Low-Velocity Impact Induced Damage in Composite Overwrapped Pressure Vessel (COPV)

Composite Overwrapped Pressure Vessels (COPVs) are critical components in the aerospace, automotive (hydrogen/CNG storage), and marine industries due to their exceptional strength-to-weight ratio. However, their layered, anisotropic structure makes them highly susceptible to Low-Velocity Impact (LVI) events. Tool drops during manufacturing, debris strikes during operation, or mishandling during maintenance can induce severe internal damage that is completely invisible from the outside (Barely Visible Impact Damage – BVID).

At BanuMusa R&D, we specialize in the advanced numerical modeling of COPVs. By utilizing high-fidelity Finite Element Analysis (FEA) in Abaqus, our engineering team predicts the exact micro-mechanical damage mechanisms and evaluates the critical residual burst pressure of the vessel post-impact, ensuring absolute safety and regulatory compliance.

Abaqus finite element software allows modeling low-velocity impact-induced damage in a composite overwrapped pressure vessel using a 2D or 3D Hashin VUMAT (user material subroutine). Low-velocity impact can cause significant internal damage in composite overwrapped pressure vessels (COPVs) that is not visible externally. Modeling such impact damage in COPVs using finite element analysis can provide useful insights. A Hashin 3D VUMAT can be implemented to predict various damage modes in the FRP composite – fiber tension/compression failure, and matrix tension/compression failure. Modeling low-velocity impact damage in COPVs using Abaqus with a composite damage model like 3D Hashin VUMAT can provide valuable insights into failure mechanisms, residual strength and impact resistance. Such models can be validated using experiments and used to evaluate design improvements for better impact performance.

Various low-velocity impact induced damages can occur on the composite structures during manufacturing, operation, and maintenance processes.

Therefore, the impact damage and residual strength of the composite structures have been important issues in the FRP compindustry. The impact damage needs to be considered in the design step and an efficient analysis tool is required to analyze the impact damage and residual strength of the composite structures.

Low-Velocity Impact Induced Damage in Composite Overwrapped Pressure Vessel  with Abaqus & 3D Hashin VUMAT

The Engineering Challenge: Progressive Failure in FRP Composites

Analyzing impact damage and residual strength in Fiber-Reinforced Polymer (FRP) composites requires moving far beyond standard built-in material models. Standard criteria fail to account for the complex interaction between different degradation modes.

To resolve this, our team applies a phenomenological continuum damage mechanics (CDM) approach. We utilize an energy-release-rate-driven damage variable to simulate progressive failure. Furthermore, to prevent the notoriously difficult problem of strain localization and mesh sensitivity during impact analysis, we incorporate a nonlocal smeared cracking approach.

After extensive benchmarking against various macroscopic failure criteria, we selected the 3D Hashin Failure Criterion as the absolute best mathematical foundation for our composite damage model.

Low-Velocity Impact Induced Damage in Composite Overwrapped Pressure Vessel with Abaqus & 3D Hashin VUMAT User Material Subroutine

The analysis results such as the impact response, impact damage, and residual strength, were compared with the test data to verify the damage model and analysis methods.

Implementation: The 3D Hashin VUMAT Subroutine

A VUMAT user material subroutine allows you to define a custom material model in finite element analysis software. It gives you more flexibility beyond the built-in material models.

The VUMAT subroutine allows you to:

• Define your own constitutive model – This could be a plasticity model (like GTN damage model for metals), a damage model, a composite damage model, etc. You define the behavior of the material.
• Calculate stress and stiffness – The VUMAT must calculate the Cauchy stress tensor and material stiffness tensor based on the input strain and state variables.
• Update state variables – You can define state variables that evolve as the analysis progresses. The VUMAT updates these state variables at each increment.
• Predict damage initiation and evolution – For a damage model, the VUMAT can determine when damage initiates based on failure criteria (for example, look at the 3D Hashin VUMAT code developed by BanuMusa R&D). It then evolves the damage as the analysis progresses.
• Consider nonlinearity – The VUMAT can model nonlinear material response due to plasticity, damage, large deformations, etc.
• Return data to Abaqus – The VUMAT returns the calculated stresses, stiffnesses, and updated state variables to Abaqus at each increment.
• Be more efficient – A VUMAT coded in a language like Fortran can be more computationally efficient than using built-in models.

The VUMAT defines the constitutive equations and material logic, while Abaqus takes care of the geometry, mesh, loads and analysis procedure. This allows sophisticated material models to be used within Abaqus.

Why use a VUMAT for COPV Impact Analysis?

A VUMAT (Vectorized User Material Subroutine) gives the FEA engineer complete, unfiltered control over the material’s constitutive behavior during highly dynamic, non-linear events. Our custom VUMAT allows us to:

• Define Custom Constitutive Logic: We bypass basic elasticity and enforce the 3D Hashin theory directly at the integration point.
• Predict 4 Distinct Damage Modes: The subroutine continuously calculates and updates damage variables for Fiber Tension, Fiber Compression, Matrix Tension, and Matrix Compression.
• Track Interlaminar Damage: Coupled with a cohesive zone model (CZM), we accurately simulate the severe delamination that occurs between composite plies upon impact.
• Update State Variables (SDVs): The VUMAT tracks the irreversible strain and stiffness degradation, feeding the weakened structural tensor back to Abaqus at every time increment.

Validating Residual Strength and Burst Pressure

The ultimate goal of modeling low-velocity impact is not just to see the dent, but to answer the critical engineering question: Will the vessel still hold pressure?

Our simulation workflow precisely maps the degraded element stiffness from the dynamic impact step into a subsequent static internal pressurization step. The analysis results—including the transient impact response, energy dissipation, and ultimate residual burst pressure—were rigorously compared and validated against experimental drop-weight test data.

For teams designing Type 3 and Type 4 COPVs, accurately mapping the filament winding angles is just as critical as the damage model. We highly recommend pairing our VUMAT with the Wound Composite Modeler (WCM) Plugin to automate the complex geodesic winding geometry generation.

Elevate Your Composite Design Workflow

Understanding the true impact resistance of your composite structures is mandatory for preventing catastrophic in-service failures.