Dynamic stress intensity factors for arresting cracks in DCB specimens

Abstract

Crack arrest toughness Kla values are usually evaluated using a static stress intensity factor analysis. According to this procedure which is advocated by Crosley and Ripling [i], the process of crack arrest is treated as mathematically identical to the process of crack initiation, with only the time scale being reversed. Dynamic effects (stress waves, structural vibrations) and their possible influence on the process of crack arrest are not considered. This concept is contested by Hahn et al. [2]. These authors analyzed the energetics of fracture arrest and concluded that, as the crack accelerates in the initial phase, kinetic energy is built up in the specimen which is subsequently available to contribute to the crack driving force during the arresting phase of crack propagation.In this work the possible influences of kinetic energy are investigated by measuring the actual dynamic stress intensity factors for arresting cracks and comparing these to the corresponding static values.The crack arrest experiments were performed in wedge-loaded DCB specimens (321x127x10 mm) made from an epoxy resin (Araldit B). Notches with different root radii were used to achieve different initial crack velocities. Dynamic stress intensity factors for propagating and subsequently arresting cracks were determined using the shadow spot method of Manogg [3] in combination with a Cranz~Schardin high speed camera; the static stress intensity factors for the corresponding crack lengths were obtained using the conventional stress intensity factor formulae from deflection measurements at the loading point.Experimental results for cracks, initiated at different values of the stress intensity factor at initiation (KI_) and thus propagating at different initial crack velocities, are summarized indFig, i. Fig. la shows values of the dynamic stress intensity factor KIyn (experimental points in the upper part of the diagram) as a function of crack length a compared with the corresponding static stress intensity factor K~ tat curve. The measured velocities are given in the lower par t of the diagram additionally. The same results as in Fig. la, but plotted in a different form, are shown in Fig. lb. Simplified curves have been drawn through the measured data points. The very early phase,of crack propagation (dashed lines), being of less importance in this context, has not been investigated.The following characteristics of the crack arrest process can be drawn from Figs. la and Ib:i. At the beginning of the crack propagation phase the dynamic stress intensity factor K~ yn is smaller than the corresponding static value K~ tat. At the end of the crack propagation phase the dynamic stress intensity factor K~yn is larger than the corresponding static value K~ tat. Only after arrest does the dynamic stress intenInt Journ of Fracture 12 (1976) 318 2.factor K~yn approach the statically determined stress intensity sity factor at ~ arrest K stat Ia "The velocity of the propagating crack stays constant for a considerable period but decreases to smaller value...

On the measurement of dynamic fracture toughnesses ? a review of recent work

“…In order to investigate the influence of dynamic effects on the mechanical behavior of cracks at arrest, the stress condition at the tip of a propagating and subsequently arresting crack was analyzed [10][11][12][13][14]. Using the epoxy resin Araldite B, the actual dynamic stress intensity factors, Kp yn, were measured by means of the shadow optical method of caustics in transmission.…”

Dynamic fracture analysis by explicit solid dynamics and implicit crack propagation

“…and that this is equal to nΓ, being a dimensionless coefficient accounting for some initial bluntness of the crack tip then, Equation (17) can be reduced to find the equilibrium point of crack arrest length at = = 0 to approximately be [28,29]:…”

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