R-curve behavior

The R-curve describes how a material’s resistance to cracking increases as a crack grows. Initially, it’s easier for a crack to start and extend, but as it gets longer, the material's internal mechanisms—like crack bridging or fiber pull-out—work to slow its progress. This results in the resistance to crack growth rising with crack length, making the material more damage-tolerant over time. Essentially, the R-curve provides a way to understand how a material’s ability to withstand cracking improves as the crack develops.