External Impact Loads Research Articles

Development of Laminated Egg-Shaped Tsunami Shelter Structure Made of Steel-Cushioning-Steel

When a tsunami is caused by an earthquake or other event, spherical shelters are developed to protect people from the tsunami. This study proposes a new egg-shaped laminated tsunami shelter with a buffer layer to improve the functionality of traditional spherical shelters. The inner and outer shells of this shelter are made from thin-walled stainless steel, using the integral hydro bulge forming (IHBF) process. The space between these two layers was filled with urethane foam, providing an elastic buffer. This resulted in a laminated egg-shaped structure designed for tsunami protection. To verify the proposed laminated egg-shaped tsunami shelter and its processing method, an egg-shaped shell with an external shape (length 660 mm, width 493 mm) was fabricated using a 1.0 mm thick stainless plate, and a laminated egg-shaped tsunami shelter with a 25 mm thick intermediate layer made of urethane foam was fabricated. The shape accuracy of the processed egg-shaped laminated tsunami shelter structure was measured, and the maximum error between the surface shape of the molded egg-shaped shell and the true egg shape was -4.13 mm, and the relative error to the maximum radius of the egg shape of 246.5 mm was -1.68%. In addition, to assess the buffering effect under external impact loads, acceleration sensors were attached to both the inner and outer layers of the fabricated egg-shaped laminated tsunami shelters. A hammer was used to apply an impact load to the outer layer, and the response acceleration values recorded by the sensors on both layers were compared. It was found that the response acceleration of the inner layer was 15.81% lower than that of the outer layer.

Microseismic comprehensive evaluation method for coal burst: a case study in the Zhaolou Coal Mine

To explore the multiparameter precursor characteristics of pre- and post-coal burst. Based on a coal burst of LW 1305 in the Zhaolou Coal Mine, an early warning method combining stress‒strain curve and microseismic multiparameter is proposed. The research results show that coal burst was induced by the intrinsic static high-stress concentration and the strong external impact loading generated by fracturing of the key stratum. The precursors mainly characterize the enhancement trend of the S value, the sudden and sharp increase in the A(t) value, the continuous and abnormal decrease in the b value, the increasing absolute value of Z sharply and larger than 2, the continuous and abnormal decrease in the Qt value, and the dominant frequency moving to the low-frequency band. Essentially, many micro-fissures inside the key stratum initiated, converged and connected to form macro-fractures, which was verified by the attenuation rate of the K value. Considering the time-varying effect of the overlying stratum movement, the curves of the six parameters agree well with those of stress vs. strain, which indicates that it is reasonable to take the observed zone as a whole system to investigate the variation in the multiple parameters and fracturing of the key stratum. The research results can be applied to the monitoring, early warning and control of coal burst so that effective safety measures can be taken in real time.

Dynamic mechanical response and failure characteristics of coal and rock under saltwater immersion conditions

The stability of coal and rock masses in water-rich mines is affected by both mine water erosion and dynamic disturbances. Thus, it is necessary to study the dynamic mechanical response and failure characteristics of coal and rock under the combination of saltwater and a high strain rate. To this end, a split Hopkinson pressure bar device was employed to investigate the effects of impact velocity, water content, and immersion liquid on the dynamic mechanical behaviours of coal and rock. The results revealed that the weakening effect of saltwater on the dynamic mechanical properties of coal and rock is much greater than that of distilled water. With increasing moisture content, the dynamic compressive strength of the coal specimens decreases monotonically, while that of the rock shows a trend of first increasing and then decreasing. The failure process and destruction of coal and rock are comprehensively affected by both the external impact load and the physical and mechanical properties of the material. The degree of damage of the coal and rock specimens increases with increasing impact velocity and water content. Moreover, the influence of various factors on the impact fracture mechanism of coal and rock under saltwater immersion conditions was revealed. These findings are highly important for the design and maintenance of underground coal and rock building structures.

On the diversity of fracture behavior in a brittle solid with sets of preexisting small-scale cracks

Understanding the multiscale fracture behavior of brittle materials is of crucial importance not only in engineering applications but also in seismology where an earthquake source, usually assumed as one single, relatively large fracture region but in reality composed of relatively smaller multiple fractures, behaves mechanically in diverse ways, emit waves with different characters and may cause a single seismic event or even a cluster of earthquakes and earthquake swarms. For understanding the complex fracture processes, by employing the experimental technique of dynamic photoelasticity with high-speed video cameras, we have been simultaneously observing global, large-scale material behavior and local, smaller-scale evolution of waves and fractures in two-dimensional linear elastic brittle polycarbonate specimens. Each specimen has sets of preexisting small-scale parallel cracks prepared by a digital laser cutter and modeling large-scale geological fault planes, and it is subjected to external (quasi-)static and impact loads. Here we show some recent examples of the diverse fracture behavior observed in brittle birefringent solid specimens under tensile/compressive external loading. The fracture behavior is considerably dependent on the loading conditions, and developing fractures do not always break the specimen in an “unzipping” way, i.e. the specimen is not always divided along a perforation line consisting of small-scale cracks. Rather, the diverse fractures can easily jump to remote places, propagate back-and-forth or reversely move in the opposite direction compared with the initial one. Our findings may play a role in comprehending the generation mechanism of a cluster of fractures in brittle solids in general.

Experimental investigation on the deformation and damage of steel ribbon wound vessel for hydrogen storage under external impact and blast loading

Steel ribbon wound vessel is a preferred type for stationary storage of high-pressure hydrogen in refueling stations. However, it is under the threat of fragment impact and blast loads caused by accidental explosions of hydrogen or other vessels. For the first time, the fragment-simulating projectile (FSP) impact tests with impact kinetic energy range from 1.8 × 103 J–4.9 × 103 J and external blast loading tests with TNT equivalent range from 157 g–4 kg of industrial-grade 50 MPa steel ribbon wound vessel were carried out. The damage of vessel under FSP impact loading was studied. The effect of concentrated and distributed blast loads on the deformation and damage of gas-filled vessel was analyzed, and the explosion resistance of vessel under near-field blast loading was explored. It is revealed that: 1) the steel ribbon wound vessels generally have good resistances to external blast and impact loads, 2) the primary damage characteristics of vessel under contact/near-field blast loading was localized depression deformation, accompanied by steel ribbons warping as the shock wave impulse increases, 3) the saddle fails earlier than the vessel under the near-field blast loading. Further, the failure modes of vessel under extreme blast loads were revealed. Results can provide reference for the safe design and validation of dynamic damage/failure analysis models of steel ribbon wound vessels.

Development of Fiber Sensors integrated with Aerospace Composites for Structure Health Monitoring

The wide-spread use of composite material in aircraft across the world is expected to create a big need to improve structure health monitoring (SHM). Optical fiber sensors offer more advantages than conventional sensors, such as light weight, small size, immune to electromagnetic interference, easily embedded in composites. Extensive applications have been reported with a few embedded technology details by Boeing and Airbus, etc. In this paper, The methods of improved optical fiber surface-bonded composites by glass cloth and epoxy resin, as well as optical fiber embedded in composites were researched. The installation of fiber and composites, composites assembly pressure adaptability on FBG sensors, influences on structure properties of composite materials due to different fiber diameters and different numbers of optical fibers by the way of microscopic interface analysis and C-san were studied. The adequate fiber embedded technology was found to obtain the most feasibility of SHM. Light intensity and wavelength of FBG sensor surface-bonded by glass cloth are affected by the structural assembly pressure. It can be seen that diameter of embedded optical fiber equal to the thickness of prepreg monolaye, the fiber embedded next to 0 degree layer, less than five fibers within a 25mm width composite unit have little impact on composite structure. Then, 12 FBG sensors were embedded in a typical aircraft structure, a carbon fiber enhanced stiffened composite plate with length 600 mm and width 600 mm, to detect position and energy of external impact load by a mathematical analysis method, cross correlation algorithm. Finally, suggestions on future study are listed from four aspects, co-forming technology of FBG sensor and composite skin, research on the extraction technology of optical fiber, new flexible wing testing technology and advanced fiber optical sensing technology.

Experimental study on impact toughness of multi-combination hybrid fiber-reinforced magnesium phosphate cement-based materials

In order to improve the brittleness of magnesium phosphate cement-based materials (MPC) and overcome the limitations of single fiber-reinforced cement-based materials, this study investigated the mechanical properties of hybrid fiber-reinforced magnesium phosphate cement-based composite materials (HFRMPC) by incorporating four types of fibers: hooked-end steel fiber (HSF), micro steel fiber (MSF), polyvinyl alcohol fiber (PVAF), and basalt fiber (BF). Cylindrical specimens were prepared by single and double hybridization of the fibers and subjected to drop hammer impact tests to analyze the effects of different fiber hybridization methods on the impact resistance of these materials. The two-parameter Weibull distribution was used to analyze the test results to predict failure probabilities and to investigate the fiber reinforcement and toughening mechanisms in MPC through microscopic structural analysis. The results showed that the impact resistance of the fiber hybridization group was much better than that of the single fiber group, and the impact resistance increased continuously with increasing fiber content. The fibers' impact resistance enhancement effects were in the following order: MSF > PVAF > BF. The fiber combination methods were ranked as follows from best to worst: HSF + PVAF > HSF + BF > HSF + MSF > PVAF + BF. The 1.5% HSF+0.6% PVAF hybridization had the best impact resistance performance, with an increase in initial cracking and impact energy dissipation at the failure of 7748.6% and 8921.2%, respectively, compared to the M0. The probability distribution of impact number of initial cracking (N1) and failure (N2) of the specimens followed the double-parameter Weibull distribution well. The multiscale strengthening and toughening zone composed of fibers with different geometric dimensions and elastic modulus was crucial to the material's performance, as the fibers acted in progressive briding under external impact loads, dissipating impact energy through the failure morphology of pull-out and breaking, significantly improving the HFRMPC's impact toughness. A comprehensive evaluation of the four indicators of impact resistance and manufacturing costs for various hybrid fiber compositions found that the combination of 1.5% HSF and 0.6% PVAF exhibits the most outstanding overall performance.

The Effect of Sole Design on Foot Stress Distribution to Runner

There is an increased stress on the metatarsal when running due to repeated loadings that cause ankle injury. The solid foam structure of the sole may not provide optimum strength and good absorption shock, as demonstrated by previous studies. So, this study aimed to design the shoe sole models of various patterns or topologies and compare the effects of shoe sole design on the foot stress distribution. This study was conducted using three different softwares which were 3-Matic, Solidworks and ANSYS. Three different topologies of sole including circular, elliptical and hexagonal patterns were designed using Solidworks software. A 23 years old female foot with 45 kg weight and 25 cm foot length was scanned using three-dimensional (3D) scanner and modified using 3 Matic software. Foot-sole simulation was carried out in finite element analysis (FEA) platform called ANSYS, considering the nonlinearity and viscoelastic properties of the sole material to reflect the stress distribution on the foot plantar that in contact with three different midsoles of various topologies. The result showed the hexagonal sole pattern has the lowest stress with a maximum of 0.1 MPa. It has the potential to enhance the area of contact between the foot and the sole. The stresses on the foot were more uniformly distributed. The highest stress was found on the elliptical design with 0.19 MPa because the struts will buckle as the compression load changes dramatically thus, it cannot avoid concentrating the stress on the foot. Meanwhile, the circular pattern has a maximum of 0.12 MPa. The increased stress caused by repeated external impact loads when running will cause ankle injury. Therefore, the hexagonal sole design is the most comfortable that will help to reduce ankle injuries. Lastly, more subjects should be involved in the future for the FEA to achieve a solid conclusion.

Electromechanical Coupling Dynamic Characteristics of the Dual-Motor Electric Drive System of Hybrid Electric Vehicles

The electric mode is the main operational mode of dual-motor hybrid electric vehicles (HEVs), so the reliability of the dual-motor electric drive system (DEDS) is particularly important. To research the electromechanical coupling mechanism of the DEDS of HEVs, firstly, considering the time-varying mesh stiffness of gears and the nonlinear characteristics of inverters, an electromechanical coupling dynamics model of the DEDS was established, including the permanent magnet synchronous motor (PMSM) and the gear transmission system. Then, the electromechanical coupled dynamic characteristics of the DEDS in the single-motor and dual-motor drive modes were analyzed under steady-state and impact load conditions, respectively. The results show that the motor stator current frequency is modulated by the complicated gear meshing frequency, and the operation state of the gear transmission system can thus be monitored in the stator current. Impact load causes the instantaneous torsional vibration of the transmission system dominated by the first-order natural frequency, and the vibration characteristic frequency appears in the form of a side frequency in the stator current signal; moreover, compared with the single-motor drive mode, the speed synchronization error in the dual-motor drive mode will aggravate torsional vibration in the gear system. The impact energy of the gear system caused by external impact load can be suppressed by reducing the speed synchronization error.

Design for Manufacturing of Cemented Carbide Coated Components Toward High Wear and Impact Resistance Performance

Abstract Wear- yet impact-resistant demand is a big challenge for coated components under heavy-load service condition. To solve this high-performance manufacturing problem, a new strategy of design for manufacturing (DFM) with integrated design and processing is developed to incorporate processing effect on final performance via the pivot role of surface integrity. An impact performance model and the impact tester are constructed for a component with coated flat block/bulk cylinder mates for potential application in hydraulic machinery. A WC-12Ni/Ni60A two-layer coating on 17-4PH martensitic steel substrate is designed with thermal spray process. Impact crater depth, surface hardening, and residual stresses are identified as major surface integrity parameters determining wear/impact performance by the modeling with testing. The design parameters of geometry, material, and structure are quantitatively linked to the final performance by a process signature (PS) correlative analysis on the identified surface integrity to internal material loading of plastic/elastic strain energies. The PS correlation posts coating thickness as a high-sensitivity parameter for design, facilitating a buffering effect of reduced peak stresses among the coating-substrate system. The DFM optimization is understood by irreversible thermodynamics as reducing energy dissipation of the internal material loading from the external impact loads. The manufacturing inverse problem is thus solved by material-oriented regularization (MOR) on the homologous PS correlations integrating the design and processing phases. The manufactured component, with optimal Ni60A interlayer thickness of 75–100 µm at a top WC-12Ni coating of 200 µm, achieves a desired performance of up to 6000 impacts under a nominal load of 15 kN.

Design and analysis of passive variable stiffness device based on shear stiffening gel

Shear thickening materials are utilized as the core of variable stiffness devices in various engineering applications in recent years. However, due to the used materials being liquid, most of these devices are suffered from the problem of liquid leakage and coagulation. To address this issue, this paper proposes a design method of a passive variable stiffness device based on shear stiffening gel (SSG), which integrates the core SSG and spring into a compact structure to obtain well stability. The used SSG can affect the force transfer within the device, thereby influencing the system’s stiffness dynamically over the external impact load. Due to the core SSG being good shear stiffening properties, the device is highly sensitive to frequency and shows variable stiffness characteristics. When the frequency of external load increased from 0.1 Hz to 5 Hz, the equivalent stiffness of the device could be increased by 105.74%. An equivalent nonlinear model is used to represent the overall stiffness of the device as a function of the frequency and displacement amplitude of the load. The output of the equivalent nonlinear model agrees well with the experimental data. The cross-validation also proves the accuracy of the model fitting, which provides a quantitative description of the dynamic performance of the variable stiffness device. By combining different types of SSGs and springs with different stiffness, the design principle of the variable stiffness device proposed in this paper can be utilized in various impact-loading applications, such as designing the exoskeletal load carriage supporting mechanism and vehicle suspension systems. The work of this paper has guiding significance for the design of adaptive variable stiffness devices.

Brittle fracture development through sets of preexisting small-scale cracks under quasi-static and dynamic impact loading

Fracture development and its possible relation with wave motion in a brittle rectangular solid material with sets of preexisting small-scale parallel cracks are investigated. Experimental observations using high-speed cameras under external quasi-static tensile or dynamic impact loading conditions indicate that depending on the inclination angle, length and distribution pattern of cracks, fractures evolve considerably differently. Quasi-static load generally produces dynamic or quasi-static primary fracture with ensuing dynamic secondary retrograde fractures that are initiated at distant positions. Fractures can be arrested and jump easily, often resulting in clusters. Impact-related waves can induce primary fracture moving in both prograde and retrograde directions.

Measurement of natural frequencies and mode shapes of transparent insect wings using common-path ESPI.

In this study, a common-path electronic speckle pattern interferometry system which upholds the natural property of transparency of insect's wings has been developed to measure the wings' natural frequencies and mode shapes for the first time. A novel base-exciting method was designed to enable the simultaneous application of sinusoidal and static forces to excite wings and introduce an additional phase. The moiré effect induced by the amplitude modulation was employed to accurately recognize the resonance state. Subsequently, the mode shapes were visualized by phase-shifting and real-time frame subtraction. Eight pairs of forewings from cicadas were investigated. The first three order natural frequencies of the wings are approximately 145 Hz, 272 Hz and 394 Hz, respectively, which are dispersed to prevent modal coupling. The cambered mode shapes exhibit a strongly spanwise-chordwise anisotropy flexural stiffness distribution, generally dominated by bending and twisting deformation. The details of the high-order mode shapes show that the tip exhibits distinct deformation, indicating more flexibility to cope with external impact load, and the nodal lines usually comply with the direction of the wing veins in higher modes, substantiating the fact that the veins play an important role as stiffeners of the membrane. The results are in excellent agreement with the dynamic performance of previous studies, which will potentially affect a broader community of optical measurement specialists and entomologists to enhance our understanding of time-averaged interferograms and insect flights.

The Effect of Organic and Inorganic Modifiers on the Physical Properties of Granite Residual Soil

As a kind of highly weathered special soil in South China, granite residual soils (GRS) feature high strength and high void ratio in a dry environment, so they tend to disintegrate in water and cause geological disasters including collapse. Therefore, modifying GRS for higher strength has become a hot spot. Glass fiber reinforced soils boast fewer cracks, higher energy absorption, and residual strength. This study aims to analyze the reinforcement effect of glass fibers on GRS with inorganic and organic solutions and its environmental feasibility. The inorganic solution contains silicon ion and sodium ion at the ratio of 1 : 4 (hereinafter referred to as Si : Na = 1 : 4 solutions), and the organic one is a modified polyvinyl alcohol solution (hereinafter referred to as SH solution). The reinforced samples were subjected to plate and impact load tests, SEM, and XRD analysis to investigate their mechanical properties, microcharacteristics, and the components produced. Results indicate that the reinforcement effect of glass fibers on GRS under Si : Na = 1 : 4 solutions was better than that of SH solutions. After being reinforced by Si : Na = 1 : 4 solutions, the samples reached maximum impact resistance. SEM results show that glass fibers bond more soil and form an integral structure; thereby the strength was improved as glass fibers share external impact load. XRD results show that geopolymer and alkali-activated materials and potassium feldspar were formed. Geopolymer and alkali-activated materials are pollution-free, inorganic polymers featuring viscosity and high compressive strength. Potassium feldspar is an aluminosilicate mineral with high strength and stable chemical properties, which can adhere to more granules and form a stronger whole structure with geopolymers playing a role. Therefore, it is feasible to reuse these soils sustainably by reinforcing them with glass fibers and the best Si : Na = 1 : 4 solutions. This study finds a new direction for recycling and reusing construction waste, GRS.

Research on the Constitutive Model of PTFE/Al/Si Reactive Material.

As a new type of energetic material, reactive materials are widely used at present; in particular, the metal/polymer mixtures type reactive materials show great advantages in engineering applications. This type of reactive material has good mechanical properties, and its overall performance is insensitive and high-energy under external impact loading. After a large number of previous studies, our team found that the energy release characteristics of PTFE/Al/Si reactive material are prominent. In order to master the mechanical properties of PTFE/Al/Si reactive materials, the quasi-static mechanical properties and dynamic mechanical properties were obtained by carrying out a quasi-static compression test and a dynamic SHPB test in this paper. Based on the experimental data, a Johnson-Cook constitutive model of PTFE/Al/Si reactive material considering strain hardening effect, strain rate hardening effect and thermal softening effect was constructed. The relevant research results will be used to guide future research on the reaction mechanism of PTFE/Al/Si reactive materials, in order to promote the engineering application of PTFE/Al/Si reactive materials.

Study on dynamic mechanical properties of carbon nanotubes reinforced concrete subjected to freeze–thaw cycles

AbstractAs a nanometer material, carbon nanotubes can inhibit the expansion of microcracks and is of great significance to improve the mechanical properties and durability of concrete. In order to explore the mechanism of dynamic mechanical properties enhancement of carbon nanotube reinforced concrete under impact load after freeze–thaw cycles, the dynamic compression test of carbon nanotube reinforced concrete samples was carried out by using the control variable method with freeze–thaw cycles and impact air pressure as variables. The results showed that under the same impact pressure, the peak stress of carbon nanotube reinforced concrete sample decreased gradually with the increase of freeze–thaw cycles. The corresponding strain increased slightly, and the stress–strain curve moved to the right and down as a whole. Under the same freeze–thaw cycle, the peak stress of carbon nanotube reinforced concrete sample increased with the increase of strain rate, and the dynamic strength growth factor DIF also increased, showing an obvious strain rate strengthening effect. In addition, under the same freeze–thaw cycles and external impact load, the dynamic strength of carbon nanotube concrete is 41.97 and 54.21 MPa, respectively, which is 22.27 and 25.81% higher than that of ordinary plain concrete. The results show that carbon nanotube reinforced concrete samples have higher dynamic mechanical properties and freeze–thaw cycle resistance than ordinary plain concrete samples.

Leg and Joint Stiffness Adaptations to Minimalist and Maximalist Running Shoes.

The running footwear literature reports a conceptual disconnect between shoe cushioning and external impact loading: footwear or surfaces with greater cushioning tend to result in greater impact force characteristics during running. Increased impact loading with maximalist footwear may reflect an altered lower-extremity gait strategy to adjust for running in compliant footwear. The authors hypothesized that ankle and knee joint stiffness would change to maintain the effective vertical stiffness, as cushioning changed with minimalist, traditional, and maximalist footwear. Eleven participants ran on an instrumental treadmill (3.5m·s-1) for a 5-minute familiarization in each footwear, plus an additional 110 seconds before data collection. Vertical, leg, ankle, and knee joint stiffness and vertical impact force characteristics were calculated. Mixed model with repeated measures tested differences between footwear conditions. Compared with traditional and maximalist, the minimalist shoes were associated with greater average instantaneous and average vertical loading rates (P < .050), greater vertical stiffness (P ≤ .010), and less change in leg length between initial contact and peak resultant ground reaction force (P < .050). No other differences in stiffness or impact variables were observed. The shoe cushioning paradox did not hold in this study due to a similar musculoskeletal strategy for running in traditional and maximalist footwear and running with a more rigid limb in minimalist footwear.

The Influence of Background Ultrasonic Field on the Strength of Adhesive Zones under Dynamic Impact Loads.

The influence of background ultrasonic field on the ultimate dynamic strength of adhesive joints is studied using fracture mechanics analysis. Winkler foundation-type models are applied to describe the cohesion zone, and the incubation time fracture criterion is used. The challenging task is to study whether relatively weak ultrasound is able to decrease the threshold values of the external impact load depending on a joint model, such as an “elastic membrane” or “beam” approximation, and various boundary conditions at the ends. The specific task was to investigate the case of short pulse loading through application of time-dependent fracture criterion instead of the conventional principle of critical stress. Three different load cases, namely, step constant force, dynamic pulse, and their combination with ultrasonic vibrations, were also studied. The analytical solution to the problem demonstrates that background vibrations at certain frequencies can significantly decrease threshold values of fracture impact load. Specific calculations indicate that even a weak background sonic field is enough to cause a significant reduction in the threshold amplitude of a dynamic short pulse load. Additionally, non-monotonic dependency of threshold amplitude on pulse duration for weak background field was observed, which demonstrates the existence of optimal regimes of impact energy input. Moreover, this phenomenon does not depend on the way in which the beam edges mount, whether they are clamped or hinged, and it could be applied for micro-electro-mechanical switch design processes as an additional tool to control operational regimes.

Impact analysis of a honeycomb-filled motorcycle helmet based on coupled head-helmet modelling

Human head is most vulnerable in road traffic accidents involving motorcyclists where the helmet is commonly used to protect human head and reduce head injuries. This study aims to develop a methodology for the analysis of a novel motorcycle helmet based on coupled head-helmet modelling and the kinematic and biomechanical head injure criteria. A representative full-face motorcycle helmet finite element (FE) model was established and validated by the standard drop tests. The validated helmet model was coupled with two types of human head models to obtain the acceleration histories transmitted to the head (e.g. peak acceleration (Amax) and Head Injury Criterion (HIC)) and the biomechanical responses of the head (e.g. intracranial pressure (ICP) and von Mises stress (σv)) under external impact load. Analyses were further conducted to investigate the use of honeycomb filler in helmets to improve their protective performance. An optimization study was implemented to find the feasibly optimised geometries of honeycomb filler and densities of liner foam for the helmet, which can significantly enhance helmet's protection performance and reduce head injury from impact scenarios. In comparison with the head responses (Amax = 215.1 g, HIC = 1985, ICP = 228.8 kPa and σv = 33.6 kPa) in conventional helmet design, the protective performance of the optimum helmet is considerably improved with a large reduction of the head responses (Amax = 137 g, HIC = 918, ICP = 146.9 kPa and σv = 18.1 kPa), representing their respective reductions of 36.3%, 53.8%, 35.8%, and 46.1%. The proposed method and procedure could be used in structural crashworthiness studies when human-structure interaction is important.

Modeling and Effect of Ultrasonic Waves on Bearing Shells in Industry by Non-Destructive Testing

Defect of roller bearings shell may have many defects for rotary machines due to alternative connection forces and external impact loads, such as surface cracks fault results from corrosion or wear. Investigating the methods of detecting surface cracking defects for roller bearings shell is very useful for maintaining these machines. The new faults detection method was based on sent signals by amplitude and frequency by a multi-probe (FDDP) device due to influence of different frequency spectrum in order to detect surface cracking fault in the outer wall of the roller bearings shell. The finite element model was obtained for the shell of roller bearings by simulating the surface cracking defect in its outer wall and then by new method to use various probes for measure in the frequency domain of high detection fault cracking echoes. Differences in amplitude and signal received from the flawless bearing shell with surface cracking defects have been investigated and it used to detect the position of the surface cracking defect in the outer bearing wall. Experimental and simulation results indicated that the differences between the previous method and the new method for measuring the surface cracking defect in the outer wall of the roller bearing shell by using the new method of more probes with different frequencies (FDDP) simultaneously and without the time delay can be used to detect the location of the crack fault and its size more quickly and reliably. High and low frequencies are sensitive to the effects of ultrasonic waves, which do not react naturally at low frequencies such as speed, and low speeds cause structural vibration, but at high frequencies, these defects will be clearly visible and less vibration. Ultrasound is an optional method for non-destructive testing by which defective echoes are detected by using normal probe by ultrasonic immersion. Experimental and simulation results indicated that the study of all previous cases for surface cracking defects of shell has less efficient than the discovery of the new current method in which the detection of defect size was clearly shown for 5 different states.