How is toughness measured

See our failure analysis page. As shown in the figure, for any particular alloy, toughness decreases as strength increases. Consequently, when high toughness and high strength are both required it is often necessary to change from one alloy to a different alloy that satisfies both requirements.

Designers are often tempted to use a material that is as strong as possible to enable them to minimize component cross-section. However, this can inadvertently lead to using a material with insufficient fracture toughness to withstand fracturing if a small crack forms in the material during component manufacturing or during use. Fatigue stress is also another cause of cracks.

The formation of cracks in components exposed to fatigue conditions is often expected. In these situations, knowledge of the fracture toughness is required to determine how long the component can remain in service before a crack grows so long that the intact cross-section of the component cannot support the load, and the component fractures.

This applies to aerospace components and pressure vessels such as boilers. Want to improve your metallurgy and metals engineering knowledge? See our metallurgy courses page for training options. For structural components exposed to fatigue conditions, designers must be concerned with both the strength and the toughness.

Toughness is the ability of a material to absorb energy before ultimate failure. Material which can absorb more energy before failure is considered more tough than another material which can absorb less energy.

There are mainly two tests which are generally used to measure toughness. Izod impact tester. Toughness of a material can be measured using a small specimen of that material. A typical testing machine uses a pendulum to strike a notched specimen of defined cross-section and deform it.

The height from which the pendulum fell, minus the height to which it rose after deforming the specimen, multiplied by the weight of the pendulum is a measure of the energy absorbed by the specimen as it was deformed during the impact with the pendulum.

At low temperatures the material is more brittle and impact toughness is low. At high temperatures the material is more ductile and impact toughness is higher. Little or no deformation in the shape of the part is observed. The fracture is usually flat and perpendicular to the stress axis. The fracture surface is shiny, with a grainy appearance. Failure occurs rapidly, often with a loud report. Often, the fracture appearance is faceted.

In a tough material, the energy absorbed by the part is substantial. Visual distortion of the part is observed. The fracture surface is dull and fibrous. The material is ductile. A variety of tests have been developed to measure the toughness of a material.

Some test methods can be used directly in the design of a part, while other test methods cannot be. The Charpy V notch test is a test for measuring impact strength in which a small notched bar is loaded dynamically in three-point bending. The specimen has a square cross section of 10mm and a length of 55mm. The notch radius is 0. Dimensions are shown in Figure 1. An example of a Charpy Impact Tester is shown in Figure 2. In this test, the test specimen is removed from its cooling or heating bath and placed on the specimen fixture.

The pendulum is released, and the specimen is broken within five seconds after removal from the bath. The calibrated dial of the impact machine is read, and the broken specimen is retrieved. If high-strength, low-energy specimens are tested at low temperatures, the specimens tend to leave the machine perpendicular to the swing of the pendulum.

This may cause errors in reading as well as pose hazards to the operator from the specimens hitting the pendulum. If the specimen hits the pendulum with enough energy, the pendulum will slow down and the machine will record a higher impact energy absorbed than truly occurred. Being hit by the pendulum forces the specimen to bend and fracture. The strain rate of loading is high, approximately 10 3 s Because of the high-strain rate, a considerable plastic constraint exists at the notch.

This plastic constraint yields a triaxial stress state at the notch tip. In steels, during impact loading, a transition from ductile to brittle fracture occurs that is dependent on temperature. The temperature at which it occurs is called the transition temperature.

Other variables such as geometry, grain size, and alloying elements affect the ductile to brittle transition temperature, but only within a given alloy. The toughness versus temperature curve Figure 3 has three basic regions: the upper shelf, the lower shelf, and the transition region. The upper shelf is characterized by primarily ductile fracture.