Dynamic Z Direction Tensile Stress Propagation in Recycled Fibers during High Speed Creasing
High speed creasing generates Z-direction tensile stress waves that fracture short recycled fibers unless matrix clearance absorbs elastic strain energy.
Impulse Loading
Dynamic Stress
Dynamic force application characterized by a high magnitude of energy delivered over an extremely short duration. Packaging materials frequently encounter impulse loading during drop tests or accidental impacts in the warehouse. The ability of the board to absorb this energy without rupturing determines the safety of the contents.
Energy Absorption
Viscoelastic properties of the paper fibers govern how the material responds to a sudden strike. Under impulse loading the fibers do not have time to rearrange, which can lead to brittle failure instead of the ductile behavior seen in slow compression. Multi ply structures are designed to dissipate this energy through the different layers of the board.
The bond between the fluting and the liners must be strong enough to resist the shear forces generated by a quick impact. Standard testing protocols use a falling weight or a pendulum to measure the resistance of a sample to these sudden forces. High moisture content generally reduces the peak force a material can withstand before permanent damage occurs.
Protective Limit
Cushioning inserts are often required to mitigate the effect of sudden shocks on fragile items. If the impulse loading exceeds the threshold of the outer container, the energy is transferred directly to the product. Design specifications must account for the highest probable impact during the shipping cycle.