Experimental characterization of tensile properties of epoxy resin by using micro-fiber specimens

Abstract

In unidirectional carbon fiber-reinforced plastic laminates, the distance between fibers can varies from submicron to micron length scales. The mechanical properties of the matrix at this length scale are not well understood. In this study, processing methods have been developed to produce high quality epoxy micro-fibers with diameters ranging from 100 to 150 µm that are used for tensile testing. Five types of epoxy resin systems ranging from standard DGEBA to high-crosslink TGDDM and TGMAP epoxy systems have been characterized. Epoxy macroscopic specimens with film thickness of 3300 µm exhibited brittle behavior (1.7 to 4.9% average failure strain) with DGEBA resin having the highest failure strain level. The epoxy micro-fiber specimens exhibited significant ductile behavior (20 to 42% average failure strain) with a distinct yield point being observed in all five resin systems. In addition, the ultimate stress of the highly cross-linked TGDDM epoxy fiber exceeded the bulk film properties by a factor of two and the energy absorption was over 50 times greater on average. The mechanism explaining the dramatic difference in properties is discussed and is based on size effects (the film volume is about 2000 times greater than the fiber volume within the gage sections) and surface defects. Based on the findings presented in this paper, the microscale fiber test specimens are recommended and provide more realistic stress–strain response for describing the role of the matrix in composites at smaller length scales.

Extreme scale-dependent tensile properties of epoxy fibers

“…Nor can defects explain the colossal ductility (plastic deformation and failure strain) seen in our experiments with epoxy fibers. Only scarce evidence exists in the literature for similar (and milder) size effects in epoxy fibers [13,16] but largely without explanation. We attempt to partly deal with this issue below.…”

“…These authors also reported, without further discussion, the observation of a highly ductile behavior of their epoxy fibers. Recently Misumi et al [16] studied the tensile behavior of fibers made of five different types of epoxies and compared the mechanical properties (yield stress, failure stress, failure strain, modulus and toughness) of fiber and bulk materials. Here too the epoxy fibers showed ductile behavior with a distinct yield point in all resin systems, whereas the macroscopic specimens exhibited brittle behavior with no yield point.…”

“…The mechanical properties of the fibers were significantly higher (apart from the modulus) than those of their bulk counterparts but no diameter effect was observed (except for the failure strain) within the 100-150 μm diameter range of the fibers. Misumi et al [16] recommended using microscale fiber test specimens, claiming that they provide a more realistic stressstrain response for describing the role of the matrix in composites at smaller length scales. Towse et al [17] analyzed the defects causing failure and found a correlation between the size of the defects and the failure strain of an aerospace grade epoxy.…”

A new approach to estimate the curing mode of thermosetting polymer films with regard to physical aging and slow chemical processes

“…In other words, for the complete curing of partially cured glassy polymers in their curing mode, a thermal postcuring stage is included. The curing mode (temperature and time) was typically selected empirically (see, e.g., the experimental sections of papers [ 5,6 ] ), and this process might consist of several stages. For example, DiFrancia et al [ 5 ] cured samples in three stages: (a) 1 hr at 150°C; (b) 1 hr at 210°C and; (c) 2 hr at 230°C.…”

“…In ref. [6], the samples were successively cured at 80°C for 1 hr, 130°C for 30 min, and 180°C for 2 hr; in some cases, the epoxy resin was cured for 1 hr at 130°C, and then for 2 hr at 180°C. Such an approach does not guarantee complete curing.…”