CAE: Differences in deformation and stress values due to rib geometry

When insufficient strength becomes an issue in product design, designers consider various reinforcement measures. One representative technique is the addition of “ribs” (reinforcing members). Ribs are an extremely effective design element for achieving both weight reduction and strength assurance, as they can enhance rigidity with relatively little additional material. However, the term “rib” encompasses a wide variety of shapes, including triangular, square, hexagonal (honeycomb), and others. The choice of shape relies heavily on the designer's expertise, making quantitative verification using CAE (Computer Aided Engineering) highly effective.

In this study, triangular, square, and hexagonal ribs, each with a perimeter of 6 cm, were placed on a 100 cm² square plate. The rib thickness was uniformly set at 1 mm for comparison. Polyethylene material properties from the CAE software's database were employed, with two load conditions set: “bending stiffness” and “torsional stiffness”. This enabled numerical assessment of the stress distribution and deformation behaviour exhibited by each rib shape.

Comparison of bending stiffness

First, we summarise the analysis results when subjected to bending loads. The triangular rib exhibited a maximum stress of 15.1 MPa and a maximum deformation of 4.77 mm. The square rib demonstrated nearly equivalent performance with a maximum stress of 15.0 MPa and a maximum deformation of 4.73 mm. Conversely, the hexagonal rib exhibited significantly inferior rigidity compared to the other two shapes, with a maximum stress of 19.5 MPa and a maximum deformation of 7.64 mm. This clearly demonstrates that triangular and square ribs are effective against bending, whereas the hexagonal rib is disadvantageous.

Comparison of torsional rigidity

Next, we examine the results when a torsional load is applied. The triangular ribs demonstrated exceptionally high rigidity, exhibiting a maximum stress of 15.2 MPa and a maximum deformation of 2.06 mm. The square rib exhibited a maximum stress of 27.6 MPa and a maximum deformation of 6.35 mm, while the hexagonal rib showed a maximum stress of 27.8 MPa and a maximum deformation of 5.80 mm. Both exhibited higher stress values and greater deformation compared to the triangular rib. In other words, the triangular rib also demonstrated the most favourable results regarding torsion.

Overall Evaluation

Summarising the above results, while hexagonal ribs (honeycomb ribs) may appear to be an effective shape for applications such as composite adhesive structures, under these conditions they clearly underperform compared to triangular or square ribs. Particularly regarding bending stiffness, they exhibit significant deformation and high stress concentration, making them unsuitable from a stiffness enhancement perspective. Conversely, triangular ribs demonstrated superior characteristics under both bending and torsional loading conditions, confirming their high effectiveness in suppressing product deformation and enhancing durability. Square ribs are also effective against bending but are inferior to triangular ribs for torsional loading; thus, triangular ribs offer an overall advantage.

Design Implications

A key insight from this study is that in rib design, ‘shape selection directly impacts stress levels and deformation’. It is crucial not merely to add ribs, but to select the optimal shape according to the load conditions. For instance, selecting triangular or square ribs is effective when the product is subjected to significant bending loads, and triangular ribs are particularly recommended for applications requiring torsional rigidity. Conversely, while hexagonal ribs may be effective in scenarios prioritising weight reduction or visual uniformity, it should be noted that they do not necessarily deliver high rigidity under the conditions examined here.

Furthermore, rib design is not solely about enhancing rigidity; it is also closely linked to factors such as mouldability, venting efficiency, and the machining difficulty of the mold. Therefore, while numerical evaluations from CAE serve as one indicator, optimisation requires balancing these with actual product conditions and production feasibility. Particularly in injection-molded products, rib thickness forming wall sections can cause sink marks or warping. Consequently, it is essential to devise methods that maximise reinforcement while keeping thickness below a certain threshold.

Summary

This comparison clearly demonstrated that rib shape differences significantly affect deformation and stress values. Triangular ribs exhibit high rigidity in both bending and torsion, making them the most effective overall shape. Square ribs are strong in bending but slightly weaker in torsion, while hexagonal ribs showed inferior performance under these conditions compared to the others. Designers can achieve lightweight yet high-strength designs by accurately understanding the product's usage environment and required load conditions, and selecting the appropriate rib shape. Quantitative evaluation through CAE analysis provides scientific justification for rib design, which traditionally relied on know-how, and will likely grow increasingly important in future design practice.