Short Duration Impact Research Articles

Characterizing Natural Frequencies of the Hybrid III and NOCSAE Headforms

The vibrational characteristics of the Hybrid III and NOCSAE headforms are not well understood. It is hypothesized that they may perform differently in certain loading environments due to their structural differences; their frequency responses may differ depending on the impact characteristics. Short-duration impacts excite a wider range of headform frequencies than longer-duration (padded) impacts. While headforms generally perform similarly during padded head impacts where resonant frequencies are avoided, excitation of resonant frequencies during short-duration impacts can result in differences in kinematic measurements between headforms for the matched impacts. This study aimed to identify the natural frequencies of each headform through experimental modal analysis techniques. An impulse hammer was used to excite various locations on both the Hybrid III and NOCSAE headforms. The resulting frequency response functions were analyzed to determine the first natural frequencies. The average first natural frequency of the NOCSAE headform was 812 Hz. The Hybrid III headform did not exhibit any natural frequencies below 1000 Hz. Comparisons of our results with previous studies of the human head suggest that the NOCSAE headform’s vibrational response aligns more closely with that of the human head, as it exhibits lower natural frequencies. This insight is particularly relevant for assessing head injury risk in short-duration impact scenarios, where resonant frequencies can influence the injury outcome.

A Biomechanical and Evolutionary Perspective of the Menisci Indicate They Are Not Shock Absorbers

Human bipedalism entails high‐magnitude, short‐duration impact transient forces that occur every heel strike. These transient forces, or ‘shock’, are attenuated as they propagate up the body, and many structures have been proposed as ‘shock absorbers’ within the body. The medial and lateral menisci of the knee are a pair of fibrocartilaginous structures that have often been cited as shock absorbers. However, despite the frequency with which this claim is made in scientific literature, medical education and clinical practice, there is little primary data that supports the notion that the menisci function as shock absorbers.Here we compile evolutionary and biomechanical data to critique this oft‐cited notion. First, we show that the origin of this idea draws on a limited number of studies, each with caveats that make that evidence for a shock absorptive function dubious. We then review evolutionary and comparative evidence to show that human menisci are unremarkable in morphology compared with most other tetrapods. Humans possess two, crescentic‐shaped menisci that are similar in morphology to most mammals and many tetrapods. This suggests that the null hypothesis of meniscal function is one of similarity among tetrapods. Furthermore, we show that among tetrapods, shock during locomotion is uncommon; humans stand out as one of the only tetrapods that regularly experience high levels of shock during locomotion. Thus, a shock‐absorption function does not explain the origin of menisci, nor are human menisci specialized in any way that would explain this unique function during human gait. We further assess the mechanical material properties of menisci and show that the menisci overwhelmingly behave like springs and are poorly suited for energy absorption. We compiled data on phase angle and loss tangent, two properties used to assess energy absorption or release. Loss tangents of the menisci are typically between 0.1–0.2, close to an ideal spring (0.0) as opposed to an energy absorbing damper (>1.0). Similarly, phase angles were generally <10°, more similar to an idealized spring value of 0°, as opposed to a pure damper at 90°. Thus, when deformed menisci release most of this energy as opposed to shock absorbers, which dissipate energy.Finally, we integrated data on material properties of the meniscus and loading data from previous studies to estimate actual meniscal energy absorption in comparison to other known energy‐absorbing structures in the body. The heel pad, a well‐known contributor to absorbing energy from heel strike, absorbed 50–69% (0.70 +/‐ 0.11 J) of impact loading when using realistic loading conditions in vitro. All knee structures combined absorbed 0.12 +/‐ 0.01 J of impact energy, only 6–17% of the energy absorbed by the heel pad alone. Available loading data suggest that energy absorption of the menisci alone likely account for only ~4% of all energy absorption at the knee. Thus, our estimates of the total shock absorption capacity of the menisci is essentially negligible.These findings indicate that the menisci did not originally develop for shock‐absorption in tetrapods and did not develop a specialized shock absorbing function in humans.

Impact of a ball with a flexible beam

The impact of a ball with a flexible beam is analysed both experimentally and theoretically. The impact generates a bending wave that may or may not return to the impact point before the ball bounces off the beam. If the ball rebounds before the bending wave returns, then the rebound speed of the ball is determined by localised bending of the beam in the vicinity of the impact point. For a short duration impact, the rebound speed is therefore independent of whether the beam is freely supported or clamped at one or both ends, unless the impact occurs close to an end of the beam. Two different values of the coefficient of restitution can be defined, depending on whether the speed of the beam is measured when the ball rebounds or at a later stage. Experimental results were obtained using a small rubber ball impacting light wood and steel beams. A video camera and two piezoelectric disks were used to capture the relevant ball speeds and vibration response of the beam. The experiment would be suitable for undergraduate students.

Prevalence and nature of heating processes in CM and C2-ungrouped chondrites as revealed by insoluble organic matter

Chondrites are exhumed from their parent bodies by impacts, which at the same time can result in heating and mechanical modification (compaction, deformation, fracturing, etc.). However, whether impacts are responsible for the occurrence of heated C2s remains controversial since radiogenic and solar heating have also been invoked to explain them. Here we report a Raman and infrared study of the composition and structure of Insoluble Organic Matter (IOM) in a series of 39 CM and C2-ungrouped chondrites. These parameters are tracers of the extent and nature of thermal metamorphism a meteorite has experienced and reflect the degree to which the thermally driven and irreversible carbonization of IOM has proceeded. We propose a carbon-based classification of heated C2 chondrites that reveals a high occurrence frequency of thermally processed C2 chondrites (>36%). This classification is in agreement with the mineralogical classification scheme of Nakamura (2005). Strongly heated C2 chondrites (PCA 02012, PCA 91008, Y 96720) display an IOM structural evolution that is dissimilar to that of type 3 chondrites that experienced long duration radiogenic thermal metamorphism. These differences almost certainly reflect kinetic constraints on IOM modification during short duration heating events. QUE 93005 is a weakly heated chondrite that experienced a retrograde aqueous alteration. Its very aliphatic-rich IOM points to a parent body hydrogenation through interactions with water. The closed-system conditions required by this mechanism could be satisfied by a kinetic confinement during a very short duration impact. MET 01072, a heavily compacted and uni-axially deformed chondrite, did not experience post-accretional heating. In this case, the deformation features probably reflect a low-velocity impact. In contrast, the weakly metamorphosed chondrite EET 96029 experienced one or several low pressure impacts that triggered mild heating and partial dehydration without deformation features. The study of a series of lithologies from the Tagish Lake C2-ungrouped chondrite confirms the coexistence of various degrees of post-accretional alteration, the most altered lithologies having experienced a moderate degree of heating. Overall, the high prevalence of heating in C2 chondrites, the evidence of short-duration heating in the most heated C2s and the ability of low velocity collisions to trigger heating favor impacts (against solar heating), as the dominant heating mechanism. Finally, our set of data does not support the action of a low temperature oxidation process that would control the aliphatic abundance in unheated primitive C2s.

Shock Isolation with Anticipating Control for External Disturbances of Various Shapes

Limiting performance analysis is performed for isolation of an object on a movable base from short-duration impact excitations by means of an active shock isolator with anticipating control. The external disturbance (excitation) is modeled by the time history of the absolute acceleration of the base. The control is performed by a force that acts between the base and the object to be protected. The absolute value of the control force is subject to a constraint. A procedure is proposed for constructing optimal anticipating controls that minimize the peak magnitude of the displacement of the object relative to the base for external disturbances from a certain class. For a number of types of external disturbances, the solution of the optimal control problem is obtained in closed form. The controls that are constructed in closed form are modified to be applicable for the disturbances for which closedform solutions are absent.

Measurement of Dynamic Responses of a Force Sensor against Small Impact Forces with Various Durations

At present, a method for evaluating dynamic characteristics of force sensors against small and short-duration impact forces has been developed. In this method, a small mass collides with a force sensor and the impact force is measured with high accuracy as the inertial force of the mass. A pneumatic linear bearing is used in order to realize linear motion with sufficiently small friction acting on the mass, i.e., the moving part of the bearing. Using this method, the dynamic characteristics of the force sensor are evaluated in detail: small and various-duration impact forces with maximum values of approximately 0.4-6.0 N and full width at half maximum (FWHM) of approximately 0.6-2.8 ms are applied to the force sensor and the impact responses of the force sensor are evaluated.

Experimental investigation of hydrodynamic loads and pressure distribution during a pyramid water entry

In the maritime environment slamming is a phenomenon known as short duration impact of water on a floating or sailing structure. Slamming loads are local and could induce very high local stresses. This paper reports a series of impact test results and investigate the slamming loads and pressures acting on a square based pyramid. In this study the slamming tests have been conducted at constant velocity impact with a hydraulic high speed shock machine. This specific experimental equipment avoids the deceleration of the structure observed usually during water entry with drop tests. Three velocities of the rigid pyramid have been used (10, 13 and 15ms−1). Time-histories of local pressures, accelerations and slamming loads were successfully measured. The relationship between the pressure magnitude and the impact velocity is obtained and the spatial distribution of pressures on pyramid sides is characterized. The impact velocity was found to have a negligible influence in predicting the maximum pressure coefficient.

Wave interaction with a vertical wall: SPH numerical and experimental modeling

Prediction of wave forces on caisson-type breakwaters is challenging, especially in the case of impact loads that strongly depend on wave reflection, and wave breaking which affects the wave characteristics near the structure. In recent years, numerical models have been further developed and their use is becoming increasingly attractive, such as the Smoothed Particle Hydrodynamics (SPH) numerical models which enable the simulation of complex free surface flows. The SPH numerical model developed at the National Laboratory for Civil Engineering (LNEC) is validated by comparing the numerical results of the wave interaction with a vertical wall using data obtained from physical modeling tests carried out in one of the LNEC׳s flumes. The numerical results presented a good agreement with the physical model tests. The free surface level was well estimated, with a concordance index between numerical results and experimental data of about 90.5–99.1%, presenting an average value of 95.5%. Pressure at the vertical wall shows high intensity and short duration impact loads, and presented a concordance index between numerical results and experimental data about 68.8–94.4%, with an average value about 86.2%. Considering the complex phenomena involved, which is highly difficult to be accurately modeled, numerically and experimentally, a good agreement between physical tests and numerical results was accomplished.

DYNAMIC CHARACTERISTICS MEASUREMENTS OF A FORCE TRANSDUCER AGAINST SMALL AND SHORT-DURATION IMPACT FORCES

Abstract A method for evaluating the dynamic characteristics of force transducers against small and short-duration impact forces is developed. In this method, a small mass collides with a force transducer and the impact force is measured with high accuracy as the inertial force of the mass. A pneumatic linear bearing is used to achieve linear motion with sufficiently small friction acting on the mass, which is the moving part of the bearing. Small and short-duration impact forces with a maximum impact force of approximately 5 N and minimum half-value width of approximately 1 ms are applied to a force transducer and the impulse responses are evaluated.

Direct and indirect measurement of rain drop size distributions using an acoustic water tank disdrometer

Several rain drop size distribution (DSD) point measurement technologies exist, but all are unable to sample either short timescales or the large drop tail of the DSD due to inherent instrumental limitations. The development of an acoustic water tank disdrometer (AWTD) is described, which improves the sampling statistics by increasing the catchment area. This is achieved by distinguishing individual drops, locating them on the surface of the tank then converting the impact pressure into a drop size. Wavelet decomposition is used to distinguish the broadband, short duration impact events and a fast multilateration method is used to position the drop. Issues relating to the different types of noise are also investigated and mitigated. Also, further work on inverting the measured acoustic intensity into a DSD, by fitting sampling distributions, is presented. Six months of data were collected in the Eastern UK. The AWTD then converted the data into DSDs and the results were compared to a commercially available co-located laser precipitation monitor. The sampling errors are far lower due to the increased catchment size, and hence the large drop sized tail of the DSD is greatly improved. DSD results compare favourably to other disdrometers for drop diameters greater than 1.8 mm. Below this size individual drops become increasingly difficult to detect and are underestimated.

Measurement of Six Degrees of Freedom Head Kinematics in Impact Conditions Employing Six Accelerometers and Three Angular Rate Sensors (6aω Configuration)

The ability to measure six degrees of freedom (6 DOF) head kinematics in motor vehicle crash conditions is important for assessing head-neck loads as well as brain injuries. A method for obtaining accurate 6 DOF head kinematics in short duration impact conditions is proposed and validated in this study. The proposed methodology utilizes six accelerometers and three angular rate sensors (6aω configuration) such that an algebraic equation is used to determine angular acceleration with respect to the body-fixed coordinate system, and angular velocity is measured directly rather than numerically integrating the angular acceleration. Head impact tests to validate the method were conducted using the internal nine accelerometer head of the Hybrid III dummy and the proposed 6aω scheme in both low (2.3 m/s) and high (4.0 m/s) speed impact conditions. The 6aω method was compared with a nine accelerometer array sensor package (NAP) as well as a configuration of three accelerometers and three angular rate sensors (3aω), both of which have been commonly used to measure 6 DOF kinematics of the head for assessment of brain and neck injuries. The ability of each of the three methods (6aω, 3aω, and NAP) to accurately measure 6 DOF head kinematics was quantified by calculating the normalized root mean squared deviation (NRMSD), which provides an average percent error over time. Results from the head impact tests indicate that the proposed 6aω scheme is capable of producing angular accelerations and linear accelerations transformed to a remote location that are comparable to that determined from the NAP scheme in both low and high speed impact conditions. The 3aω scheme was found to be unable to provide accurate angular accelerations or linear accelerations transformed to a remote location in the high speed head impact condition due to the required numerical differentiation. Both the 6aω and 3aω schemes were capable of measuring accurate angular displacement while the NAP instrumentation was unable to produce accurate angular displacement due to double numerical integration. The proposed 6aω scheme appears to be capable of measuring accurate 6 DOF kinematics of the head in any severity of impact conditions.

Mass-plate impact parameters for the elastic range

When an impact load is applied laterally to a plate, the deflection gradually spreads over time until the entire plate radius is affected. The determination of effects for a short-duration impact presents substantial mathematical difficulties. The normal-mode superposition is not only awkward but it does not lead to closed-form solutions. Some of the past works gave a possibility of such a solution by assuming a certain flexural wave spreading from the impact point. The complexity involved in that was simplified in this paper. This main part of this work is based on a shear wave approach, by relating the lateral stiffness to the propagation of a shear wave from impact point. This, along with some other simplifications, makes it possible to obtain the peak contact force by using compact expressions. The development is limited to impactor mass not exceeding one-half of the plate mass. The evaluation of rebound velocity or the coefficient of restitution was successfully resolved for only a limited range of parameters. The other subtopic is the quantification of the contact problem itself, which is nonlinear by nature. A transparent method of linearization is proposed, based either on experiment or on Hertzian formulation. The results presented in this paper are compared with the answers from finite-element simulations, physical experiments and with some cases available from literature.

A dynamic approximation of balanced gymnastics landings

A gymnast can balance after an uneven parallel bar dismount by contacting the ground with an appropriate body position and angular velocity. Balanceable gymnastics landings were predicted by dividing landing into two parts: a short duration impact phase when foot-mat contact forces are high, and a subsequent longer balancing phase during which the gymnast exerts active control at the joints to attenuate the resulting post-impact velocities and achieve a desired motionless terminal configuration. This paper identifies controllable sets of pre-impact landing positions and velocities for a typical female collegiate gymnast that are feasible targets for the end of flight following a single- and double-backward somersault uneven bar dismount with forward mass centre (CM) velocity. Simple algebraic impulse-momentum models for the impact phase determine impulses necessary to stop the feet and slow angular velocity. After impact, a dynamic analysis is needed to determine stability because non-negligible post-impact angular and linear velocities are possible. For almost all successfully balanced landings, ground contact occurs with the CM positioned above the feet and between the toe and heel, because the linear impact impulses that stop the feet also contribute to an angular impulse that slows angular momentum. Only at the largest pre-impact angular velocities following a double-somersault dismount can contact occur with the CM in front of the toe. The gymnast is in balanceable preimpact orientations longer when rotating more slowly and with a smaller angular momentum; therefore balance may be more likely after performing a single-somersault dismount with a large moment of inertia.

Top-side ionosphere response to extreme solar events

Abstract. Strong X-flares and solar energetic particle (SEP) fluxes are considered as sources of topside ionospheric disturbances observed by the ROCSAT-1/IPEI instrument during the Bastille Day event on 14 July 2000 and the Halloween event on 28 October–4 November 2003. It was found that within a prestorm period in the dayside ionosphere at altitudes of ~600 km the ion density increased up to ~80% in response to flare-associated enhancements of the solar X-ray emission. Ionospheric response to the SEP events was revealed both at sunlit and nightside hemispheres, where the ion density increased up to ~40% and 100%, respectively. We did not find any prominent response of the ion temperature to the X-ray and SEP enhancements. The largest X-ray and SEP impacts were found for the X17 solar flare on 28 October 2003, which was characterized by the most intense fluxes of solar EUV (Tsurutani et al., 2005) and relativistic solar particles (Veselovsky et al., 2004). Solar events on 14 July 2000 and 29 October 2003 demonstrate weaker impacts with respect to their X-ray and SEP intensities. The weakest ionospheric response is observed for the limb X28 solar flare on 4 November 2003. The topside ionosphere response to the extreme solar events is interpreted in terms of the short-duration impact of the solar electromagnetic radiation and the long-lasting impact of the SEP.

Tsunami science before and beyond Boxing Day 2004

Tsunami science has evolved differently from research on other extreme natural hazards, primarily because of the unavailability until recently of instrumental recordings of tsunamis in the open ocean. Here, the progress towards developing tsunami inundation modelling tools for use in inundation forecasting is discussed historically from the perspective of hydrodynamics. The state-of-knowledge before the 26 December 2004 tsunami is described. Remaining aspects for future research are identified. One, validated inundation models need to be further developed through benchmark testing and instrumental tsunameter measurements and standards for operational codes need to be established. Two, a methodology is needed to better quantify short-duration impact forces on structures. Three, the mapping of vulnerable continental margins to identify unrecognized hazards must proceed expeditiously, along with palaeotsunami research to establish repeat intervals. Four, the development of better coupling between deforming seafloor motions and model initialization needs further refinement. Five, in an era of global citizenship, more comprehensive educational efforts on tsunami hazard mitigation are necessary worldwide.

Simulation of dynamics of interacting rigid bodies including friction II: Software system design and implementation

This is the second of two papers that deal with the problem of modeling contact (impact, sliding, rolling) between unconstrained rigid bodies, including friction. In a companion paper [1] we showed that the main underlying problem concerns the ability to do efficient contact mechanics when bodies interact through impact and/or sustained contact. Contact mechanics involves two aspects: detection of contact between bodies and estimation of contact forces. These forces are complicated in character and difficult to estimate because they depend on the material response of the contacting objects, on the duration of contact (very short duration impact, or more sustained contact), frictional interaction at the surfaces, geometry of contact, etc. In [1] we proposed a conceptual model in which linear elastic (springs) and viscous (dampers) elements acting at points of contact between objects generate all contact forces. In this paper we describe how the contact model has been implemented in the software of a working computer simulation system. The major aspects of this process are: formulation of a discrete version of the contact model; calculation of model parameters to reflect material properties; geometric representation of objects (in our system, objects are modeled as convex polyhedra); algorithms to detect and evaluate contacts among objects (a process called contact analysis); and estimation and control of model response for stable numerical integration of equations of motion. A graphical user interface displays a three-dimensional (3-D) perspective animation of the solution using full color shaded surface images. While the simulation may not be accomplished in real time, solutions can be saved in files for real-time visualization.

Short duration impact loading on fibre composite laminates

The response of a cross-ply laminated plate subjected to a time-dependent line load is examined. The solution is obtained by solving the full three-dimensional equations of elasticity in each layer and then applying the appropriate interface and boundary conditions. Time histories of the upper and lower surface disturbance are presented for each of three different impulses. The dependence of the response on the duration and orientation of the line load impulse is discussed.

The anatomy of underwater rain noise

When rain falls onto a large body of water it produces dominating underwater sound over a broad range of audio frequencies. Laboratory studies using more than 1000 single drops, covering the complete size range of actual rain drops at their terminal speeds, have now shown that the complete underwater spectrum of rainfall sound can be dissected into the impact and microbubble sounds produced by four acoustically distinctive ranges of drop diameters D. These are defined as ‘‘minuscule’’ drops (D≤0.8 mm), ‘‘small’’ drops (0.8 mm≤D≤1.1 mm), ‘‘mid-size’’ drops (1.1≤D≤2.2 mm), and ‘‘large’’ drops (D≥2.2 mm). A minuscule raindrop produces only a very weak, almost undetectable, short duration impact noise. A small drop at terminal speed and at local, near-normal incidence, radiates measurable broadband impact sound followed by the very much stronger sound of a ‘‘type I’’ damped microbubble oscillating at frequencies near 15 kHz. A mid-size raindrop radiates only impact sound. Large raindrops, which comprise the major volume of moderate to heavy rainfall, produce an impact sound and a dominating, ‘‘type II,’’ ‘‘primary’’ oscillating microbubble of characteristic frequency 2 to 10 kHz depending on the drop diameter. Also, large drops often generate weaker sounds from ‘‘secondary’’ bubbles. The average acoustic energy spectra of large raindrops are distinctive functions of their diameters, the salinity of the surface water, and the temperature difference between the drop and the surface water. When the underwater acoustic intensity spectrum during heavy rain is calculated from the single drop acoustic energy spectra and the drop size distribution, it compares quite well with ocean measurements. The gas injection at the air–water interface is calculated from the probability of bubble formation during a heavy rainfall.

Computer Simulation of Head Impact: Estimation of Head-Injury Risk during Soccer Heading

With a validated mathematical model of the head-neck consisting of nine rigid bodies (skull, seven cervical vertebrae, and torso), we simulated head impacts to estimate the injury risk associated with soccer heading. Experimental data from head-linear accelerations during soccer heading were used to validate the nine-body head-neck model for short duration impact loading of the head. In the computer simulations, the mass ratios between head mass and impacting body mass, the velocity of the impacting body, and the impact elasticity were varied. Head-linear and angular accelerations were compared to standard head-injury tolerance levels, and the injury risk specifically related to soccer heading was estimated. Based on our choice of tolerance levels in general, our simulations showed that injury risk from angular head accelerations was greater than from linear head accelerations, and compared to frontal impacts, lateral impacts had greater angular and less linear head accelerations. During soccer heading, our simulations indicated an unacceptable injury risk caused by angular head accelerations for frontal and lateral impacts at relatively low impact velocities for children, and at medium range impact velocities for adults. For linear head accelerations, injury risk existed for frontal and lateral impacts at medium range to relatively larger impact velocities for children, while no injury risk was shown for adults throughout the entire velocity range. For injury prevention, we suggest that head-injury risk can be reduced most substantially by increasing the mass ratio between head and impacting body. In soccer with children, the mass of the impacting body has to be adjusted to the reduced head mass of a child, that is, it must be clearly communicated to parents, coaches, and youngsters to only use smaller soccer balls.

Theoretical determination of damage risk criteria for impulse noise.

It is widely accepted that for continuous noise, a relationship exists between the energy of sound (A-weighted) and its deleterious effects on hearing. However, when the exposure is to high intensity short duration impact or impulse noise, a hitherto unpredictable degree of damage can occur to the inner ear. The present study has set out to establish damage risk criteria for a variety of impulsive waveforms. A mathematical model of the auditory periphery has been developed and verified for sinusoidal stimulation using existing experimental data. Experimental results, obtained using guinea pigs under impulsive stimulation, were used to calibrate the model. Once calibrated the model was used to give information as to the damage likely to occur from other types of impulsive stimuli, hence reducing the need for extensive animal experimentation. It has been shown that the model can predict the potential auditory hazard from various impulsive stimuli, quantitatively in the guinea pig and qualitatively in man. Both the experimental and theoretical results indicated that the intensity of the first pressure wave of an impulse, above a certain value, is the most significant determinator of hearing damage. The machine designer thus has guide lines as to where to concentrate his efforts in reducing potentially traumatic machine noise.