Core Bond ETFs
A good choice here is iShares iBonds Mar 2020 Corporate (IBDC), it holds intermediate term corporate bonds that all will expire before March 31, 2020, which means they are all less sensitive to interest rate hikes what will happen soon. The fund will close and return all of the assets to shareholders by March 31, 2020.
Speculative Bond ETFs
If you want to be speculative on bonds, especially on bonds that fall out of favor - dropped from investment grade to junk status - a good candidate is Market Vectors Fallen Angel High Yield Bond (ANGL) - its yield is 5.34% and expense ratio is only 0.4%.
The Bottom Line
In short, with some well diversified ETFs, and well diversified Stocks/Bonds ETFs that range from stable core to speculative ETFs, you can construct a portfolio that could sustain market fluctuation and grow in the long term.