Thermal shock on interfacial adhesion of thermally conditioned glass fiber/epoxy composites

In recent years, structural composites manufactured by carbon fiber/epoxy laminates have been employed in large scale in aircraft industries. These structures require high strength under severe temperature changes of -56° until 80 °C. Regarding this scenario, the aim of this research was to reproduce thermal stress in the laminate plate developed by temperature changes and tracking possible cumulative damages on the laminate using ultrasonic C-scan inspection. The evaluation was based on attenuation signals and the C-scan map of the composite plate. The carbon fiber/epoxy plain weave laminate underwent temperatures of -60° to 80 °C, kept during 10 minutes and repeated for 1000, 2000, 3000 and 4000 times. After 1000 cycles, the specimens were inspected by C-scanning. A few changes in the laminate were observed using the inspection methodology only in specimens cycled 3000 times, or so. According to the found results, the used temperature range did not present enough conditions to cumulative damage in this type of laminate, which is in agreement with the macro - and micromechanical theory.The fiber/matrix adhesion is most likely to control the overall mechanical behavior of fiber-reinforced composites. An interfacial reaction may result in various morphological modifications to polymer matrix microstructure in proximity to the fiber surface. The interactions between fiber and polymer matrix during thermal conditioning and thermal shock are important phenomena. Thermal stresses were built-up in glass fiber-reinforced epoxy composites by upthermal shock cycles (negative to positive temperature exposure) for different durations and also by downthermal shock cycles (positive to negative temperature exposure). The concentration of thermal stresses often results in weaker fiber/matrix interface. A degradative effect was observed in both modes for short shock cycles and thereafter, an improvement in shear strength was measured. The effects were shown in two different crosshead speeds during short-beam shear (SBS)

The intermediate modulus carbon fiber/epoxy composites were manufactured via autoclave processing. HexPly® F155 prepreg fabric tapes (Plain Weave), supplied from Hexcel Company, were stacked in a way to keep the weft and warp fiber pointing at the same direction, accounting for 10 plies and 3.30 mm thickness.Cure preparation primarily involves the bagging of the preformed part and the cutting and placement of many ancillary materials. After the lay-up process, the laminate is bagged under vacuum. The laminate is then placed in an autoclave in order to cure the resin system. During the cure cycle, a low heat ratio of 2.5 °C/min was employed in order to remove the bubbles until it reaches 121 °C, kept for 180 minutes. A constant pressure of 0.69 MPa and the vacuum being at 0.083 MPa were employed during the entire process.

The thermal cycling was performed in an Envirotronics device (model TVS.5-2-2-2-AC), known as Two Zone Vertical Thermal Shock, which is specifically for the aforementioned purpose. A vertical elevator changed the temperature zone ranging from -60 to 80 °C, and it was kept for 10 minutes in each zone prior to reaching the equilibrium. It was cycled 1000 times and then, the laminate integrity was inspected using an ultrasonic test. The procedure was repeated until it reached 4000 cycles using a duplicate specimen.

The ultrasonic inspection was performed by using the pulse-echo mode Microscopy Inspection Acoustic System-Model PSS-600, which applied the software MUIS-32 for data acquisition, and immersing the duplicate specimen (specimen 1 and 2) in a water bath. It was kept a 127 mm distance between the flat specimen and the transducer to achieve an optimum performance. The results were provided in a C-scan map or an A-scan graph, which can detect discontinuities in the composites, e.g. resin-rich regions, voids or cracks.

According to the results obtained in this work, microstructural changes could be observed using the ultrasonic inspection in the laminates cycled 3000 times, or so. This study also verified a low void content in the composites as it can generate cracks, easily detectable by C-scan method, due to the influence of the temperature changes on the entrapped gases (thermal expansion). From the macromechanical model applied to temperature of -60 °C, it resulted of theoretical stresses below the necessary one in order to nucleate cracks, thus, not reaching minimum stresses to modify the features of the C-scan maps from a health laminate. The quality inspection methodology appears to have low accuracy, as the tests depended on the perfect position of the laminate in the water bath. However, for the crack detection purpose, the ultrasonic inspection is reliable, and it offers a complete diagnosis regarding the position of the crack (in-plane and out-of-plane) and size.


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