Durability analysis 101

July 12, 2007
Structural components such as a steering knuckle might be strong enough to withstand a single applied load.

But what happens when the part operates over and over, day after day? To predict component failure in such cases requires what's called fatigue or durability analysis. Computer simulations determine how well parts will hold up during cyclic loading. Results are important in calculating and verifying safe part lifetimes.

In the past, durability analysis was largely the province of research. For starters, processes that cause part failure are complex. They involve progressive material changes, often highly localized, that end in cracks or complete fracture. For an introduction to durability analysis, it's helpful to start with a few basics. First, because failure is progressive, time is an important element of design. And because failure is localized, it's necessary to understand local stresses.

For our purposes, a constant or static load does not cause failure. What counts is the impact of working or fluctuating loads. Failures, or fractures, take place when cracks get so large that remaining material can no longer endure stresses and strains. In classic structural analyses, failure predictions are based solely on material strength or yield strength. Durability analysis goes beyond this, evaluating failure based on repeated simple or complex loading.

Programs with capabilities for durability analysis range from basic tools with simplified loading assumptions to advanced applications that target specialized engineering-analysis tasks. A basic durability application can be efficient and useful when comparing designs or design options. UGS NX Design Simulation, for example, is an add-on to CAD that lets users perform basic part-level analysis.

Take the steering knuckle, for instance. The user defines the basic load the part will carry, such as a compression load. It is defined by identifying the appropriate face and applying a pressure to it. The user then performs a durability analysis to see part areas likely to fail under loading over time. Moreadvanced simulations are stand-alone and contain CAD features along with analysis functions.

Like many CAE simulation programs, durability-analysis software follows the usual routines of preparing data (preprocessing), solving equations (solving), and studying results (postprocessing).

Results are viewed as contour plots of damage and number of cycles until crack initiation.

To perform a durability analysis, a novice designer might use a basic tool with built-in best-practice wizards that help set up an initial finite-element (FE) model of the structure under consideration. More-experienced users might bypass the wizards and build the FE model by setting boundary conditions, applying loads, and meshing the model with an advanced application.

Next comes adding fatigue conditions or defining the load cycle. A basic durability analysis lets users define "static-event duty cycles." These are simple load cycles. Users type in variables such as the static load, number of cycles, and scaling factor. The latter is a magnification factor in structural mechanics that depends on the material and damping in the system. Advanced simulation tools let users assign complex load cycles, perform fatigue analysis with power spectral density (PSD) data, and consider multiaxial loading effects, such as applying loads at different directions.

After defining the load cycle, users first perform an FE structural analysis to calculate stresses and strains. For a follow-on durability analysis, results are combined with what's called a durability event. To understand the term, image you are designing a motorcycle frame. The structure is tubular and branched, and stresses concentrate at each joint. A more-advanced simulation might be to perform an FE analysis on the bike model as it travels down a road headed for a pothole. The goal is to see what happens to the stresses in the frame during the impact with the pothole. The durability event is the definition of the pothole.

Besides the number of cycles, a basic durability analysis uses data from S-N curves that are based on uniaxial loading. Here, the primary load is in one direction.

S-N curves plot the magnitude of cyclical stress (S) against the number of cycles to failure (N). An S-N curve flattens out at what is called the fatigue or endurance limit. Stresses below this limit are assumed to do no damage.

A useful function closely related to durability analysis is the damage estimate. It is the inverse of the life-estimate value. Damage for one cycle is 1/life, and damage for n cycles is n/life. An algorithm sums the number of valleys and peaks, ignoring loading not affecting fatigue. A damage estimate thus allows estimating total damage. A user might determine, say, that at 60,000 cycles, a part is at 60% of its life.

The Navigator bar lets users manage and view on-screen results. Results for Solution 1 include displacement, stresses, and strains. This data is then used in durability analysis. A number of results in Setup 1 have been computed, including fatigue life and fatigue safety factor.

A wizard in UGS NX Design Simulation lets users define a static-event duty cycle for a durability analysis. In this case, the user typed in the number of cycles (1,000,000) and the scaling factor (1.0000), and then applied a static load.

Results of a fatigue analysis of a yoke shows component life in terms of load cycles. A title block (upper left) lets users document the contentof the image. Element Nodal means results are element results that were generated at node points. Red areas of the color plot show where failure will start, at around 30,000 cycles.

Simple static-event duty cycles show that loads can be applied and removed (left), or reversed. Either would be adequate for basic durability analysis.

A complex static-event duty cycle shows stress magnitude as a function of time. Users can generate definitions such as this in more-advanced applications.

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