Failure of a “Toggle Bar” (Part of a hand-brake system of a motor van)

The bar, formed by flattening one end of a cylindrical tube, was a connection between the brake lever and the cable operating the brakes. The hand lever was bolted into the hole in the flattened end of the bar. The top end is shown in Figure 1. The hollow end of the cylindrical portion of the bar was threaded to receive the cable, representing the lower end of the bar shown in Figure 1.

Figure 2 shows that the bar had broken across the centre of the hole, the side marked B giving way before that marked A, so that the fracture at A was completed by bending as shown in Figure 3.

Figure 4 shows a complete transverse section at LM in Figure 1 and Figure 5 shows the microstructure of a transverse section cut through PQ in Figure 1.
It was estimated that the original dimensions of the metal surrounding the hole were:

Thickness (uniform): 5.080 mm Effective width at A: 1.905 mm Effective width at B: 1.270 mm

The hardness of the steel at PQ was 110 Hv, and at LM it varied between 100-140 Hv. It was estimated that the maximum effort which the driver could apply to his brake lever was equivalent to a force of 445-534 N which would be magnified by the lever system between 8-10 times.

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