2020 design crack 10.5 cracked#
The above discussion is predicated on cracked body geometries of moderate to relatively high constraint such as a thick double edge cracked panel, single edge cracked bar or a compact toughness specimen. This has important implications for hydrogen embrittlement problems, where the amount of stored hydrogen scales exponentially with the hydrostatic stress. The hydrostatic stress does peak near r ≈ 3 δ t but the peak is relatively broad and maintains a high value over a considerable distance ahead of the crack tip. 10.2 it is seen very high strain hardening, n = 2, leads to a normal stress that steadily increases as the crack tip is approached, in contrast to lower hardening results which reach a peak value near r ≈ 3 δ t. As a practical matter, that is for n of 0.2 or less, assuming a characteristic blunted crack tip stress–strain environment is reasonable when limiting cracked body dimensions are greater than (20 – 100) δ t.Ī very large degree of strain hardening in the range of n = 2 is usually the province of low strength alloys where the ratio of ultimate tensile strength to yield strength is on the order of 3, such as austenitic stainless steels and low strength nickel–base alloys (e.g. The lower value is appropriate to the onset of ductile tearing while the larger value is appropriate to cleavage fracture particularly in ferritic steels. When the thickness meets these values it is reasonable to assume near tip plane strain conditions. Calculations and fracture toughness tests indicate that the characteristic blunted crack tip stress/strain environment is a useful approximation when the limiting dimensions of a crack body are greater than about (10–30) J/ σ o. The question of what is small depends on both the degree of strain hardening and overall geometry of the cracked body. However, these fields persist for larger-scale yielding provided that the crack opening, δ t, remains small compared with other specimen dimensions. The plastic zone size is about 10% of the K zone. Landis, in Gaseous Hydrogen Embrittlement of Materials in Energy Technologies: Mechanisms, Modelling and Future Developments, 2012 10.5.1 Application of crack tip fields to gain insight into cracking mechanismsīlunted crack tip stress–strain fields in Figs 10.6–10.10 were calculated for small scale yielding conditions. The resulting crack growth along the surface and in the through thickness direction as a function of the number of cycles (time) is shown in Fig. 6.1. A total of about 150 increments is used for the complete analysis. (The stress intensity factor, K I, is a function of the crack depth and length.) Since the stress intensity factor is, of necessity, calculated based on the initial crack size for the current increment of crack growth, this stress intensity factor must be updated by a small percentage to account for the increase in crack size during the previous increment.
The number of cycles in each increment was adjusted to limit the total crack growth to a value that results in an increase in the crack tip stress intensity factor of less than about 2%. The crack extension along the surface was calculated using a similar procedure.
The calculated d a/d N from equation 6.1 was presumed to be essentially constant over an increment of cycles (Δ N) so that the crack extension in the depth or through thickness direction (Δ a) in this increment was evaluated from the product (d a/d N) × (Δ N). The crack tip stress intensity factors were used to calculate crack growth rates both along the surface and in the through thickness direction using equation 6.1.