Mechanical Properties of Steel

Strength of materials deals with the relations between the external forces applied to steel and the resulting deformations and stresses. In the design of structures and machines, the application of the principles of strength of materials is necessary if satisfactory steel is to be utilized and adequate proportions obtained to resist functional forces.

Forces are produced by the action of gravity, by accelerations and impacts of moving parts, by gases and fluids under pressure, by the transmission of mechanical power, etc. In order to analyze the stresses and deflections of a steel piece, the magnitudes, directions and points of application of forces acting on the piece must be known.

The time element in the application of foce on a steel piece is an important consideration. A force may be static or change so slowly that its maximum value can be treated as if it were static; it may be suddenly applied, as with an impact, or it may have a repetitive or cyclic behavior.

The environment in which forces act on a machine or part is also essential. Factors such as high and low temperatures; the presence of corrosive gases, vapors, liquids, radiation, etc. may have a significant impact on how effectively parts are able to resist stresses.

Many mechanical properties of steel are determined from tests that show relationships between stresses and strains. These values can be charted to show specific relationship changes as the values progress.

Stress

Stress is defined as the force per unit area and is usually expressed in pounds per square inch (psi). Tensile stress will stretch or lengthen steel. Compressive stress will compress or shorten steel. Shearing stress will break or tear steel into pieces. Tensile and compressive stresses always act at right angles to (normal to) the area being considered; shearing stresses are always in the plane of the area (at right angles to compressive or tensile stresses).

Unit Strain

Unit strain is the amount by which the dimension of steel changes when it is subjected to a load, divided by the original value of the dimension. The simpler term “strain” is often used instead of unit strain.

Proportional Limit

The Point on a stress/strain ratio curve at which it begins to deviate from the straight line relationship between stress and strain. This is a mathematical point and is not directly observable.

Elastic Limit

Elastic limit is the maximum stress to which a test specimen may be subjected and still return to its original length upon release of the load. Steel is said to be stressed within the elastic region when the working stress does not exceed the elastic limit, and to be stressed in the plastic region when the working stress exceeds the elastic limit. The elastic limit for steel is for all practical purposes the same as its proportional limit.

Yield

Modes of Fatigue Failure

Fatigue is tested on fixtures that are unique to the application. These tests should account for all modes of failure, including thermal causes and the presence of corrosive elements. Several modes of fatigue failure are:

Low/High-Cycle Fatigue

This fatigue process covers cyclic loading in two significantly different domains with different physical mechanisms of failure. One domain, known as “high‑cycle fatigue” is characterized by relatively low cyclic loads, strain cycles confined largely to the elastic range, and long lives or a high number of cycles to failure.

The other domain, known as “low‑cycle fatigue” or cyclic strain‑controlled fatigue, has cyclic loads that are relatively high, significant amounts of plastic strain induced during each cycle, and short lives or a low number of cycles to failure.

The transition from low‑ to high‑cycle fatigue behavior occurs in the range from approximately 10,000 to 100,000 cycles. Typical low‑cycle fatigue is defined as failure that occurs in 50,000 cycles or less.

Thermal Fatigue

Cyclic temperature changes in a machine part will produce cyclic stresses and strains if natural thermal expansions and contractions are either wholly or partially constrained. These cyclic strains produce fatigue failure just as though they were produced by external mechanical loading. When strain cycling is produced by a fluctuating temperature field, the failure process is termed “thermal fatigue.”

Corrosion Fatigue

Corrosion fatigue is a failure mode where cyclic stresses and a corrosion producing environment combine to initiate and propagate cracks in fewer stress cycles and at lower stress amplitudes than would be required in a more inert environment. The corrosion process forms pits and surface discontinuities that act as stress raisers to accelerate fatigue cracking. The cyclic loads may also cause cracking and flaking of the corrosion layer, baring fresh metal to the corrosive environment. Each process accelerates the other, making the cumulative result more serious.

Surface or Contact Fatigue

Surface fatigue failure is usually associated with rolling surfaces in contact, and results in pitting, cracking, and spalling of the contacting surfaces from cyclic contact stresses that cause shear stresses to be slightly below the surface. the cyclic subsurface shear stresses generate cracks that propagate to the contacting surface, dislodging particles in the process.

Creep Failure

Unlike fatigue failure, creep failure is the result of strain developing from long‑term stress. A solid material can imperceptibly flow or deform when subjected to high temperatures or stresses over a long period. Common examples of creep are the visible distortions in antique glass, the slow movement of glaciers, and the filament pulling apart in household light bulbs.

Combined Creep and Fatigue

In this failure mode, all of the conditions for both creep failure and fatigue failure exist simultaneously. Each process influences the other in producing failure, but this interaction is not well understood.