What is the Ultimate Tensile Strength of Materials?

Civil Guide

Ultimate Tensile Strength?

The ultimate tensile strength of a material is the maximum stress that it can withstand and resist tearing due to tension. This is an important material property used in the design of beams, vehicles and in other engineering industries.

The ultimate tensile strength is found through experimental data on specimens. A steel rod is placed in a tensile testing machine (universal testing machine), which involves applying a tensile stress to the metal rod. The metal rod is held by 2 clamps (top and bottom), with an applied tensile load at one end. The strain in the rod is recorded against the applied stress at each instance until failure, and this is recorded as a stress strain graph.

The ultimate tensile strength (denoted \(\sigma_{uts}\)) as  can be seen as the highest point in the graph below, and we can see the stress decreases after this point. It is located in the plastic zone of the material.

The SI units for Ultimate tensile strength is MPa (N/mm2).

Typical Ultimate Tensile Strength Table

Steel, structural ASTM A36 steel 250 400–550
Steel, 1090 mild 247 841
Chromium-vanadium steel AISI 6150 620 940
Steel, 2800 Maraging steel 2617 2693
Steel, AerMet 340 2160 2430
Steel, Sandvik Sanicro 36Mo logging cable precision wire 1758 2070
Steel, AISI 4130, water quenched 855 °C (1570 °F), 480 °C (900 °F) temper 951 1110
Steel, API 5L X65 448 531
Steel, high strength alloy ASTM A514 690 760
High-density polyethylene (HDPE) 26–33 37
Polypropylene 12–43 19.7–80
Cast iron 4.5% C, ASTM A-48 130 200
"Liquidmetal" alloy[citation needed] 1723 550–1600
Beryllium99.9% Be 345 448
Aluminium alloy2014-T6 414 483
Polyester resin (unreinforced) 55 55
Polyester and chopped strand mat laminate 30% E-glass 100 100
S-Glass epoxy composite 2358 2358
Aluminium alloy 6061-T6 241 300
Cupronickel 10% Ni, 1.6% Fe, 1% Mn, balance Cu 130 350
Brass 200 + 500
Tungsten 941 1510
S-Glass N/A 4710
Basalt fiber N/A 4840
Marble N/A 15
Concrete N/A 2–5
Carbon fiber N/A 1600 for laminates, 4137 for fibers alone
Carbon fiber (Toray T1100G) (the strongest human-made fibres) 7000 fibre alone
Bamboo 350–500
Spider silk (see note below) 1000
Spider silk, Darwin's bark spider 1652
Silkworm silk 500
Aramid (Kevlar or Twaron) 3620 3757
UHMWPE 24 52
UHMWPE fibers (Dyneema or Spectra) 2300–3500
Vectran 2850–3340
Polybenzoxazole (Zylon) 2700 5800
Wood, pine (parallel to grain) 40
Bone (limb) 104–121 130
Nylon, molded, type 6/6 450 750
Rubber 16
Boron N/A 3100
Silicon, monocrystalline (m-Si) N/A 7000
Ultra-pure silica glass fiber-optic strands 4100
Sapphire (Al2O3) 400 at 25 °C, 275 at 500 °C, 345 at 1000 °C 1900
Diamond 1600 2800
Carbon nanotube (see note below) N/A 11000–63000
Carbon nanotube composites N/A 1200
High-strength carbon nanotube film N/A 9600
Iron (pure mono-crystal) 3

What is Tensile Strength?

The tensile strength of a material is its ability to resist tensile stress (i.e. force required to pull something, beam, or rope) before it fails/factures.

When steel enters into plastic deformation, it undergoes strain-hardening (also known as work hardening). The strength of the material increases during the phase and the magnitude of tensile strength is at its highest before necking occurs.

What is Necking?

Necking is a phenomenon which occurs only to a few materials, and it is at this point the cross-sectional area reduces and the material weakens. This process occurs until failure.

Necking and Ultimate Tensile Strength
Necking

What is Yield Stress?

Yield Stress or yield strength is the point in the stress strain curve of a material when the strain becomes permanent, and any deformation in the material is permanent. 

Note – As a material is loaded, it initially undergoes, elastic deformation and strain, which is reversible once the load is removed. Once it goes past the yield stress/strength, the deformation is permanent, even if the load is removed.

This is important in the design of any structure, infrastructure, or product, as engineers generally design in the elastic region.

We keep the load in the elastic area to keep any engineering products/structures safe from failure (i.e, product is safe from plastic deformation).

In design calculations, we use factors of safety for the applied load and the material. The applied load can be multiplied in the region between 1 and 2. Also, the stress the material (allowable stress) can take is also factored and this can be in a region between 1 and 2.

These factors of safety ensure contingency for any miscalculations or unforeseen additional loads, and also any material defects or imperfections. 

Engineers do undertake designs using the ultimate tensile strength for structures or mechanical parts that are subjected to large loads. However, they need to take into account permanent deformation and strain, as the structure/parts, will not be the same due to a change in its crystal structure.

An example is a metal plate with bolt holes. If the plate is subjected to large loads greater that its yield stress, it will undergo permanent strain. Once the load is released the metal plate will not go back into its original shape and the bolt holes will have changed shapes. This may result in the bolts not fitting anymore!!

Stress strain curve
Stress strain curve

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