How to Measure Residual Stress

Measurements – Imperial and Metric explanation

Ksi means kilopound per square unit and is derived from psi(lbs/square inch).  Kilo, which is often used in the metric system basically means a thousand (1000).  It comes from the Greek word for 1000. Therefore, a kilopound is 1000 pounds.

A Megapascal (or MPa) is a variation on a pascal.   Mega in this case means 1,000,000 and again it means “great”.  As you may guess, when using MPa you are usually working with something heavy or with a high pressure!

Both ksi (kilopound per square inch) and megapascal (MPa) are units of pressure measurement – the former is an Imperial unit of measurement and the latter a metric unit.
Ksi values are commonly converted to MPa, as the metric system is the more widely used system of measurement.

Kilopound per square inch (ksi)

Ksi (kilopound per square inch) is a unit of stress or pressure resulting from one kilopound force acting on an area of one square inch. It is derived from one pound square inch (psi). Kilo is widely used in the metric system and means one thousand. Therefore 1 kilopound is 1000 pounds.  1 ksi = 1 psi * 1000 1 psi = 1 lbf / 1 in2 (pounds per square inch)

Ksi are used where the pressure measured is a large value of psi. Ksi finds its application mostly in materials science to measure the tensile strength of metal beams. For example, A36 steel has a tensile strength of 58 ksi.

The psi is an imperial unit of pressure measurement most commonly used in the United States, Burma and Liberia.

One kilopound per square inch (ksi) is approximately 6.894759 MPa.

Megapascal (MPa)

Megapascal is a multiple unit of Pascal (1 MPa = 1,000,000,000 Pa). The Pascal is an SI (Système International d’Unités) unit of pressure measurement. It is more commonly used in many countries. It is named after the French mathematician, inventor and physicist Blaise Pascal. The Pascal is a measure of force per unit area is defined as one Newton per square metre.  1 Pa = 1 N / 1 m2

As base units of SI, one Pascal can be expressed as 1 Pa = 1 kg / 1 m * s2

The standard atmospheric pressure is 101,325 Pa or 0.101325 MPa.

As with ksi, megapascal is also widely used in materials science to measure the tensile strength and the stiffness of materials.

Below we show some common conversions:

  • 1 ksi to MPa = 6.89475908677537
  • 2 ksi to MPa = 13.789518173550741
  • 25 ksi to MPa = 172.36897716938424
  • 30 ksi to MPa = 206.8427726032611
  • 40 ksi to MPa = 275.7903634710148
  • 45 ksi to MPa = 310.26415890489164
  • 80 ksi to MPa = 551.5807269420296
  • 100 ksi to MPa = 689.475908677537

Click on links to read

1 – What Will Peening Do For My Components?
2 – Measurement Of Compressive Residual Stress Using X-ray Diffraction
4 – MPa and Relationship to Almen Strips

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Haydn Kitchen New APost written by
Haydn Kitchen
Shot Blasting Technical Manager

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