PSI - Issue 60

6

M Mohan Kumar et al. / Procedia Structural Integrity 60 (2024) 177–184 M Mohan Kumar et al. / Structural Integrity Procedia 00 (2024) 000 – 000

182

4. Results & Discussions In the present work, linear static analysis is carried outinitially for an unstiffenedpanelandthe stressintensity factor for a progressive central crack is established by MVCCI technique. It is a method is used to determine stress intensity factor for different crack lengths in the panel which is based on Irwin assumption, that the energy released in the process of crack expansion is equal to work required to close the crack to its original state as the crack extends by a small amount Δ a. Further, SIF for both integral and riveted stiffened panels are estimated for growing crack along the width of the panel, maintaining the same loading conditions. A graph of stress intensity factor (K 1 ) vs. half crack length (a) is plotted for unstiffened panel and both types of the stiffened panels as shown in Fig.5. The fatigue crack growth rate (da/dn) is calculated using fatigue crack growth rate equation comprising of da/dn, ∆ K with C and m material constants defined in Paris law to predict life of the stiffened panels for stress ratio R=0. Further, crack length is plotted as a function of number of cycles as shown in the Fig 6. From the Fig 5. It may be noted that the stress intensity factor tends to increase with same magnitudes with the progressive crack lengths for both types of the stiffened panels till it reaches the vicinity of first stringer, after which the stress intensity factor starts to increase with the crack propagation for an integrally stiffened panel, in comparison to the riveted stiffened panel, where the stress intensity factor starts to decreases, as the crack propagates through the first stringer and a similar response is observed further, as crack tip approaches the second stringer as well. The decrease in the stiffening effects is seen in the riveted stiffened panel, when the crack progresses away from the stringer and eventually causes the SIF to increase and also the crack growth rate. This trend is reflected by the plots in Figs.5 and 6. The von-mises stress is calculated for certain crack length for both stiffened and unstiffened panels and the von-mises stress distribution near the crack tip for unstiffened and stiffened panels are depicted in the Fig.7. It can be observed from the Fig. 7 that the orientation of crack in the panel is in longitudinal direction and crack widens due to loading in transversedirection. In the case of integral stiffeners, the skin crack progresses simultaneously with the stringers which are integrated to it. The plot in Fig. 6 shows that crack propagation rate in the integrally stiffened panel is lower due to stiffening effects, compared to the riveted stiffened panel, as the crack grows through the stiffened panel. It may be noticed that the magnitude of stress intensity factor in panel without stiffeners is higher compared to the integrally stiffened panel for the similar crack length. In riveted panel, the crack growth in the skin causes a high stress concentration in the stringer rivet holes thereby increasing the chances of stringer failure. The fracture of both types of the stiffened panels and unstiffened panel takes place as stress intensity factor reaches critical stress intensity factor (Kc) corresponding to critical crack length. The load bearing capability of both types of stiffened panel and unstiffened panel is analyzed by comparing the critical stress intensity factor of each stiffened panel and unstiffened panel with fracture toughness of the Aluminumalloy. It is observed that 6,20,060 number of cycles are required to propagate crack of 280 mm in a integrally stiffened panel compared to riveted stiffened panel which require 7,52,725 fatigue cycles to grow same length of the crack. Also the unstiffened panel requires very less cycles of 1, 67,027 to reach the maximum crack length of 280 mm.