Impact Loading Conditions Research Articles

Low-velocity impact characteristics of closed cell AA2014-SiCp composite foam

Deformation behaviour of closed cell aluminium alloy (AA2014) −10 wt % silicon carbide particles (SiCp) composite foam is studied under static and impact loading conditions. Foam is fabricated through liquid metallurgy route. Static compression tests are performed at an equivalent strain rate of 0.01/s and drop-weight impact tests are performed at an impact velocity of about 0.80 m/s. Based on these tests, Young's modulus, plateau stress, and densification strain of the foam are determined. Finite element analysis is used to simulate the static and impact tests and numerical simulation results are compared with the experimental results for fracture behavior of the foam. Results show the brittle but high strength characteristic of the foam. Further, experimental and simulation results shows good conformity. Based on this study, it is found that a peak load of about 520 N is required to cause the foam fracture and complete perforation under the impact test.

Strength of the Ti–6Al–4V Titanium Alloy under Conditions of Impact and Short Pulse Loading

The strength and resistance of the Ti–6Al–4V titanium alloy to solid particle erosion have been studied in as-delivered state and after the equal-channel angular pressing. The ultrafine-grained material and the initial material have been tested for dynamic tension using an Instron drop tower testing machine and an aerodynamic setup for erosion testing with corundum particles as an abrasive material. Despite a substantial increase in the static strength properties, the ultrafine-grained material demonstrates the properties similar to those of the initial material under dynamic loads, which, in turn, has been analyzed based on a structural–temporal approach using the incubation time criterion.

Properties and destruction of anisotropic composite materials under static deformation and impact loading conditions

A new approach toward understanding failure mechanisms of anisotropic fiber-reinforced composite materials due to low velocity impact is discussed. The dependence of failure mechanisms and mechanical properties of such composites on loading velocity are examined by Impact Break method. It has been shown experimentally that especially large change in the CM properties occurs in the transition from static to impact loading conditions. CM destruction was observed at the first moment of a shock load application. The relaxation of the stresses in CM and the energy dissipation from breaking fibers are limited the short duration of impact value equal to 1-2 ms. Failure mechanism is based on the fibers stretching and stress-wave propagation through the CM under impact. The processes of multi-breaking and crushing of the filaments are imposed on the process of multi-stage stretching deformation. It led to decrease CM properties as compared with that under static. In a static situation, the deformation of CM is mostly stretching deformation. It gradually grows as the load increases. It has been found out that specific absorbed-in-fracture energy of CFRP and OFRP under impact loading conditions is significantly reduced by factors of 3.7 and 3.2, respectively, as compared with that under static ones. As a result, the choice of CM to create structures based only on the static properties of the material does not guarantee the impact resistance of structures upon low-velocity impact.

Dynamic stress intensity factors of an interfacial crack in orthotropic elastic strips under impact loading conditions

AbstractThe present article deals with the investigation of elasto‐dynamic response of a finite crack under normal and shear impact loading situated at the interface of two semi‐infinite orthotropic strips. Laplace and Fourier Integral transforms are employed to reduce the transient problem to the solution of a pair of dual integral equations in the Laplace transform plane which are solved using iterations in the low frequency domain. The analytical expressions of stress intensity factors of the crack at the interface problem are found. To determine time dependence of the parameters, these equations are inverted to yield the dynamic stress intensity factors for some fiber‐reinforced orthotropic composite materials using Bellman method and the method given by H. Dubner and J. Abate. The results are depicted through graphs for different particular cases.

Distribution of Brain Strain in the Cerebrum for Ice Hockey Goaltender Impacts.

Concussions are among the most common injuries sustained by goaltenders. Concussive injuries are characterized by impairment to neurological function which can affect many different brain regions. Understanding how different impact loading conditions (event type and impact site) affect the brain tissue response may help identify what kind of impacts create a high risk of injury to specific brain regions. The purpose of this study was to examine the influence of different impact conditions on the distribution of brain strain for ice hockey goaltender impacts. An instrumented headform was fitted with an ice hockey goaltender mask and impacted under a protocol which was developed using video analysis of real world ice hockey goaltender concussions for three different impact events (collision, puck, and fall). The resulting kinematic response served as input into the University College Dublin Brain Trauma Model, which calculated maximum principal strain in the cerebrum. Strain subsets were then determined and analyzed. Resulting peak strains (0.124 - 0.328) were found to be within the range for concussion reported in the literature. The results demonstrated that falls and collisions produced larger strain subsets in the cerebrum than puck impacts which is likely a reflection of longer impact duration for falls and collisions than puck impacts. For each impact event, impact site was also found to produce strain subsets of varying size and configuration. The results of this study suggest that the location and number of brain regions which can be damaged depend on the loading conditions of the impact.

Damage and failure modes of railway prestressed concrete sleepers with holes/web openings subject to impact loading conditions

Prestressed concrete sleepers are essential to the structural integrity of railway track structures, redistributing wheel loads from the rails to underlying ballast bed while securing rail gauges for safe train traffics. In practice, drilled holes or web openings are usually generated ad hoc in sleepers to enable signalling equipment and cables at a construction site. These holes and web openings could however affect the structural integrity of sleepers, especially when they are exposed to impact loading. In fact, statistically, 15–25% of dynamic loading conditions are of transience and high-intensity by the nature of wheel-rail interaction over irregularities. This study is thus the world first to rigorously investigate the impact behaviours of railway sleepers with hole and web openings, which is critical to railway safety and reliability. In this study, three-dimensional finite element modelling using ABAQUS Explicit is used to comprehensively design and analyse the behaviour of prestressed concrete sleepers with various types of holes and web openings upon impact loading. Two different modelling techniques including concrete damaged plasticity model and brittle cracking model are also exercised to aid in this study. The results obtained show that the brittle cracking model provides better damage results as it can illustrate crack propagation very well until reaching the failure mode under impact loading. The findings illustrate a pathway to use brittle cracking model instead of concrete damaged plasticity model for dynamic impact analysis. Moreover, the outcome of this study will provide a better and new insight into the influences of holes and web openings on sleepers’ failure modes under impact loading so that appropriate guidance can be proposed to rail and track engineers in order to generate holes and web openings ad hoc in prestressed concrete sleepers without compromising their structural performance.

Characterization of coarse and ultrafine-grained austenitic stainless steel subjected to dynamic impact load: XRD, SEM, TEM and EBSD analyses

A comparative study of the dynamic impact responses of coarse (37 µm) and ultrafine-grained (0.24 µm) AISI 321 austenitic stainless steel was conducted using a split Hopkinson pressure bar system. An increase in firing pressure of the system's projectile results in the corresponding increase in impact momentum, strain rate, total strain and maximum flow stress of the specimens. Although ultrafine-grained (UFG) specimens show higher compressive strength, coarse-grained (CG) specimens possess higher strain hardenability than the UFG specimens under the same impact loading conditions. UFG specimens exhibit lower critical strain and strain rate at which adiabatic shear bands (ASBs) are observed; hence, UFG specimens are more susceptible to ASB formation than CG specimens. Using the XRD, SEM, TEM and EBSD characterization techniques, the underlying deformation and strengthening mechanisms were studied. Slip and twinning are the active deformation mechanisms in CG specimens and they are highly suppressed in UFG specimens due to spatial restriction effect. At higher strain rates, CG specimens are only susceptible to the formation of multiple transformed shear band (MTSB), while UFG specimens are susceptible to both MTSB and TSB bifurcation. Although the mechanism behind the formation of MTSB is not completely clear, bifurcation of TSB was geometrically necessary and the presence of precipitate at the bifurcation point could play major role. EBSD analysis of inside, interface and outside the TSB revealed the development of equiaxed ultra-fine grain structure (∼0.17–0.21 µm in CG and ∼0.14–0.15 µm in UFG specimens) inside TSB by rotational dynamic recrystallization mechanism.

The Energy Absorption Capability of Composite Materials and Structures: Influence of Impact Loading

In this paper, the energy absorption capability of composite materials and performance-critical structures made from using these materials under conditions of impact loading is presented and discussed. An overview is provided of the key events associated with detonation and decomposition of an explosive that eventually culminates in the generation of a shock wave that exerts an impact type of loading on structures both in contact and in the immediate vicinity. The blast loads that culminate from an explosive often tend to generate very high strain rates in the range of 102/sec to 104/sec. The resultant high rate of loading does tend to exert an influence on dynamic mechanical properties of the targeted structure besides exerting an influence on damage mechanisms experienced by the structural element. The progressive use of composite materials for structures, such as sandwich panels, that essentially comprise of a mixture of composite face sheets and foam cores was shown by researchers, based on actual field test data, to offer few to many advantages over usual metal counterparts when it came to the purpose of offering acceptable blast resistance. Thus, it became both essential and desirable to assess the blast response of composite structures made using appropriate selection of composite materials. Through the years several independent research studies have been conducted to understand the influence of blast loading on the response kinetics of sandwich panels. Key highlights of the research done and resultant findings obtained from these studies is presented and briefly discussed The importance of both material selection and resultant structure for providing adequate protection against an impact type of loading caused by an explosive device, such as Improvised Explosive Device (IED), is highlighted through appropriate summary of research conducted and published in the open literature. The need and necessity for developing new and improved materials, composite in nature, that can be used for performance-critical structures that can also offer an enhanced level of safety to all personal involved in emphasized.

Dynamic failure risk of coal pillar formed by irregular shape longwall face: A case study

Irregular shape workface would result in the presence of coal pillar, which leads to high stress concentration and possibly induces coal bumps. In order to study the coal bump mechanism of pillars, static and dynamic stress overlapping (SDSO) method was proposed to explain the impacts of static stress concentration and tremors induced by mining activities. The stress and deformation in surrounding rock of mining face were analyzed based on the field case study at 1303 workface in Zhaolou Coal Mine in China. The results illustrate that the surrounding rock of a workface could be divided into four different zones, i.e., residual stress zone, stress decrease zone, stress increase zone and original stress zone. The stress increase zone is prone to failure under the SDSO impact loading conditions and will provide elastic energy for inducing coal bump. Based on the numerical modelling results, the evolution of static stress in coal pillar as the size of gob increasing was studied, and the impact of dynamic stress was investigated through analyzing the characteristics of tremor activities. The numerical results demonstrate the peak value of vertical stress in coal pillar rises from about 30 MPa with mining distance 10 m to 52.6 MPa with mining distance 120 m, and the location of peak stress transfers to the inner zone of coal pillars as the workface moves forward. For the daily tremor activities, tremors with high energy released indicate high dynamic stress disturbance on the surrounding rock, therefore, the impact of dynamic stressing is more serious during workface extension period because the tremor frequency and average energy after workface extension are higher than those before the workface extension.

Shear properties of polyurethane ductile adhesive at low temperatures under high strain rate conditions

The shear properties of the polyurethane adhesives at room temperature (RT) and low temperatures under different loading speeds were experimentally studied by thick adherend shear test (TAST) specimens. The shear strength of the polyurethane (PU) adhesive decreases with the temperature reducing at quasi-static conditions. The ultimate shear strain also shows a decreasing trend with the decrease of the temperature. With the loading speed increases, the shear strength of the adhesive increases significantly. When the loading speed increases to 1000 mm/s, compared with the data under quasi-static conditions, the strength of the adhesive increases by 177%. Under the impact conditions, the strength of the adhesive at low temperatures decreases a lot compared with that at RT. The mechanical behavior of the TAST specimens under impact loadings at different temperatures were reasonably predicted by the 3D finite elements method by ABAQUS. It is concluded that the shear strength of adhesive at RT under quasi-static cannot be used to design and analyze the adhesive joints at low temperatures under impact loading conditions.

A through-thickness damage regularisation scheme for shell elements subjected to severe bending and membrane deformations

This research work proposes and validates a damage regularisation model for shell elements used in large-scale simulations. The model evaluates the ratio of bending to membrane loading in the elements based on the through-thickness gradient of the through-thickness plastic strain. The Cockcroft–Latham failure criterion is adopted, whose parameters are modified according to the length-to-thickness ratio of the shell elements in order to reduce the mesh dependency. This regularisation scheme is validated against experimental component tests on a double-chamber profile in an AA6005-T6 recrystallised aluminium alloy under quasi-static and impact loading conditions. The results show that the model is able to accurately predict fracture initiation under all tested conditions.

High Strain Rate Behavior of Epoxy Graphene Oxide Nanocomposites

The present work investigates the novel impact loading response of two-dimensional graphene oxide (GO) reinforced epoxy nanocomposites at high strain rate. The testing was performed up to 1000[Formula: see text]s[Formula: see text] of high strain rate, where maximum damage occurs during the impact loading conditions. The Split Hopkinson Pressure Bar (SHPB) was used for the impact loading of the composite specimen. The nanofiller material GO was synthesized by chemical oxidation of graphite flakes used as the precurser. Synthesized GO was characterized using FTIR, UV-visible, XRD, Raman Spectroscopy and FE-SEM. Solution mixing method was used to fabricate the nanocomposite samples having uniform dispersion of GO as confirmed from the SEM images. Strain gauges mounted on the SHPB showed regular signal of transmitted wave during high strain rate testing on SHPB, confirming the regular dispersion of both the phases. Results of the transmission signal showed that the solution mixing method was effective in the synthesis of almost defect-free nanocomposite samples. The strength of the nanocomposite improved significantly using 0.5[Formula: see text]wt.% reinforcement of GO in the epoxy matrix at high strain rate loading. The epoxy GO nanocomposite showed a 41% improvement in maximum stress at 815[Formula: see text]s[Formula: see text] strain rate loading.

Influence of Stress State on the Mechanical Impact and Deformation Behaviors of Aluminum Alloys

Under impact loading conditions, the stress state derived from the contact between the projectile and the target, as well as from the subsequent mechanical waves, is a variable of great interest. The geometry of the projectile plays a dertermining role in the resulting stress state in the targeted structure. In this regard, different stress states lead to different failure modes. In this work, we analyze the influence of the stress state on the deformation and failure behaviors of three aluminum alloys that are commonly used in the aeronautical, naval, and automotive industries. To this purpose, tension-torsion tests are performed covering a wide range of stress triaxialities and Lode parameters. Secondly, the observations from these static tests are compared to failure mode of the same materials at high impact velocities tests with the aim of analysing the role of stress state and strain rate in the mechanical response of the aluminum plates. Experimental impacts are conducted with different projectile geometries to allow for the analysis of stress states influence. In addition, these experiments are simulated by using finite element models to evaluate the predictive capability of three failure criteria: critical plastic deformation, Johnson-Cook, and Bai-Wierzbicki.

Viscoelastic response of apple flesh in a wide range of mechanical loading rates

Abstract The cylindrical samples of ‘Beni Shogun’ apples cultivar in a range of deformation velocities from 0.0002 to 1 m s−1 were studied using stress relaxation tests. In the work, experimental courses of the force response were described via the Maxwell model, and the effects of deformation velocity on the Maxwell model parameters as well as the maximum and residual force were determined. The maximum force increased with the increase of the deformation velocity, which proved the response of apple flesh to be of viscoelastic nature. The residual force described the state of the material after the strain and was much higher under the quasi-static than impact loading conditions. The three relaxation times decreased with the increasing deformation velocity. For the shortest relaxation time (order of magnitude 0.1 s) there was a rapid decrease in the velocities under the quasi-static loading conditions and it remained on a steady and low level under the impact loading conditions. A definite limit was observed between the medium relaxation time (order of magnitude 1 s) for the lowest deformation velocity of 0.0002 m s−1 and the other relaxation times obtained at higher deformation velocities. The values of the longest relaxation time (order of magnitude 100 s) were much larger under the quasi-static than the impact loading conditions.

Crashworthiness Performance of Stiffened Foam-filled Tapered Structures under Axial and Oblique Dynamic Loads

Abstract This paper aims to explore crashworthiness performance of new designed foam-filled tapered structures under axial and oblique impact load conditions. The structures consisted of inner and outer tapered tubes connected together by four stiffening plates. They also included square, hexagonal, octagonal, decagonal and circular cross-sectional shapes with different ratios of a/b (ratio of the inner tube side length to the outer tube one). The ratio of a/b was assumed to be 0, 0.25, 0.5, 0.75 and 1. Crashworthiness criteria of specific energy absorption (SEA), peak collapse force (Fmax) and crash force efficiency (CFE) were obtained for these structures using the experimentally validated model in LS-DYNA. Based on the numerical results, semi-foam filled structures (i.e. those with the ratio of a/b=0.5) demonstrated greater crashworthiness performance than foam filled single-cell ones. Furthermore, according to the TOPSIS calculations, cross sectional shape of decagonal was found to be an outstanding potential profile for the automobile structural components.

Effect of fiber orientation on the compressive response of plain weave carbon fiber/epoxy composites submitted to high strain rates

Composite materials undergo intricate damage processes, which are accentuated when these materials are exposed to impact loading conditions. In this context, this work aims to study the behavior of fiber reinforced polymer composites submitted to high strain rate in compression. A composite laminate plate was obtained using a plain weave carbon fiber fabric and an epoxy resin as matrix. The Split Hopkinson Pressure Bar (SHPB) technique was used to apply compressive loads at three different strain rates and in six different directions (0°, 15°, 30°, 45°, 60° and 75°) relative to the warp. The compressive failure modes were studied using a high speed camera to record the SHPB test. With increasing strain rates, it was possible to identify a transition from longitudinal cracking failure to delamination buckling failures. Similarly, the off-axis loading conditions (mainly for 45°) resulted in extension-shear coupling effects, which promoted the delamination buckling failures.

Energy absorption characteristics of composite tubes with different fibers and matrix under axial quasi-static and impact crushing conditions

The collapse characteristics and energy absorption capability of composite tubes with different fibers and matrix were studied in present article under axial quasi-static and impact crushing conditions. The sensitivity to fibers, matrix and loading conditions were thoroughly discussed for the crushing modes and energy absorption capability. Experimental results showed specimens with different matrix and fibers exhibit three typical types of crushing modes. Specimens G803/3234 and G827/3234 had better energy absorption performance than the specimens with 5224 matrix. Impact loading condition led to lower energy absorption capability as compared to quasi-static loading condition. Moreover, impact loading condition also caused the crushing mode transition from splaying mode to fragmentation mode for G803/3234 and G827/3234.

Thermomechanical behavior of adhesively bonded joints under out-of-plane dynamic compression loading at high strain rate

In this study, a new experimental approach in which the deformation, the damage kinetic, and the temperature are measured simultaneously during a high strain rate on adhesively bonded composite joints. Especially, our goal is to quantify the amount heat dissipation during impact and to identify the mechanisms that induce this dissipation. Out of plane dynamic compression tests were conducted on assembled specimens over a range of strain rate from 372 s−1 to 1030 s−1 using the Split hopkinson Pressure Bars technique. The specimen surface temperatures were monitored using an infrared camera. The increase in the strain rate has a dramatic effect on the stress–strain behavior producing a significant heat dissipation in the material. The infrared monitoring provides the spatial distribution of temperature that increase near the adhesive/adherent interfaces of the specimen. The observed temperature increase profiles clearly show that the stress concentration appears in the adhesive area and provide valuable information regarding the damage mechanisms and their role in the heat dissipation during dynamic loading conditions. The dependence of these results on strain rate indicates that there exists a correlation between the thermo-mechanical behavior and the strain rate effect, which might be useful when developping damage models taking into account the energy balance for adhesively bonded joints under impact loading conditions.

Influence of reversing impact load on performance of a two-step unloading pilot-operated check valves

Reversing impact load conditions may cause severe vibration in large-flow pilot-operated check values (POCVs). Cavitation erosion may also occur in valve ports after long-term operation. To prevent cavitation erosion, improve the performance of POCVs, enhance working reliability, and performance stability thereof, firstly, the cavitation index and working principles of large-flow POCVs were introduced. Then, a theoretical model was established to analyse the flow field characteristics of a large-flow POCV. The flow field characteristics during the opening of the pilot poppet, and those under different load conditions, were investigated. Then an experiment was conducted to study the influence of controlling pressure on the impact characteristics, and the effect of impact load and controlling pressure on cavitation, through the use of a high-pressure impact bench. The results showed that the variation of controlling pressure had a minor effect on cavitation. With increasing controlling pressure, the vibration of the poppet decreased and the valve unloading capacities also decreased. The cavitation index showed a linear relationship with impact pressure, but was nonlinear with hydraulic flow under different impact parameters. It indicated that the variation of impact pressure exerted a significant influence on cavitation under reversing impact load conditions in POCVs, in particular, cavitation mainly occurred while the pilot poppet of the POCVs was fully open. Finally, pilot poppets with three different cone angles were designed, and experiments were carried to verify that the pilot poppet with a 45° cone angle was optimal.

Structural response and performance of hexagonal thin-walled grooved tubes under dynamic impact loading conditions

This paper numerically investigates the structural response and crashworthiness performance of a hexagonal thin-walled grooved tube subjected to axial and oblique impact loading conditions. First, an analytical formulation that estimates the mean crushing force and total energy absorption are obtained for both hexagonal and circular tubes subjected to high dynamic impact loading conditions. The solutions of the analytical model are used to verify and compare the approximate solutions of the finite element model. The inclusion of grooves to the conventional hexagonal thin-walled tubes shows the great potential for improving the crashworthiness performance such as the energy absorption capacity, crushing force efficiency and specific energy absorption. In further, parametric studies are performed, deformation modes of different models are obtained and their crushing force-displacement characteristics are described. The improved crashworthiness performance of the hexagonal thin-walled grooved tubes make them good candidate used as energy absorbers.