There are essentially three types of cracks: Fatigue, stress-corrosion and acoustic fatigue. Each has a different mode of behaviour. Fatigue cracks typically grow perpendicular to major loads, while stress corrosion often grows parallel to the major loads, and acoustic fatigue grows in thin material often in a non-linear fashion (as buckling modes change). Fatigue is directly related to loads, so that is a design and certification testing issue. Stress-corrosion occurs at grain boundaries in rolled, forged or extruded alloys and is usually a heat-treatment or materials selection issue. Acoustic fatigue is related to buffeting and may be difficult to design and test.
Do we know what alloy was used and the directions the cracks are growing?