Elbowing allowed it isnt listed as illegal at wtf.

I want to know if elbowing to the chest area is fine because there are no rules aginst it and it could do some damade to the open area next to the armpits, when you are inside ofcourse

No its not allowed, precisly why you stated, it could do some serious damage. Elbowing is kept in step sparring practice where no or very light contact is made.

According to WTF official sparring rules, only hand and foot techniques are allowed to the chest protector, not any other parts of the body, such as knees, shins or elbows.

thanks for telling me it would be bad if I had went and done it anyway...