This picture shows the photoelastic fringe patterns of the nut and the spanner while the spanner is used to tighten the nut. Stresses at the contact points are very high. Note that the fringes at the outer and inner curves of the spanner are tangential to these. The geometry of the spanner is purposefully kept like this, so that minimum number of fringes cross the boundary and thus minimising stress concentration. The spanner thus has streamline fillets .

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