Advanced CT Inspection and Data Analysis

June 1, 2021

Composite materials for aerospace & defense continue on a path of market growth fueled by tightening environmental and economic targets. This trend is occurring alongside rapid innovations in design, manufacturing, automation and cost reduction.

Although slowed by the global pandemic, the increasing role of composites for many industries is, in fact, a foregone conclusion. More and more, composites performance—expressed in the elegant functionality of a curved wingtip on an airliner or in the fewer, thinner, winding fan blades of a GE9X engine—demonstrate an accelerating freedom-of-form and utility that makes composites essential to the future.

Rapid innovation and success in composite design and manufacturing also means a required step-up in the technology used to ensure that new designs using enhanced production methods are structurally sound. End-use safety in aerospace is critical above all else and goes hand-in-hand with achieving business sustainability.

Advanced Design Needs Advanced Inspection

Aerospace manufacturers are experienced in visual, acoustic, ultrasonic, and coordinate measuring machines (CMM) for surface-dimension checks and, more recently, radiographic-based inspection methods. These are all tools in the non-destructive testing (NDT) kit that can be called upon depending on part complexity, time available, cost, and the level of safety needed.

Figure 1. (Left) Near-vertical winglet of an Airbus A380 illustrates the design freedom carbon-fiber structures can achieve in pursuit of improved drag efficiency. Image courtesy of Airbus. (Right) The GE9X is the company’s most quiet, fuel-efficient, clean-burning and powerful commercial aircraft engine to date. (Image courtesy of General Electric)

Relatively new to the field of composite inspection is X-Ray Tomography or Computed Tomography (CT). This inspection approach, propelled forward by significant advances in CT analysis software, can look deeply into materials of almost any kind, from resin-fiber panels to aerospace aluminum. Revealing characteristics that cannot be captured and visualized with any other technology, these three-dimensional inspections are the foundation for trust in high-performance, high-safety aerospace parts and assemblies.

Why is this software technology particularly important now? Aside from the intense economic pressures on the aerospace industry for efficiency and fast, early-stage successes, both in design and production, new material and manufacturing strategies need to be fully quantified and understood. More than ever, the tools are ready to provide validation of designs to their manufactured versions at the end of (and increasingly even during) production lines, as well as to help predict the behavior of materials and construction strategies in the R&D stage.

State of Computed Tomography (CT)

Today’s CT analysis software has benefited from giant leaps made in high-performance computing and corresponding development of powerful algorithms and user-interfaces. Technical challenges are much more easily solved than even a few years ago. For example, leading scan-data analysis software can now peer into very complex or dense materials to create high-resolution, dimensionally accurate, 3D volume (voxel) images and numerical analyses that address pressing engineering questions.

Together, the numerical outputs and color-coded, 3D images derived from analysis software expose the microstructure of composites, offering As-Designed and As-Manufactured comparisons critical to quality, closed-loop inspections and the archiving of digital twin representations at each point of creation.

Figure 2. (Left) Orientation analysis carried out on a glass-fiber-reinforced sheet-molding compound (SMC). Each direction is coded by a certain color. This orientation data can be used to obtain fiber orientation distributions over the thickness to validate flow simulations and to provide reliable data for the structural simulation. Image courtesy of IAM-WK / KIT (Institute for Applied Materials/ Karlsruhe Institute of Technology), Germany. (Right) Local orientation histograms of a woven fabric are shown in yellow. Using the principal orientations of these histograms allows engineers to measure the local shear angle of the material after the draping process. (Image courtesy of Volume Graphics)

Figure 3. Porosity (upper left and top images) and fiber orientation (upper right image) can be visualized and quantified using a single CT-scan. Both results can be mapped onto a FE-Mesh (bottom-center image) to be used locally as inputs for the material modeling for structural analyses or in order to validate process simulations. (Image courtesy of Volume Graphics)

Programs exist as standalones or in a turnkey package that will analyze material density, orientation of reinforcement structures, internal defects (adhesive failure, delamination, cracking, etc.) originating from design and manufacturing flaws or overload, and strain patterns calculated from multiple scans of different states of a sample using digital volume correlation.

Any type of structure can be captured and characterized against its design intent—from autoclave to additively manufactured hybrid composites. Templates can be created to rapidly and repeatedly analyze part features and problems automatically. This includes porosity analysis (e.g., the pore volume and distance from the surface); fiber and resin analysis for local fiber, fabric and roving orientations; and derived statistics like orientation histograms or orientation tensors, fiber volume fractions, porosity within the resin, and more.

Figure 4. Mechanical *in-situ* testing combined with digital volume correlation (DVC) on samples or entire parts paves the way for full three-dimensional model validation in order to see if the model is correct or if it should be adjusted (upper route). Moreover, the analytical process can be used as a crack-detection tool to see where damage occurs. Mechanical and material engineers can do an entire microstructure characterization on the first volume of the unloaded sample and then feed that data into finite element analysis (FEA) (bottom route), either to predict specific part behavior or to create a full comparative analytical model for product validation. (Image courtesy of Volume Graphics)

Figure 5. Strain field of an *in-situ* test using a novel SMC-Material. The arising cracks are clearly visible and can be associated with the microstructure characterization, which can be carried out using the same data. All results can be mapped on an FE-mesh to easily validate strain fields of FE-simulations. (Image courtesy of IAM-WK / KIT (Institute for Applied Materials/ Karlsruhe Institute of Technology), Germany)

All the scan data can undergo simulation from outside FEA tools and be captured and used for material modeling and structural simulations, helping to build comparative models for R&D purposes and component design. Where appropriate, these analyses are even applicable for inline inspection (ILI).

Materials New and Old (CFRP to Natural Fibers)

Polymer matrix compounds (PMC), which include advanced carbon- and glass fiber-reinforced plastics (CFRP and GFRP), are the most common composite materials found in aerospace and defense. Ceramic matrix composites (CMC), made with long, multi-strand ceramic fibers, are also gaining recognition for high-heat applications. They exhibit excellent crack and fracture resistance as well as strong thermal behavior. All-in-all, there is a wide selection of materials and structural strategies that are being deployed within aerospace to custom match material behavior to application goal.

Figure 6. As seen above, composite materials of various types make up approximately 50 percent of modern aircraft structures. (Image courtesy of Airbus)

Hybrid composite materials, for instance, are under considerable study in aerospace to help advance performance within the larger goal of lightweighting. Although their design and production method may at times be similar to traditional layup structures and curing approaches, they can be composed of inorganic nano/micro materials and particulates, along with organic substances such as flax, hemp and cotton fibers. The pressure is on with organic fibers to improve the environmental viability of composites. Recycled polymers can be reinforced with natural fibers to both improve material behavior and reduce their carbon footprint, research shows.

Glass Laminate Aluminum Reinforced Epoxy (GLARE), a material found in the outer shell of the Airbus A380, is certainly a hybrid success story. After considerable research and iteration, the company, among others, proved that it is possible to skillfully combine very different individual materials to achieve high-strength and stiffness at low density while also suppressing potential crack growth.

Such applications reinforce the need for 3D-scanning, visualization, and data analysis to understand if combinations of materials were distributed appropriately and perform as intended. Researchers need ongoing information about the reinforcement distributions and orientations involved and how to control manufacturing outcomes.

With past successes in hand and competitive challenges ahead, universities and industry R&D labs are exploring new and existing material combinations to address specific, desired performance envelopes. Of course, each new material and microstructure combination brings new mechanical behaviors. These conditions must be well understood early-on in the R&D space.

R&D and Production Inspection

R&D and prototype production is where CT analysis first helps quantify performance and yield, defect types and rates, and quality-inspection states related to both design instances and variations that may occur in manufacturing. Because of inner part complexity and increasing economic emphasis on automation and production output, understanding what happens in the Process Event, as well as in design, is particularly important to composites. Porosity, delamination, and cracking, to name a few conditions, are some of the problems that can result from process settings and their innate variables—not just design flaws. Advanced inspection software can capture these pre- and post-production states for review.

Typical Capabilities of Advanced CT-Scan Data Analysis Software:

Calculate local fiber orientations and generate statistics such as orientation tensors, or histograms locally or globally per mesh-cell, and linear over-the-material thickness.
Display local fiber orientation in color code or as vectors or tensors in 3D.
Detect porosity within a material – either in the matrix or the reinforcement.
Determine local volume fractions of the reinforcing phase for all kinds of composites.
Measure the cell size and elongation of foams for each cell or use the same analysis to quantify highly filled particle-reinforced composites or powders.
Calculate displacements and strain fields with the digital volume correlation by comparing multiple CT-scans carried out during in-situ tests.
Compare in-situ datasets at multiple load states for a robust crack detection.
Import FE meshes from any FE software to map microstructure information or strains measured by in-situ testing directly on the mesh.

Following the R&D and prototype-production stage, successful composite designs and process plans move to full manufacturing, where CT analysis software can serve either in batch or inline inspection. The frequency of these CT inspections depends on the use case and mission-critical nature of the component. For critical part areas—mating and joining points on large, connected surfaces, or for post-installed high-stress geometries—CT inspection can be applied via robotics. In this way, the most critical areas of even large surfaces are inspected for maintenance or suspected situational failures such as a bird strike or other stress contacts.

The Future of CT-Scan Data Analysis in Aerospace

The numerical and visual outputs from CT analysis software will continue to improve and make reporting easier and problem solving faster in R&D, production and aerospace and defense ground operations. Automation is sweeping design, manufacturing and inspection, providing improving costs and performance for all facets of composites creation. CT analysis specifically will help protect R&D and production investments, and capture life-cycle knowledge from field service. This will aid in advancing composite safety and business viability in the aerospace industry.

This article was written by Pascal Pinter, Product Manager, Material Research & Development, Volume Graphics (Charlotte, NC). For more information, visit here .