Issue 46

A. Deliou et alii, Frattura ed Integrità Strutturale, 46 (2018) 306-318; DOI: 10.3221/IGF-ESIS.46.28 313 Figure10: Fatigue crack growth rates for as a function of ΔK for the three zones studied The results of Fig. 10 have shown that the welded metal and heat affected zone have an important higher crack propagation resistance at the small ΔK values (The mechanical properties of weld material are better (Tabs. 3 and 4). The Paris equation coefficients obtained for the different X70 welded joint regions mentioned in Tab. 6 indicate that the growth rate of the crack influenced by the microstructure of the welded joint zones. The high resistance to propagation of heat affected zone can be related to the existence of acicular ferrite in microstructure (Acicular ferrite microstructure in heat-affected zone give high mechanical behaviors in terms of strength and toughness) [47-49]. At the high values of ΔK, the affinity between cracking rate plots can be related for the decrease of crack closure. For low values of ΔK this phenomenon as more marked by the crack tip plasticity following its advance (thus forming a wake plasticized). These results are in good agreement with the results of Beltrao et al.[50] but, they noticed that the fatigue crack propagation is get more independent from the high load ratio values(R=0.5) E VOLUTION OF ENERGY PARAMETERS arious experimental techniques have been developed to measure the surface creation energy U. Most of these techniques are based on stable mechanical hysteresis loops, which measure work in an area near the crack. Ikeda et al. [51] had measured the quantity for steel of low carbon content and for high resistance aluminium alloy from hysteresis loops in the plastic zone using strain micro gages. Using the differential method of Kikukawa et al. [52], Ranganathan measured the hysteric work U [53], which represents the dissipated energy in the plastic zone by unit created surface on the aluminium alloy (2024 T351), testing CT specimens modified in order to be able to measure the crack tip opening displacement (CTOD) in the loading axial direction. The progression of the crack tip opening displacement δ, and δ’ with respect to the load P were registered at a frequency of 0.05 Hz ( δ evaluated by a clip gage). The differential displacement δ’ is calculated by the expression: δ δ α P    (5)  : The specimen compliance at a singular crack length. The measurements were realized during one cycle for constant amplitude. Characteristic δ and δ’ with respect to the load P layouts for constant amplitude loading are shown in Fig. 11. (Crack opening load Pop was measured at the beginning of the horizontal segment on δ’ with respect to P diagram [54]. Fig. 12 shows the evolution of the hysteretic energy Q dissipated during a cycle as a function of ΔK, for a load ratio R = 0.1, in the three zones studied. This energy is determined by a numerical integration of the cycles (P- δ‘) , its expression is obtained by calculating the area of this loop obtained by acquisition and processing by a program written under LABVIEW. V