Carbon and stainless steel solutions

  • 13-Mar-2014 10:03 EDT
fig 3 2014-01-1061.jpg

SEM images of all samples discussed in the Polymeric Composites Laboratory/MIT and University of Nevada at Reno technical paper. The images are a CFRP control sample (a-b), CFRP with epoxy foam interlayer (c-d), CFRP with CNT reinforced epoxy foam (e-f), CFRP with electrospun fiber interlayer (g-h), and CFRP with CNT reinforced electrospun fiber interlayer (i-j).

Out of the nearly 1500 technical papers being presented this year at the SAE 2014 World Congress & Exhibition, about 200 of them will focus on materials, and from a wide range of angles. In all, there will be 38 technical sessions with an emphasis on materials.

On Wednesday April 9 at 1 p.m. during Part 2 of a two-part technical session on Automotive Composites (Part 1 is the same day and starts at 8 a.m.), researchers from the Polymeric Composites Laboratory/MIT and University of Nevada at Reno will present a paper (2014-01-1061) titled “Featherweight Composites Manufactured by Selective Nanobridization with Potential Applications in the Automotive Industry.”

Nanobridization is a “nano-inspired process” by which scalable material structures can be designed and manufactured by combining the concept of “nano free volume” with specific material molecules defining a systemic density (nano-density). According to the paper’s authors, this approach explores nanotechnology from a porosity perspective rather than nanoparticles, thus minimizing health concerns with nanotechnology, while providing nanoporosity throughout the entirety of the composite system.

Nanobridization may be viewed as a density system transformation of material heterogeneity utilizing a unified class of materials such as polynanomers and in developing next generation structures such as featherweight carbon fiber reinforced polymers (CFRP). Polynanomers are further defined by the incorporation of hollow carbon fibers, electrospun nano-fibers, nano-pores and carbon nanotubes (CNT) into this newly established type of matrix. Nanobridization involves fractal structural design and constitutes a scalable structure from the nano- to the macro- scale and vice versa, resulting in a spatial density with significant overall weight reduction.

Featherweight composites are a characteristic example product of a nanobridization process, as they include novel “bio-inspired fractal structures that are combined with unprecedented mechanical and transport properties.” Porosity is designed both at the macro- (honeycomb/foam type) and micro- (hollow carbon fiber foamed matrix and interphase) scales and can find immediate applications such as tooling and nonstructural or failure-critical applications. Featherweight composite manufacturing introduces a new production process that includes novel steps, such as electrospinning of carbon fibers and aligned CNT incorporation into the novel polynanomeric matrix system, and an innovative, integrated, roll-to-roll process sequence.

The main objective of the research behind the paper was the exploration of the manufacturing scalability of polynanomeric composites through nanobridization and to highlight potential relevant uses for this technology in the automotive industry. This type of material establishes a unique framework for creating the next-generation of composites technology that will be 25 to 40% lighter, while maintaining structural load-bearing characteristics such as stiffness and strength.

Future work with featherweights will investigate the overall recyclability, further define cost-effective production means including out-of-autoclave production and fast-curing resins and the applicability to thermoplastics.

Other papers to be presented during this technical session include “Production of a Composite Monocoque Frame for a Formula SAE Racecar” from the U.S. Naval Academy and “Effect of Fiber Orientation on the Mechanical Properties of Long Glass Fiber Reinforced (LGFR) Composites” from Ford and Nanjing University of Aeronautics and Astronautics.

On Thursday April 10 at 1 p.m., Tenneco and Ford will team to present a paper (2014-01-0975) titled “Fatigue Behavior of Stainless Steel Sheet Specimens at Extremely High Temperatures” during the Fatigue Research and Applications technical session. In particular, the authors will focus on the timely subject of active regeneration systems for diesel engines.

Active regeneration utilizes heat sources, such as electric heaters or fuel burning to initiate the combustion of particulate matter. During an active regeneration cycle, the temperature of structural components can be extremely high, at temperatures up to 1000°C (1832°F). The extremely high temperatures create a unique challenge for the design of regeneration structural components near their melting temperatures. Thermal fatigue, creep, and oxidation then become critical mechanisms to be considered for durability of the components.

Stainless steels are widely used for high-temperature automotive applications due to their high-temperature performance, durability, and cost. Much previous research on high-temperature fatigue of stainless steels has focused on thick sections of austenitic stainless steels for use in power plants. However, most austenitic grades may be less suitable for high-temperature automotive applications due to their higher cost, poor oxidation resistance above about 850°C (1562°F), and high coefficients of thermal expansion.

For this reason, ferritic grades are considered for use in automotive applications. Further, exhaust system components are typically thin-walled structures with wall thickness of 3 mm (0.12 in) or less. Relatively few researchers have investigated the high-temperature fatigue behavior of ferritic stainless steels with thin cross sections.

For example, one researcher investigated the thickness effect on high-temperature bending fatigue life for ferritic stainless steel plates up to 800°C (1472°F) and found that an increase in thickness from 1 to 2 mm (0.04 to 0.08 in) deteriorated the fatigue life at a given stress level for the tested materials.

 Another researcher presented high-temperature properties of an oxidation-resistant ferritic stainless steel sheet in comparison with those of austenitic grades up to 1000°C (1832°F). To accurately predict the durability of the structural components of an active regeneration system, the fatigue, creep, and oxidation behavior of thin stainless steel sheet material must be characterized accurately in a wide range of conditions.

Tensile test results indicated that the yield strength and tensile strength of the candidate steel decrease with increasing temperature for room temperature up to 1100°C (2012°F). The yield strength and tensile strength were observed to decrease with decreasing strain rate in tests conducted at 900 and 1100°C (1652 and 2012°F), but no strain rate dependence was observed in the elastic properties for tests conducted below 900°C. Cyclic fatigue test data show that the stress-life relations are influenced by the addition of a tensile hold time at elevated temperatures.

For tests conducted at 700 and 1100°C (1292 and 2012°F), the failure behavior is predominantly time-dependent. For tests conducted at 1100°C (2012°F), the failure behavior seems to be driven by creep, but the time to failure is reduced for tests without a hold time at the maximum stress. This may be due to the effect of increased oxidation at 1100°C (2012°F) combined with possible repeated breaking of the oxides with faster cycling that can contribute to crack growth.

Further metallographic and fractographic analyses are needed to determine the failure mechanisms of the candidate stainless steel at elevated temperatures.

Other papers during this technical session include “Fatigue Behavior of Aluminum Alloys under Multiaxial Loading” from General Motors and “A Study on a Visualization of Fatigue Behavior Near the Glass Transition Region” from NOK.

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