Engineering stress assumes that the area a force is acting upon remains constant, true stress takes into account the variation in the cross sectional area as a result of the stress induced deformation (strain) of a material.
For example a steel bar in tension once it's yield point or stress is reached will start to "neck". Necking is the localised concentration of strain in a small region of the material, causing a reduction in cross sectional area at this point.
To calculate the engineering stress in the above case, the applied load is divided by the original cross sectional area, however the true stress would be equal to the load divided by the new deformed cross sectional area. Therefore true stress is likely to be significantly higher than engineering stress. Note that while the material is deforming elastically before thwe yield point is reached there will be some difference between true and enginnering stress (as the material is changing shape) but it will be much smaller than the difference after the yield point is reached.
A rock core in a uniaxial compression test will typically expand radially under loading. Therefore in this case, the engineering stress (based on the original diameter) will be larger than the true stress within the material.
For example a steel bar in tension once it's yield point or stress is reached will start to "neck". Necking is the localised concentration of strain in a small region of the material, causing a reduction in cross sectional area at this point.
To calculate the engineering stress in the above case, the applied load is divided by the original cross sectional area, however the true stress would be equal to the load divided by the new deformed cross sectional area. Therefore true stress is likely to be significantly higher than engineering stress. Note that while the material is deforming elastically before thwe yield point is reached there will be some difference between true and enginnering stress (as the material is changing shape) but it will be much smaller than the difference after the yield point is reached.
A rock core in a uniaxial compression test will typically expand radially under loading. Therefore in this case, the engineering stress (based on the original diameter) will be larger than the true stress within the material.
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