Modern companies must deliver high-quality goods and services against the minimal cost to remain in the competitive market. Clients also look for multi-functional and high-performing mechanical components. So, manufacturers continue to draft and craft parts with constantly increasing geometric complexity. This tends to complicate the load transfer mechanism thus resulting in stress concentration in parts. Complicated load-transferring paths call further attention to stress reduction in modern die casting design. Keeping the component functional and durable is impossible without resolving stress concentrations.
What is Stress Concentration?
It refers to a particular location within a component where the stress is higher than the average value. This point or section mostly remains in places having abrupt changes or discontinuity. Therefore, stress concentrations in machine parts occur near corners, holes, cracks, notches, etc.
Stress Concentration Factor
Quantitative measurement can determine the possible development of high-stressed points. It calculates the stress concentration factor (Kt) with the respective ratio of the highest stress (σmax) to the reference stress (σavg) in the casting.
In mathematical terms, the measurement goes by –
where,
σmax = maximum or peak stress
σavg = average or reference stress
The factor directly depends on the casting geometry. Parts having more significant discontinuities and irregularities result in higher factors. Utilizing a geometric curve can also help determine the concentration factor. The curve is drawn from the corner radii to the critical part dimensions ratio.
However, the formula keeps changing based on the geometry and load applications. So, other charts are necessary to understand the full geometry for stress-reduced products.
Causes of Stress Concentration
Complex geometry and subsequent load-transferring complication is the primary reason. But there are more causes associated with this phenomenon, outlined with examples below –
- Variations in Material Properties: inner cracks/flaws, welded cavities, blow holes, foreign inclusions, air/gas pores.
- Application of External Load: cam – follower, meshing teeth – driving gear, balls – ball bearing, rail – wheel, and hook – chain contact points.
- Machining Scratches: machined and automated stamps, inspection, or labels on the die-cast surfaces.
- Abrupt/Sudden Shape Changes: changed cross-section in mount gears, pulleys, sprockets, and ball bearings due to cut steps.
- Discontinuity of Components: oil grooves/holes, keyways, screw threads, or splines to discontinue the component’s cross-section.
Effects of Stress Concentration in Die Casting
A stress concentration increases the chance of failure of the castings. The stress maneuvering near the irregularities produces localized concentrations. It contributes to the stress value in the concentrated regions, making the area vulnerable.
Static loading in brittle materials like die castings severely affects durability and performance. Cracks appear in the concentrated sections to gradually increase the stress intensity throughout the part. Any changes in the section must feature fillets to avoid this from happening.
However, cyclic loading affects ductile materials due to the poor relieving capability of stress. Cracks, flaws, sharp discontinuity, or surface roughness lead to eventual failure. Based on the design and properties, castings are also affected to a limited extent.
How to Reduce Stress Concentrations
Eliminating stress concentrations in die casting is a task for the designing as well as the production team. The following suggestions work great on nearly all geometric shapes used in machine manufacturing.
· Gradual Transition of Geometry
An abrupt change in shape or geometry is the most common trigger for concentrations. Specific measures like fillet radii and tapers can ease the transition. Smooth surfaces of critical parts where shape changes occur can help this issue. It keeps the stress flow uninterrupted, preventing accumulation in one region.
· Additional Groove or Relief Notch
Removal of cast materials near the notches can further aid concentrations. It creates an extra groove or a relief notch to alter the existing geometry. Still, these notches can keep the load paths in line for castings. But this measure requires successive FEA analysis to ensure its perfect placement.
· Multiple Notches Instead of One
However, a long notch can severely alter the geometry to cause significant deviation of load transfers. So, using multiple notches to replace unavoidable holes/notches can reduce issues. Surrounding every notch/hole with intentional notches can smooth out potential stress distribution without concentration.
· Avoiding Sharp Corners/Edges
Geometries of CNC machining, especially the internal parts, must remain free of sharp corners. Unless specified, having no sharp corners or edges is a general rule in the industry. Using a fillet radius is the most popular and recognized solution. It ensures a gradual decrease in targeted cross-sectional areas.
Further Design Tips for Die Casting
- Try to make the fillet radius as large as possible on the load-transferring path.
- The same-sized radius doesn’t work for all features as it’s based on ratios.
- Always refer to the stress concentration charts concerning the radius size.
- Do not create any large-size change between the loaded features for stiffness.
- Put a limit to all the design ratios between larger features and smaller features.
- Drill to add stress-reducing holes at slits, cracks, and sharp angles for relief.
- Avoid stress concentrations in high cyclic loads for a must-use sharp corner.
This guideline isn’t exactly comprehensive by all means for every die-cast manufacturing process. Still, it should clarify the fundamentals of stress concentration reduction.
Sunrise Metal’s talented team of engineers and designers ensures high-quality functional parts by adhering to the latest DFM practices. We thoroughly review the design and run simulations to eliminate stress concentration in aluminum castings. Feel free to contact us for consultation on the design optimization of your parts.