  With the prime aim of better probing and understanding the intimate link
between star formation activity and the presence of bars, a representative
sample of 32 non-interacting late-type galaxies with well-determined bar
properties has been selected. We show that all the galaxies displaying the
highest current star forming activity have both strong and long bars.
Conversely not all strong and long bars are intensively creating stars. Except
for two cases, strong bars are in fact long as well. Numerical simulations
allow to understand these observational facts as well as the connection between
bar axis ratio, star formation activity, and chemical abundance gradient: Very
young strong bars are first characterized by a raging episode of star formation
and two different radial gaseous abundance gradients, one steep in the bar and
one shallow in the disc. Then, principally due to gas consumption, galaxies
progressively fall back in a more quiescent state with a nearly flat abundance
gradient across the whole galaxy. On the contrary, weak bars are unable to
trigger significant star formation or to generate flat abundance gradients. The
selected galaxies have tentatively been classified in four classes
corresponding to main stages of secular evolution scenario.
