WHAT IS METAL FATIGUE?

Metal fatigue is one of the subtlest types of metal failures, and one of the most dangerous. Although metal failure can occur from excessive tensile loads, shear loads and impact loads, to name a few, metal fatigue is a failure type that can go unnoticed right up until the point when it is too late.

 

What is Metal Fatigue?

Metal fatigue occurs when metal parts are weakened due to repeated stresses. There are three stages to metal fatigue:

 

Stage One: After a certain amount of load cycles, micro-cracks begin to form on the metal during the metal fatigue process. The micro-cracks tend to form around stress concentrating geometric features such as edges. The stress required to create these micro-cracks can actually be less than the ultimate tensile strength and yield tensile strength of the metal.

Stage Two: These micro-cracks continued to be stressed by cyclic loading, causing them to increase in size.

Stage Three: Eventually, the enlarged micro-cracks reach a size where the stresses are enough to cause rapid crack propagation, leading to metal failure. The crack surface will be different in appearance depending on metal type and metal tensile strength.

Metal Fatigue Strength

In order to stop metal fatigue failures, it is important to understand how the characteristics of a metal affect its ability to resist fatigue. The most obvious and important of these is the fatigue strength of a metal. Fatigue strength is measured through fatigue testing, a group of evaluation methods that run material specimens of a determined size and shape through repeated cycles of a certain stress level. This is done for many stress levels, and a graph is plotted that highlights how many cycles, at a given stress level a material can withstand without fracturing. Being informed about the fatigue strength of a metal is extremely important to avoid metal fatigue failures.

 

Common Applications Subjected to Risk of Metal Fatigue Failure

Metal fatigue should be a consideration for all applications. However, there are several components and structures that are especially at risk for metal fatigue failure. Examples of some of these include:

 

Bridges

Automotive suspension equipment

Metal stamping equipment

Airplane body parts

High-vibration parts

Furniture

How to Prevent Metal Fatigue

Metal fatigue is a phenomena that can be lessened or altogether avoided with proper engineering considerations. One key method of preventing metal fatigue is by running a software fatigue analysis on component or structure designs. By running analysis and iterating the design process each time, metal failure can be avoided. For instance, software analysis could reveal an area subjected to substantial fatigue loads. If this was near a geometric feature such as a hole, the hole may be able to be moved elsewhere. Also, redesign could focus the stresses to an area that would be less subject to metal fatigue failure.

 

Another method to prevent metal fatigue is through appropriate material selection. Different materials have different fatigue strengths. For instance, steel can typically withstand higher amounts of stress cycles than aluminum can. By determining the fatigue strength of a material and evaluating how that will match up with the application, metal fatigue failure can be avoided.