Frequently Asked Questions¶

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General¶

What is XdTd?¶

XdTd (Extended Damage Tolerance Design) is a desktop application for crack growth analysis and damage tolerance design of metallic aerospace structures. It uses the Dual Boundary Element Method (DBEM) to compute stress intensity factors, predict crack growth paths, and evaluate residual strength — all within an interactive graphical environment.

Who is XdTd for?¶

XdTd is designed for:

Aerospace structural engineers performing damage tolerance assessments (FAR 25.571, EASA CS 25.571)
MRB/MRO engineers evaluating repairs with fastened or bonded patches
Researchers and students working on fracture mechanics, BEM, or fatigue crack propagation
Mechanical engineers analyzing cracked 2D structures with stiffeners or reinforcements

Is XdTd free?¶

Yes — XdTd is free for academic and non-commercial use. Students, researchers, and educators can use the full application at no cost. Commercial use requires a yearly contribution. See the Downloads page for details.

What platforms does XdTd run on?¶

XdTd is available for Windows 10+ (64-bit) and Linux (Ubuntu-based distributions). See System Requirements for details.

Analysis Method¶

What is the Boundary Element Method (BEM)?¶

The Boundary Element Method is a numerical technique that only requires discretization (meshing) of the boundaries of the problem — edges, holes, and crack surfaces — rather than the entire domain. This makes it particularly efficient for fracture mechanics, where crack surfaces are boundaries by nature.

XdTd uses the Dual Boundary Element Method (DBEM), which handles cracks by applying displacement equations on one crack face and traction equations on the other. This avoids the need for special crack-tip elements and allows crack growth to be modeled by simply adding new boundary elements — no remeshing of the domain is required.

Why use BEM instead of FEM for crack growth?¶

In the Finite Element Method (FEM), crack growth requires remeshing the entire domain at every increment, which is computationally expensive and error-prone. In DBEM:

Only the boundaries are meshed — far fewer elements are needed
Crack extension is natural — new boundary elements are added at the crack tip without touching the rest of the mesh
Mixed-mode analysis is straightforward — crack direction is determined by stress intensity factors at each increment
Multiple cracks and multi-site damage are handled without special treatment

This makes BEM the method of choice for 2D crack growth simulation in panel-type structures.

What crack growth laws does XdTd support?¶

XdTd supports the following fatigue crack growth equations:

Paris law — the classic power-law relationship (da/dN = C * DK^m)
Forman equation — accounts for stress ratio (R) and fracture toughness effects, capturing the acceleration near critical SIF

How does XdTd compute stress intensity factors?¶

XdTd computes Mode I and Mode II stress intensity factors using two independent methods:

J-Integral — a path-independent energy integral evaluated around the crack tip
Crack Opening Displacement (COD) method — based on the relative displacement of the crack faces near the tip

The J-Integral is generally within 1% of analytical solutions, while CROD (COD) is within 5%. Both are demonstrated in the Benchmarks.

Capabilities¶

What types of structures can XdTd analyze?¶

XdTd handles 2D structures including:

Unstiffened panels — flat sheets with arbitrary boundary shapes, holes, and cutouts
Stiffened panels — sheets reinforced with stringers and frames (riveted, bonded, or integral)
Repaired panels — cracked sheets with fastened or bonded repair patches
Multi-site damage — panels with multiple cracks growing simultaneously

What types of cracks does XdTd support?¶

Edge cracks — originating from any boundary (external edge, hole, cutout)
Embedded cracks — located within the sheet interior
Multiple cracks — several cracks growing simultaneously with interaction effects

Can XdTd handle stiffeners and repair patches?¶

Yes. Stiffeners (stringers, frames) and patches are core features:

Stiffeners can be fastened (riveted/bolted) or bonded to the sheet, with material and cross-section properties from built-in databases
Patches can be single-sided or double-sided, fastened or bonded, with full control over adhesive or fastener properties
The analysis captures load transfer between reinforcements and the sheet, including fastener flexibility and adhesive shear behavior

Can I compare different designs?¶

Yes. A case study can accumulate multiple result sets from different configurations. You can also open multiple case studies simultaneously and overlay their results in the same chart — for example, comparing crack growth curves for different stiffener spacings or patch sizes.

Comparison with Other Tools¶

Why use a 2D tool instead of a 3D tool?¶

To build a good 3D fracture mechanics model, it is always wise to start with a simpler 2D model first. A 2D model is much easier to build, much faster to analyze, and much easier to interpret — you can explore design alternatives rapidly without the overhead of 3D meshing and computation.

This simplicity and ease of use makes 2D tools like XdTd perfect for early design work, when the main structural layout and sizing decisions have to be made. At this stage, engineers need to evaluate many configurations quickly — stiffener spacing, patch sizes, material choices — and a 2D approach gives reliable answers in a fraction of the time. The detailed 3D analysis can then be reserved for final verification of the chosen design, informed by the understanding gained from the 2D study.

How does XdTd compare to NASGRO and AFGROW?¶

NASGRO and AFGROW use predefined stress intensity factor solutions (handbook solutions) for standard geometries. They are powerful for those configurations but limited when the geometry deviates from the catalog.

XdTd computes stress intensity factors directly from the geometry using BEM — no handbook solutions or beta corrections needed. This means:

Arbitrary panel shapes, hole patterns, and stiffener layouts are handled naturally
Crack path is predicted, not assumed
Multiple interacting cracks are analyzed without workarounds
Repair patches are modeled explicitly, not approximated

XdTd is complementary to NASGRO/AFGROW: use handbook tools for standard configurations and XdTd when the geometry requires a numerical approach.

How does XdTd compare to FRANC2D and FRANC3D?¶

FRANC2D/3D are FEM-based fracture mechanics codes. They share the goal of numerical crack growth analysis but differ in method:

FRANC uses FEM (domain mesh) while XdTd uses BEM (boundary-only mesh)
XdTd requires significantly fewer elements for equivalent accuracy in 2D panel problems
Crack extension in XdTd does not require domain remeshing

How does XdTd compare to ABAQUS or ANSYS for fracture mechanics?¶

General-purpose FEM codes like ABAQUS and ANSYS can perform fracture mechanics analysis, but:

They require specialized crack modeling techniques (XFEM, cohesive elements, contour integrals)
Crack propagation typically requires scripting or third-party plugins
Mesh management around the crack tip is complex and labor-intensive

XdTd is purpose-built for crack growth — the entire workflow from geometry to results is streamlined for damage tolerance engineers. The Benchmarks section shows direct comparisons with ANSYS results.

File Format and Data¶

What file format does XdTd use?¶

Case studies are saved as .xdtd files, which are SQLite databases. This means:

Files are self-contained — geometry, properties, mesh, and results are all in one file
Files can be inspected with any SQLite browser if needed
Multiple case studies can be open simultaneously

Does XdTd include material databases?¶

Yes. XdTd ships with built-in databases for:

Materials — aluminum alloys and other metallic materials with elastic and crack growth properties
Fasteners — rivet and bolt properties
Adhesive bonds — adhesive shear properties

Important: these databases are supplied only for completeness. The use of the properties therein is the sole responsibility of the users. XdTd provides tools for users to easily create and manage their own databases with verified, project-specific data.

Still have questions? Visit the Support page or contact us.