Failure Analysis and Prevention: Composites
Sponsored by: MS&T Organization
Program Organizers: Andrew Spowage, The University of Nottingham, Malaysia Campus; Tom Ackerson, IMR Metallurgical Services; Larry Hanke, Materials Evaluation and Engineering, Inc
Wednesday 2:00 PM
October 19, 2011
Room: D235
Location: Greater Columbus Convention Center
Session Chair: Ronald Parrington, IMR Test Labs; Joseph Rakow, Exponent Failure Analysis Associates; William Rossey, GE Aviation
2:00 PM
Composite Failure Analysis: A Step-by-Step Overview: William Pinnell1; 1University Dayton Research Institute
Failure analysis has always been an important tool to not only determining the cause of a failure but to also to help prevent future failures. In order to perform a failure analysis investigation, it is important to understand the properties of the material, characteristics of the material’s failures, and the techniques needed to identify and analyze both. Basic procedures are used in any failure analysis investigation. However, composite materials require specific types of analyses that require specific investigative techniques. The fundamental steps in performing a failure analysis investigation of composite materials shall be presented. The steps include obtaining background information, performing visual examinations, conducting nondestructive evaluations, performing materials characterization, examining fracture surfaces, and reporting documentation. Various techniques involved with each of these steps will be discussed.
2:40 PM
An Interactive Case Study in Composite Failure Analysis: Joseph Rakow1; 1Exponent Failure Analysis Associates
This presentation will provide a case study in composite failure analysis, wherein the audience will get a chance to handle and analyze a failed composite structure and identify the root cause of its failure. The case study involves a carbon fiber bicycle frame that failed catastrophically, resulting in serious injuries to the professional rider. In the case study, the audience will be presented with the failure scenario and key observations from the physical examination of the failed components. The audience will gain an understanding of how these observations integrate to identify the root cause of the failure. The failed components will be available for the audience to examine at their convenience.
3:00 PM
Failure Analysis of Composites at the NTSB: Matthew Fox1; 1NTSB
The National Transportation Safety Board (NTSB) conducts investigations into accidents in all modes of transportation including aviation, rail, pipeline, highway, and marine. Investigations focusing on components made of composite materials are growing in number as more components are being manufactured from these materials. The author will share recent experiences examining failures in composite materials at the NTSB Materials Laboratory. In one investigation, a section of a main rotor blade fractured, leading to a forced landing of a sightseeing helicopter in the Hudson River. The glass-fiber reinforced composite main rotor blade skin showed evidence of fatigue crack progression emanating from the trailing edge where a manufacturing defect was found. In separate investigations, two helicopters experienced reduced yaw directional authority and subsequent emergency landings after failure of their anti-torque fans. Kevlar-reinforced tension-torsion straps that retain the fan blades under centrifugal loads fractured in areas that showed evidence of fiber kinking.
3:20 PM Break
4:00 PM
Catastrophic Failure of Fiber-Reinforced Polypropylene Lawnmower Wheels: Ronald Parrington1; 1IMR Test Labs
Three instances of riding lawnmower fiber-reinforced polypropylene wheel failures resulting in serious injuries have been examined. The wheels fractured violently due to the presence of fatigue cracks. The failure mechanism is fatigue initiation and growth during normal service followed by brittle overload fracture upon handling (e.g., tire inflation). Several design and manufacturing issues contribute to the failures: (1) cracks initiate and grow by fatigue at weak points in the design; (2) glass fiber orientation is nonrandom and predominantly in a direction that is ineffective in preventing fatigue crack propagation; (3) the polypropylene wheel material is very susceptible to embrittlement by weathering; (4) voids in the plastic weaken the part by reducing the effective cross sectional area; and (5) weld or knit lines further cause local weakening of the material. Crack path analysis was a very valuable tool used to understand the fractures.
4:20 PM
Stress Rupture Failure of a Continuous Fiber Composite Water Holding Tank: John DeFranks1; William DeLaurier1; Ronald Parrington1; 1IMR Test Labs
Stress rupture failure of a large (57,000 gallon), continuous fiber composite water holding tank occurred just hours after the initial fill at a power plant. The manway, located near the bottom of the tank, completely separated at the tank-to-manway joint. Furthermore, the vacuum resulting from water rushing out caused extensive secondary damage to the upper portions of the composite tank, which collapsed inward. The failure resulted in significant schedule delays and repair costs. The tank was fabricated from a fiberglass / vinyl ester system and the manway was attached using a fiberglass / epoxy system. Extensive visual examination, thermal analysis, chemical analysis, mechanical testing, and materialography were performed to characterize the tank failure. Contributing factors to the stress rupture failure include issues with incomplete cure, insufficient resin penetration, improper material selection, and substandard workmanship.
4:40 PM
Failure of 3D Woven Composites: Brian Justusson1; Mark Pankow1; Anthony Waas1; Miranda Rudolph1; 1University of Michigan
Composite materials are becoming more utilized in structural applications due to their relatively high stiffness to weight ratio. Typical unidirectional and woven ply laminates have shown adequate strength characteristics, but their application is limited due to delamination between the plies. 3-dimensional woven textile composites have been developed to address these concerns. A primary advantage of 3D woven composites is the strong relationship between the design of the woven architecture and its resistance to undesired failure modes. These composites can be designed in such a way that typical failure modes seen in laminated composites are avoided by reinforcement through either the addition of through thickness fibers, known as Z-fibers, or by weaving fibers between one another, known as an angle-interlock. Experimental results that examine the dependence of architecture on failure modes and strengths through the use of digital image correlation and other novel testing techniques will be presented.
5:00 PM
Fracture Study of Polymer and Polymeric Composite Materials Using the Spiral Notch Torsion Test: Fei Ren1; Jy-An Wang1; Ting Tan1; Edgar Lara-Curzio1; Pancasatya Agastra2; John Mandell2; Williams Bertelsen3; Carl LaFrance4; 1Oak Ridge National Laboratory; 2Montana State University; 3Gougeon Brothers, Inc. ; 4Molded Fiber Glass Companies
Wind turbines are subject to complex loading conditions during service. Since turbine blades are usually fabricated from polymeric composite materials, understanding the fracture behavior of these composite materials is very important to design highly reliable blades. In this project, a new testing method, the spiral notch torsion test (SNTT) has been used to study the fracture behavior of epoxy and glass fiber-reinforced epoxy under combined mode I (opening mode) and mode III (tearing mode) loading condition. The effect of mixed mode condition on the fracture behavior under both monotonic loading and fatigue loading is discussed. Fracture surfaces were quantitatively characterized using optical and electron microscopy, while a finite element stress analysis was used to calculate fracture toughness under different loading conditions.
5:20 PM Cancelled
Lessons Learned from Aging Studies and Teardown of Composite Structures
: LAMIA SALAH1; John Tomblin1; 1NIAR
As more commercial and military airplanes are required to maintain operational capability beyond their original design life objectives, it has become necessary to assess the structural health of aging aircraft to ensure their airworthiness and structural integrity for continued service in commercial and military applications. Most of the aging aircraft studies have focused so far on metallic structures; however, as more composite components are being certified, it is crucial to address this aging concern for composite components as well. This paper provides an overview of the lessons learned from teardowns conducted on two composite structures, presents the investigative plan, details of the teardown activities, and results of the NDI, physical, thermal and mechanical test results. Current NDI, mechanical, and physical test methods are compared with those used in the development program to assess differences in capabilities between the late 70’s and today and a summary of the findings is provided.