# Calculating the deformation gradient for a 3D surface

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Amin Kassab-Bachi on 15 Jan 2022
Commented: Amin Kassab-Bachi on 15 Jan 2022
I'm trying to calculate strain from deformation field, for which I need the deformation gradient. What I have is the x,y,z displacements of the nodes, and I found that the the deformation gradient F = I + u , but I don't understand how the gradient function is applied (i.e. what the input (u) should look like).
I tried using this function [Grad] = gradient(u), Where u is the [x,y,z] disaplacment of the size (#nodes x 3).
is this the correct way for using this function in this case?
Thanks.

Friedel Hartmann on 15 Jan 2022
You can calculate the gradient of the displacement field u (3 components), which is a function of (x,y,z) and which is an expansion in terms of the shape functions of the mesh. The nodal displacements cannot be differentiated, they are vectors, they are the value of the displacement field at the nodes.
So, you must understand how the displacement field is defined, which shape functions the program uses.
Amin Kassab-Bachi on 15 Jan 2022
Thanks for the answer. My understanding is that the displacement field is just the nodal displacements as defined here. I probably also need to input the points' x,y,z spacings.
What I'm trying to do is based on spam python package

R2021b

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