Materials

This chapter is designed to give some useful data for choosing armour materials. It gives values for several materials for density, hardness, strength, and flexibility. The materials may be used on their own, or sandwiched into layers. After the table is a section on what the properties actually mean, and how they are useful for the purpose of armour. Following that are links to useful mechanical engineering / materials science sites.

2. The data table

Material	type	Ultimate tensile strength (MPa)	Yield strength (MPa)	Young’s (tensile) modulus of elasticity / Elongation at yield (GPa)	Shear Modulus (GPa)	density (kg/m³)	Hardness^A ^{Brinell unless otherwise stated}	Machine / Welding	Max working temp / Melting Point (MP)
Material	type	Ultimate tensile strength (MPa)	Impact strength (Charpy J/cm²)		Shear Modulus (GPa)	density (kg/m³)	Hardness^A ^{Brinell unless otherwise stated}	Machine / Welding	Max working temp / Melting Point (MP)
Steel	1020	380	207	207 /	79.3	7750	222	(2)
Steel	Stainless (201)	696	301	207 /	73.1	7860	171	(5)
Hardox	400	1250	1000 / 45				370-430	(6)
	450	1400	1200 / 35				425-475
	500		? / 30				470-540
	600						570-640
Titanium	Grade 2	345	275	103 /	41.4	4510	175	(3)
	Grade 5	552	830	110 /	41.4	4420		(3)
	10% Vanadium	1193	1103	109.6 /	42.1	4650	369
Aluminium	Alloy type 0	90	48.3	73.1 /	69.0	2768	23	(4)
Brass	C36000	403	217	110 /	40.1	8498	127
Copper	C110	331	310	108 /	44.7	8885	92
Polycarbonate	Moulded	64	69.7 / 0.9	2.3 /	2.3	1200	HRM: 71	(1)	138ºC
Wood	Plywood	31	13.8	12.4 /	0.62	615
Wood	Douglas fir	130		11.0 /	4.1	443
Fibre glass	Veil surface mat	55		4.8 /		1246
Carbon Fibre	Thermoset composite	810	200	190 /		1570	Barcol 63.3
Kevlar		338		34.5 /	19	1384			425ºC
Peek	Aramid Fiber Filled	9.5	10 / 0.35	3.8 /		1420	HRM: 99		250ºC
UHMW Polyethelyne	Garland GAR-DUR	44.1	23.4	? / 14%		941	HRR: 64 ShoreD: 67		MP: 130ºC
Glass	96% silica	~2000		46.2 /	18.6	2602	550

Notes

A. There are several different measures of a material's hardness, which can make it difficult to compare one material with another. There are conversion tables around which give approximate equalities:

2.1. Machining:

1. Parts can be easily machined from standard metal working tools. No special tools are needed, and finished parts can be polished to a high gloss. Water or water-soluble cutting oils should be used when machining polycarbonate, since some standard cutting oils will attack the material. Polycarbonate can be machined on standard metalworking or woodworking equipment. Its unique properties permit it to be machined without chipping, splitting, or breaking.

2. Machinability is good at 65% compared to 1112 carbon steel as 100% baseline. Readily weldable by all of the standard methods

3. As a family, titanium and its alloys have developed a mystique as a nightmare to machine. This is simply not the case. Experienced operators have compared its characteristics to those found in 316 stainless steel. Recommended practice includes high coolant flow(to offset the material's low thermal conductivity), slow speeds and relatively high feed rates. Tooling should be tungsten carbide designations C1-C4 or cobalt type high speed tools.

4. Readily machined, preferably using carbide tooling. Lathe cutting may be done dry, unless heavy cuts are made in which case oil lubricants may be used. Cutting tools should have large rake angles (top and side), similar to tooling for the cutting or machining of high speed steels. Readily welded by commercial techniques such as electric resistance, inert arc, gas welding with inert-gas shielded arc preferred. Filler metal of Aluminum 1100 alloy should be used. If welding AL 1100 to a higher alloyed aluminum alloy, such as 6063 or 5052 then the filler rod should be Aluminum 4043 alloy

5. All conventional welding methods are applicable here. Filler metal of similar composition, or of standard 18-8 stainless steels may be used. 201 is sensitive to intergranular attack along weld boundaries. If heavy welding is required, many producing mills can provide a low carbon version of the alloy.

3. Meaning of the parameters

Elastic limit. Greatest stress that can be applied to a material without causing permanent deformation. For metals and other materials that have a significant straight line portion in their stress-strain diagram, elastic limit is approximately equal to proportional limit. For materials that do not exhibit a significant proportional limit, elastic limit is an arbitrary approximation (apparent elastic limit).

Hardness. Measure of a material's resistance to localized plastic deformation. Most hardness tests involve indentation, but hardness may be reported as resistance to scratching (file test), or rebound of a projectile bounced off the material (scleroscope hardness). Some common measures of indentation hardness are Brinell hardness number, Rock well hardness number, ASTM hardness number, diamond pyramid hardness number, durometer hardness, Knoop harness and Pfund hardness number. Hardness often is a good indication of tensile and wear properties of a material. A hardness conversion table can be found by following the appropriate link in the index frame here.

Impact strength. Energy required to fracture a specimen subjected to shock loading, as in an impact test. Alternate terms are impact energy, impact value, impact resistance and energy absorption. It is an indication of the toughness of a material. For more details of impact testing, see here.

Plastic deformation. Deformation that remains after the load causing it is removed. It is the permanent part of the deformation beyond the elastic limit of a material. It also is called plastic strain and plastic flow.

Shear modulus of elasticity. Tangent or secant modulus of elasticity of a material subjected to shear loading. Alternate terms are modulus of rigidity and modulus of elasticity in shear. Also, shear modulus of elasticity usually is equal to torsional modulus of elasticity.

Strain. Change per unit length in a linear dimension of a part or specimen, usually expressed in %. Strain as used with most mechanical tests is based on original length of the specimen. True or natural strain is based on instantaneous length and is equal to In l/lo where l is instantaneous length and lo is original length of the specimen. Shear strain is the change in angle between two lines originally at right angles.

Stress. Load on a specimen divided by the area through which it acts. As used with most mechanical tests, stress is based on original cross section area without taking into account changes in area due to applied load. This sometimes is called conventional or engineering stress. True stress is equal to the load divided by the instantaneous cross section area through which it acts.

Stress-strain ratio. Stress divided by strain at any load or deflection. Below the elastic limit of a material it is equal to tangent modulus of elasticity. An alternate term is secant modulus of elasticity. Stripping strength. Alternate term for peel strength.

Strain energy. Measure of energy absorption characteristics of a material under load up to fracture. It is equal to the area under the stress strain diagram, and is a measure of the toughness of a material.

Tensile strength. Ultimate strength of a material subjected to tensile loading. It is the maximum stress developed in a material in a tension test.

Toughness. Extent to which a material absorbs energy without fracture. It is usually expressed as energy absorbed in an impact test. The area under a stress-strain diagram also is a measure of toughness of a material. (ASTM D-256, plastics and ASTM E-23, metals).

Ultimate strength. Highest engineering stress developed in material before rupture. Normally, changes in area due to changing load and necking are disregarded in determining ultimate strength.

Yield strength. Indication of maximum stress that can be developed in a material without causing plastic deformation. It is the stress at which a material exhibits a specified permanent deformation and is a practical approximation of elastic limit.

Offset yield strength is determined from a stress-strain diagram. It is the stress corresponding to the intersection of the stress-strain curve and a line parallel to its straight line portion offset by a specified strain. Offset is usually specified as 0.2 %, i.e., the intersection of the offset line and the 0-stress axis is at 0.2 % strain.

4. Links

4.1. The materials database web site

4.2. Titanium

4.3. Various metals

Mechanical properties of metals

4.4. Plastics & composites

USA based supplier of carbon fibre laminates. Does custom jobs. Some technical information on web page. May provide sponsorship!
http://www.dragonplate.com