Quasi-static Loading Conditions Research Articles

Effect of ZnO-decorated electrospun veils on the damage tolerance of CFRP laminates

In this work, multifunctional carbon fiber reinforced polymer (CFRP) laminates were manufactured by interleaving electrospun veils decorated with ZnO nanorods (NRs) (CFRP_NY_ZnO). The laminates were tested under dynamic (low velocity impact at 5.0 J and 7.5 J) and quasi-static (pre- and post-impact four-point bending tests) loading conditions. The delamination resistance of CFRP_NY_ZnO was compared with those of CFRP (without veils) and CFRP_NY (with four non-decorated veils) laminates. At 5.0 J, ZnO NRs enhanced the delamination resistance (the lowest delaminated area was around 232.3 ± 38.7 mm2) due to additional energy consuming mechanisms. When the impact energy was increased (7.5 J), the debonding of ZnO NRs from the nanofibers was promoted and a rapid failure propagation was observed (increase in delaminated area of about 27% with respect to CFRP_NY). In quasi-static condition, ZnO NRs proved to be effective in increasing flexural strength (914.4 MPa (±18.8 MPa)) and modulus (83.6 GPa (±0.3 GPa)) of the laminate with respect to CFRP and CFRP_NY and in preventing laminate stiffness losses even after impacts at 5.0 J and 7.5 J.

Experimental investigation of the quasi-static and dynamic compressive behavior of polymer-based 3D-printed lattice structures

Because additive manufacturing (AM) allows the creation of complex structures that can be tailored or optimized based on mechanical and material specifications, it is increasingly used for various applications. Among others, it gives access to new designs of lightweight shields exhibiting good capabilities for absorbing dynamic loads and kinetic energy. In the present work, the quasi-static and dynamic compressive behavior of 3D polymeric lattice structures manufactured by the fused deposition modeling (FDM) process were evaluated. Two different materials were considered: ABS (Acrylonitrile Butadiene Styrene) and Tough-PLA (PolyLactic Acid). First, the tensile quasi-static mechanical properties of precursor filaments and 3D-printed dog-bone specimens were evaluated. Through these tests, it was observed that the printing process and orientation have a significant effect on the mechanical behavior of AM-produced samples. Then, quasi-static and dynamic compressive tests were conducted on AM-produced cylindrical specimens using two distinct printing topologies. The dynamic tests were performed with a split Hopkinson pressure bar (SHPB) apparatus. The results indicated that the compressive strength increases with the loading rate, indicating a strain rate sensitivity of the studied materials. Finally, three different 3D lattice structures were experimentally evaluated against crushing at two different deformation rates. It was noted that the geometrical design and relative densities of these structures have major effects on their energy absorption capability. However, no clear correlation seems to exist between energy absorption capabilities under quasi-static and dynamic loading conditions.

A Model-Assisted Probability of Detection Framework for Optical Fiber Sensors

Optical fiber sensors (OFSs) represent an efficient sensing solution in various structural health monitoring (SHM) applications. However, a well-defined methodology is still missing to quantify their damage detection performance, preventing their certification and full deployment in SHM. In a recent study, the authors proposed an experimental methodology to qualify distributed OFSs using the concept of probability of detection (POD). Nevertheless, POD curves require considerable testing, which is often not feasible. This study takes a step forward, presenting a model-assisted POD (MAPOD) approach for the first time applied to distributed OFSs (DOFSs). The new MAPOD framework applied to DOFSs is validated through previous experimental results, considering the mode I delamination monitoring of a double-cantilever beam (DCB) specimen under quasi-static loading conditions. The results show how strain transfer, loading conditions, human factors, interrogator resolution, and noise can alter the damage detection capabilities of DOFSs. This MAPOD approach represents a tool to study the effects of varying environmental and operational conditions on SHM systems based on DOFSs and for the design optimization of the monitoring system.

Effect of interlayer carbon nanotube films on the quasi-static and dynamic mode Ⅰ fracture behavior of laminated composites – An experimental and numerical investigation

A combined experimental and numerical study was conducted to investigate the quasi-static and dynamic mode Ⅰ fracture behavior of laminated composites toughened by carbon nanotube (CNT) films. Fracture tests under quasi-static and dynamic loading conditions were carried out by using electronic universal testing machine and electromagnetic Hopkinson bar device respectively. It’s illustrated that the CNT films can suppress the propagation of interlayer cracks effectively. Dynamic strengthening effect was also observed for CNT toughened composites during tests. For numerical simulations, the three-linear cohesive zone model (CZM) was proposed to simulate the Mode I delamination of CNT films toughened composites under quasi-static loading condition. Furthermore, rate-dependent parameters were introduced into this model by user-defined subroutine to describe the dynamic fracture behavior of toughened composites. This developed numerical model was validated by comparing with dynamic crack propagating speeds obtained from experimental tests. During the dynamic delaminating process, the average crack propagation rate of toughened composites decreased while the energy release rate increased, indicating that the interlayer CNT films can hinder the dynamic delamination effectively.

Study of resistant spot welding and its effect on the metallurgical and mechanical properties _ a review

One of the most significant sectors of all industrialised economies today is the automotive sector. It offers several people at all economic levels the chance to find employment. In the automotive sector, resistance spot welding is the most used type of welding. The resistance spot weld quality has a significant impact on the structural integrity of a vehicle. Because to complex interactions between electrical, mechanical, thermal, and metallurgical processes, the process is both quick and effective while also being sophisticated. Much effort is always being put forth to produce lightweight materials with excellent strength-ductility combinations because of the growing demand for increased fuel efficiency, decreased CO2 emission, and superior crashworthiness. This paper reviews metallurgical and mechanical performance of resistance spot welds in terms of weld nugget size, load bearing capacity and failure mode, under quasi static loading conditions. It also reviews the effect of process parameters such as welding current, welding time, electrode force and mechanism on the mechanical performance of spot welds.

Discontinuous Yielding of Fe E420 under High Strain Rate Loading

The discontinuous yielding behaviour of Fe E420 steel was experimentally investigated through tensile tests conducted on laminated sheets. In order to assess the response under both quasi-static and dynamic loading conditions, uniaxial tensile tests were performed according to ASTM E8 standards, and dynamic tests were conducted on the Hopkinson bar using a specially designed flat specimen. Three nominal strain rates (2×10−4, 500, and 1000) s−1 were considered. The effect of heat treatment on the material was also considered. For this purpose, the proposed experimental campaign was carried out on both the as-is material and on one treated by water quenching. All dynamic tests were performed with the direct tensile split Hopkinson pressure bar and were recorded via a high frame rate camera. The post failure fracture surfaces were investigated using scanning electron microscopy technique.

Compressive and splitting tensile impact properties of rubberised one-part alkali-activated concrete

This paper presents an experimental assessment of the compressive and splitting tensile properties of rubberised one-part alkali-activated concrete under quasi-static and low-velocity impact loading. An optimised mix design, employing blast furnace slag and fly ash as precursors and anhydrous sodium metasilicate as a solid activator, is used as a reference. Rubber contents of up to 60% volumetric replacement of total natural aggregates are considered. Quasi-static tests are performed using servo-hydraulic machines, whilst the impact tests are performed in an instrumented drop-weight loading rig. Digital image correlation is used to get displacement measurements under both quasi-static and impact loading conditions. Three impact velocities of 5, 10, and 15 m/s are considered, giving rise to strain-rates in the range of 3–270 s−1. The quasi-static results show shape- and size-dependency and characteristically lower compressive and splitting tensile strengths with higher rubber content. The dynamic properties are notably influenced by the rubber content, with a higher ratio resulting in greater impact duration under compressive loading, reduced peak compressive strength, and reduced peak splitting tensile strength. The shape of the stress-strain response under compressive loading changes with rubber addition, showing two major peaks as opposed to a single peak for the non-rubberised specimens. The dynamic mechanical properties are also strain-rate dependent, exhibiting an increase with higher strain-rates. The rubberised specimens exhibit higher strain-rate sensitivity in splitting tension than compression, signified by higher dynamic increase factors for a given strain-rate and lower critical transition strain-rates. A higher rubber content in the mix also result in reduced critical transition strain-rates for the compressive strength, axial crushing strain, and splitting tensile strength. Based on the results of this study, analytical expressions are provided for predicting the dynamic increase factors for the compressive strength, axial crushing strain, elastic modulus, and splitting tensile strength.

Mechanical behaviour of fabric-reinforced plastic sandwich structures: A state-of-the-art review

The use of fibre-reinforced plastics (FRPs) in sandwich structures increased for various industrial applications thanks to their strength-to-weight ratio which provides designers with advanced options for modern structures. FRP Sandwich Structures (FRPSS) are often used in aerospace, biomedical, defence, and marine products, where their high structural performance is required to sustain complex in-service loads and withstand varying environmental conditions. Progressive degradation of FRPSS under such circumstances has been a subject of interest for researchers owing to safety requirements for products with FRP. This paper reviews the state-of-the-art of the mechanical behaviour of FRPSS subjected to various loading regimes. It highlights the variation in structural performance, viscoelastic properties, damage resistance, and sequence of environmental degradation of FRPSS. Numerical methods and damage algorithms used to predict failures are also presented to provide sufficient knowledge for the design of FRPSS. This review contributes to further research on characterizing the properties of FRPSS under quasi-static and dynamic loading conditions.

Design and Characterization of 2.5D Nature-Inspired Infill Structures under Out-Plane Quasi-Static Loading Condition

The 2.5D (2.5-dimensional) structures are acquired to enhance safety and lightweight designs with better energy absorption in the aerospace and automobile sectors. Additive manufacturing (AM), which is practical to build suitable components in industrial and transportation applications, may produce these structures more efficiently. The current study aims to improve the 2.5D infilled structures mean crush force (MCF) and energy absorption capabilities. Under compression loading, the proposed novel nature-inspired 2.5D infilled structure is compared to six existing 2.5D geometries that are inspired by nature. These structures are made of cylindrical shells that are filled with various infill configurations and maintained at a consistent volume. Photopolymer resin is used as the material for the structures, which are created using a digital light processing (DLP) method under AM technology. The characterization of the constructed models was done under compressive out-plane quasi-static stress conditions. ANSYS numerical simulations have been carried out to confirm the dependability of experimental data. The impact of supporting ribs and infill designs on crushing behaviour is thoroughly discussed. Mean crush force (MCF) and specific energy absorption (SEA) under quasi-static compression loading are provided to the proposed unique nature-inspired 2.5D infilled structure to significantly boost crushing qualities, axial collapse, and energy absorption behaviours.

A fast adaptive PD-FEM coupling model for predicting cohesive crack growth

In this study, nonlocal theory of peridynamic (PD) is coupled with finite element method (FEM) to simulate cohesive crack growth in quasi-brittle materials. The adaptive dynamic relaxation method is adopted to implement quasi-static loading conditions. The advantage of peridynamic method in the modeling of crack propagation is taken into consideration to be coupled with that of finite elements which is computationally less expensive and covers various boundary value problems. In this novel method, the whole domain of the problem is initially dominated by FEM solver with coarse mesh grids. Coupling process is then introduced to those regions which have critical conditions according to damage criterion. The capability of the proposed coupling method in the modeling of cohesive crack growth in quasi-brittle materials is demonstrated. In comparison to other conventional PD-FEM coupling methods, the computational cost of the proposed method is considerably improved and total running time of the solution is significantly attenuated. The validity of the method is confirmed through comparing the results with experimental ones.

Dynamic fracture toughness of cellular materials with different microstructures

Ultra-lightweight cellular materials have been widely utilized owing to their excellent specific stiffness, strength and fracture resistance ability. However, current understanding of the fracture resistance of cellular materials is mostly limited to quasi-static loading conditions, and their resistance to dynamic fracture is comparatively less understood. In this study, we investigate the dynamic fracture behaviors of the triangular, kagome, and hexagonal honeycomb cellular materials using numerical methods. Both single-edge notched bend and single-edge notched tension specimens under dynamic loading are considered, and the J values of dynamic fracture toughness of the three configurations are determined. Simulation results demonstrate that the fracture resistance of the triangular honeycomb increases with increasing loading velocity, whereas the fracture toughness of the two other structures exhibits slight variation with respect to the loading rates. After examining the fracture response by changing the strain-rate-dependence of the constituent material, we concluded that the microstructure has a more significant effect on the rate-dependence of fracture toughness than the constituent material. A mechanism based on crack deviation is proposed to account for the distinct rate-dependence of fracture toughness in specimens with different microstructures.

On the mechanical performance and damage evolution of rivet-bonded composite T-joint under three-point bending: A combined experimental and numerical study

This paper presents an experimental and numerical study on the damage characteristics and failure modes of carbon fiber reinforced thermoplastic (CFRTP) T-joints subjected to quasi-static bending load. First, tensile loading tests were conducted on 6061-T6 aluminum alloy and 304 stainless steel specimens at various triaxial stress levels. Then, on the basis of these tensile tests, Finite Element Modeling (FEM) was used to determine the relationship between the metal’s stress triaxiality and failure strain, allowing for the parameter determination of the Johnson–Cook​ failure criteria. After fabricating the T-joint in vehicle body using thermoplastic carbon fiber reinforced composites and 6061-T6 aluminum alloy with rivet-bonded joining method, the mechanical performance of the structure was evaluated under bending load. The T-joint was tested up to failure under quasi-static loading conditions and load–displacement curves were obtained to evaluate the load-bearing capacity and bending resistance. In the FE analysis, the Johnson–Cook, Hashin, and Cohesive Zone Model were used to numerically simulate the bending failure mode and mechanical properties of the T-joint subjected to bending loading. Good agreement was achieved between numerical and experimental results, which can serve as a reference value for the study of rivet-bonded composite joints with dissimilar materials.

Structural behaviour of RC-ECC beam-column connections under quasi-static loading

This research investigates the quasi-static cyclic behaviour of a special moment-resisting frame (SMRF) structure with the application of engineered cementitious composites (ECC) in the beam-column joint. For this purpose, three full-scale beam-column connections were cast and tested under quasi-static loading conditions. The control specimen was cast with the conventional concrete having 21 MPa compressive strength and detailed as per special moment resisting frame (SMRF) requirement. On the contrary, transverse reinforcement was removed from the remaining two (02) specimens with ECC replacing concrete in the joint region and the interface portion of the beam-column connection. Strain gauges and LVDTs were installed at appropriate locations to record the connection's response. After the careful acquisition of data, hysteresis loops, load-displacement envelop and energy dissipation curves were obtained. The test results demonstrated that models with ECC in the joint outperformed the control specimen in ductility, load-carrying capacity and energy dissipation. As compared to the control specimen, the use of ECC in beam-column connection increased the peak displacements by 28.44 %, peak load carrying capacity by 27.13 % and energy dissipation by 58.06 %. Due to high shear strength and ductility, the use of ECC in the joint region was found to address the shear stress requirements without using any transverse reinforcement.

Experimental and numerical investigation of 3D printed bio-inspired lattice structures for mechanical behaviour under Quasi static loading conditions

This research focuses on investigating the design and mechanical aspects of unique bio-inspired structures inspired by the biological elements basal body and nautilus shell. Centriole, nautilus, and cartwheel structures have been designed by CAD software and stereolithography (SLA) 3d printing was applied to manufacture these lattice structures. Finite element analysis was applied to optimize and compressive tests were carried out for the evaluation of the mechanical properties of bioinspired structures. Bio-inspired designs have a combination of qualities, which may create unique mechanical, electrical, thermal, and acoustic characteristics by controlling various factors, which have attracted a lot of study interest. The results of experiments show that the cartwheel structure is stiffer and stronger than other lattices with almost the same volume. According to the mass-to-strength ratio, both cartwheel structures are attractively lightweight and strong. Furthermore, this is the sole study that discusses centriole, nautilus, and cartwheel structure design, manufacturing, and deformation.

Numerical modelling of the mechanical behaviour of Aluminosilicate Glass: A comparison between two simulation approaches

This paper presents a comparison between two numerical methods for modelling the mechanical and failure behaviour of aluminosilicate glass: Cohesive Elements Method (CEM) and Finite Element Method coupled to Smooth Particle Hydrodynamics (FEM-SPH). The failure behaviours, provided by these two approaches, are herein compared under i) quasi-static tests on material coupons, ii) dynamic tests (compressive and tensile tests) also on material coupons and iii) structural impact loading conditions in which the ballistic perforation of aluminosilicate glass tiles due to the impact of a flat-nosed steel projectile are considered. Both methods provide comparable results under quasi-static loading conditions, while more significant differences arise in the ballistic impact part, in which some limitations of CEM are shown.

Impact loading of additively manufactured metallic stochastic sheet-based cellular material

The mechanical response of additively manufactured sheet-based stochastic cellular materials under impact loading was investigated in this work. Samples with four different relative densities (13%, 16%, 19%, and 21%) were fabricated out of 316 L stainless steel using the powder bed fusion (PBF) additive manufacturing (AM) technique. The samples were then tested under quasi-static and dynamic (impact) compressive loading conditions to evaluate their mechanical behavior. All analyzed samples showed a similar progressive plateau stress behavior at all analyzed strain rates. The Crash Force Efficiency (CFE) values of samples at analyzed strain rates were in the range of 67–70%, indicating the uniformity of their mechanical response, which is a vital application feature. The samples exhibited excellent Specific Energy Absorption (SEA) capacity ranging between 5 and 9.2 J/g in the quasi-static deformation mode while achieving up to 35 J/g in the shock deformation mode. Thus, the proposed design approach has great potential for impact and blast mitigation applications. The first and second critical loading velocities of analyzed samples are proportional to the relative density and are in the range of 25–32 m/s and 43–56 m/s, respectively. Minor strain rate hardening was observed in quasi-static deformation mode, while a significant strain rate hardening was observed in shock deformation mode achieved by powder gun testing. The developed and validated computational models offered a more detailed analysis of the deformation mechanism and provided means to predict the sample's behavior at very high strain rates.

The damage effect on the dynamic characteristics of FRP-strengthened reinforced concrete structures

The present work aims to assess the modal properties of reinforced concrete structures strengthened with FRP-based systems subjected to progressive damage. In particular, an integrated numerical fracture model, based on a cohesive crack approach and an embedded truss model, has been used to simulate the damage phenomena of such kinds of structures under quasi-static loading conditions. Subsequently, at predefined damage levels, the dynamic response, in terms of natural vibration frequencies, has been obtained by solving the problem of small amplitude free oscillations and then compared with the numerical and experimental results. The comparisons show the beneficial strengthening effect of the FRP system on both static and dynamic structural response, in terms of load-carrying capacity and natural vibration frequency degradation, highlighting the applicability of the proposed numerical model in the framework of model-based damage identification methods for FRP-strengthened concrete structures.

Investigation of the applicability of single cantilever beam test for the evaluation of fracture toughness of sandwich composites

Sandwich composites are widely used in load-carrying structures and there is a need for a testing method that will allow for qualitative and quantitative assessment of the bond between the facesheets and the core. The presented research investigates the applicability of the single cantilever beam (SCB) test to the evaluation of the mode I dominated fracture toughness of such structures under both quasi-static and fatigue loading.The SCB method uses a relatively simple fixture, the debond front loading conditions are independent of the debond length and the propagation takes place near the facesheet-to-core interface. It allows for the determination of the initiation and the propagation fracture toughness and can also be used for fatigue testing.A sandwich composite, made of carbon-epoxy facesheets and an aramid honeycomb core, was tested in quasi-static and fatigue loading conditions. The initiation values of the Energy Release Rate (ERR) were determined using the Modified Beam Theory (MBT) approach for different definitions of the initiation point. The propagation tests were conducted along with the visual observation of the debond growth. The propagation fracture toughness was determined using two approaches – MBT and the Area Method. The fatigue testing was realized, using the same setup, by cycling the specimens through a range of ERR values. The debond length was measured during the test which allowed for the determination of a relationship between the ERR and the debond length growth rate.The research confirmed that the SCB method can be used for the quantitative measurement of the fracture toughness of sandwich structures.

Investigation of mechanical properties of Al3Zr intermetallics at room and elevated temperatures using nanoindentation

This work deals with the measurement of mechanical properties of single and polycrystalline Al3Zr specimens from ambient to elevated temperatures using nano-indentation experiments. In this study, we employed three kinds of intermetallic specimens produced from Al3Zr crystals chemically extracted from an Al-3 wt% Zr alloy. The properties such as elastic modulus and hardness were determined under quasistatic loading conditions. Constant multicycle indentation testing (MCT) was further performed using a Vickers indenter to understand the fatigue response of intermetallics at high load low cycle conditions. The results showed that hardness and elastic modulus of Al3Zr intermetallics depended on the crystal structure/orientation, with polycrystalline samples showing higher elastic modulus than single crystal specimens at room temperature conditions. MCT experiments revealed that contact pressure of more than 7 GPa was needed to fracture a crack-free crystal under dynamic loading conditions. Consequently the properties of intermetallics at temperatures up to 700 °C were determined for the first time, using high-temperature nano-indentation technique. Elevated temperature measurements indicated that intermetallics had high creep resistance at low and intermediate temperatures, but exhibited significant plastic deformation and creep close to the melting point of pure aluminium.

Multiscale, multiphysics modeling of saturated granular materials in large deformation

We propose a multiscale, multiphysics approach by coupling two-phase material point method (MPM) with discrete element method (DEM) (MPM-DEM) to simulate the hydro-mechanical coupling responses of saturated granular media from small strain en route to large deformation under either quasi-static or dynamic loading conditions. The multiscale scheme is featured by (a) using a two-phase MPM in conjunction with the u−v−p formulation to solve the solid–fluid interactions in the macroscopic domain of a boundary value problem of saturated porous media, and (b) employing a DEM assembly comprised of arbitrarily shaped particles to provide path-dependent effective constitutive responses for each material point under complex loading conditions. A semi-implicit integration scheme based on the fractional step algorithm is further implemented in the proposed multiscale approach to improve its overall efficiency. The proposed approach is validated by the one-dimensional consolidation test before being further used to simulate more challenging problems, including the cyclic shaking test, the column collapse, and the wave propagation in anisotropic saturated porous media. We demonstrate that the proposed MPM-DEM approach is powerful and versatile in capturing the complicated static and dynamic multiphysics interactions exhibited in saturated granular media that could be of practical importance in various engineering settings. We further establish connections between these macroscopic observations with their underlying microstructural mechanisms to offer multiscale insights into the complicated dynamic responses of saturated sand.