Ultimate tensile strength

Two vises apply tension to a specimen by pulling at it, stretching the specimen until it fails. The maximum stress it withstands before failing is its ultimate tensile strength.

Ultimate tensile strength (UTS), often shortened to tensile strength (TS) or ultimate strength, is the capacity of a material or structure to withstand loads tending to elongate, as opposed to compressive strength, which withstands loads tending to reduce size. In other words, tensile strength resists tension (being pulled apart), whereas compressive strength resists compression (being pushed together). Ultimate tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. In the study of strength of materials, tensile strength, compressive strength, and shear strength can be analyzed independently.

Some materials break very sharply, without plastic deformation, in what is called a brittle failure. Others, which are more ductile, including most metals, experience some plastic deformation and possibly necking before fracture.

The UTS is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point of the stress–strain curve (see point 1 on the engineering stress/strain diagrams below) is the UTS. It is an intensive property; therefore its value does not depend on the size of the test specimen. However, it is dependent on other factors, such as the preparation of the specimen, the presence or otherwise of surface defects, and the temperature of the test environment and material.

Tensile strengths are rarely used in the design of ductile members, but they are important in brittle members. They are tabulated for common materials such as alloys, composite materials, ceramics, plastics, and wood.

Tensile strength is defined as a stress, which is measured as force per unit area. For some non-homogeneous materials (or for assembled components) it can be reported just as a force or as a force per unit width. In the International System of Units (SI), the unit is the pascal (Pa) (or a multiple thereof, often megapascals (MPa), using the SI prefix mega); or, equivalently to pascals, newtons per square metre (N/m²). A United States customary unit is pounds per square inch (lb/in² or psi), or kilo-pounds per square inch (ksi, or sometimes kpsi), which is equal to 1000 psi; kilo-pounds per square inch are commonly used when measuring tensile strengths.

Concept

Ductile materials

"Engineering" stress–strain (σ–ε) curve typical of aluminum
1. Ultimate strength
2. Yield strength
3. Proportional limit stress
4. Fracture
5. Offset strain (typically 0.2%)

"Engineering" (red) and "true" (blue) stress–strain curve typical of structural steel.

• 1: Ultimate strength
• 2: Yield strength (yield point)
• 3: Rupture
• 4: Strain hardening region
• 5: Necking region
• A: Apparent stress (F/A₀)
• B: Actual stress (F/A)

Many materials can display linear elastic behavior, defined by a linear stress–strain relationship, as shown in the left figure up to point 3. The elastic behavior of materials often extends into a non-linear region, represented in the figure by point 2 (the "yield point"), up to which deformations are completely recoverable upon removal of the load; that is, a specimen loaded elastically in tension will elongate, but will return to its original shape and size when unloaded. Beyond this elastic region, for ductile materials, such as steel, deformations are plastic. A plastically deformed specimen does not completely return to its original size and shape when unloaded. For many applications, plastic deformation is unacceptable, and is used as the design limitation.

After the yield point, ductile metals undergo a period of strain hardening, in which the stress increases again with increasing strain, and they begin to neck, as the cross-sectional area of the specimen decreases due to plastic flow. In a sufficiently ductile material, when necking becomes substantial, it causes a reversal of the engineering stress–strain curve (curve A, right figure); this is because the engineering stress is calculated assuming the original cross-sectional area before necking. The reversal point is the maximum stress on the engineering stress–strain curve, and the engineering stress coordinate of this point is the ultimate tensile strength, given by point 1.

The UTS is not used in the design of ductile static members because design practices dictate the use of the yield stress. It is, however, used for quality control, because of the ease of testing. It is also used to roughly determine material types for unknown samples.

The UTS is a common engineering parameter when designing brittle members, because there is no yield point.

Testing

Round bar specimen after tensile stress testing

Main article: Tensile testing

Typically, the testing involves taking a small sample with a fixed cross-sectional area, and then pulling it with a tensometer at a constant strain (change in gauge length divided by initial gauge length) rate until the sample breaks.

When testing some metals, indentation hardness correlates linearly with tensile strength. This important relation permits economically important nondestructive testing of bulk metal deliveries with lightweight, even portable equipment, such as hand-held Rockwell hardness testers. This practical correlation helps quality assurance in metalworking industries to extend well beyond the laboratory and universal testing machines.

It should be noted that, while most metal forms, such as sheet, bar, tube, and wire, can exhibit the test UTS, fibers, such as carbon fibers, being only 2/10,000th of an inch in diameter, must be made into composites to create useful real-world forms. As the datasheet on T1000G below indicates, while the UTS of the fiber is very high at 6,370MPa, the UTS of a derived composite is 3,040MPa - less than half the strength of the fiber.

Typical tensile strengths

Typical tensile strengths of some materials

Material	Yield strength (MPa)	Ultimate tensile strength (MPa)	Density (g/cm³)
Steel, structural ASTM A36 steel	250	400-550	7.8
Steel, 1090 mild	247	841	7.58
Human skin	15	20	2
Steel, Micro-Melt 10 Tough Treated Tool (AISI A11)	—	5205	7.45
Steel, 2800 Maraging steel	2617	2693	8.00
Steel, AerMet 340	2160	2430	7.86
Steel, Sandvik Sanicro 36Mo logging cable precision wire	1758	2070	8.00
Steel, AISI 4130, water quenched 855 °C (1570 °F), 480 °C (900 °F) temper	951	1110	7.85
Steel, API 5L X65	448	531	7.8
Steel, high strength alloy ASTM A514	690	760	7.8
Acrylic, clear cast sheet (PMMA)	72	114	1.16
High-density polyethylene (HDPE)	26-33	37	0.85
Polypropylene	12-43	19.7-80	0.91
Steel, stainless AISI 302 - cold-rolled	520	860	8.19
Cast iron 4.5% C, ASTM A-48	130	200
"Liquidmetal" alloy	1723	550-1600	6.1
Beryllium 99.9% Be	345	448	1.84
Aluminium alloy 2014-T6	414	483	2.8
Polyester resin (unreinforced)	55
Polyester and chopped strand mat laminate 30% E-glass	100
S-Glass epoxy composite	2358
Aluminium alloy 6061-T6	241	300	2.7
Copper 99.9% Cu	70	220	8.92
Cupronickel 10% Ni, 1.6% Fe, 1% Mn, balance Cu	130	350	8.94
Brass	200 +	550	8.73
Tungsten	941	1510	19.25
Glass		33	2.53
E-Glass	N/A	1500 for laminates, 3450 for fibers alone	2.57
S-Glass	N/A	4710	2.48
Basalt fiber	N/A	4840	2.7
Marble	N/A	15	2.6
Concrete	N/A	2-5	2.7
Carbon fiber	N/A	1600 for laminates, 4137 for fibers alone	1.75
Carbon fiber (Toray T1000G) (the strongest man-made fibres)		6370 fibre alone	1.80
Human hair		10
Bamboo		350-500	0.4
Spider silk (see note below)		1000	1.3
Spider silk, Darwin's bark spider	1652
Silkworm silk	500		1.3
Aramid (Kevlar or Twaron)	3620	3757	1.44
UHMWPE	24	52	0.97
UHMWPE fibers (Dyneema or Spectra)		2300-3500	0.97
Vectran		2850-3340
Polybenzoxazole (Zylon)	2700	5800	1.56
Wood, pine (parallel to grain)		40
Bone (limb)	104-121	130	1.6
Nylon, molded, type 6/6	45	75	1.15
Nylon fiber, drawn		900	1.13
Epoxy adhesive	-	12 - 30	-
Rubber	-	16
Boron	N/A	3100	2.46
Silicon, monocrystalline (m-Si)	N/A	7000	2.33
Silicon carbide (SiC)	N/A	3440	3.21
Ultra-pure silica glass fiber-optic strands		4100
Sapphire (Al2O3)	400 at 25 °C, 275 at 500 °C, 345 at 1000 °C	1900	3.9-4.1
Boron nitride nanotube	N/A	33000	?
Diamond	1600	2800	3.5
Graphene	N/A	130000	1.0
First carbon nanotube ropes	?	3600	1.3
Colossal carbon tube	N/A	7000	0.116
Carbon nanotube (see note below)	N/A	11000-63000	0.037-1.34
Carbon nanotube composites	N/A	1200	N/A
Iron (pure mono-crystal)		3	7.874
Limpet Patella vulgata teeth (Goethite)		4900 3000-6500

^a Many of the values depend on manufacturing process and purity/composition.

^b Multiwalled carbon nanotubes have the highest tensile strength of any material yet measured, with labs producing them at a tensile strength of 63 GPa, still well below their theoretical limit of 300 GPa. The first nanotube ropes (20mm in length) whose tensile strength was published (in 2000) had a strength of 3.6 GPa. The density depends on the manufacturing method, and the lowest value is 0.037 or 0.55 (solid).

^c The strength of spider silk is highly variable. It depends on many factors including kind of silk (Every spider can produce several for sundry purposes.), species, age of silk, temperature, humidity, swiftness at which stress is applied during testing, length stress is applied, and way the silk is gathered (forced silking or natural spinning). The value shown in the table, 1000 MPa, is roughly representative of the results from a few studies involving several different species of spider however specific results varied greatly.

^d Human hair strength varies by ethnicity and chemical treatments.

Typical properties for annealed elements

Element	Young's modulus (GPa)	Offset or yield strength (MPa)	Ultimate strength (MPa)
silicon	107		5000–9000
tungsten	411	550	550–620
iron	211	80–100	350
titanium	120	100–225	246–370
copper	130	117	210
tantalum	186	180	200
tin	47	9–14	15–200
zinc (wrought)	105		110–200
nickel	170	140–350	140–195
silver	83		170
gold	79		100
aluminium	70	15–20	40-50
lead	16		12

See also

• Flexural strength
• Strength of materials
• Tensile structure
• Toughness
• Failure
• Tension (physics)

Further reading

• Giancoli, Douglas, Physics for Scientists & Engineers Third Edition (2000). Upper Saddle River: Prentice Hall.
• T Follett, Life without metals
• George E. Dieter, Mechanical Metallurgy (1988). McGraw-Hill, UK