James G. Ratcliffe

Investigation of the effect of single wall carbon nanotubes on interlaminar fracture toughness of woven carbon fiber—epoxy composites:

Single wall carbon nanotubes (SWCNTs) were introduced in the interlaminar region of woven carbon fiber—epoxy composites and the mode-I delamination behavior was investigated. Pristine (P-SWCNT) and functionalized (F-SWCNT) nanotubes were sprayed in the mid-plane of these laminates and delamination was initiated using a teflon pre-crack insert. The composite laminates were produced using vacuum-assisted resin transfer molding process. The interlaminar fracture toughness (ILFT) represented by mode-I critical strain energy release rate (GIc) for the initiation of delamination was measured using double cantilever beam tests. The specimens with pristine nanotubes and functionalized nanotubes showed a small effect on the ILFT. The specimens with P-SWCNTs showed stable crack growth and the potential for enhanced crack bridging along with slightly higher GIc than F-SWCNT specimens. Scanning electron microscopy images showed enhanced fiber—matrix interfacial bonding in the specimens with F-SWCNTs. However, large unstable crack propagation was observed in these F-SWCNT specimens from load—displacement curves and crack propagation videos. This research helps in understanding the differences in mechanisms by addition of functionalized and unfunctionalized (pristine) nanotubes to the woven carbon fiber—epoxy matrix composite laminates.

Sizing a single cantilever beam specimen for characterizing facesheet–core debonding in sandwich structure

This article details a procedure for sizing single cantilever beam (SCB) test specimens that are used to characterize facesheet–core debonding in sandwich structure. The characterization is accomplished by measuring the critical strain energy release rate, Gc, associated with the debonding process. The sizing procedure is based on an analytical representation of the SCB specimen, which models the specimen as a cantilever beam partially supported on an elastic foundation. This results in an approximate, closed-form solution for the compliance–debond length relationship of the specimen. The solution provides a series of limitations that can be imposed on the specimen dimensions to help ensure the specimen behaviour does not violate assumptions made in the derivation of the data reduction method used to calculate Gc. Application of the sizing procedure to actual sandwich systems yielded SCB specimen dimensions that would be practical for use. The method is specifically configured for incorporation into a draft testing protocol to be developed into an ASTM International testing standard.

Redesign of the ECT Test for Mode III Delamination Testing. Part I: Finite Element Analysis

A series of finite element analyses (Part I) and tests (Part II) on various modifications of the edge crack torsion (ECT) test have been conducted in an effort to render the test more suitable for characterizing mode III delamination in laminated composites. To this end, two ECT specimen configurations were considered. The first configuration involves loading the specimen at a single location, and the second configuration involves a symmetric double load point application. Investigations were conducted on the effect of specimen overhang, along the length and width direction, on the inferred strain energy release rate distributions across the delamination front. Stress distributions in the vicinity of the delamination front were used to infer the corresponding modes I, II, and III strain energy release rate distributions. Results indicate the single and double loading configurations exhibiting similar stress distributions along the delamination front. Specimens with short crack lengths and small amounts of overhang yielded the most uniform distribution of mode III loading along the delamination front.

Modification of the edge crack torsion specimen for mode III delamination testing. Part II – experimental study

Experimental studies of carbon/epoxy edge crack torsion specimen have been conducted using a specially designed twist test fixture. Of particular concern was verification of the recommendations expressed in the analytical part of this study (Part 1), where it was suggested that overhang (sections of specimen laying outside of the loading and support pins) in the x- and y-directions should be minimized, and fracture testing at longer delamination lengths should be avoided. The experimental test results verified that the specimens with the smallest overhang produced the most consistent delamination toughness data, GIIIc. Specimens with large overhangs exhibited high apparent GIIIc values at long delamination lengths. This was most likely due to nonuniform loading and associated nonuniform delamination extension.

AN OVERVIEW OF INNOVATIVE STRATEGIES FOR FRACTURE MECHANICS AT NASA LANGLEY RESEARCH CENTER

Engineering fracture mechanics has played a vital role in the development and certification of virtually every aerospace vehicle that has been developed since the mid-20th century. NASA Langley Research Center s Durability, Damage Tolerance and Reliability Branch has contributed to the development and implementation of many fracture mechanics methods aimed at predicting and characterizing damage in both metallic and composite materials. This paper presents a selection of computational, analytical and experimental strategies that have been developed by the branch for assessing damage growth under monotonic and cyclic loading and for characterizing the damage tolerance of aerospace structures

An Overview of Durability and Damage Tolerance Methodology at NASA Langley Research Center

The NASA Langley Research Center’s Research Directorate provides many of the research and technology development capabilities required by the present and future needs of NASA across three encompassing technology areas, namely, aerodynamics, aerothermodynamics and acoustics (AAA); structures and materials (SM); and Airborne Systems (AirSc). Researchers contribute to nine primary areas of expertise which include structures, hypersonics, materials, flight dynamics and control, measurement sciences, crew systems and aviation operations, aerodynamics, safety critical avionics systems, and acoustics. These areas of expertise cover virtually all of the important disciplines related to flight, including the agency’s main thrusts within structures and materials. Researchers in the structures and materials technology area are constantly working to develop advanced materials to enable efficient, high-performance aerospace concepts; efficient, physics-based analytical and computational methods for multidisciplinary design and analysis; and methods to quantify the behavior, durability, damage tolerance, and overall performance of advanced materials and structures.As part of the structures and materials technology area, the Durability, Damage Tolerance and Reliability Branch (DDTRB) conducts research and technology development of efficient, physics-based analytical and computational methods to enable multidisciplinary design and analysis of advanced materials and structures for aerospace applications, including evaluation of concepts, quantification of behavior, durability, and damage tolerance, and validation of performance.DDTRB has contributed to the development and implementation of many fracture mechanics methods aimed at predicting and characterizing damage in both metallic and composite materials. Engineering fracture mechanics plays a vital role in the development and certification of virtually every aerospace vehicle that has been developed since the mid-twentieth century. This chapter presents a selection of computational, analytical, and experimental strategies and methodologies that have been developed by the branch for simulating and assessing damage growth under monotonic and cyclic loading and for characterizing the damage tolerance of aerospace structures. It includes continuum-based mechanics as well as a new paradigm focused on simulating and characterizing fundamental damage processes, called damage science.