You can create a custom field with an expression in report creator to get the bend cut length. Implementation and exact expression text depends on your report setup but basically you need to check if the item is a bend and if so, report property [Pipe_CutLength]. If not, report a different property. This field should be bound to a 'Group, Sum' cell.
Example:
Iif([Engineering Items_ShortDescription]=='PIPE BEND',
[Pipe_CutLength],
<your else here>
)

Here's another example of an expression that does not address bends but might be helpful to understand how to use the code:
Iif(IsNullOrEmpty([BoltSet_NumberInSet])
And IsNullOrEmpty([Pipe_Length]),
1,
Iif(IsNullOrEmpty([BoltSet_NumberInSet]),
Round([Pipe_Length]/12, 2),
[BoltSet_NumberInSet])
)
This one checks if the item is a bolt set or pipe by seeing if certain properties return null. If they are null, it returns 1. If they are not null, it returns the desired property value.