If the limit of proportionality is not exceeded then stress σ is directly proportional to strain ε.The ratio of stress to strain is called the Young modulus E.The unit is the same as the unit for stress i.e pascal (Pa) or Nm-2The Young modulus is the gradient of the linear region of the stress-strain graph.Unlike the spring constant the Young modulus depends on the material only.You can compare the stiffness of two materials using the Young modulus.