Numerical calculation of blast effect on human and on the mannequin for the assessment of blast incapacitation and lethality

Transcription

1 Numerical calculation of blast effect on human and on the mannequin for the assessment of blast incapacitation and lethality Amal Bouamoul DRDC Valcartier Defence R&D Canada Valcartier Technical Memorandum DRDC Valcartier TM November 2008

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3 Numerical calculation of blast effect on human and on the mannequin for the assessment of blast incapacitation and lethality Amal Bouamoul DRDC Valcartier Defence R&D Canada Valcartier Technical Memorandum DRDC Valcartier TM November 2008

4 Principal Author Amal Bouamoul Defence Scientist Approved by Dennis Nandlall Section Head, Protection and Weapons Effects Approved for release by Christian Carrier Chief Scientist This study was done at DRDC Valcartier between March and June 2006, under the Numerical Modelling WBE of the 12RA Soldier Protection against Blast ARP. Her Majesty the Queen in Right of Canada, as represented by the Minister of National Defence, 2008 Sa Majesté la Reine (en droit du Canada), telle que représentée par le ministre de la Défense nationale, 2008

5 Abstract.. There are multiple test methodologies currently in use to assess the performance of current and novel protective equipment and to investigate the thoracic injuries caused by blast overpressure. Among these methodologies, there is the mannequin for the assessment of blast incapacitation and lethality (MABIL), an instrumented human upper body surrogate that can be used to assess the performance of personal protective equipment. The objectives of the current study were first to develop and validate with experimental data a finite element (FE) model of MABIL for the purpose of investigating and predicting its response to blast overpressure. The second aim was to compare the DRDC MABIL FE model responses with a human FE torso model. The MABIL mannequin and the human torso numerical models consist of a simplified two-dimensional slice taken at the mid-sternum level, and subjected to different blast loadings. Experimental data on MABIL captured during field trials were used to validate the numerical models. In general, the numerical chest wall acceleration, velocity and the viscous criteria were higher than the experimental ones. However, the ratio between the numerical and the experimental chest wall velocity was the same across the range of blast loads studied. Résumé... Il y a plusieurs méthodes expérimentales présentement utilisées pour évaluer la performance de nouveaux équipements de protection personnelle conçus afin de réduire les blessures induites par des armes à effet de souffle. Parmi ces méthodes, il y a le mannequin for the assessment of blast incapacitation and lethality (MABIL), une membrane instrumentée représentant le haut du corps humain qui peut être utilisée pour des tests expérimentaux sur le terrain. Les objectifs de cette étude consistaient premièrement à développer et valider à l aide de tests expérimentaux un modèle d éléments finis de MABIL dans le but de comprendre et prédire sa réponse aux effets de souffle. Le deuxième objectif était de comparer la réponse de MABIL à un autre modèle d éléments finis du torse humain. Les modèles numériques du mannequin MABIL et du torse humain représentent une tranche tridimensionnelle horizontale située au milieu du torse. Des données expérimentales sur MABIL ont été utilisées pour valider les modèles numériques. En général, la vitesse et l accélération maximale de la cage thoracique ainsi que le critère visqueux maximal sont plus élevés que celles enregistrées expérimentalement. Cependant, le rapport entre les vitesses de la cage thoracique du modèle numérique et celles enregistrées expérimentalement est resté constant pour les différents cas étudiés. DRDC Valcartier TM i

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7 Executive summary Numerical calculation of blast effect on human and on the mannequin for the assessment of blast incapacitation and lethality: Amal Bouamoul; DRDC Valcartier TM ; Defence R&D Canada Valcartier; November Until very recently, the major developments in personal protective equipment for the soldier s torso has been focused on the protection against ballistic and fragmentation threats. However, recent developments and proliferation of blast weapons, whose primary injury mechanism is blast overpressure, have led to primary blast injuries being recognized as a significant threat to soldiers. The organs vulnerable to blast overpressure effects are principally those containing air such as the lungs, the auditory system, and the gastrointestinal tract. The DRDC mannequin for the assessment of blast incapacitation and lethality (MABIL) has been developed to assist the development of efficient personal protection systems against blast loading. The MABIL surrogate is a representation of the human torso and head used to measure primary blast injuries. Even though the DRDC MABIL surrogate is a useful tool to test blast personal protection equipment, experimental testing is time consuming and expensive. It is also impractical to set up testing procedures to cover all blast scenarios of interest. As a result, numerical simulations are advantageous. With a representative model, several simulations can be performed to complete or complement the experimental test matrix. Consequently, a two-dimensional finite element model of the DRDC MABIL torso has been built. A validation of the DRDC MABIL numerical model has been performed using experimental trial data. The measured maximum mid-sternum acceleration, velocity, and viscous criterion were used to validate the numerical model. The ratio between the experimental and the numerical, maximum mid-sternum acceleration and the viscous criterion were not constant for the three blasts used in this study. However, the ratio between the experimental and the numerical maximum mid-sternum velocity was constant. In addition to the experimental loading scenarios studied, six other blast overpressure histories, representing three different levels of injury at two load durations according to the Bowen curves, were simulated. Through the numerical simulations, some contradictions were found for the same level of injury on the Bowen curves. In fact, blast with a short duration generated higher mid-sternum acceleration than a blast with long duration, even though the Bowen curves would suggest that the injuries should be similar. On the other hand, blast with a long duration induced higher midsternum velocity than blast than those with a short duration. These two contradictory conclusions illustrate the need for more experimental and theoretical studies. DRDC Valcartier TM iii

8 Sommaire... Numerical calculation of blast effect on human and on the mannequin for the assessment of blast incapacitation and lethality: Amal Bouamoul; DRDC Valcartier TM ; R & D pour la défense Canada Valcartier; novembre Depuis plusieurs années, les développements majeurs concernant la protection personnelle au niveau du torse ont été concentrées sur les vestes anti-fragments et les plaques balistiques. Cependant, d autres types de blessures ont été observés comme les blessures causées par l effet de souffle (blast). Les parties vulnérables du corps humain aux effets de souffle sont principalement celles contenant de l air, comme les poumons, le système auditif et le système gastro-intestinal. Afin de mettre au point une protection personnelle efficace pour contrer cette menace, le mannequin for assessment of blast incapacitation and lethality (MABIL) a été développé. RDDC MABIL est un mannequin ayant une géométrie réelle du torse et de la tête humaine, ce qui le rend utile pour tester expérimentalement des protections personnelles. Même si MABIL est un bon outil pour tester la protection personnelle, les tests expérimentaux restent coûteux aux points de vue argent et temps. Un nombre limité de tests doivent être choisis. Il est alors difficile de tester tous les prototypes de protection dans tous les scénarios. Pour résoudre ce type de problème, la simulation numérique est avantageuse. Avec un modèle numérique représentatif, plusieurs simulations peuvent être faites pour compléter la matrice d essais expérimentale. En tenant compte de cela, un modèle d éléments finis d une tranche horizontale du MABIL a été construit ainsi, qu un modèle bidimensionnel par éléments finis du torse humain. La validation des résultats numériques du MABIL a été effectuée avec ceux de l expérience. L accélération, la vitesse de la cage thoracique ainsi que le critère visqueux du torse sont les réponses dynamiques utilisées pour valider le modèle numérique. Le rapport entre les valeurs numériques et expérimentales concernant l accélération maximale de la cage thoracique et du critère visqueux maximal n était pas constant pour les types des chargements testés. Par contre, celui de la vitesse de la cage thoracique est demeuré constant. En plus des effets de souffle étudiés, six autres blasts ayant trois différents niveaux de blessure et deux différentes durées selon les courbes de Bowen ont été simulés. Même si les différents blasts ont le même niveau de blessure, les réponses du modèle numérique ont été différentes. Pour le même niveau de blessure sur les courbes de Bowen, les chargements caractérisés par une courte durée créaient une accélération de la cage thoracique plus grande que ceux ayant une longue durée. Cependant, les chargements de longues durées engendraient une vitesse maximale plus grande que celle à courte durée. Ces contradictions ont montré la nécessité d effectuer plus d études numériques et expérimentales. iv DRDC Valcartier TM

9 Table of contents Abstract..... i Résumé i Executive summary... iii Sommaire... iv Table of contents... v List of figures... vi List of tables... vii 1 Introduction Mannequin of assessment of blast incapacitation and lethality Description Geometry Human and MABIL FE torso models Torso model Material properties MABIL model Material properties Loading conditions Experimental loading Numerical loading Scenarios considered during the numerical simulations Numerical results Validation of the FE MABIL model with experimental data Comparison between MABIL and the torso FE models Simulated blast loading Blast used Results Conclusion Recommendations References Annex A PU70 material properties Distribution list DRDC Valcartier TM v

10 List of figures Figure 1: DRDC MABIL surrogate... 2 Figure 2: Human torso model components, top view [Ref. 3] Figure 3: DRDC MABIL FE model... 6 Figure 4: PU70 experimental stress-stretch curves for high rate and quasi-static experimental data [Ref. 13]... 7 Figure 5: A typical experimental setup using the DRDC MABIL surrogate Figure 6: Typical Friedlander curve Figure 7: ALE model Figure 8: Boundary conditions Figure 9: Experimental and numerical blast overpressure curves for 5 kg C4 at 2 m (a), 2.5 m (b) and 3.5 m (c) Figure 10: Experimental and numerical MABIL maximum mid-sternum wall acceleration Figure 11: Experimental and numerical MABIL maximum mid-sternum wall velocity Figure 12: Experimental and the numerical MABIL VC max for the MABIL mid-sternum Figure 13: Experimental and numerical MABIL mid-sternum wall velocity, 5 kg C4 at 2m Figure 14: Experimental and numerical MABIL mid-sternum wall velocity, 5 kg C4 at 2.5m Figure 15: Experimental and numerical MABIL mid-sternum wall velocity, 5 kg C4 at 3.5m Figure 16: Experimental and numerical MABIL and human torso mid-sternum wall acceleration Figure 17: Experimental and numerical MABIL and human torso mid-sternum wall velocity Figure 18: Experimental and numerical MABIL and human torso mid-sternum wall viscous criteria vi DRDC Valcartier TM

11 List of tables Table 1: Material properties used for each component of the human torso model Table 2: The numerical test matrix Table 3: Blast duration response DRDC Valcartier TM vii

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13 1 Introduction Since World War I, primary blast injuries have been recognized as the most significant reported injury to the lungs [Ref. 1]. A simple definition of the blast is an overpressure wave, usually created by an explosive detonation, travelling in air at supersonic velocity. Blast waves are usually characterized by their peak overpressure and their positive phase duration. Typically, the blast induces injuries to the organs containing air like the lung, the auditory system, and the gastro-intestinal system. There are multiple test methods currently in use to assess the qualitative and quantitative performance of novel protective equipment against blast weapons. Each of these methods has their advantages and disadvantages. The surrogate used in the present study is the mannequin of assessment of blast incapacitation and lethality (MABIL), which is developed exclusively to assess torso and head blast-loading injuries. MABIL is a representation of the human torso and it has been developed by Defence Research & Development Canada (DRDC) Valcartier [Ref. 2]. Even if the DRDC MABIL is a good tool to test blast protection, experimental testing is time consuming and expensive. A limited test numbers have to be selected to record a maximum of data from the surrogate. It is then difficult to set up a testing procedure, which covers all the protection concepts in different blast scenarios. To solve this problem, the numerical simulation is advantageous. With a representative model, several simulations can be performed to complete the experimental tests matrix. Considering this, a finite element (FE) model of a simplified three-dimensional MABIL model slice has been built. The first aim of this study was to validate the MABIL model using different blast intensities. The maximum mid-sternum velocity, acceleration and the viscous criterion were used to validate the numerical model. The second aim was to compare the DRDC MABIL FE model responses with a human FE torso model. The torso model was developed to quantify the level of injury induced by blast loading [Refs. 3, 4, 5 and 6]. Finally, the third aim of the present study was to subject the DRDC MABIL model to six new blasts to investigate its mechanical response. DRDC Valcartier TM

14 2 Mannequin of assessment of blast incapacitation and lethality 2.1 Description The DRDC MABIL is a human surrogate that was designed to assess primary blast injuries and burns. The model incorporates an instrumented responding torso model and an instrumented head. A complete description of the MABIL surrogate is given in [Ref. 2]. The torso was instrumented with two single axis accelerometers. One accelerometer was located at the mid-sternum and the other was at the level of the navel in the centre of the abdomen. In the experiments referenced in this report, the DRDC MABIL torso was placed facing the charge with the thelion (nipple) 1.27 m from the ground to represent a standing 50 th percentile soldier. Figure 1 shows the complete DRDC MABIL mannequin prototype and its support. Figure 1: DRDC MABIL surrogate 2.2 Geometry A requirement for the DRDC MABIL was to have an external shape representative of the human body to ensure proper fit with personal protection equipment (e.g. a fragmentation vest). The anthropometry of the model was based on 1988 US Army 2 DRDC Valcartier TM

15 anthropometric database [Ref. 2]. Overall, the DRDC MABIL model is considered approximately representative of the 50th percentile Canadian Forces soldier [Ref. 7]. As shown in Fig. 1, the DRDC MABIL torso is supported at the neck and at the hips. This restrained configuration was designed to assess only primary blast injuries. Usually, blast injuries are classified as primary, secondary, tertiary, and quaternary [Ref. 2]. Primary blast injuries are caused by the direct interaction between the blast wave and the body. Secondary injuries include those caused by environmental debris propelled by the blast and/or fragments from the warhead. Tertiary injuries are those injuries due to displacement of the body such as skull fracture caused by head impact on the ground. Finally, quaternary blast injuries include burns and injuries caused by detonation products and toxicity. The current prototype of the DRDC MABIL torso surrogate is made from Shore A 70 (PU70) polyurethane [Ref. 8]. This visco-elastic material has been used in the past to represent the behaviour of the human thorax under dynamic loading [Ref. 7]. DRDC Valcartier TM

16 3 Human and MABIL FE torso models 3.1 Torso model The human model described in this section was initially developed by University of Waterloo under contract number W /001/QCA [Refs. 3, 4, 5, 6] based on the work done by O Brien et al and Cooper et al. [Refs. 9, 10]. Figure 2 shows a top view of the numerical and CT-scanned human torso components. The model was created by the University of Waterloo using pictures from the Visible Human Project, National Library of Medicine [Ref. 11]. The picture is for an 82- kilogram, 1.8 meter high man and was taken at the fifth and sixth thoracic vertebrae level. The human model was modelled with four layers of elements through its thickness to approximate a 1.4 cm section of the mid thorax. The human torso model is constituted of approximately 42,000 nodes and 35,000 solid elements. The mean dimension of each solid element is approximately 2 mm. To reduce the computation time, no contact surfaces are used in the model and the interface nodes of all the components are merged together. The numerical simulations were run using the hydrocode LS-DYNA [Ref. 12]. Figure 2: Human torso model components, top view [Ref. 3] 4 DRDC Valcartier TM

17 3.1.1 Material properties An extensive literature review for human material properties was performed by the University of Waterloo [Ref. 4]. Based on this review, Table 1 lists the material models used for the components of human torso model. The MAT_SIMPLIFIED_RUBBER model, which is defined by a family of curves at discrete strain rates, was used to model muscle tissue, the heart, inner tissues and the intercostals. The ribs, the costal cartilage, the vertebrae and the sternum were modelled using the MAT_ELASTIC model. This model allows an isotropic elastic behaviour to be modelled without fracture or plastic strain. Lung tissue was modelled with MAT_NULL and the Gruneisen equation of state (EOS). MAT_NULL do not account for the deviatoric stresses caused by changes of shape therefore the Gruneisen EOS was used to predict the pressures resulting from volumetric strain. The input required for the Gruneisen EOS are the speed of sound in the uncompressed material and the slope of the shock velocity and the particle velocity (U s -U p ) curve described by S 1 and S 2 parameters which are equal to 1.92 and 0, respectively [Ref. 2]. Table 1: Material properties used for each component of the human torso model PART MATERIAL MODEL DENSITY (kg/m 3 ) YOUNG MODULUS (Pa) POISSON RATIO BULK MODULUS (Pa) Tissue (Muscle) Heart Inner tissue Intercostal MAT_SIMPLIFIED _RUBBER 1050 N/R N/R 2.2 x 10 9 Ribs MAT_ELASTIC x x 10 9 Lungs MAT_NULL 200 N/R N/R N/R Vertebrae MAT_ELASTIC x x 10 9 Costal Cartilage MAT_ELASTIC x x 10 6 Sternum MAT_ELASTIC x x 10 9 N/R: not required DRDC Valcartier TM

18 3.2 MABIL model The numerical model of the DRDC MABIL represents a quasi two-dimensional slice of the membrane, as shown in Fig. 3. This simplified representation of the torso membrane was used in order to keep the computational times reasonable. The slice, 1.4 cm thick, is situated approximately at the mid-sternum level of the torso where the accelerometer is located in the experimental rig. The model is composed of 436 hexahedron elements with an average dimension of 4.5 mm. As shown in Fig 3, the DRDC MABIL front chest wall is thinner than the back. This geometry has been designed to approximate the rigidity of the spine while modelling the dynamic response of the chest wall. Figure 3: DRDC MABIL FE model Material properties The MABIL membrane was made from polyurethane shore A 70 (PU70) and was modelled using the *MAT_QUASILINEAR_VISCOELASTIC material model. This model represents a quasi-linear isotropic, viscoelastic material and it was introduced by Fung in 1993 [Ref. 9]. The model is often used to represent biological soft tissues such as brain, skin and spleen. Doman [Ref. 10] has investigated experimentally the mechanical properties of polyurethane rubber in the high strain rate range. Doman used a compressive split polymeric Hopkinson bar to characterize the PU70 at high strain rates (up to 1300 s -1 ). These data complemented quasi-static results he also obtained with traditional characterization tests. Mechanical properties and high strain rate experimental tests [Ref. 13] used throughout the numerical study are given in Figure 4. In Figure 4, stretch ratio is a measure of the deformation and is also known as relative elongation. Annex A gives the *MAT_QUASILINEAR_VISCOELASTIC parameters for the PU70 used to model the DRDC MABIL membrane. 6 DRDC Valcartier TM

19 Figure 4: PU70 experimental stress-stretch curves for high rate and quasi-static experimental data [Ref. 13] DRDC Valcartier TM

20 4 Loading conditions 4.1 Experimental loading In the experiments that have been conducted using the DRDC MABIL, the surrogate was placed at different distances from bare explosive charges in order to reproduce loading conditions that are associated with varying probabilities of lung primary blast injury. The blast was usually produced by a 5 kg C4 charge suspended at a 1.5 m height of burst with the mannequins located between 2 m to 5 m from the charges. Several lollipop pressure transducers and pitot tubes were used to record static and dynamic blast overpressure histories, respectively, as shown in Figure 5. Figure 6 shows a typical incident pressure-time curve for an ideal free field blast where P s is the peak pressure, T d the positive duration, t is the time and t a is the time of arrival. Figure 5: A typical experimental setup using the DRDC MABIL surrogate 8 DRDC Valcartier TM

21 ps Pressure Pressure p0 ta td Time Figure 6: Typical Friedlander curve 4.2 Numerical loading The current study was performed using an Arbitrary Lagrange Eulerian (ALE) formulation to capture the interaction of the blast overpressure in the air and the DRDC MABIL membrane. The blast loading was modelled as a material flowing through a fixed finite element mesh (Eularian representation) while the DRDC MABIL was modelled using a Lagrangian approach where the elements are deformable. The two models were coupled using a penalty-based method, with the interaction force between the fluid/solid interface determined by the distance of separation and the contacting material properties [Ref. 12]. The blast was created by inducing a time-pressure dynamic curve in the boundary ambient elements. Applying a pressure history to a series of ambient elements along the boundary of the Eularian mesh results in a more or less planar blast wave propagating through the mesh towards the DRDC MABIL model at supersonic speed, as shown in Fig. 7. The complete numerical model contains approximately 150,000 nodes and 100,000 elements. Boundary conditions include fixed nodes along the (y-z) and (z-x) planes of the explosive and air boundaries. All nodes on the upper and lower (x-y) planes are fixed with respect to z-translation, x-rotation and y-rotation. In addition, non-reflecting boundary conditions were applied to the three free air edges to avoid the reflection of the blast wave at the edge of the Eulerian domain, as shown in Figure 8. DRDC Valcartier TM

22 Figure 7: ALE model Figure 8: Boundary conditions 10 DRDC Valcartier TM

23 4.3 Scenarios considered during the numerical simulations Three experimental scenarios were reproduced in order to compare the response of the numerical model of the DRDC MABIL with experimental measurements. These three configurations correspond to the DRDC MABIL placed at 2, 2.5, and 3.5 m from a 5 kg C4 explosive charge. Figure 9 shows a comparison between the measured blast overpressure histories and the loadings used in the numerical model. The height of burst used in the experimental results in the shock wave being reflected from the ground, which in turn creates a complex blast wave with two peaks. Even if these experimental blasts cannot be considered to be ideal Friedlander waves, it is possible to use the peak overpressure and an estimate of the positive phase duration to obtain a rough estimate of the expected lethality using the Bowen curves [Ref. 14]. Bowen curves were developed using free field blast testing on animals and different blast direction scenarios. The results have been scaled to be representative of a 70 kg man. The level of injury is dependant on the duration of the positive phase and the peak overpressure. Based on these curves, the pressure histories from the 2, 2.5, and 3 m tests correspond approximately to 50% survivability (LD50), 99% survivability (LD1) and threshold lung injury (LTH). a b c Figure 9: Experimental and numerical blast overpressure curves for 5 kg C4 at 2 m (a), 2.5 m (b) and 3.5 m (c) DRDC Valcartier TM

24 5 Numerical results 5.1 Validation of the FE MABIL model with experimental data The comparison of the experimental and numerical results for the DRDC MABIL response to blast loading was based on the mid-sternum acceleration, velocity and the viscous criterion (VC). The (VC) criterion was developed by Viano and Lau [Ref. 15]. It incorporates the compression (C) and the rate at which the compression occurred (V) into a single time-varying parameter, that has been shown to correlate to the risk of anatomical and functional injury. The prediction of injury risk is based on the VC max, which is the maximum of the VC history recorded during the loading. Velocity and displacement were then computed from the acceleration histories. The acceleration signal was computed numerically at an equivalent 200 khz sampling rate and filtered with a 40 khz cut-off frequency, which is the same as the anti-aliasing filter applied to the experimental signals. Velocity and displacement were then computed from these acceleration histories. In general, the experimental and numerical MABIL mid-sternum acceleration histories for the three selected scenarios (2, 2.5 and 3.5 m) were qualitatively similar. Figures 10, 11 and 12 show respectively a comparison between the experimental and the numerical results for the maximum mid-sternum wall acceleration, velocity and VC max. In general, the predicted peak acceleration, velocity and VC max. of the FE MABIL model were higher than those measured experimentally. However, the ratio between the mid-sternum wall experimental and numerical acceleration, velocity and VC max remains reasonably constant across the range of loading investigated. The dynamic response of FE MABIL was also compared with the experimental measurements. Figures 13, 14 and 15 show a comparison between the experimental and the numerical mid-sternum wall velocity for three different blast scenarios. In general, a good correlation between the experimental and the numerical values was found. 12 DRDC Valcartier TM

25 Maximum acceleration (m/s2) 275,0E+3 225,0E+3 175,0E+3 125,0E+3 75,0E+3 Experimental Numerical 25,0E+3 1,5 2 2,5 3 3,5 4 Distance from the charge (m) Figure 10: Experimental and numerical MABIL maximum mid-sternum wall acceleration Maximum velocity (m/s) Experimental Numerical 10 1,5 2 2,5 3 3,5 4 Distance from the charge (m) Figure 11: Experimental and numerical MABIL maximum mid-sternum wall velocity DRDC Valcartier TM

26 Maximum viscous criterion (m/s) Experimental Numerical ,5 2 2,5 3 3,5 4 Distance from the charge (m) Figure 12: Experimental and the numerical MABIL VC max for the MABIL mid-sternum Experimental Numerical Velocity (m/s) ,005 0,006 0,007 0,008 0,009 0,01 Time (s) Figure 13: Experimental and numerical MABIL mid-sternum wall velocity, 5 kg C4 at 2 m 14 DRDC Valcartier TM

27 40 30 Experimental Numerical Velocity (m/s) ,0065 0,0075 0,0085 0,0095 0,0105 0,0115 Time (s) Figure 14: Experimental and numerical MABIL mid-sternum wall velocity, 5 kg C4 at 2.5 m Velocity (m/s) Experimental Numerical 0-5 0,007 0,009 0,011 0,013 0,015 Time (s) Figure 15: Experimental and numerical MABIL mid-sternum wall velocity, 5 kg C4 at 3.5 m DRDC Valcartier TM

28 5.2 Comparison between MABIL and the torso FE models Comparisons between the maximum mid-sternum wall acceleration, velocity and the VC max for the FE MABIL and the human torso models are given in Figures 16, 17 and 18. From the numerical simulations, it was found that the response of the MABIL membrane (velocity, displacement and VC max was higher than the human torso model. Only the ratio between the FE MABIL and the human torso model predictions of the maximum mid-sternum acceleration was constant. No correlation was observed in peak velocity, or VC max Maximum acceleration (m/s2) MABIL model Torso model ,5 2 2,5 3 3,5 4 Distance from the charge (m) Figure 16: Experimental and numerical MABIL and human torso mid-sternum wall acceleration 16 DRDC Valcartier TM

29 Maximum velocity (m/s) MABIL model Torso model ,5 2 2,5 3 3,5 4 Distance from the charge (m) Figure 17: Experimental and numerical MABIL and human torso mid-sternum wall velocity Maximum viscous criterion (m/s) MABIL model Torso model 0 1,5 2 2,5 3 3,5 4 Distance from the charge (m) Figure 18: Experimental and numerical MABIL and human torso mid-sternum wall viscous criteria DRDC Valcartier TM

30 6 Simulated blast loading 6.1 Blast used A parametric study was performed to predict DRDC MABIL responses to different Friedlander blast intensities. The three parameters that typically describe a Friedlander curve are the peak overpressure, duration and the decay coefficient, which was set equal to 3.3 in this parametric study. Loading curves that represented lung threshold (LTH), 1% (LD1) and 50% (LD50) probability of lethality, based on the Bowen curves [Ref. 14], for durations equal to 1 ms and 5 ms, were selected and are listed in Table 2. Table 2: The numerical test matrix Bowen injury level 1 ms 5 ms Lung Threshold (LTH) 210 kpa 130 kpa 1% Lung damage (LD1) 560 kpa 350 kpa 50% lung damage (LD50) 770 kpa 500 kpa 6.2 Results Table 3 shows the MABIL numerical results for short (1 ms) and longer (5 ms) duration Friedlander type blast overpressure histories. Based on the Bowen curves, the level of injury for the 1 ms and 5 ms durations should be the same for each of the lung threshold (LTH), LD1 and LD50 cases. However, the mid-sternum acceleration and mid-sternum velocity are not the same for the same level of injury. For all the injury levels (LTH, LD1 and LD50), the maximum mid-sternum acceleration is higher for blasts with short duration than for the longer duration loading histories. In experimental tests performed by Cooper on pigs [Ref. 16], the lung injury was found to be proportional to the peak acceleration of the lateral thoracic wall. Using the data from Table 2 and the Cooper results, blast with short duration would induce more injuries than a one with long duration. To predict non-auditory blast injury from complex waves, Axelsson et al [Ref. 17] used their own experimental data to correlate values from a mathematical model. In this mathematical model, the Adjusted Severity of Injury Index (ASII) is proportional to the model chest-wall velocity. Applying the Axelsson conclusions to the numerical results (Table 3), blasts with long duration but lower peak overpressure would induce more 18 DRDC Valcartier TM

31 injuries than blasts with short duration. From these two conclusions, which are contradictory, it is difficult to state which one is correct. Chest wall acceleration, velocity and blast impulse are used currently to design protection systems. More experimental and theoretical studies have to be done in order to find the right injury predictor to be used during design process. Table 3: Blast duration response Short blast duration (1 ms) Long blast duration (5 ms) Blast Acceleration (m/s 2 ) Velocity (m/s) Acceleration (m/s 2 ) Velocity (m/s) TH LD LD DRDC Valcartier TM

32 7 Conclusion The aim of the present study was to validate the mechanical response of the MABIL FE model, subjected to different blast loadings, using the experimental results from the DRDC MABIL membrane. The comparison was based on the maximum mid-sternum acceleration, maximum velocity and viscous criteria. Even if the numerical results were different from the experimental ones, the ratio between the experimental and the numerical results for the different blast scenarios was constant suggesting that the constitutive model used under-predicts the stiffness of the polyurethane used to construct the surrogate. The maximum mid-sternum wall velocity was the one that gave the most consistent results. The DRDC MABIL FE response under blast was compared with that of a human FE torso model. The predicted responses were different for all the blast scenarios studied. In all cases, the MABIL FE model response (velocity, acceleration and viscous criteria) was higher than the FE human torso model. Since both models have yet to be validated with biological response data, it is difficult to state which model reacts correctly. Friedlander type blast overpressure histories that represent a lung threshold injury, 1%, and 50% probabilities of lethality for both short (1 ms) and long (5 ms) durations were used to perform a parametric study on the FE model. Through the numerical simulations, some contradictions were found for the same level of injury on the Bowen curves. In fact, blast with a short duration generated higher mid-sternum acceleration than a blast with long duration, even though the Bowen curves would suggest that the injuries should be similar. On the other hand, blast with a long duration induced higher mid-sternum velocity than blast than those with a short duration. These two contradictory conclusions illustrate the need to more experimental and theoretical studies. 20 DRDC Valcartier TM

33 8 Recommendations In order to correctly model the DRDC MABIL surrogate, it is necessary to characterize the PU70 over a range of high strain rates. The DRDC MABIL model was calibrated with three experimental blast scenarios. Even if these curves cover a reasonable range of blast overpressures, more tests are necessary to cover other scenarios. The DRDC MABIL response should also be calibrated using a repeatable, distributed load. One way to do this would be to use experimental tests and results from the automotive industry. The automotive industry has performed extensive experimental studies on both animals and post-mortem human subjects subject to impact loading. These tests recreate the forces acting on the human body when a crash occurs. Some of these tests were performed with an impactor, which strikes the chest wall at velocities varying from 8 to 30 m/s [Ref. 18]. These velocities appear to be comparable to those observed in experimental measurements of the DRDC MABIL surrogate when subjected to blast loadings. Finally, the FE MABIL model response has been validated based only on the response of the mid sternum (i.e. the front of the chest wall at the mid torso). However, an accurate assessment of the FE models prediction of the global deformation of the membrane must include the response of the sides of the MABIL mannequin. To obtain the additional calibration data, at least two additional accelerometers should be placed on the DRDC MABIL torso s sides. DRDC Valcartier TM

34 References... [1] Cripps, N.P.J. and Cooper G.J., The Influence of personnal blast protection on the distribution and severity of primary blast gut injury, Journal of Trauma, Vol. 40, No. 3, p. S206-S211, [2] Anctil, B., Keown, M., Williams, K., Manseau, J., Dionne, J.P., Jetté, F.X. and Makris, A., Development of a mannequin for assessment of blast incapacitation and lethality, Personal Armour Systems Symposium (PASS) 2004 Proceedings, p , The Netherlands, [3] Cronin, D., Salisbury, C. and Greer, A., Numerical modeling of blast injuries phase I report, Technical Report for DRDC Valcartier, March [4] Salisbury, C., Cronin, D. and Greer, A., Numerical modeling of blast injuries phase II report, Technical Report for DRDC Valcartier, October [5] Salisbury, C., Cronin, D. and Greer, A., Numerical modeling of blast injuries phase III report, Technical Report for DRDC Valcartier, March [6] Salisbury, C., Cronin, D. and Greer, A., Numerical modeling of blast injuries phase IV report, Technical Report for DRDC Valcartier, October [7] Bourget, D., Anctil, B., Doman, D., Cronin, D., Development of a surrogate thorax for BABT studies, Personal Armour Systems Symposium (PASS) 2002 Proceedings, p.69-78, The Netherlands, [8] Smooth-on, Liquid Rubbers and Plastic for Artists and Industry, (May 2006). [9] O Brien, W.D. and Zachary, J.F., 1996, Rabbit and pig lung damage comparison from exposure to continuous wave 30-kHz ultrasound, Ultrasound in Med. And Biol. Vol. 22, No. 3. [10] Cooper, G., Townend, D, Cater, S. and Pearce, B., 1991, The role of stress waves in thoracic visceral injury from blast loading: modification of stress transmission by foams and high-density materials, Journal of Biomechanics Vol. 24, pp [11] February, [12] Hallquist, J., LS-DYNA users manual Version 970, Livermore Software Technology Corporation, Livermore, CA, [13] Doman, D.A., Modeling of the high rate behaviour of the polyurethane rubber, Master of Applied Sciences Thesis, Waterloo University, Ontario, Canada, DRDC Valcartier TM

35 [14] Bowen, I.G., Fletcher, E.R., Richmond, D.R., Hirsch, F.G. and White, C.S., Biophysical mechanisms and scaling procedures applicable in assessing responses of the thorax energized by air-blast overpressures or by nonpenetrating missiles, Ann. N.Y. Acad. Sci. 152, pp , [15] Viano, D.C. and Lau, I.V., Thoracic impact: A viscous tolerance criterion. Tenth International Conference on Experimental Safety Vehicles. Oxford, England, [16] Cooper, G.J., Protection of the lung from blast overpressure by thoracic stress wave decouplers, Journal of Trauma, Vol. 40, No.3, S , [17] Axelsson, H. and Yelverton, J.T., Chest wall velocity as a predictor of nonauditory blast injury in complex wave environment, the Journal of Trauma, Vol. 40, No.3, S31-S37, [18] Kroell, C.K., Allen, D.S., Warner, C.Y. and Perl, T.R. Interrelationship of velocity and chest compression in blunt thoracic impact to swine II. 30 th Stapp Car Crash conference. SAE Paper No DRDC Valcartier TM

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37 Annex A PU70 material properties *MAT_QUASILINEAR_VISCOELASTIC material parameters that were used to model the PU70. MID RO K LC1 LC2 N GSTART M SO 2 G1 BETA1 G2 BETA2 G3 BETA3 G4 BETA G5 BETA5 G6 BETA6 G7 BETA7 G8 BETA8 G9 BETA9 G10 BETA10 G11 BETA11 G12 BETA12 C1 C2 C3 C4 C5 C DRDC Valcartier TM

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39 Distribution list Document No.: DRDC Valcartier TM LIST PART 1: Internal Distribution by Centre 1 Director General 3 Document Library 1 Dr. Amal Bouamoul (Author) 1 Dr. Dennis Nandlall 1 Dr. Claude Fortier 1 Dr. Kevin Williams 1 Simon Ouellet 1 Daniel Bourget 10 TOTAL LIST PART 1 LIST PART 2: External Distribution by DRDKIM 1 Library and Archives Canada 1 Director R&D Knowledge and Information Management (PDF) 1 LCol Bodner, Director Land Requirements 5 (DLR-5), LSTL 4, 101 Colonel by Drive, Ottawa, Ontario, K1G 0K2 1 LCol Levesque, Directorate Soldier System Program Management (DSSPM-10), 101 Colonel by Drive, Ottawa, Ontario, K1G 0K2 1 LCol Ross, Director Armoured Vehicle Program Management, 101 Colonel by Drive, Ottawa, Ontario, K1G 0K2 5 TOTAL LIST PART 2 15 TOTAL COPIES REQUIRED DRDC Valcartier TM

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41 DOCUMENT CONTROL DATA (Security classification of title, body of abstract and indexing annotation must be entered when the overall document is classified) 1. ORIGINATOR (The name and address of the organization preparing the document. Organizations for whom the document was prepared, e.g. Centre sponsoring a contractor's report, or tasking agency, are entered in section 8.) 2. SECURITY CLASSIFICATION (Overall security classification of the document including special warning terms if applicable.) Defence R&D Canada Valcartier 2459 Pie-XI Blvd North Quebec (Quebec) G3J 1X5 Canada UNCLASSIFIED 3. TITLE (The complete document title as indicated on the title page. Its classification should be indicated by the appropriate abbreviation (S, C or U) in parentheses after the title.) Numerical calculation of blast effect on human and on the mannequin for the assessment of blast incapacitation and lethality 4. AUTHORS (last name, followed by initials ranks, titles, etc. not to be used) Bouamoul, A. 5. DATE OF PUBLICATION (Month and year of publication of document.) November a. NO. OF PAGES (Total containing information, including Annexes, Appendices, etc.) 40 6b. NO. OF REFS (Total cited in document.) DESCRIPTIVE NOTES (The category of the document, e.g. technical report, technical note or memorandum. If appropriate, enter the type of report, e.g. interim, progress, summary, annual or final. Give the inclusive dates when a specific reporting period is covered.) Technical Memorandum 8. SPONSORING ACTIVITY (The name of the department project office or laboratory sponsoring the research and development include address.) Defence R&D Canada Valcartier 2459 Pie-XI Blvd North Quebec (Quebec) G3J 1X5 Canada 9a. PROJECT OR GRANT NO. (If appropriate, the applicable research and development project or grant number under which the document was written. Please specify whether project or grant.) 9b. CONTRACT NO. (If appropriate, the applicable number under which the document was written.) 10a. ORIGINATOR'S DOCUMENT NUMBER (The official document number by which the document is identified by the originating activity. This number must be unique to this document.) 10b. OTHER DOCUMENT NO(s). (Any other numbers which may be assigned this document either by the originator or by the sponsor.) DRDC Valcartier TM DOCUMENT AVAILABILITY (Any limitations on further dissemination of the document, other than those imposed by security classification.) Unlimited 12. DOCUMENT ANNOUNCEMENT (Any limitation to the bibliographic announcement of this document. This will normally correspond to the Document Availability (11). However, where further distribution (beyond the audience specified in (11) is possible, a wider announcement audience may be selected.)) Unlimited DRDC Valcartier TM

42 13. ABSTRACT (A brief and factual summary of the document. It may also appear elsewhere in the body of the document itself. It is highly desirable that the abstract of classified documents be unclassified. Each paragraph of the abstract shall begin with an indication of the security classification of the information in the paragraph (unless the document itself is unclassified) represented as (S), (C), (R), or (U). It is not necessary to include here abstracts in both official languages unless the text is bilingual.) There are multiple test methodologies currently in use to assess the performance of current and novel protective equipment and to investigate the thoracic injuries caused by blast overpressure. Among these methodologies, there is the mannequin for the assessment of blast incapacitation and lethality (MABIL), an instrumented human upper body surrogate that can be used to assess the performance of personal protective equipment. The objectives of the current study were first to develop and validate with experimental data a finite element (FE) model of MABIL for the purpose of investigating and predicting its response to blast overpressure. The second aim was to compare the DRDC MABIL FE model responses with a human FE torso model. The MABIL mannequin and the human torso numerical models consist of a simplified two-dimensional slice taken at the mid-sternum level, and subjected to different blast loadings. Experimental data on MABIL captured during field trials were used to validate the numerical models. In general, the numerical chest wall acceleration, velocity and the viscous criteria were higher than the experimental ones. However, the ratio between the numerical and the experimental chest wall velocity was the same across the range of blast loads studied. 14. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Technically meaningful terms or short phrases that characterize a document and could be helpful in cataloguing the document. They should be selected so that no security classification is required. Identifiers, such as equipment model designation, trade name, military project code name, geographic location may also be included. If possible keywords should be selected from a published thesaurus, e.g. Thesaurus of Engineering and Scientific Terms (TEST) and that thesaurus identified. If it is not possible to select indexing terms which are Unclassified, the classification of each should be indicated as with the title.) Blast effects, numerical simulations, experimental trials, human torso, MABIL

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44 Defence R&D Canada Canada s Leader in Defence and National Security Science and Technology R & D pour la défense Canada Chef de file au Canada en matière de science et de technologie pour la défense et la sécurité nationale