Aircraft Material Testing Methods

Hardness Testing on Aircraft

• Portable hardness testers are used to test material hardness on aircraft items like wheels, especially after an overheat condition.
• Overheat conditions can cause wheel material to become soft or partially annealed.

Impact Testing

• The impact test determines the toughness of a material.
• Two commonly used types are Charpy and Izod tests.
• Both tests use notched bar test pieces of standard dimensions struck by a fast-moving, weighted pendulum.
• The energy absorbed by the test piece on impact measures toughness.
• Brittle materials break easily, absorbing little energy, while tough materials absorb considerable energy and greatly reduce the pendulum swing.
• Most materials show a drop in toughness with a reduction in temperature.
• Some materials (certain steels) show a rapid drop in toughness as the temperature is progressively reduced, known as the Transition Zone.
• Parts designed for low-temperature use should be operated above the material Transition Temperature.
• Nickel is an effective alloying element for lowering the Transition Temperature of steels.

Other Forms of Material Testing

• Fatigue Testing and Creep Testing are other important forms of material testing.

Creep

• Creep is the continuing deformation with time in materials subjected to prolonged stress.
• Deformation is plastic and occurs even though the acting stress may be below the yield stress.
• Creep rate is very low at temperatures below 0.4T (where T is the melting point in Kelvin) but increases at higher temperatures.
• Creep is commonly associated with high-temperature phenomena like superheated steam plant and gas turbine technology.
• Some soft, low-melting-point materials creep significantly at ambient temperatures, and some aircraft materials may creep during overheat conditions.

Creep in Metals

• Metallic materials undergo immediate elastic deformation, followed by plastic strain in three stages: Primary Creep, Secondary Creep, and Tertiary Creep.
• Primary Creep begins at a relatively rapid rate, decreasing with time as strain hardening sets in.
• Secondary Creep has a fairly uniform rate of strain, at its lowest value.
• Tertiary Creep has a rapidly increasing rate of strain, leading to rupture and coinciding with gross necking of the component prior to failure.
• The rate of creep is at a maximum in the Tertiary Creep phase.