Back to basics – Toughness testing of material

The Charpy impact test determines the amount of energy absorbed by a material during fracture. 

0
494

In the last series of articles, we discussed hardness testing and tensile testing. In these applications, the loading is a static load over a short time duration. All the forces on the body are in equilibrium. However, in many applications, a static load is not what the part experiences in service. In dynamic loading, the load is applied rapidly, and the forces do not reach equilibrium within the body. Part momentum is considered as the part is accelerated. The stresses are static but are applied in a short time interval. An example would be the forces in a hammer as it is striking a nail.

Toughness Testing

Toughness, or the resistance of a material to tearing, is related to the energy absorbed by the part during fracture. If very little energy is absorbed, then the fracture is brittle. 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.

Charpy Impact Test

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 bar contains a sharp notch with an included angle of 45° and a depth of 2 mm. The notch radius is 0.25mm. Dimensions are shown in Figure 1. The use of sub-size specimens is permitted if they conform to ASTM E23-18 [1]. An example of a Charpy Impact Tester is shown in Figure 2.

Figure 1: Dimensions of the Charpy Impact Specimen per ASTM E23-18.
Figure 2: Pendulum Charpy impact tester. (Courtesy: Instron, Inc., Norwood, Massachusetts)

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. Because of conservation of energy, the specimens may leave the machine at speeds in excess of 50ft/s. 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 103 s-1. 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. High impact energies are associated with this regime. The lower shelf is a region where fracture is brittle; low impact energies are found in this region. The third region is the transition region, which displays a reduction in impact energies required to fracture the specimen. It is in this region that the fracture changes from ductile to brittle. Because this transition often occurs over a wide temperature range, various criteria have been developed to define the transition temperature.

Figure 3: Charpy impact strengths as a function of testing temperature for three 0.40 percent C steels [2].

The most common method of defining the ductile to brittle transition temperature is called the ductility transition temperature. This temperature is the minimum temperature that the steel can absorb a specific amount of energy without fracturing. Often it is defined as the temperature at which the steel absorbs 20 J (15 ft-lbs) of impact energy during fracture.

Changes in the ductile to brittle transition temperature of up to 40°C can be made by simple changes in the chemistry of the steel [3]. The greatest chemical changes are due to the addition of carbon and manganese. Other alloying additions also tend to change the ductile to brittle transition temperature in steels. Nickel decreases the transition temperature, whereas chromium has little effect. Silicon raises the transition temperature, as does phosphorus. Phosphorus in excessive amounts will also tend to form grain boundary precipitates, further increasing the transition temperature.

Grain size of the steel plays an important role in the temperature of the ductile to brittle transition. Decreasing the grain size significantly decreases the transition temperature [4]. Changes in processing that create or promote fine ferrite grain size, such as normalizing, with subsequent tempering will decrease the transition temperature. The rolling temperatures and other processing variables all play important roles in the finished grain size of the product and therefore the grain size.

Unfortunately, the Charpy impact values cannot be used in design but can give the designer a minimum value for design. It is most often used for quality assurance purposes, with specification indicating a ductile to brittle transition temperature.

Conclusions

In this short article, we have described the principles of impact testing, and some of the factors that affect Charpy impact values. Chemistry, microstructure, and processing can have large changes in the ductile to brittle transition temperature. By understanding the factors, designers can specify minimum values for processing.

Should you have any questions on this article, or suggestions for further articles, please contact the author or editor. 

References

  1. ASTM International, “Standard Test Methods for Notched Bar Impact Testing of Metallic Materials,” ASTM International, Conshohocken, PA, 2018.
  2. A. Pineau, “Fracture: Cleavage,” in Encyclopedia of Materials: Science and Technology (Second Edition), K. J. Buschow, R. W. Cahn, M. C. Flemings, B. Ilschner, E. J. Kramer, S. Mahajan and P. Veyssière, Eds., Elsevier, 2001, pp. 3279-3283.
  3. G. E. Dieter, Mechanical Metallurgy, New York, NY: McGraw-Hill, 1976.
  4. W. S. Owen, P. H. Whitman, M. Cohen and R. L. Averbach, “Relation of Charpy Impact properties to microstructure of three ship steels,” Weld J., vol. 36, p. 503s, 1957.