Pleaes help me with "tensile-strength"

Date 08-14-2004 00:56

Ultimate tensile strength is essentially the stress at which the bolt breaks when it is being pulled apart. Engineers generally do not count on being able to load things up to this limit. They are usually more interested in yield strength. The bolt will start to stretch at some stress below this. If the stress is less than the yield strenth, the bolt will return to its original shape when the stress is removed. If the stress exceeds the yield strength, the part will permanently deform. Most designs limit tensile stress to well below the yield point.

IIRC you can estimate ultimate shear strength to be about 60% of ultimate tensile strength. But don't rely on that. Use the manufacturer's specs.

There are many kinds of steel made for many different uses. Grade-8 bolts are made from medium carbon alloy steel. It has alloying ingredients that increase the hardness. And it has enough carbon that it can hardened by heat treating.

By MBSims

Date 08-14-2004 02:31

Put another way, the tensile strength is the amount of tension force (in pounds or Pascals) required to break the bolt, divided by the original cross sectional area (in square inches or square millimeters) at the fracture location (usually the smallest diameter at the root of the threads). So PSI is pounds per square inch, or pounds of force divided by square inches of material.

The point at which the bolt begins to neck down as it stretches under force (tension) is the yield point. The yield strength is the force at the yield point divided by the original cross-sectional area.

The shear strength is proportional to tensile strength. The shear strength of steel is approximately the tensile strength divided by the square root of 3 (or tensile strength divided by 1.732 or app. 58% of the tensile strength). For a bolt with 150,000 psi tensile strength, the shear strength is approximately 86,600 psi.

By swnorris (****)

Date 08-16-2004 11:59

Hi RadiationBurn,

In simple terms, tensile strength is a material's resistance to breaking when it is subjected to pulling forces.

By G.S.Crisi (****)

Date 08-16-2004 20:04

MBSims, don't get upset if I make a couple of corrections on your statement.
In the SI metric system, Pascal is the unit of pressure, not force. The unit of force is the Newton (simplified N). One Pascal is one Newton per square meter. As it's a very small amount of pressure, it's usual to use kilo or megaPascals (kPa or MPa).
Also, the point at which the specimen under tensile stress starts to neck down is not the yield point. In carbon and low alloy steel, the yield point comes right after the elastic limit, i.e., the point at which the specimen looses its elastic properties and elongations are permanent, as Malcolm correctly says.
In the yield point, carbon and low alloy steels continue to elongate at an approximately constant tensile strength. In the tensile test this point lasts a few seconds, then the specimen enters the plastic region, in which the elongations are permanent. It is in the highest point of this region that the specimen starts to neck down.
In the tensile test diagram the yield point is recognized because it consists of a short "sawteeth" horizontal line located right after the elastic region and before the plastic one.

Unfortunately for Radiation, ASTM standards seldom specify a minimum shear strength for the materials they cover. They ususally specify minimum tensile strengths. In this case, Radiation can go after Malcolm's and MBSlims' reccomendations, in that the shear stress is approximately 60% (or 58%, if you prefer) of the ultimate tensile.

Giovanni S. Crisi
Sao Paulo - Brazil

By MBSims

Date 08-17-2004 00:24

No offense here Giovanni! The only thing worse than no information is bad information. Thanks for the corrections.