Is expiation obligatory for someone who said, "I will fast every Sha'ban," but did not fast, without intending a vow?

A vow is not binding unless expressed in definitive terms, such as "It is Allah's due upon me to do so-and-so." As for mere intention, or saying "I will" or similar expressions that do not entail an absolute commitment, these do not constitute a vow, and nothing is due on their account.