C172 Flight Instructor reported smelling strong fuel fumes in the cockpit during a training flight. The flight crew performed an air turnback with the Flight Instructor assuming control as the Student Pilot was affected by the fumes. Maintenance found the aircraft's filler tube developed a crack in the area welded to the gas tank.

Narrative

On or about Day 0; I departed ZZZ in a C172 with a student working on his instructor rating. Shortly after takeoff we smelled raw fuel fumes and returned for a landing. The scent of the fumes got stronger in our descent. The student asked me to land. On the ground he told me he got a migraine headache from the fumes. It appeared he was incapacitated from the fumes. If he was by himself this could have been deadly.The plane was withdrawn from service and the fuel tank sent to a welding shop. Apparently the filler tube developed a crack where it is welded to the gas tank. We were lucky that there was no fire or explosion; as in Aircraft Y. This was the second time this year that this fuel tank leaked. Last [eight or nine months ago] I refused to fly the plane after smelling fumes on another training/demonstration flight. The plane continued to be operated by other instructors and students; despite my emailed warnings to them; until it was pulled from service for its 100-hour inspection. The fuel tank was apparently sent to a welding shop for repair.I spoke with another Aircraft Inspector who works for a different school and was told that this was a common problem in their Cessna aircraft. This Inspector believes the problem is caused by the fuelers letting the fuel nozzle apply too much force on the filler neck. Our Inspector/Director of Maintenance thinks is caused by the fuel tank walls or top flexing; and causing stress cracks where the nozzle is welded. Person A at Company confirmed that a lot of Cessna fuel tanks have cracks where the filler neck joins the top of the tank. They seem to think it is from the fuelers letting the nozzles put too much pressure on the filler necks. However; they said the top of the tanks also develop cracks.The Cessna leaking fuel tanks appear to be a systemic problem and it is inconceivable that the FAA is unaware of the problem and obscene that there have been no ADs issued to warn pilots to have mandatory fuel tank inspections [and] ground the aircraft anytime there is the smell of fuel in the cockpit or fuel stains behind the filler caps or under the wing above the door. Some operators have taken the step of not filling the fuel tanks to the top. However; in a descent; there will be fuel behind and consequently above the filler neck to create a pressure head to drive fuel through the crack in the neck weld. Partially filling a tank with a known leak should be considered operating an aircraft in a reckless manner and consequent violation of FAR 91.13.