Any person who, within the twenty-four calendar months immediately preceding a week in which he files a valid claim for benefits, has made a false statement or representation of a material fact knowing it to be false, or knowingly failed to disclose a material fact with intent to obtain benefits under this chapter, shall be disqualified for the week for which the claim was filed and for not more than the fifty-one weeks immediately following such week as determined by the commission according to the circumstances in each case.