After an unexpectedly good year for fixed-income assets, corporate bond exchange traded fund investors should tone down their expectations.

The iShares iBoxx $ Investment Grade Corporate Bond ETF (NYSEArca: LQD), which has a 7.85 year duration and a 3.12% 30-day SEC yield, has increased 7.5% year-to-date.

Fund managers anticipate the bull market in bonds will weaken in 2015 due to slowing growth overseas, lower oil prices and potentially higher U.S. interest rates, reports Mike Cherney for the Wall Street Journal.

Moreover, any potential gains could be muted after companies took on more risks to expand businesses during an improving U.S. environment, which could lead to potential credit risks down the line. Moreover, mergers and acquisitions are at the highest in years, forcing companies to raise yields to entice investors.

The corporate borrowing spree has already left some areas of the market vulnerable, namely junk bonds exposed to the energy industry. The U.S. shale oil boom has been fueled by heavy borrowing, with many companies highly leveraged as they expand drilling operations. Consequently, the low oil prices could make it hard for many of these companies to repay their debt obligations. [There Will be Blood: Warnings for Junk ETFs With Energy Exposure]

The iShares iBoxx $ High Yield Corporate Bond ETF (NYSEArca: HYG) an allocates about 14% of its weight to oil and gas issuers. Approximately 120 of the high-yield bonds held by SPDR Barclays High Yield Bond ETF (NYSEArca: JNK) are issued by companies engaged in the exploration, production or transportation of coal, natural gas or oil. Additionally, the AdvisorShares Peritus High Yield ETF (NYSEArca: HYLD) includes 14% oil & gas producers, 2% oil refining and manufacturing and 3% oil services. Over the past three months, HYG dropped 3.1%, JNK decreased 4.5% and HYLD fell 16.5%.