I found
this article interesting. It says
== Composite materials are generally laminates, sheets of high-performance fibers, such as carbon, glass or Kevlar, embedded in a polymer resin matrix. Chou said that the traditional composite materials have inherent weaknesses because the matrix materials--plastics--surrounding the fibers are "strong, but far less strong than the fibers."
This results in "weak spots in composites in the interface areas in the matrix materials, particularly where there are pockets of resin," Chou said.
As a result, defects, including tiny microcracks, can occur. Over time, those microcracks can threaten the integrity of the composite ==
And there maybe more reliable way to detect it using carbon nanotube mixed (not for the strength but for detection purpose) into it in the future.
In other words, the current methods available are not quite perfect yet.