Quasi-static Loading Conditions Research Articles

Maximum energy absorbing capacity and structural stability for energy dissipation devices exploiting a hybrid cutting/clamping deformation mode

There is significant and accelerating demand for novel, lightweight sacrificial energy absorbers which can exceed the performance of the current state-of-the-art (i.e., axial crushing), however, the risks of structural instability, practical limitations and mitigation strategies are unexplored in the literature. High-capacity hybrid cutting/clamping deformation modes considering 10 or more blades were newly investigated for their potential to eclipse the mechanical performance of traditional progressive folding deformation under quasi-static loading conditions, with further emphasis on maximizing energy absorbing capacity without exceeding acceptable limits on stability. Analytical, numerical and experimental studies were conducted for circular AA6061 extrusions in T6 and T4 temper conditions with multiple diameter/wall thickness combinations to quantify the influence of these engineering parameters, and the number of blades, on unfavorable transitions from steady-state cutting to progressive folding. Transitions were attributed to two primary instability mechanisms: peak force saturation as the cutting force exceeded the maximum collapse force of an extrusion, and petalled sidewall binding followed by geometric transformations to unilateral lobes. Temper condition was the most sensitive parameter, 6061-T4 extrusions in multiple geometric configurations regularly experienced transitions to progressive folding compared to limits between 13 and 15 blades for complementary 6061-T6 extrusions. These limits were analytically predicted with an average tolerance of ±1 blades. Numerical modeling considering an Eulerian material-and-void approach was suitable to predict steady-state cutting/clamping forces with 11.8% average cumulative error. The extrusions subjected to cutting/clamping dissipated up to 65.3% more energy (28.9% average) compared to axial crushing before geometric instabilities were observed.

Fatigue Threshold Analysis of Adhesives: Displacement Control vs. Load Control Strategy

The aim of this paper is to study the response of a two-part polyurethane-resin adhesive under quasi-static and fatigue loading conditions, to compare load control and displacement control approaches for the mode I fatigue threshold analysis of the tested material. To achieve this, double cantilever beam (DCB) joints were manufactured and tested. For the post-processing of the raw data, a compliance-based beam method was used. Out of this analysis, R curves and Paris law curves were obtained. Both approaches showed a similar Paris law slope meaning a low sensibility of the crack growth rate between them. As the displacement control load decreases gradually during the test, it can give more precise threshold energy than the load control technique.

Response of Apple Flesh to Compression under the Quasi-Static and Impact Loading Conditions.

Compression tests of cylindrical samples were carried out using two 'Rubin' and 'Red Jonaprince' apple cultivars with flesh firmness differing in a statistically significant way. The tests were conducted under both the quasi-static and impact loading conditions, which required the use of two test stands. For this purpose, an impact measuring stand was designed and built. The tests proved that the firmness of the apple flesh influenced the mechanical response under both the quasi-static and impact loading conditions. The elasticity modulus had much higher values under the impact than quasi-static loading conditions. This indicates that the stiffness of the apple tissue was larger during the impact than at the low-speed compression. Different failure mechanisms of cylindrical apple flesh samples were found depending on the loading conditions. Under the quasi-static loading conditions, the apples of both cultivars were damaged at the same strain value. However, during the impact, apples were apt to damage at a constant stress value regardless of the firmness of the tested cultivar. The toughness of the apple tissue depended on firmness and was larger under the quasi-static loading conditions for the apples with larger firmness. However, under the impact loading conditions, the toughness was greater for the apples with smaller firmness.

Modelling the effect of lateral inertia on the dynamic response of aluminium square tube extrusions

Crashworthiness of automotives require the design of crush tubes installed as a frontal structure to absorb energy by deforming plastically during collision. The crushing behaviour of aluminium extrusions modelled as square tubes is investigated under quasi-static and dynamic loading conditions using the non-linear explicit finite element code LS DYNA. Previous works have attributed the dynamic increase of the peak and mean crushing load of aluminium thin-walled structures under axial compression, to inertia effects. Results of our investigation revealed that there is a considerable thickening of the shell side walls before and during progressive folding. This shell thickening is established as the cause of lateral inertia which consequently results in increased crush strength. In addition, a positive correlation was established between the shell thickening and crushing speeds. 

Role of build orientation on quasi-static and dynamic fracture responses of additively manufactured AlF357 and AlSi10Mg alloys

In this work, a popular additive manufacturing (AM) aluminum alloy - AlSi10Mg and a new AM aluminum alloy - AlF357 are fabricated using Laser-Powder Bed Fusion ( L -PBF) approach to evaluate quasi-static and dynamic fracture behaviors. Four build orientations - horizontal, vertical, flat, and diagonal - are assessed. A Kolsky pressure bar apparatus along with ultrahigh speed photography is used to carry out the dynamic fracture experiments. In-plane surface displacements near cracks are measured using Digital Image Correlation (DIC) to evaluate the fracture parameters directly. A hybrid experimental-numerical approach that combines DIC measurements and finite element analysis is implemented to extract critical energy release rate and crack growth resistance behaviors. The effect of build orientation is found to be significant on the quasi-static fracture behavior with the horizontal and flat orientations having the best and worst fracture responses, respectively. Under dynamic loading conditions, however, the effects of build orientation are marginal. Both AlF357 and AlSi10Mg show strain-rate sensitivity and change in fracture mechanism under dynamic loading conditions with ~175% and ~200% increase in critical energy release rate. AlF357 consistently outperforms AlSi10Mg in both quasi-static (~30%) and dynamic (~10%) loading conditions.

Discrete Element Modelling of Cold Crushing Tests Considering Various Interface Property Distributions in Ordinary Refractory Ceramics.

The microstructures and local properties of ordinary refractory ceramic materials are heterogeneous and play a role in the fracture behavior of ordinary refractory ceramic materials. It is important to consider them in numerical modeling. Herein, the discrete element (DE) method was applied to determine the influences of heterogeneity of ordinary refractory ceramic materials by applying statistically distributed interface properties (uniform, Weibull), as opposed to constant interface properties, among the elements. Uniaxial cold crushing tests were performed as a case study. A reasonable loading strain rate for receiving quasi-static loading conditions and computation efficiency was evaluated. The loading wall displacement was recorded to present the stress-strain curves of cold crushing tests. Furthermore, the effects of the interface property distributions on the load/displacement curve, fracture energy, cold crushing strength, and fracture events were investigated. The results reveal that the DE method is a promising method for visualizing and quantifying the post-peak fracture process and crack events in ordinary refractory ceramics. Different interface property distributions contribute to significant variances in the load/displacement curve shape and fracture pattern. The heterogeneity of ordinary refractory ceramics can be further determined by comparing the experimental curves and fracture propagation along with an inverse identification approach.

Dynamic beam-end tests: Investigation using split Hopkinson bar

The bond between concrete and steel reinforcement is a highly investigated topic under both quasi-static and dynamic loading conditions. Strain rate sensitivity and dynamic increase factor are the key aspects affecting the resulting response of the structure and represent crucial parameters for modelling the impact-related events. Measuring the bond properties under dynamic conditions, however, is a complex task requiring advanced experimental techniques and methodology. In this paper, the bond stress-slip relationships during dynamic pull-out was measured using an innovative technique based on a tensile split Hopkinson bar and a near-to-full-scale beam-end specimen representing an end-part of a reinforced concrete beam.A tensile split Hopkinson bar was used to generate a tensile stress wave with a long duration that pulls a reinforcement bar with a short bond length out of the concrete block. As a very complex wave propagation behavior was observed during the impact, the forward and backward propagating waves have to be separated and subsequently used to properly calculate the bond stress-slip relationship. Two redundant methods utilizing strain gauge signals and digital image correlation for separation of the longitudinal stress waves were employed to solve the problem. The proof-of-concept and applicability of the method which are the main focus of the contribution were demonstrated with a numerical simulation performed in LS-DYNA. An exemplary evaluation of the results of the real tests performed at medium impact velocity with full pull-out of the rebar was performed. The presented method is considered suitable for the evaluation of the dynamic bond stress-slip relationship and produced high-quality results during the initial impact phase and reasonable quality results up to the full pull-out.

On the inverse identification of wood elastic properties using a DIC-based FEMU approach

PurposeThis work aims to identify the linear elastic orthotropic material paramters of Pinus pinaster Ait. wood, using full-field measurements and an inverse identification strategy based on the finite element (FE) method updating technique.Design/methodology/approachCompression tests are carried out under uniaxial and quasi-static loading conditions on wood specimens oriented on the radial-tangential (RT) plane, with different grain orientations. Full-field displacements and strains are measured using digital image correlation (DIC), which are then used as a reference in the identification procedure. A FE model is implemented assuming plane stress conditions, where wood is modelled as an orthotropic homogeneous material. Based on the numerical results, a synthetic image reconstruction scheme is implemented to synthetically deform the reference experimental image, which is then processed by DIC and further compared to the experimental results.FindingsThe results for both approaches were similar when both specimen configurations were used in a single run. However, when using the DIC-based FEMU approach with the on-axis configuration, the identified modulus of elasticity in the tangential direction and shear modulus are closer to the reference values.Originality/valueThis approach ensures a fair comparison between both sets of data since the full-field strain maps are obtained through the same filter and therefore have the same strain formulation, spatial resolution and data filtering. Firstly, the identification is performed using a single configuration, either the on-axis or the off-axis specimen. Secondly, the identification is carried out by merging data from both on-axis and off-axis configurations.

Superior energy dissipation mechanisms compounded within composite AA6061/H130 foam structures

Significant demand exists for sacrificial energy absorbers which can achieve ideal mechanical performance (i.e., near-constant reaction forces) utilizing lightweight, readily available materials to simultaneously improve occupant safety and minimize vehicle mass to address growing calls to further improve efficiency in the transportation sector. A novel energy dissipation system comprised of AA6061 extrusions subjected to hybrid cutting/clamping and axially compressed H130 PVC foam, which could compound the ideal mechanical performance of each lightweight constituent component, was conceptualized and experimentally tested under quasi-static loading conditions. Since the extrusions remained intact above the blades the foam cores experienced uniform compression without in-plane deformation, leading to uniquely negligible extrusion/foam interaction effects. Extrusions with a T4 temper condition further benefited from resistance to circumferential distortion attributed to internal support from the foam cores. This behavior was in stark contrast to complementary structures subjected to a conventional axial crushing mode which experienced unstable combinations of lateral kinking, brittle lobe fracture and highly fluctuating reaction forces due to inconsistent interaction effects. The average TEA and energy absorbing effectiveness factor (EAEF) improved by 13 % and 38 %, respectively, for the composite structures subjected to novel compounded cutting/clamping compared to the traditional crushing mode, quantifying the enhancements over the current state-of-the-art. An analytical modeling procedure which could accurately (> 5 % error) replicate the newly observed, compounded force/displacement responses was derived and utilized in a parametric study identifying maximizing extrusion diameter and minimizing wall thickness and/or foam density as effective approaches to balance load bearing capacity and SEA.

Modeling impact fracture in a quasi-brittle solids using a 3D nonlocal graph-based finite element analysis: Theory, finite element simulations, and experimental verification

In this work, the authors have developed and implemented a novel nonlocal three-dimensional graph-based finite element approach for simulating fracture in quasi-brittle solids as an extension of their previous work in two dimensions. In order to validate the nonlocal aspects of the model, the authors have also fabricated a gypsum-based particulate composite with silica particles of specific dimensions and mass fractions, thus the length scale of the material is fixed by the particulate media. The GraFEA fracture model is implemented in a graphics processing unit (GPU) parallel computing environment that allows substantial speed-up of the simulations in both cases of impact and quasi-static loading conditions. The improvement in computational performance is especially essential for carrying out the simulation of parametric study. Comparison of the physical response of this specially designed composite with the three-dimensional nonlocal GraFEA shows that the model is capable of simulating fracture in such materials. Finally, the efficacy of simulating impact response of concrete including crack closure behavior is tested by simulating hammer drop test for the concrete beam sample and cyclic shear loading on the circumferentially-notched concrete cylinder sample.

Tuning the Mechanical Behavior of Density-Graded Elastomeric Foam Structures via Interlayer Properties.

The concept of density-graded foams has been proposed to simultaneously enhance strain energy dissipation and the load-bearing capacities at a reduced structural weight. From a practical perspective, the fabrication of density-graded foams is often achieved by stacking different foam densities. Under such conditions, the adhesive interlayer significantly affects the mechanical performance and failure modes of the structure. This work investigates the role of different adhesive layers on the mechanical and energy absorption behaviors of graded flexible foams with distinct density layers. Three adhesive candidates with different chemical, physical, and mechanical characteristics are used to assemble density-graded polyurea foam structures. The mechanical load-bearing and energy absorption performances of the structures are evaluated under quasi-static and dynamic loading conditions. Mechanical tests are accompanied by digital image correlation (DIC) analyses to study the local strain fields developed in the vicinity of the interface. Experimental measurements are also supplemented by model predictions that reveal the interplay between the mechanical properties of an adhesive interlayer and the macroscale mechanical performance of the graded foam structures. The results obtained herein demonstrate that the deformation patterns and macroscale properties of graded foam composites can be tuned by selecting different bonding agents. It is also shown that the proper selection of an adhesive can be a practical way to address the strength–energy dissipation dichotomy in graded structures.

Finite Fracture Mechanics extension to dynamic loading scenarios

The coupled criterion of Finite Fracture Mechanics (FFM) has already been successfully applied to assess the brittle failure initiation in cracked and notched structures subjected to quasi-static loading conditions. The FFM originality lies in addressing failure onset through the simultaneous fulfilment of a stress requirement and the energy balance, both computed over a finite distance ahead of the stress raiser. Accordingly, this length results to be a structural parameter, thus able to interact with the geometry under investigation. This work aims at extending the FFM failure criterion to dynamic loadings. To this end, the general requisites of a proper dynamic failure criterion are first shortlisted. The novel Dynamic extension of FFM (DFFM) is then put forward assuming the existence of a material time interval that is related to the coalescence period of microcracks upon macroscopic failure. On this basis, the DFFM model is investigated in case a one-to-one relation between the external solicitation and both the dynamic stress field and energy release rate holds true. Under such a condition, the DFFM is also validated against suitable experimental data on rock materials from the literature and proven to properly catch the increase of the failure load as the loading rate rises, thus proving to be a novel technique suitable for modelling the rate dependence of failure initiation in brittle and quasi-brittle materials.

Optomechanical design and analysis of lens assembly for space application

This study illustrates how commercial off-the-shelf optics may be employed in space exploration cameras. The most frequent loads on optical systems are self-weight, which is caused by gravitational orientations, vibration, and temperature ranges, which is caused by exposure to quickly changing temperatures ranging from extremely cold to very hot and during launch. This study describes the modulation transfer function (MTF) performance of the selected lens assembly, followed by a top-down method to disassemble the optical parts of the commercial lens housing. The lens enclosure was designed and modelled by retaining the interelement lengths determined from physical measurement. The performance of the lens barrel and optical elements have also been validated in ANSYS using a quasi-static loading and thermoelastic condition. As a post-performance assessment owing to stress and vibrations during satellite launch, a tilt-separation simulation was conducted. As a result of this research, it has been demonstrated that optomechanical design of commercial lens assembly is feasible for use in space application camera systems.

Microstructure evolution and shear band formation in an ultrafine-grained Ti–6Al–4V alloy during cold compression

It is generally accepted that dynamic recrystallization (DRX) is the result of shear banding in metallic materials. However, we observed the microstructure evolution in an ultrafine-grained Ti–6Al–4V alloy under quasi-static compression and found the formation of DRX induced micro shear bands at the initial stage of plastic deformation. It is of interest that the strain-localized regions experienced a two-step recrystallization process, thus resulting in the formation of refined dynamic recrystallized (DRX) grains in transition zones and nanograins in core regions of micro shear bands successively. It is suggested that the DRX taking place at local regions led to the strain softening, which subsequently accelerated the formation of micro shear bands. Micro shear bands nucleated, grew wider, multiplied and propagated by means of gradually consuming the DRX regions with the increase of local strains. Those micro shear bands are considered to contribute to synergetic plastic deformation in addition to dislocation slipping at lower strains, while some macro shear bands are regarded as the precursor of fracture at higher strains. Based on the observation results a new DRX mechanism of shear band was proposed. The experimental results in the present work can provide new evidences and novel insights of the shear-band formation, plastic deformation as well as failure mode in UFG alloys under quasi-static loading conditions.

Cyclic behaviour of demountable metallic corrugated shear panel dampers

This study aims to develop a new demountable corrugated shear panel damper (DCSPD). The developed device is to upgrade the flat shear panel of the classical shear panel damper (classical SPD) using a corrugated shear panel. And the energy dissipating parts of the damper are demountable, allowing for easy processing, transport, and assembly onsite. Moreover, the new damper can make full use of the geometrical properties of the corrugated shear panel to overcome the shortcomings of the classical SPD, such as low initial stiffness and poor energy dissipation. A series of tests were conducted to systematically study the seismic performance of the proposed damper under quasi-static cyclic loading conditions. The effects of three key design parameters are discussed, including the corrugated direction and thickness of the shear panel and the thickness of the flange plate. The experimental results demonstrate that the direction of the corrugated shear panel changes the plastic failure modes of the dampers, resulting in the overall out-of-plane buckling failure of demountable horizontally corrugated shear panel damper (DCSPD-H) and the local buckling failure of demountable vertically corrugated shear panel damper (DCSPD-V). Therefore, the shear bearing capacity, ductility coefficient, and energy dissipation capacity of DCSPD-H are 5.56%, 10.23%, and 35.37% higher than those of the DCSPD-V, respectively. Moreover, an increase in the thickness of the shear panel will significantly improve the seismic performance of the damper, whereas an increase in the thickness of the flange plate may hinder the deformability of the damper. Further, the finite element models of tested dampers were established, and the plastic collapse mechanisms and the sharing of energy-absorbing of each part were investigated. Finally, simple design equations are presented through parametric studies of dampers with different geometric dimensions.

Compressive strength of the plastic-bonded explosive surrogate idoxuridine under quasistatic and dynamic compression

Inert simulant materials, or “mocks”, are often used as surrogates for plastic-bonded explosives (PBX) in non-detonative tests in order to mitigate hazards. Mocks should reproduce as many non-detonative properties of the explosive as possible, including structural behavior in a variety of thermal and mechanical environments. Recently, the molecular crystal idoxuridine (IDOX) has been identified as an ideal mock for the main component in the explosive polymer-matrix composite PBX 9501, and has performed favorably under quasistatic loading conditions. Here, in order to assess robustness over a range of mechanical environments, plastic-bonded IDOX was compression tested from 0.001/s to 2000/s strain rates and compared to PBX 9501 historical data. Plastic-bonded IDOX showed good agreement to PBX 9501 across these strain rates, justifying continued development and production as a mock.

Seamless coupling of bond- and state-based peridynamic and finite element analyses

This study presents a variational approach to couple PeriDynamic (PD) and Finite Element (FE) analyses to take advantage of their salient features. The PD and FE domains share the same nodes along their interface without an overlap region and any constraint conditions. Therefore, this approach does not require a morphing or a blending function that facilitates coupling over a transition zone. Although this approach is general and not limited to any commercial FE software, the PD interactions for bond-based (BB), ordinary state-based (OSB) and non-ordinary state-based (NOSB) are modeled with MATRIX27 elements native to ANSYS, a commercial software, for the purpose of demonstration. It is a general framework to perform BB, OSB and NOSB PD coupled with traditional elements. It provides the ability to practicing engineers in industry for integrating PD into their FE based design tools. The PD domains can share a boundary with FE domains or be completely embedded in the FE domain. The resulting global system of equations includes the contributions arising from the PD nodes and FE nodes. These equations are solved simultaneously without requiring an iterative procedure through implicit methods. Therefore, it is a direct (seamless) coupling approach without an overlap zone. The results are free of displacement kinks; thus, unphysical stress concentrations do not appear near the interface between the PD and FE regions. Its accuracy is demonstrated by considering isotropic elastic plates subjected to various types of boundary conditions under quasi-static and dynamic loading conditions. Also, its accuracy is demonstrated for quasi-static crack propagation by considering double cantilever beam and three-point bend test specimens. The results from the coupled PD-FE approach agree well with those of FEM for all boundary and loading conditions.

Co-cured carbon fibre/epoxy composite joints by advanced thermoplastic films with excellent structural integrity and thermal resistance

Carbon fibre/epoxy composite joints were assembled with Poly-etherether-ketone (PEEK) and Poly-ethylenimine (PEI) films using a co-curing process to prepare single-lap joint specimens. The joints were tested under quasi-static loading conditions at 22 °C and 130 °C and a fatigue loading condition. The experimental results demonstrated better or comparable structure integrity of the composite joints co-cured by PEEK and PEI films than the reference joints bonded by aerospace FM300 adhesives. In particular, the PEEK co-cured joints exhibited extraordinary mechanical performance at 130 °C and excellent fatigue resistance. For instance, the lap-shear strength at 130 °C and the fatigue life of the composite joints co-cured by 200μm PEEK films was 2.1 times and 2.7 times higher than that of the aerospace adhesive joints, respectively. Overall, the results of this work proved that advanced thermoplastic films are promising alternatives to epoxy adhesives for the co-cure joining of thermoset composites with significantly enhanced structural integrity and thermal stability.

A novel seismic energy dissipation device: Laboratory tests, mathematical modeling, and numerical analysis

Seismic energy dissipation devices are used in buildings to dissipate seismic energy in the event of an earthquake. These devices use steel and its yielding properties to dissipate the input seismic energy and protect the building’s structure from damage and failure. There are many types of seismic energy dissipation devices, but all share the same goal of dissipating seismic energy. As part of an ongoing research program, a novel steel seismic energy dissipation device has been proposed. The device is made of a curved diaphragm plate made of mild steel. The diaphragm plate is welded to flanges and transverse stiffeners and can be connected to beam–column joints with a number of bolts. The damper and its geometric variants have been designed and tested in the lab under quasi-static cyclic loading conditions. In the present study, we briefly discuss the experimental program and its main outcomes; however, the main objectives will be mathematical modeling and numerical simulations. The proposed dampers are found to exhibit a stable nonlinear hysteretic behavior with small and large load–deflection loops. This behavior is an indicator of the damper’s capabilities in dissipating significant amounts of input energies with varying intensities. Mathematical modeling of the proposed damper was performed using the Bouc–Wen–Baber–Noori (BWBN) model of hysteresis to simulate the degradation of the device after repetitive loading cycles. The identification of the BWBN model parameters is discussed in detail. Nonlinear finite element buckling analysis was found to be in good agreement with test results of the best performing variant. Interesting results about the shear stress distribution at different buckling stages are presented and discussed in detail.

Güneş Enerjili Hafif Aracın Arka Salıncak Kolunun Topoloji Optimizasyonu Çalışması

In this study, the design improvement of the rear swing arm used in the independent rear suspension system of a single-wheel drive solar-powered vehicle was carried out by topology optimization. First of all, the quasi-static standard load conditions for the driving conditions of the solar-powered vehicle were examined, and the static analysis of the rear suspension system was performed using the finite element method. Considering the critical stress regions in the rear swing arm, a rough model was created with the same geometrical boundaries. Multi-step topology optimization is applied to this model. Finite element analysis was performed under the same boundary conditions to validate the optimized design. The main purpose of this study is to provide an improved model by reducing the stress concentration of the rear swing arm.