More Than Nuts & Bolts: Potential New Tests for Ballistic Helmet Materials

More Than Nuts & Bolts: Potential New Tests for Ballistic Helmet Materials

September 14, 2018

 

New Testing Methods to Evaluate Tactical Helmets in the Lab

 

It’s been a few weeks since we wrote about a scientific study relating to helmets. You might remember the last study we discussed tested the kinds of head injuries that can occur when different kinds of ammunition are fired into the head, and this was tested by shooting ammunition into pig heads. No pigs will be harmed in the making of this study, as it deals with one of our favorite subjects: helmet testing. There is no gore at all in this study, though if you appreciate adolescent humor this might be the study for you. The words “screw”, “nuts”, and “hardness test” are used frequently throughout, often in the same sentence. We’re not saying that’s funny, because it’s not. Get your mind out of the gutter.

Our study was conducted by Ronald L. Holtz and Peter Matic of the Naval Research Laboratory in Washington, D.C., at the request of Marine Corps Systems Command.  As you know, the U.S. Army uses the Advanced Combat Helmet (ACH) as its standard-issue helmet, and the Marine Corps uses the Lightweight Helmet (LWH). Both of these helmets go through extensive ballistic testing, in line with current Department of Defense (DoD) practice. At Hard Head Veterans we put our ATE and MICH helmets to the test against the National Institute of Justice’s ballistic standards, and the results are on our website so you can see we mean business. It is an extra cost, but it’s worth it to guarantee quality and keep you safe. There is no substitute for testing helmets against live ammunition, though the authors of this study would have us believe they can develop one. Read more about how our helmets performed in laboratory testing here.

 

But back to the study. The authors point out that the screws and nuts that are used to attach the retention system to the shell are required to perform to specific ballistic testing standards, but that there are no materials standards for the screws and nuts. In other words, DoD tells manufacturers how the equipment must perform, but does not indicate what the equipment must be made of to perform to their requirements. This lack of materials standards requires DoD to conduct extensive (and expensive) ballistic testing in order to ensure quality. The authors think that defining the range of acceptable materials for the screws and nuts, and then developing non-ballistic tests for them, would be beneficial in several ways. It would save significant amounts of time and money, by replacing ballistic testing with simpler and less costly tests. Also, developing standards for this equipment creates baseline reference measures that can be used in the future to evaluate screws and nuts made from new and different materials.

We agree that there should be materials standards available for helmet manufacturers to utilize in their designs, but we are skeptical that it will ever supplant traditional ballistics testing. Ballistics testing is designed to mimic the conditions faced while under fire on the battlefield, and other laboratory tests can’t mimic those conditions. If a manufacturer tells you it has tested its helmets, but hasn’t ballistic-tested them, you should rethink buying their product!

ACH and LWH tactical helmet screws like these were tested in the study. Photo Credit: Holtz et al.

To conduct the test, the authors obtained three LWH and three ACH helmets that had successfully passes ballistic testing, and took screws and nuts out of these helmets. They also obtained replacement nuts and screws that had not been subjected to testing.  Once these were assembled, the authors evaluated their properties. This included measuring the dimensions of the screws and nuts, as well as the hardness of their exterior and in cross section, evaluating the composition and grain of the metals used, measuring the tensile strength of the screw and nut threads, and measuring their toughness to impact. The tensile strength and toughness were measured using tests created by the authors themselves, because there was no approved pre-existing standard for evaluating these properties in screws and nuts. The authors hypothesized that examining screws and nuts in this way would give them a set of standards by which to judge the quality of helmet screws and nuts, and would further suggest which methods would be suitable for evaluating those standards.

After exhaustive measuring and testing, the authors did come up with a set of standards for screws and nuts in ballistic helmets. The testing itself is fairly technical and makes for dry reading, but if you’re a science whiz read more about it here. They also outline methods for testing the nuts and screws, as well as recommendations for future research.

How dimensions of the screws and nuts are taken. Photo Credit: Holtz et al.

TABLE IIA –REFERENCE HARDWARE DIMENSIONS

Hardware Item

Dimension

Average [inch]

+/- St.Dev. [inch]

Minimum* [inch]

page11image8232

Maximum* [inch]

Reference LWH Screws

Head diameter

0.492

0.002

Head height

0.131

0.001

0.129

Slot depth

0.050

0.004

0.062

Slot width

0.064

0.002

0.070

Slot base

0.075

0.004

0.063

Shoulder diameter

0.216

0.001

0.213

Shoulder length

0.323

0.006

Stem length

0.479

0.006

Overall length

0.608

0.002

Thread outside diameter

0.186

0.001

0.181

0.189

# of fully-formed threads

>3

3

Reference ACH Screws

Head diameter

0.563

0.001

Head height

0.139

0.004

0.127

Slot depth

0.058

0.004

0.070

Slot width

0.063

0.001

0.066

Slot base

0.076

0.002

0.070

Shoulder diameter

0.245

0.001

0.242

Shoulder length

0.323

0.007

Stem length

0.482

0.003

Overall length

0.618

0.003

Thread outside diameter

0.184

0.001

0.181

0.189

# of fully-formed threads

>3

3

Reference LWH Nuts

Major diameter

0.283

0.001

Minor diameter

0.250

0.001

Overall length

0.168

0.001

Thread inside diameter

0.158

0.002

0.152

0.164

Slot depth

0.035

0.005

0.050

Slot width

0.065

0.001

0.068

Slot base

0.128

0.002

0.122

Reference ACH Nuts

Major diameter

0.283

0.002

Minor diameter

0.252

0.001

Overall length

0.170

0.003

Thread inside diameter

0.158

0.002

0.152

0.164

Slot depth

0.037

0.003

0.046

Slot width

0.061

0.002

0.067

Slot base

0.129

0.003

0.120

* Estimated minimum and maximum values = average +/- 3 standard deviations.

Table Credit: Holtz et al.

TABLE IIB –REPLACEMENT HARDWARE DIMENSIONS

Hardware Item

Dimension

Average [inch]

page12image5520

+/- St.Dev. [inch]

Replacement LWH Screws

Head diameter

0.493

0.001

Head height

0.132

0.002

Slot depth

0.049

0.003

Slot width

0.057

0.001

Slot base

0.081

0.002

Shoulder diameter

0.212

0.001

Shoulder length

0.327

0.002

Stem length

0.484

0.001

Overall length

0.613

0.001

Thread outside diameter

0.186

0.001

# of fully-formed threads

>3

Replacement ACH Screws

Head diameter

0.562

0.001

Head height

0.139

0.001

Slot depth

0.046

0.001

Slot width

0.060

0.001

Slot base

0.092

0.001

Shoulder diameter

0.243

0.001

Shoulder length

0.329

0.002

Stem length

0.481

0.002

Overall length

0.622

0.003

Thread outside diameter

0.185

0.001

# of fully-formed threads

>3

Replacement Nuts

Major diameter

0.282

0.001

Minor diameter

0.252

0.001

Overall length

0.168

0.001

Thread inside diameter

0.163

0.002

Slot depth

0.042

0.003

Slot width

0.059

0.002

Slot base

0.127

0.004

page12image102088

Table Credit: Holtz et al.

 

For screws, they recommend a minimum of seven and a maximum of ten tests in the evaluation. They are as follows, and in this order:

  1. Exterior Examination for any flaws or damage visible to the naked eye.
  2. Dimensional Measurements to ensure the screw is within the standard dimensions listed in Table II.
  3. Surface Vicker’s Microhardness test to ensure average surface hardness of 110 to 180 HV.
  4. Grain Structure Examination to ensure the grain structure of the metal is similar to the reference screws. If it is not, Test 7 is recommended.
  5. Rockwell Hardness of Screw Cross Section to ensure average Rockwell hardness of B80-B90. If the average is outside this range, Tests 9 and 10 are recommended.
  6. Thread and Head Examination of the screw cross sections for any flaws or damage visible to the naked eye.
  7. Chemical Analysis via an appropriate method to discern the levels of manganese, chromium, and other elements if the grain structure does not match the reference screws. If the chemical composition differs significantly from the reference screws, Tests 8-10 are recommended.
  8. Screw/Nut Thread Proof Test to ensure that the threads have strength of at least 1000lbf, that being the minimum amount of force at which the threads strip off the screw.
  9. Screw Tensile Strength to ensure average tensile strength of 90,000 to 110,000 psi.
  10. Drop Weight Impact Test “with a peak impact force of about 3200 lbf for LWH screws and 3600 lbf for ACH screws...to detect any visible indications of cracking.” Depending on the level of cracking on the screw, different impact forces should be tested to figure out the threshold at which the screws will crack.

Stripped screws, as you would see after administering Test 8. Photo Credit: Holtz et al.

TABLE X – SUMMARY OF REFERENCE VALUES FOR SCREWS

Test # in Text

Test Description

page27image8168

Reference Values or Condition

(S1)

Exterior Examination

Screws free of defects, burrs, scratches. Rounded fillet at head to shoulder corner, threads well-formed.

(S2)

Dimensional Measurements

See Table II

(S3)

Surface Vicker’s Hardness

Screws 110 to 180 HV50

(S4)

Grain Structure

Similar to reference screws. See Section 2.2 for detailed criteria.

(S5)

Rockwell Hardness - Screws

Average between Rockwell B80 and B90

(S6)

Thread and Head Examination

No grain structure distortions in head, area around head slot or shoulder to head transition.
Free of cracks, voids, notches, other defects.

(S7)

Chemical Analysis

Low-carbon steel with nominal 1% Mn Traces of other elements can be present

(S8)

Screw/ Nut Thread Proof Test

At least 1000 lbf to strip threads from screws. No stripping of nut threads.

(S9)

Screw Tensile Strength

Between 90,000 to 110,000 psi

(S10)

Drop Weight Impact Test

Per NRL test method described herein:
Minor damage for impact peak force of 3,200 lbf to LWH screw heads; or 3,600 lbf to ACH screw heads.

Table Credit: Holtz et al.

 

TABLE XI – SUMMARY OF REFERENCE VALUES FOR NUTS

Test # in Text

Test Description

Reference Values or Condition

(N1)

Exterior Examination

Nuts free of defects, burrs, scratches, etc. Threads well-formed.

(N2)

Dimensional Measurements

See Table II

(N3)

Screw/ Nut Thread Proof Test

At least 1000 lbf to strip threads from screws. No stripping of nut threads.

(N4)*

Grain Structure

Similar to reference nuts. See Section 2.2 for criteria.

(N5)*

Vicker’s Microhardness – Nuts

Average between 210 to 330 HV200

(N6)*

Thread Examination

No grain structure distortions.

Free of cracks, voids, notches, malformations.

(N7)*

Chemical Analysis

Nuts nominally 1% Mn and 1% Cr Traces of other elements can be present

 

*These tests are optional but recommended.

Table Credit: Holtz et al.

For nuts, the authors recommend just three tests as an acceptable basis for evaluation. They are the equivalents of Tests 1, 2 and 8 outlined above: Exterior Examination, Dimensional Measurements, and Screw/Nut Thread Proof Test. Nuts require fewer tests because they are not likely to be hit by bullets, so are not required to be tested for their ballistic properties.

To their credit, the authors recognize that this is a preliminary study, and that more research needs to be done. They conducted their experiments using a small sample size of screws and nuts, so they would like further testing of screws to make the reference values more robust. In addition, they would like testing of screws and nuts that failed ballistic tests, to establish a range of unacceptable values. They also would like a standard impact test to be created, as the one they created had not been used before and did not account for all possible variables. Lastly, they would like the scientists that conduct these follow-up studies to use their research to make explicit recommendations about what materials should be used in screws and nuts.

This study represents a first step in the development of alternatives to old-fashioned ballistic testing, but we’re not at the point to replace ballistic testing yet, and we are skeptical we ever will be. It is definitely good to expand our knowledge of what materials are best suited for building the best bullet-resistant tactical helmets available, and hopefully this line of research will be useful in that regard.  We will keep looking out for new developments on this front, and will continue to bring you the finest tactical helmets subjected to the most comprehensive ballistic testing.





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