The problem of brittle fracture in structural steel has been vigorously attacked by powerful research efforts in many countries over the past sixteen years or so, but remains in some respects intractable. The main difficulty seems to lie in defining the problem itself, and in isolating the essential features. The investigations have been characterized by conflicts of ideas on these essentials, and on their interpretation, possibly because the problem has brought together into one forum, as it were, several branches of scientific and technical endeavor which in the past have functioned, to a large extent, independently. This has led to misunderstandings due to differences in terminology and other difficulties due to conflicts of interest.

In such circumstances it is often desirable to re-examine fundamentals, and the present notes are an attempt to do this. Such re-examinations often entail the repetition of "obvious" facts, and while this may appear tedious, it is essential to the process.