I strongly suggest that people google "The difference between a bond and a bond fund"
If you own an "individual bond" and hold it to maturity, you get your principle back plus interest. Regardless of the stock market's up and down. This is because a bond is a "loan" to the government or corporation with a fixed maturity date and interest rate.
For an individual bond, you earn interest and 100% of the principle is returned at the maturity date, provided the government or corporation does not declare bankruptcy...which is why they categorize bonds as AA, A, B and junk.
Granted the value of the bond may flucturate due to market condition but ONLY IF YOU SELL PRIOR TO THE MATURITY DATE. Once you hold it until the maturity date, you get 100% of your principle back. The classical example is a Government Savings Bond which grandparents like to give to their grandchildren.
A Bond fund is a completely different animal because the fund manager has to buy and sell bonds due to buy orders and sell orders. Hence the price of the Bond fund flucturates. Investors are sometimes confused about the behavior of a bond fund versus the behavior of an individual bond.
If you want better security, I suggest buying individual bonds from the government or a AAA rated corporation and hold it to maturity date. I prefer short term individual bonds which reduces the risk but the return is small. A diversified portfolio should have some individual bonds...and not a bond fund to reduce the volatility.