The researchers studied how a type of aluminum alloy commonly used in airplanes behaves when subjected to different types of stress. They used a special testing setup and computer simulations to analyze the stress levels at the cracks in the material. The results showed that when different types of stress were combined, it had a significant impact on the material's ability to resist breaking. By looking at the fracture surfaces under a microscope, they confirmed that their predictions matched what they observed in the experiments.