Today we discuss how to evaluate the stress concentration factor in a plastic part, actually in last blog, you can find a small illustration, please look at it carefully. there is a Mathematical relationship between radius and thickness for the stress concentration factor. it means when R/T values lesss than 0.5, the stress concentration factor is quite high, and when values of R/T over 0.5 the stress concentration factor gets lower. So at the inside radiuses of a plastic part, a minimum of 1 x thickness would be better.

Today’s product complexity, along with increasing downward pressure on overall pricing, has led to an inevitable increase in the geometric complexity of mechanical components. The increased geometric complexity allows components to be utilized in an advanced way, typically performing multiple functions. This multi-functional utilization has led to complex load transfers, resulting in new stress concentrations. the pressure to design components that can run for the longest number of cycles at the lowest price is significantly demanding.plastic part

A stress concentration is defined as high localized stress, compared to the average stress of the body, and is typically found in a region that has an abrupt geometric change or discontinuity. The body tends to fail at the location of a high-stress concentration or stress riser. This article will walk through the basics of stress concentration, offer real-world examples to illustrate the concept and outline methods for reducing stress concentrations in your designs.

Stress Concentration Factor
The effect of a stress riser can be determined quantitatively, utilizing mathematical methods such as the stress concentration factor. The stress concentration factor, Kt, can be defined as the ratio of the highest stress in the part to the reference stress, expressed as the following mathematical equation. Stress concentrations are relatively straightforward when it comes to identifying where they will be located and whether they have contributed to the failure. They will be located in the small radii and sharp corners that are in a load path.

The stress concentration factor is dependent on the geometry of the part. Components with more significant discontinuities in geometry will have higher stress concentration factors.

Stress Concentration-It’s All Geometry
Determining the location of stress concentrations and identifying whether they tend to cause failure is a relatively straightforward process. For example, there will be a stress concentration in geometric features such as small radii or sharp corners that are in a load path.

It’s important to note that the specify formula changes based on the load case and geometry, so further charts will need to be utilized for additional geometries. A good resource for finding curves for other geometries is “Stress Concentration Factors”. Now that we understand the basics, we can step into some examples of correcting stress concentrations.

By now, the idea of locating and mitigating the risk of stress concentrations should be clearly defined. While the FEA program was utilized here to determine the magnitude of stresses, the same general guidelines can be used to improve a design.

General Design Tips and Issues to Avoid
When it comes to common methods of reducing stress, the following list includes some simple guidelines to follow:

Make radii in a load path as large as possible.
Limit the ratio of the large feature to the small feature.
Add stress-reducing holes at the end of slits, sharp angles, or cracks to relieve high-stress concentrations.
Refer to stress concentration charts to understand when you are in a region of diminishing returns concerning radius size.
Some common issues to avoid are:

Do not use sharp corners along a load path.
Do not make a large-size transition between loaded features. The stiffness mismatch will drive the stress concentration much higher.
Don’t assume that the same size radius works for all features. Remember that the stress concentration is based on a ratio, not a magnitude.
Don’t place a stress concentration in a high cyclic load if you absolutely must use a sharp corner.
As you may have guessed, most of the “don’ts” are inverse to the “dos”. This list is not fully comprehensive, but it should cover the basic concepts and design tips that every designer should know to improve their design skills. Understanding the stress concentration effect on product or component longevity is important in areas like stress concentration in machine design.

Through the examples and analysis above, it should be clear exactly why we need to be concerned with stress concentrations. By incorporating these concepts through your design, you should be able to achieve higher load ratings, reliability, and fatigue life.

I encourage design teams to talk through product requirements and design tips and choices to ensure the proper blend of aesthetics and function while mitigating the effect of stress concentration. There will always be tradeoffs, but proper analysis can help achieve an optimized solution.

Stress Concentration FAQs:

Stress concentrations occur because of abrupt changes in geometry, such as a sharp radius or edge.
As the radius of curvature approaches zero, the stress concentration factor approaches infinity. Increase the radius of curvature to reduce the stress concentration factor.
The stress concentration factor is generally calculated by dividing the maximum stress by the average stress in a member, under loading.
A fillet provides lower stress concentration than a chamfer.
A stress concentration factor is a ratio of the maximum stress in a member to the average stress in a member.