Hi, engineering stress is the applied load divided by the original cross-sectional area of a material. Also known as nominal stress. True stress is the applied load divided by the actual cross-sectional area ( the changing area with respect to time) of the specimen at that load.
hi True stress is obtained by dividing the test load by the instantaneous (varying true area) whereas the engineeing stress is the result of dividing the test load by the original (constant) area@