Elastoplastic Fracture Behavior of Structural Adhesives Under Monotonic Loading

Abstract

Elastic-plastic fracture behavior of a structural adhesive in the bulk and bonded forms is discussed. The model adhesive chosen, Metlbond 1113 (with scrim carrier cloth) and 1113-2 (neat resin) solid film adhesives exhibit a relatively brittle material behavior to justify the use of LEFM methods. The solid film adhesives are first cast in the form of tensile coupons to determine the bulk fracture properties with the use of single-edge-cracked specimen geometry. K_Ic evaluation is done using the procedure suggested by the ASTM standard. A K-calibration method based on application of boundary collocation procedure to the William's stress function is utilized to relate the measured critical loads to the K_Ic values. The yield stresses and elastic moduli values in the bulk tensile mode are also evaluated. The availability of K_Ic à _y E and v (Poisson's ratio) values makes the calculation of crack tip plastic zone radii (r_yc ) and fracture energy (G_Ic ) values possible on the basis of Irwin's theory. The bulk casting procedure is done under different cure (temperature, time and cool-down) conditions to determine optimum properties. The fracture behavior of the same adhesives in the bonded form is studied with the use of Independently Loaded Mixed Mode Specimen (ILMMS) geometry. This specimen allows independent measurement of P_I and P_II (and consequently G_I and G_II ) values. Since the fracture energy values are affected by the thickness of the adherend and the bondline, an experimental program is executed first by varying these geometrical parameters to determine the plane strain conditions. The relationship between the bondline thickness and the crack tip plastic zone radius values calculated earlier is also studied. Expressions developed on the basis of LEFM assumptions are utilized to calculate G_Ic and G_IIC values in the bonded form. The G_IC values obtained in this manner are compared to the bulk G_IC values obtained earlier. With the availability of P_I and P_II (G_I and G_II ) values that result in failure in the bonded form, the fracture condition (i.e. the fracture failure criterion) in mixed mode (modes I and II) loading is determined for adhesively bonded joints. The use of both 1113 and 1113-2 adhesives also reveals the effects of the carrier cloth on the mechanical phenomena cited above.