L-shaped specimen set in this test, it can rotate around the geometric center composed of four composite dowels.

Experimental and theoretical study on seismic behavior of slab-wall joints in a new large-span slab-shear wall structural system

This paper proposes a new structural system named large-span slab-shear wall structure, which has prominent advantages in the architectural function and structural construction compared to the traditional shear wall structure. Due to the larger span and greater thickness of slabs, as well as the elimination of many beams and internal walls, the thick slab-shear wall joint becomes a key component in the new structure. This paper focuses on three typical types of thick slab-shear wall joints and conducts cyclic loading tests to reveal their seismic performance, including the exterior joints on the top floor (three l-shaped specimens), exterior joints on the intermedia floors (two T-shaped specimens), and interior joints on the top floor (two T-shaped specimens). Variables including the anchorage method of the slab bars and the reinforcement ratio of the walls are studied. Thereafter, based on the test results, a new calculation method for the joint strength of the slab-wall joints is proposed based on the strut-and-tie model from ACI318-19. The test results demonstrate that the proposed three types of slab-wall joints exhibit adequate strength and ductility under seismic loading despite the joint shear failure. The proposed simplified anchorage method can greatly reduce the congestion in the joint zone without weakening the seismic performance. The strut-and-tie model can provide an accurate prediction for the shear strength of the slab-wall joint zone.

Phase-field regularised cohesive zone model for interface modelling

A discrete interface is represented as a smeared interface in the framework of phase field regularisation. Due to the numerical challenge when imposing boundary conditions we prescribe the phase field in the domain using analytical solutions. We obtain the displacement and the strain from the variational form of the energy functional, but also in the framework of the regularised extended finite element method. Indeed, both approaches result in identical forms, validating the rigour of the method. We have examined different forms of the phase field model and conclude that models with a compact support are more appropriate for modelling interfaces. The method combines advantages of discrete and smeared approaches. Compared to discrete interface models, the method holds similar properties as the extended finite element method (XFEM): no need to treat cracks as geometric discontinuities and avoiding mesh refinement around crack tips. Different from the XFEM, however, the method does not introduce enrichment functions to describe cracks. An advantage compared to the phase field method is that this method directly employs the cohesive zone law from the discrete model, which is physically relevant. The accuracy of the approach for cohesive interface modelling is demonstrated by several numerical examples, including a bar, an L-shaped specimen, and a fibre embedded in an epoxy matrix.

An XFEM‐strain gradient damage model for efficient modeling of materials with reinforcement particles

A coupling of extended finite element method (XFEM) and strain gradient enhanced damage method is proposed to model damage initiation and growth in the materials with reinforcement particles. The proposed model inherits the advantages of both methods i.e., simple definition of reinforcement particles from XFEM and numerical stability during damage evolution from the strain gradient damage model. The capability of the proposed XFEM-strain gradient damage model has been demonstrated by simulating mode-I, mode-II, and the mix-mode failure mechanism in the SENB, SEN, and L-shaped specimen with reinforcement particles respectively. The numerical simulations are performed by considering hard and soft reinforcement particles in a matrix. Mesh-sensitivity and particle-sensitivity analysis are performed to quantify effect of various parameters like mesh size, particle size and volume fraction on behavior of SENB specimen. The proposed XFEM model is at least 10% efficient (at higher mesh densities) than the standard FEM model for solving the SENB specimen with reinforcement particles. In the particle sensitivity analysis, proposed XFEM model used only one mesh density (unlike FEM model that needs different conformal meshes) to simulate set of 400 realizations with random spatial distribution of reinforcement particles. The analysis shows that the computational demand for the creation of conformal mesh is alleviated with the use of XFEM for defining reinforcement particles. The mesh-sensitivity analysis shows that difference in the solution obtained using proposed XFEM model and standard FEM model is negligible (less than 1% at higher mesh densities). Therefore, the proposed XFEM-strain gradient damage model can substitute FEM-strain gradient damage model with above mentioned efficiencies.

The analysis on the seismic behavior of L-shaped prefabricated special-shaped CFST with rectangular multi-cell columns

In this paper, a prefabricated special-shaped concrete-filled steel tube (CFST) with rectangular multi-cell column has been subjected to low cyclic reversed loading experiment and finite element (FE) parameter analysis. A total of three full-scale L-shaped specimens were tested under constant axial compression load and transverse cyclic load. The main variable in the experiment was the direction of loading. The results show that the failure mode of the special-shaped column specimen is compression-bending failure. Within 300 mm of the bottom of the column, the steel tube bulges or even cracks, the concrete powder fells and fails to exit the work. Among them, the L-135° specimen has the highest peak bearing capacity while the L-0° specimen has better ductility. The main variables of FE parametric analysis are axial compression ratio, concrete grade, steel tube thickness and loading angle. The analysis results show that the lower axial compression ratio and higher concrete grade are conducive to the improvement of bearing capacity. As the thickness of the steel tube increases, the peak bearing capacity and displacement of the specimen increases but the ductility decreases. The special-shaped column model has better seismic behavior when loaded along various loading angles. The peak bearing capacity of the special-shaped column specimen loaded at 135° is consistent with the test results and compared with the L-45° specimen, its bearing capacity is increased by up to 62.04%, which has better comprehensive seismic behavior.

Survey of Localizing Gradient Damage in Static and Dynamic Tension of Concrete.

The continuum damage model should be regularized to ensure mesh-insensitive results in simulations of strain localization, e.g., for concrete cracking under tension. The paper confronts the conventional gradient damage model with its upgrade including a variable internal length scale. In these models, the Helmholtz free energy depends additionally on an averaged strain measure and its gradient. In the formulation for dynamics the equations of motion are discretized simultaneously with an averaging equation. If gradient regularization is employed with a constant internal length parameter, then an artificially expanded damage zone can occur in the strain softening analysis. This broadening effect can be inhibited by a gradient activity function. The localizing character of the gradient activity has physical motivation—the nonlocal interactions in the fracture zone are reduced with the damage growth. The internal length can decrease exponentially or as a cosine function. After presentation of the theory, including the free energy definition, the finite element analyses of three different examples connected with tensile cracking in concrete are discussed: static tension of a double-edge-notched specimen, dynamic direct tension for a configuration without or with a reinforcing bar and tension of an L-shaped specimen under static and dynamic loading.

Experimental study on L-shaped precast concrete superposed shear walls under quasi-static cyclic loading with different axial compressive load ratios

This study experimentally investigates the seismic behavior of L-shaped precast superposed shear walls with cast-in-place boundary elements. Five full-scale L-shaped specimens were constructed and quasi-static cyclic loading applied to them. A new vertical joint connecting the superposed wall panels and a boundary element was designed. The influence of different vertical joints, boundary elements, and axial compressive load ratios on the seismic behavior of the L-shaped superposed shear wall were analyzed. The failure mode, strength, hysteresis feature, ductility, energy dissipation capacity, and stiffness degradation of each specimen were compared. The test results showed that the specimens with boundary columns exhibited a flexural failure characterized by concentrated crushing of the bottom of the free end of the webs, whereas the specimens with special boundary elements exhibited shear compression failure characterized by a principal inclined crack in the center of the webs. The vertical connecting longitudinal steel bars were effective in resisting load under the action of an earthquake, and the force transfer performance of the connecting reinforcement was reliable and stable. The transverse confinement at the free end of the specimens with special boundary elements ensured a stable bond stiffness to prevent the crack from opening at the horizontal joint. The increase in the axial compressive load ratio may lead to an increase in positive shear deformation. The story drift ratios of the L-shaped superposed shear walls with cast-in-place boundary columns and special boundary elements met the limit value requirements of the story drift ratio in Chinese code JGJ/ 3-2010. The seismic performance of L-shaped superposed shear walls with boundary columns was slightly lower than that of L-shaped cast-in-place shear walls. The seismic performance of L-shaped superposed shear walls with special boundary elements was significantly better than that of the reference cast-in-place shear walls. Furthermore, based on the characteristics of shear compression failure, a calculation formula for the shear bearing capacity of an L-shape precast concrete superposed shear wall was established, and the proposed equations were validated.

Testing method for evaluating mechanical properties of 3D printed CFRP with curved fibers by four-point bending test of L-shaped specimen

In this study, we propose a method to evaluate the mechanical properties (circumferential fracture stress, circumferential elastic modulus) of 3D printed carbon fiber reinforced plastics with a curved section by a four-point bending test of an L-shaped specimen. Several L-shaped specimens with different radii were printed to evaluate the relation between the radius and mechanical properties. Additionally, the internal structure was observed by X-ray CT, and the fiber orientation angle was examined by cross-sectional observation of the curved section to clarify the fracture mechanism and causes that affect the mechanical properties.In the four-point bending test of the L-shaped specimen, a compressive fracture first occurs at the outer surface of the curved section, and the compressive fracture stress and the elastic modulus decrease with the inner radius are less than 10 mm and 5 mm, respectively. The evaluation of the fiber orientation angle suggests that the decrease in the elastic modulus is caused by the fiber bundle twisting that deviates the fiber direction from the circumferential direction. Furthermore, the fiber breakage and voids generated in the curved section can affect its mechanical properties.

Experimental study on the seismic behavior of prefabricated L-shaped concrete-filled steel tube with rectangular multi-cell columns under different lateral loading directions

Prefabricated residential buildings are becoming the focus of the construction industry. Simultaneously, in order to improve the seismic behavior of traditional RC special-shaped columns, a prefabricated special-shaped concrete-filled steel tube with rectangular multi-cell (S-CFST-R) column was proposed for residential systems. The seismic test of prefabricated L-shaped concrete-filled steel tube with rectangular multi-cell (L-CFST-R) columns was conducted in this study. Three full-scale L-shaped specimens with high practical guidance were designed which were based on the common parameters of residential buildings. A quasi-static test method under a low cycle load was used to simulate the earthquake action. Taking different loading directions (0°, + 45°, and + 135°) as the main research factors, the failure state, bearing capacity, hysteretic behavior, ductility, energy dissipation capacity, strength degradation, stiffness degradation, and strain change of each specimen were studied and analyzed. Moreover, a finite element model was developed to simulate L-CFST-R columns at different loading directions, and the simulation results agreed well with the experimental results. The influence of more loading directions on the ultimate bearing capacity and skeleton curve is analyzed comprehensively. The results indicated that multi-cell special-shaped steel tubes had a strong restraining effect on the core concrete, so the L-CFST-R columns under different lateral loading directions exhibited favorable seismic behavior, which is suitable for prefabricated residential buildings.

Experimental study of insulated sandwich concrete wall connections under cyclic loading

An innovative connections of insulation sandwich concrete panels (ISCPs) is presented in this study. The connection uses reinforced concrete (RC) column connecting wall panels, and steel rectangular tube is used as the boundary element at the free end of the web. To evaluate the hysteretic property of the connection and the effectiveness of the boundary element, the cyclic loading test is performed with four full-scale specimens including two T-shaped and two L-shaped specimens. The failure mode, bearing capacity, ductility, degradation property and energy dissipation capacity are investigated. The results show that the failure of the connection is due to the cracks and local crushing in the web. The structural integrity remains intact during the test, and there is no separation between RC column and wall panel. The specimens with boundary elements have better hysteresis performance, better ductility, more stable stiffness degradation and better energy dissipation capacity. For T-shaped and L-shaped ISCPs wall connections, steel rectangular tube do not significantly increase their bearing capacity, but do substantially increase their ductility.

Experimental Study and Damage Model on the Seismic Behavior of Reinforced Concrete L-Shaped Columns under Combined Torsion

Due to the advantage of saving indoor space, a special-shaped column frame attracted more attention of the engineers and researchers. This paper presented a quasi-static cyclic loading experiment of six specimens of reinforced concrete (RC) L-shaped columns under compression-flexure-shear-torsion combined loadings to investigate the effect in the ratio of torsion to moment (T/M) and axial compression ratio (n) on their seismic performance. The results showed that the failure modes of L-shaped specimens included bending failure, bending-torsion failure, and torsion-shear failure with the hysteretic curves exhibiting S shape. With the increase of T/M ratio, cracks on the flange developed more fully, and the height of plastic hinge decreased and torsion bearing capacity improved. Besides, as the T/M ratio increased the twist ductility increased, while displacement ductility decreased. On the other hand, with a higher axial compression ratio, torsion bearing capacity and bending stiffness were both increased. Moreover, the equivalent viscous damping coefficient of bending and torsion were 0.08~0.28 and 0.13~0.23, respectively. The average inter-story drift ratio met the requirements of the Chinese standard. Finally, two modified models were proposed to predict the progression of damage for the L-shaped column under combined loading including torsion.

Corner inspection method for L-shaped composite structures using laser ultrasonic rotational scanning technique

Composite primary structures that can sustain high loads are used to reduce the weight of structures in various industries. Composite primary structures that are L-shaped or T-shaped have a right-angled corner, and phased-array ultrasonic inspection or water squirt robotic C-scan is mainly used for the corners. However, using a customized transducer has restrictions depending on the size and shape of the corner. In this paper, we propose a novel approach based on rotational laser ultrasonic inspection for even narrow corner sections regardless of size and angle. In the rotational scanning method, the target structure is rotated for inspection, which is different from conventional scanning methods in which transducers or a scanning head are moved along the inspection area. The rotational scanning method is designed to solve the problem of signal-to-noise ratio deterioration that occurs when the ultrasonic sensing laser scans a curved surface. The corner sections of two L-shaped specimens fabricated by autoclave curing and resin transfer molding were inspected using the rotational scanning method. Using the proposed method, all of the artificial defects inserted into the corner section of the specimens were clearly visualized. Then, the inspection results were compared with water squirt C-scan inspection results. The test results showed that the proposed method is highly reliable and useful for inspecting the corner sections.

Study of the effect of shape processing of a specimen on the modal properties

When the dynamic design of a structure is carried out by using finite element method, the dynamic characteristics of the member of the structure have to be identified in advance. Especially, the damping property of the actual complex shaped member is sometimes assumed to be the same as the one of a simple shaped member with the same material. The damping properties often become a function of the frequency, so it is not certain whether we can apply the dynamic characteristics of the simple shaped member to the complex shaped one or not. In this study, a beam specimen made of steel is adopted, and the modal properties of both uniform shaped specimen and L-shaped one are identified; then the influence of the shape processing is investigated on the damping property. As a result, it is recognized that the damping property is high in the low-frequency band and low constant in the high-frequency band. Furthermore, the damping property of the L-shaped specimen is higher in the low-frequency band than the case of uniform specimen, while it is almost the same in the higher-frequency band.

Efficient Deep Learning for Gradient-Enhanced Stress Dependent Damage Model

This manuscript introduces a computational approach to micro-damage problems using deep learning for the prediction of loading deflection curves. The location of applied forces, dimensions of the specimen and material parameters are used as inputs of the process. The micro-damage is modelled with a gradient-enhanced damage model which ensures the well-posedness of the boundary value and yields mesh-independent results in computational methods such as FEM. We employ the Adam optimizer and Rectified linear unit activation function for training processes and research into the deep neural network architecture. The performance of our approach is demonstrated through some numerical examples including the three-point bending specimen, shear bending on L-shaped specimen and different failure mechanisms.

Computational multi-scale analysis of simultaneous processes of delamination and damage accumulation in laminated composites

When studying mechanical behavior of structures made of laminated polymer composite materials, issues related to their relatively high susceptibility to damage, such as delamination, fiber breakage and matrix failure, play significant role. Key aspects are also related to accurate description of the internal structures and heterogeneity of the material. This work is aimed at a numerical study of the processes of damage accumulation in plies as well as development of delamination in laminated polymer composite L-shaped specimen taking into account microstructural parameters. The influence of contact conditions between the plies on the model’s behavior has been studied. It was established that the delamination processes inside the specimen can initiate at the early stages of deformation, which is reflected in the difference of the results obtained using models with various numbers of separated plies. Comparison of the results of modelled elastic behavior of the specimen and models with the developed delamination leads to conclusions about influence of a number of model parameters on the results obtained before and after propagation of delamination. In particular, such factors as the geometry and mechanical properties of models, the number and conditions for specifying contact pairs in the model were analyzed.

Modeling and Validation of Bolted Joints Based on Nonlinear Virtual Material Method

The bolted joint provides more than half of the stiffness and nearly all of the damping of the whole machine tool. It is necessary to study the dynamics mechanism and establish accurate model of joint surface. The paper proposes the model of nonlinear virtual material based on fractal theory and Hertz contact theory, the contact surface can be divided into several sections based on the pressure distribution, the elastic modulus and stress of each section can be calculated. An L-shaped specimen is designed to validate the accuracy of presented model. The maximum errors of natural frequencies between theoretical and experimental is 8.8%, suggesting that nonlinear virtual material method is appropriate for the bolted joint.