One of the surprising phenomena in the metals industry is that normally ductile materials, such as mild steel, can become brittle under certain conditions. Three basic factors contribute to a brittle-type fracture: the presence of a triaxial stress state (defined below), a low temperature and a high strain rate, or rapid rate of loading.1

Interestingly, not all three need be present at the same time for a brittle fracture to occur. For example, the presence of stress risers at sudden changes in part geometry, such as surface imperfections as well as the presence of low temperature, is often responsible for many of the brittle failures experienced in service. Three-dimensional (volumetric) defects, while they create a lesser notch effect, can also amplify stress (since they reduce the load-bearing area). The following characteristics must be taken into account when assessing the significance of a defect:2

  • Size
  • Sharpness
  • Orientation (with respect to the principle working stress and residual stress)
  • Location

Furthermore, the tendency toward brittle fracture cannot necessarily be determined by tension or torsion tests, which are conducted at slow strain rates because materials that have identical properties can show pronounced differences. For this reason, impact tests at high rates of loading have been developed.
 

Impact Tests

The impact test provides us with a simple method of ascertaining the change in the fracture mode of a material as a function of temperature (Fig. 1). Note that in the graph shown there is an absence of a sharp transition from ductile to brittle failure modes but rather one that occurs over an extended range of temperatures. Thus, an analysis of the fracture surface of an impact specimen can characterize the fracture mode3,4

Impact tests simply measure the resistance to failure of a material to a suddenly applied force. Therefore, the test measures the impact energy, which is the energy absorbed by the material prior to fracture. The two most common tests are the Charpy test and the Izod test Next time, we will take an in-depth look at the two types of impact tests.
 

Triaxial Stress

Engineered components often experience more than one type of stress at the same time, and this is simply called the combined stress state. In normal and shear stress, the magnitude of this stress is maximum for surfaces that are perpendicular to a given direction (and zero across any surfaces that are parallel to that direction). Uniaxial, biaxial and triaxial stresses refer to conditions where stress is applied on one, two or all three of the principal axes of a component. Thus, triaxial stress has normal and shear stresses that are applied in three dimensions or planes (i.e., the stress is nonzero across every surface element).

 

References

1.    Dieter Jr., George E., Mechanical Metallurgy, McGraw-Hill Book Company, 1961

2.    Wilby, A.J. and D.P. Neale, “Defects Introduced into Metals During Fabrication and Service,” Vol. III, Materials Science and Engineering, Encyclopedia of Life Support Systems

3.    Herring, Daniel H., “Understanding Component Failures Parts 1 & 2,” Industrial Heating, July/August 2013

4.    Herring, Daniel H., Atmosphere Heat Treating, Volume II, BNP Media Group, 2015

5.    Reed-Hill, Robert E., Physical Metallurgy Principles, D. Van Nostrand Company Inc., 1964