For a material to have good impact resistance it must be able to withstand high loading rates and absorb the energy by deformation rather than fracture. For joints, peel and cleavage stresses are the most damaging and so adhesives with a high peel strength are typically recommended for good impact resistance. A dual phase toughened adhesive will be preferable, with the rubbery phase dissipating the loads.
The viscoelastic and viscoplastic nature of polymeric adhesives make their properties dependent on strain rate as illustrated by the tensile hardening curves below for a rubber-toughened epoxy. Yield and flow stresses increase progressively with strain rate. This type of behaviour can be explained by models for plastic deformation through molecular relaxation processes that involve thermally activated transitions across an energy barrier whose height is modified by an applied stress. At high loading speeds, these processes become more inhibited which can lead to reductions in the strain at failure.
The impact resistance of bonded structures relies on joint integrity being maintained at high rates of loading so that high energy can be absorbed by the deformation and fracture of the substrate materials. This is achieved for many applications by ensuring sufficient toughness is maintained in the adhesive under these conditions. The wedge impact test has been developed for automotive applications and the impact force measured in this test has been shown to relate to the impact toughness of the adhesive.
Standard tests that determine impact properties
ASTM D950-03 Standard Test Method for Impact Strength of Adhesive Bonds
ISO 11343:1993 Determination of dynamic resistance to cleavage of high strength adhesive bonds under impact conditions - Wedge Impact method
