The relationship between resin mechanical properties and Mode I delamination fracture toughness

Abstract

The purpose of this investigation has been to define better the relationship between resin mechanical properties and the resultant Mode I delamination toughness. Five resin systems have been selected for study: (i) Hercules 3502 (a highly crosslinked epoxy); (ii) Hexcel F 155 (a rubber-toughened, moderate crosslink density system with epoxy equivalent weight of 280); (iii) Hexcel 155 without the rubber particle additions; (iv) Hexcel F185 (a rubber-toughened, low crosslink density system with epoxy equivalent weight of 410); and (v) Hexcel F185 without the rubber particles. The F155 system had 5.7% carboxyterminated butadiene acrylonitrile (CTBN) added to the resin. It was added in the form of 2.3% liquid and 3.4% solid rubber additions. The F185 had 8.1% liquid Hycar CTBN and 5.4% solid Hycar 1472 CTBN added to the resin [1]. It is possible that not all of the liquid CTBN precipitated as particles, but that some of it may have been retained in solution as a plasticizer. The neat resin mechanical properties which might correlate with delamination fracture toughness include tensile elongation, modulus, and neat resin critical energy release rate G~c. These neat resin properties have been measured for each of the resins along with the tensile strain to failure for transverse fracture of unidirectional laminates. These results have been correlated with Mode I delamination fracture toughness measurements made from unidirectional symmetric split laminate tests [2]. The neat resin fracture toughness was determined using compact tension specimens. The experimental results are summarized in Tables I to III. The non-linear component of the strain to failure was calculated by subtracting from the total elongation the linear elastic portion of the strain (the ratio of the strength to modulus). Fig. 1 shows the expected monotonic increase in fracture toughness with both non-linear strain to failure and total strain TAB LE I Mechanical properties of resins (measured at 22 ° C) Resin Resin Resin non-linear Resin Resin tensile system elongation strain to failure UTS modulus (%) (%) (MPa) (GPa)