Short Answer:

The selection of failure criteria plays a very important role in machine design because it directly affects the safety, performance, and cost of the component. Failure criteria help engineers decide the stress limit a material can handle before it starts to deform or break. Choosing the correct failure theory ensures the part works safely without being over-designed or under-designed.

If the wrong failure theory is used, it can lead to unsafe machines that may break unexpectedly, or it can cause unnecessary material use, making the machine heavier and costlier. So, selecting the right failure criteria helps maintain a good balance between safety, efficiency, and economy in mechanical design.

Detailed Explanation:

Selection of failure criteria and its effect on machine design

In machine design, components face various types of loads such as tension, compression, bending, torsion, and combinations of these. These loads create different stress conditions inside the material. If the stresses go beyond a safe limit, the material will fail either by yielding (plastic deformation) or by breaking (fracture). To prevent such failure, engineers use failure criteria—rules or theories that predict when and how a material will fail under stress.

The correct selection of failure criteria is essential to designing safe, cost-effective, and long-lasting machines. Different materials (ductile or brittle) and different loading conditions require different failure theories to be applied. If the wrong theory is used, the machine part may either fail too soon or be unnecessarily overbuilt.

How failure criteria affect machine design

1. Safety of the component

Choosing the right failure theory ensures that the machine part remains safe during operation. For example:

• If a ductile material is used and Maximum Principal Stress Theory is applied (which is for brittle materials), it may not predict yielding properly.
• On the other hand, using von Mises Theory (for ductile materials) gives more accurate results and prevents early failure.

A correct failure criterion helps avoid accidents, damage, or injuries, especially in critical systems like airplanes, bridges, or medical devices.

2. Material selection and cost

Using the correct failure theory helps in choosing the right material with proper strength. It also avoids over-designing, which leads to extra material usage and higher costs. For example:

• If von Mises Theory shows that the material can handle the stress safely, there is no need to increase size or strength unnecessarily.
• But if an incorrect or very conservative theory is used, the design may become too heavy and expensive.

Proper theory selection leads to cost-effective and efficient design.

3. Performance and weight of the machine

In modern industries like automobile and aerospace, weight reduction is very important. Selecting accurate failure criteria ensures that:

• The component is lightweight but strong.
• The performance remains high without risking failure.

For example, using Distortion Energy Theory instead of Maximum Shear Stress Theory can result in a more accurate and lighter design for rotating shafts or structural frames.

4. Design simplicity and ease of analysis

Different failure theories have different levels of complexity:

• Tresca Theory is simple and easy to apply.
• Von Mises Theory is more accurate but slightly more complex.

Choosing the correct balance between accuracy and ease of calculation helps in faster and better machine design.

5. Suitability for material behavior

• For ductile materials, theories like von Mises or Tresca are suitable.
• For brittle materials, Maximum Principal Stress Theory or Coulomb-Mohr Theory is better.

If the wrong theory is used, the material might fail unexpectedly, causing danger or loss.

Example scenario

Suppose a designer is working on a crankshaft made of steel:

• Using Tresca Theory will provide a simple and safe design.
• Using von Mises Theory may allow for a slightly lighter and more optimized design.
• Using Maximum Stress Theory (meant for brittle materials) may lead to wrong predictions and result in early failure.

This shows how critical the selection of failure theory is in design.

Conclusion

The selection of failure criteria strongly affects the strength, safety, cost, and overall performance of a machine. Applying the right failure theory ensures that the design is neither too weak nor too heavy. It helps in choosing suitable materials, avoiding unexpected failures, and achieving long-lasting and economical designs. Therefore, every mechanical designer must understand the nature of loading, the type of material, and apply the appropriate failure criteria for successful machine design.