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case). The same pattern has been found by [9] on hotrolled strips. This proves the relevance of thisphenomenon, tentatively attributed to the presenceof significant shear stress (die edge / friction) whichinduces the rotation of principal axes.zx3.3 Interface3.3.a Delamination / spallingThis is another failure mechanism whereby oxidefragments may be spalled off the sample surface; inrolling, such fragments may then be embeddedinside the strip surface by the contact pressure.zyFigure 5: oblique cracks (top) near the PSCT die edge,(bottom) on a rolled strip [9].3.2 Roughness transferFigure 6: comparison of the oxide surface state after PSCT1with rough / smooth dies. Die width 12 mm. ε = 0. 4 , & ε = 1.s− ,T = 900°C, oxide thickness 50 µm.Figure 8: two examples of delamination. Left: interfacialdelamination on the flank of a groove. Right: delaminationwithin the oxide layer.In figure 8 (left), bending of the sample in the flankof a grooved die (representing the “roll banding”defect, i.e. peeling of an orthoradial strip of rolloxide) has resulted in a normal, through-thicknesscrack which has bifurcated along the interface; insuch situations, the formation and embedding of afragment becomes highly probable. The die grooveis oriented in the die width direction, equivalent tothe rolling direction. Such a crack would thus belongitudinal, contrary to those shown above. Figure8 (right) shows delamination within the oxide. Linesof pores have occasionally been found in oxidelayers, parallel to the interface; they may be theorigin of such defects. Yet delamination might alsohave taken place at the interface, with subsequent reoxidationduring cooling in imperfectly pure N 2 .Here again, fragmentation and embedding of thespalled layer in the next rolling stand is inevitable.Figure 7: cross-section of the samples shown in figure 5,showing the smooth metal (white) – oxide (grey) interface,whatever the die roughness.Another possible defect is interface waviness due totool roughness printing, in particular when rolls areseverely worn. Figure 6 shows that the oxidesurface, to a large extent, takes the roll roughness,which suggests a certain degree of plasticity of theoxide, but the oxide – metal interface remainssmooth (figure 7). This may not be a generalconclusion however, certainly dependent (i) on theratio of the tool roughness to the oxide thickness,and (ii) of the temperature-dependent mechanicalproperties of the oxide (toughness, oxide-to-metalyield stress ratio). Here again, numerical simulationcan contribute in the study of these parameters.3.3.b Interface instabilityIn a series of tests devoted to varying strain rate, asinusoidal interface waviness has been observed atthe lowest strain-rate, decreasing and disappearingas ε & increases (figure 9).Figure 9: Interface waviness without cracking. Strain rateincreases from top to bottom (0.1, 1 and 10 s -1 ).There is no evidence of any crack / micro-extrusionphenomenon, hence the interpretation by plastic codeformationinstability (see [10] e.g.). It is not surethat this can occur in strip rolling, since it has been