Due to this blogs mentioning the importance of medium integrity or md shear performance of the medium and its effect on box performance I have had a number of questions raised about how the effect is seen and how severe it is. As a result I thought I would start an occasional series on md shear performance as people seem to be interested in general. This week I’m reviewing some initial data I presented some years ago that related md shear and board crushing to the performance of boxes in transport. The results are not as you might expect!
This graph shows some preliminary data presented at the International Creep symposium in 2001. The broad effect on box performance of a transport stage. The boxes are equivalent except that one has been damaged (crushed) to give a BQM (md shear) value of around 8, while the other “good quality” box was maintained at 27. The transport simulation was undertaken at level II for 10 minutes on a vibration table, and is equivalent to a 165 km truck trip.
The graph shows the measured box strength after the 10 minute vibration simulation for various loads. Interestingly as the load increases the measured box strength sampled after the vibration stage does not change greatly. Eventually, at a particular load during the vibration test the box fails before the 10 minute simulation ends.
In the case of the quality (BQM=27), the load needed to make the box fail during the transport test is around 1500N. For the damaged box this drops to 900 N. In other words a box strength drop due to crushing damage of 16% equates to a drop in applied load during transport of 40%!
There are two take aways from these results that underline the importance of avoiding unnecessary damage to the corrugated medium of the box during manufacture and conversion:

• During transport there is no drop in box strength along the journey – until the box fails and this is likely to be sudden and catastrophic. So, at least for the boxes in this test, it is no good looking at the truck load to see if the boxes are close to failure as their strength is maintained until failure. They do quite well … until they don’t.
• Small drops in box strength due to crushing equate to large drops in performance during transit.