Stress Concentration Factor versus Stress Intensity Factor
Stress concentration factor and stress intensity factor are completely different concepts. In this article, we’ll briefly explain what they are and how they differ from each other.
What is a stress concentration?
A stress concentration refers to a spot in a component where the stress is significantly higher than the stress in the surrounding material.
When there are irregularities or weak points in a material, the stress equations in mechanics of materials may not accurately represent the actual stress. These irregularities, also known as stress raisers, cause sudden increases in stress (stress peaks) near the stress raisers.
The term stress gradient describes the rate at which stress increases as the stress raiser is approached. The stress gradient may affect the damaging impact of the peak stress value. Often, large stresses caused by irregularities only occur in a small portion of a structure. These localized stresses are referred to as stress concentrations.
The stress concentration factor depends on the specific geometry and loading conditions of a component. It can be determined through experimentation, analysis, or computational methods.
What is a stress concentration factor?
A stress concentration factor is simply the ratio of the highest stress in a component to a nominal reference stress. The highest stress usually occurs at a stress concentration point, while the nominal stress represents what the stress would be if the stress raiser were absent.
Stress concentration factors are influenced by part geometry and loading. They can be determined through experimental testing, analytical calculations, or numerical methods.
What is a stress intensity factor?
Stress intensity factor is a key concept in fracture mechanics theory. It’s important to note that stress intensity factor is not related to the equivalent stress quantity called stress intensity (Tresca Stress).
When analyzing cracks under elastic stress, we introduce the concept of stress intensity factor K. This factor helps describe the elastic stress distribution around the crack tip. As mentioned earlier, crack surfaces can be categorized into three types, each with its corresponding stress fields. Therefore, we use three stress intensity factors—KI, KII, and KIII—to characterize the stress fields for these three modes. The dimensions of stress intensity factor K are [stress] x [length]^1/2.
The value of factor K depends on the dimensions of the specimen and the applied loads. Generally, K is proportional to the product of the average stress and the square root of the crack length.
Knowing the stress intensity factor K for a specific mode (let’s say K1), we can calculate the stresses and displacements near the crack tip.
What is a Stress Intensification Factor (SIF)?
It’s crucial not to confuse stress intensity factor with stress intensification factor, as they are completely different and unrelated concepts.
Stress intensification factors are fatigue correlation factors used to compare the fatigue life of piping components with that of girth butt welds in straight pipes under bending moments.
What about Stress Singularities?
Engineers and analysts often use the term “stress singularity,” which is distinct from the concepts discussed in this article. While the aforementioned concepts are based on real phenomena, a stress singularity is merely a numerical artifact and not a “real” stress.
A stress singularity occurs at a point in a finite element model where the stress value doesn’t converge to a specific point. Theoretically, the stress at a stress singularity is infinite. Such singularities may arise at nodes where point loads are applied or at boundary conditions.
Stress singularities are typically disregarded when studying the behavior of structures. However, analysts should exercise caution and verify that stress singularities do not significantly impact the stresses in the areas of interest within the model. In other words, stress singularities should be located far from the regions of study to have minimal or negligible impact on the stresses and strains of the geometry under investigation.