Why Do DIC Results Differ from Finite Element Simulations?

In material and structural mechanics tests, more and more researchers use DIC systems to acquire full-field displacement and strain data, and compare them with simulation results from finite element software such as Abaqus and ANSYS.

In actual comparisons, the following situations often occur:

▪ The deformation trends of the two are basically consistent, but the specific values differ;

▪ Strain concentration positions are similar, but peaks differ obviously;

▪ Finite element contour maps are smoother, while DIC contour maps contain more local variations;

▪ Simulation results are basically symmetric while actual measurements show deviation;

▪ The curve trends are consistent, but they cannot coincide point by point.

The appearance of these differences does not necessarily meanDIC measurement being inaccurate does not necessarily mean the finite element model is wrong. In many cases, the root cause is that the objects described and the calculation conditions used are not exactly the same.

What Do DIC and Finite Elements Reflect Respectively?

The DIC system analyzes speckle images on the specimen surface to obtain surface displacement and strain under real test conditions.

The measurement results may include:

▪ Local material inhomogeneity;

▪ Specimen machining dimensional errors;

▪ Thickness change;

▪ Clamping eccentricity;

▪ Fixture slippage;

▪ Contact friction;

▪ Initial defects;

▪ Equipment and environmental vibration.

Finite element simulation, by contrast, numerically predicts the structural response under a defined geometric model, material parameters, loads and boundary conditions.

Simulation usually requires idealizing the real test. For example, assuming the material is homogeneous, fixtures are fully fixed, loads are strictly centered, and the specimen has no machining errors.

Simply put, DIC records what actually happened to the real specimen, while finite elements compute what would theoretically happen under the set conditions.

There is a certain difference between the two, which is not abnormal in itself.

Which Factors Affect the Comparison Between DIC and Simulation?

1. The coordinate systems are not unified

DICsystemsand the finite element software may use different coordinate directions, origin positions and positive/negative conventions.

For example, the X direction in DIC may correspond to the Y direction in the simulation model, and the DIC surface normal may be opposite to the Z direction of the finite element model.

Before the formal comparison, unify:

▪ Coordinate origin;

▪ X, Y and Z directions;

▪ Positive/negative direction of displacement;

▪ Units;

▪ Engineering strain or true strain;

▪ Principal strain or strain in specified directions.

If the coordinates and physical quantities are not unified, even two essentially identical sets of data may show obvious differences.

2. Real boundary conditions differ from model settings

In the finite element model, the clamped end is usually set as fully fixed.

But in real tests, fixtures may undergo slight slippage, deflection or deformation, and the specimen may have clamping eccentricity.

For thin sheets, flexible materials and easily bending structures, even small boundary differences may change the overall strain distribution.

If the DIC results show obvious asymmetry while the finite element results are highly symmetric, first check the clamping state, load position and boundary conditions, rather than directly assuming a problem with the DIC contour map.

3. The material model is too idealized

Finite element results depend heavily on the material model.

If the model only defines elastic modulus and Poisson's ratio while the actual material has entered plasticity, viscoelasticity, damage or large deformation, the simulation naturally struggles to agree with the test.

For composites, rubber, foam, welded joints and additive manufacturing materials, there may also be:

▪ Anisotropy;

▪ Material zoning;

▪ Residual stress;

▪ Interface damage;

▪ Pore defects;

▪ Performance differences between regions.

The full-field data obtained by DIC can help users discover in which areas and at which load stages the material model deviates.

4. Different spatial resolutions

Finite element results are computed from mesh nodes and elements, while DIC results are affected by image resolution, field of view, subset, step size and strain window.

DIC strain is not read directly from a single pixel, but computed from displacement changes within a certain spatial range.

Therefore, the local strain obtained by DIC usually has a certain spatial averaging characteristic.

Near hole edges, notch roots, sharp corners and crack tips, finite element models may show very high local peaks, while DIC results are affected by actual spatial resolution and the strain calculation window, so peaks are usually relatively smooth.

This does not mean DIC missed the deformation; the two methods use different spatial scales.

5. Comparison positions are not exactly identical

DIC usually measures the visible surface of the specimen, while finite element results may be extracted from:

▪ Structural mid-surface;

▪ Internal nodes;

▪ The back side;

▪ Element integration points;

▪ Nodal averaged results.

If the specimen has bending, torsion or a through-thickness strain gradient, the strain on different surfaces may differ significantly.

Therefore, when comparing the two, ensure:

▪ Corresponding to the same surface;

▪ Corresponding to the same spatial position;

▪ Corresponding to the same load moment;

▪ Corresponding to the same strain component;

▪ Use similar spatial averaging scales.

6. Loads and images are not accurately synchronized

In dynamic, fatigue, vibration or fast loading tests, a time deviation between images and testing machine loads can cause curves and contour maps to fail to correspond.

Even for slow tests, if images and force values are recorded independently by different devices, it is necessary to confirm whether the time origin and sampling period are consistent.

How Should DIC and Finite Elements Be Correctly Compared?

The correct way to compare is not simply putting two contour maps side by side, nor only comparing maximum strain values.

First, unify coordinates, units, load moments and result types.

Second, you can compare at the following levels:

▪ Whether the overall deformation direction is consistent;

▪ Whether the main displacement areas are consistent;

▪ Whether strain concentration locations are consistent;

▪ Whether the deformation modes are similar;

▪ Whether representative path or area curves are consistent;

▪ Whether the peak difference can be explained by spatial resolution.

For local strain concentration areas, it is recommended to compare regional averages, path changes and development trends, rather than directly comparing single extreme points.

How Does the Haytham DIC System Support Simulation Verification?

Data comparison between DIC and finite elements is not just equipment shooting and software-generated contour maps; a spatial correspondence between the specimen surface and the simulation model must also be established.

Haytham TechnologyDIC visual strain measurement system can acquire full-field displacement, strain and 3D point cloud data of the specimen surface, and supports exporting measurement results and point clouds for comparison with CAD models and simulation results such as Abaqus.

For larger structures, complex curved surfaces or multi-area measurement, multi-camera measurement and point cloud stitching schemes can be configured according to the test object, establishing a more complete spatial correspondence between the experimental measurement area and the simulation model.

In project implementation, Haytham usually comprehensively considers:

▪ Specimen dimensions;

▪ Surface curvature;

▪ Deformation speed;

▪ Target strain range;

▪ Local feature size;

▪ Simulation model form;

▪ Data output requirements.

Configure 2D DIC, single-camera 3D DIC, single-camera high-speed 3D DIC or stereo 3D DIC according to the actual test objective, instead of choosing a system merely by the number of cameras.

What Is the Value of DIC for Finite Element Analysis?

The value of DIC is not just proving the simulation right or wrong, but helping users identify the sources of differences between the ideal model and the real test.

Through full-field experimental data, one can further judge:

▪ Whether the boundary conditions are reasonable;

▪ Whether the material parameters are accurate;

▪ Whether the load has eccentricity;

▪ Whether the actual structure has deformed asymmetrically;

▪ Whether local strain concentration is underestimated;

▪ Whether damage locations are predicted accurately;

▪ Whether the simulation model needs further correction.

For material parameter inversion, structural design verification and performance analysis of complex components, DIC provides more complete experimental evidence than single-point measurement.

Frequently Asked Questions

Must DIC and Finite Element Results Be Fully Consistent?

No. Reasonable validation should focus on comparing overall trends, deformation modes, strain concentration positions and curve development, rather than requiring all local values to coincide exactly.

Why Is the Finite Element Peak Strain Higher Than DIC?

It may relate to mesh singularity, local theoretical peaks, DIC spatial resolution and the strain window. Especially near sharp corners and crack tips, single-point peaks are usually not suitable for direct comparison.

Can DIC Be Used to Update Finite Element Models?

Yes. Full-field displacement and strain data obtained by DIC can be used to adjust material parameters, load positions, boundary conditions and contact relationships, improving the consistency between the model and real tests.