A crystalline solid exhibits thermally induced localised {\em non-affine}droplets in the absence of external stress. Here we show that upon an imposedshear, the size of these droplets grow until they percolate at a criticalstrain, well {\em below} the value at which the solid begins to yield. Thiscritical point does not manifest in bulk thermodynamic or mechanicalproperties, but is {\em hidden} and reveals itself in the onset ofinhomogeneities in elastic moduli, marked changes in the appearance and localproperties of non-affine droplets and a sudden enhancement in defect pairconcentration. Slow relaxation of stress and an-elasticity appear as observabledynamical consequences of this hidden criticality. Our results may be directlyverified in colloidal crystals with video microscopy techniques but areexpected to have more general validity.
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