Since early 1999, the Canadian and U.S. organizations responsible for delivering science and technology to the humanitarian demining community have cooperated on several research and development (R&D) programs in the area of personal protection equipment (PPE). This paper presents an overview of the work performed cooperatively, focusing on key lessons learned during this joint effort.
This paper gives an overview of the R&D work performed by Canada and the United States since early 1999 to improve the personal protection of deminers. It should be said that this is still “work in progress” and that more contributions will be made in coming years. For clarity, the paper is divided in several parts that correspond to three broad phases within the overall program. First, work was done to understand the injuries to the lower extremities due to blast mines and the basic protection mechanisms that footwear should provide, and to define a test methodology to support future development of mine-protective footwear. The second phase saw the development of a test protocol to evaluate how well PPE protects the upper body of a deminer during excavation drills and applied this protocol to compare existing PPE. The third phase identified that small changes in body position could have a significant effect on the forces transmitted to the body. This led to the idea of mapping out the blast field of buried landmines and measuring the forces transmitted to the human body shape so that recommendations could be made to improve body position during excavation drills, or at the very least to advise deminers about what positions are detrimental. Finally, there was a need to determine the physiology of the injuries to the upper body, since this was poorly documented.
Mapping Out Field Injuries
In 1998, through the United States-sponsored consultation process, representatives of the humanitarian demining community wanted to look into the issue of PPE for deminers. It was suggested that priority be given to blast mines for accidents against the foot while walking or against the upper body during excavation drills. However, there were few solid facts to justify spending the limited R&D dollars in this way. There was a need to quantify the problem in terms of threats, activities most likely to cause a mine accident and the resulting injuries. It was necessary to map out the situation that actually prevailed in the field.
Fortunately, one individual, Mr. Andrew V. Smith, had been gathering demining injury information. In 1998, the U.S. Humanitarian Demining R&D Program decided to fund the efforts of Mr. Smith by contracting him to assemble the limited information he already had and to gather additional information. The resulting database1 consists of contributions from organizations in nine countries on four continents, totaling 232 accidents that resulted in 295 victims.
The deminer injury survey demonstrated that gathering field data was difficult, often because the data had not been acquired in the first place. The database nevertheless provided a much clearer picture of what was happening. The database indicated that the threat was definitely from APLs. APLs were involved in 79 percent of all accidents, accounting for 78 percent of all injured people and 81 percent of fatalities. The results from the survey definitely provided support that the R&D effort should focus on the APL threat.
|Figure 1: Activity being carried out at time of accident.|
Another important piece of information relating to the threat distribution was the ratio of APL accidents involving blast versus fragmentation landmines. Blast APLs were involved in 83 percent of the accidents, but only seven percent of the victims died from their wounds. On the other hand, 38 percent of the victims from fragmentation APLs died, nearly six times more than for blast APLs. This reflects the different nature of the threat. A blast APL is designed to maim its victims, thereby inflicting psychological as well as physical trauma to opposing forces. On the other hand, a fragmentation APL is designed to kill its victims and maximize the damage to opposing forces. This has a strong influence when selecting suitable PPE to defend against these threats. Another important fact regarding the deminer injury data was the very high incidence of the PMN blast landmine, which was involved in 66 out of 153 (43 percent) blast APL accidents. This ratio is unusual and is likely an artifact of the large contribution from organizations in Afghanistan to this database rather than a reflection of the prominence of this particular mine throughout the world.
Figure 1 provides some insight into the activity that was taking place at the time of accident, which was documented in over 94 percent of the cases. Excavation and Missed Mines accounted for 34 percent and 37 percent of all accidents, respectively. It should be noted that Missed Mines is not an activity on its own; it indicates that some mine clearance or minefield survey activity had not resulted in an area that was clear of landmines. This reflects either a failure of the detection equipment or human error in marking the extent of the minefield.
|Figure 2: Distribution of injury on the body for the two most frequent activities during accidents.|
It is useful to look into the type of injuries associated with the two categories that result in the most accidents. In particular, Missed Mine accidents yielded 3.5 times more leg injuries than Excavation accidents. This suggests that most of these accidents occurred as a result of stepping on a buried landmine and that injury could be reduced through better protection of the lower extremities. The data from Figure 2 also shows that a victim was 3.6 times more likely to suffer injury to the ears during an Excavation accident than during a Missed Mine accident. Injuries to the eyes, head and neck were also from 1.6 to 2 times more likely with Excavation accidents. This data suggests that special emphasis should be put on better protection for the upper body during excavation drills, particularly to the arms and head.
Figure 3 shows the position of the victims at the time of accident. It is immediately evident that this information is not always recorded, since body position was Unknown for 39 percent of the victims. It is interesting to correlate the numbers for body positions to the 78 Excavation accidents (34 percent of 232 accidents) that were reported.
|Figure 3: Position of body during accidents.|
The position most recommended during excavation is lying prone, but only 11 victims (3.7 percent) were reported to be in that position at the time of accident. The database indicates that deminers were 1.5 times and 3.2 times more likely to be kneeling or squatting at the time of accident, respectively. This agrees well with the information coming back from the field that deminers prefer the more upright positions because of comfort, better field of vision and generally because of improved ergonomics in carrying out prodding and soil-removal tasks. There might also be some cultural bias, as squatting is widely used in countries such as Afghanistan and Cambodia. The design of PPE must take this fact into account.
The survey results confirmed and quantified the feedback that had been coming from the field. It led to two main decisions in regards to the U.S. and Canadian2 R&D programs. First, the work would continue to concentrate on the blast APL threat; second, two principal areas of work would be addressed: protection of the lower extremities while standing and protection of the upper body when lying prone, kneeling or squatting during excavation drills. It was felt that focusing the R&D on those needs would yield the most benefits to deminers.
The Lower Extremity Assessment Program
The Lower Extremity Assessment Program (LEAP)3, 4 was born in 1998, before completing the deminer injury survey. It was designed to answer questions about the effectiveness of existing mine-protected footwear, but more importantly to document the process of injury to the lower limb due to landmine blast. It was generally agreed that by documenting and understanding this process, insight would be gained that might yield clues towards designing better protective footwear. LEAP was quite bold in its approach because it proposed to use human cadavers for this research. The LEAP data formed the most complete dataset about landmine injury to the lower extremities.
The LEAP tests used three blast APLs representing mines with a small (M-14, 29-g), medium (PMA-2, 100-g) and large (PMN, 240-g) explosive content. A small but representative range of footwear was tested. The unprotected references included an improvised sandal and the standard U.S. Army Combat Boot (CB). Two representative mine-protected boots were used: the Wellco® Blast Boot (BB) and the BFR-40 boot. These boots have a blast deflector in the rear portion of their sole while the forward portion of the sole is unprotected. Finally, two boot supplements were also used in the test program. The Wellco® Over Boot (OB) consists of the BB sole containing a blast deflector but mounted on a Kevlar® strapping system so that the OB can be worn over another boot. The second boot supplement is the Med-Eng Spider Boot™ (SB), which consists of a platform supported approximately 10 cm above ground by four legs that protrude fore and aft of the platform. The SB differs from conventional boots as it is designed to move the point of mine detonation away from below the foot.
|Table 1: Mine Trauma Score for the lower extremity.|
The fracture patterns observed during LEAP correlate well with reports from the field5 about landmine injuries to deminers. The extent of injury clearly depended on the amount of explosive contained in the landmine, but also on footwear. One challenge that the medical staff involved in LEAP had to overcome was to define a scoring system that would adequately describe the medical outcome from each test while retaining sufficient sensitivity6 to capture differences in performance due to footwear. The result was the Mine Trauma Score (MTS), as listed in Table 1.
The MTS consists of nine distinct scores built from a number from 0 to 4 that describes the amputation level, from no injury to above-knee amputation. In addition, the letter A or B indicates the level of soft tissue contamination. Using the numbers 1 and 2 without a letter refers to an injury where there is no visible break or laceration of the skin, albeit there might be internal bone fractures. The letter A refers to an injury where the skin is broken but contamination from external agents has been minimized because the footwear was not breached. The letter B refers to an injury where the footwear was breached and external agents such as dirt or the detonation products visibly contaminate the wound.
|Table 2: Summary of MTS results from LEAP.|
It is useful to summarize the knowledge gained from LEAP. First, this program provided much-needed insight into the process that takes place during mine blast injury to the lower leg. LEAP showed that using mine-protected boots on their own did not provide any advantage as compared to unprotected footwear. Only when some form of standoff was introduced, with the OB or the SB, was there any significant difference in outcome. The OB results suggest that this protective equipment can make a difference, but only for the smallest mines. More standoff, or at least a different protective sole is required to extend the benefit of the OB to larger landmines. The limited number of tests with the SB showed that this might be an effective means of limiting lower extremity injury, but there are many deminers who are reluctant to use this footwear because of its ergonomics.
Tools to Assess Footwear Performance
|Figure 4 (a) and (b): Components of the FSL
include: (a) accurate reproductions of the bones and a reconstruction of
the knee and ankle joints and with strain gauges and (b) bones enclosed
c/o DSTO, Australia
The strength of the LEAP dataset is its high degree of fidelity because it used human cadavers. Paradoxically, it is also its main weakness because the use of cadavers for this type of test can only be conducted at specialized institutions within a strict legal and ethical framework, making them very difficult to work with. Recognizing this, the Canadian Centre for Mine Action Technologies (CCMAT) decided in early 1999 to look for a tool that can replace cadavers but still produces results in terms of the MTS score without needing medical staff and facilities. The answer was a product called the Frangible Surrogate Leg (FSL), which was then under development at the Defense Science & Technology Organisation (DSTO) in Australia.
|Figure 5: Sequence of flash x-ray pictures showing early deformation of the FSL with CB against an M-14 mine|
The FSL is a reproduction of the human lower extremity that uses geometrically accurate bones cast from synthetic materials with x-ray visible dye. As of early 1999, an early version of the FSL had been subjected to explosive tests, but there had been no focus to calibrate its mechanical response against landmine injuries to human limbs. Thus, the main objective of the CCMAT program was to purchase FSL specimens, subject them to the same test conditions as in LEAP, compare the results and develop a correlation through MTS scores. Close cooperation with the personnel that performed the LEAP tests ensured that the test conditions were reproduced as closely as possible.
The FSL program did not attempt to duplicate all LEAP tests. Only 16 of the 37 tests corresponded one-to-one with LEAP tests. These were with CB, BB and OB footwear against the M-14 and PMA-2 landmines. The MTS scores and details of the soft and bony tissue damage were compared10 between the LEAP and FSL/S models. Figure 6 displays the MTS values from the tests involving the M-14 mine against an unprotected combat boot, which revealed that one of the main differences between the FSL/S and LEAP models is in the soft tissues.
|Figure 6: Comparison of MTS scores for LEAP and FSL M-14 versus CB.|
The LEAP tests produced MTS scores of 3 and 4, while the FSL/S model produced consistent MTS scores of 2B. This limitation with the soft tissues does not curtail the usefulness of the FSL model to assess the effectiveness of protective footwear. The level of bone damage was similar for the two models with complete traumatic amputation of the foot up to the ankle level. The aft portion of the foot was pulverized, severing the forward portion of the foot, which was still recognizable. The tibia and fibula bones remained intact over most of their length for each model.
Figure 7 displays the data for the M-14 mine against the CB/OB combination. This combination produced a roughly even split between amputation and salvageable limbs, making this dataset particularly useful to assess the fidelity of the FSL model. The OB absorbed or deflected sufficient blast energy to preserve the structural integrity of the inner footwear, thereby preventing gross contamination of the injury; none of the MTS scores had a B qualifier. Comparing the bone damage data, the behavior of the calcaneus and talus bones was identical for the LEAP and FSL/S models, i.e., these bones were fractured each time. The differences in bone and soft tissue behavior are documented and do not curtail the usefulness of the FSL/S model.
|Figure 7: Comparison of MTS scores for LEAP and FSL tests M-14 versus CB/OB.|
Figure 8 shows the MTS scores for the PMA-2 mine against the BB/OB and CB/OB footwear combinations. All scores were either 2B or 3 and the scores for the three models overlapped. Flash x-ray photography showed that for this mine/footwear combination, the heel and ankle literally burst during the explosion, resulting in the extrusion of soft tissues and bone fragments through the inner and outer footwear. Bone damage from one model to the next was consistent, although damage to the FSL tibia and fibula bones extended farther up the leg, often with segmental breaks almost up to knee level, which did not appear to be the case during the LEAP study.
|Figure 8: Comparison of MTS scores for LEAP and FSL tests PMA-2 versus CB/OB and BB/OB.|
The LEAP dataset provides an excellent reference from which the usefulness of the FSL as a tool to assess the performance of mine-protected footwear could be evaluated. It was found that although the behavior of the FSL model differed from human injuries, it was sufficiently similar to justify the use of the FSL in the future, particularly now that the differences have been documented. There is still a need to refine the post-test analysis so that it can be performed without involving medical staff. This might be achieved by generating a rating chart based on the severity of bone fracture and level of soft tissue disruption.
Test Protocol for Upper-Body Protection
In late 1998, representatives of the humanitarian demining community were asking questions about the effectiveness of PPE during excavation drills. An opportunity for help had just opened with Med-Eng Systems, Inc., a specialist in personal protection with a long history of working with the explosive ordnance disposal (EOD) community (bomb squads). Med-Eng had won funding from the Canadian and U.S. programs to develop a protective ensemble for humanitarian demining. This led to a very successful three-way partnership that advanced the understanding of mine blast injuries to the upper body and how to better protect against these injuries.
|Figure 9: Sequence of frames from high-speed video of landmine detonation showing different stages of event.|
When describing injuries to the upper body, demining organizations were talking about burns, fragmentation and blast injuries. These three injury mechanisms are closely connected with the physics of APL explosions; thus, it is important to understand the physics of the APL threat to properly quantify the injury mechanisms. Previous work to that effect has shown that the soil has a considerable influence on the explosion, confining the expansion of the detonation products to create a conical danger zone above the ground, as shown in Figure 10.
This figure depicts three stages in the development of a buried mine explosion. Early during the explosion, the hot gas pushes hard on the surrounding soil, propelling particles from the soil cap directly above the mine at great speed. The hot gas breaks through the surface and jets upward at supersonic speed. In the process, it pushes the air ahead and creates an air shock, which is visible in the second frame of Figure 10. The gas slows down as the expansion develops. The initial push of the gas on the soil also creates soil ejecta, a stream of soil particles that flow as a conical sheet surrounding the gas core, as shown in the third frame of Figure 10. It is useful to define this conical zone in terms of the angle φ about a line perpendicular to the soil surface. The angle φ = 0 corresponds to the direction along this perpendicular.
Let us come back to the three injury mechanisms, starting with burns. The fireball from a typical shallow-buried (less than 10 mm) APL lasts about 15–30 milliseconds, but its temperature is certainly high enough to cause burns. When a mine is deeply buried, the hot gases cool down too much before encountering fresh air and the chemical reaction cannot be sustained. The result is a dark cloud.
Fragmentation injuries from a buried APL result from the collision of high-speed soil particles, small pebbles or rocks, mine-casing fragments and broken tool parts with the victim. Let us consider soil particles in the first place. Individual soil particles have a small mass, which limits their ability to penetrate the human body, but their large number has an abrasive effect that can injure the skin and sensitive organs such as the eyes. Larger fragments from rocks, mine casings and tool parts can usually pierce and penetrate the body. The wounding mechanisms for these larger projectiles are fairly well understood by the medical community. From the perspective of protection, these fragments need to be stopped with armor, a process fairly well developed for bullets and other high-speed projectiles.
The third injury mechanism to consider is blast, which is not as well-understood as the other two. There are in fact at least two physical sources for blast injury: the air shock and the jetting of the detonation products. The physics of air shocks has been extensively documented for large blast weapons (tens to thousands of kg of explosive). The passage of a strong air shock results in a sudden change of local pressure, a change that the human body is ill-equipped to cope with. Immediately after the passage of the shock, the air starts to flow outward from the source of the explosion. The flow from a large explosion can literally propel a person. The second physical source of blast injury is due to the high-speed flow of the detonation products. The conical shape of the flow zone is such that the streaming gas often impinges on the upper body. Even if the detonation products are gaseous, they travel at a considerable velocity and can exert great force on objects in their path.
Let us now see how the environment created by the explosion of an APL relates to evaluating the performance of PPE during excavation drills. The threat can consist of a combination of an air shock, the jetting of detonation products that might still be burning, and the impacts from soil particles and larger fragments. Furthermore, it is important that components of the PPE be in the proper location and proper orientation within the blast cone, because geometry is important to blast and because of the variation of soil ejecta patterns. Given these considerations, it became apparent to the authors that the most realistic method to test PPE performance was to use the actual PPE on a human surrogate that offered fidelity in terms of mass and geometry. A decision was made to build upon decades of research by the automobile industry and adopt their crash test dummies, the Hybrid III mannequin.
The Hybrid III mannequin was found to be sufficiently robust for the task, although special care must be taken to protect the rubber skin and the neck, particularly when the mannequin is used in an unprotected configuration. These components are susceptible to damage from sand particles. This mannequin produced repeatable and reliable results for a given threat level and a given body position relative to the blast source. Another advantage is that this mannequin is available in a range of sizes that have mass characteristics similar to the human body they represent. Handling the mannequins requires work, but they can assume the kneeling and prone positions in a repeatable manner with the help of a positioning fixture.
The Hybrid III mannequin can be instrumented with a broad range of sensors for automotive testing, but not all of these sensors are relevant to the mine blast scenario. For example, it is not appropriate to use a leg load cell when data from the field indicates that most of the trauma is to the upper body. Table 3 lists the instrumentation used to develop the current test methodology.
|Table 3: Mannequin sensors used during development of upper-body test methodology.|
Most of the instrumentation proved robust enough for the application. The measurements were used to assess whether or not several automotive injury criteria applied to mine blast trauma. The following are the findings for the two positions and threat levels used in this study:
The Head Impact Criterion (HIC) showed that there exists a significant risk of serious head injury from blunt trauma for larger mines. However, questions remained about applying this automotive injury criterion to mine blasts. The HIC requires filtering of the acceleration signals with a 1650-Hz low-pass filter, but a significant portion of the energy from mine blast damage to the mannequin resides in frequencies higher than 1650 Hz. The study therefore recommended that a limited number of tests be performed with an injury model under blast loading to determine the boundaries of applicability of this injury criterion.
The neck sensor showed loads similar to those seen in automobile impacts using the Nij blunt impact injury criterion. The data was repeatable and showed good differentiation between mine threat levels.
The thoracic loads measured in terms of chest displacement and chest acceleration did not produce injurious values in terms of blunt trauma, even for an unprotected dummy. It is questionable whether or not this criterion should be used in future tests.
The burn sensors used on the hand and chin registered only small increases of temperature due to the very short duration of the event and the depth of burial selected. This indicates that there is only a small risk of serious burns, with the caveat that this applies for these mine blast scenarios.
The pressure transducers are not part of the regular instrumentation package for the Hybrid III. They were added to measure the likelihood of eardrum rupture and blast lung. The latter could result from coupling of the shock wave with poorly designed torso armor, as reported for British operations in northern Ireland. However, it was found that the loads generated by an APL for the body positions considered were well below the threshold required for blast lung injury. On the other hand, ear pressure was found to often exceed the acceptable threshold for eardrum rupture.
|Figure 10: Sketch of the location of the head relative to the blast cone for the low- and high-kneeling positions.|
The positioning fixture was essential to the test methodology. It greatly decreased the physical work required to position a mannequin and produced a positional accuracy better than ±5 mm. Tight control over position demonstrated the importance of the position of the body within the blast cone. Two positions were selected for the mannequin, kneeling and lying prone, as depicted in Figure 10.
Comparative Testing of PPE Performance
The test methodology described in the previous section was applied in a landmark series of tests13 in October 2000. More than 100 tests were done at the Aberdeen Test Center, Maryland, USA, to measure and compare the protective performance of five commercially available PPE outfits. The five ensembles are depicted in Figure 14 while some key characteristics are listed in Table 4. These five PPE systems represent a range of protective equipment available to the demining community. All ensembles provide some form of protection to the face and thorax, although there are significant differences in the implementation of these protective measures, e.g., extent of the facial coverage of the visor. Three ensembles also offer protection to the groin area and three use a helmet to further protect the head.
|Table 4: Mass characteristics and visor/helmet projected area for the five PPE.|
The mannequin position was set so that the nose of the mannequin would be located 70 cm from the mine on the φ = 25° line for the kneeling position and 45 cm from the mine on the φ = 43° line for the lying prone position. The mannequins were positioned first and any head protection gear added afterward. Each piece of PPE was exposed three times to the blast from 100-g and 200-g C4 charges for each combination of body position and charge mass. Additionally, two pieces of PPE were exposed two or three times to the blast from the PMN mine in order to validate the use of the 200-g surrogate charge; the blast field from these two explosive devices was found to be very similar, but there remained questions about fragmentation from the thick Bakelite casing. The data was recorded in accordance with the test protocol. Damages to the PPE were noted from inspection and the physical response of the mannequin was recorded. The instrumentation records were post-processed and compared to injury criteria reference values for head injury, eardrum rupture and neck injury.
|Table 5: Statistics about loss and structural failure of visors (all PPE).|
The structural integrity of the equipment was generally good. The aprons and vests remained in place for all tests, although abrasion of the materials, rips and some partial penetration were often observed, particularly for the 200-g and PMN shots. Fasteners often failed and Velcro® straps became loose, but the equipment generally remained in position on the mannequin. The combination of kneeling and lying prone positions with sand as the soil medium meant that the trouser and pant components were never really challenged during these tests. The ensemble components that proved most susceptible to damage from blast and soil ejecta were helmets and visors. All visors were covered with soot and dust, and most were pitted by high-speed soil particles. The pitting was most prominent on the upper portion of the visors, and in some cases, on the helmet as well. Visors failed to remain in place for a significant portion of the tests, either as a consequence of structural failure of the visor itself or failure of the retaining system on the headband or helmet. Table 5 shows that this phenomenon increased with the threat level.
There was also a dependence on body position with the lying prone position resulting in more severe conditions. The reader should note that the two positions were selected on a basis of body dimensions and ergonomics, not to generate equivalent challenges. Each position subjects the headgear to different loading conditions. The lying prone position is 25 cm closer to the source of the blast, which increases the strength of the blast loading and exposes the equipment to a greater cross-section of soil ejecta. This is in a region where the soil ejecta velocity is less but soil density is higher. For the kneeling position, the upper section of the headgear is closer to the φ = 0° line where soil particles travel at greater speed, which likely explains the prominence of pitting on the upper part of the visors. The difference in damage to the equipment highlights the importance of taking into account all damage mechanisms (i.e., blast, ejecta and fragmentation) when designing PPE.
Differences in visor/helmet performance were noted from one PPE design to the next. The PPE with large facial visors retained by a headband remained on the mannequin only three times out of 28 tests (11 percent). When the visor was attached to a helmet, it remained in place 31 times out of 42 tests (74 percent). In regard to structural failure, one PPE failed 67 percent of the time and the type of failure was the visor itself breaking into several large pieces. The second worst performer failed 28 percent of the time with most of the failures involving the visor breaking into several large pieces as well. The reader should be careful in interpreting the structural failure; it does not mean that the visors did not perform their function. With one exception, none of the tests resulted in a “clean” perforation of the visor with resulting damage to the mannequin face. In other words, it appears that the visors performed their primary function, i.e., to stop penetration of the facial area by fragments. The extent of pitting on the upper portion of the visors also attests to the importance of this function, and without it, sensitive organs such as the eyes would be worst off.
In addition to soil ejecta and case fragments, the head is also vulnerable to the blast. Impingement of the shock and the following high-speed gas flow essentially deliver a blow to the head, which is akin to a blunt impact. Figure 11 shows the HIC results as a function of PPE, threat level and body position. The onset of severe head injury occurs at a HIC value of 1000. For the 100-g charge, the mean HIC value was 379 for PPE1 in the kneeling position, which indicates that this threat level is unlikely to produce severe injury through blunt trauma to the head. The largest mean HIC value for the 200-g charge was 5756 for PPE1 in the kneeling position, which indicates a definite probability of severe injury from blunt trauma to the head.
|Figure 11: HIC values produced for each PPE ensemble as a function of threat and body position.|
It is interesting to note that for the kneeling position, the visor design for PPE1 increases the probability of head injury from blunt trauma relative to the unprotected mannequin. Another important aspect of the HIC data is that the heavy helmet of PPE5 combined with the lower projected surface area of the visor never produced an HIC value greater than 100. In the kneeling position, the headgear with greater mass generally resulted in lower HIC values, while the two headband designs with lower mass and larger surface area produced large HIC values. In the prone position, only PPE5 resulted in a significantly lower HIC value. This behavior can be partially explained by the fact that a visor/helmet combination increases the projected area of the head against which the air shock and the transient flow from the expanding gas can push, thereby increasing head acceleration. Increasing the mass of the headgear has the opposite effect of reducing head acceleration. Of course, the air shock and the transient gas flow are only responsible for part of the overall loading on the headgear. Impacts from soil particles also play an important role, but the relative contribution of these three loading components of mine blast requires further work to be fully understood.
Another aspect of head protection that should be considered is its effect on the probability of ear injury. For an unprotected mannequin, the sensors mounted on the side of the mannequin head record a signal of the Friedlander type, characteristic of free-field side-on pressure. The presence of headgear affects the shape of the pressure pulse, indicating a more complex flow around the visor and/or helmet. Figure 16 shows the average peak pressure measured as a function of PPE, body position and charge size.
|Figure 12: Peak value of ear pressure for each PPE ensemble as a function of threat and body position.|
It is seen that the threshold of eardrum rupture is reached easily, even for the 100-gram charges. The effect of PPE design is relatively small for all 100-g tests and for the 200-g tests in the prone position. However, the 200-g tests in the kneeling position show a significant increase of peak pressure for the two PPE utilizing open-style helmets. It is speculated that for the kneeling position, an open helmet captures a part of the streaming flow, acting as a reflector to increase pressure at the ear.
Injury to the neck due to blunt trauma is either due to a direct impact on the neck or due to relative motion between the head and chest. During a mine blast, the head and chest are subject to different accelerations as a function of their location within the blast cone. The probability of injury due to blunt impact can be evaluated from the forces and moments recorded by the neck sensor and the Nij criterion. A value of Nij = 1.0 corresponds to a 22 percent probability of severe neck injury. Figure 13 shows the results as a function of PPE, body position and charge size.
|Figure 13: Neck injury criterion Nij for each PPE as a function of threat and body position.|
Note that data is missing because the neck on one of the two mannequins used for testing became loose during the test program. With the highest value of Nij being 0.55, it is apparent that none of the test conditions resulted in a high probability of severe neck injury. The values of Nij are generally larger for the unprotected mannequin, indicating that the use of PPE reduces the probability of neck injury. The larger charges generate larger values of Nij. The values of Nij are also larger for the lying prone position, consistent with the closer distance to the blast. It is suspected that the same factors that influence head acceleration (i.e., visor surface area and helmet/visor mass) play a similar role with respect to neck injury.
Body Position Versus Probability of Injury
While developing the upper body test protocol, the mine-to-chest (dc) and mine-to-nose (dn) distances were measured for each test to provide a rough measure of body orientation. These parameters were varied to assess their effect on mannequin response. As more test data were coming in, it became clear that body position relative to the blast cone has a strong influence on the transfer of force to the mannequin. Our measurements indicated that the head acceleration often reached levels indicative of a high risk of blunt trauma injury. Nerenberg et al.14 examined the relation between head acceleration, the distance dn and the location of the nose within the blast cone, i.e., the angle φ mentioned earlier. They demonstrated that deminer safety could be improved through better position during excavation. Low-kneeling results in a larger value of φ than high kneeling, as illustrated in Figure 10.
By avoiding the blast cone, position 1 with a smaller value of dn generated less head acceleration than position 2. This is a clear indication that the transient flow produced by the vertical expansion of the detonation products is one of the load-producing mechanisms that must be taken into account in the design of head-protective gear. This phenomenon was repeated for charge sizes of 50, 100 and 200 g. Applying the HIC criterion to the acceleration values for the 200-g charge against the unprotected mannequin, the low-kneeling position predicted a 1.7 percent probability of fatal injury. In the high-kneeling position, a 100 percent probability of fatal injury was predicted despite the larger standoff.
The above difference due to body position relative to the blast cone is very significant. Another factor is also significant. Our tests were performed with the 50th percentile North American male Hybrid III mannequin, which measures 1.75 m and weighs 77 kg. Deminers are often people of smaller stature, e.g., in Asia, and there might be physical limitations that prevent a person from achieving sufficient standoff from the mine. When taking into account the consequences of body position combined with body size, it was clear that mannequin response as a function of body position and body size needed to be mapped out. This map might prove useful to improve deminer position during excavation drills.
CCMAT sponsored this study of body position. Starting in June 2001, body positions for two mannequin sizes, the 50th percentile Hybrid III male and the 5th percentile Hybrid III female (1.57 m and 54 kg), were carefully mapped out using 3-D laser measurements. Nine positions were carefully defined for each mannequin size (three distances x three φ angles). The distance dn was set at 60, 70 and 80 cm for the larger mannequin, and 50, 60 and 70 cm for the smaller mannequin. Testing then started in September 2001 and was completed later that year. A subsequent series of tests in June 2002 explored the effect of body position for the lying prone position. More than 130 tests were performed and data analysis is ongoing. The results will be published in the near future.
To solve personal protection issues, the scientific approach was applied, first to identify and study the basic injury-producing mechanisms and second to look into appropriate protection strategies, working with contractors and manufacturers that had expertise to contribute towards practical solutions. A large portion of the work concentrated on the development of test methods so that protective products could be evaluated objectively. These test methods were then used to determine the performance of a few footwear products and five PPE outfits designed to protect the upper body. The results from these tests were published, and it is now proposed to contribute these test methods to organizations such as the International Test & Evaluation Program (ITEP) for their consideration.
The LEAP study produced several outcomes, including the development of a specialized mine trauma score for the lower extremities and a detailed epidemiology database for blast mines. Tests with the FSL and comparison of the results against those from LEAP generated a validation of this new tool for use in future tests. Test results from the two programs demonstrated that existing mine-protected footwear did not perform any better than unprotected footwear unless additional standoff from the mine is added. This can be achieved through the use of an overboot or a raised platform such as the Spider Boot®. Through our test programs, the injury mechanisms were largely quantified, and this new knowledge is already being used to develop better footwear. It might soon be time to conduct a new assessment of performance improvements using the test methods developed under the Canadian/U.S. programs.
Our work in upper-body protection produced several outcomes. Through the study of basic mine blast physics, it is now understood that injury to the upper body can be caused by a combination of air shock, high-speed gas flow around the body, soil ejecta and fragments from mine casings or other sources. Burns can also occur under some conditions. The existence of a conical zone where the risk of injury is greatly increased has also been demonstrated experimentally. The implication is that small changes in body posture can have a large effect. Further work is underway to map out this zone, and the results will be published in the near future; this might lead to safer body positions during excavation drills. Quantifying these injury-producing mechanisms should lead to improvements in PPE design.
A test methodology using automotive anthropomorphic mannequins has been developed and extensively tested. Two main body positions, lying prone and kneeling, have been adopted. Further work with cadaver models is underway to validate these criteria, particularly when the body is unprotected. The results from this study will be published in the near future.
The upper-body test methodology was used to evaluate the performance of five commercially available PPE ensembles. This equipment generally fared well in protecting the torso and neck from soil ejecta, while more testing would be required to fully assess protection against mine-case fragments from larger mines, e.g., the PMN. There were large differences in head protection. Some helmet designs tended to increase the probability of ear blast injury. It was found that the addition of mass (e.g., a helmet) reduced head acceleration, while large visor surface area tended to catch more of the blast and increase head acceleration. Some visors often suffered structural failure or failed to remain in place, but it is unclear whether or not these failures occurred late enough in the event to prevent sufficient protection against most of the injuring elements. The main outcome from these tests is that protection of the head through improved visor and helmet design requires further work.
*All figures courtesy of the authors unless otherwise noted.
The authors gratefully acknowledge the funding from their sponsoring organizations, U.S. Department of Defense SO/LIC, U.S. Army Communications and Electronics Command and the Canadian Center for Mine Action Technologies, without which none of this work would have been possible. The authors also recognize the high-quality contributions from their co-workers from academia, government and industry. The work reported herein resulted from the efforts of a wide range of dedicated staff, as reflected by the list of references below, but the few names listed below do not reflect the excellent support received from those people that too often carry out their daily functions in the background: technicians, photographers, etc. Let it be known that your enthusiasm and care are essential to the quality of the work. All findings and views reported in this paper are those of the authors and do not necessarily reflect the consensus of views from the funding organizations.
Denis M. Bergeron
DRDC Suffield / CCMAT
U.S. Army CECOM