It is no secret that composite materials are extremely attractive to the aviation and aerospace industries, where there has been a significant increase in the use of carbon fiber. What’s not to like? Carbon fiber material has a proven track record of providing superior strength with the added benefit of substantial weight savings. Lighter airframes contribute to greater fuel efficiency and lower emissions. But there are other advantages that have driven the aerospace industry to heavily invest in carbon fiber technology.
Carbon fiber has also ushered in a number of other engineering and design improvements for widebody and narrow-body aircraft, including fewer parts, less maintenance and a longer design life. As technology adoption continues to evolve, new applications for carbon fiber are steadily emerging.
There are two types of composite materials: (1) cloth or a woven textile with fibers typically running in both 0° and 90° orientations, and (2) unidirectional fiber with fibers running in the same direction.
The most common method for composite production is hand lay-up, which consists of manually placing dry fabric layers (plies) or pre-preg plies directly onto a tool to form a laminate stack. Resin and hardeners are applied to complete the process and create a versatile composite material.
Solving Carbon Composite Challenges
As in most cases with engineering and production, nothing is “free.” Carbon composite technology comes with its challenges and two main issues: (1) carbon fiber strength as it is very sensitive to tape layup variation, and (2) the overall manufacturing cost is high.
To maximize carbon fiber strength, engineers design carbon fiber orientation in a very specific manor. If the carbon fiber orientation varies just 10 degrees from design it can reduce the part’s strength by 50%. Due to the carbon fiber orientation sensitivity, aerospace and aviation designs have exacting specifications which drives very tight fiber orientation tolerances.
Secondly, today’s manufacturing cost for carbon fiber verse sheet metal is approximately six times higher. Carbon fiber manufacturers have a huge opportunity to drive down costs. To solve these two challenges, an efficient and effective inspection methodology is necessary.
“Time-consuming” and “tedious” describe today’s inspection procedure, which is a manual process and comprises about half of the carbon fiber part-making time. Manual and visual surface inspection processes for defect detection have greater potential of introducing human error and wreak havoc on the bottom line. Complex material properties make testing difficult for the human eye, and overlooked defects are the result.
Technicians also conduct manual quality assurance with rulers and gauges resulting in varying dimensional measurements. Without automation and the acquisition of 3D coordinate data, there is also no statistical process control leading to unknown process capability.
Not only are manual inspection processes time-consuming and prone to error, they are also executed after the product is manufactured or the tape layer is completed. When defects are detected, a lengthy and expensive rework process must be initiated. In addition, complex aerospace and aviation carbon fiber parts often cannot be reworked thus leading to extremely high cost scrap.
These challenges have compelled original equipment manufacturers (OEMs) and Tier 1 aerospace manufacturers to drive and develop automated lay-up process inspection.
The goals of the automated inspection process are to significantly shorten the inspection time, increase defect detectability, and bring the inspection process ‘upstream’ of the manufacturing process. These critical issues are driving innovation in the metrology world and several inspection solutions are being evaluated including laser technologies and photogrammetry.
Automated Inspection Emerges
New and advanced methods of producing composite materials are emerging to automate tape laying and fiber placement operations.
In an automated fiber placement (AFP) lay-up process, tape is laid next to each other, lane by lane, which forms plies. Using a photogrammetric measurement technology known as Apodius (Hexagon Manufacturing Intelligence, North Kingstown, RI), real-time inspection of this lay-up process can take place. This optical inspection occurs during the lay-up of plies, which includes process-specific and quality-assuring checks. These quality control evaluations include the geometry of the tape, cutting edges, fiber orientation, gaps, missing tows, folds, overlapping tapes and foreign bodies. Due to the complex material properties and the large number of different quality criteria, automatic visual inspections are currently only used in a small number of applications.
Getting Closer with Laser Inspection
Another promising inspection-automation option is based on laser technology. This type of measurement system uses triangulation to translate the recorded laser data as geometric data. The system then interprets the generated data based on patterns that describe certain error types.
With the aid of positional data provided by the robot or an external tracking system, the geometric data can be displayed according to global coordinates. Due to the complex reflections of the tape, however, laser systems exhibit inaccuracies. The system’s interpretation in the micrometer range can not be regarded as robust and some error groups cannot be detected or classified.
With more research and development, a sensor fusion system combining the two optical measuring systems, laser technology and photogrammetry, now addresses the missing error groups and eliminates the inaccuracies. An Apodius vision sensor, which includes an illumination unit, can be added to the laser technology.
This advancement makes it possible to capture images that can be evaluated and interpreted independently. The laser data can be superimposed to achieve a robust and fused data statement for precision inspection. The vision sensor is used to determine the fiber orientation and classify the defect characteristics. The combination of both technologies has garnered progress in the pursuit of robust carbon composite inspection.
To create an inline-capable automated measurement system, laser and vision sensors are mounted directly behind the AFP head. This placement allows the recording of all process properties produced and occurring immediately after each ply lay-up. By calibrating the laser and vision sensor to the position-giving medium—robot or external tracking system—the acquired data can be transformed into a global 3D coordinate system.
The detected errors are viewed in real-time via an easy-to-interpret color map that provides instantaneous feedback and actionable intelligence. In other words, the solution utilizes the geometric data to create a mesh and generate a complete reconstruction and digitalization of the material properties for 3D visualization.
An Eye on the Horizon
Both aerospace and automotive manufacturers are certainly open to the benefits of composite parts. Carbon fiber is 75% lighter than sheet metal and has a significant strength advantage over traditional materials.
However, production speed and limited quantities keep it out of the mainstream. Currently, carbon fiber costs nearly 85% more to produce than traditional sheet metal. Manufacturing cost is a deterrent to expanding carbon fiber use beyond very high value products in aerospace applications.
However, the future is starting to fuse with the present, as both automation and robotics in composites manufacturing are heading toward removing major obstacles.
There are huge gains to be made in this industry with nearly 50% of manufacturing time being dedicated to inspecting the carbon fiber lay-up plies. Automated inspection solutions can dramatically improve the throughput rate and supplant the manual inspection process. Quality assurance and direct process improvement can also be harnessed using a modular, production-integrated inspection system, saving time and money.
Consequently, the larger benefit may lie in the increased accuracy and improved composite materials.