Strength of Steel

[Cat Whisperer]

LetÂ’s look at a few terms relating to STRENGTH of steel.  ThereÂ’s a lot of information here, but much more to come.  I think this will help in understanding how complex the issues of picking a steel is.  The issue is how the steel reacts to the applied forces.  Here we're looking at the types of strength there are.  Look carefull at Yield strength and Tensile strength.  When we get to testing and determining how to measure strength in particular geometries, this will be important.

Comparison of strengths between several types of steels is next; then more terms and finally some tests to determine characteristics of steel.

The following is reprinted from MATERIALS ENGINEERING, June 1967 issue, Reinhold Publishing Corporation, 430 Park Avenue, New York, N. Y. 10022.  

Bending strength. Alternate term for flexural strength. It is most commonly used to describe flexure properties of cast iron and wood products.

Breaking strength. Tensile load or force required to rupture textiles (e.g., fibers, yarn) or leather. It is analogous to breaking load in a tension test. Ordinarily, breaking strength is reported as lb or lb/in. of width for sheet specimens.

Cohesive strength. Theoretical stress that causes fracture in tension test if material exhibits no plastic deformation.

Compressive strength. Maximum stress a material can sustain under crush loading. The compressive strength of a material that fails by shattering fracture can be defined within fairly narrow limits as an independent property. However, the compressive strength of materials that do not shatter in compression must be defined as the amount of stress required to distort the material an arbitrary amount. Compressive strength is calculated by dividing the maximum load by the original cross section area of a specimen in a compression test.

Creep strength. Maximum stress required to cause a specified amount of creep in a specified time. Also used to describe maximum stress that can be generated in a material at constant temperature under which creep rate decreases with time. Alternate term is creep limit.

Fatigue strength. Magnitude of fluctuating stress required to cause failure in a fatigue test specimen after a specified number of cycles of loading. Usually determined directly from the S-N diagram.

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.

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

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.

[CAV Trooper]

Tim,

Good info. Thanks for your effort. When I'm researching materials for gun tubes, I usually go by the tensile strength to determine if something will be strong enough.

[Cat Whisperer]

I used to, untill I realized it wasy yield strength that was the important one.  This relates directly to the EBWhite tests I posted links for in the sticky's.  

More to come.  Lots of reading before we get to the charts.  Hopefully this will provide a common ground of BASIC understanding of terms.  There are other factors influencing design than what we'll be able to cover here.

[Cat Whisperer]

Here are some terms related to brittleness, toughness, ductility and stress & strain.

It is important to understand the difference between toughness and ductility.  One refers to the resistance to fracture the other to the ability of the material to be formed.  These become important when comparing two (or more) different types of steel (or different hardness of steels of the same type).  

The following terms are reprinted from MATERIALS ENGINEERING, June 1967 issue, Reinhold Publishing Corporation, 430 Park Avenue, New York, N. Y. 10022

Brittle fracture. Failure or rupture of a material with little or no plastic flow or deformation of a metal's crystal lattice. Usually this type of failure is associated with impact loads. However, many materials at low temperatures also show brittle fracture failures under static loads. Two common methods for determining resistance to brittle fracture are the Izod and Charpy impact tests. (ASTM E-23).
Charpy impact test. Method for determining behavior of materials under conditions favorable to brittle fracture. Used where results of a tension impact test would not be significant. It is the most popular impact test. Test is performed by striking a notched specimen supported as simple beam with a falling weight. Results are reported as energy absorbed in fracture and a description of the fracture. ASTM E-23 describes test for metals, ASTM A 327 for cast iron, ASTM D-256 for plastics and ASTM D-758 for plastics at subnormal and elevated temperatures.
Drop ball impact test. Method for determining the energy absorption characteristics of a material subjected to shock loading. Metal ball of known weight is dropped on specimen from regularly increasing heights and height of drop, producing failure is reported. Test is used for hard metals, ceramics and plastics.
Drop weight test. Method for determining the nil-ductility transition temperature of steel. Results are reported as temperature above which specimens no longer show brittle fracture after specified shock loadings. (ASTM E-208).
Ductility. Extent to which a material can sustain plastic deformation without rupture. Elongation and reduction of area are common indices of ductility.
Embrittlement. Reduction in ductility due to physical or chemical changes.
Flattening test. Measure of the ductility of metal pipe. A short section of pipe is crushed diametrically between parallel plates to a specified extent and examined for failure.
Fracture stress. True stress generated in a material at fracture.
Fracture stress. True stress generated in a material at fracture.
Fracture test. Visual test wherein a specimen is fractured and examined for grain size, case depth, etc.
Fracture toughness. Ability of a material to resist crack propagation when subjected to shock load as in an impact test.
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.
Nominal stress. Stress calculated on the basis of the net cross section of a specimen without taking into account the effect of geometric discontinuities such as holes, grooves, fillets, etc.
Notch brittleness. Phenomena by which brittle fracture occurs more readily in notched specimens than in notch free specimens.
Notch ductility. Reduction in area of a notched specimen at fracture in a tension test.
Notch sensitivity. Measure of reduction in load-carrying ability caused by stress concentration in a specimen.
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.
True strain. Instantaneous % change in length of specimen in mechanical test. It is equal to the natural logarithm of the ratio of length at any instant to original length.
True stress. Applied load divided by actual area of the cross section through which load operates. It takes into account the change in cross section that occurs with changing load.
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).