It is not enough to know only how strong a metal is in tensile strength, or how ductile it is; information as to how it reacts under sudden impact also is of prime importance. This quality is known as toughness. This is determined by impact test. Information about impact test is given in this article.
Resilience and Toughness
Ductility is a measure of how much something deforms plastically before fracture, but just because a material is ductile does not make it tough. The key to toughness is a good combination of strength and ductility. A material with high strength and high ductility will have more toughness than a material with low strength and high ductility. Therefore, one way to measure toughness is by calculating the area under the stress strain curve from a tensile test. This value is simply called “material toughness†and it has units of energy per volume. Material toughness equates to a slow absorption of energy by the material.
As shown above, it is possible to divide a stress – strain diagram into two parts. The part on the left of elastic limit may be called the elastic range and on the right of the elastic limit the plastic range. The area under the curve OP in the elastic range (area OPR, shown in red color) is a measure of the energy per unit volume which can be absorbed by the material without permanent deformation. This value is known as the modulus of resilience. The energy per unit volume that can be absorbed by a material (the area under the entire stress – strain diagram) up to the point of fracture (point B) is known as toughness. This is mainly a property of the plastic range as only a small part of the total energy absorbed is elastic energy.
Comparison between various carbon steels is shown in above figure.
Following variables have a profound influence on the toughness of a material.
- Strain rate (rate of loading)
- Temperature
- Notch effect
A metal may possess satisfactory toughness under static loads but may fail under dynamic loads or impact. Ductility and therefore, toughness decrease as the rate of loading increases. Temperature is the second variable to have a major influence on its toughness. As temperature is lowered, the ductility and toughness also decrease. The third variable, termed notch effect is due to the distribution of stress. A material might display good toughness when the applied stress is uniaxial; but when a multiaxial stress state is produced due to the presence of a notch, the material might not withstand the simultaneous elastic and plastic deformation in the various directions.
Difference Between Strong and Tough
A material requiring large force before rupture/deformation is called strong material. Thus material having higher tensile strength is stronger material.
Toughness is the property of a material to resist failure by impact and is measured by the area under the entire stress – strain diagram up to the point of fracture.
Thus as shown in the above diagram, material having highest tensile strength is the strongest material where as material having largest area under stress-strain diagram is the toughest material.
From above it can be seen that stronger does not mean tougher. In case of comparison between two materials, a stronger material may / may not be a tougher material.
Impact Test
Impact test is carried out to find out relative toughness by a Charpy or Izod test method. These tests are named after their inventors. Notch-type specimens are used for the test. Generally two types of notches are used, the V notch (most commonly used) and the keyhole notch. The two types of specimens used (Charpy and Izod) are shown below.
The Charpy type of specimen is placed on an anvil as shown below so that it is a simple beam supported at the ends.
The Izod specimen is held in a vise so that its one end is free and is therefore a cantilever beam.
As shown in figure below, in impact testing, a notched specimen is broken by a single blow of a freely swinging pendulum. The pendulum is released from a fixed height. Since the height to which the pendulum is raised prior to its swing, and the mass of the pendulum are known, the energy of the blow is predetermined. In case of Charpy type specimen, the pendulum hits the specimen behind the V notch. In Izod type specimen, the specimen with v notch facing the pendulum is hit above the V notch. In either case, some of the energy of the pendulum will be used to rupture the specimen so that the pendulum will rise to a height lower than the initial height on opposite side of the machine. A means is provided to indicate the energy absorbed in breaking the specimen.
Charpy machines used for testing steel generally have capacities in the 220 to 300 ft lbs (300 to 400 J) energy range. Sometimes machines of lesser capacity are used; however, the capacity of the machine should be substantially in excess of the absorbed energy of the specimen.
Many times, testing temperatures other than room (ambient) temperature are often specified.
Significance of Test Temperature
Since toughness is greatly affected by temperature, a Charpy or Izod test is often repeated numerous times with each specimen tested at a different temperature. This produces a graph of impact toughness for the material as a function of temperature. Impact toughness versus temperature graph for a steel is shown in the figure given below.
It can be seen that 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. The transition temperature is the boundary between brittle and ductile behavior and this temperature is often an extremely important consideration in the selection of a material.
Body centered cubic or ferritic alloys exhibit a significant transition in behavior when impact tested over a range of temperatures. At temperature above transition, impact specimen fracture by a ductile (usually microvoid coalescence) mechanism, absorbing relatively large amounts of energy. At lower temperature, they fracture in a brittle (usually cleavage) manner absorbing less energy. Within the transition range, the fracture will generally be a mixture of areas of ductile fracture and brittle fracture. The temperature range of the transition from one type of behavior to the other varies according to the material being tested.
Above figure shows effect of temperature on strength of a material at different temperatures. It may be noted that at room temperature it behaves like a ductile material but at low temperature (- 200° C) it behaves like a brittle material.
In view of above, many times material specification requires it to be tested for absorbed energy, fracture appearance, lateral expansion, or a combination thereof, at a specified test temperature.
An Example
The steel used in the Titanic hull had adequate strength but low toughness in cold temperatures. When the Titanic hit the iceberg, instead of the steel bending and causing small cracks (ductile failure), a crack grew very fast and very large (due to brittle failure).
For more information on impact test, please refer following ASTM specifications.
E23 – Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
A370 – Standard Test Methods and Definitions for Mechanical Testing of Steel Products
