PSI - Issue 10

V.N. Kytopoulos et al. / Structural Integrity Procedia 00 (2018) 000 – 000

6

V.N. Kytopoulos et al. / Procedia Structural Integrity 10 (2018) 272–279

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tion increases in presence of initial hole damage. This could be explained again by the general fact that delay effects of crack initiation and propagation caused by “mild“ stress -strain concentration fields around holes, compared to strong fields around “sharp” edge crack, increases the probability of development of “surplus” of damage sites taking place within the virgin material up to final fracture. In other words holes presenting energy sinks with reduced energy inflow rate, compared to edge crack energy sinks with increased inflow rate, permit a higher portion of the supplied strain energy to be directed for development of new damage sites. In Fig.5 the evolution of the structural instability number with initial damage is reflected. This number is expressed by the inverse of total absorbed strain energy up to fracture ωf. This would mean that low (high) fracture strain should reflect a high (low) structural instability of the material. One can observe in this figure the general expected trend of the increase in microstructural instability with increasing initial damage. This instability seems to be slightly higher for initial hole damage, a fact which could be associated with the increased population of fracture damage sites as explained earlier.

Fig. 5. Evolution of structural instability number with initial damage D 0 .

In Fig.6, the complex evolution of failure strain energy efficiency number with initial damage was obtained. This number expresses the ratio of the total dissipated fracture strain energy to the total supplied elastic energy. At this place it should be noted that damage is a dissipation process always associated with strain and which also involves dissipation of energy. It is therefore important to find any kinds of dependence of damage on strain energy. In this sense this number could reflect, in other words, the strain energy dissipation capacity of the material for fracture damage formation. The initial edge-crack damage seems to present very different trend compared to hole damage. This behavior can reasonable be explained as follows: Delay effects of initiation of crack propagation due to “milder” stress concentration fields around hole sites allow “adequate” time for “surplus” strain energy dissipation by microdamage formation within the material which in this manner can absorb, in a monotonic fashion and with increasing rate, a corresponding high strain energy from the total (externally) supplied elastic strain energy. On the other hand, in the case of initial edge-crack damage, due to stronger stress concentration fields and hence strong energy sinks, a high portion of supplied energy is absorbed for crack initiation and propagation. However the absorbed-dissipated strain energy can with decreasing rate attain a maximum, after which at certain initial damage must become falling. One can also observe that beyond a “critical” initial edge-crack damage a rapid microstructural instability or disintegration, associated with a high strain energy release rate takes place. In this aspect, the initial slant edge-crack damage presents similar behavior accompanied with slightly increased values of fracture strain energy absorption. Finally, in Fig.7 the evolution of energy dissipation deficiency of damage formation number with initial damage is presented. This parameter reflects the ratio of heat convertible energy to new surface or crack opening energy. From this figure the general falling trend of this parameter with initial damage can, at first, be stated. This means a