Does divorce take effect, or is it merely an oath, if the wife washes clothes after the husband said to himself: "You will be divorced if you wash this month; I want to wash," without her hearing it, and this was intended to intimidate her during a moment of anger?

Divorce does not occur merely by thinking or intending it unless it is uttered. If you uttered it in a low voice, this is considered a conditional divorce, and its ruling is that of an oath because you intended to frighten her, not to divorce her. If she washes your clothes, you owe an expiation for breaking an oath (feeding ten poor persons, or clothing them, or freeing a slave, or fasting three days if you cannot find these). This ruling is based on your utterance of the words, and giving the expiation for the oath is more appropriate. However, if you did not utter it, then there is nothing upon you.