Application of polyethylene air-bubble cushions to improve the shock absorption performance of Type I construction helmets for repeated impacts

1. Introduction

Traumatic brain injury (TBI) is one of the leading causes of death and disability in North America [1]. The data from the Ontario Trauma Registry [2] showed that approximately 7.3% of TBI cases were work-related. Work-related traumatic brain injury (WrTBI) accounted for approximately 20% of traumatic occupational injuries and 60% of work-related deaths in the state of Washington [3]. In the United States, WrTBIs have been identified as one of the most serious workplace injuries among all work-related injuries from 1998 to 2007 [4,5]. Most WrTBI cases have occurred in the construction industry [6,7]. Epidemiological studies indicated that industrial or construction workers who were wearing safety helmets were less likely to sustain an intracranial injury compared with workers not wearing a safety helmet [2]. The use of helmets has been considered to be one of the important prevention strategies on construction sites [8]. Shock absorption performance is a construction helmet’s most important performance parameter.

Industrial or construction helmets are categorized as Type I and Type II, according to international standards [9–11]. The Type I construction helmet is the most commonly used helmet model in the construction and manufacturing industries. A Type I helmet is designed to provide top impact protection. These helmets typically consist of a polyethylene or polycarbonate hard shell and a suspension system. The helmet’s suspension system is an essential component for shock absorption. Therefore, research and development efforts to improve helmets’ shock absorption performance have been mainly focused on the improvement of the suspension system [12,13].

Although it is not regulated by OSHA or in any test standards, it is well-accepted and recommended by helmet manufacturers (e.g., 3M Occupational Health and Environmental Safety [14], MSA Safety [15], Columbia Safety and Supply [16], and ED Bullard Co [17]) that an industrial helmet should be disposed of when it is subjected to a significant impact. However, in real workplaces, it is likely that industrial helmets are abused and subjected to multiple impacts, causing structural damage to the helmets. Construction helmets are not required to be tested with repeated impacts by international standards [9–11]. A previous study [18] indicated that an industrial helmet will experience structural damage under repeated impacts of a significant impact magnitude. A new parameter, “the endurance limit”, has been proposed to describe the helmet’s shock absorption performance for repeated impacts [18]. If a helmet receives repeated impacts of a magnitude greater than the endurance limit, it will experience cumulative structural damage, resulting in a degradation of the helmet shock absorption performance [18]. Improvement of the helmets’ endurance limit is important to improve workers’ safety. However, the helmets’ endurance performance under repeated impacts has not yet become a design consideration by helmet manufacturers or in standardized tests.

Air cushions have the advantages of being lightweight and low cost, together with their unique mechanical performance, compared with other conventional materials, such as rubbers and polymers. Air cushions have been widely used where humans interact with the equipment/environment [19–21]. For example, air cushion soles have been widely used in shoes to improve shock absorption performance and comfort [22]. In air-cushioned gloves, the finger segments are cushioned by separated air bubbles to absorb vibrations transmitted to the hand [23,24]. In packing industries, air-bubble cushioning sheets have been widely used. Compared to other packing materials, the air-bubble wrapping has the advantages of excellent shock absorption characteristics, being lightweight and insensitive to climate conditions (e.g., temperature and humidity), and high flexibility [25]. Malasri et al.’s [26] tests showed that bubble wrapping sheets (with air bubble sizes 5 mm and 8 mm) helped to reduce about 34% of impact acceleration compared to a viscoelastic foam wrapping. An air-bubble wrap sheet, which is widely used for packing materials in industries [27], is usually made of low-density polyethylene (LDPE) film with a shaped side bonded to a flat side to form air bubbles. Air-bubble wrapping sheets are commercially available in different thicknesses, bubble sizes, and bubble densities. The air bubble size can be as small as 6 mm to as large as 25 mm in diameter. The most commonly used air-bubble wrapping sheet has a bubble diameter of 10 mm [25]. Since air-bubble wrapping sheets are commercially available and can easily be cut to fit into any desired shape and space, we proposed to test an air-bubble cushioning in helmets using commercially available air-bubble wrapping sheets.

The suspension systems of construction helmets have different designs and have utilized different shock absorbing materials. Despite widespread adoption of air-bubble cushions in ergonomic designs and in commercial packaging as shock absorption materials, an air-bubble cushion has never been used in industrial helmets. The goal of this study was to test if air-bubble cushions can be used to improve the shock absorption performance of Type I construction helmets subjected to repeated impacts. Our hypothesis was that the air-bubble cushion, when added to a typical Type I construction helmet, will effectively reduce the impact forces transmitted through the helmet during repeated impacts and that the effects of the air-bubble cushioning will be dependent on the magnitude of the impact force. A representative basic Type I construction helmet model and popular air-bubble wrapping sheets were selected for the study. Top drop impact tests were performed at different drop heights with an impactor mass of 3.6 kg. The knowledge obtained in this study could be useful to improve Type I helmet design and the existing helmet test standards.

2. Methods

2.1. Experimental setup

Helmet impact tests were performed according to the Type I impact protocol as outlined in the ANSI Z89.1 standard (ANSI/ISEA Z89.1, 2014) [9]. The free-fall impactor impacts onto the fixed helmet, as illustrated in Fig. 1A. The experimental set-up is similar to those used in our previous studies [18,28,29]. A commercial drop tower test machine (H.P. White Laboratory, Street, MD, USA) was used in the tests. An aluminum impactor (mass = 3.6 kg) was raised to a predetermined height, dropped freely in a guided rail, and impacted onto the helmet at the top of the crown. The helmet was fixed on an aluminum headform (mass = 3.64 kg). The transmitted impact forces were measured via a force sensor (Model 925M113, Kistler, Amherst, NY, USA) installed between the base plate and the headform (Fig. 1B). The accelerations of the impactor were collected via a single axial accelerometer (Model 357B03, PCB Electronics, Depew, NY, USA) installed near the impactor’s mass center (Fig. 1A). Both force and acceleration data were collected at a sampling rate of 25 kHz. The velocity of the impactor immediately before impact was measured via an optical sensor built into the drop tower system.

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Fig. 1.

Experimental set-up. (A) The control parameter was the drop height and the transmitted forces through the helmet at the base of the headform were measured. (B) Test Group A consists of original Type I helmets. (C) Test Group B consists of Type I helmets equipped with an air-bubble cushioning liner between the suspension and headform. A basic Type I construction helmet model was selected for the drop impact tests.

2.2. Test procedure

Typical off-the-shelf Type I basic construction helmets were used in the study. The effects of the air-bubble cushioning on the helmets’ shock absorption performance were evaluated via two Test Groups, A and B (Fig. 1B-C). Test Group A contained original, unmodified helmet samples. They served as a reference group. Helmets in Test Group B included helmets equipped with air-bubble cushioning liners. Commercially available air-bubble cushioning wrap sheets (Blue Hawk, Gilbert, AZ) were used for the cushioning liner. Air-bubble sheets had a dimension of 30.5 cm × 30.5 cm (1 ft × 1 ft), as illustrated in Fig. 2. At an undeformed state, an air bubble had a diameter of approximately 9 mm and a height of approximately 4 mm. The air-bubble cushioning liner consists of two layers of air-bubble cushioning wrap sheets, with their bubble-sides being placed against each other. The air-bubble cushioning liner had a thickness of approximately 5 mm at an undeformed state. The air-bubble cushioning liner was wrapped on the headform and the helmet was placed onto the wrapped headform (Fig. 1C), such that impact force was transmitted to the headform through the air-bubble cushioning liner.

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Fig. 2.

Air-bubble cushioning structure. The air-bubble cushioning consists of two layers of commercially available air-bubble packing sheet (1-foot width), with the side of the bubbles against each other. The air-bubble cushioning has a thickness of 5 mm at an undeformed state.

The drop impact tests were performed at four drop heights: 0.61, 1.52, 1.63, and 1.73 m. For the tests in Group A, a new helmet was placed on the headform and was impacted ten times at each of the predetermined drop heights. The test procedure for Test Group B was the same as that for Test Group A. A new helmet and a new air-bubble cushioning liner were used for the repeated impacts at each of the predetermined drop heights. In order to protect the test equipment, the drop impact test was terminated once the peak transmitted force exceeded 20 kN. After the repeated impact tests, the helmets and the air cushioning liners were visually examined for structural damage. Four replicate trials were performed at each of the drop heights for both Test Groups A and B. A total of 32 helmet samples (i.e., 4 drop heights × 4 replicates × 2 Test Groups) were used.

Before data collection, a pre-conditioning process was performed for each of the helmets [18,28]. During pre-conditioning, a helmet was placed on the headform, as in the impact test, and impacted three times by the impactor at a drop height of about 10 cm (4 in). The transmitted force to the headform during the pre-conditioning was monitored. The force-time histories of each helmet during the pre-conditioning were examined to make sure that the helmets and/or the air-bubble cushioning liners reached a “steady state” before the repeated impacts were applied. The collected data of the transmitted forces became more repeatable after the pre-conditioning impact treatment.

3. Results

The representative time-histories of the transmitted forces for Test Group A (original helmets) and Test Group B (helmets with air-bubble cushioning liners) for ten repeated drop impacts and for three different drop heights, 0.61 m, 1.52 m, and 1.63 m, are shown in Fig. 3A-B, 3C-D, and 3E-F, respectively. The left-column plots (Fig. 3A, C, and E) are the representative results for Test Group A and the right-column plots (Fig. 3B, D, and F) are the representative results for Test Group B. The results show that at a lower drop height (h = 0.61 m, Fig. 3A-B), the measured transmitted forces varied little and were well under the force limit F = 4.45 kN for ten repeated impacts (n = 1–10) for both Test Groups. The helmet test was considered a failure if the peak transmitted force was greater than 4.45 kN. For Test Group A at drop heights of 1.52 m and 1.63 m (Fig. 3C and E), the peak transmitted forces (F _max ) were smaller than 4.45 kN for only the first two drop impacts (n = 1–2), and the helmets failed after the third drop impact (n ≥ 3). In comparison, Test Group B helmets survived ten repeated impacts (n = 1–10, F _max < 4.45 kN) with a drop height of 1.52 m (Fig. 3D) and nine repeated impacts (n = 1–9, F _max < 4.45 kN) with a drop height of 1.63 m (Fig. 3D). The helmets finally failed at the last drop impact with a drop height of 1.63 m (n = 10, Fig. 3D).

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Fig. 3.

The representative recorded time-histories of the transmitted forces for three different drop heights (h = 0.61 m, 1.52 m, and 1.63 m). The plots of the left column (A, C, and E) are results collected from a representative sample in Test Group A (original helmets). The plots of the right column (B, D, and F) are results collected from a representative sample in Test Group B (helmets with air-bubble cushioning liner). F = 4.45 kN is the maximal allowable transmitted impact force to pass the ANSI Z89.1.

For each of the time histories of the transmitted forces obtained in the experiments, we determined the maximal peak force values. The mean peak transmitted forces for the four different drop impact heights (0.61 m, 1.52 m, 1.63 m, and 1.73 m) and for ten repeated impacts for Test Group A are compared with those for Test Group B in Table 1. The helmets failed when F _max > 4.45 kN, as shown by the highlighted values in the table. When the recorded force values were greater than 20 kN, the tests were terminated, as the forces were beyond the capacity of the force sensor equipped in the drop tower system. It is seen that the standard deviations are very small (mostly SD < 0.5 kN) when F _max < 4.45 kN; once the helmets failed (F _max > 4.45 kN), the standard deviations became larger (mostly SD > 2 kN).

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Table 1

The mean peak transmitted forces (F _max in kN) for Test Groups A (original helmets) and B (helmets equipped with air-bubble cushioning liners) for different impact numbers and different drop heights (h in m). The values shown are means of four replication tests. The impact tests were stopped once the measured peak force values reached 20 kN. The highlighted force values are higher than the force limit of 4.45 kN, which is the maximum allowable value to pass the standardized test ANSI Z89.1

For the helmet to pass the ANSI Z89.1 standard (ANSI/ISEA Z89.1, 2014) [9], the peak transmitted impact force must be less than 4.45 kN when impacted by an impactor (mass = 3.6 kg) at a velocity of 5.5 m/s. In our test set-up, the drop height of 1.73 m resulted in an impact velocity of 5.5 m/s. The peak transmitted force at h = 1.73 m for the original helmets was 2.94 (±0.66) kN. The measured peak transmitted forces of the tested helmets were only about 66% of the maximal allowed force value (4.45 kN), indicating the quality of the helmets is acceptable.

The peak transmitted forces as a function of impact number for each drop impact height (i.e., 0.61 m, 1.52 m, 1.63 m, and 1.73 m) are plotted in Figs 4A, B, C, and D, respectively. At a lower drop height (h = 0.61 m, Fig. 4A), the air-bubble cushioning liners had little effect on the helmets’ shock absorption performance. The peak transmitted forces as a function of impact number for Test Group A are identical to those for Test Group B. The peak transmitted force varied little with increasing impact number for both Test Groups and they were well below 4.45 kN. At a drop height of 1.52 m (Fig. 4B), the mean peak forces and the corresponding standard deviations for Test Group A increased with increasing impact numbers. After the second drop impact, the peak forces for Test Group A became greater than 4.45 kN. However, the peak transmitted forces for Test Group B varied little with increasing impact numbers for ten repeated impacts. For tests with drop heights of 1.52 m and 1.73 m (Fig. 4C and D), the helmets in Test Group A could withstand only the first impact and they failed starting from the second impact (n ≥ 2). In comparison, the peak transmitted force for helmets in Test Group B started to increase only after the 9th and 6th drop impact at drop heights of 1.63 m and 1.73 m, respectively.

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Fig. 4.

The peak transmitted forces as a function of impact number tested at four different drop heights for Test Group A (original helmets) compared with those for Test Group B (helmets with air-bubble cushioning liners). (A) h = 0.61 m. (B) h = 1.52 m. (C) h = 1.63 m. (D) h = 1.73 m. F = 4.45 kN is the maximal allowable transmitted impact force to pass the ANSI Z89.1.

The peak transmitted forces (F _max ) as a function of drop impact heights (h) for ten impacts are shown in Fig. 5A and Fig. 5B for Test Groups A and B, respectively. For Test Group A, the peak transmitted force (Fig. 5A) did not vary with increasing impact number at a drop height of 0.61 m. At drop heights ≥ 1.52 m, the peak transmitted force increased dramatically with increasing impact number and the helmets failed after the second drop impact (n ≥ 2). For Test Group B, the peak transmitted force (Fig. 5B) did not change during the ten repeated drop impacts at drop heights of 0.61 m and 1.53 m; the peak transmitted forces increased gradually with increasing impact number at h ≥ 1.63 m.

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Fig. 5.

The peak transmitted forces as a function of drop height for ten repeated impacts. A: Test Group A (original helmets) B: Test Group B (helmets with air-bubble cushioning liners). F = 4.45 kN is the maximal allowable transmitted impact force to pass the ANSI Z89.1.

The shock absorption performance for repeated impacts is more clearly demonstrated by the dependency of the peak transmitted force (F _max ) on the impact number (n), as illustrated in Fig. 6. The peak transmitted force (F _max ) for four drop heights is plotted as a function of impact number in Fig. 6A and Fig. 6B for Test Groups A and B, respectively. For Test Group A (Fig. 6A), the peak transmitted force did not vary with increasing impact number at h = 0.61 m, but it increased with increasing impact number at h ≥ 1.53 m. For Test Group B (Fig. 6B), the peak transmitted force varied little with increasing impact number at drop heights from 0.61 to 1.52 m. The peak transmitted force increased only at the 10th drop impact at h = 1.63 m, and began to increase gradually with increasing impact number starting at the 6th impact at h = 1.73 m.

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Fig. 6.

The peak transmitted forces as a function of impact number for four different drop heights. A: Test Group A (original helmets) B: Test Group B (helmets with air-bubble cushioning liners). F = 4.45 kN is the maximal allowable transmitted impact force to pass the ANSI Z89.1.

Shock absorption performances of air cushions have been widely recognized in many applications, such as air beds, shoes, seats, and other ergonomic designs. However, air-bubble cushioning has not been considered in the design of construction helmets. In the current study, we tested the effects of an air-bubble cushioning liner on the shock absorption performance of Type I construction helmets subjected to repeated impacts. Our results show that the endurance for a typical helmet under repeated impacts was improved substantially by adding an air-bubble cushioning liner. A typical construction helmet can be subjected to repeated impacts of a magnitude of approximately 22 J (a drop height of 0.61 m) without compromising its shock absorption performance. In comparison, the same construction helmet when equipped with an air-bubble cushioning liner can be subjected to repeated impacts of a magnitude of approximately 54 J (drop height 1.52 m) without losing its shock absorption performance. Using the concept of a helmet endurance limit for repeated impacts as proposed in a previous study [18], the current results indicate that the helmet’s endurance limit has been increased from 22 J (drop height 0.61 m) to 54 J (drop height 1.52 m)– an increase of 145% in shock absorption performance, by adding an air-bubble cushioning liner.

Our results showed consistently that the recorded peak force (F _max ) data have a narrower scattering with the relative standard deviation (ST∕F _max ) values being mostly smaller than 2% (Table 1), when F _max < 4.45 kN, which is the maximal allowable transmitted peak force specified in ANSI Z89.1 standard [9]. Once F _max > 4.45 kN, the standard deviation of the test data becomes substantially larger, with the relative standard deviation (ST∕F _max ) values mostly greater than 25% (Table 1). In addition, our results showed that the standard deviations for F _max results for group B tests are much smaller than those for group A tests. A low standard deviation of F _max data means that the F _max value is predictable within a small margin for a given drop height, whereas a large standard deviation for F _max results mean that it is not possible to precisely predict the F _max value for a given drop height. Therefore, the standard deviations may indicate the mechanical stability of the helmets’ suspension system. An elevated standard deviation would suggest that the mechanical system becomes unstable, meaning that the mechanical characteristics of the system become less predictable and the helmets’ safety or reliability is degraded. Our results suggest that the mechanical performance of the helmets during impacts become more stable by adding air-bubble cushion liners.

Visual examinations of the tested helmets found that, for the helmets without an air-bubble cushioning, the conjunction sites between the suspension belts and hard shell were damaged after the tests. For the helmets with the air-bubble cushioning liners, the air-bubble cushioning sheets seemed to be damaged (i.e., air bubbles deflated), but no structural damage to the helmet suspension system was observed after the tests. In this current, we used commercial air-bubble cushioning sheets only for proof of concept. If the proposed approach is adopted by manufacturers, it would be suggested to helmet designers to apply customer-made air cushioning liners using durable materials, such as high-density polyethylene (HDPE) and polypropylene (PP). The helmets’ shock absorption performances would be expected to be improved further if custom-made air-bubble cushioning liners with durable and high strength membranes were used. In addition, commercial air-bubble packing materials are mostly made of LDPE, which is not designed for durable applications. They would likely lose their shock absorption performance after approximately six months. Customer-made air cushioning liners using durable materials would be expected to last five years.

The mean maximal transmitted force for the original construction helmets was 2.94 (±0.66) kN for an impact speed of 5.5 m/s (drop height 1.73 m), which is well below the maximal allowable value of 4.45 kN as specified in ANSI Z89.1 [9]. However, the helmets cannot withstand the repeated impacts of these impact force levels. For any drop height greater than 0.61 m, the helmets in Test Group A failed just after the second impact. The endurance limit of the original helmets is only around 22 J (drop height 0.61 m), much smaller than 61 J (impact speed of 5.5 m/s or drop height 1.73 m). That is why helmet manufacturers (e.g., 3M Occupational Health and Environmental Safety [14], MSA Safety [15], Columbia Safety and Supply [16], and ED Bullard Co [17]) recommend replacing an industrial helmet after a “significant” impact. In in situ conditions, the impact forces on the helmets are not monitored to make sure that the helmets do not experience impacts greater than the endurance limit. Workers may not have a way to know when the helmets should be replaced and when they are still safe to use. If damaged helmets are used, workers will be at a risk for injury. Increasing the endurance limit of the helmets may help reduce workers’ injury risk. According to our observations, the helmets that are equipped with air-bubble cushioning liner failed when the air bubbles busted or deflated, however, the helmets did not show any structural damage. Therefore, the status of the air bubbles in the cushioning liner could also serve as an indicator of the helmet safety for continued use. If the air bubbles in the cushioning liner are busted or deflated, the helmet could be deemed no longer safe to use and therefore should be replaced.

Air-bubble cushioning sheets are known to have outstanding thermal insulation performances. A good construction helmet may help reduce the heat strain of construction workers during the summer [30,31]. Adding an air-bubble cushioning liner to a construction helmet may also improve its thermal insulation performance. This would be especially beneficial when the helmets are used in extreme weather conditions. However, we are only focused on the helmets’ mechanical performance in the current study. We have not evaluated the thermal insulation performances of the helmets. This aspect would be one of our future research directions.

It is well known that the mechanical behaviors of most engineering materials are defined based on strict physical parameters. For example, the strength, stiffness, and ductility of structural steel are characterized using its yield stress, Young’s modulus, and elongation, respectively. These physical parameters of common engineering materials are well defined and can be directly measured in materials tests. From a mechanical point-of-view, the air-bubble cushion, as used in the current study, is not a pure engineering material, but a complex composite structure. The mechanical performance of such a composite structure can only be characterized by observing how it interacts with the surrounding structure under external loadings. Technically, it is difficult to directly define the mechanical performance of a complex structure using well-defined physical parameters, as treated in testing traditional engineering materials. In the current study, the mechanical behaviors of the air-bubble cushion liners in the helmets have been characterized by evaluating how the air-bubble cushion liners affect the peak maximal impact forces of the modified helmets. In future study, when we have performed more comprehensive tests and studies, we may be able to design a more stringent approach to define the mechanical characteristics of the air-bubble cushioning liners in helmets.

A further limitation of the current study is that we tested only one selected helmet model from a particular manufacturer. The shock absorption performance of an air-bubble cushioning liner may be different for different helmets tested using this proposed approach. In addition, we tested the helmets only in top impacts. Type I helmets are not required to be tested for lateral impacts by any test standards [9–11]. However, if the proposed approach is applied to Type II helmet models, the shock absorption performances for lateral impacts should be evaluated. The shock absorption mechanisms for Type I and Type II helmets may be different. In a Type I helmet, the shock impact is absorbed by the belt suspension system, whereas the shock impact is mainly absorbed by the foam liner materials in Type II helmets [32]. It is not known if adding an air-bubble cushioning liner will also improve the shock absorption performance for Type II helmets. The proposed test methodology needs to be further verified using Type II helmets and also using Type I helmets by different manufacturers.

5. Conclusion

In the current study, we found that adding an air-bubble cushioning liner to a typical Type I construction helmet will substantially increase its endurance limit for repeated impacts. Our findings may help manufactures to improve the helmet designs, thereby reducing work-related traumatic brain injuries. The concept of the air-bubble cushioning liner may also be used for sports helmets to increase the shock absorption performance.