We have the benefit of controlling quite a few printing parameters during FDM printing, through which we can control a few mechanical properties of the printed parts. We can control the strength & toughness of your parts by varying the Infill Percentage parameter and the number of shells / perimeters to be printed.
Increasing the Infill Percentage
parameter makes your 3D printed parts denser & stronger while increasing the number of perimeters used in the printed parts increases the impact resistance of the printed parts. We can use a lesser infill amount for parts that won't undergo stress and a higher infill amount for parts that would undergo stress.
Also, depending on the printing material used, decreasing the width of the perimeter / shell of the printed part may increase the flexibility of the part.