Improve Shell Model Accuracy for Torsion in Abaqus

Drill DOF tuning improves torsional stiffness in composite shell models.

When simulating wind turbine blades under torsional loads, shell models are often preferred over solid models for efficiency. But standard shell formulations can significantly underestimate torsional stiffness, especially when offsets and layered orthotropic materials are involved. 

A joint study by Ghent University and 4RealSim, published in Composite Structures, Volume 280 (2022), demonstrates that tuning the drill degree-of-freedom (DOF) stiffness in shell models reduces the torsional stiffness mismatch from 35% to just 5%. 

Shell Models Underestimate Torsional Stiffness Under Load 

Initial simulations on a composite wind turbine blade revealed that baseline outer mold line (OML) shell models predicted roughly 30–35% lower torsional stiffness compared to full solid element models. Even using shells without offset didn’t close the gap; the models remained overly compliant in torsion. 

To isolate the root cause, a simpler geometry, a cylindrical tube, was analyzed. Since an analytical solution is available for this case, it allowed direct comparison and made it possible to trace the issue to the out-of-plane rotational stiffness, or drilling DOF, of shell elements. 

What is Drill DOF Stiffness and Why Does it Matter? 

Shell elements in Abaqus are available in 5 or 6 DOF versions. The 6 DOF formulation enforces rotational compatibility across fold lines and intersections, which is critical in 3D structures. But for smooth shell surfaces, it introduces a singularity in the drilling DOF, the rotation normal to the shell surface. 

Abaqus resolves this by adding a small artificial stiffness to the drill DOF. However, in models with offset shells (where nodal planes do not coincide with the shell mid-plane), this default stiffness is often too low. The result: 

• Unwanted deformation modes 

• Non-physical energy contributions 

• Underestimated torsional response 

To correct this, Abaqus allows the drill stiffness to be scaled by a user-defined factor. This gives users direct control over the rotational stiffness in the shell formulation. 

Practical Workflow: From Benchmark to Improved Blade Model 

1. Benchmark the Effect: A composite cylindrical shell was simulated under torsional loading. With standard drill stiffness, the model significantly underestimated torsional rigidity. 

2. Apply User-Defined Drill Stiffness: By increasing the drill stiffness factor in Abaqus, the model’s torsional behavior aligned more closely with analytical expectations, especially for layered orthotropic materials. 

3. Transfer to Real Geometry: The modified drill stiffness was then applied to the original wind turbine blade models. The results: 

   1. Torsional stiffness improved significantly 

   2. Offset shell models now closely matched non-offset shells 

   3. The difference with the solid model dropped from 35% to ~10% 

4.  Validate with Refined Solid Model: A new solid model was built with corrected thickness transitions to match the shell geometry more closely. This reduced the solid model’s predicted stiffness by 5%, bringing the remaining difference with the improved shell model to just 5%. 

Why This Matters for Simulation Workflows 

• Design Accuracy: Underpredicting torsional stiffness by 30% can result in flawed design decisions, especially in dynamic or fatigue-sensitive structures like wind turbine blades. 

• Efficiency Without Compromise: Shell models are faster and lighter than solids. With calibrated drill DOF stiffness, they become a practical and reliable alternative even under torsional loads. 

• Better Material Representation: Especially in composite layups, proper rotational stiffness helps capture realistic torsional resistance and interlaminar shear behavior. 

More Information 

This blog is based on a study conducted by Ghent University and 4RealSim, and published in:   

Composite Structures, Volume 280, 15 January 2022, Article 114905  

Let’s Improve Your Torsional Predictions

Accurate torsional behavior in shell models is crucial when working with composites or offset geometries. If you're looking to improve stiffness predictions in Abaqus, our team can guide you through effective drill DOF calibration. Reach out via the contact form or send an email to sales@4realsim.com.