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D. A. Hills et alii, Frattura ed Integrità Strutturale, 25 (2013) 27-35 ; DOI: 10.3221/IGF-ESIS.25.05 30   p x . Thus, when I K is negative and II K is positive, closure is implied in both mode I and mode II regions. Conversely, when I K is positive and II K is negative, Williams' solution implies separation throughout the whole of the region controlled by the asymptote. This implication of gross separation means that Williams' solution cannot yield any further characterisation of contact edge behaviour, therefore we do not consider this case in the subsequent analysis. R EGIONS OF F RICTIONAL S LIP o estimate the implied extent of slip, based on violations of the slip condition, the adhered interfacial tractions must be substituted into the slip condition     q x fp x   , where f is the coefficient of friction. This calculation reveals the position of all the implied boundaries between stick and slip within the edge region controlled by the asymptote, where we denote the distance from the corner to any point at which slip condition is just met as s x . For simplicity, we begin by considering the case when I K and II K are both positive , such that a small region of edge separation is implied, with an adjacent slip region. Explicitly, the slip extent, 0 / x d , is given by 1 0 I II II II s r I I r x f f f d f f f                   (16) Plots of the implied slip extent (from Eq. (16)) as well as a line showing the implied separated region (from Eq. (15)), for the case when I K and II K are both positive , are shown in Fig. 2, for three sample punch angles   60 , 90 ,120      . Figure 2 : Plots of the implied regions of slip, stick, and separation, when both I K and II K are positive, for punch angles of   60 , 90 ,120      , where the black line denotes the boundary between closure and separation, the red line the position at which the     q x fp x   condition is met, and the blue line the position at which the     q x fp x   condition is met. For the case when the remote loads excite stresses that result in a negative I K and a positive II K , the parameters 0 d and 0 G must be defined differently from Eq. (11), to avoid raising a negative number to a fractional power. Thus, we define a new parameter n n K K   , and substitute it in place of the negative stress intensity factor, so that, for example when I K is negative and II K is positive we have stick - slip + slip separation 60° Punch K I ,  K II 0.5 1.0 1.5 2.0 2.5 3.0 3.5 x s d 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 f 90° Punch stick + slip separation K I ,  K II 1 2 3 4 x s d 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 f 120° Punch K I ,  K II + slip separation 0.5 1.0 1.5 2.0 2.5 x s d 0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 f T