Assessment of Human Thermal Response when Wearing Body Armor

Transcription

1 Assessment of Human Thermal Response when Wearing Body Armor Yuan M. Q.*, Ji T. C., Jiang J. H., Qian X. M. Beijing Institute of Technology, School of Mechatronical Engineering, Beijing, China *Corresponding author ABSTRACT With the frequent occurrences of violent and terrorist incidents, the research and development of body armor is getting more attention. Thermal comfort of ballistic body armor directly affects users work efficiency and health conditions especially at high ambient temperature. Therefore assessment of human thermal response wearing body armors (BA) is essential in that it provides basis for BA development and ensure the policeman life safety when discharging duties. In this paper, the thermo-physical properties (i.e., thermal insulation and vapor resistance) of two BAs were assessed via conducting the thermal manikin experiment. An evaluation model to assess the subject thermal comfort was designed to estimate the changes of physiological indexes (i.e., core temperature). The effects of environment temperature, relative humidity, clothing thermal insulation, vapor resistance, and personal physiological conditions were analyzed. Ergonomics experiments were conducted with four healthy male undergraduates wearing the BA in a climate chamber. The core temperature and skin temperature were recorded when subjects conducting activities of different intensities. The evaluation was validated through comparing the simulated results with the ergonomics experiment results. The thermal manikin experiment shows the total vapor resistances calculated by the parallel method are higher than by the serial method; the thermal insulation is higher in room temperature, and smaller in high temperature; the vapor resistance increases with the chamber temperature increasing, and decreases with the chamber humidity increasing. Simulated results show the core temperature and skin temperature increase with ambient temperature increasing or relative humidity increasing. Fine agreement was achieved between the manikin experiment, the simulation and a serial of human experiment results. This paper provide a basis for assessing human thermal response when wearing ballistic body armors made from different materials, which maybe carry out a new approach for BA material development and comfort design. KEYWORDS: Ballistic body armor, thermal manikin, thermal comfort evaluation model, ergonomics experiment, hot environment. INTRODUCTION Ballistic body armor (BBA) vest provides upper body protection for a variety of personnel such as soldiers, security forces customs patrol, etc [1-3]. In the typical battles of 20th century, 75%-80% of fatal injury was caused by the high-velocity fragments of bullets [4]. The death rate was remarkably reduced in recent years since the ballistic body armor was applied. The application of ballistic body armor is not limited to the army, policeman, but used for diplomats, people with special duties etc. The market and development of customized and personal-used ballistic body armors are broadened widely. Maintaining the stable rectal temperature during work is the basic need for human beings. The body armor is an important media for heat exchange between human and the environment. The weight of BBA could reach 4-9 kg, depending on the protection level, which greatly inhibits the heat and sweat loss of human [5-7]. Wearing body armor when performing physical activity reduces convective and evaporative heat loss, in part because it reduces sweat evaporation and airflow on a substantial portion (> 50%) of the upper-body surface [8]. As special personal protective clothing, Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8), pp Edited by Chao J., Liu N. A., Molkov V., Sunderland P., Tamanini F. and Torero J. Published by USTC Press ISBN: DOI: /c.sklfs.8thISFEH

2 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) the heat and moist transfer properties of BBA are unique. Multi-layered protection material leads to heat stress, which results in human psychological exhaustion and fatigue [9]. Thus, it is important to study the heat and moisture transfer within the human-body armor-environment, to prevent the fatigue danger caused by heat strain and poor protection due to the low wearbility [10]. Thermal insulation and vapor resistance are two essential parameters to assess the clothing thermal comfort, which represent the abilities of heat and moist transfer of clothing [11]. To evaluate the BBA thermal comfort, it is of importance to investigate the thermal insulation and vapor resistance [12, 13]. It is demonstrated the study on BBA via ergonomics experiment is drawn more attention in recent years; however not too much research have been conducted to show the relation between the thermal comfort simulation model and the ergonomics experiment result, which could provide new means to evaluate the human physiological condition when the BBA is too heavy or the environment is too harsh to conduct the ergonomics experiment. In this paper, the thermal insulation and vapor resistance of two BBA were analyzed by conducting the thermal manikin experiment. The thermal manikin experiment results provide basic data for establishing the human thermal comfort evaluation model, which simulated results were confirmed by performing the ergonomics experiment. The thermal comfort evaluation model was then used to estimate the changes of human physiological indexes when subject wearing BBA. SUBJECTS AND METHOD Manikin experiment Experimental Device and Materials The thermal manikin Newton (Measurement Technology Northwest. Inc, USA) was used to measure the thermal insulation and vapor resistance of body armors. It has an averaged Asian adult size, with each zone individually controls the temperature, heat flux and sweating rate from the software ThermDAC [10]. Experiments were conducted in a climate chamber, in which the temperature and humidity (RH) could independently controlled at range of ( ± 0.5 ) and 30%-80% ( ± 5%), respectively. The size of the chamber is 6 m 5 m 2.7 m (length width height). Ten WZP-PT100 thermocouples (accuracy: ± 0.1 ) were distributed in the chamber to detect the air temperatures, and an AM-101 humidity sensor (± 3%) was placed in the center of the climate chamber, to measure the humidity. Two types of bulletproof body armor were used, meeting the demand of the China GA 141 Standard [14]. One is named as BA1, which protection layer is made of ultra-high molecular modulus weight polyethylene (UHMWPE) and aramid. Another one is named as BA2, which protection layer is made of alloy steel plates. Experimental Design and Calculation Method Thermal Insulation Experiment: Experiments were conducted in the climate chamber with air temperatures of 20, 30 and 40, and RH of 55%. According to ISO 9920 and ASTM F1291, the experiments were firstly performed with the manikin wearing well-fitted 0.1 mm thick knit cotton fabric, then performed wearing two BBA. A constant skin temperature (35 ) was used without sweating when measuring the thermal insulation. Each experiment lasted for 1 hour. Each measurement was repeated three times, the average value and standard deviation were obtained. Serial and parallel calculation methods were used to acquire the total thermal insulation. The thermal insulation of each zone is obtained: 788

3 Part III Explosion R T T = (1) skin, i amb ct, i, Qi where R ct,i is the localized heat insulation (m 2 K/W); Tskin,i is the localized skin temperature (K); T amb is the air temperature of the climate chamber (K); Q i is the localized heat flux (W/m 2 ). The total thermal insulations are calculated by serial and parallel calculation methods [15]: R = ( R a ), (2) R serial parallel where: N i= 1 ct, i i = 1 ct, i i N ai = 1, (3) R Rserial and Rparallel are the total thermal insulation calculated by serial and parallel calculation method, respectively. N is the total number of the measured zones of the manikin. a i is the local zone surface area ratio. Vapor Resistance Experiment: Experiments were conducted with air temperatures of 20, 30 and 40, and RH of 40%, 55% and 70%. The constant sweating mode was selected, with the sweating rate of ml/hm 2. The ThermDAC output was selected and processed. The vapor resistance was calculated as: R et, i Pskin Pamb = Qi ( T T ) / R skin amb ct, i, (4) where: R is the localized vapor resistance, (kpa m 2 /W), P et,i amb and P skin are saturated vapor pressures of the climate chamber and the skin surface of the thermal manikin, respectively (kpa): ( ( T )) / 235+ skin Pskin = , (5) ( ( T )) / 235+ amb Pamb = RH , (6) Simulation model Assumptions The model was built based on the assumptions: (1) the ambient temperature is constant; (2) human central core and the skin are two concentric cylinders; (3) the heat transfer in the direction of parallel to the fabric is not considered; (4) the wind speed in the simulation is 0 m/s. (5) the core temperature and skin temperature in human thermal balance are set as 36.1 and 34.1, respectively. Heat transfer between human body and the environment Maintaining a stable core temperature is dependent upon the balance between the heat produced from human body metabolism, the heat gained from the environment, and the loss of heat by conduction, convection, radiation and evaporation [16]. The heat balance can be represented by the heat exchange equation: = ( ) ( ), (7) S M W RD CV E E sk res 789

4 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) where S is heat storage, M is the metabolic rate, W is the work accomplished, CV is the heat flow by convection at the skin surface, R D is the heat flow by radiation at the skin surface, E sk is the heat flow by evaporation at the skin surface, E res is heat flow by respiratory. 1) Metabolic rate, M, and work accomplished, W: Brake [17] et al. have measured the metabolic rate of human under different conditions: 40 W/m 2 during sleep; 58.2 W/m 2 during quietly sitting; W/m 2 during moderate work; W/m 2 during heavy work. Metabolic rate was set as 130 W/m 2 in this model following the ISO Standard 8996 [18]. Human bodyʼs work accomplished W is about 0~20% of the metabolic rate. It was set as 0.1M in this model. 2) The heat flow by evaporation at the skin surface E sk can be calculated by the following equation: Esk = Ediff + Ersw, (8) where E diff is the heat loss of insensible perspiration, which means the heat loss from the evaporation of the water through the skin; E rsw is the heat loss of sensible perspiration, which means the evaporation of sweat at the skin surface. 3) The heat flow by radiation, RD, and the heat flow by convection CV at the skin surface can be calculated [19] by the following equation: + = ( ), (9) RD CV h T T F f sk a cl c where h is the integrated heat transfer coefficient, which is the sum of radical heat transfer coefficient and the heat convection coefficient ( h= hr + hc ); T sk is skin temperature; T a is ambient temperature; F cl is costume area; f c is xerothermic transmission efficiency coefficient. According to ISO7399 [20], the costume area can be given as: F cl = R. (10) et Xerothermic transmission efficiency coefficient is used to measure heat transfer efficiency of radical and convection from external environment to human skin. The equation was first presented by Burton and Edholm[21]. c = ( + ), (11) f R R R F α et α cl where R a is the thermal resistance of the air cushion at the costume surface. = 1 ( + ). (12) R h h a r c 4) The heat flow by respiratory E res: Fanger [22] presented the equation of E res through numerous physiological experiments: 5 = ( ). (13) Eres M 5852 P φ α α Heat transfer between core and skin Gagge [23] regarded the human central core and the skin as two concentric cylinders, m sk is the mass of the thin skin shell, m cr is the mass of the core body. The heat flow from core body to skin can be given as: = ( ) ( ) ( ), (14) S M E W K T T c V T T cr res min cr sk bl bl cr sk where S cr is heat storage of core body; K min is minimum heat conductance of skin tissue, which is 5.28 W/(m 2 ); c bl is specific heat capacity of blood, Wh/(kg ); is rate of skin blood flow. 790

5 Part III Explosion The heat flow at skin surface can be given as: = ( ) + ( ) ( + ), (15) S K T T c V T T E RD CV sk min cr sk bl bl cr sk sk where: S sk is heat storage of skin. Core temperature, T cr, and skin temperature, T sk, are calculated as: = + ( (0.97 )), (16) T T A m S t st sk0 sk sk = + ( (0.97 )), (17) T T A m S t cr cr0 cr cr where A is the body surface area, T cr0 is core temperature in thermal balance, which is set as 36.6 ; T sk0 is skin temperature in thermal balance, which is set as Thermoregulation Mechanisms Thermoregulation of human body relies mainly on vascular contraction or expansion and sweating [24]. Blood flow rate can be calculated by equation (18) ( ) ( ) Vbl = Tcr Tcr Tsk Tsk0. (18) When the body temperature is adjusted through sweating, the sweating rate of sweat glands is influenced by core and skin temperature. And it is calculated by equation (19) [25]: ( ) rsw cr cr0 ( ) sk sk0 m 170 T T e T T /10.7 =. (19) Bullard et al. [26] found that sweat glands secretion was affected by skin temperature to a certain ( sk sk0 ) extent. And the effect coefficient was /3. So the heat loss of sweating evaporation is: E rsw 2 T T ( sk sk0 )/3 0.7 rsw 2 T = m T (20) where 0.7 is the latent heat evaporation of sweat. Based on the equations above, MATLAB is used for programing calculation. Inputs of the simulation model contain human basic physiological data, workload, temperature and relative humidity of environment, thermal insulation and vapor resistance of the protective clothing. Experimental Participants Four healthy young male participants were volunteered for the study. Participants were required to be physically active, without history of cardiovascular disease, neurological deficits, or musculoskeletal injury. Subjects had been told the procedures and relative potential risks, by providing written informed consent prior to the experiments. Participants were told to refrain from any caffeine, alcohol or exercise 24 h prior to experimentation. The mean participant characteristics were as follows. Age: ± 0.5 years; total body mass: ± 7.86 kg; height: ± 6.58 cm; BMI: ± Experimental Design Experiment was conducted in a climate chamber in which the ambient air temperature and relative humidity were set as 30 ± 1 and 50% ± 5%, respectively. Participants wearing the body armor BA1 were requested to walk on the treadmill with a speed of 3 km/h for 45 min. 30 min of rest was required before the experiment to obtain the baseline of subjectsʼ core temperature and skin temperature without BBA. The rectal temperature and skin temperature were measured by using a 791

6 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) rectal thermometer (± 0.2 ) and a thermocouple (PT100, ± 0.1 ), respectively. And all the results are averaged for 4 volunteers. RESULTS Thermal Insulation and Vapor Resistance (a) Total thermal insulation (b) Total vapor resistance Figure 1. Total thermal insulation and vapor resistance of BBA under different environment conditions. Fig. 1(a) shows the comparison of the total thermal insulation under different conditions. The total thermal insulation reaches m 2 K/W in 20, m 2 K/W in 30 and around 0.1 m 2 K/W in 40. The total thermal insulation decreases with the air temperature increasing. Fig. 1(b) shows the comparison of the total vapor resistances under different conditions. The total vapor resistance reaches m 2 Pa/W, m 2 Pa/W in 30, m 2 Pa/W in 40. Under the specific relative humidity (RH = 55%), the total vapor resistances increase with the ambient temperature increasing. However, the total vapor resistances decrease with the relative humidity increasing under the specific ambient temperature of 30. Results of the total thermal insulations and vapor resistances obtained from the serial calculation method are larger than those from the parallel calculation method. Core Temperature in simulation Fig. 2(a) shows the comparison of core temperature under different ambient temperatures from 20 to 40, with 55% RH. At 20, the core temperature nearly takes no change due to the relative high thermal insulation of the two BBA and the fact that human retains the low intensity activities (130 W/m 2 ). At 30, the core temperature increases progressively to around 38. At 40, the core temperature would reach 38.5 after around 40 minutes. Figure 2. Comparison of simulated core temperature when a subject wearing BBA; (a) different ambient temperatures at 55% RH; (b) The same ambient temperature but different RH. 792

7 Part III Explosion Fig. 2(b) shows the comparison of core temperatures under different environment conditions of RH. The core temperatures first present nearly the same growth trend, then rises differently in 0.2 hour. For BA1, core temperatures increase 1.16, 1.22, and 1.31 when relative humilities are 40%, 55%, and 70%, respectively. For BA2, core temperatures increase 1.46, 1.6, and 1.65 under the same environment conditions, respectively. In addition, it is shown that the core temperature growth of BA2 is generally higher than that of BA1 in Fig. 2. The vapor resistance and thermal insulation of BA2 is higher than that of BA1, so a heavier barrier was built on BA2 to hinder the vapor evaporation, causing higher core temperature. Skin Temperature in simulation Fig. 3(a) shows the comparison of skin temperature under different ambient temperature. At 20, human skin temperature decreases rapidly to around 33.4 within 10 minutes, then increases slowly to around 34 and remains steady. At 30, the skin temperature shows a rapid growth, reaching At 40 after 1.5 hours, the trends increases quickly in the first 15 minutes, reaching around 37. Slower trends are then presented that peak at 40 after 1.2 hours. Figure 3. Comparison of simulated skin temperature when wearing BBA; (a) different ambient temperatures at 55% RH; (b) The same ambient temperature but different RH. Fig. 3(b) shows the comparison of skin temperature when RH is different. Human skin temperature increases in 1.5 hours when wearing protective body armor. When wearing BA1, the skin temperatures grow to 37, 37.1, and 37.3 from 34.2 with the ambient temperature of 30 and RH of 40%, 55% and 70%, respectively. It grows to 37.5, 37.6 and 37.8, respectively, when the subject wearing sample BA2, which are higher than those of BA1. Model Validation Since the BBA are designed as vest-style, the skin temperatures of the chest and back sections are typical in the analysis. That is because this two part are completely covered by the armor. In this way, the thermal insulation and vapor resistance in the simulation are unanimous with that in the experiment. Skin temperatures obtained from the model, and the averaged values and corresponded standard deviations obtained from the experiments for the chest and back sections are shown in Fig. 4(a). The simulated skin temperature increases from 34.1 to in 0.75 h, while the experimental skin temperature from the chest section increases from to 36.38, and that from the back section increases from to 36.33, with averaged standard deviations of 0.68 and 0.59, separately. It is shown that the simulated results fit well with the experimental data, especially for the results from the chest section. It shows a temperature difference of 0.13 with the simulated results in 0.75 h. The core temperatures simulated by the model were compared to the rectal temperatures measured in the experiments, shown in Fig. 4(b). The increments of experimental rectal temperature and simulated core temperature are 0.3 and And the averaged standard deviations of rectal temperature is The simulation results agree well with the experimental data, where the maximum discrepancy 793

8 Proceedings of the Eighth International Seminar on Fire and Explosion Hazards (ISFEH8) is 0.55 from the very beginning. The phenomena could be explained due to the constant initial temperature settings (36.6 ) in the simulation model, which should be a variable when the ambient temperature is different. (a) Skin temperature (b) Core temperature Figure 4. Comparison between the simulated results and experimental results under the same environment condition (ambient temperature = 30 ± 1, RH = 50 ± 5%). DISCUSSION The much higher total thermal insulation and total vapor resistance indicate that wearing the body armor would cause heavier heat accumulation. The total thermal insulation decreases with the air temperature, since the thermal conductivity of the materials increases with the air temperature. With the ambient temperature increasing from 20 to 40, it gets gradually close then beyond the skin surface temperature, which results into reducing of the heat evaporation and diffusion. So the total vapor resistances increase with the ambient temperature increasing. Complete and proper functioning of the body is dependent on maintaining a body core temperature of between 36.5 to 38.5 [27]. The simulation indicates that under the hot environment (40, 55% RH), human core temperature would reach 38.5 after around 40 minutes when wearing BBA with low intensity activities. The normal core temperature of an adult during resting remains at , to ensure physiological comfort [28]. Thus an adult with a core temperature of 38.5 C indicates the very high possibilities of suffering a fever or hyperthermia [29, 30]. Thermal responses, such as, nausea, vomiting, headaches, low blood pressure and latter can lead to fainting or dizziness [31]. The effect of RH on human physiological conditions is partially dependent on the human perspiration status [32]. Heat exchange between the skin and the environment can be convection, radiation, and evaporation [33]. In the high temperature environment of 30, with the loss of sweat accumulating, the heat stored under the skin dissipates mainly by the evaporation of the sweat instead of convection or radiation. However the high RH of 55% and 70% in the environment prevents sweat evaporating, leading the heat stored under the skin accumulated and resulting to the situation that human core temperatures rapidly increase. Comparing to the core temperature, human skin temperature is more sensitive to the varying of the environment temperature. Human skin temperature remains at around [34] in comfortable conditions. The ambient temperature of 30 (when RH = 55%) beyond the range of thermal comfort temperature [35], thus lots of sweat is excreted and body heat is accumulated, resulting into skin temperature increment and discomfort on the subject. The ambient temperature of 40 beyond the range of hot temperature [36], of which is higher than the human skin temperature at the initial state. Thus the essential approaches of heat transfer between the skin and the environment are heat conduction and radiation, leading a rapid increase of skin temperature. After a short period (15 minutes), the heat accumulated under the skin is dissipated through the evaporation of the sweat, causing the skin temperature slower increment. 794

9 Part III Explosion Under the same ambient temperature but different RH(40%, 55%, 70%), the skin temperatures in simulation grow rapidly to in the first 12 minutes, since few sweat was produced at this initial stage and the vapor evaporation is slow, the environment RH show less effect on the skin temperature increment. With the increasing rate of sweat evaporating, skin temperature shows slower growth rate under the same RH. With the RH increasing, the effect of skin vapor evaporation is weakened and less heat is dissipated, leading to an increase of skin temperature. CONCLUSIONS The much higher total thermal insulations and total vapor resistances indicates that wearing the body armor would cause heavier heat accumulation. BA1 shows improved heat and moisture transfer properties due to the higher thermal conduction properties of the UHMWPE and aramid. The evaluation model of thermal comfort, which considers the environment condition, BBA thermophysical properties and human physiological responses were proved to be reasonable after compared with the ergonomics experimental results. The growth rates of core temperature and skin temperature in simulation increase with the ambient temperature or relative humidity when the ambient temperature is high (i.e., 30 and 40 ). Once the thermal insulation and vapor resistance are known the model could be applied to evaluate human thermal responses when wearing BBA. REFERENCES 1. Adams, J. D., McDermott, B., P., Ridings, C. B., Mainer, L. L., Ganio, M. S., and Kavouras, S. A. Effects of Air-Filled Vest on Exercise-Heat Strain when Wearing Ballistic Protection, The Annals of Occupational Hygiene, 58(8): , Lehmacher, E. J., Jansing, P., and Kupper, T. Thermophysiological Responses Caused by Ballistic Bullet- Proof Vests, Annals of Occupational Hygiene, 51(1): 91-96, Ricciardi, R., Deuster, P. A., and Talbot, L. A. Metabolic Demands of Body Armor on Physical Performance in Simulated Conditions, Military Medicine, 173(9): , Wei, H. J. Simulated Study of Defensive Capability of Composite Bullet-Proof Vest, Postgraduate Thesis, Shanghai Jiao Tong University, Ryan, G. A., Bishop, S. H., Herron, R. L., Katica, C. P., Elbon, B. A. L., Bosak, A. M., and Bishop, P. Clothing Adjustments for Concealed Soft Body Armor during Moderate Physical Exertion, Journal of Occupational and Environmental Hygiene, 12(4): , Larsen, B., Netto, K., and Aisbett, B. The Effect of Body Armor on Performance, Thermal Stress, and Exertion: A Critical Review, Military Medicine, 176(11): , Larsen, B., Netto, K., Skovli, D., Vincs, K., Vu, S., and Aisbett, B. Body Armor, Performance, and Physiology during Repeated High-Intensity Work Tasks, Military Medicine, 177(11): , Ryan, G. A., Bishop, S. H., Herron, R. L., Katica, C. P., Elbon, B. A. L., Bosak, A. M., and Bishop, P. Ambient Air Cooling for Concealed Soft Body Armor in a Hot Environment, Journal of Occupational and Environmental Hygiene, 11(2): , Parsons, K. C., Havenith, G., Holmer, I., Nilsson, H., and Malchaire, J. The Effects of Wind and Human Movement on the Heat and Vapour Transfer Properties of Clothing, Annals of Occupational Hygiene, 43(5): , Weng, W. G., Fu, M., Han, X. F., et al. Experimental Investigate of the Thermal Insulation and Evaporative Resistance of Protective Clothing on a Thermal Manikin in Hot Environment, J. THU. Uni. (Sci. and Tech.), Liu, Y., and Dai, X. Q. Measurement and Calculation of Clothing Thermal Resistance and Evaporative Resistance, China Personal Protective Equipment, 1: 32-36, Mahbub, R. F. Comfort and Stab-Resistant Performance of Body Armour Fabrics and Female Vests (Ph. D, Thesis), RMIT University, Zwolinska, M., Bogdan, A., Delczyk-Olejniczak, B., and Robak, D. Bulletproof Vest Thermal Insulation Properties vs. User Thermal Comfort, Fibres & Textiles in Eastern Europe, 21(5): , Standards Press of China. GA Police Ballistic Resistance of Body Armor, Beijing,

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