MEASURING STEEL STRENGTH

Determining the strength of steel is an examination of the relationship between external forces applied to steel and the resulting deformation and stresses. These forces are produced by the action of gravity, by accelerations and impacts of moving parts, by gases and fluids under pressure, and by the transmission of mechanical power. They can occur separately, in combination, or progressively. Time is also a critical element in determining the effects of external forces ‑ 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.

Types of Stress

• Tensile Stress ‑ Pulls steel apart, resulting in stretching and elongation
• Compressive Stress ‑ Pushes steel together, leading to shortening and spreading in the lateral direction
• Shearing Stress ‑ Pushes steel from opposite sides, resulting in failure of the steel in the form of breakage

Stress

Stress is defined as force per unit area and in the U.S. 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 the applied force; shearing stresses always act in the same plane.

Fatigue

When steel is subjected to many cycles of stress reversal or fluctuation (variation in magnitude without reversal), failure may occur, often at levels considerably less than if the stress were constant. Fatigue properties are determined by subjecting test specimens to stress cycles and counting the number of cycles to failure.

Ductility

Ductility is the ability of steel to undergo permanent changes in shape (such as stretching into a wire) from tensile stress without fracturing at room temperature or losing its toughness. Brittleness is the opposite of ductility. Malleability refers to deformation under compressive stress (such as being pressured into a sheet). Ductility is sometimes used to refer to both types.

When steel is subjected to many cycles of stress reversal or fluctuation (variation in magnitude without reversal), failure may occur, even though the maximum stress at any cyle is considerably less than the value at which failure would occur if the stress were constant. Fatigue properties are determined by subjecting test specimens to stress cycles and counting the number of cycles to 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.

Modes of Fatigue Failure

Low-/High-Cycle

This fatigue process covers cyclic loads in two significantly different domains with different physical mechanisms of failure. "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. "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.

Corrosion

Corrosion fatigue is a failure mode where cyclic stresses and a corrosion producing environment combine to initiate and propagate cracksn 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.

Thermal

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 load. When strain cycling is produced by a fluctuating temperature field, the failure process is termed "thermal fatigue."

Surface or Contact

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.

Shear strength is tested by measuring the force necessary to sever a sample into two pieces for single shear, or three pieces for double shear. In a single shear test the workpiece is supported on only one end, whereas in a double shear test the workpiece is supported from both ends, which requires greater force to break a middle piece free. Both tests result in strength ratings that categorize the metal.

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