an internal reinforcement structure of a plastic fuel tank resists deformation of opposing walls of the fuel tank and provides an integral , and directionally sensitive , stress relief feature when pre - determined forces are exceeded . the stress relief feature is contained within a fuel chamber of the fuel tank defined by the opposing walls . each wall has an inward projecting indentation of the structure which engage one - another at their distal ends or bottom portions , preferably , via a welded plastic engagement area . the indentations have a consistent wall thickness which has a higher cross - sectional area than the stress relief feature causing the stress relief feature to tear as opposed to the tank walls thereby assuring fuel tank integrity .