Why is the force feedback not zero after the specimen fractures (Figure 1)?
Figure 1 – Negative load at fracture
If there was a manual offset in the force signal in the software, once the specimen broke and separated, the feedback would be equal to the offset. For example, loading and clamping a specimen in the grips can sometimes apply a small force to the sample. If you don’t command force to zero before starting the test, but instead use auto-offset to zero the force, you will read zero force at the start of the test but the actual force will be equal and opposite of the offset value. When the specimen finally breaks the feedback will be equal to the software offset value.
There could be other reasons why an offset may have been entered in the software. Someone may have changed fixtures but didn’t auto-offset the new fixture weight, so it created a zero-load offset in the feedback. In any case, you can check to see if an offset is responsible by looking in Station Manager Station Setup under the Offset/Zero tab > Manual Offset (Figure 1).
Figure 1 – Manual Offset in Station Setup