3pc-sandfreq

3pc-sandfreq is a Python library for free vibration analysis of shear deformable anisotropic sandwich plates. It provides a Sandfreq class to calculate natural frequencies and mode shapes, considering shear deformation effects in composite sandwich construction.

Installation

Use the package manager pip to install 3pc-sandfreq. Since the package and its dependencies are available on PyPI, you can install them directly:

pip install 3pc-sandfreq

Alternatively, you can install it from the local source directory:

pip install .

API Reference

Sandfreq Class

The primary component of this package is the Sandfreq class, which takes a panel, a load case, and analysis grid settings to compute natural frequencies and mode shapes.

from sandfreq.sandfreq import Sandfreq

Required Parameters

When initializing Sandfreq, the following parameters are required:

• panel (Panel): An instance of the Panel class (from 3pc-panel) representing the sandwich plate panel.
• loads (Loads): An instance of the Loads class (from 3pc-loads) containing the applied in-plane loads ($N_{xx}$, $N_{yy}$, $N_{xy}$).
• number_of_terms (int): The number of terms to use in the Ritz/analytical solver.
• number_of_points_x (int): Number of reporting points along the length (x-axis).
• number_of_points_y (int): Number of reporting points along the width (y-axis).

Calculated Properties

The class calculates the following properties:

• pairs: List of $(m, n)$ pairs for all $i, j = 1, 2, \dots, M$, where $M$ is the number of terms in the analysis.
• K33: Loads dependent part of the stiffness matrix $K_{33}$ for all pairs.
• lambdaEV: Right eigenvalues of the generalized eigenvalue problem: $K = \lambda_{EV} \cdot M$.
• vr: Right eigenvectors of the generalized eigenvalue problem.
• lambda_cr: Critical eigenvalue.
• vr_cr: Critical right eigenvector.
• mn_cr: Critical $(m, n)$ pair.
• lambdaP: Sorted list of positive eigenvalues.
• lambda1 to lambda5: First to fifth critical (lowest positive) eigenvalues (natural frequencies in Hz).
• mn1 to mn5: Dominant $(m, n)$ mode shapes corresponding to natural frequencies lambda1 to lambda5.

Methods

Sandfreq.fromDict(**kwargs)

Initializes a Sandfreq instance using a dictionary of keyword arguments.

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Usage Examples

Basic Initialization

Below is an example of performing a sandwich free vibration analysis.

from panel.panel import Panel
from loads.loads import Loads
from sandfreq.sandfreq import Sandfreq

# Assuming pnl (Panel) and ld (Loads) are already defined
sf = Sandfreq(
    panel=pnl,
    loads=ld,
    number_of_terms=4,
    number_of_points_x=11,
    number_of_points_y=11
)

# Access natural frequencies and their dominant mode shapes
print("First Natural Frequency (Hz):", sf.lambda1)
print("Dominant (m, n) for lambda1:", sf.mn1)

Commands

The package provides a command-line interface entry point. After installing, you can run the application using:

sandfreq input.cfg

(This entry point is defined in sandfreq.__main__:main)

Options

• config (positional): Specify the configuration file (.cfg) to run the analysis.
• -h, --help: Show the help message and exit.
• -v, --version: Show the version number.
• -s, --sample: Copy example files (materials, plies, cores, laminates, panels, loads, config) to the current directory.

Configuration File (sandfreq.cfg)

The sandfreq command requires a configuration file (typically named sandfreq.cfg). Below is an example based on sandfreq/examples/example001/sandfreq.cfg:

[files]
materials = materials.json
plies = plies.json
cores = cores.json
laminates = laminates.json
panels = panels.json
loads = loads.json
output = output.txt

[analysis]
number_of_terms = 4
number_of_points_x = 11
number_of_points_y = 11

[files] Section

This section defines the paths to the required input JSON files and the desired output file:

• materials: Path to the materials JSON file. Follows the format expected by 3pc-material.
• plies: Path to the plies JSON file. Follows the format expected by 3pc-ply.
• cores: Path to the cores JSON file.
• laminates: Path to the laminates JSON file. Follows the format expected by 3pc-laminate.
• panels: Path to the panels JSON file. Follows the format expected by 3pc-panel.
• loads: Path to the loads JSON file. Follows the format expected by 3pc-loads.
• output: Path to the text file where the sandwich free vibration analysis results will be written.

[analysis] Section

This section specifies the analysis settings:

• number_of_terms: The number of terms to use in the Ritz/analytical solver.
• number_of_points_x: Number of reporting points along the length (x-axis).
• number_of_points_y: Number of reporting points along the width (y-axis).

Citation

If you use this library in your research or work, please cite it as follows:

APA Format:

Rastogi, N., & Seresta, O. (2026). 3pc-sandfreq: Python library for free vibration analysis of shear deformable anisotropic sandwich plates. PyPI. https://pypi.org/project/3pc-sandfreq/

BibTeX:

@software{3pc_sandfreq,
  author = {Rastogi, Naveen and Seresta, Omprakash},
  title = {3pc-sandfreq: Python library for free vibration analysis of shear deformable anisotropic sandwich plates},
  year = {2026},
  url = {https://pypi.org/project/3pc-sandfreq/}
}