Strength and toughness relationships are perhaps the most important ones when it comes to advances in materials and engineered products. Toughness is often determined by impact testing, and every heat treater needs to know how these tests are performed and what they mean. Let’s learn more.

Brittle Failure

One of the surprising phenomena in the metals industry is that normally ductile materials, such as mild steel, can become brittle under certain conditions. There are three basic factors that contribute to a brittle-type (or cleavage-type) fracture, namely: the presence of a triaxial stress state (defined later), a low temperature (relative to the ductile-to-brittle transition temperature) for the material in question and a high strain rate or rapid rate of loading.[2]

Interestingly, not all three need be present at the same time for a brittle fracture to occur. For example, the presence of low temperature, stress risers – such as sudden changes in part geometry – or other “metallurgical notches” – such as nonmetallic inclusions or surface defects/imperfections (seams, laps, etc.) – is often responsible for the brittle failures experienced in service. Other three-dimensional (volumetric) defects (e.g., porosity or other macro-imperfections), while they create a lesser stress 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 usually conducted at slower strain rates. Materials that have similar tensile properties can show pronounced differences in toughness. For this reason, impact tests at high rates of loading have been developed.

Triaxial Stress

Engineered components often experience more than one type of stress at the same time. This is known as the combined stress state. In normal and shear stress, the magnitude of this stress is maximum for surfaces that are perpendicular to a given loading direction (and, in some instances, 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).

Impact Tests

Impact testing 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 isn’t a sharp transition from ductile to brittle failure modes but rather one that occurs over an extended range of temperatures. An analysis of the fracture surface of an impact specimen can characterize the fracture mode.[4,5]

Impact tests measure both the energy required and the resistance to failure of a material subjected to a sudden applied load. The test measures the impact energy; that is, the energy absorbed by the material prior to fracture. The two most common tests are the Charpy test and the Izod test.

The Charpy Test

This test is named for its inventor, Georges Augustin Albert Charpy (1865-1945). The Charpy test measures the energy absorbed by a standard notched specimen while breaking under a three-point bending impact load. The most common method of measuring impact energy in steels today is the Charpy V-notch test (Fig. 2a). Other notch configurations (U-notch, keyhole, etc.) can be used. The importance of the Charpy impact tests lies in the fact that it can reproduce the ductile-brittle transition transformation (DBTT) in essentially the same temperature range as it is actually observed in engineering structures.[5]

What does the Charpy test involve?

The Charpy tester (Fig. 3) involves striking a suitable test piece with a shaped tup mounted at the end of a pendulum. The test piece is fixed at both ends in a horizontal orientation and the striker impacts the test piece immediately behind a machined notch. In this case the “V” notch points opposite the direction of load approach. A significant amount of testing is conducted at room temperature. When DBTT testing is required, the testing usually involves six temperatures, one of which is room temperature. The non-room-temperature testing involves removing a test specimen from its medium and positioning it on the specimen supports. The pendulum is to be released as vibration-free as possible within five seconds after the material is removed from the thermal medium.

Fracturing the specimen removes energy from the hammer, and the height at which the hammer rises after the specimen is broken is measured on the tester. For a ductile fracture, the energy expended is high, and the energy will be low for a brittle fracture. The information obtainable from this test includes the impact energy, lateral expansion and fracture appearance.

The Izod Test

The Izod impact test was named for its inventor, Edwin Gilbert Izod (1876-1946), and consists of a pendulum with a determined weight at the end of its arm swinging down in a circular arc and striking a specimen while it is held securely in a vertical position (Fig. 2b). It is a cantilever beam test in which the notch is oriented to point in the direction of load approach. The impact strength is determined by the loss of energy of the pendulum as calculated by precisely measuring the loss of height in the pendulum’s swing. The specimen that is usually notched is gripped at one end only, which is the principal difference between it and the Charpy test.

The principal advantage of this test is that a single specimen can be used multiple times since the ends are broken off one at a time. The principal limitation is the lengthy setup time required. Thus, both elevated- and low-temperature testing are not options, which is a major disadvantage due to the importance of the ductile-to-brittle transition temperature characteristics of most steels.

The size and shape of the specimen varies according to what materials are being tested. Specimens of metals are usually square, and polymers are usually rectangular and struck perpendicular to the long axis of the rectangle. The specimen-holding fixture is usually part of the machine and, as such, cannot be readily cooled (or heated). For this reason, Izod testing is not recommended at other than room temperature. The information obtainable from this test includes the impact energy, lateral expansion and fracture appearance.

Next Time: A discussion about the variables that affect transition temperature and a look toward the future of impact testing. Click here for part 2.

 

References

  1. Mr. Craig Darragh, AgFox LLC, technical and editorial contributions, private correspondence.
  2. Dieter Jr., George E., Mechanical Metallurgy, McGraw-Hill Book Company, 1961
  3. 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
  4. Herring, Daniel H., “Understanding Component Failures Parts 1 & 2,” Industrial Heating, July/August 2013
  5. Herring, Daniel H., Atmosphere Heat Treating, Volume II, BNP Media Group, 2015
  6. Reed-Hill, Robert E., Physical Metallurgy Principles, D. Van Nostrand Company, Inc., 1964
  7. Wulpi, Donald J., Understanding How Components Fail, 3rd Edition, ASM International, 2013
  8. ASTM E23 (Standard Test Methods for Notched Bar Impact Testing of Metallic Materials)
  9. ASTM A370 (Standard Test Methods and Definitions for Mechanical Testing of Steel Products)

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