Unidirectional Loading Research Articles

Extrusion-Based 3D Printing of Stretchable Electronic Coating for Condition Monitoring of Suction Cups.

Suction cups (SCs) are used extensively by the industrial sector, particularly for a wide variety of automated material-handling applications. To enhance productivity and reduce maintenance costs, an online supervision system is essential to check the status of SCs. This paper thus proposes an innovative method for condition monitoring of SCs coated with printed electronics whose electrical resistance is supposed to be correlated with the mechanical strain. A simulation model is first examined to observe the deformation of SCs under vacuum compression, which is needed for the development of sensor coating thanks to the 3D printing process. The proposed design involves three circle-shaped sensors, two for the top and bottom bellows (whose mechanical strains are revealed to be the most significant), and one for the lip (small strain, but important stress that might provoke wear and tear in the long term). For the sake of simplicity, practical measurement is performed on 2D samples coated with two different conductive inks subjected to unidirectional tensile loading. Graphical representations together with analytical models of both linear and nonlinear piezoresistive responses allows for the characterization of the inks’ behavior under several conditions of displacement and speed inputs. After a comparison of the two inks, the most appropriate is selected as a consequence of its excellent adhesion and stretchability, which are essential criteria to meet the target field. Room temperature extrusion-based 3D printing is then investigated using a motorized 3D Hyrel printer with a syringe-extrusion modular system. Design optimization is finally carried out to enhance the surface detection of sensitive elements while minimizing the effect of electrodes. Although several issues still need to be further considered to match specifications imposed by our industrial partner, the achievement of this work is meaningful and could pave the way for a new generation of SCs integrated with smart sensing devices. The 3D printing of conductive ink directly on the cup’s curving surface is a true challenge, which has been demonstrated, for the first time, to be technically feasible throughout the additive manufacturing (AM) process.

Influence of tooth root cracks on the mesh stiffness of asymmetric spur gear pair with different backup ratios

Gears are critical machine elements that transmit power and motion in diverse implementation fields. Over time, gears may produce a series of faults due to harsh operating conditions, fatigue, manufacturing errors, etc., leading to severe performance degradation. During the meshing process, the stiffness of a single tooth controls the load sharing, vibration, and noise characteristics of a geared system. An undetected fault could decrease the gear stiffness and thus may lead to a fatal breakdown, substantial economic losses, or even human casualties in safety-critical applications such as helicopters, high-speed trains, and wind turbines. Hence, the accurate quantification of the gear stiffness emerges as an important research area to obtain reliable gear designs. With this in mind, the asymmetric tooth concept offers superior bending strength, fatigue propagation life, and the ability to lessen vibration over the standard (symmetric) designs in applications where unidirectional loadings are predominant. This study investigates the influence of tooth root cracks on the single-tooth and meshing stiffness characteristics of the standard and asymmetric involute spur gears. To this end, the numerical crack propagation paths obtained in our previous works were introduced to the created 3D CAD geometries. Subsequently, the single tooth stiffness of both healthy and cracked (2 5%-50%-75%-100%) gears was calculated through the ANSYS® Workbench, and the time-varying mesh stiffness was obtained. The present study evaluated the effects of backup ratio and the tooth asymmetry on the spur gears’ meshing stiffness characteristics simultaneously and further expanded the scope of the research work. The results indicated that the single tooth stiffness and mesh stiffness could be improved by 35% and 22%, respectively, as the drive side pressure angle increased from 20° to 35°. It has been noted that the gear stiffness decreased as the crack level increased, while the increment of the backup ratio further increased the reduction in the stiffness. The findings could provide significant outputs for a better understanding of the influence of tooth asymmetry on the gear dynamics characteristics, life prediction, and early fault diagnosis.

Construction and Verification of Spherical Thin Shell Model for Revealing Walnut Shell Crack Initiation and Expansion Mechanism

Walnut shell breaking is the first step of deep walnut processing. This study aims to investigate the mechanical properties and fracture state of the Qingxiang walnut shell under unidirectional load and guide the complete separation of the walnut shell and kernel. The spherical thin shell model of the walnut (the fitting error is less than 5%) was established and verified. The process from the initiation to the expansion of walnut cracks was analyzed. The crack expansion rate was estimated in terms of the crack fracture regularity on the shell’s surface. Based on the momentless theory and finite element simulation analysis, we found that the stress on the shell surface in the concentrated force action region was gradient distributed from inside to outside and that the internal forces were equal in all directions in the peripheral force action region. The unidirectional impact shell-breaking experiments confirmed the reliability of our spherical thin shell model and verified our hypothesis of walnut shell fracture along the longitudinal grain. Our results can provide a theoretical basis for the development and structural optimization of shell-breaking machinery.

Effect of boundary conditions on values of homogenized fabrics’s elastic parameters and inelastic response

The effect of displacement, periodic and surface traction boundary conditions (BCs) on predicted elastic parameters and inelastic response of a homogenized material equivalent to either a unidirectional or a randomly distributed short fibers reinforced composite has not been extensively studied. A few such studies have focused on their elastic analyses. Here, we analyze the effect of the three types of BCs on both elastic and inelastic deformations of weaves and braids with continuum damage mechanics used in the inelastic analyses. We employ simple unit cells, Hill’s theorem, and the displacement, periodic and uniform traction BCs to deduce from the finite element stiffness matrix K the matrix C of homogenized moduli. For both elastic and inelastic problems, it is seen that predictions using the displacement and the periodic BCs are close to those reported in the literature while those from the traction BCs are not. For unidirectional loading, it is found that the nonlinearity in the axial stress - axial strain curve, the localized deformations and the resin damage occurring in regions unsupported by fibers are significantly more for the traction BCs than that for the other two BCs. Also, there is more damage for the periodic BCs than that for the affine displacement BCs.

Bearing failure mechanism of rock-socketed monopile foundation for offshore wind turbine in weathered-granite seabed

Large-scale constructions of offshore wind power are carried out in southeast coast of China, where the seabed is covered in weathered granite. However, the bearing capacity and failure mechanism of monopile foundation in weathered-granite seabed are rarely researched. In this paper, the stress-strain characteristics and strength parameters of granite with different weathering degrees were obtained through laboratory tests, and the pre-strain test method was used to study mechanical parameter variation of moderately weathered granite in the crushing process. Experiment found that the strength parameters decrease mainly after the strain reaching 50% of the ultimate strain of moderately weathered granite. In addition, the bearing capacity and failure mode of rock-socketed monopile under unidirectional horizontal loading was investigated numerically. Analysis showed that the failure mode of rock-socketed monopile under unidirectional horizontal loading is progressive and presents two obvious failure zones. More critical, the existence of vertical loading effectively improves the horizontal bearing capacity of monopile foundation in the weathered rock seabed, and the failure envelope of the monopile foundation in H-V loading space is elliptic-shaped. This provides further understanding of the mechanical properties of submarine weathered granite, and the helpful reference for stability evaluation of rock-socketed monopile foundation in weathered granite seabed.

Test and mechanism analysis for crack propagation by blasting in rock under the condition of unidirectional load

In deep rock blasting, there are different constraint conditions such as unidirectional loading, bidirectional loading and tridirectional loading due to in-situ stress. The similar model test of rock blasting under unidirectional load was carry out to study the law of crack propagation in rock by blasting in this study. A transparent material which conformed to the mechanical properties of hard rock is used to make specimens. A high-speed camera was used in some model blasts. After blasting the surface cracks on the specimen were measured, including the near zone and middle zone of blasting. The results showed that: (1) the radial main cracks in the specimens under initial static load propagated along the principal stress direction, which is different from that without initial stress, and there is almost no radial main crack in the direction perpendicular to the principal stress. (2) The average length of radial crack and diameter of circumferential fracture circle gradually decrease with the increase of unidirectional initial stress, but the diameter of compression crushing circle increase. (3) The maximum and average initial velocity of radial crack growth decrease with the increase of unidirectional initial stress.

Cylindrical Bidirectional Strain Sensors Based on Fiber Bragg Grating.

To realize continuous real-time monitoring of the large-scale internal strain field of coal and rock mass, a bidirectional strain sensor based on FBGs encapsulated using a hollow cylindrical steel tube was designed. The sensor’s structural parameters were optimized through unidirectional loading, and the strain change laws of the sensor were analyzed under unidirectional and bidirectional loading conditions, in which the stress-strain fitting curves of the sensor and the relationships of the strain in the vertical and horizontal directions were obtained under different lateral pressure loading conditions. A similar theoretical model was established to verify the accuracy of the linear relationship between the surrounding rock stress and the strain measured by the sensor system.

Five-stage tensile constitutive model of hybrid fiber reinforced polymer bar

Tensile constitutive response of hybrid fiber reinforced polymer (HFRP) bar plays a crucial role to characterize its tensile properties. An innovative model with five stages, including the initial linear elastic stage, stress sudden drop stage, stress recovery stage, platform stage and strain hardening stage, was proposed to describe the stress-strain response of HFRP bar under the unidirectional tensile load. The proposed tensile constitutive model considered the influence of hybrid effect on the yield tensile strength and strain, while the post-yield tensile stress sudden drop was also taken into account. Subsequently, a series of carbon/glass-HFRP bars with different characteristics were independently produced by the pultrusion-winding process, and tested under unidirectional tensile load to verify the validity of proposed tensile constitutive model. The outcomes have indicated that the proposed tensile constitutive model is in good agreement with the experimental results.

Test results for a monopile in medium dense sand subjected to unidirectional lateral cyclic loading

It is necessary to understand the response of monopile foundations for large offshore wind turbines (OWTs) with rated powers of under cyclic loading. For this purpose, a monopile model test study was conducted in saturated medium dense sand under long-term unidirectional lateral sinusoidal cyclic loading for cycles. The model pile was constructed with displacement transducers on the pile head and strain gauges along the pile length. The experimentally obtained bending moment, displacement, soil reaction evaluation along the pile length, pile head displacement accumulation, and lateral soil-pile stiffness were substantially different from the results of the American Petroleum Institute (API)-recommended method of determination and were significantly affected by the number of loading cycles. The experimentally based soil reaction -pile displacement curves obtained considering the maximum soil reactions for particular cycles were compared with the - curves recommended by the API. The maximum soil reaction during the increase in pile displacement obtained experimentally was significantly higher than the soil reaction obtained with the API method. The p–y curves obtained experimentally show similarities with those determined with the API method at only small pile displacements in the initial stage of the curves.

Analysis of Initial Cracking of an Interface between a Bundled Lipped Channel–Concrete Composite Wall and an Infill Wall

The bundled lipped channel–concrete (BLC-C) composite wall structure is a new structure with several advantages such as a high bearing capacity and good seismic performance. However, interface cracks between a BLC-C composite wall and the infill wall (non-structural wall) are a severe problem and need to be urgently resolved. Interface cracks affect not only the esthetics, but also the normal use of a building. The presence of interface cracks changes the perceptions of the owners of a structure, forcing them to question its safety and even take legal action against its developer. Therefore, in this study we aimed to investigate the initial cracking of the interface between a BLC-C composite wall and an infill wall. Unidirectional horizontal loading tests were conducted on two infill wall specimens constrained by BLC-C composite walls on both sides. The finite element analysis software ANSYS was used to simulate the loading process of the tests. The test results were compared to verify the accuracy of the finite element model. A finite element analysis was conducted to determine the effect of the horizontal displacement of the specimens when the interface initially cracked under different parameters such as the widths of the BLC-C composite wall, infill wall, and opening as well as the strength grade of the bricks and maximum normal contact stress. The results showed that a decrease in the width of the BLC-C composite wall or a rise in the width of the infill wall delayed the appearance of interface cracks. A large opening also delayed the occurrence of interface cracks. An enhancement in the strength grade of the bricks led to an earlier appearance of interface cracks. Interface cracks occurred later with an increase in the maximum normal contact stress between the BLC-C composite wall and the infill wall.

Influence of Unidirectional Cyclic Loading on Bond between Steel Bars Embedded in Lightweight Aggregate Concrete

The topic of present research is the experimental investigation of pull-out resistance of B500B-type ribbed reinforcing steel bars embedded in lightweight aggregate concrete (LWAC) under unidirectional cyclic loading. Only a limited amount of standardized data on bond strengths are available in the case of cyclic loading, especially for lightweight aggregate concrete. This paper deals with the experimental comparison of bond behavior of steel bars embedded in LWAC with expanded clay (Liapor) aggregate and normal weight aggregate concrete (NWAC) specimens by means of standard (non-cyclic) and cyclic pull-out tests. The LWAC and NWAC specimen’s mixes were identical except for the type of the coarse aggregate. It was found that the bond strength determined by non-cyclic pull-out tests showed a higher standard deviation compared to that for compression and splitting tensile strength tests. The shape of the characteristic bond strength–the slip curve was similar in the case of LWAC and NWAC and the failure mode was the same for both types of aggregate. Moreover, the maximum value of bond stress was identical for LWAC and NWAC and also no significant difference was found in the low cycle number fatigue. Both mixes were able to resist the maximum pull-out force multiple times in the case of cyclic loading because there was no time for the development of cracks.

Analysis of Vertically Oriented Coupled Shear Wall Interconnected with Coupling Beams

The nonlinear static response of a vertically oriented coupled wall subjected to horizontal loading is presented in this research article. The 3 storey vertically oriented coupled wall interconnected with coupling beams is modelled as solid elements in a finite element (FE) software named Abaqus CAE and the steel reinforcement is modelled as a wire element. For simulation of concrete models, a concrete damaged plasticity constitutive model is taken into consideration in this research. Moreover, with the help of concrete damage plasticity parameters, validation of two rectangular planar walls was executed with an error of less than 10 percent. Finally, these parameters are used for modeling and analyzing the static behavior of coupled walls connected with coupling beams. Furthermore, the maximum unidirectional horizontal loading helped in obtaining the compression and tensile damage as well as scalar stiffness degradation. Significantly, the research also found the plastic hinge location in the coupled wall as well as in the coupling beam, which are of utmost importance in nonlinear analysis. Doi: 10.28991/HIJ-2022-03-02-010 Full Text: PDF

Structural performance of cross-laminated timber-concrete composite floors with inclined self-tapping screws bearing unidirectional tension-shear loads

This study investigated the structural performance of cross-laminated timber (CLT)-concrete composite floor with inclined self-tapping screws bearing unidirectional tension-shear loads. Five groups of push-out tests were conducted to evaluate the shear behavior of the inclined screw connector. The results showed the embedded depth of the inclined screw in the CLT had a significant influence on the shear performance of the connectors, and the vertical screws demonstrated enhanced ductility. Next, six groups of specimens were subjected to four-point bending tests to study their flexural behavior under various parameters. A screw spacing that was too small led to premature failure, whereas an increased section height greatly improved the bending performance of these specimens. One specimen in particular exhibited a plate-end shear enhancement accompanied by excellent performance in terms of flexural stiffness and bearing capacity. Furthermore, a computational optimization method for determining an effective bending stiffness of the composite floor was proposed based on ‘γ-method’. This approach expanded the application scope of traditional methods, and the calculated values were in good agreement with the experimental values. Finally, a simplified finite element model was established, and parametric finite element analysis verified that improving the shear efficiency of the connector could bring economic benefits.

Mechanical characteristics of circular hollow section damper under bidirectional cyclic loading

During earthquake events, hysteretic dampers installed on the elevations composed of the columns and beams of structural frames can resist forces in any direction other than the in-plane direction because of the three-dimensional behavior of buildings and the interrelationship between the dampers and the adjacent members. Circular hollow section dampers (CHSDs), which form a type of steel hysteretic dampers, can be employed as seismic devices because they provide equal structural performance in any horizontal direction according to the geometrical characteristics of the cross-sectional shape. However, the cumulative hysteretic characteristics of CHSDs for multidirectional behavior remain unelucidated; therefore, an empirical review is needed. This study experimentally analyzed the cumulative hysteresis behavior of a CHSD repeatedly loaded sequentially in two horizontal directions (bidirectional cyclic loading). The results obtained were compared with previous experimental results under unidirectional cyclic loading conditions. The comparative analysis using empirical data revealed that the hysteretic curve of the CHSD under bidirectional cyclic loading maintained a relatively constant strength increase ratio and effective stiffness along with a stable spindle shape until the damper reached the ultimate state despite the continuous change in the loading direction. Further, compared with the unidirectional cyclic loading condition, the CHSD under bidirectional cyclic loading exhibited similar structural trends considering the damper aspect ratio and material influence. Moreover, the total accumulated plastic deformation in each loading direction was approximately the same as the cumulative plastic deformation caused by unidirectional cyclic loading.

Pressure-resistant cylindrical shell structures comprising graded hybrid zero Poisson's ratio metamaterials with designated band gap characteristics

Deep submersible technology is currently an important part of oceanic advanced technologies while developing new pressure-resistant materials has become its inevitable trend. Mechanical metamaterials can be flexibly designed to exhibit a unique range of mechanical properties, among which zero Poisson's ratio (ZPR) metamaterials can yield no transverse displacements under in-plane unidirectional tensile or compressive loadings. Thus, it is projected to be applied to the pressure-resistant cylindrical shell structures to avoid circumferential extrusion stress when subjected to radial forced compression due to underwater distributed pressure. Inspired by nonhomogeneous or graded composite materials and structures, this work proposes a novel pressure-resistant cylindrical shell cover comprising graded ZPR metamaterials with vibroacoustic attenuation capabilities. The periodic boundary condition (PBC) and Bloch's Theorem are implemented with the finite element method (FEM) to optimally design the ZPR metamaterials with specified load-bearing properties and band gap characteristics. The mechanical and vibroacoustic performance of the corresponding homogeneous and graded metamaterial structures is finally discussed. The results indicate that the designed structure made of titanium alloy can safely bear the hydrostatic distributed pressure of 10 MPa with a weight-bulk ratio of 0.540 g/cm3 and also yield higher STL within specified frequency regions corresponding to the designable band gaps, which strongly demonstrates the effectiveness of the optimal design for multi-functional ZPR functional elements. Moreover, both acoustic mediums and preloads affect its underwater vibroacoustic performance. Therefore, this work offers an innovative design idea of pressure-resistant protective covers for marine structures and also extends the route to enhance their underwater vibroacoustic behavior via designing the band gap characteristics of the graded metamaterials.

Dynamic Analysis of the Seismo-Dynamic Response of Anti-Dip Bedding Rock Slopes Using a Three-Dimensional Discrete-Element Method

Earthquakes are a major external factor that induce landslides. In order to systematically study the dynamic effects and failure mechanism of anti-dip bedding rock slopes (the slope trend is the same as the joint trend, while the slope dip direction is opposite to the joint dip direction) under seismic action (as well as the spatial effects of the structural planes in the anti-dip bedding rock slopes), three-dimensional (3D) discrete-element numerical calculations were performed to analyze anti-dip bedding rock slopes with different slope angles, joint angles, and joint trends subjected to the action of natural seismic and sinusoidal waves. The results were analyzed to investigate the amplification effect, change in Fourier spectrum, failure mechanism, and permanent displacement of the slope under the applied seismic action. The permanent displacement of the slope was calculated using Newmark’s method and the results obtained were discussed and compared with those obtained from a dynamic analysis performed using the 3D discrete-element method. The results showed that the regularity of the spatial distribution of the amplification effect was less clear than that encountered in the planar problem (unidirectional or bidirectional dynamical loading), and this leads to the effect of having an overall rhythmical nature. The seismic wave decays in the high-frequency part from the bottom up of the slope, while the dominant frequency of the seismic wave decreases. The value of the permanent displacement obtained using Newmark’s method is much smaller than that obtained using the dynamic 3D discrete-element analysis approach. The angle between the joint and slope trends has a significant effect on the amplification effect, failure mode, permanent displacement, and stability of slopes subjected to seismic action.

Liquefaction behavior of fiber-reinforced calcareous sands in unidirectional and multidirectional simple shear tests

A study on the liquefaction resistance of calcareous sands reinforced with polypropylene fibers was reported. Stress-controlled cyclic simple shear tests were conducted on specimens prepared at a relative density of 50%, with and without fiber reinforcements. The liquefaction behavior was investigated by considering the effects of fiber contents ranging from 0% to 1%, fiber lengths varying from 3 mm to 12 mm and loading patterns. The results indicated that increasing fiber content and fiber length resulted in a decrease in the deformation, a reduction in pore pressure accumulation rate, and improved the liquefaction resistance of calcareous sands. Additionally, the risk of soil liquefaction could be significantly reduced when the fiber content was greater than 0.8%. The multidirectional loading had a considerable effect in reducing the liquefaction resistance compared to unidirectional loading. Further, the stiffness degradation of calcareous sands decreased with increasing fiber content and fiber length. The pore pressure generated in the cyclic tests was analyzed and was found to be affected by fiber content. A pore pressure prediction model was proposed to obtain the pore pressure characteristics of fiber-reinforced calcareous sands under various fiber content conditions.

The impact of walking posture on trabecular structure in the femur of Rattus norvegicus

Bone responds to mechanical loads through growth. This response to external loading has been important for understanding skeletal form and function relationships. However, uncertainty still exists regarding how different locomotor behaviors, (e.g., bipedalism, quadrupedalism) are reflected in bone morphology. This study assesses the effects of locomotor posture on femoral bone structure in a rodent model (Rattus norvegicus). The study individuals come from a previously‐executed experiment (Foster, 2019) that utilized a treadmill‐mounted harness to induce bipedal walking postures in rats over 12‐weeks. The sample includes individuals from two experimental groups plus a control: bipedal walking, quadrupedal walking, and no exercise controls. Proximal and distal femora were microCT scanned at 20µm and trabecular bone volume fraction (BV/TV) and degree of anisotropy (DA) were analyzed and compared among groups to assess the impacts of differences in hip loading. In the proximal femur, the bipedal group had higher DA inferior to the neck and greater trochanter than the quadrupedal group, suggesting more unidirectional loading in bipeds. However, the quadrupedal group had greater BV/TV throughout the proximal metaphysis; particularly at the greater trochanter, which may be due to a higher demand for hip extension. In the distal femur, the bipedal group had higher mean BV/TV (0.39) when compared to the quadrupedal group (0.36), with a larger concentration of BV/TV in the medial condyle. These results suggest that the femoral epiphyses are responsive to changes in hip postural angle due to locomotion, however cortical bone structure and a larger sample size may be more informative.

Effects of Freeze–Thaw Cycles on Volume Change Behavior and Mechanical Properties of Expansive Clay with Different Degrees of Compaction

Freeze–thaw (F-T) cycles are one of the main deterioration factors for the performance of expansive clay in seasonally frozen soil regions. This study designed a unidirectional environmental boundary loading device for F-T cycle tests. Laboratory tests were conducted to quantitatively evaluate the effects of the F-T cycle on volume change behavior and mechanical properties of expansive clay with different degrees of compaction below the canal in Xinjiang. The SEM image processing was performed with the Image-Pro Plus image processing software to get pore parameters. The results showed that during the F-T cycle, the volume change of the sample gradually changes from contraction to expansion as the degree of compaction increases. As the degree of compaction increases, the hardening characteristics of the expansive clay are weakened, and the F-T cycle promotes the softening of the stress–strain relationship. The first F-T cycle resulted in a significant decrease of failure strength, and the failure strength of different compaction degrees decreased by 56.07%–67.54% after seven cycles. The failure strain and resilient modulus of the samples are attenuated to varying degrees after F-T cycles. The correlation rate between the porosity and the attenuation rate of the failure strength is 0.719–0.882, the corresponding weight reaches 0.419–0.445, and there is an excellent linear relationship between them. The research offers some important insights into the construction and maintenance of expansive clay engineering such as the Xinjiang Canal.

Mechanical behavior and recoverable properties of CFRP shape memory alloy composite under different prestrains

This paper explored a prestressed CFRP/SMA composite system, which can be applied to add prestress to the civil structural members. The composite system was composed of carbon fiber reinforced polymer (CFRP) sheets and the embedded shape memory alloy (SMA) wires. The prestress in CFRP sheets was formed by the recovery of SMA wires. Four groups of fully composite (Type I) specimens were fabricated to bear a unidirectional static load to investigate their tensile mechanical properties. Three groups of partial composite (Type II) specimens and five groups of SMA wire specimens were used to study their recoverability. The static test results showed that two main failure modes of the Type I specimens were observed, and the strength of the material was affected by the structural form of the composite. The recovery test results showed that the SMA wire could generate different levels of recovery stress under various amounts of prestrain, and the maximum recovery stress can reach 394.2 MPa for the SMA wire with 6% prestrain by heating it to 131℃. More importantly, the recovery stress can be introduced into the CFRP sheets. In addition, an analytical model for predicting the stress–strain relationship and material strength of the fully composite was proposed, and an improved segmented recovery stress–temperature model of the SMA wire under constraint conditions was also put forward. The results indicated that the calculated values are in good agreement with the experimental values, and the proposed model can effectively predict the mechanical and recoverable properties of the composites.