1 Answers
Differential stress is the difference between the greatest and the least compressive stress experienced by an object. For both the geological and civil engineering convention σ 1 {\displaystyle \sigma _{1}} is the greatest compressive stress and σ 3 {\displaystyle \sigma _{3}} is the weakest,
σ D = σ 1 − σ 3 {\displaystyle \!\sigma _{D}=\sigma _{1}-\sigma _{3}}.
In other engineering fields and in physics, σ 3 {\displaystyle \sigma _{3}} is the greatest compressive stress and σ 1 {\displaystyle \sigma _{1}} is the weakest, so
σ D = σ 3 − σ 1 {\displaystyle \!\sigma _{D}=\sigma _{3}-\sigma _{1}}.