Observable collection

Hello, i'd like to know/understand why the event Collectionchanged isn't triggered when i change a value in my datagrid ? In the following picture we have in the datagrid material an item with the name "mat23", and in the second picture the same thing but after renaming the first material "mat23" to "mat".
The datagrid.ItemSource is set to my observableCollection "MaterialObservable".