Experimental and finite element study on stable crack growth in three point bending

Abstract

Experimental and finite element results are presented on mode I and mixed mode (involving I and II only) stable crack growth under static loading through an aircraft grade aluminium alloy (D16AT) in three point bending. The results include load-displacement diagrams, J-integrals, plastic zones, tunneling (or crack front curving), etc. During experiment a substantial amount of tunneling is observed, the extent of which increases as the extension progresses in both mode I and mixed mode. The tunneling reduces as ao/w increases. The crack extends initially almost along a straight line at an angle with the initial crack in a mixed mode. The maximum load is observed to be as high as 1.6 times the initiation load in the whole range examined. From the finite element study it is seen that, in a mixed mode, the J-integral at the onset of extension is the lowest compared with the values at the later stages. The plastic zone size grows as the stable extension progresses; the growth is approximately the maximum along the crack extension line. The direction of initial crack extension in a mixed mode can be predicted through an elastic finite element analysis and using the criterion of maximum tangential principal stress. The study also indicates that the load-displacement diagram associated with a mixed mode stable crack growth can be predicted reasonably accurately using the criterion of crack opening angle.

Influence of state of stress on mixed mode stable crack growth through D16AT aluminium alloy

“…The theoretical investigations have been done with a view to predicting the load-displacement diagrams, plastic zones and the crack edge profiles using a finite element scheme [44][45][46][47] The algorithm works with a variable load increment per step, which is sufficient to bring one or two Gauss points to the yield surface. Further, at the end of each step, any Gauss point whose equivalent stress is greater than 0.995 ~ry is treated as yielded subsequently.…”

“…Each discretisation is made taking note of the initial direction of crack growth so as to permit a study of the whole span of the stable growth through a single discretisation. The direction of initial crack growth can be predicted [10,[44][45][46][47] or obtained experimentally. The elements ahead of the crack tip are squares of size d and those behind it are rhombuses of the same size.…”

“…The study of mixed mode, both theoretically and experimentally, is lacking. We have shown through both theoretical and experimental investigations [9][10][44][45][46][47] that the COA criterion can be used to characterize the whole stable crack growth under quasistatic loading in both CT and TPB configurations, provided there is a small crack growth from the initiation to instability.…”