PSI - Issue 5

Andrzej Katunin et al. / Procedia Structural Integrity 5 (2017) 416–421 Andrzej Katunin/ Structural Integrity Procedia 00 (2017) 000 – 000

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Analysing the obtained visualizations it can be noticed that the first flaws become observable for specimens that reached a maximal self-heating temperature values of 60°C. A very little tiny cracks and delaminations located very close to the top and bottom surfaces of the specimen are observable in Fig. 2a. Comparing this result with the maximal self-heating temperature history curve (Fig. 1) one can notice that around 60°C region the self-heating temperature increases monotonically, without any disturbances. The previous studies based on analysis of a character of a self heating temperature (Katunin (2012)) as well as other measurement methods and techniques described in Katunin et al. (2017a) indicated that the critical self-heating temperature value is at level of 65-70°C. Nevertheless, considering the existence of initial damage at the self-heating temperature of 60°C observed during XCT studies one can conclude that this value should be considered in further studies as a critical self-heating temperature. Comparing these observations with the ultimate tensile strength results acquired during tensile tests of specimens subjected to fatigue with self-heating (Katunin et al. (2017a)) presented in Fig. 3 it can be observed that the current results coincide well with the values of the ultimate tensile strength.

Fig. 3. Residual ultimate tensile strength of specimens subjected to fatigue loading accompanied with the self-heating effect.

While the self-heating temperature increase the structural damage propagates, which is clearly visible in Figs. 2b d. In Fig. 2b one can observe periodically distributed delaminations. The observed periodicity of damage is a result of delamination in a region of interlacing of fibres in a reinforcing fabric, and following the performed observations it can be assumed that these locations are the initiators of further development of delaminations and cracks, which finds a proof while observing the damage patterns for higher self-heating temperature values (see Figs. 2c,d). Comparing the XCT results with the related self-heating temperature on the history plot presented in Fig. 1, it can be observed that the temperature growth starts to be non-linear, which indicates a beginning of formation of a macrocrack. In this case, damage still has a surface character. The two last cases shown in Fig. 2 represent advanced damage state with a formation of macrocracks along the width of the specimens, which is confirmed both by a position on a self-heating temperature history curve (Fig. 1) and by the tensile testing results (Fig. 3), where the significant drop of an ultimate tensile strength is observed. In both cases the delaminated areas cover the surface of region of stress concentration both at the top and at the bottom of a specimen. Moreover, both delaminations and cracks are observable not only on top and bottom surfaces, but also inside the specimens. These two examples show the character of damage propagation during fatigue of polymeric composites accompanied with the self-heating effect. It is interesting that damage propagates along the thickness from the surfaces to the middle plane of the specimen. This observation looks evident when considering stress distribution along the thickness following the theoretical statements of bending of plates. However, considering the generated heat and low thermal conductivity of polymers used as a filler in industrial composites, the heat may be stored inside a structure and cause damage initiation on the level of a middle plane. The performed studies confirmed that in damage mechanism of composite structures loaded in the above-described conditions the mechanical stress on surfaces of a