Hair is a dead structure, like finger-nails, produced by the hair follicle.
The hair follicle will produce a good strand of hair if the body was healthy 40 days back. No fever, no calcium and iron vitamin deficiency, and no major disease, stress, lack of sleep.
Eat well, sleep well, exercise and your hair will grow strong.
The strands may be weakened by infections, chemicals and dyes, but the fibers cannot be strengthened.
The follicles can be strengthened with good scalp care and the above activities.