Question

30 pts) The broken shaft shown below has failed in service, the part was lo ion in a static fashion, dynamic loading was not present. N aded in pure o stress concentrations exist e to make it in the part, the shaft was smooth and quite polished. It has been painted whit easier to see. The arrows show the direction of torque loading on the shaft. Your investigation has revealed the shaft was purchased overseas from a cheaper vendor your company has never used before. The maintenance department says the part is too soft, with not enough heat treatment, they think it is too weak and just twisted off. Due to the failure of the shaft, an injury occurred to an employee, so an engineering review must be conducted. That task has fallen on your shoulders as a licensed Professional Engineer in Kentucky Use the skills of ME 344 to review the fracture and make a conclusion about the failure and the heat treatment statement. Document your work with appropriate figures and diagrams and make a decision about the level of heat treatment in the shaft. Do not just make statements, create figures and diagrams to support your work in a manner lawvers can nderstand.

Answer 1

The shaft failed due to brittleness of the shaft. The shaft material is not soft, it is brittle in nature. The type of failure shown in figure occurs in case of twisting of brittle materials. To see it practically, take some chalks and break them off by twisting them. you will notice that everytime the chalk breaks like this.

This happens because the brittle materials are weak in tension. If we apply twisting moment to a brittle shaft or a chalk, due to tension at 45 degree plane, the material breaks along 45 degree axis.

t tangential 45 2- ス

As we can see here, there is normal tensile stress at plane AB whose magnitude is similar to the shear stress in the stress element. Due to tension applied over to a brittle material, the material breaks off at 45 degree.

So , the claim by maintenance department that material is soft is completely wrong. The material is actually brittle. May the heat treatment of the shaft has been done in a wrong way. It is not supposed to be soft. It should be rather tough.

If a shaft fails, it should fail like this(ideally) which is almost impossible though. It is supposed to have a cup and cone type of failure