A co-operative effect of closely spaced intruding objects puncturing into a soft solid

When two or more closely spaced indenters puncture a soft solid, the puncturing load diminishes from that required when they are far apart. We have developed a method to examine this effect by embedding a planar tracer grid within a block of an elastic gel and by driving closely spaced syringe needles of different inter-tip spacing through this plane. These experiments show that in addition to a primary crack ahead of the needle-tip, there occurs also a periodically appearing secondary radial crack from its side. The strain field around the side crack remains non-monotonic with a characteristic length scale which varies with both solid modulus and needle diameter. For closely spaced multiple indenters the overlapping strain profiles cause compressive stress at the vicinity of the needles, which arrest the side cracks, the diminishing effect of which leads to stress concentration for the primary crack and the resultant decrease in required load for driving the indenters through the solid. We have shown that such a co-operative effect remains valid for materials of different types, e.g. brittle and ductile and for multiple-tip needles consisting of constituent tips of equal and unequal diameters.