Thermal and Failure Analysis of Industrial and Cricket Helmets

Transcription

1 Thermal and Failure Analysis of Industrial and Cricket Helmets 1 Aditya A. Deshmukh and 2 D.V.Burande, 1 U.G. Student, 2 Professor, 1,2 Department of Mechanical Engineering, Smt. Kashibai Navale College of Engineering, Pune, India Abstract- The document specifies the requirements and testing methods suitable to industrial and cricket helmets. The important parameters considered during designing a helmet are its shock absorption capacity and thermal comfort while wearing it. The performance of a safety helmet involves the shock-absorbing capacity of a striking object associated with a deformation of the shell and cradle, as well as an increase in the transferred force to the user's head. The protective helmets used in cricket now-a-days offer sufficient protection to players against impact. Discomfort due to heat is one of the cricket player s main complaints when wearing the helmets. Keywords: Helmet, Impact energy, Thermal comfort, Ventilation І. INTRODUCTION Headgear is widely used in both occupation and leisure; it is used as a fashionable accessory or as an optional/mandatory means of protection. Primary review of thermal properties and helmet comfort revealed the following similarities between cricket and industrial helmets: 1. The two helmets should protect the wearer s head against direct impact with an object; 2. Both workers and cricketers are exposed to extreme environmental conditions for a long duration of time; 3. Most users find the existing helmets are heavy, inadequately ventilated and uncomfortable; 4. Most international cricket games are played in environments with low air velocity similar to low building construction sites. Here, for the failure test of industrial helmet we use a methodology of testing shock absorption capacity. A helmet placed on a head form is hit by a 5 kg hemispherical striker freely falling from a height of 1 m. During the impact, the value of the force acting on the head form beneath the helmet is measured. According to the requirements, the maximum value measured of that force must not exceed 5 KN. As a result, the testing method involves a check on whether the safety of helmet sufficiently reduces the transferred force to the user's head during the impact of a moving object. In view of the above, the question arises of how the currently available industrial safety helmets behave during impacts exerted with higher energy. For the thermal analysis of cricket helmets, the main aim of this was to study the thermal properties of selected, cricket helmets, which are widely used in international cricket games in order to develop knowledge and understanding of the thermal behaviour of cricket helmets. Also, the newly acquired knowledge can be applied in future to improve thermal comfort of cricketers. This research aimed at quantifying 1. The net heat transfer, 2. The average temperature under the helmet, 3. Average radiant heat with the help of thermal imaging. The readings acquired were then used to describe the thermal properties of the helmets at normal conditions for low to moderate intensity activity games, where the ambient temperature was found to be 23 ⁰C and relative humidity was around 65%. The radiation, convection and total heat transferred between the helmeted head form and the surrounding environment, under steady-state conditions, were calculated. It is found that insight into the thermal distribution of the helmeted head form will allow design improvement of cricket helmets and this can lead to an improvement in thermal comfort. ІІ. TESTING OF INDUSTRIAL HELMET A. Testing method Due to the impact exerted by a moving object, also occurs the deformation of the shell and cradle of the For the impacts to the highest point of the shell and directed vertically downwards the most significant deformation occurs within the area of height X (Fig. 1). Due to the deformation of helmet elements, absorption of the impact of the striking object is observed, also an increase in the force transferred to the head form is seen. So to find the value of the energy absorbed by the helmet when hit by a moving object generating a force of F = 5 kn, the following equation was used (1) E 5 = 0 x5 F ( x) d x (1) B. Experimental Setup The setup for testing the industrial safety helmets mainly consist of test object, falling weight, monitoring system with sensors. The setup is shown in fig. 1 Figure 1: Setup for energy absorption of industrial safety helmets [2] Available Online@ 159

2 1 Monolithic base, 2 slide ways, 3 4 trolley, 5 hoist, 6 electromagnetic latch, 7 striker, 8 button for striker disengagement, 9 head form, 10 test helmet, 11 force transducer, 12 amplifier, 13 analog filter, 14 digital oscilloscope, 15 high speed camera, 16 computer, 17 device for measuring striker speed, 18 laser diode, and 19 marker on striker. In the figure shown above, (1) is the monolithic base on which the head form is placed. This base not only supports the stand of the system but also absorbs the shocks generated during the testing of the Vertical stand (2) has a trolley (4) which slides on the vertical bars of the stand on this trolley, a striker (7) is attached. Initially the trolley is locked magnetically by an electromagnetic latch (6), its vertical position is set by another trolley (3). This helps in gaining the desired kinetic energy for the striker. The position of this latch is varied using the push buttons (8) and it is also used to release the latch. The test helmet is positioned on the head form. The stand is fitted with electronic system for measuring the transferred force to the head form, which is achieved by a transducer (11) at the base. The axis of this transducer coincides with the axis of the striker. The transducer is connected to the display unit using an amplifier and filter circuit. The digital oscilloscope (14), records the time course of the force acting on the transducer (12). The second measuring system has a high speed digital camera (15), which records the displacement of the marker (19) attached to the striker surface (7) during the impact. The camera (15) is coupled with a computer (16), used for programming the desired mode for the camera, saving the images recorded by the camera, and to process them after they are saved. The camera is such positioned that the laser diode (18), the test helmet, and the striker (7) are in its line of sight at the moment of contact. The system measures the striker speed for the last 20 mm of its drop prior to impact on the helmet and simultaneously generates signals for the data recording by the oscilloscope (14) and switching on the laser diode (18). In this way the data recorded by the oscilloscope and by the camera (the displacement of the striker) is achieved in synchronous with each other. The industrial test helmet used is shown in fig. 2 exceed 5kN. The damages caused to the helmet at three temperatures are very less that is there no perceptible damage was caused. The F vs. x characteristics which were obtained for the above test is as follows in the fig. 3 Figure 3: Sample F( x) characteristics of helmet based on test results obtained by conditioning at 30⁰ C, +20⁰ C, and +50 ⁰C. [2] D. Summary of the test results During the test, the value of impulse recorded was about 10KN for the The steep increase in the graph for all three temperatures shows that the contact of internal surface of shell and cradle has occurred that means all the force has transferred to the user's head. In this case the helmet broke at the connecting junctions of the cradle to the shell. Due to increase in conditioning temperature, the capability of the helmet to absorb the energy was reduced. Material softening occurred due to increased temperature which was found to affect both, the shell and the cradle. Due to this the contact of internal surface of shell and cradle took place at lower force values. ІІІ. TESING OF CRICKET HELMETS A. Experimental setup The set-up used in this study consists of a slim rectangular flat plate and a timber plank, on which the aluminium manikin head form was securely mounted (Fig. 4). Figure 2: Section of an industrial safety helmet 1 shell, 2 cradle, X distance between the internal surface of shell and cradle, and F force acting during an impact. [2] C. Test Results The test was carried out at three different temperatures -30, 20, 50. The height of the striker was so adjusted that the force acting on the head form would not Figure 4: Experimental set-up. [1] Two insulated pipes were fitted to the head form. One of the two pipes was connected to the water boiler and another pipe was connected to a water pump through which used water was Available Online@ 160

3 removed from the head form and again supplied to the water boiler that mean the head form, water boiler and the pump are in closed loop. An infrared camera which is used to capture the thermal images of the head form was installed in front of the head form to capture its images. For the first hour of the test, the warm water was continuously fed to the head form so that the steady state was achieved. The temperature on the surface of the head form was maintained within the range of 34.9 ± 0.1⁰C so that it matches the actual skin temperature. To measure the rise in head form temperature total 8 k-type thermocouples with an accuracy of ±1.5⁰C or ±0.25⁰C were used which were located to the right side of the head form as shown in the fig. 5. above is dotted with small 96 round holes (diameter 5 mm, approximate total surface area of the ventilation holes is 1884 mm^2) which allows airflow through the outer layer and allows final dissipation of heat to the environment. 2. The Elite helmet is made of a polypropylene moulded shell, has four radial openings across the shell, and a small, cross shaped opening at the crown. (Total surface area of the ventilation opening is 3460 mm^2.) 3. The Premiere helmet is prepared using a high-impact injection-moulded shell, which has three circular holes (diameter 13 mm) surrounding the crown area to facilitate ventilation. (Total surface area of the ventilation opening is 950 mm^2.) 4. The Masuri helmet is made using a strong carbon-fibre reinforced shell, has two circular holes (diameter 11 mm) on each side of the crown to facilitate ventilation. (Total surface area of the ventilation holes is 190 mm^2.) 5. The Ultimate helmet includes three circular vents (diameter 16 mm, total surface area of the holes is 600 mm^2) located at the front and both sides of this carbonfibre reinforced polymer shell. C. Results 1. Thermal distribution The heat exchange between each helmet was depicted by using a graph shown in fig. 7 Figure 5: Thermocouple sampling zones. [1] Total five types of helmets were selected for the study. B. Helmet Details: Following helmets were used for the study (Fig. 6): Figure 6: Details of helmets used in the study [1] 1. The NXT helmet contains two layers of shell: the inner layer has linear cuts (length 20 mm) from front to back, to increase the airflow through the helmet; the layer Figure 7: Temperature measurements on Al. head form [1] (1) Phase 1 (0-2 min): temperature of head form without (2) Phase 2 ( min s): temperature of head form with helmet properly fastened and steady state attained. (3) Phase 3 (30-45 min s): temperature of head form after the helmet was removed. It can be clearly seen from the graph that once the helmets were put on then the temperatures started to increase beneath the helmet for about 8 min s, then during the test periods they remained stable. The recorded temperatures increased by an average of 1.4 C for all the helmet types. The highest temperature increase was found at the Location 8 where on average, the temperature increased by 1.7 ± 0.1 C, followed by Position 7 were the temperature increased by about 1.4 ± 0.1 C. The temperatures at the forehead area i.e. position 3 incremented by 1.2 ± 0.2 C. The average temperature increment at position 4 was 0.5 ± 0.1 C and was Available Online@ 161

4 least than the other locations. The recorded temperature measurements showed differences among the various types of 2. Thermal imaging: The infrared camera used showed us the images of the surface temperatures on the helmets tested. Repeatability of surface temperature was determined and the standard deviation of the temperature measurement was 0.22 C. The obtained results are shown in the fig. 8 The formula used for the total heat transfer is given by, q = q conv + q rad q = h.a(t sk T sur ) + ε.a.σ(t 4 sk T 4 Hei) (2) Where, q conv (W) is the convection heat transfer given by Newton s law of cooling and q rad (W) is the radiation emitted from a surface at a thermodynamic temperature given by the Stefan-Boltzmann law. T sk (K) is the surface temperature of head form, T Hei (K) is the mean radiant temperature of the helmets, T sur (K) is the temperature of surrounding A is the surface area of the head form (estimated to be m 2 ), σ = Stefan-Boltzmann constant = 5.67 * 10-8 Wm -2 K -4, and ε = emissivity of the head form (determined as 0.98). D. Design suggestions: For the obtained results, temperatures in the parietal and the frontal regions were higher by about 1.5 C. This was probably because most of the test helmets did not have ventilation holes at these regions. Following are the suggestions to improve the ventilation. 1. Inserting circular vents: It is believed that the general thermal comfort of the helmet can be improved by increasing round ventilation holes at the parietal and forehead zones to allow cooing air to circulate through the helmet as shown in fig.10. As cricket is mostly played between 11 and 5 pm, when the sun is high in the sky, larger ventilation holes could cause more heat gain from the hot sun through radiant heat. Figure 8: (left) Thermal images at the start of 2-30 min phase, (right) thermal images after 30 min. [1] Thermal imaging showed temperature variations on the surface areas for the same helmet, depending on the design and construction of ventilation holes of the helmet, where heat tended to radiate through the vents; the mean temperatures near the ventilation holes were approximately 1-2 C higher than surface areas that are mm away from the holes. Figure 10: Circular ventilation holes to improve heat transfer according to hotspots (highlighted in black dots). [1] 2. Inserting grooves: Several parallel grooves with several vents in each groove could be added to provide space or channels for air to flow. Ventilations holes or slots are need in a minimum of three positions: in the front and rear of the helmet for air to flow in or out of the helmet, and on the top of the helmet to take advantage of the convective mode of heat transfer, in which warm air rises and is replaced by cooler air that flows into the helmet via rear and front vents as shown in fig. 11 Figure 9: Mean values for heat transfer for the helmets. [1] The values for the heat transfer for all five helmets is shown below with the help of a bar chart (fig. 9) Figure 11: Parallel grooves to improve airflow according to hotspots (highlighted in black dots) [1] Available Online@ 162

5 3. Insert additional pads: Additional pads may be added to the inside surface of the helmet which will allow more customization to ensure a snug fit, while creating a gap or space between the helmet and the wearer s head in improve ventilation. Comfort foam of mm thickness provided along the circumference sides of the helmet only, but not on the top as shown in fig. 12. Figure 12. Additional pads to create air gap and space to improve ventilation.[1] 4. Insert suspension straps: The suspention straps are positioned perpendicular to each other so that they intersect crown of the head. These increase air gap and to promote airflow between helmet shell and head, thus increasing the potential for heat dissipation from the head as shown in fig. 13 Figure 13: Suspension straps to increase air gap and to promote airflow. [1] CONCLUSION The article gives detailed experimental study of failure test of industrial safety helmet and heat transfer and its influence on the thermal comfort in cricket The following conclusions can be drawn from this study: 1. Failure test of industrial safety helmets: With increasing temperature the capability of the helmets to absorb impact energy decreased. This was mainly due to softening of both the shell and the cradle materials. This phenomenon actually reflects the actual conditions of use at worksites. These effects can be reduced if helmet shells and cradles are produced using materials whose mechanical characteristics do not dependent on temperature. The deterioration of the mechanical strength of helmet elements, such as attachments connecting the shell to the cradle, can also be responsible for reduced capability to absorb impact energy. Strength deterioration, breaking of elements, occurs mostly at low temperatures. 2. Thermal analysis of cricket helmets: The total heat transfer from the head form to the surrounding environment decreased, on average by 15%, after putting on a cricket helmet; hence, the helmets resists the heat dissipation from the head. Experimental results confirmed that the helmets cause head temperature to rise by 1.5 ± 0.1 C around the frontal and parietal regions, which can cause discomfort. Thermal imaging results showed that the helmets tended to radiate heat through the side rim and ventilation outlets depending on their design and construction. i. By inserting circular vents at the parietal and forehead zones to allow cooing air to circulate through the ii. By inserting four grooves from front to the rear of the helmet with two centrally located and the remaining two on each side. iii. By inserting additional pads throughout the circumference of the iv. By inserting suspension straps fastened respectively at the opposite ends of the circular ring-shaped head strap and intersecting each other at the crown of the head. Acknowledgment I am very thankful to those who have helped me in completing this study. I am also thankful to the authors of the references I have referred, as their work has greatly helped and influenced me. I am extremely grateful to these authors. I am also thankful to Prof. D.V. Burande for his support and guidance. References [1] Toh Yen Pang, Aleksandar Subic, Monir Takla, A comparative experimental study of the thermal properties of cricket helmets, International Journal of Industrial Ergonomics, vol. 43 (2013) , January [2] Krzysztof Baszczynski, The effect of temperature on the capability of industrial safety helmets to absorb impact energy. Engineering Failure Analysis, vol. 46 (2014) 1 8, July [3] Cornelis P. Bogerd, Jean-Marie Aerts, Simon Annaheim, Peter Brode, Guido de Bruyne, Andreas D. Flouris, Kalev kuklane, Tiago Sotto Mayor, Rene M. Rossi, Areview on ergonomics of headgear: Thermal effects, International Journal of Industrial Ergonomics, vol. 45, pp 1-12, November 2014 Available Online@ 163