Federal Reserve Chair Janet Yellen was clearer than ever on Friday that the central bank was poised to raise interest rates this year, as the U.S. economy was set to bounce back from an early-year slump and as headwinds at home and abroad waned. Yellen spoke amid growing concern at the Fed about volatility in financial markets once it begins to raise rates, and a desire to begin coaxing skeptical investors toward accepting the inevitable: that a 6-1/2-year stretch of near-zero interest rates would soon end.