One of most important technical issues that a fracture analyst must deal with is evaluating whether the crack was ductile or brittle. Here we are talking about visible characteristics revealed to the human eye, and we are strictly discussing structures with features that are readily viewed with the human eye.

“Ductile versus brittle” is arguably the most essential characteristic of what is called mechanical behavior. That’s because even if the material is very strong, it may not get to act strong if it’s also brittle. A faster-than-anticipated load event or traces of corrosion may prevent the full strength from manifesting.

Charpy V-notch (CVN) testing is a popular way to evaluate toughness in steel products.[1] Toughness is a result of both strength and ductility. The CVN test allows us to make multiple measurements of different characteristics that allow us to tease out the contribution of the ductility to the apparent strength.

Figure 1 shows two sets of steel Charpy V-notch test coupons. The untested coupons are 10-mm square bars 50 mm long. A v-shaped notch is cut parallel to the small ends in the center of the length. We see the faces of the “v” notches on the outer edges of all of the broken fragments (marked VF).

The bars were broken by hitting them with a freely swinging hammer on the surface opposite the notch. We can see that the ductile column has noticeable deviations from the nice 10-mm square bar described as the starting point for these tests. The brittle coupons have very smooth crack surfaces and very little evidence of deformation. They still appear nice and square.

When we complete the impact test, we read how much energy was absorbed by the deformation and fracture processes by noting how much lower the hammer swung back up after falling down to the position where it encounters and breaks the test coupon. The value is shown in foot-pounds or Joules.

But we’re not done! We also perform a measurement of the widest portion of the deformed bar. That allows us to calculate a lateral expansion percentage, which is one measure of ductility. Zero lateral expansion is as brittle as it gets.