What's the difference between Stress Test and Scalability Test?

The stress testing is used to analyze the stress develope in the material under applied forces(stress), and the scalability testing is the to represent the stress on paper(stress strain diagram).

Scalability can be considered as a part of performance testing which checks if the system is scalable or not. To find if the functionality and the performance of a system will scale up to meet specified requirements. Load testing is to check if the system can take load or not. To check the expected usage of a software program by simulating multiple users accessing the program's services concurrently.