The following is the Collectors' Clearinghouse column from the Dec. 19, 2016, issue of Coin World:

When the collar (the retaining ring that establishes the final diameter of a newly-struck coin) experiences brittle failure, several types of collar breaks (collar cuds) may result (Collectors’ Clearinghouse, Nov. 22, 2010).

A collar chip (chipped collar) occurs when a small piece detach­es from the collar’s working face.

A vertical collar crack involves no loss of metal, but does result in a slight widening of the collar’s diameter. Single, paired, and multiple collar cracks are known.

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A bilateral split collar occurs when two collar cracks at opposite poles widen considerably, causing the collar to expand along one axis.

A retained collar break develops when the area between two collar cracks sinks into surrounding softer steel or when the mobilized portion of the collar is restrained by a surrounding clamp or bolts. Retained collar breaks often measure 180 arc degrees, although smaller examples are known.

An irregular collar break occurs when a substantial portion of the upper margin of the collar’s working face breaks away. This breakage can be spontaneous or can be caused by a collision with a misaligned hammer die. An irregular collar break will display a ragged lower margin where it meets the intact portion of the collar’s working face.

Finally, a full collar break entails the complete loss of the collar’s working face along an arc that ranges from a few arc degrees to well over 180 arc degrees.

Plain (smooth-faced) and reeded issues tend to develop different types of collar breaks. For example, the presence of a tungsten carbide steel liner in many plain collars may predispose these collars to develop more irregular and more extensive forms of brittle failure. The presence of ridges and valleys on the working face of reeded collars presumably facilitates formation of vertical collar cracks and related forms of brittle failure.

The defect that can only be found on 2002-P Kennedy half dollars in modern era: A collar break is expected to appear in the same spot on the edge of every coin struck within that collar. The 2002-P half dollar defies that expectation.

While die chips occur in both plain and reeded issues, their location and etiology differ. In reeded issues they form when a part or all of a ridge on the working face of the collar breaks off. In plain collars they develop along the upper margin of the collar’s working face. Shown in the second image in the slider above is the edge of a 1982 Lincoln copper-alloy cent with two substantial collar chips.

Vertical collar cracks are known in both plain and reeded issues, although I sense they’re more common in the latter. In reeded issues the line of breakage is presumably guided and facilitated by the vertically oriented valleys that form the reeding.

Bilateral split collars occur far more commonly among reeded issues, like the 1967 Roosevelt dime shown in the third image above. However, the widest example I’ve seen occurs in a 1964-D Jefferson 5-cent coin (see previous reference).

Retained collar breaks are similar in both appearance and etiology to bilateral split collars. One way to tell them apart is to look at the points of breakage. In a bilateral split collar, both points of breakage will be marked by raised nibs. In a retained collar break, there is usually a step at one or both ends testifying to a horizontal shift or pivot of the mobilized segment. Nibs, when present, tend to be rather small.

Retained collar breaks are much more common among reeded issues. An exception exists among 1973-D Lincoln cents, where three different 180 degree retained collar breaks are known (see fourth photo above).

Irregular and full collar breaks are almost completely confined to plain-edge issues (cents and 5-cent coins). This may be due to the presence of a tungsten carbide steel liner. I suspect that this hard, brittle alloy is both difficult to machine and vulnerable to breakage should grooves be machined into its working face. Shown in the fifth image above is a 1981 Lincoln cent with a 180 degree full collar break that extends from 4:00 to 10:00.

The only reeded coin I know of with a full collar break is a 1998-D Washington quarter dollar shown in the sixth image above in which the collar break extends from 1:30 to 5:30. The coin also features catastrophic die damage on both faces and a large corner die break (cud) on the obverse face (Collectors’ Clearinghouse, Oct. 27, 2014).

More on the U.S. Coins mentioned in this article:

Lincoln cent:  The popular Lincoln cent has gone through several reverse updates since it was introduced in 1909 to honor the nation's 16th president on the 100th anniversary of his birth. How much are Lincoln cents worth?

Jefferson 5-cent coin: If one word sums up the Jefferson 5-cent coin, it would be "change" because throughout its history it has endured many changes, from Mint marks and designer intitials, to portraits of its namesake president. How much are Jefferson 5-cent coins worth?

Roosevelt dime: Franklin Delano Roosevelt's sudden death of a cerebral hemorrhage on April 12, 1945, compelled officials to recommend the late president's portrait to be immediately placed on a coin of regular issue. How much are Roosevelt dimes worth?

Washington quarter: The Washington quarter dollar, which has been circulating since 1932, was born out of the Treasury Department's desire to produce a coin to mark the bicentennial of the birth of the first president of the United States. How much are Washington quarters worth?