Ultimate Tensile Strength

Ultimate tensile strength (or tensile strength for short) is an important property of materials to determine their mechanical performance. It is the ability of a material to resist cracking due to stress. This parameter is applicable to all types of materials such as wires, ropes, metal beams, etc.

What is tensile strength?

Imagine a strip of paper that you pull the two ends of with your fingers. You exert a tensile force on the strip. When this tensile force exceeds a certain threshold, the paper tears. The tensile stress at which this happens is the tensile strength of that material, in this case paper.

When excessive tension is applied, both tough and brittle materials will approach a breaking point. Initially, uniform deformation will be observed. Over the entire body of the material, the length will increase while the width will decrease by the same amount.

The ultimate tensile strength is the amount of stress that causes materials to pass from the state of uniform plastic deformation to localized concentrated deformation. The phenomenon of necking begins at this point.

Necking process

The final tensile strength is an intensive property. In other words, it does not depend on the size of the sample. The same material with a varying cross section will have the same value of tensile strength.

Since this type of fracture in a system can lead to failure and potentially endanger lives, it is imperative that this parameter be taken into account when selecting appropriate materials for an application.

Ultimate Tensile Strength on a Stress-Strain Curve

There are 4 main areas into which a stress-strain curve can be divided..

1. Proportional limit
2. Yield limit
3. Strain hardening
4. Necking

Proportional limit

In the proportional limit, the specimen material behaves like a spring and any strain induced is fully reversible. On the stress-strain curve, this region is called the Hooke's region. The reason lies in the applicability of Hooke's law for forces that fall in this region.

Yield limit

Once the specimen passes the proportional limit, it enters the region of the yield strength. At this point, permanent deformation occurs. From this point, it does not matter if you release the tensile force or apply a force in the opposite direction, the specimen will not return to its original dimensions.

Strain hardening area

With further increase in tensile stress, the specimen enters the stress hardening region. This is a very unique section because the crystal structure of the material changes. The material is under enough stress that the microstructure itself is altered.

As the name implies, the material becomes harder and tougher. This hardening can be very useful and so is not necessarily a bad thing (cold hardening, cold forming processes actually use this area to impart strength to the workpiece).

Necking area

Just before the necking phase, the material is the strongest it will ever be. The material is stretched to its maximum. When we get to the neck phase, the material begins to weaken. It is characterized by a local reduction in cross-sectional area.

Beyond this point, the material is only going to fail. It can handle less stress with increasing strain.

We can sort of go back to the original equation that says stress is equal to force per unit area and infer that the smaller the area, the higher the stress. The material moves beyond this point until rupturing.

Why Is Tensile Strength Important?

It is imperative to know the tensile strength of a particular metal or any material to ensure it is the right choice for an application. This ensures an incident-free service life.

The results of choosing materials with lower tensile strength than what the application demands can be disastrous.

Engineers turn to yield strength in the design phase to make sure the stress never reaches any higher than that. Otherwise, the structure suffers permanent deformations. But ultimate tensile strength tells us the value that is necessary for complete failure and breaking.

Thus, a roof construction that comes under more stress because of a higher than normal snow load may bend the structure. At the same time, surpassing the tensile strength value means that the roof may fall in.

Tensile Strength vs Yield Strength

Engineers use yield strength when designing products. Keeping the load within this area ensures the product is safe from failure. This means that the maximum load has to stay below the yield strength limit at all times.

A common way of doing so is by determining the maximum load first. Taking the specifics of the chosen material into account, calculations give the answer for the necessary cross-sectional area. Geometry plays an important role in how high loads a part can withstand.

As an extra precautionary measure, a safety factor is added. The safety factor usually falls somewhere between 1.5 and 2. The simplest way of using it is just multiplying the maximum load value by the factor. Adding the safety factor ensures that unexpected loads and material imperfections will not result in broken parts.

Designing for ultimate tensile strength means your part will permanently deform once subjected to the load it was designed for. The material’s crystal structure may change and it will probably lose an important property. This means that the product no longer has the same characteristics that may have been the very reason for its selection.

An important point to note here is that some tools like knives and spanners are strain hardened so that they can be stronger and closer to their ultimate tensile strength value before they can potentially fracture.

Tensile test on ductile metallic materials

Tensile strength is measured by elongating a specimen in a Universal Testing Machine (UTM). A UTM is a tensile testing machine.

The specimen is held on opposite ends using clamps. One of the ends is stationary while pulling the other with real-time monitoring of the forces. A steady increase of force takes place until reaching a point where the specimen breaks. The recording of tensile test data is constant all through the process.

This tensile tester consists of features such as servo automation control (electro-hydraulic), data acquisition, automatic measurement, screen display and test result calculation.

The maximum force that was applied is then divided by the cross-sectional area to obtain the maximum stress it was subjected to. This maximum stress is the value of ultimate tensile strength.

The SI unit of ultimate tensile strength is N/m2 or Pascal with large numbers being expressed in megapascals.

Reference(s).. fractory.com

Typical Tensile and Yield strengths of some materials

 Material Ultimate tensile strength MPa Yield strength MPa Acrylic, clear cast sheet (PMMA) 87 72 Aluminium alloy 2014-T6 483 414 Aluminium alloy 6061-T6 310 270 Aramid (Kevlar or Twaron) 3757 3620 Bamboo 350-500 - Basalt fiber 4840 - Beryllium 99.9% Be 448 345 Bone (limb) 130 104-121 Boron 3100 - Boron nitride nanotube 33000 - Brass 500 200+ Carbon fiber (Toray T1000G)(the strongest man-made fibres) 6370 fibre alone - Carbon nanotube 11000-63000 - Carbon nanotube composites 1200 - Cast iron 4.5% C, ASTM A-48 200 130 Chromium-vanadium steel AISI 6150 940 620 Colossal carbon tube 7000 - Concrete 44683 - Copper 99.9% Cu 220 69 Cupronickel 10% Ni, 1.6% Fe, 1% Mn, balance Cu 350 130 Diamond 2800 1600 Epoxy adhesive 11293 - First carbon nanotube ropes 3600 - Glass 33 - Graphene 130000 - High-density polyethylene (HDPE) 37 26-33 High-strength carbon nanotube film 9600 - Human hair 200-250 140-160 Human skin 20 15 Iron (pure mono-crystal) 3 - Liquidmetal alloy 550-1600 1723 Marble 15 - Nylon fiber, drawn 900 - Nylon, molded, type 6/6 750 450 Polybenzoxazole (Zylon) 5800 2700 Polyester and chopped strand mat laminate 30% E-glass 100 100 Polyester resin (unreinforced) 55 55 Rubber 16 - Sapphire (Al2O3) 1900 400 at 25 °C, 275 at 500 °C, 345 at 1000 °C S-Glass epoxy composite 2358 2358 Silicon, monocrystalline (m-Si) 7000 - Silkworm silk 500 Spider silk (see note below) 1000 - Spider silk, Darwin's bark spider 1652 Steel, 1090 mild 841 247 Steel, 2800 Maraging steel 2693 2617 Steel, AerMet 340 2430 2160 Steel, AISI 4130, water quenched 855 °C (1570 °F), 480 °C (900 °F) temper 1110 951 Steel, structural ASTM A36 steel 400-550 250 Steel, API 5L X65 531 448 Steel, high strength alloy ASTM A514 760 690 Steel, Sandvik Sanicro 36Mo logging cable precision wire 2070 1758 Steel, stainless AISI 302 - cold-rolled 860 520 Tungsten 1510 941 UHMWPE 52 24 UHMWPE fibers (Dyneema or Spectra) 2300-3500 - Ultra-pure silica glass fiber-optic strands 4100 - Vectran 2850-3340 - Wood, pine (parallel to grain) 40 -

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