Quasi-static Three-point Bending Research Articles

Experimental and numerical investigation of damage in multilayer sandwich panels with square and trapezoidal corrugated cores under quasi-static three-point bending

Multilayer composite sandwich panels with corrugated cores have the potential for structural applications requiring high stiffness and strength-to-weight ratios. This study aims to experimentally and numerically investigate the bending response of sandwich panels with novel square and trapezoidal corrugated core configurations fabricated from E-glass fiber-reinforced epoxy composites, both with and without polyurethane foam filling. Quasi-static three-point bending tests were conducted to evaluate and compare the flexural stiffness and maximum load capacity of the panel designs. The cores were manufactured using vacuum-assisted resin transfer molding. Key results showed that changing the core geometry from square to trapezoidal improved the bending stiffness by up to 26 %. Incorporating polyurethane foam further increased the bending stiffness by up to 41 % and maximum load by up to 47 % compared to solid cores. Failure initiation and progression were governed by matrix cracking in the face sheets and cores, followed by core cell wall buckling and delamination. Finite element modeling using ABAQUS captured the progressive damage behavior, exhibiting good agreement with experimental force-displacement responses. Failure modes included matrix cracks, fiber fractures, core buckling and delamination. This study provides valuable insights into the mechanical performance of innovative corrugated core sandwich panel designs under quasi-static bending loads. The validated FE approach also enables virtual testing and optimization of composite sandwich structures.

Crashworthiness optimization of variable stiffness B-pillar with thermoplastic composites

This study focuses on enhancing of crashworthiness and lightweight of the automotive B-pillar structure by using continuous carbon fiber reinforced thermoplastic composites (CFRTP) and optimized layer design. Firstly, the B-pillar is divided into four regions with variable stiffness. Quasi-static three-point bending test and numerical simulation are performed to calculate the stiffness of the structure. Then, the four regions of the B-pillar with different CFRTP layers are structurally designed to realize variable stiffness. Considering manufacturing requirements, a draping simulation is employed to predict potential layer defects. The results demonstrate that B-pillar structural performance is enhanced after the reorientation of CFRTP prepreg. Afterward, the crashworthiness of the CFRTP B-pillar considering the ply drop-off is evaluated and compared with the traditional Q235 steel counterparts. The results indicate that the CFRTP B-pillar exhibits superior crashworthiness but a substantial reduction in mass by 53.1%. Finally, the multi-island genetic algorithm (MIGA) is employed to achieve the optimal layer design scheme for lay-up optimization of the CFRTP B-pillar, and the crashworthiness of the CFRTP B-pillar with variable stiffness is further improved. The current work highlights a structural design strategy for CFRTP components for vehicles, which is crucial for the updating of automobile engineering.

Mesoscopic simulations of a fracture process in reinforced concrete beam in bending using a 2D coupled DEM/micro-CT approach

In this study, a complex fracture process in a short rectangular concrete (RC) beam reinforced by one longitudinal bar (without vertical reinforcement) and subjected to quasi-static three-point bending was numerically explored in 2D conditions. A critical diagonal shear crack in the beam caused it to fail during the experiment. The numerical simulations were conducted with a classical particle discrete element method (DEM). A three-phase concrete description (aggregate, mortar, and interfacial transitional zones (ITZs) around aggregates) accounted for the concrete heterogeneity. In mesoscopic DEM calculations based on a 2D X-ray CT scan, the actual shape and placement of aggregate particles in concrete were taken for granted. In the study, the steel bar with ribs was replicated. ITZ was also assumed between the bar and mortar. Without imposing any bond-slip law, a geometrical bar/concrete interface condition was explicitly considered. The focus was on the force–deflection diagram, fracture process, contact forces, and stresses along the bar. A good level of agreement about the evolution of the vertical force versus the deflection and failure mechanism was attained between DEM analyses and in-house laboratory tests despite simplified 2D conditions. A strong effect of concrete mesostructure on the crack pattern was found.

Investigation of the Bending Properties of Ex situ Functional Metal Foams

In each industry, compromises have to be made when choosing materials. Lower-density materials have a significant advantage in the automotive sector, especially due to rising fuel prices, since weight reduction can lower overall fuel consumption. The advantageous properties of metal foams, such as low density, high specific strength, and excellent energy absorption, should be researched and exploited in as many areas and ways as possible. This research aims to perform and evaluate the bending tests of ex situ functional metal foams: aluminum alloy matrix (AlSi7Mg) was used, which was filled with Ø2.5–3.0 mm lightweight expanded clay aggregate particles and surrounded by thin-walled aluminum tubes (AlMgSi0.5) with a wall thickness of 2 mm and an outer diameter of Ø32 mm. Empty tubes, foam-filled tubes (with and without structural epoxy adhesive) and metal matrix syntactic foams were compared based on their flexural strength and energy absorption capacity. Quasi-static three-point bend tests were carried out up to 25 mm deflection. The foam-filled tubes with epoxy adhesive showed an average of 6% increase in flexural strength compared to the foam-filled tubes without adhesive and a 145% increase compared to the metal matrix syntactic foams.

Impact of composite and aluminium face sheets on the properties of the 3D-printed cores under quasi-static three-point bending

3D printers have been the focus of many researchers in recent years. Many thin-walled structures can be produced using 3D printers. One of the thin wall structures that can be made with 3D printers is the core of sandwich panels. In this research, cores with rectangular cross section have been made using Acrylonitrile Butadiene Styrene (ABS) and Polylactic Acid (PLA) filaments. These cores were reinforced using aluminum and composite face sheets and subjected to a three-point bending test. Glass fibers with a density of 200 g/m2 were used to make composite shells. The results showed that the addition of aluminum and composite face sheets, although increasing the flexural strength, greatly reduces the flexibility of the core.

Performance of 3D printing biomimetic conch shell and pearl shell hybrid design composites under quasi-static three-point bending load

The failure process of biomimetic hybrid design composite composed of layers of conch shell and pearl shell was studied through quasi-static three-point bending experiments and numerical simulations. The biomimetic conch shell structure with inclined angles serves as the upper layer of the hybrid structure, while the biomimetic pearl shell structure with traditional brick and mud structure serves as the lower layer of the hybrid structure, forming a hybrid design structure. Four inclined angles were designed for the structural units of the conch shell, namely 15°, 30°, 45°, and 60°. Twenty-four specimens (six specimens for each inclined angle) were prepared using 3D printing technology using both soft and hard matrix materials. The influence of different inclined angles on the fracture strength, fracture toughness, and energy absorption of hybrid design structures was experimentally studied. The biomimetic hybrid design composite specimen with a notch is placed between two supporting rollers, and a loading indenter acts at mid-span. All twenty-four specimens were notched with a triangular tip and a rectangular bottom. A loading rate of 1 mm/min is used to avoid the viscoelastic effect of the composite materials. Details of the specimens, the experimental set-up and procedure are discussed in this paper. Complementary to the experimental studies, an extensive numerical investigation was carried out to study the influence of the aspect ratio of brick and mud units on the fracture initiation and failure of hybrid design structures. The causes of crack initiation and propagation, and failure modes in biomimetic hybrid design structures were postulated. These numerical findings help in reinforcing the experimental results and provide crucial information to enhance further research in this exciting area.

Quasi-static three-point bending of sandwich panels with Miura-ori cores

Collapse mechanisms of sandwich panels with Miura-ori cores are analysed and classification of the deformation regimes under three-point bending is proposed. It is revealed that the homogenisation approach to the core material properties is not applicable to the deformation of panels with relatively strong origami cores due to the immediate simultaneous local deformations of the core and face sheets. To describe this deformation regime, an analytical model based on the development of new stationary hinge lines in Miura-ori cells and rigid motion rotation of the adjacent planes is proposed. The model pertains to a single degree-of-freedom model governed by the global bending angle of the panel. On the other hand, the homogenisation approach to the core material properties proved to be applicable to bending analysis of panels with relatively soft Miura-ori cores. This approach is used to analytically obtain the collapse load related to several deformation modes. Parametric FE analysis is conducted to further explore the deformation regimes of panels with different face sheet thicknesses.Experiments were conducted to validate the finite element studies and analytical models. A good agreement between the analytically predicted collapse forces and those obtained by the FE simulations is shown. Deformation mode maps are constructed based on the analytical and FE analyses in terms of the face sheet thicknesses and core strength.A brief comparison between the bending strength of sandwich panels with Miura-ori cores and panels with PVC and honeycomb cores is presented, demonstrating the superiority of the honeycomb cores loaded in the out-of-plane direction. On the other hand, the possibility of the development of high shear strains in metallic Miura-ori cores without damage gives them advantages compared to three-point bending of panels with low density PVC foam cores.

Effect of fill pattern on flexural performance of FDM printed PLA

ABSTRACT In most biomedical devices, such as fracture fixation devices and medical tools, the printed part is subjected to complex loadings that involve large deflections instead of large strains. Among various FDM tool path parameters, the fill pattern has a critical role in controlling voids and raster bonding. This work highlights the flexural performance of the PLA-printed part based on various fill patterns, such as concentric, linear, Hilbert, raster angle 45º, alternate pattern (0º/45º), and concentric pattern with holes resembling bone plate. Based on deposition paths, the dimensional changes and interlayer adhesion were analyzed before and after annealing. The flexural performance was evaluated using quasi-static three-point and four-point bend tests and dynamic flexural tests under temperature sweep. The annealed samples exhibit improved flexural strength regardless of infill pattern and loading conditions, with the concentric pattern showing better performance in both static and dynamic tests.

Bending behavior of 3D printed sandwich structures with different core geometries and thermal aging durations

In recent years, the Fused Deposition Modeling (FDM) method has been employed in the production of small-scale structural elements with moderate loads, such as small-unmanned aerial vehicles, sports equipment, dental implant molds, and similar applications. In these cases, the FDM technique can utilize polymers like Polyamide (PA), Polylactic Acid (PLA), and Acrylonitrile Butadiene Styrene (ABS) to manufacture structural components. Lightweight sandwich structures are utilized as structural elements in various industries due to their unique characteristics, high stiffness-to-weight ratio, and energy absorption capabilities. Although the number of studies on the mechanical properties of sandwich structures manufactured with FDM has increased in recent years, experimental data on the mechanical characteristics of sandwich structures manufactured with FDM under different thermal aging durations are still insufficient. Driven by this motivation, the energy absorption capabilities of sandwich structures with different four core geometries (i.e., circular, hexagonal, square, and triangular) were experimentally investigated under different thermal aging durations. The sandwich structures were manufactured from PLA material by the FDM method. Four different thermal aging durations, 0, 15, 30 and 45 days, were considered for environmental conditions. Quasi-static three-point bending experiments were conducted to assess the energy absorption capability of lightweight sandwich structures featuring diverse core topologies. The bending test results demonstrate that the core topology significantly affects the energy absorption abilities of sandwich structures. Moreover, fractographic analysis using scanning electron microscopy (SEM) was conducted to gain deeper insights into the impact of thermal aging on aged specimens. In addition, it was concluded that the energy absorption performances of all sandwich structures with different core structure topologies were adversely affected by increasing the thermal aging time.

The effect of accretion temperature on microstructure and bending strength of atmospheric ice

Accurate determination of the mechanical response of atmospheric ice is key to understanding the risks associated with ice impact on aircraft during flight. Two types of atmospheric ice which are of particular interest to the aerospace industry are studied. Rime and Glaze ice are each manufactured in an icing wind tunnel facility under controlled conditions. Rime ice is accreted at a temperature of −20°C, and Glaze ice is accreted at −5°C. Quasi-static three-point bend tests are performed on both types of ice to understand the effect of accretion temperature, and therefore microstructure, on strength. The results indicate that the ice accretion temperature, and thus microstructure, has a significant influence on the bending strength. On average, the bending strength of Rime ice is 9.0±0.18MPa compared to 4.4±0.093MPa for Glaze. The comparatively lower accretion temperature of Rime results in smaller grain sizes and higher bending strength. In contrast, the effective modulus appears insensitive to ice microstructure, with an average value of 3.5±0.12GPa for Rime compared to 3.6±0.098GPa for Glaze. Furthermore, the results indicate that both the bending strength and effective modulus are insensitive to the ice storage time.

Three-point bending response and energy absorption of novel sandwich beams with combined re-entrant double-arrow auxetic honeycomb cores

The response and energy absorption of novel sandwich beams with combined re-entrant double-arrow auxetic honeycomb (RDAH) cores subjected to three-point bending were studied experimentally and numerically. Two typical sandwich beams loaded at different loading positions were considered. Quasi-static three-point bending experiments were conducted to obtain the failure modes and force–displacement curves. The reliable numerical simulation models were further established based on experimental validations. The results indicate that when the loading roller is located directly above the re-entrant cell, the RDAH core sandwich beam has better load-carrying and energy absorption capacity. Subsequently, the influence of face sheet distribution, cell-wall thickness, impact velocity and cell configuration on the structural response were explored. For the sandwich beams with same total mass, the arrangement where the thickness of the front face sheet is larger than that of the back face sheet is beneficial for improving the load-carrying and energy absorption capacity. In addition, the cell-wall thickness has an influence on the local deformation mode of the sandwich beam, and increasing its value can produce more stable deformation and improve the load-carrying capacity. Increasing impact velocity has a significant influence on the initial deformation but little influence on the final deformation of the sandwich beams. As the impact velocity increases, the total energy absorption of the sandwich beam gradually increases, and the negative Poisson's ratio characteristic of the core still exists. Compared to the traditional re-entrant honeycomb (RH) core sandwich beams, RDAH core sandwich beams have better energy absorption capacity and bending resistance.

Sinusoidally architected helicoidal composites inspired by the dactyl club of mantis shrimp

ABSTRACT The impact region of the dactyl club of mantis shrimp features a rare sinusoidally helicoidal architecture, contributing to its efficient impact-resistant characteristics. This study aims to attain bioinspired sinusoidally architected composites from a practical engineering way. Morphological features of plain-woven fabric were characterized, which demonstrated that the interweaving warp and weft yarns exhibited a sinusoidal architecture. Interconnected woven composites were thus employed and helicoidally stacked to achieve the desired structure. Quasi-static three-point bending and low-velocity impact tests were subsequently performed to evaluate their mechanical performance. Under three-point bending condition, the dominant failure mode gradually changed from fiber breakage to delamination with the increase in the pitch angle. Failure displacement and energy absorption of the helicoidal woven composites were, respectively, 43.89% and 141.90% greater than the unidirectional ones. Under low-velocity impact condition, the damage area of the helicoidal woven composites decreased by 49.66% while the residual strength increased by 10.10% compared with those of the unidirectional ones, exhibiting better damage resistance and tolerance. Also, effects of fiber architecture on mechanical properties were examined. This work will shed light on future design of the next-generation impact-resistant architected composites.

Crack modes and toughening mechanism of a bioinspired helicoidal recursive composite with nonlinear recursive rotation angle-based layups

The rotation angle is an important parameter affecting the performance of helical structures, and helical structures with nonlinearly increasing rotation angles have been studied. The fracture behavior of a 3D-printed helicoidal recursive (HR) composite with nonlinear rotation angle-based layups was investigated by performing quasistatic three-point bending experiments and simulations. First, the crack propagation paths during the loading of the samples were observed, and the critical deformation displacements and fracture toughness were calculated. It was found that the crack path that propagated along the soft phase increased the critical failure displacement and toughness of the samples. Then, the deformation and interlayer stress distribution of the helical structure under static loading were obtained by finite element simulation. The results showed that the variation in the rotation angle between the layers caused different degrees of shear deformation at the interface between adjacent layers, resulting in different shear stress distributions and thus different crack modes of the HR structures. The mixed-mode I + II cracks induced crack deflection, which slowed the eventual failure of the sample and improved the fracture toughness.

Crack modes and toughening strategies of bioinspired 3D printed double-helicoidal architectures

The crack modes and toughening strategies of 3D printed double-helicoidal (DH) composites were fully characterized by performing quasi-static three-point bending experiments and simulations. Notch-containing DH specimens were fabricated with dual-material curing 3D printer PolyJet technology, while single-helicoidal (SH) specimens were also prepared for comparison. The rigid glassy polymer VeroWhite and the soft rubbery polymer TangoBlackPlus were selected as the brick and adhesive of the specimens, respectively. First, the crack propagation path of the specimens under three-point bending loading was observed, and the fracture toughness and critical deformation displacements were calculated. The double twisting pattern of DH composite makes cracks deflect along complex deflection paths, resulting in increased energy dissipation required to produce deflected crack surfaces, which delays catastrophic damage. Then, the deformation and interlayer stress distribution under static loading were analyzed by finite element analysis. It was found that the interlaminar shear stresses σ13 and σ23 generated in adjacent layers were responsible for the crack deflection of the DH specimens. Toughening mechanisms of the DH structure inspired by coelacanth will promote the design of next generation fracture-resistant composites.

Flexural Behaviour of Foam Cored Sandwich Structures with Through-Thickness Reinforcements

Composite sandwich structures are well-suited for applications requiring high bending strength, flexural rigidity, crashworthiness, and light weight. However, skin–core debonding and core failure remain a barrier to optimal structural performance when polymeric foams are used as core materials. Suppressing or compartmentalising these failure modes can enhance the structural integrity of sandwich structures. In this paper, the flexural response of a sandwich structure was improved by adding carbon fibre-reinforced plastic in the form of through-thickness ribs during the manufacturing process. The effect of the position of the ribs was investigated using a quasi-static three-point bend test. A camera was used to capture failure events, while the digital image correlation technique provided the full-strain field at different stages of loading. Improved flexural performance was obtained when a reinforcement was placed on either side of the loading roller. With this configuration, skin–core debonding was restricted to a confined portion of the panel, resulting in a more localised and stable fracture process, which involved enhanced foam crushing and hardening. A simple FEA approach has been adopted in this paper and has proven to be an effective approach for capturing the details of the failure process, including the debonding in the composite foam structures, without the need for complex and computationally expensive interface modelling.

Early-age fracture behavior of sandstone-concrete interface with different sand/cement ratios

To understand the influence of sand/cement (S/C) ratio on the mode I fracture characteristics of the sandstone-concrete interface (SCI) at different ages, the quasi-static three-point bending test was carried out on semi-circular bi-material specimens. Experimental results indicate that the fracture toughness (KIC) of SCI increases first and then decreases with the increase of S/C ratio. Furthermore, when the age of concrete is between 0.5 and 7 days, the fracture energy (Gf) of SCI with different S/C ratios fall in the following order: G1.0>G0.0>G3.5. However, when the age of concrete ranges from 14 to 28 days, the order of fracture energy is changed to G3.5>G1.0>G0.0. Additionally, many obvious acoustic emission signals appear before the final failure of the specimen when the S/C ratio is 3.5. However, when the S/C ratios are 0.0 and 1.0, only few apparent acoustic emission signals appear at the final failure. In addition, when the S/C ratios are 0.0 or 1.0, the load–displacement curve of specimens in the age interval [0.5d, 28d] has one single peak point. However, when the S/C ratio is 3.5, the load–displacement curve of specimens in the age interval [7d, 28d] shows double-peak points. Moreover, the S/C ratio also controls the failure pattern of SCI. Compared to those with S/C ratios of 0.0 and 1.0, the broken concrete or rock with S/C ratio of 3.5 is more dispersed.

Micro-CT Imaging and Mechanical Properties of Ovine Ribs.

The use of ovine animal models in the study of injury biomechanics and modeling is increasing, due to their favorable size and other physiological characteristics. Along with this increase, there has also been increased interest in the development of in silico ovine models for computational studies to compliment physical experiments. However, there remains a gap in the literature characterizing the morphological and mechanical characteristics of ovine ribs. The objective of this study therefore is to report anatomical and mechanical properties of the ovine ribs using microtomography (micro-CT) and two types of mechanical testing (quasi-static bending and dynamic tension). Using microtomography, young ovine rib samples obtained from a local abattoir were cut into approximately fourteen 38mm sections and scanned. From these scans, the cortical bone thickness and cross-sectional area were measured, and the moment of inertia was calculated to enhance the mechanical testing data. Based on a standard least squares statistical model, the cortical bone thickness varied depending on the region of the cross-section and the position along the length of the rib (p < 0.05), whereas the cross-sectional area remained consistent (p > 0.05). Quasi-static three-point bend testing was completed on ovine rib samples, and the resulting force-displacement data was analyzed to obtain the stiffness (44.67 ± 17.65 N/mm), maximum load (170.54 ± 48.28 N) and displacement at maximum load (7.19 ± 2.75mm), yield load (167.81 ± 48.12 N) and displacement at yield (6.10 ± 2.25mm), and the failure load (110.90 ± 39.30 N) and displacement at failure (18.43 ± 2.10mm). The resulting properties were not significantly affected by the rib (p > 0.05), but by the animal they originated from (p < 0.05). For the dynamic testing, samples were cut into coupons and tested in tension with an average strain rate of 18.9 strain/sec. The resulting dynamic testing properties of elastic modulus (5.16 ± 2.03 GPa), failure stress (63.29 ± 14.02MPa), and failure strain (0.0201 ± 0.0052) did not vary based on loading rate (p > 0.05).

Explosion responses of movable [formula omitted]-shaped box bridge: Experimental and numerical investigations

For military bridges, the damage evaluation under close-in explosion is critical to judge their load capacity and service life. In this study, the blast response and damage mechanism of a movable light-weight metallic bridge model, composed of thin-walled skins, π-shaped diaphragms and pin-jointed hinges model, were investigated by close-in explosion experiments and numerical simulations. Quasi-static three-point bending experiments were performed to evaluate the residual load capacity of the exploded bridge model. The research reveals that the bridge model has overall flexural deformation and complex local deformation​ patterns. Based on the critical scaled distance, the explosion response can be intuitively divided into elastic deformation, plastic deformation, and local fracture. The load capacity of the bridge model is mainly determined by the pin-jointed hinges and the webs. The π-shaped design of diaphragms can reduce the discreteness between the bridge deck and the web, and result in effective energy absorption and good anti-blast performance for the bridge. The study can work as an instruction for the application of movable light-weight bridges under explosion conditions.

Blast responses of BFRP-bar reinforced AAC panels

Basalt fiber reinforced plastic (BFRP) bars can replace steel bars in concrete protective structures because of their advantages of corrosion resistance and high load-bearing capacity. In this paper, the BFRP-bar reinforced autoclaved aerated concrete (AAC) panels (BFRP-AACPs) were designed and prepared. Based on close-in explosion experiments, the dynamic responses and failure modes of the BFRP-AACPs were revealed. Through the quasi-static three-point bending experiment, the residual load capacity of each BFRP-AACP was evaluated, which can quantitatively reveal the damage degree of the BFRP-AACP after explosion. Calculated by the equivalent static load method, the critical scaled distance of the designed panel is 2.08 m/kg1/3, which provides a design basis for the application of the BFRP-AACP.

Failure mechanism and heat treatment effect of 3D-printed bio-inspired helicoidal CF/PEEK composites

The bio-inspired helicoidal structure has displayed a unique combination of lightweight, high strength, and high impact resistance. This study presents an experimental and numerical investigation of the failure mechanism and heat treatment effect of 3D-printed helicoidal composites by adopting a helicoidal laminate stacking configuration found in the dactyl club in Homarus americanus. The helicoidal specimens were additively manufactured with short carbon fiber reinforced Polyether ether ketone (PEEK) composites and their mechanical performance was characterized through quasi-static three-point bending. The results showed that the flexural strength, flexural modulus, and absorbed energy of the helicoidal composites with the twisted crack pattern were 6.9%, 23.9%, and 5.9% higher than those of the quasi-isotropic composites with the flat crack pattern. Stress analysis was then carried out to investigate the underlying failure mechanism of helicoidal materials. The stress component of σ33 and σ13 in helicoidal specimens showed a milder distribution than that of quasi-isotropic specimens, which plays a major role in the resulting fracture-resistant behavior. Further results showed that the helicoidal specimens under heat treatment of 250 °C over 6 h achieved 24.9% and 20.9% enhancement in flexural strength and modulus. The cracking surface area was increased with extensive translaminar twisted cracks observed with a scanning electron microscope. Experiment results show that heat treatment can exert a significant influence on the crack path of the CF/PEEK helicoidal specimens. Meanwhile, the increased degree of crystallization and crosslinking at the interface could inhibit the initiation of delamination failure.