Material Card Data: The Foundation of Product Development

A critical and often underappreciated factor in successful product development programs is the availability and validity of material card information. The term “material card” refers to the collection of input data that design engineers must enter into their simulation programs prior to modeling their designs. As the level of complexity of the design exercise increases, the amount and fidelity of the material card data must also increase to achieve an acceptable level of modeling predictability.

Simply put, without valid material card data, a given material/process combination cannot be modeled, and therefore, parts/systems cannot be designed incorporating those materials and processes.

CFRP Stress Modeling

It is understood that numerous testing standards exist to characterize a given material’s performance in tensile, compressive, shear and bending modes (e.g. ASTM/DIN/ISO/EN protocols). Testing methods conducted within this project are based on a subset of these standards. Certain test methods are expanded beyond the test standards given the unique nature of FRP composite materials and the associated manufacturing processes. The challenge for the nascent automotive FRP composites industry has been selecting, conducting and interpreting the results from these standard tests to generate consistent and usable data.

Recognizing that a lack of capable standardized material card data for automotive applications presented barriers to adopting FRP composites, Forward Engineering, together with industry partners Hexion and Zoltek, initiated a program to organize the testing procedures, develop a set of best practices and a methodology for translating test results into material card formats that are compatible with commercially available simulation-solver programs.

With the launch of this program, participants can now accelerate their FRP composite part development, reducing program costs, as well as provide visibility to material-testing budget and timing requirements for the evaluation of new materials and processes.

 

STANDARDIZED MATERIAL CHARACTERIZATION TESTING PROGRAM

At the outset, the team was focused on establishing a flexible platform that was cost effective and scalable. To that end, the new program was structured in a modular format. Each application, where a given material will be employed on the vehicle and what job the material needs to perform (e.g. dynamic stiffness, crash-energy absorption), will dictate which modules are required to support the design program. In some cases, a proxy material is available for preliminary design concept development, allowing specific material testing to be conducted in parallel, reducing the overall product development schedule.

Figure 2 provides a graphic overview of the program. In its latest form, the modules are organized in three different levels. Each module builds upon the others. As a product development program advances and higher resolution in the design performance evaluation is required, adding subsequent modules to the investigation increases the fidelity of the results with a corollary increase in modeling predictability.

 

LEVEL 1—BASIC MATERIAL CARD

At the base of the program, the Module 1a package provides basic material card information required for modeling simulation up to first failure of the material. For the most basic applications, where stiffness and dynamic loading before failure are the design constraints, the testing program associated with Module 1a is sufficient. These are applications where the component or system is not factored into the overall vehicle-crash performance.

Module 1a enables a multitude of design calculations and simulations that are not crash-energy related. All stiffness and strength-related design topics such as bending, torsion, shear and normal (tensile/compressive) forces both globally and locally rely on material card information produced in Module 1a. In addition to stiffness and strength-related design challenges, data from Module 1a material card testing is used for vibration/NVH (noise, vibration and harshness) simulation (modal analysis) and vehicle-dynamics simulation.

Module 1b, crush screening, is a series of tests designed to provide a quick assessment of crash-energy performance. For those materials where post-failure crash performance is important, a relatively quick and cost-effective crush-screening evaluation can be performed to evaluate combinations of materials (resins, fibers, additives, “systems”) to assess their relative crash-energy behavior. Outputs from crush screening include specific energy absorption (SEA) rate and crash-stability behavior. This screening process provides a relatively quick assessment of the targeted performance level that could be achieved or how close the samples are to achieving the desired performance, prior to investing in more complex component and system testing, which can be much more resource intensive.

An example of employing Module 1b to establish initial relative performance of a material system could be as follows. Evaluation of a 5-x-5 resin fiber matrix, five resin formulations and five fibers (mix of fiber types and/or fiber treatments). The crush-screening testing provides a look ahead at the compatibility of the resins and fibers in the context of crash-energy absorption.  The combination of resin and fiber alone can lead to drastic differences in performance between composite materials, even if the basic values of resin and fiber are mostly the same. The results of the crush screening provide a relative measure of energy-dissipation capability of the mix of materials as well as benchmarking relative to other material combinations in the database.

Although the results are qualitative, the greatest value of the crush screening is providing a quick and cost-effective preliminary indication as to whether the material combination is, or is not, suitable for crash-energy absorption applications. This reduces wasted cost and time associated with more advanced (e.g. Module 2a and 2b) testing of materials that will ultimately not deliver the required crash-energy absorption performance.

 

LEVEL 2—POST FAILURE MATERIAL CARD  

For applications where understanding crash performance, post-first failure behavior, is a requirement, then advancing to Module 2a and 2b will be required. These modules focus on the material behavior after the first failure when cracks begin to form and propagate through the material with increasing load and deformation.

In principle, Module 2a is comprised of several coupon-level hardware tests that will provide initial insight into post-first failure performance based on crack-energy release upon crack formation. The level of fidelity derived from this testing will support initial structural-component level crash modeling. While limited, this level of testing will provide a basic indication of how the material will perform under crash conditions.

To achieve a more accurate picture of the post-failure energy absorption behavior, Module 2b is required. Module 2b is a part-level hardware test of more complex three-dimensional components. Results from this module are critical to pole impact/side impact testing. The complex multi-axis deformation behavior in these cases require, at a minimum, the more advance data set provided by Module 2b.

Although described as part-level testing, the testing doesn’t have to be conducted on an actual “part.” A generic component, like a quadratic cross-section bending beam, is sufficient to provide the material card data required. The goal is to capture the interactions of multiple failure modes. In 2a coupon testing, only one failure mode can be observed at a time. In Module 2b, basically everything is mixed together, with the four standard failure modes—tension/compression/shear/bending—evaluated concurrently. Given the more complex sample used for this testing, a more accurate picture of the failure mode in three dimensions can be derived.

The additional fidelity provided by Module 2b results supports accurate modeling of larger more complex subsystems (e.g. front-end modules/crush zones). The improved correlation between sample testing and modeling results can be seen in Figure 3. This graph clearly highlights the increased forecast quality in post-failure performance modeling when employing results from Module 2b.

With each module that is performed, a considerable increase in simulation-forecast quality is observed. For crash-energy absorption applications, Module 2a should be considered as an absolute minimum requirement for part concept development. For real-world component engineering, Module 2b will provide the level of insight required to ensure the most effective pilot-part validation program. Module 2b level of fidelity provides engineers with the inputs required to support accurate part-modeling, mitigating risks associated with physical component/system pilot validation testing.

 

LEVEL 3—ADVANCED POST-FAILURE MATERIAL CARD

Module 3a is strain rate-dependent material testing. This is very advanced level of testing and is still under development at leading universities and institutions around the globe.

Although development work continues in this area, there are several established ways to characterize the strain-rate dependency (e.g., “Split-Hopkinson bar” or more standard dynamic tensile tests using servo-hydraulic machines) in lab tests today. While work is ongoing to refine and advance these lab-based tests, an alternate approach to generate this data is to incorporate an implicit strain-rate test into a substructure test.

Testing a vehicle-crash structure under real-world velocities and energies can provide real-world strain rates within the test results. This kind of testing will not generate additional data usable for the material card, but provides a realistic insight to the material behavior at a given strain rate.

A more resource-intensive approach to developing strain-rate dependent data is conducting coupon testing through a range of strain rates. Depending on solver and material model, strain-rate dependent curves for strength and stiffness can be included in a simulation.

Regardless of which path is chosen, the strain-rate topic is complex, and testing can be very resource intensive. At this time, strain-rate dependency remains a “special” topic that must be addressed on a case-by-case basis.

Module 3b—axial crush testing is focused on emulating crash-structure performance often seen in high-speed vehicle crash testing. Module 2a and 2b are not sufficient to model this behavior. Module 3b includes drop-tower testing as well as other impact type tests.

 

STANDARDIZED MATERIAL-CHARACTERIZATION TESTING PROGRAM UPDATE 

The program outlined above was launched with Hexion Epoxy resins for high-volume manufacturing of automotive parts in combination with Zoltek large-tow carbon fibers. The material cards for CAE simulation developed from the testing completed to date is available through project partners Forward Engineering, Hexion and Zoltek. The partners are focusing this program on resins and fibers suitable for high-volume automotive component production applications. To learn more about the results to date, as well as how to participate in this program, please contact the authors.

In summary, the development of a comprehensive, flexible, modular material-card development program for automotive industry FRP composite applications is providing a road map for engineers and program managers to guide new product development, accelerating the adoption of new FRP composites by the automotive industry. Additionally, the support and participation from composites-industry leaders Zoltek and Hexion played a critical role in launching this program, and their continued support has been key to its advancement and continuous development.

Written by Adam Halsband

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