What happens to a material when it reaches its proportional limit?

When a material reaches its proportional limit, the key phenomenon observed is that stress and strain are no longer proportional. Up to this limit, the relationship between stress and strain follows Hooke's Law, which states that strain is directly proportional to stress within the elastic range. This means that if you double the stress, you will double the strain, and the material will return to its original shape when the load is removed.

Once the proportional limit is reached, the material begins to exhibit non-linear behavior. This indicates that small increases in stress can lead to increasingly larger amounts of strain, and the material may not return to its original form after the load is removed. This transition marks the onset of plastic deformation where further loading can lead to permanent changes in the material's structure.

The other options do not accurately describe what occurs at the proportional limit. The material does not yet enter a phase of permanent deformation or fracturing, nor does it necessarily mean it can withstand larger loads beyond this point; rather, it signifies the limits of elastic behavior.