Oblique Impact Research Articles

Understanding the impact dynamics of droplets on superhydrophobic surface

The dynamics of droplets impacting superhydrophobic surfaces is investigated using experiments and numerical simulations. The superhydrophobic surface is fabricated by growing the carbon nanotube forest on stainless steel mesh in order to perform experiments. To numerically model the surface, Kistler dynamic contact angle model is used. Depending on different physical properties such as drop Weber number, the inclination of the surface and Ohnesorge number, the impact outcomes are categorized into symmetric bouncing, asymmetric bouncing, breakup, and tail impingement. It is observed that with the increase in the inclination angle of the surface, the contact time of droplets on the surface decreases drastically. The energy budget of impacting drops is analyzed for the entire evolution cycle to delineate the process of drop deformation and its bouncing. It is observed that the droplets bouncing off the inclined surface carried more kinetic energy compared to those of their counterparts on horizontal surfaces. To address the bouncing phenomena, co-efficient of restitution of the drop is also explored. The inclination of the substrate is found to increase the co-efficient of restitution of the drop, which is directly related to the energy budget of the bouncing drop. Furthermore, the force exerted by a drop on such surfaces is also analyzed, showing two peaks, making superhydrophobic surfaces more prone to damage in comparison to hydrophilic surfaces, which showed only a single peak. It is observed that the magnitudes of the peak forces are smaller for oblique impact as compared to horizontal impact.

Analysis of the spinal 3D motion of postmortem human surrogates in nearside oblique impacts

Objective: The objective of this study is to analyze the 6 degrees of freedom (DOF) motion of the spine using the finite helical axis (FHA) in three postmortem human surrogates (PMHS) sled tests. Methods: The sled test configurations corresponded to a 30° nearside oblique impact at 35 km/h. Two different restraint system versions (RSv) were used. RSv1 was used for PMHS A and B while RSv2 was used for PMHS C. The 6 DOF motion of the head and three selected vertebrae have been analyzed using the FHA which describes the 3 D motion of a rigid body between two instants of time as a rotation about and a translation along a unit vector. A minimal amount of rotation is necessary to the FHA calculation, thus the FHA components have been calculated based on a pre-defined interval of 8° of rotation. Results: The analysis of the FHA components demonstrated right lateral bending until around 100 ms, when the rebound phase was reached and the head and the lower spine undergoes left lateral bending. The three PMHS exhibited, in general, flexion movement of the whole body and torsion to the right side of the occupant. This general motion can be associated to the effect of the seatbelt acting as a fulcrum of the rotational movement of the bony landmarks. The interaction of the PMHS with the retention system can be noted by analyzing the time in which the head and the upper spine initiated the rotation and the sudden changes of rotational direction of the three PMHS’s head. Conclusions: The rotational analyses have shown to be more sensitive to experimental events than the trajectory analyses for the studied physical tests. Additionally, the results presented in the present study contributes to the analysis of the body kinematics during an oblique impact and adds new experimental data for Human Body Models (HBM) and Anthropometric Test Devices (ATD) benchmarking.

Sitting posture influence in autonomous vehicles for the evaluation of occupant safety in side impact

IntroductionAutonomous vehicles will give vehicle drivers greater freedom to engage in other activities while the vehicle is in operation that will lead to changes in the seating configurations as well as driver posture. Although it is expected that these vehicles will prevent crashes, collisions will still happen. Ensuring the adequate protection of occupants in these new unconventional positions is the main challenge. The objective of this study was to evaluate and analyse occupant responses in non-standard seating positions that may be expected to occur in automated vehicles occupants under side impact conditions. MethodsFinite Elements Method simulations were performed using the WorldSID 50th male dummy positioned in the driver position. A validated simplified vehicle model with fully deformable driver-side components was used. To explore the sensitivity to side impact across various occupant activities, five scenarios were defined: normal driving (S0), fall-back ready (S1), work (S2), leisure (S3) and relax/sleep (S4). All scenarios were subjected to the Euro NCAP Side Tests Protocols (deformable barrier and oblique pole impact). ResultsAs the distance from the pole impact point to the B-pillar decreased, the structure deformation increased due to the B-Pillar deformation. In cases S2 and S3 predicted thorax injury values were the highest, whereas scenarios S3 and S4 were the worst situation in deformable barrier impact test. Good pelvis protection was provided in all pole impact scenarios, but pelvis injury values were the highest in all scenarios in deformable barrier impact test. ConclusionsAttending to the new scenarios due to the new available driver activities, current safety standards are not enough to guarantee the highest level of protection. Reclined seating scenarios required a wide range of impact positions to be covered since injury responses were influenced by the initial impact alignment of the dummy relative to the vehicle and the pole. Large rotations of the seatback lead to high chest compressions.

Comparison of free-surface and conservative Allen–Cahn phase-field lattice Boltzmann method

This study compares the free-surface lattice Boltzmann method (FSLBM) with the conservative Allen–Cahn phase-field lattice Boltzmann method (PFLBM) in their ability to model two-phase flows in which the behavior of the system is dominated by the heavy phase. Both models are introduced and their individual properties, strengths and weaknesses are thoroughly discussed. Six numerical benchmark cases were simulated with both models, including (i) a standing gravity and (ii) capillary wave, (iii) an unconfined rising gas bubble in liquid, (iv) a Taylor bubble in a cylindrical tube, and (v) the vertical and (vi) oblique impact of a drop into a pool of liquid. Comparing the simulation results with either analytical models or experimental data from the literature, four major observations were made. Firstly, the PFLBM selected was able to simulate flows purely governed by surface tension with reasonable accuracy. Secondly, the FSLBM, a sharp interface model, generally requires a lower resolution than the PFLBM, a diffuse interface model. However, in the limit case of a standing wave, this was not observed. Thirdly, in simulations of a bubble moving in a liquid, the FSLBM accurately predicted the bubble's shape and rise velocity with low computational resolution. Finally, the PFLBM's accuracy is found to be sensitive to the choice of the model's mobility parameter and interface width.

Crashworthiness Analysis of Square Aluminum Tubes Subjected to Oblique Impact: Experimental and Numerical Study on the Initial Contact Effect

This study investigates the crashworthiness behavior of square aluminum thin-walled tubes subjected to both axial and oblique impact loading, emphasizing the effects of crushing angle and initial contact between impactor and tube on the plastic collapse initiation and energy absorption capacity. A parametric study in crushing angle is conducted until 15°, while the two examined types of initial contact between impactor and tube consist of a contact-in-edge case and a contact-in-corner one, aiming to capture the effect of initial contact on both plastic collapse and energy absorption. Both experimental quasi-static tests and numerical simulation via finite element modeling in LS-DYNA software are carried out for the evaluation of the crushing response of the tested tubes. The 5° oblique cornered crushing revealed the greatest energy absorption, reflecting the most efficient loading case as significant tearing failure occurred around the tube corners in axial crushing due to a higher peak crushing force, while the increase in crushing angle caused a drop in energy absorption and peak force regarding the oblique loading. Finally, an initial contact-in-corner case revealed higher energy absorption compared to both axial and edged oblique loading, while peak force showed a slight decrease with crushing angle in that case.

Classification of the collapse of a composite fairing during the oblique high-speed water entry

The crushing behaviors of the buffer devices for the projectile in the high-speed oblique water entry process were investigated numerically. The high-speed water entry tests of the aluminum alloy projectile with a sabot were conducted to study the cavity formation, also used to validate the numerical method. Detailed material parameters of three kinds of foams including expanded polystyrene foam (EPS), closed-cell aluminum foam, and rigid polyurethane foam (PU), and the composite material for the fairings were obtained from a series of statics and dynamics mechanics experiments. Twelve collapse modes of the thin-walled fairings were identified during the oblique water impact. The effects of the length of the fairing, foam type, and water entry angle on these failure modes, and the peak impact acceleration of the projectiles were revealed. The results showed that the load reduction efficiency (LRE) is dependent on both entry angle θ and dimensionless length LFA/DPJ. The critical load reduction water entry angle (CLRA) is linearly related to LFA/DPJ. Among the three kinds of foam materials, the closed-cell aluminum foam has the best load reduction performance (up to 72.82%) and the widest range of load reduction water entry angles. Meanwhile, the EPS foam makes the projectile withstand the largest impact overload (up to 11.43%).

Complex Crater Formation by Oblique Impacts on the Earth and Moon

AbstractAlmost all meteorite impacts occur at oblique incidence angles, but the effect of impact angle on crater size is not well understood, especially for large craters. To improve oblique impact crater scaling, we present a suite of simulations of complex crater formation on Earth and the Moon over a range of impact angles, velocities and impactor sizes. We show that crater diameter is larger than predicted by existing scaling relationships for oblique impacts; there is little dependence on obliquity for impacts steeper than 45° from the horizontal. Crater depth, volume and diameter depend on impact angle in different ways—relatively shallower craters are formed by more oblique impacts. Our simulation results have implications for how crater populations are determined from impactor populations and vice‐versa. They suggest that existing approaches to account for impact obliquity may underestimate the number of complex craters larger than a given size by as much as one‐third.

Modelling of woven CFRP plates subjected to oblique high-velocity impact and membrane loads

In this work, an analytical model was employed to model the perforation of woven plates made of AS4/8552 (carbon/epoxy) laminates exposed to oblique impact and preload in plane. The model assumed six mechanisms for the absorption of the kinetic energy of projectile: the laminate acceleration, elastic deformation of fibres, fibres failure, shear plugging, delamination and matrix cracking. The model was validated with numerical and experimental data from previous works and, was used to determine the influence of an oblique and perpendicular impacts on laminates with and without the in-plane preload.

Dynamic response of aluminium matrix syntactic foams subjected to high strain-rate loadings

This paper presents experimental work to characterise the dynamic behaviour of aluminium matrix syntactic foams subjected to compression, Split Hopkinson Pressure Bar and terminal ballistic impact tests as well as blast loading. Numerical models have also been developed to simulate the dynamic response of the composite foams. The effect of strain-rate on their compressive crush behaviour has been investigated, given that the rate-dependent characteristics of these materials are required for designing dynamically loaded structures. Characterisation of the behaviour of the foam under high strain-rate loadings and the identification of the underlying failure mechanisms were also undertaken to evaluate their effective mechanical performance. The results show that the aluminium syntactic foam is sensitive to strain-rate in terms of initial stiffness, peak stress and plateau stress and show a pronounced high-rate dependence at a strain rate above 1000 s-1. The concrete damage plasticity model with rate-dependent features were used to simulate the dynamic behaviour of the foams, with the failure modes being captured. The model was verified and validated against the experimental results, and predictions were made for the normal and oblique ballistic impact response. Overall, the level of agreement between the numerical simulations and the experimental results is encouraging.

An update of Tritium co-deposition in ITER using WallDYN3D

The retention by co-deposition of fuel species in Be-layers, forming due to migration of Be eroded from the 3D-shaped main chamber wall in ITER, was modeled with WallDYN-3D. The goal was to investigate the influence of the 3D shaping on Be layer formation location, compared to previous 2D calculations for a perfectly toroidally symmetric main chamber Be wall. The calculation showed that the shaping results in a large number of partially plasma shadowed regions in the main chamber, where Be can deposit and form layers. Depending on the main chamber plasma solution, this results in a shift of the primary Be layer deposition location from the divertor in 2D to the main chamber in 3D calculations. The deposition location is important because the higher average hydrogen isotope (HI) particle energy in the main chamber affects HI content in the co-deposited layers. However, for the currently available HI/Be scaling laws the resulting increase in HI/Be is small. A comparison of the 3D-shaped Be-main chamber erosion calculated recently by ERO2.0 and by WallDYN-3D in this work, shows very similar erosion patterns. However, differences exist in the deposition locations because, in contrast to ERO2.0, WallDYN-3D takes re-erosion of deposited Be into account. To match the absolute amount of Be erosion the Be-sputter yields used by WallDYN-3D have to be increased by a factor of 1.5, due to a different erosion yield database used by ERO2.0 which results in higher yields at oblique impact angles. This uncertainty in the sputter yield, but also in the HI/Be scaling laws, make an uncertainty quantification necessary. Applying polynomial chaos expansion to the Be layer growth rate and HI-retention rate by co-deposition, yields an uncertainty of ≈ 100 % in both quantities, due to the uncertainties in the sputter yields and the fit parameters of the HI/Be scaling laws. • WallDYN-3D was applied to Be erosion/migration/layer deposition for the 3D shaped ITER main chamber wall. • 3D shaping leads shifts the location of Be layer deposition from the divertor to main chamber • A comparison with recent ERO2.0 calculations shows very similar erosion patterns. • WallDYN and ER02.0 use different Be erosion yield database which results in factor 1.5 increased erosion at oblique angles for ER02.0. • Uncertainty quantification results in a 100% uncertainty in both Be layer growth rate and D/T retention rate.

Spinal Cord Boundary Conditions Affect Brain Tissue Strains in Impact Simulations.

Brain and spinal cord injuries have devastating consequences on quality of life but are challenging to assess experimentally due to the traumatic nature of such injuries. Finite element human body models (HBM) have been developed to investigate injury but are limited by a lack of biofidelic spinal cord implementation. In many HBM, brain models terminate with a fixed boundary condition at the brain stem. The goals of this study were to implement a comprehensive representation of the spinal cord into a contemporary head and neck HBM, and quantify the effect of the spinal cord on brain deformation during simulated impacts. Spinal cord tissue geometries were developed, based on 3D medical imaging and literature data, meshed, and implemented into the GHBMC 50th percentile male model. The model was evaluated in frontal, lateral, rear, and oblique impact conditions, and the resulting maximum principal strains in the brain tissue were compared, with and without the spinal cord. A new cumulative strain curve metric was proposed to quantify brain strain distribution. Presence of the spinal cord increased brain tissue strains in all simulated cases, owing to a more compliant boundary condition, highlighting the importance of the spinal cord to assess brain response during impact.

Microstructure and mechanical properties investigation of explosively welded titanium/copper/steel trimetallic plate

In this work, a systematic investigation of microstructure and mechanical properties of explosively welded Ti/Cu/Fe trimetallic plate is presented. Smoothed particles hydrodynamics (SPH) methods is applied to simulate the high-velocity oblique impact welding process. The simulation results reproduce the formation of the wave boundary, vortex zones, and jetting, supporting and extending the presented experimental results. Combined with the SEM, EBSD, TEM and XRD examinations, Cu4Ti intermetallics are identified in the vortices and solid-solid bonded regions at Ti/Cu interface, while ultra-fine Cu grains are formed along the entire Cu/Fe interface. During explosive welding, heat transfer between the vortex zones and severely deformed layers of the parent plates induces the formation of above microstructures. Microhardness contours depict well the characterized hardness distribution across the entire Ti/Cu/Fe joints. The nanoindentation tests reveals the individual properties of the typical phases formed, i.e., Cu4Ti ∼ 11.8 GPa, ultra-fine Cu grians∼3.9 GPa. The Ti/Cu/Fe trimetallic plates show the average tensile strength of 366 MPa, with brittle fracture observed at Ti/Cu interface region.

Delamination in GLARE laminates subjected to oblique low velocity impact considering friction

Study of oblique low velocity impact and the subsurface delamination caused in fibre metal laminates is not well reported in literatures. The present paper numerically investigates the oblique low velocity impact behaviour of clamped GLARE plates by a spherical impactor to study the influence of obliquity and coefficient of friction on the impact mechanism and on the extent of interfacial delamination. A three dimensional finite element formulation employing Newmark-β method has been developed for analyzing the oblique contact impact response of GLARE plate where Hertzian contact model is used for the normal component of contact and Mindlin and Deresiewicz theory is used for the tangential component. Results show that besides impactor velocity, obliquity of incidence and the coefficient of friction significantly influence the contact force history, contact duration and the delamination at the interfaces. In addition, the oblique angle also decides the instant and duration of reversal of tangential contact force where increasing obliquity leads to delayed instant and reduced duration of such reversal.

Analyzing ballistic impact of bullet over laminated test specimen

Impact analysis of an object over a test specimen is a major design related aspect related to toughness of the material. Based on review, different parameters affecting the ballistic impact were analyzed. Parameters having a major effect on impact were found to be apex angle, thickness, and composite material parameters. A projectile impact of bullet on laminated test specimen was carried experimentally and simulated with explicit dynamics approach using Ansys®. The bullet shears certain amount from laminated test specimen on impact and passes out. The ballistic limit of laminated test specimens of varied thickness was analyzed for projectiles with 7.6 AP against oblique and normal impact. Projectile's experienced resistance is seen to be greater with greater conical portion's surface area in contact with target.

Tension-bending risk curves for the ATD lower lumbar spine subjected to oblique impact under FAA emergency landing conditions

With the increasing use of obliquely oriented airline seating configurations, the objective of the present study was to develop injury risk curves for the lower lumbar spine load cell of the FAA-H3 dummy. A new spinal criterion, termed FAA-LLtb, which is a linear combination of tensile load with forward flexion, and lateral bending moments, was developed to predict the injuries occurring to the lower lumbar spine and sacrum regions. The injury definition required for the metric was obtained from the matched PMHS tests. The loading conditions included variations in peak sled accelerations, the presence or absence of an armrest, the belt type (single and dual lap-belt systems), and seat orientation relative to impact vector (45° and 30°). The developed ATD risk curve based on the combined metric represents AIS = 3+ injury probability for the lower lumbar spinal levels. The survival analysis estimated normalized confidence interval size (NCIS) values were in fair and good categories at all levels of probability. At 5%, 25% and 50% risk levels, the combined loading metric values were 1.6, 1.8, and 2.0, respectively. The study estimated that the combination of bending moments and tensile load was a better injury criterion than any individual metric for assessing the injury to the lower lumbar spine and pelvis regions under oblique loading.

Oblique drop impact: can one infer the angle of impact?

During solid surface impact, a falling drop's energy is transformed into oscillations of its liquid/gas interface. We consider drop deposition during oblique impact in the capillary-ballistic regime characterized by high Reynolds number and moderate Weber number. We treat this as an inverse problem showing that post-impact observations of the frequency spectrum and modal partition of energy allow one to determine a drop's pre-impact characteristics and wetting properties. Our analysis is useful for quantifying contact-line dissipation during inertial spreading and can be used as a diagnostic technique for determining substrate wetting properties.

Crashworthiness of lantern-like lattice structures with a bidirectional gradient distribution

Oblique impact loading conditions strongly influence the crashworthiness of energy-absorbing structures. In the present work, inspired by the distribution characteristics of spider webs, a bidirectional gradient configuration of cell wall thickness was introduced to enhance the crashworthiness of lattice structures based on the functional similarity of longitudinal gradient distribution and lateral gradient distribution. A comparative study of different gradient configurations was carried out using the finite element (FE) simulations. It was found that the longitudinal gradient distribution was beneficial to improve the comprehensive energy absorption capacity of the lattice structure, especially under small-angle impact conditions. With the increase of impact angle, the effect of the lateral gradient distribution on the stability of energy absorption became more prominent. In comparison to the uniform structure and the single gradient structure, the lattice structure with the positive longitudinal gradient and the negative lateral gradient distribution exhibited great potential in enhancing the energy absorption capacity and reducing the peak crushing force. A detailed parametric study was executed to explore the effects of the gradient layer number and the gradient thickness increment of the bidirectional gradient distribution on the comprehensive energy absorption capacity and energy absorption stability of the structure under multi-angle impact conditions. The findings of this research provide valuable guidelines to design bio-inspired lattice structures subjected to complex loading conditions.

Oblique impact responses of Hybrid III and a new headform with more biofidelic coefficient of friction and moments of inertia.

New oblique impact methods for evaluating head injury mitigation effects of helmets are emerging, which mandate measuring both translational and rotational kinematics of the headform. These methods need headforms with biofidelic mass, moments of inertia (MoIs), and coefficient of friction (CoF). To fulfill this need, working group 11 of the European standardization head protection committee (CEN/TC158) has been working on the development of a new headform with realistic MoIs and CoF, based on recent biomechanics research on the human head. In this study, we used a version of this headform (Cellbond) to test a motorcycle helmet under the oblique impact at 8 m/s at five different locations. We also used the Hybrid III headform, which is commonly used in the helmet oblique impact. We tested whether there is a difference between the predictions of the headforms in terms of injury metrics based on head kinematics, including peak translational and rotational acceleration, peak rotational velocity, and BrIC (brain injury criterion). We also used the Imperial College finite element model of the human head to predict the strain and strain rate across the brain and tested whether there is a difference between the headforms in terms of the predicted strain and strain rate. We found that the Cellbond headform produced similar or higher peak translational accelerations depending on the impact location (−3.2% in the front-side impact to 24.3% in the rear impact). The Cellbond headform, however, produced significantly lower peak rotational acceleration (−41.8% in a rear impact to −62.7% in a side impact), peak rotational velocity (−29.5% in a side impact to −47.6% in a rear impact), and BrIC (−29% in a rear-side impact to −45.3% in a rear impact). The 90th percentile values of the maximum brain strain and strain rate were also significantly lower using this headform. Our results suggest that MoIs and CoF have significant effects on headform rotational kinematics, and consequently brain deformation, during the helmeted oblique impact. Future helmet standards and rating methods should use headforms with realistic MoIs and CoF (e.g., the Cellbond headform) to ensure more accurate representation of the head in laboratory impact tests.

Dynamic behaviors of double-column RC bridge subjected to barge impact

This paper aims to study the dynamic responses and damage patterns of the prototype inland double-column RC bridge subjected to barge impact. Firstly, by utilizing a horizontal collision facility, a 1/5 scaled Jumbo Hopper (JH) barge bow repeated impact test with various impact velocities is carried out on three double-column RC bridge pier (DCBP) specimens. The impact force-time histories, crushing process of barge bow, and dynamic behaviors of both the impacted and adjacent piers are obtained and assessed. Then, by establishing the refined finite element (FE) models of the barge bow-DCBP collision, the present test is numerically simulated by adopting the FE program LS-DYNA. The corresponding FE analyses approach as well as the material models and parameters, are validated by comparing with the experimental impact forces, crush depths of barge bow, lateral displacement-time histories, and damage patterns of the DCBP specimens. Finally, the prototype barge-bridge collision is further studied with considering the material nonlinearity of the bridge pier and superstructure, soil-pile interactions, as well as the strain-rate effect. The impact process and damage pattern of bridge during collisions are discussed, and the influences of impact velocity, barge mass, and oblique angle on the dynamic behaviors of the prototype bridge are examined. It derives that, the impact velocity is more dominant on the peak impact force than the barge mass; the oblique barge impact amplifies the overall collapse risk of the bridge superstructure; the oblique angle has a remarkable aggravating effect on the induced damage level of bridge under identical impact kinetic energy.

Armature dynamics - an aspect of rail gouging mechanisms in electromagnetic railgun

The purpose of this study is to explore the causes leading to the rail gouging from the perspective of armature dynamics. The axial, lateral motion and rotation equations were derived, and a set of calculations according to the experimental conditions were conducted. The calculated in-bore processes of the armature exhibit apparent non-linear features. Especially with the contact interface of armature being worn rapidly, the armature gradually leans toward one side of the bore, and forms a trend of oblique impact to the rail that may lead to the gouging. This study offered an insight on the transient processes of armature in bore, and helped to better understand the gouging failure mechanism.