An experimental and analytical study of micro-laser line thermography on micro-sized flaws in stitched carbon fiber reinforced polymer composites

doi:10.1016/j.compscitech.2016.02.007

Composites Science and Technology

Volume 126, 1 April 2016, Pages 17-26

https://doi.org/10.1016/j.compscitech.2016.02.007 Get rights and content

Abstract

Stitching is used to reduce incomplete infusion of T-joint core (dry-core) and reinforce T-joint structure. However, it might cause new types of flaws, especially micro-sized flaws. In this paper, a new micro-laser line thermography (micro-LLT) is presented. X-ray micro-computed tomography (micro-CT) was used to validate the infrared results. The micro-LLT and micro-CT inspection are compared. Then, a finite element analysis (FEA) is performed. The geometrical model needed for finite element discretization was developed from micro-CT measurements. The model is validated for the experimental results. Finally a comparison of the experiments and simulation is conducted. The infrared experimental phenomenon and results are explained based on the FEA results.

Introduction

Three-dimensional (3D) carbon fiber reinforced polymer matrix composites (CFRP) are increasingly used for aircraft construction due to their exceptional stiffness and strength-to-mass ratios. Composites made from 3D textile preforms can reduce both the weight and manufacturing cost of advanced composite structures within aircraft, naval vessels and the blades of wind turbines [1]. The in-plane stiffness and strength of 3D woven composites are lower; while the out-of-plane properties are higher compared to conventional 2D laminates [2]. Assembly of 3D complex composite structures requires efficient joining methods. The most frequently used joint found in structural applications is the T-joint.

The purpose of T-joints is to transfer flexural, tension and shear loads to the skin. T-stiffeners are used extensively in aircraft wings in order to prevent skin buckling during wing loading. However, designing composite joints is more difficult than metallic joints due to the mechanical properties of composite materials [3].

In the design of T-joints, filler is inserted in T-joints and resin is used to reinforce the structure. The fiber insertion technique has the potential of creating a low-cost T-joint with improved damage tolerance and failure strength [4]. However, incomplete infusion of T-joints core (dry-core) is a typical issue. Fig. 1 shows typical dry-core in a non-stitched CFRP T-joint.

Stitching [5] is used to reduce dry-core and reinforce T-joint structure [6]. However, stitching might lead to new types of flaws due to the characteristics of its structure.

Non-destructive testing (NDT) of composite materials is complicated due to the wide range of flaws encountered (including delamination, micro-cracking, fiber fracture, fiber pullout, matrix cracking, inclusions, voids, and impact damage). The ability to quantitatively characterize the type, geometry, and orientation of flaws is essential [7], [8]. The ability to identify and characterize such micro-sized flaws accurately is challenging [9].

Infrared thermography (IRT) is becoming increasingly popular in the recent years as a NDT technique due to its fast inspection rate, contactless, spatial resolution and acquisition rate improvements of infrared cameras as well as the development of advanced image processing techniques. It is used for diagnostics and monitoring in several fields such as electrical components, thermal comfort, buildings, artworks, composite materials and others [10].

In this paper, a new micro-laser line thermography (micro-LLT) is used to detect a stitched CFRP T-joint. An 18 μm resolution micro-CT is used to validate the infrared results. A comparison of the micro-LLT and micro-CT is conducted. Then, a finite element analysis (FEA) simulating the infrared results is performed. The geometrical model needed for finite element discretization is developed from the micro-CT. Finally a comparison of the experiments and simulation is conducted. There is little information in the open literature on FEA for IRT on micro-sized flaws. The infrared experimental results are explained on based of the FEA.

Section snippets

Materials system

The T-joint specimen selected for this evaluation was sewn using stacked TC-06-T 3K carbon fiber. The 3D architecture was woven using 3K/12K carbon fiber. A continuous row of stacked 12K tow fiber was used for insertion. A toughened epoxy resin infusion system was selected.

Fabrication

The T-joint specimen was fabricated using 3D preform consisting of multiple layers of woven fabric. The complete 3D fabrication model is shown in Fig. 2(a). A high-resolution photograph of the fabrication is shown in Fig. 2

Micro-LLT

In conventional IRT set-up, a relatively homogenous heat source such as flash or halogen lamps is used to heat sample surface, and then temperature distribution on the surface is recorded with an infrared camera. Conventional IRT can detect a broad variety of defects, such as voids, pores, or delaminations [13]. However, it is difficult to detect micro-sized flaws due to CFRP structural complexity, which leads to heat diffusion abnormity, especially for interlayer detection.

To tackle this

Summary

A new micro-LLT was presented. Micro-CT was used to validate the micro-laser line thermography results. A comparison of micro-LLT and high-resolution x-ray tomography was conducted. Then a FEA was performed on the micro-LLT results. The geometrical model needed for finite element discretization was developed from micro-CT measurements. The comparison of the experiments and simulation was conducted. As a conclusion, micro-LLT can detect the micro-sized internal defects in the sample. However,

Future work

Compared to micro-CT, micro-LLT is a lower-cost, mobilizable, more robust alternative technique. Micro-LLT has advantage of LST, and can effectively reduce its disadvantage - larger detection area and less detection time. Micro-LLT can detect a 3.3 times larger area (3 mm × 10 mm) to LST (3 mm × 3 mm) per testing, which means 30% detection time of LST. This makes micro-LLT a more practical technique for large surface sample. However, micro-LLT cannot detect through-depth when a sample is thick

Acknowledgements

This research is supported by Consortium for Research and Innovation in Aerospace in Québec (CRIAQ) through Project No. CRIAQ COMP-501–CRDPJ 408010-10 and Natural Sciences and Engineering Research Council of Canada (NSERC).

This research is also supported by governments of Québec and Bavaria, Ministère des Relations Internationales and Ministry of External Affairs through bilateral Project No. 13.311.

The authors would like to acknowledge the financial support to the project provided by Bell

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An experimental and analytical study of micro-laser line thermography on micro-sized flaws in stitched carbon fiber reinforced polymer composites

Abstract

Introduction

Section snippets

Materials system

Fabrication

Micro-LLT

Summary

Future work

Acknowledgements

Compos. Sci. Technol.

Compos. Struct.

Compos. Struct.

NDT E Int.

Compos. Part B Eng.

Mater. Sci. Eng. A

Compos. Sci. Technol.

Compos. Sci. Technol.

Compos. Sci. Technol.

Compos. Sci. Technol.

Infrared Phys. Technol.

Compos. Struct.

Compos. Sci. Technol.

3d-woven Reinforcement in Composites