A simple methodology for observing mechanical properties of nanocomposite

Part 1: interfacial properties in an Epoxy Clay Nanocomposites

Jae-Hyun Kim and Gale A. Holmes

Polymer Division

Characterization and Methods Development Group

Abstract

Research by the Pinnavaia Group and Mülhaupt et al. on elastomeric polyurethane and elastomeric epoxy clay hybrid nanocomposites have shown that the tensile strength, tensile modulus and strain-to-failure of these nanocomposites are increased relative to the neat matrices. These results contrast the behavior observed in fibrous composites and fulfill the promise, as envisioned by early researchers, of using nanotechnology to achieve significant increases in materials performance. However, similar results on glassy epoxies, which are often used in structural composites, by Pinnavaia and Kornmann, Mülhaupt, et al. indicate that, like conventional composites, modulus and strength increases in the nanocomposite are achieved at the expense of its strain-to-failure. In addition, research on thermoplastic polyolefin (TPO) nanocomposites also shows a significant reduction in strain-to-failure with nanomaterial incorporation. TPO nanocomposites have been targeted for use in semi-structural automotive applications.

The performances of these nanocomposites are achieved without the establishment of significant adhesive forces between the nanomaterial and the matrix. This suggests that the potential detrimental effects of substrate-matrix debonding that characterize the behavior of conventional composites may be offset in some cases by the morphology of the nanocomposite and/or the intrinsic toughness/ductility of the matrix and interphase region. To understand and quantify the affect of these factors on nanocomposite strain-to-failure, a methodology was developed that detects the onset of clay-matrix debonding in transparent nanocomposites.
 
 


Part 2: Investigation of interfacial properties in E-glass fiber model composites

Jae-Hyun Kim, Gale A. Holmes and Chad R. Snyder

National Institute of Standards and Technology, Polymer Division

Characterization and Methods Development Group

Gaithersburg, MD 20899-8541



Abstract

The fiber/matrix interfacial shear strength (IFSS) and the fiber/matrix interphase fracture toughness are critical parameters that control failure initiation and crack propagation in composites. In spite of this importance, most IFSS test methods ignore these parameters. As a result, output results from IFSS tests cannot be used as input parameters for modeling the failure behavior of real or model composites. are not used to quantify the failure behavior of real fiber interaction, and this ignorance cause the difference between the measured values determined by single fiber-model composites (i.e. fragmentation test) and multi fiber-real composites (i.e. direct shear and short beam test).

For this reasons, we have sought to obtain in situ IFSS parameters through the testing of micro-composites that consist of 2-D and 3-D multi-fibers arrays, whose inter-fiber spacing is comparable to the spacing observed in typical composites. This testing methodology admits the direct observation how IFSS, inter-fiber spacing, deformation rate, and matrix cracks influence fiber break clustering, critical flaw nucleation, and the in situ IFSS. The in situ IFSS obtained from this testing approach provides an experimental link to the ineffective length parameter that is used in statistics based micromechanics models. These models seek to quantify the strength and failure behavior of unidirectional laminae, the basic building block of composite structures, using micromechanics input parameters. The key goal of this research is to develop a predictive composite failure model by linking micromechanics parameters that quantify the local properties that control failure initiation and crack propagation in a composite lamina to computationally efficient failure criteria.

Using this measurement technique, the IFSS of single fiber and 2-D multi fiber composites having 15?m interfiber distance were measured using testing conditions where the interval between successive deformations was 10 min or 1h. The experimental results showed that the average IFSS of the cluster fibers measured with the 10 min time interval is lower than those tested using the 1 h time interval. Moreover, the IFSS obtained from a single fiber was found to be lower than the in situ IFSS that is obtained from the 2-D multi-fiber micro-composite. It was concluded that the reduction in IFSS is due to the interaction between the closely spaced fibers and the clustering of fiber breaks that occurs in the adjacent fibers that surround the fiber that contains the initial break.
 
 
 
 

Jae-Hyun Kim, Ph.D

Polymer division

Characterization and Methods Development Group

National Institute of Standards and Technology

Building 224, Room B-120

100 Bureau Dr. Mail Stop 8541

Gaithersburg, MD 20899-8541

Phone: 301- 975-2315

E-mail: jaehyun@nist.gov