Applied Composite Materials (v.16, #3)

This article is the third of three papers describing a study of the monitoring of filament wound composite cylinders for underwater applications. Part I described the technological issues and the development of specimens instrumented with embedded gratings and thermocouples, with the aim of monitoring the temperature and strain changes during the cylinder manufacturing presented in Part II. Residual strains are not negligible, over 1,000 axial micro-strain at the end of the curing cycle. Part III describes the response of these cylinders to hydrostatic pressure loading. The same embedded fiber optical Bragg gratings (FBGs) used for parts I and II of the study are here used as strain gauges. Their response is compared to that of classical resistive strain gages bonded to the inner surface of the tube. Results from these initial tests demonstrate the embedded FBG sensor’s capability to monitor structural health of an underwater structure from fabrication throughout its service life. Embedded instrumentation records strains during pressure cycles up to final failure, without affecting the cylinder response.
Keywords: Polymer-matrix composites (PMCs); Smart materials; Residual stress; Filament winding; Underwater application

Modeling Inter-Laminar Failure in Composite Structures: Illustration on an Industrial Case Study by M. Bruyneel; J.-P. Delsemme; Ph. Jetteur; F. Germain (149-162).
This paper presents a solution procedure made available at an industrial level to study delamination in composites. Two approaches are presented. The first one is based on an original VCE (Virtual Crack Extension) method used to provide a quick estimate of the propagation load and the critical inter-laminar cracks, in a linear finite element analysis. The second approach relies on cohesive elements, and implies a non linear analysis. More general than the fracture mechanics (VCE) approach, the cohesive elements technique allows to provide the value of the maximum load that can be sustained by the structure, and to predict the residual strength and stiffness over the fracture process. Those two methods are first compared in a DCB test case to show that the results agree well with those from the literature or with the analytical solutions. Finally, a multi-delaminated industrial test case is solved with both approaches.
Keywords: Delamination; Industrial test case

On the Rigidity in Bending of a Sandwich with Thick CFRP Facings and Thin Soft Core by G. Caprino; P. Iaccarino; A. Langella; A. Lamboglia (163-172).
Flexure tests in three-point bending were performed in the elastic domain on sandwich specimens whose facings were made of T800H/3900-2 laminates, and the core by a soft rubbery layer. The contribution of the shear and flexural deformations to the overall deflection was varied by varying the slenderness ratio. The rigidities yielded by the load-displacement curve were corrected for the indentation occurring at the points of load introduction, using an experimentally determined calibration curve. Due to the thinness of the sandwich, indentation negligibly affected the precision of the results, with the apparent rigidities differing from the actual ones by less than 2%. By an analytical formula previously developed for sandwich structures, a prediction of the rigidities in flexure was attempted, adopting elastic constants available in the literature. The correlation with the data points was poor, with the theoretical results largely overestimating the actual rigidities. However, the reliability of the closed-form formula was supported by finite element analysis, carried out modelling the facings by 2D plate elements, and the core by 3D brick elements. Through the formula, the core shear modulus was individuated as responsible of the discrepancies observed. Assuming a suitable value for this parameter, both the analytic solution and the finite element models were able to match with accuracy the rigidities measured.
Keywords: Polymer-matrix composites; Mechanical properties; Finite element analysis; Elastic properties; Sandwich with soft core

Entire Life Time Monitoring of Filament Wound Composite Cylinders Using Bragg Grating Sensors: I. Adapted Tooling and Instrumented Specimen by H. Hernández-Moreno; F. Collombet; B. Douchin; D. Choqueuse; P. Davies; J. L. González Velázquez (173-182).
This paper is the first of three describing the monitoring of filament wound cylinders using Bragg grating sensors. Part I describes the technological issues and the development of specimens instrumented with embedded gratings and thermocouples. The aim is to monitor the temperature and strain changes during cylinder manufacture (see Part II) and in-service behaviour (see Part III). Specimens are filament wound glass reinforced epoxy composites, so two technological problems have to be solved: one is to collect data during fabrication and the second is to remove the specimen from the mandrel without damaging the sensors. These were accomplished by design of a specially adapted split mandrel and a rotating interface between the filament winding machine and the composite cylinder in fabrication. Immediately after sensor insertion it was possible to monitor the fabrication process, by collecting Bragg grating wavelength and temperature response, using this specially adapted tooling.
Keywords: Polymer-matrix composites (PMCs); Residual/internal stress; Non-destructive testing; Filament winding

Ultrasonic imaging in the C-scan mode was used in conjunction with the amplitude of the reflected signal to measure the temperature dependence of resin flow rate in single layers of woven carbon fabric. The RFI samples were vacuum-bagged and scanned in a water tank at 50°C, 60°C, 70°C, and 80°C. The measured flow rates were plotted versus inverse viscosity to determine the permeability in the thin film, non-saturated system. The permeability values determined in this work were consistent with permeability values reported in the literature. Capillary flow was not observed at the temperatures and times required for pressurized flow to occur. The flow rate at 65°C was predicted from the measured flow rates, and then measured in a 10-layer laminate. The investigation demonstrates that ultrasonic imaging in the C-scan mode in conjunction with the amplitude of the reflected signal is an effective method for measuring resin flow through fabric.
Keywords: Carbon fabric; Ultrasonic imaging; Optical microscopy; Out-of-autoclave