The Federal Reserve is once again facing unsettled markets as it prepares to raise interest rates for the first time in nine years, and as investors grapple with a deepening slump in junk bonds and plunging energy prices.

Expectations that the Fed will go ahead and raise rates at its meeting Tuesday and Wednesday weren’t shaken by a rough week that ended with a 1.8% decline for the Dow industrials on Friday. Still, the sense of stress in markets was unmistakable after oil prices fell 11% for the week in the U.S., the deepest...