Rate and time dependent behavior of structural adhesives

Studies on two adhesives (Metlbond 1113 and 1113-2) identified as having applications in the bonding of composite materials are presented. Constitutive equations capable of describing changes in material behavior with strain rate are derived from various theoretical approaches. It is shown that certain unique relationships exist between these approaches. It is also shown that the constitutive equation derived from mechanical models can be used for creep and relaxation loading.

Modifications to the constitutive equations for constant strain rate loading are proposed. Nonlinear constitutive equations are derived using a nonlinear perturbation technique in conjunction with a modified Bingham model. Using the modified Bingham model, constitutive equations describing loading-unloading behavior are also proposed.

The stress-strain behavior of the adhesives is shown to be significantly rate dependent. Further it is shown that a rate dependent stress-whitening (crazing) phenomenon occurs prior to the maximum stress. A region of elastic behavior, a rate and time dependent region, and a region of perfectly plastic flow are identified in the stress-strain behavior. Information regarding variations of Poisson's ratio with rate and time is also presented.

The elastic limit stress and strain, and maximum stress are shown to be rate dependent and agree well with an empirical equation proposed by Ludwik. Analytical predictions based on modified Ramberg-Osgood equations are shown to agree well with experimental stress-strain-strain rate results. It is shown that the coefficients of these equations are different before and after stress-whitening due to changes in the properties of the adhesives. Analytical predictions based on the modified Bingham model are shown to agree well with the constant strain rate results. It is also shown that the nonlinear model indicates that the coefficients of the modified Bingham model may vary due to the change in material properties before and after stress-whitening.

A creep to failure phenomenon is shown to exist and is correlated with a delayed yield equation proposed by Crochet. Loading-unloading results are presented and are shown to correlate well with the proposed form of the loading-unloading equations for the modified Bingham model.

Experimental results obtained for relaxation tests above and below the glass transition temperature are presented. It is shown that the adhesives obey the time-temperature superposition principle.