Quasi-static Three-point Bending Tests Research Articles

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.

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.

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.

Experimental investigation of an alternative approach to temporarily repair Nomex honeycomb sandwich structures in aerospace applications

In this study, the effectiveness of an alternative approach to temporarily repair an open-hole damaged Nomex honeycomb sandwich structure is experimentally investigated. The repair process consists of a resin-infused carbon fabric wet layup patch adhesively bonded to the facesheets and Airex structural foam used as a core replacement plug. The specimens of intact, damaged and repaired Nomex honeycomb sandwich panels are subjected to the quasi-static three-point bending test. The panel's flexural performances are determined by analyzing the obtained load-displacement responses. The effectiveness of the proposed repair technique is evaluated by comparing the responses of the repaired panels with the ones obtained from the intact and damaged panel specimens. The comparison is effectuated in terms of mean values of flexural elastic modulus, peak force, energy absorption, and specific energy absorption. In addition, the observed failure modes of the different panel configurations are pointed out and discussed.

Development of three-dimensional woven weft interlock carbon-fiber-reinforced plastic composites and experimental investigation on their out-of-plane flexural performance

Developing three-dimensional (3D) woven composites using various woven structures is important in the advanced composite research field. In this study, carbon-fiber 3D woven textile-reinforced epoxy–resin composites using four different woven structures (such as 2.5D, 3D-a, 3D-b, and 3D-c) were designed and fabricated based on a dobby-weaving loom, and vacuum-assisted resin transfer molding technology. In these 3D woven structures, part of weft insertions serves as binder yarns with a special weft interlock structural design, different weft-to-binder yarn ratios are obtained. Out-of-plane quasistatic three-point bending tests and dynamic low-velocity drop-weight impact tests were conducted on these woven composites to study the effect of woven structure on their flexural performance. Nondestructive ultrasonic C-scan and X-ray microcomputed tomography were applied to characterize the failure modes of the post impacted composites. We found that woven structures affect the flexural performance and failure mode of the developed composites, these structures with lower in-plane and binder yarn waviness exhibited higher quasistatic–flexural performance; 3D-c structure with serious in-plane yarn waviness combined with through-thickness binder yarn path exhibited superior impact resistance than other structures and survived more from composite through-thickness cracks and binder yarn breakage failures. Proper structural design is a key to improving flexural performance in specific engineering applications.

Bending Performance of Aluminum Sandwich Panel with Polyurethane Foam Core

Although rigid foam core structures have gotten a lot of attention, there have only been a few researches on foam reinforced sandwich panels with aluminum alloy A6061 sheets as face-sheets. In this research, the sandwich concept was applied to develop lightweight panels for roofing system. Analysis on the influences of core thickness, density, and foam layer arrangement on energy absorption, bending strength and displacement of sandwich panel under the quasi-static three-point bending test were investigated. Sandwich panel core is made of closed-cell polyurethane foam with densities of 40 kg/m3, 60 kg/m3, and 80 kg/m3. The quasi-static three-point bending tests were conducted in accordance to ASTM C-393 Standard and the polyurethane foam cores are design accordingly to the guideline of National Institutes of Standards and Technology (NIST). Load–displacement curves and mechanical properties are shown using data from experimental works. Results demonstrate that increased in thickness of the sandwich panel, also increased the bending strength, energy absorption and displacement. Furthermore, the sandwich panel with 50 mm thickness and 60 kg/m3 density foam core has the maximum bending strength.

Fracture Properties of Double Skin Structure including Friction Stir Welds used in Aluminum Alloy Railway Carbody in Three-point Bending Tests.

Recently friction stir (FS) welds are applied to aluminum alloy railway carbody. FS welds may have lower mechanical strength than base metal. It is important to identify their characteristics when evaluating the strength of carbody. Therefore, quasi-static three-point bending tests were conducted to determine the fracture characteristics of double skin structure to which FS welds are applied under bending loads. Specimens are double skin structure with FS welds and were fabricated from the side structure of aluminum alloy railway vehicles. Furthermore, we carried out a FE simulation under the same condition as the experimental test, and the differences between the experimental results and the simulation results were verified. The simulation result was in good agreement with the experimental one in terms of load before crack initiation or buckling.

Research on development of 3D woven textile-reinforced composites and their flexural behavior

In this study, a new weaving technology with a modified heddle position system based on a self-built three-dimensional (3D) weaving loom is designed, and four typical 3D woven-structure textile groups are manufactured: layer-to-layer orthogonal weaving, through-thickness orthogonal weaving, layer-to-layer angle-interlock weaving, and through-thickness angle-interlock weaving. The new weaving technology has great potential for manufacturing various 3D woven structures effectively and efficiently. The fabricated 3D woven textile-reinforced epoxy-resin composites undergo quasi-static three-point bending tests to study the influence of the woven structure on the flexural performance and failure modes along the textile warp and weft directions. The composites along the weft direction (weft-direction beams) have a larger flexural modulus but smaller failure strain compared with the warp direction (warp-direction beams) for all woven-structure types. Among the designed 3D textile composites, the angle-interlock woven structures have a larger flexural strength (50%), modulus (40%), and failure resistance than have the orthogonal-woven structures. Overall, the through-thickness angle-interlock woven structure has the best flexural-failure resistance among all textile structures, and is the optimal structural design based on this modified weaving technology.

Flexible Fiber Fabric for FRP-Concrete Connection of Thin Hybrid Slabs.

In order to combat corrosion issues, several studies on progressively replacing steel reinforcement elements with composite ones have been conducted in recent years. Hybrid steel–concrete thin slabs in which the steel acts as formwork are also candidates for update in the coming years. Achieving a reliable connection between fiber-reinforced polymer (FRP) and cast-in-place concrete is key to promoting this technology. This study analyzed different connection systems and proposes the novel approach of embedding a flexible fiber fabric as a superficially distributed connector between concrete and FRP. Eight specimens with four different connection strategies were tested using an experimental modal analysis and a quasi-static three-point bending test. The impact of the connection system on the vibrational response, flexural ultimate load, moment response, neutral axis position, shear and dissipated energy was obtained and compared. The results show that the use of an embedded mesh increases the frictional mechanism and produces the best performance in terms of load-bearing capacity and ductility.

On the mechanical performance of honeycomb-core sandwich L-joint under bending load: an experimental-numerical study

The mechanical properties of sandwich structures have drawn much attention from research and industrial application, while sandwich structures in the form of L-joint have been rarely addressed. In this paper, L-joint sandwich structures were experimentally tested under bending load to investigate the mechanical performance during structural deformation and damage evolution. The corresponding testing process was numerically simulated using finite element (FE) modeling. Nomex® honeycomb was selected as the core material of the sandwich structure, with the outer facesheets made of thermosetting Carbon Fiber Reinforced Polymer (CFRP) prepreg. Sandwich structures in the form of thin-walled beams and L-joint were then manufactured with specially-designed molds. Quasi-static three-point bending tests up to failure were conducted on the sandwich panels. Experimental results revealed that the main failure mode of the sandwich panel was the fracture in the honeycomb wall. The stiffness and strength properties of the sandwich structure were analyzed with FE modelling results, which showed good agreement with the experimental data. The adhesively bonded and hybrid rivet-bonded sandwich L-joints were investigated under vertical crushing load, showing the main failure region located in the corner area of the L-joint. The proposed experimental-numerical method on the mechanical performance of sandwich structures can provide reliable guidance on the connection of the caravan floor and side walls in automotive industries.

The role of ply angle in interlaminar delamination properties of CFRP laminates

The paper describes the fracture toughness of biomimetic architected carbon fiber reinforced polymer (CFRP) laminates via experimental and numerical methods. High-fidelity Finite Element (FE) models in conjunction with modified Mises criteria have been developed to evaluate the effect of the interlaminar architectures on the fracture toughness of bio-inspired helicoidal composites. A bi-linear cohesive zone model (CZM) with Hashin damage criterion has been used here to simulate the interlaminar and intralaminar damage behaviors during quasi-static three-point bending (3 PB) tests of biomimetic architected CFRP laminates, and 3 PB test results extracted from open literature and obtained in this work have been used to validate the load-displacement response of the specimens predicted from the proposed FE model. Mode I and mode II interlaminar fracture toughness of these layered composites have also been identified as semi-analytical functions of the stacking angle of two adjacent layers, and the predictions show a good agreement with the corresponding experimental data. This work provides guidelines about the use of the bionic design CFRP composite in applications where toughness is critical.

Mixed Mode Crack Propagation in Polymers Using a Discrete Lattice Method

The fracture behavior of polymeric materials has been widely studied in recent years, both experimentally and numerically. Different numerical approaches have been considered in the study of crack propagation processes, from continuum-based numerical formulations to discrete models, many of the latter being limited in the selection of the Poisson’s coefficient of the considered material. In this work, we present a numerical and experimental analysis of the crack propagation process of polymethylmethacrylate beams with central and eccentric notches subjected to quasi-static three-point bending tests. The developed discrete numerical model consists of a regular triangular lattice model based on axial and normal interaction springs, accounting for nearest-neighbor interactions. The proposed model allows solving the above mentioned limitation in the selection of Poisson’s coefficient, incorporating a fracture criterion defined by a bilinear law with softening that includes the fracture energy in the formulation and allows considering a progressive damage. One of the main objectives of this work is to show the capacity of this lattice to simulate quasi-static fracture problems. The obtained results show that the proposed lattice model is capable of providing results close to the experimental ones in terms of crack pattern, peak load and initial stiffening.

Surface energy in impact testing of unnotched specimens: Influence of carbon nano-fillers in epoxy matrix

This work focuses on the mechanical response of an epoxy-based system submitted to Charpy low-velocity impacts with unnotched specimens. The use of samples without notch provokes scattered results on the accumulated energy of the impact. The dispersion is reduced if the energy involved in the failure is calculated per unit of the fractured area. This leads to the estimation of the surface energy in the impact of the material, which is defined as the energy required to create a new free surface. Therefore, the crack paths of epoxy unnotched specimens after breakage are analysed and measured with normalization purposes. A possible correlation of the failure pattern with the action of the positioning tab of the Charpy support is pointed out. As well, the non-negligible effect of modifying the post-curing process of the neat resin is highlighted. The influence of introducing a low percentage of carbon-based nanofillers in the polymeric system is reviewed in terms of the surface energy and its elastic response. To assess the stiffness and strength of the material, the results of quasi-static three-point bending tests are analysed combining the so-called Homogenised Section Technique and the full strain fields acquired by Digital Image Correlation.

Investigation into different numerical methods in predicting the response of aluminosilicate glass under quasi-static and impact loading conditions

This paper presents a comparison of three numerical approaches for modeling brittle materials in both quasi-static and dynamic loading conditions. These methods include the Finite Element Method coupled to Smooth Particle Hydrodynamics (FEM-SPH), Discrete Element Method (DEM) and the elastic bond-based Peridynamics (PD). Numerical models for each method were built in the commercial software LS-DYNA. The parameters, associated to the mechanical behavior of aluminosilicate glass, were calibrated for each numerical model by means of experimental tests on material coupons. The experimental results from quasi-static three-point bending tests and ballistic impact tests were utilized for numerical models’ assessment. All methods provide comparable results under quasi-static flexural loading condition and for ballistic impact cases, the numerical results show the particular capability of each approach to capture the projectile residual velocity and the damage morphology of aluminosilicate glass tiles. However, considering all the loading conditions the FEM-SPH method replicates the mechanical response of aluminosilicate glass best.

Toughening mechanism of coelacanth-fish-inspired double-helicoidal composites

The scales of coelacanth fish feature a rare double-helicoidal structure of collagenous fibrils, exhibiting great toughness. In this study, inspired by such biomaterials, engineering composite materials are designed and fabricated. Quasi-static three-point bending and Charpy impact tests are performed to evaluate their mechanical performance compared with the single-helicoidal counterparts (emulated from the Bouligand structure in Odontodactylus scyllarus). Multiple delaminations are observed in the double-helicoidal composites, which preclude tensile crack propagation and then prolong the failure displacement; meanwhile, for single-helicoidal composites, the fiber architecture at the mid-plane serves as a vital role for hindering translaminar cracks propagation across the mid-plane. The dynamic energy absorption of double-helicoidal composites can reach 163.44 kJ m−2, 26.5% greater than that of the corresponding single-helicoidal samples, which is attributed to the more complex failure modes including fiber breakage and matrix cracking that both contribute to energy absorption. Double-helicoidal composites are not sensitive to fiber orientations on failure mode and such attribute enables more design freedoms. Additionally, the interlaminar stress is analyzed, and a greater value of the interlaminar stresses σ13 is discovered to be responsible for the delamination of double-helicoidal composites. Results reveal the underlying toughening mechanism of coelacanth-fish-inspired double-helicoidal composites and promote the next-generation impact-resistant composites design.

Mechanical behavior and collapse mechanisms of innovative aluminum foam-based sandwich panels under three-point bending

Recent studies on innovative solutions in structural engineering fields highlight the effectiveness of closed-cell aluminum foams as traditional material/architecture core replacement in sandwich structures. This study aims to investigate the mechanical behavior and the collapse modes of innovative sandwich panels with aluminum foam as the core and stainless steel wire mesh-grid as the skins; the manufacturing process was based on the powder compact melting technique. The results of quasi-static three-point bending tests point out the beneficial effects, in terms of mechanical performance, of the reinforcement skin on the sandwiches, compared to the plain foams; this joins by their effectiveness in withstanding thermal and mechanical loads. Moreover, the investigation of the collapse mechanisms of the sandwiches with different thicknesses allows for testing the goodness-of-fit of the experimentally observed collapses with the ones predicted by the failure map from analytical models available in the literature.

Dynamic characteristics of three-layer beams with load-bearing layers made of alumino-glass plastic

Aluminum-fiber-reinforced plastics (GLARE) are promising aviation materials with increased characteristics of specific stiffness and strength, fatigue strength, impact resistance, and residual strength after impact. Nowadays, GLARE are used for making elements of the fuselage of the long-haul passenger aircraft Airbus A380, as well as some other elements of aircraft structures. The results of experimental studies of eigenfrequencies and damping coefficients of three-layer beams made with face sheets of five-layer GLARE and with a core made of polyimide foam are presented. The tests were performed using the method of free bending vibrations of cantilever samples. The dynamic parameters of the three-layer beams were calculated based on the analysis of amplitude-frequency characteristics obtained by the fast Fourier transform method. Mechanical characteristics of GLARE and filler samples are preliminary determined in static and dynamic tests. The damping factor of the core is determined by the dynamic mechanical analysis method. The core shear modulus is determined by measuring the flexural rigidity of manufactured three-layer beams in a quasi-static three-point bending test. Based on a comparison of the design data and the results of the experiments, it is shown that in dynamic tests, the flexural rigidity of three-layer specimens is reduced in comparison with the estimated values, which may be due to the peculiarities of changing the characteristics of the foam core under dynamic loading. The value of the damping factor of GLARE samples was ~0.02, the foamed core was ~0.08 and three-layer beams were ~0.067 in the range of vibration frequency up to 60 Hz.

Experimental and numerical investigations on the quasi-static structural properties of fibre metal laminates processed by thermoforming

Recently, increasing development efforts of components made of fibre metal laminates (FMLs), with thermoplastics due to their numerous advantages are remarkable. One-step adhesion and forming approach is an innovative solution to process such laminates reducing the cycle time and delivering failure-free components simultaneously. Hat shaped sections were the chosen geometry in this paper. This process is described considering the temperature, pressure and time progress in addition to the temperature distribution inside the FMLs in different locations. In order to describe the structural behaviour of the hat profiles, top-hat crashboxes were produced and tested via quasi-static three-point bending tests. The investigations were carried out by experimental and finite element simulation. Several parameters are discussed in detail, the top layer (steel and aluminium), the core layer thickness (0.5 and 1.0 mm) and glass fibre orientations (0°/90° and ±45°). The results revealed that by doubling the core thickness, the forming forces and accordingly the absorbed deformation energy were doubled at reduced weight increase. Comparing the experimental and numerical results for the Al-based FMLs, a good agreement regarding the energy absorption deformation mode and the force–displacement progress was reached.

Failure mechanism of geopolymer composite lightweight sandwich panel under flexural and edgewise compressive loads

This paper proposes a new lightweight sandwich panel manufactured with Fiber-Reinforced Geopolymer Composite (FRGC) skin layers and PUR foam core for prefabricated buildings. The failure mechanisms of the developed sandwich panel are investigated under flexural bending and edgewise compressive loading to validate the performance of the developed panel. Digital Image Correlation (DIC) technique is used to obtain the deformation and strain during the tests. Two typical failure mechanisms, i.e. shear failure of the core and tensile failure of the skin layers of the one-way sandwich panel under flexural loading are studied by conducting quasi-static three-point bending tests. The influence of the core thickness-to-span ratio of the sandwich panel on shear failure of the core is studied. The test results show that the failure mode transforms from the shear failure of the PUR foam core to delamination between the FRGC skin and PUR foam core, when the thickness-to-span ratio increases from 0.1 to 0.3. The influence of skin thickness-to-span ratio on the skin tensile failure is also studied. Analytical models for the shear failure of the core and the tensile failure of the skin layer are proposed. Furthermore, the failure mechanism of the developed sandwich panel with different thickness-to-length ratios under edgewise compressive loading is studied. It is found that the failure mode changes from the material failure of the FRGC skin to the global buckling when the thickness-to-length ratio reduces from 0.90 to 0.30. Analytical models for predicting the critical failure load under edgewise compressive loading are also proposed.

Removable camera based 2D digital image correlation for planar slender area quasi-static deformation measurement

As one of the non-contact full-field measurement techniques, digital image correlation (DIC) has attracted much attention and has gained rapid acceptance in various fields, owing to its well-known advantages of a simple optical setup and low environmental vulnerability. In practical applications, the effective resolution of image is reduced due to the large difference in the size of the specimen in different directions, which makes it impossible to make effective full-field measurement. To significantly improve DIC-based deformation measurement for planar slender specimen without losing resolution, a removable camera based DIC method is proposed in this study for quasi-static high-resolution deformation and strain measurement. An industrial camera was mounted on a stable sliding table structure composed of linear sliding guide rail and ball screw. Under the control of the program, the camera moved with the sliding table to capture parts of the specimen at different positions. A correlation based image stitching method and overlapped region minimization method were proposed and were applied to these images, and their corresponding displacement fields and strain fields was calculated based on DIC. To verify the feasibility and effectiveness of the proposed removable camera based DIC method, static error test and quasi-static three-point bending test were performed. The results indicate that compared with a single fixed camera based DIC, the strain resolution could be increased by approximately 5 times by stitching 5 images while ensuring the same displacement measurement accuracy by using the proposed method. It concluded that the use of the removable camera array has great potential to perform deformation measurements of slender area without losing resolution.