Yes, and here’s why …
Let’s start by explaining how composite materials fail.
Composite materials fail from the opposite side from where force is applied. For example, if you take a tree branch (also a composite material) and try to snap it with your knee, pulling both ends towards you, you’ll notice the branch fails on the opposite side from where the force is being applied. The same applies to a helmet. But a helmet usually has a gel coat applied to protect the aesthetics. This gel coat can pull the shell back to its normal shape, and essentially leave it looking undamaged – bar cosmetic scratches etc.
Let’s say you release the pressure on the branch when you first hear it beginning to crack. It will likely return to its normal form and not display any evidence of damage. However, if you were to repeat the exercise, the branch’s structural integrity has been compromised and it will not be capable of taking the same amount of force again before breaking.
Like a branch, the shell of your helmet may or may not have fractured during your accident. Additionally, with the exception of user misuse, there is no condition where the EPS liner can be damaged if the outer shell is not damaged. However, it is possible for the outer shell to be damaged and for the EPS liner to remain undamaged.
There’s also the strain that may have been applied to the rivet points where your chin straps are fixed to the helmet. These are naturally weak points at best, and we often find the area around the chin strap fixings have been damaged, where other parts of the helmet remain good.
Fractures in composite materials will very often remain invisible to the human eye, which is why we use Shearography to test helmet shells. Our instruments are capable of identifying a hairline fracture as small as 100 nano metres. Even the slightest fracture will compromise the structural integrity of your helmet, because with further use even a hairline crack will increase in size.
For context, the common flu virus measures 80-100 nano metres. It’s also worth noting that composite fractures are transparent to x-ray, so x-ray is not an effective method to test for structural damage.
In short, a visual inspection is wholly inadequate, and where the helmet has knowingly been involved in an accident or a minor drop, we would always recommend our test.