There will not be a problem if your visa expires while you are in the United States. When you entered the U.S., the Department of Homeland Security, Customs and Border Protection immigration officer at the port of entry (likely an airport) authorized you to stay in the U.S. for a set period of time when he/she admitted you into the country. He/she should have noted that period of stay on your admission stamp or on your Form I-94, Arrival/Departure Record. You can stay for the entire period noted, even if your visa expires during that period of time.
It is important, however, to keep your I-94 inside your passport so that you have an official record of your permission to be in the United States, particularly if your visa does not reflect your entire authorized period of stay.
As always, please do not hesitate to contact Garfinkel Immigration Law Firm by phone at 704-442-8000 or via email with any additional questions and to receive more information.