www.3ders.org

Mar 22, 2016 | By Alec

The nature of war has been changing for a long time, and though soldiers have more access to advanced technologies than ever before, it’s obvious that the manufacturing of weapons and armor needs to evolve with it. That’s why the US Army has been looking into adopting ceramic 3D printing for the redevelopment of body armor for its ground forces, which could enable more efficient design and more effective and speedy production. That’s why they have been collaborating with HotEnd Works on a research project, to see exactly what 3D printing can bring to a soldier’s protective gear. And while 3D printing offers numerous production advantages, they found that the technology still needs to improve slightly to meet the quality demands required.

The conclusions of this interesting study are set out in a forthcoming study entitled “The First Static and Dynamic Analysis of 3-D Printed Sintered Ceramics for Body Armor Applications” by Tyrone Jones, Jeffrey J. Swab and Benjamin Becker, with Becker coming from HotEnd Works. That company, as you might know, was founded in 2012 by Becker and Jessica Whittaker with a focus on advanced material 3D printing. They are the masterminds behind the HDfab 3D printer, which uses proprietary high density deposition technology to 3D print high resolution ceramics with properties that make it very interesting for aerospace, defense, and medical applications. Though they have worked and are working on various research projects with external partners, they are also planning to finalize and commercialize their 3D printer by mid-2017.

But this latest collaboration with the US Army Research Laboratory is one of their most significant projects. As they explain in their study, it is obvious that 3D printing offers a myriad of advantages over traditional ceramics already. “Ceramic 3-D printing offers engineering-grade ceramic components in approximately 90% less time than traditional ceramics. Typical turn around can be in days, instead of weeks, depending on the complexity of the part. This not only allows for faster time to market, but also allows for more iterations during the design process, resulting in a better end product. Additionally, 3-D printed parts can have a higher degree of complexity for weight reduction, while saving on the cost of the part because of the reduction in material used,” they write. But the real question is: can the results compare in terms of static and quasi-static parameters, such as density, hardness, and fracture strength, and penetration?

Figure 1: Typical Die Press Assembly

Figure 2.  Representative Microstructure of AD-995 (A) and HotEnd Works Alumina (B)

To find out, the researchers tested the extent to which 3D printed ceramics stand up to the higher performance ceramics currently used in US army body armor. Traditionally manufactured, sintered Alumina AD-995 (also sometimes called CAP3) from CoorsTek was compared to HotEnd Works’ 3D printed and sintered alumina.

Figure 3: PSD Technology

As you might know, HotEnd Works actually relies on an unconventional 3D printing technology called Pressurized Spray Deposition (PSD), which involves the use of advanced ceramic raw material with a unique polymeric binder, with the binder servicing as a temporary support structure during part formation to accommodate overhangs and other intricate features. The materials are separately 3D printed from two external hoppers, and can be placed in very high precision, in patterns from 0.127mm up to 3.81mm (0.005” up to 0.150”) in diameter. That is done in a layered sequence. After formation, thermal debinding takes place over an average cycle of 24 hours, at a temperature of less than 150ºC. Thanks to this unique combination of materials and debinding, the resultant parts don’t need to be cleaned, but can simply be transferred to a traditional electric or gas furnace to complete the densification. Afterwards, some diamond grinding might be necessary, but not in all cases.

For the test, rod-shaped specimens of both materials were obtained, with very comparable properties. Both were3 mm in diameter and 50 mm long, and as can be seen in the table below, they exhibited very similar characteristics.

Table 1.  Property summary

So how would they stand up to a bullet? In scientific terms, are their Depth of Penetration (DOP) or residual penetration properties comparable? Experiments were set up to find out. “For DOP testing, a projectile is fired into a ceramic tile attached to a thick metal backer plate such that the projectile penetrates into the metal plate without deforming the back surface. These experiments avoid the fundamental problem of V50 ballistic dependence on armor design (e.g., front-to-back plate ratio and material), require fewer shots than V50 tests, and have a sensitivity equivalent to that of other ballistic test methods,” they explain, with the change in penetration into the metal plates enabling comparison.

Figure 4. Alumina Tiles before impact

Figure 5. Sketch of Ceramic Composite Samples.

The targets themselves were 90-mm x 90-mm ceramic tiles at a nominal thickness of 8 mm, backed by two aluminum alloy 6061 plates of 2-inch thickness, glued together with an epoxy resin. The alloy in question is a well-characterized and readily available backer material, and were expected to provide better resolution than steel. These were shot at with 12.7-mm APM2 bullets, including a hardened steel core penetrator, with length of 47.6 mm a diameter of 10.87 mm. For all experiments, the bullet hit the plates at 848 m/s (2782 ft/s), although there was some practical variation. “The velocity was chosen in order to produce a range of practical residual penetrations, while being consistent with normal operating conditions,” they explain.

Figure 6. Cross section of a 12.7-mm APM2.

(a) Front view

Figure 7. Initial conditions of ceramic composite samples in fixture.

DOP = T_b – a

Figure 8. Measurement of Residual Penetration

So how do you actually measure penetration? Essentially, by sectioning the plates with the help of electrical discharge machining (EDM) and making measurements with vernier calipers. Firstly, a baseline was established with the backup plates, the results of which can be seen in Table 2 below. The process was subsequently repeated for all targets. “In order to adjust for variations in the actual strike velocity, all residual penetration values were normalized to a striking velocity of 848 m/s,” they say, using an established technique for measuring penetration. The results of the six shots can be seen below.

Table 2. Ballistic Impact Measurements for Alumina Tiles

While the ceramic tiles were largely similar in properties, they did actually find that the average DOP for the AD-995 was quite a bit lower than for the 3D printed part. With correction, the DOP of the AD-995 tile was 14.43mm (with a standard deviation of 3.01mm), while it was 24.01mm (standard deviation of 2.06mm) for the 3D printed part by HotEnd Works. “In these limited experiments, the AD-995 tiles caused more damage to the penetrator than the HEW Alumina tiles. The penetrator underwent two failure mechanisms, fragmentation and erosion, when it impacted the AD-995 tiles. The penetrator underwent one failure mechanism, erosion, when it impacted the HEW Alumina tiles,” they say. “The CoorsTek and HotEnd Works Alumina both started failing with tensile fracture, then continued into comminution to dissipate the energy of the penetrator. The extent of the ceramic damage was very similar for both types of alumina,” they added.

Table 3. Front Photos of Reference Material

Figure 9. Penetration of 12.7mm APM2 into AA6061

To fully comprehend the results, a comparative performance value was calculated for both materials, using aluminum alloy 6061 (AA6061) plates as the reference material. Ergo, how many more times is the plate effective than simply using AA6061 plates? Through calculations, a relative comparison to of the ceramic to AA6061 is established, with the number showing that the ceramics are that many times more effective than AA6061 (a Cp of 5 meaning its five times as effective). The results can be seen in the table below.

Figure 10. Residual Mass of 12.7mm APM2 into AA6061

Figure 11. Residual Length of 12.7mm APM2 into AA6061

Figure 12. CoorsTek Alumina AD-995 after Impact.

Figure 13. HotEnd Works Alumina after Impact

Table 4. Comparative Performance of Ceramics Based on Cp

In short, both materials are significantly more effective than just AA6061 plates as body armor, but the 3D printed ceramics by HotEnd Works are slightly less effective than the established ceramic option. “This program was a preliminary investigation into the viability of using a 3-D printed alumina ceramic for body armor applications. The study revealed that CoorsTek Alumina AD-995 yielded a lower DOP, and hence performed better, than the HotEnd Works Alumina,” the researchers say. While the 3D printed plates thus certainly offer a number of manufacturing advantages, their properties will need to be improved before adoption. “The 3-D deposition and sintering processes will need to be improved to provide properties that match, or exceed, those of conventionally sintered alumina,” they conclude. Perhaps something for the future?

 

 

Posted in 3D Printing Application

 

 

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John H. wrote at 3/24/2016 4:09:04 PM:

Good article. This is surprisingly good for 3d printing, especially ceramics. It sounds like there is a good potential for improving the material further. What was the cost difference between the two samples? How long does something like this take to 3D print?

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