Aircraft Material Testing Methods

10 Questions

What is the purpose of the impact test described in the text?

To assess the toughness of a material

Why is it not normal to use Brinell, Rockwell, or Vickers testing methods on aircraft in the hangar?

These methods may not be portable

What happens to the swing of the pendulum in an impact test when a brittle material is struck?

Remains unchanged

Which type of test uses notched bar test pieces of standard dimensions struck by a fast moving, weighted pendulum?

'Charpy' test

What happens to the toughness of some materials when the environmental temperature drops?

It decreases significantly

What is the term used to describe the continuing deformation in materials under prolonged stress, even when the stress is below the yield point?

Primary Creep

Which alloying element is mentioned as one of the most effective for lowering the Transition Temperature of steels?

Nickel

At what temperatures is creep commonly regarded as a phenomenon associated with superheated steam plant and gas turbine technology?

Well above 0.4T

In metals undergoing creep, which stage experiences a rapid increase in the rate of strain leading to rupture?

Tertiary Creep

What is the temperature range where some materials, especially certain steels, show a rapid drop in toughness as the temperature decreases?

Transition Zone

Study Notes

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.