When I defended my PhD in 2013 at Georgia Tech Chemistry, the first half was the public defense, including Q&A. The second half was closed-door, consisting of just the committee and the PhD student. This second half was known to be more combative and it was common for the committee to request additional work. It was very rare for any one to flat out fail, since the student’s advisor wouldn’t allow them to defend until they had sufficient novel research.

In my program, many of us would strategize for the 30-60 minutes of the closed door grilling. We sought to give our committee members obvious things to criticize with the PhD student having prepared arguments to defend against these criticisms. E.g., I ashamedly included quite a few spelling and grammar errors in the first few pages of the summary section of the thesis (the only part anyone would actually read) and we spent at least 15 minutes on my horrible writing ability.

In general, the main outcome of the closed door portion of the defense was requests for additional work. It was common for committee members to suggest additional things that could “improve” the thesis work. Not surprisingly, many of these suggestions involved applying a committee member's methods, even if not plausibly applicable, so that one would publish another paper citing the committee member’s work. Some students, including myself, would have a job lined up before the defense to timebox the amount of additional work that could be requested.