Composites in Aircraft and NDE J Prasad

Abstract Advances in fibers such as Glass, Carbon or Kevlar and Polymers to produce high performance resins

as matrix, has led to wide application of composites in civil and military aircraft. However, inherent

weakness of composites owing to high anisotropy and a number of manufacturing variables, introduce

defects and deficiencies. This necessitates careful control, evaluation and certification of material and

components through NDE methodologies. Under service and environmental constraints composites get

damaged and need repair and Nondestructive evaluation to ensure strength, stiffness, contour and

dimensions. The paper gives an overview of composite materials and process technologies. It brings out

various defects occurring during manufacture and service and emphasizes the need to produce evidence

through NDE methodologies to establish integrity of material and structure. Finally, areas of NDE

disciplines requiring attention is emphasized through integration of NDE into design, risk assessment

and life cycle management philosophy, for lighter, cost effective, design, maintenance and repair of

composites.

jaimangalprasad2000@yahoo.co.in

SINCE2011Singapore International NDT Conference & Exhibition , 3-4 November 2011

Introduction Development of strong and stiff reinforcements like glass, aramid and carbon along with advances in

polymers to produce high performance resin as matrix has accelerated application of composites in

military and civil aircraft (2). Composite materials, however, have their own complexities in design,

analysis, fabrication and Nondestructive test and evaluation (NDE).Inherent weakness of composite

materials owing to high anisotropy and large number of manufacturing variables, invariably introduce

defects and deficiencies. This necessitates careful control, of material, evaluation and certification of

material and component through NDE methodologies. Further, during service and environmental

constraints composites get damaged and need repair and Nondestructive evaluation to ensure

restoration of strength, stiffness, contour and dimensions.

This paper examines defects and deficiencies introduced during fabrication and service and relevance of

NDE in evaluating composites for “fitness for use”.

Composite materials in Aircraft Tables 1and 2 give reinforcing fibers, their properties and application and composite material systems, respectively.

Table 1— Reinforcing fibers commonly used in aerospace application

Fiber Density Modulus(Gpa) Strength(Gpa) Applications

Glass

E-glass

2.55

65 - 75

2.2 - 2.6

Civil aircraft parts and interiors and secondary parts

S-glass

2.47

85 - 95

4.4 - 4.8

Radoms, Rocket motor casing,

Fairings

Aramid Low

Modulus

Intermediate Modulus

High

Modulus

1.44

80 - 85

2.7 - 2.8

Non load bearing parts

1.44

120 -128

2.7 - 2.8

Radoms, Structural parts, Rocket

motor casing

1.48

160 - 170

2.3 - 2.4

Highly loaded parts

Carbon Standard Modulus

Intermediate

Modulus

High Modulus

1.77-1.80

220 - 240

3.0 - 3.5

For all parts

1.77-1.80

270 - 300

5.4 - 5.7

Satellites, Antenna dishes, missiles

1.77 - 1.80

390 - 450

4.0 – 4.5

Table 2- Typical composite material system in Aerospace application

Material System Application Area

175°c Curing high strength Carbon-epoxy.

Structural components of fighter aircraft and Helicopters.(wing skins, spars, fin, rudder, frames, Rotor blades).

175°c Curing intermediate high modulus carbon with epoxy

Inter stage cases, launch vehicles, solar panels.

120°c Curing high strength Carbon epoxy

Structural components of helicopter or transport aircraft (spars, fins, rudder, elevons, doors, frames, stiffeners).

Aramid fibers in low loss Polyester

Radom

Cu - mesh epoxy pre preg

Lightning strike protection, wing skin.

E-glass fabric in epoxy resin

Fairings, fin-radom, drop tanks, fuselage wing.

Experience has shown that large number of process variables, constraints of service and environment induce a range of defects and deficiencies in these components (1) . Defects induced during fabrication and services are given in Table 3.

Table 3—Defects induced during fabrication and service.

Defects induced During fabrication

Defects induced during service

Voids, porosity, Delamination, Broken

fibers, Inclusions, Insufficient curing,

Missing plies, Handling damage, Crazing,

Cracks, Scratches, Nicks, Blisters, Pitting,

Air-bubbles, Resin rich,

Resin starved areas,

Discolouration,

Open voids

Service loading condition Corresponding defects

Fatigue Matrix cracking, Crazing, Fiber failure, Delamination

Impact

Delamination, Fiber damage

Lightening

Debond between fiber and matrix, Delamination, Burning and puncture.

Environmental

Matrix plasticization, Debonding, Irradiation effects.

Erosion

Reduction in Thickness

Moisture absorption

Reduction in compression strength

Application of NDE In view of the fact that composites are susceptible to multiplicity of defects as indicated above, it is

invariably required to produce evidence through NDE methodologies to establish integrity of materials

and structure in terms of :-

1. Elastic properties and material homogeneity

2. Dimensional deviation

3. Level of defects(discrete and volume dispersed)

As the property of composites is highly process dependent, evaluation of these materials through NDE

involves:-

1. Evaluation of repeatability of fabrication in achieving strength and stiffness, resulting from design

of lay up and cure operation.

2. Ensuring thickness variation as per design stipulations.

3. Detection, evaluation and acceptability of discrete and volume dispersed defects.

Complexity of geometry, size of components and state of assembly, dictate choice of testing facility as

well as technique of testing. Usually a combination of complimentary NDT facilities is required to test and

evaluate composites.

Most widely used and preferred tool for test and evaluation for composites is Ultrasonics (3) .Both,

through transmission and pulse echo methods with capability of presentation of information in A, B, and

C-scan modes are used. Ultrasonic system with capability to measure attenuation and velocity can

effectively detect and evaluate various defects mentioned above. A few examples of effectiveness of

Ultrasonics in detecting and evaluation defects is shown in figs 1 to 8

Among other important NDE methods that have been used successfully are - Low energy radiography for

detection and evaluation of voids, fiber damage, inclusions, and core damage in honeycomb structures.

Acoustic emission methods have been used for evaluating integrated effect of flaws produced during

microscopic failure related to resin fracture, fiber fracture or failure of resin to fiber bond.

High frequency eddy current methods have been used to detect and evaluate surface and subsurface

and present the information in C-Scan mode. Method is reliable for inspection of composite skins during

service.

Acoustic impact methods are used to detect and evaluate large defects like debond, and delamination.

To reduce dependence on human factors robot based automatic NDE devices are finding increasing

application.

Repair of composites

Composite structures may suffer damage at any stage of its life cycle: production, transportation,

assembly or service and need suitable repair action. The role of NDE in pre repair stage is to determine

the nature of damage and demarcate extension of damage and ensure satisfactory removal of defective

areas. In post repair stage, NDE ensures freedom from harmful defects and in-homogeneities. Further,

restoration of thickness, strength and stiffness is validated through NDE methods. Repaired honey- comb

structures are evaluated using through transmission ultrasonic or radiographic methods. All repair

activities need to be documented, giving design requirement, repair procedure, evaluation and

acceptance criteria.

Thrust Areas

NDE is an indispensable part of modern technology. It has emerged as a critical component of the

engineering environment where cooperation and collaboration is a necessity. Multi-discipline character of

NDE technology, changing paradigms of design, complexities of fabrication processes as well as

requirement of health monitoring, call for constant upgrading of knowledge base, test facilities data

evaluation techniques, approach to integrate NDT activities in to design and product support philosophy.

Intimately connected with these activities are procedures of standardization and documentation.

It is obvious that range of activities mentioned above require a committed effort from academic

institutions, R&D organizations and industries.

Conclusions

The technology of NDE has a critical role to play in ensuring quality and improving productivity. An

integrated approach is vital to develop this technology including NDE systems, signal processing

algorithms, standards applicable for production as well as field usage.

Acknowledgments

Thanks are due to Drs. Kota Harinarayan, P.D.Mangalgiri and Krishnadas Nair, for discussions during various stages of NDT activities References

1. Nondestructive Test and Evaluation of Materials - J. Prasad and C. G .K. Nair, Tata McGraw-Hill Publishing Company limited, New Delhi – Second Edition – 2010

2. Kota Harinarayan and P.D.Mangalgiri, Composite development in LCA program National seminar

on composites development, Allied Publishers Ltd, New Delhi - 1996. 3. J Prasad and C.G .K .Nair - Usage of Ultrasonics in Evaluation of composites - International

conference on composite materials - University of Naples, Italy – May 1994. 4. J. Prasad, Carbon fiber composites, NDE for Aeronautical usage, Proceedings of the Third

Japan International SAMPE symposium VOL – 2 - 1993.

A few examples of effectiveness of Ultrasonics in d etecting and evaluation defects

Fig – 1.

Typical photo Micrograph of satisfactory homogenous material condition

Fig – 2.

Porosity due to vacuum failure

.

Fig – 3

Delamination at the junction of ± 45º plies

Fig – 4

Impact damage by 1kg mass object dropped from a height of 2 meters

Fig – 5

Inclusion in monolithic composite component

Fig – 6

High energy impact damage

Fig – 7

Effect of low energy impact damage

1 - 1´: 8 Joule Impact damage 2 - 2´: 17 Joule impact damage 3 - 3´: 30 Joule impact damage

Fig – 8

Effectiveness of Ultrasonic in evaluating thickness variation