Picture Frame DIC Analysis Workflow

This document outlines the workflow for processing grid data in the Direct Image Correlation (DIC) analysis of a picture frame test. The core logic is encapsulated in the process_grid_data method of the DICGrid class.

Overview

The processing pipeline converts raw coordinate data from tracking markers into full-field strain maps through the following four stages:

  1. Data Ingestion: Raw image paths and point coordinates are stored.

  2. Displacement Calculation: Computes the vector difference between current and reference configurations.

  3. Field Interpolation: Converts sparse marker data into a dense grid.

  4. Strain Computation: Derives strain tensors from the displacement field.

Detailed Workflow

Step 1: Displacement Calculation

The method first determines the displacement field (\(\mathbf{u}\)) based on the input coordinates.

  • Input: current_points and reference_points.

  • Logic:

    • If remove_rigid_transform is True: The method calculates displacement after subtracting rigid body motions (translation/rotation) via compute_disp_and_remove_rigid_transform.

    • Otherwise: It calculates raw displacement via compute_displacement.

  • Output: self.disp

Step 2: Grid Interpolation

To move from discrete marker points to a continuous field, the data is passed to the GridInterpolator.

  • Class: GridInterpolator

  • Method: interpolate_displacement

  • Action: Maps the irregular self.disp data onto the regular mesh defined by self.grid_x and self.grid_y.

  • Output: self.disp_x, self.disp_y (Dense displacement fields).

Step 3: Strain Calculation

Finally, the dense displacement fields are differentiated to obtain strain.

  • Class: StrainCalculator

  • Method: compute_strain

  • Parameters:

    • method: Accepts cauchy, 2nd_order (Green-Lagrange), or log (Hencky).

  • Output:

    • Normal Strains: self.strain_xx, self.strain_yy

    • Shear Strain: self.strain_xy

API Reference

process_grid_data(self, reference_image, image, reference_points, current_points, interpolation_method, strain_type, remove_rigid_transform=False)

Orchestrates the DIC processing pipeline.

Parameters:

  • reference_image (pathlib.Path | str) – Filename or path of the reference (undeformed) image.

  • image (pathlib.Path | str) – Filename or path of the current (deformed) image.

  • reference_points (np.ndarray) – Coordinates of markers in the reference configuration.

  • current_points (np.ndarray) – Coordinates of markers in the current configuration.

  • interpolation_method (str) – The algorithm used to grid the scattered data (e.g., ‘linear’, ‘cubic’).

  • strain_type (str) – Definition of strain to compute (‘green_lagrange’, ‘cauchy-eng’, ‘2nd_order’, ‘log’).

  • remove_rigid_transform (bool, optional) – If True, rigid body rotation and translation are removed before strain calculation.

Returns:

The computed displacement vector.