Automotive composites, part 3: Quality, inspection and standardsGary Kardys | January 25, 2019
The quality and inspection of automotive parts, subassemblies and vehicles are impacted by increased use of composite materials. Engineers control quality and consistency through adherence to standards coupled with standardized testing and inspection methods to confirm that compliance, material conformance and design specifications are attained during manufacturing.
During integration of composite parts into an automobile under development, manufacturing and design engineers need answers to the following questions:
- How can quality be built into part designs and manufacturing processes?
- What types of flaws occur in composite materials?
- What flaw sizes or defect levels result in an unacceptable component?
- Are test methods available to detect and quantify defects in composite parts?
- Can the bond soundness of composite-composite or composite-metal joints be assured?
- What are the material specifications, inspection standards and process codes to form a foundation for composite quality?
While metals are mostly isotropic in properties, composites vary greatly in properties due to fiber orientation. Continuous fiber composites have the highest level of anisotropy. Variations in the textile weave can alter final properties of a composite made by vacuum infusion of epoxy into a glass or carbon fiber fabric.
Short fiber polymer matrix composites can also have fiber orientation variations. Flow during composite molding or fabrication can locally alter the resin content or alter the alignment of the reinforcing fibers. Fabric can have wrinkles, folds, bunching, spreading and tearing defects. Incomplete resin fill or dry spot in resin transfer molding (RTM) composites is a major problem for assuring consistent quality. Resin matrix cracks, debonding, blisters, voids, porosity and broken fibers are additional composite quality defects.
Bond and assembly quality
Adhesive joint bond integrity can vary due to component mixing errors, incorrect placement, void formation, inclusions (dirt and oil), poor surface pretreatment, partial mold release removal, incomplete adhesive levels (dry spots or overfill) and improper curing. Vehicles in the near future will continue to consist of a combination of metal and composite components, so maintaining the quality of metal-composite mechanical joints and adhesive bonds is paramount.
Isolating the metal from the nobler carbon fiber is key to avoiding galvanic corrosion that plagues multi-material vehicles. BMW only uses CFRP in dry areas in their 7 series vehicles for this reason. Coatings and adhesive can be used to isolate the galvanic couple members and electrolytes.
The BMW i3 electric vehicle is among the first production automobiles to extensively utilize a carbon fiber reinforced plastic composite (CFRP) structural frame, which the company calls a “Life Module." BMW manufactures the i3 with dry carbon fiber fabric and resin transfer molding (RTM) methods. The molded carbon fiber composite parts are adhesively bonded together to form the Life Module or passenger cell. The Life Module is then attached to an aluminum chassis or “Drive Module” with adhesives and screws.
A composite part or assembly can consist of a complex laminate structure where the outer skin is a fiber reinforced polymer matrix sheet and the interior, a foam or honeycomb core. The BMW i3 has an injection molded thermoplastic honeycomb crash-protection system and braided carbon fiber roll with a foam core. Hybrid fiber-metal laminates (FMLs) such as glass laminate aluminum reinforced epoxy (GLARE) are currently used in several aerospace applications because they have better damage tolerance compared to non-metallic brittle composites. FMLs will likely be deployed in future automotive applications to improve impact tolerance and crashworthiness. Hybrid composites, composite-metal joints and composite structure complexity further complicate quality analysis.
Nondestructive testing (NDT) methods
Nondestructive testing (NDT) engineers and inspectors are familiar with detecting surface and subsurface flaw detection of metal parts, castings, forgings and welds. NDT personnel will have to build a knowledge base for detecting automotive composite defects such as voids, delamination, incomplete bonding, voids, fiber orientation, degree of cure and flow-induced defects. The heterogeneous nature of composites and many matrix-filler and matrix-fiber interfaces reduces flaw resolution compared to homogeneous materials like metals, glasses and ceramics.
NDT of composite parts and assemblies with high attenuation material remains a challenging problem. High attenuation materials can include CFRP/GFRP hybrids, high porosity layers, foam cores, hollow bead or filler particles, and flexible adhesives. Detecting flaws in multilayer sandwich composite structure is also difficult.
Another challenge is inspecting structures where physical access is so limited that cameras or probes cannot contact the part’s surface. Certain composites are moisture sensitive, so wetting and immersion for ultrasonic inspection are not an option. Metal alloys are electrical conductors, which enable non-destructive testing using eddy current sensors or electromagnetic probes, or electromagnetic acoustic transducers (EMAT). Composites tend to be non-conductive unless special fillers are added to the resin matrix or conductive fiber reinforcements like carbon fiber are used.
Ultrasonic methods will likely increase in application because the technique is already widely applied and it can be used on both metals and composites (see “What Are Ultrasonic Flaw Detectors?”). Olympus’s “Ultrasonic Flaw Detection Tutorial — 7.6 Fiberglass and Composites” briefly reviews some key aspects. Ultrasonic inspection uses sound waves to locate the size and depth of features in a composite. Advanced ultrasonic systems can even provide a subsurface image of the composite part.
Infrared imaging or thermal pulse thermography can detect flaws through thermal wave imaging where a laser or flash lamp heat pulse is applied to a composite part. The presence of flaws disrupts the normal heat flow pattern, which is detected with infrared image sensors. Passive thermography or stress pattern analysis by measurement of thermal analysis (SPATE) detect the heat generated at interfaces and crack from the frictional movement after vibrational excitation. ASTM E3045 — Standard Practice for Crack Detection Using Vibroacoustic Thermography provides accepted methods for passive thermography for flaw identification. Thermography is also useful in monitoring matrix and adhesive curing processes to assure uniform heating and temperature profiles.
Digital radiography and computed tomography is another common technique that is utilized for high-resolution flaw detection and quantification in metals and composites. In radiography, X-rays or gamma rays are passed through a composite material. Transmission or opacity varies with material composition, which allows imaging of internal structures and defects.
Raman spectroscopy is a powerful technique for analyzing resins, plastics and polymer matrix composites. The technique is nondestructive and noncontact. Raman spectroscopy or a Raman microscope can analyze stress in carbon fiber or graphene reinforcements, determine crystallinity levels in polymers, characterize laminate films and quantify subtle chemical variations with resins matrixes.
Different compositions or chemical structure absorb and transmit distinct spectra. A Raman spectrum collected from a material can be compared to a library of known materials and chemicals to determine the composition. A Raman microscope collects information from a series of points across a part’s surface, which are combined to provide a chemical image or map showing compositional variations over the area analyzed. Raman microscope can be useful in understanding chemical bonding or lack of bonding of specific fibers or reinforcements to resin matrixes. Raman methods can be used to understand if sizing and coupling agents applied to fibers fully encapsulate or coat the fiber surface, which might lead to poor bonding in a composite. The degree of curing or polymerization is another factor impacting strength, which Raman can characterize.
The American Society of Nondestructive Testing (ASNT) holds an annual NDT of Composites conference focusing on existing and developing methods of NDT for composite materials and fabrication techniques. The upcoming NDT of Composites 2019 — taking place April 30 to May 2 — will cover new and developing NDT methods for composite materials and emerging product forms as well as in-process inspection techniques.
(Read more about NDT of composites at the National Composites Networks and 2008 World Conference on Nondestructive Testing.)
Standards and regulations
The composition and properties of engineering alloys are controlled by industry standards or materials specification from organizations such as ASTM, CDA, AA and SAE. But the lack of standardized grades of composite raw materials continues to hinder the widespread use of composites. Standards like ASTM D 578 and ISO 2078 exist for glass fibers, but carbon fibers are produced in closely held proprietary processes. A wide selection of epoxy, polyester and high performance laminating or matrix resins are available today, but the performance of the same resin can vary from supplier to supplier.
A 2016 U.K. Composite Strategy report from the Composites U.K. Trade Association noted that “One of the major inhibitors to the uptake of composites in new sectors is that regulations, codes, and standards are often inappropriate for composites.” Composites U.K.’s Modernising Composite Materials Regulations (2017) encourages companies to either prove composite materials “equivalence” to traditional engineering materials (steel and aluminum) or prove composites “performance” to required standards under operational conditions. The report claimed that the aerospace industry utilized a “proving performance” approach to speed up the adoption of composites.
Composite standards landing pages from some of the major technical societies and standards developing organizations (SDOs) can be found here:
Developing and issuing industrial standards is a positive step toward expanding composite adoption. However, widespread industry acceptance of composite standards, material test methods and NDT inspection techniques throughout the automotive supply chain will foster more rapid use of the composite materials in vehicle applications. New standards and evaluation methods will be needed to assure the quality of complex composite structures.