British Journal of Non-Destructive Testing, November 1990, 32(11) pp 568-577




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British Journal of Non-Destructive Testing, November 1990, 32(11) pp 568-577.

Non-destructive testing of advanced composites:
a review of recent advances



John Summerscales

Advanced Composites Manufacturing Centre

Department of Mechanical and Marine Engineering

The University of Plymouth

Plymouth, Devon, PL4 8AA


ABSTRACT

This paper presents an overview of the many possibilities for the non-destructive testing of advanced polymer matrix composites. The techniques are considered under three major headings: electromagnetic, chemical spectroscopy and mechanical vibration. Finally the use of computer tomography is briefly introduced.


INTRODUCTION

In the time since World War II, fibre reinforced plastics (FRP) have developed from a laboratory curiosity to become an industry which delivered 1208 900 tonnes of composites in the USA alone during 1988, a record year [1]. New highly stressed components of FRP construction are regularly announced in most structural design areas. The bulk of these materials use glass fibres as the reinforcements, but the use of carbon and aramid fibres is expanding rapidly. The technology of such materials has been well reviewed in a number of recent books [....] to which the reader unfamiliar with composite materials is referred.


Over the corresponding period aspects of the physics and chemistry of materials have been allied with the rapid advances in electronics and computing to produce the new discipline of non-destructive testing (NDT). Abbreviations also used include NDE (examination/evaluation) and NDI (inspection). The subject of non-destructive testing has an extensive literature [12].


Composite materials must be regarded as very different media from metals when considering which NDT methods are appropriate. Generally the reinforced plastics have poor electrical conductivity, low thermal conductivity, high acoustic attenuation and most importantly significant anisotropy of the mechanical and physical properties. The life of a metal component in determined by the nucleation and growth of cracks or damage in the material. The development of linear elastic fracture mechanics is often adequate as a basis for the definition of the size of sub-critical flaws which must be identified.


However, a fibre reinforced composite is a heterogeneous medium which can contain multiple defect geometries. No single failure model can adequately describe the level of damage which is critical. A multiplicity of models have been developed to describe the various failure modes: interlaminar debonding, matrix degradation, fibre fracture and fibre-matrix interface separation. These in turn may be caused by improper cure, fibre misalignment, inclusions, poor reinforcement distribution, machining damage, fastener fretting and environmental degradation.


Kardos et al [13] have reviewed the growth of voids during manufacture of polymer matrix composites. Judd and Wright [14] have presented brief summaries of:


  • void types and their causes

  • methods of measurement of void contents, and

  • the influence of voids on the mechanical properties of composites


They concluded that, regardless of resin, fibre type or fibre surface treatment, the interlaminar shear strength of a composite decreases by about 7% for each 1% of voids up to a total void content of about 4%. Other mechanical properties are also affected but not to the same extent.


Chatterjee et al [15] have examined the criticality assessment techniques for various types of defects in composites, and fracture mechanics based methodologies for assessing the significance of delamination defects. The latter appear to be the most critical type of defect limiting the strength and lifetime of composites.


Non-destructive evaluation has three major functions for research, development and applications testing in composite materials. They are:


a Initial inspection of test specimens

and confirmation of the structural integrity of new components.

b Monitoring sample tests in progress,

or components subjected to service loads.

c Analysing test results after failure,

or proof loading of components during their service life.


Heslehurst and Scott [16] reviewed defect types by time of occurrence, their relative size and general structural effect. The failure mechanisms of the structurally significant defects (delaminations, matrix cracks and holes) were discussed with respect to their load response.


In the case of industrial products, the design and production have always been considered the pre-eminent engineering challenges, with inspection, testing and defect diagnosis relegated to very subsidiary roles. Walkden and Boutoussov [17] have recently presented the case for feedback mechanisms between NDT and manufacture to maximise the efficiency of the overall production process. There is also an increasing awareness of the potential for in-service monitoring by NDT methods to improve the reliability of components in service and permit prolonged safe operation.


The state-of-the-art in non-destructive testing techniques as applied to the complete range of fibre reinforced plastics has been comprehensively reviewed in two volumes by Summerscales [18, 19] and updated by a recent journal paper [19a]. The only other books known to address this subject are a short volume by Wegman [20] and a volume which is in Russian with no known translation [21]. Published information for this subject is distributed across a diverse selection of journals ranging from the pure sciences to engineering. A number of conferences with a theme of NDT of FRP have recently taken place [....].


The NDT technologies applicable to fibre reinforced plastics can be divided into three broad categories: electromagnetic, spectroscopic and mechanical (Figure 1). This paper briefly summarises the techniques under the above headings, sequenced from high to low frequency. Finally it is important to realise the full potential of the NDT techniques by a consideration of signal processing and image analysis techniques.


ELECTROMAGNETIC

Radiography [18, 23]

Absorption

The term radiography encompasses several methods of using high energy radiation for the examination of opaque objects. The beam energy can be generated electrically (eg. X-rays with a broad spread of wavelengths), by radioactive isotopes (eg. gamma rays with discrete wavelengths), and by nuclear reactors. The output is typically recorded as a shadowgraph on photographic film. The molecules in FRP composite materials are usually of low atomic weight nuclei, and hence absorption of X- and gamma-rays is low and contrast is poor especially for thin laminates. Sufficient contrast may be developed with 'soft' low-voltage X-rays. The use of penetrant (sulphur, tetrabromoethane, silver iodide) enhancement produces improved images of surface breaking cracks, but care must be taken to avoid penetrant systems which degrade the performance of the composite. Glass fibres are often added as tracers in CFRP for X-ray examination.


Beta (electron) radiation attenuation can be used to determine volume fractions. The difference between absorption coefficients of the fibre and the matrix enables accurate determination of the resin content. Beta gauging may be used for process monitoring, particularly in the preimpregnation operation.


Neutrons

Unlike X-rays, neutrons pass through metals and are absorbed by lighter elements (especially hydrogen) typical of those in polymeric materials. Neutrons do not affect X-ray film directly and thus require an intermediate sensitive screen. Neutrons generated in a nuclear reactor may be used, but for convenience moderated (slowed-down) fast neutrons can be produced electrically or by isotopes (eg. Californium - 252). The method may be useful for the NDT of composites [25] especially when the material is encased within a metal. Neutron gauging should be capable of measuring changes in matrix volume fraction of 1%. It may be practical to use neutron radiography to follow long-range flow processes (eg resin transfer moulding) in metal mould tools.


Backscatter

Roye [24] has described a detector head which limits divergent X-rays to a pencil beam which is then subjected to diffuse scattering in the specimen by the Compton effect. The backscatter intensity is measured, with depth differentiation provided by a slit diaphragm system. The "Comscan" image clearly shows delaminations in a CFRP box-section.


Visible light [18]

Observation

Visual testing can be one of the simplest of the NDT techniques for defects in transparent and translucent materials and surface errors in opaque materials. Various aids (magnifying glasses, microscopes, high-level illumination or optical fibre borescopes) can enhance visual tests. For example, the impacted surface of a CF/PEEK composite usually has a readily identifiable dent [25a].


D-sight

The D-sight method [25b] may be used to locate indentations on the surface of damaged composites. Light is directed towards the part under examination and a reflected image obtained on a retroflective screen behind the part. Surface perturbations (scratches or dents) cause interference of the reflected light waves, resulting in a dark zone on the monitoring screen. Delaminations in thin laminates, and surface scratches as small as 10 m deep, can be detected.


Transmitted light

Hayward and Harris [25c] have produced plates by resin transfer moulding and photographed them on a light table. The difference in wetting when vacuum assistance is used or not used in the process can be clearly seen.


Liquid penetrants and brittle lacquers

Liquid penetrants rely on capillary action causing the liquid to enter surface breaking openings (cracks, delamination or exposed porosity). Application may be by brush, spray or dipping of a precleaned surface. The surface is rinsed, (dried), and a developer applied to draw the penetrant to the surface from the openings. Interpretation and analysis may require considerable experience. Specific applications of penetrant testing of composites have been described [26, 27]. Brittle lacquer techniques have a similar capability to follow cracking as the specimen is loaded [28].


Edge replication

Harris and Morris [29] used edge replication with enhanced x-ray for NDT of fatigue specimens where edge effects dominated.


Liu et al [30] extended the use of edge replication from identification of matrix cracks in damaged composites to the study of delamination. Results from carbon fibre/epoxy plates were in good agreement with other NDT techniques.


The deplying technique

A destructive procedure which can complement NDT methods is the deplying technique, which provides a method for sizing and precise location of interlaminar separations and fibre bundle fractures. The techniques were developed by Freeman [31-32a] and permit the characterisation of impact damage at every interlaminar interface. The impact zone is saturated with a solution of gold chloride, which penetrates into the regions of matrix-cracking and delaminations formed by the impact. The composite is then heated (418`C for 90 min) to partially pyrolyse the resin matrix and thus allow the lamina-by-lamina separation of the laminate. Cantwell and Morton [%17a] have found that application of the gold chloride solution is not necessary for a brittle matrix carbon fibre composite.


Jamison [33] has conducted a systematic examination of microdamage development in graphite/epoxy composites using penetrant-enhanced and stereo radiography, edge replication and laminate deplying.


Optical fringes

If a regular pattern of fringes is projected or 'printed' onto a surface the fringes may be used for the analysis of the surface form. If two superposed systems of lines are dense enough and show some regularities, they form a pattern known as a Moiré pattern. Sciammarella [34] has reviewed the use of Moiré as a tool to measure displacements, contours, slopes and strains.


Photoelasticity

The velocity of polarised light in a material is altered by the stress state in the material. In isotropic materials, the isoclinic fringes are known to give the principal stress or strain directions. In combination with the principal stress difference, from the isochromatics, it is possible to separate the principal stresses.


Pih and Knight [35] reported the use of transmission photoelasticity as a whole-field method of stress analysis for glass fibre composites. Sampson [36] developed a stress optic law for photoelastic materials based on a Mohr circle of birefringence, and obtained close agreement between his theoretical values and the isochromatic fringes of Pih and Knight. The isoclinic fringes showed only qualitative agreement. Prabhakaran [37] found that the Sampson concept was useful in interpreting isochromatic fringes in orthotropic material (unidirectional E-glass/polyester). He also showed that the isoclinic parameters could be well predicted by the Sampson concept if the fringes were modelled as principal strain angles which are not necessarily the principal stress angles in a composite.


Burger et al [38] used dynamic photoelasticity to visualise the behaviour of elastic waves in orthotropic media. The effect of damage zones could be seen, and was used to interpret ultrasonic signals.


Optical caustics

The method of caustics [39] uses geometrical optics to transform a singular stress field to a dark field bounded by a highly illuminated curve: the optical caustic. Scattered light reflections from a region of rapidly varying stress are concentrated along a singular spatial surface. Theocaris [40] has presented the use of optical caustics for the analysis of the stress field at a corner in a multilayer wedge, the stress field at interfacial cracks, the stress state around inclusions and for crack propagation and arrest in bi-material plates.


Coherent light [18]

The coherent light techniques are equally as applicable to conventional materials as to composites, as they monitor surface deformations. Surface finishes (smoothness and reflectivity) may affect the usefulness of these techniques, and the anisotropy may affect subsequent analysis.


Interferometry

The interaction of beams of coherent light produces fringes which may be used for the measurement of surface deformation, and by subsequent analysis can produce strain contour plots.


Moiré of Moiré interferometry

Asundi [40a] has applied laser Moiré interferometry in a study of the deformation around a small hole in woven glass-fibre/epoxy composites. The high sensitivity of the method was clearly demonstrated with excellent fringe contrast even at large strain levels. Moiré of Moiré interferometry was shown to be capable of enhancing the non-uniformity in the deformation field by subtracting the uniform component of the deformation.


Holography

Holography detects defects by cancellation and interference between laser beams. Inhomogeneities or discontinuities affect the way that the material surface deforms and cause a differential interference pattern to that in a 'good' composite. The technique is very sensitive and hence only minimal stresses, typically one or two degrees Celsius or a few millibars pressure change, are necessary to indicate defects. Detectable defects include cracks, impact damage, delaminations, water absorption and core/skin debonding.


Television holography (TVH) can be used to produce fringes of lower quality than conventional holography. However TVH has the advantage of speed, real-time operation and on-line image processing. Vikhagen and Løkberg [41] have recently described the use of such a system for the detection of defects in composites.


Speckle

Under illumination by coherent light any diffusely reflecting surface appears to be covered by a myriad bright and dark points. This speckle field is recorded in coherent addition with either a reference beam or a further speckle field. Speckle interferometry is more convenient than holographic interferometry as only coarse detail is recorded, and hence faster films may be used. The technique is very sensitive (nanometre to micrometre movements) but suited only to small changes. The photographic processing may be eliminated by the use of video-recording and electronic image comparison in the ESPI (electronic speckle pattern interferometry) technique.


Speckle photography records a speckle image before and after deformation, without a reference beam. Movement along the optical axis has little effect on the image. Translation across the beam moves the speckles sideways by an amount related to the movement and to the magnification. If the specklegram is illuminated by laser light, then the pitch of the Young's fringes indicates the magnitude of the local displacement and the inclination describes the direction of the displacement.


Shearography

Shearography is a particular form of speckle shearing interferometry (SSI). Two coherent images of the surface are formed simultaneously with one image shifted relative to the other. After deformation the recording is further double exposed. The fringes obtained are contours of the derivative of displacement (rather than the displacement). The technique is insensitive to whole body motion and reacts only to differential movement within the body.


Embedded optical fibres [42]

Embedded optical fibres in composites have been used for monitoring interfacial corrosion, damage assessment (broken fibres transmit less light), strain measurement and as acoustic emission transducers. A fibre optic chemical spectroscopy technique has been used for resin cure monitoring.


Waite [43-45] has recently reported the use of embedded optical fibres for damage indication and as a strain gauge in the flexural testing of glassfibre beams.


Aframowitz [45a] has proposed the use of a fibre optic sensor to monitor changes in the refractive index of epoxy or polyimide resin during cure. A short fibre of the cured resin was embedded in the curing resin, and illuminated with light from a GaAs laser. In air, the transmitted light level was 470 (relative units). Immersed in freshly mixed resin this fell to 90 units, and as the resin gelled and hardened the signal fell to 6 units. At full cure, both sensor and resin have the same refractive index and the sensor is unable to guide any light at all.


Thermography [18, 46]

Defects in composites exposed to either externally applied thermal fields (EATF) or stress generated thermal fields (SGTF) can be imaged by colour change coatings (liquid crystals) or by infrared cameras. The defect acts as a barrier to thermal conduction through the material or the stress concentration generates heat under stress, and hence the thermal field differs from that of good material. Thermographs also clearly show the anisotropy of heat flow in the composite. Vibration is often used to produce SGTF in a technique commonly known as vibrothermography.


There have recently been significant advances in thermographic equipment notably PVT (pulsed video thermography) at Harwell [47, 48] and SPATE/TSA (stress pattern analysis by thermal emission/thermoelastic stress analysis) at SIRA/Ometron [49]. Potter [50, 51] has developed equations relating the output of a SPATE/TSA system to the strains in a multiaxial laminate and has demonstrated their applicability.


Microwaves [19]

Microwaves are the band of frequencies from 225 MHz to 100GHz and are capable of penetrating most non-metallic materials, reflecting and scattering from internal boundaries and interacting with the molecules. Three factors (shape/dimensions, dielectric constant and loss tangent) influence the penetration of materials by microwaves.


The sensitivity to the dielectric properties of the material makes microwave testing particularly effective for internal structure (orientation), homogeneity (resin-rich or resin-poor areas), state of cure, moisture content, ageing and porosity. Microwave systems are also capable of detecting internal flaws (delaminations, voids and inclusions) and of analysis of complex motions, as well as precise measurement of distance, thickness and surface waviness. Recent Soviet work has demonstrated the relationship between the microwave frequency dielectric properties and the deformation/strain in the composite materials.


Eddy current [19]

The eddy current method is limited to composites with a high fibre volume fraction of conductive fibre (boron, carbon) reinforcement. High fibre contents are required for establishment of continuous conduction paths for the eddy currents. In the presence of these materials, the eddy current coil suffers a reduction in inductance and an increase in the reactive part of the impedance. Eddy currents can reveal variation in fibre volume fraction, and by use of a horseshoe probe can indicate fibre orientation. Thickness measurement of non-conducting composites can be achieved by placing a metal item behind the composite.


Dielectric [19, 52]

All dielectric measurements involve the determination of the electrical polarisation and conduction properties of a sample, usually in a varying electric field. The dielectric properties of a resin during cure change through a very large range of values. Commercial equipment using microfabricated interdigitated (comb-like) electrodes is now available which can follow the cure of resin matrix systems from the liquid state until solidification has occurred.


Foley [52a] has shown through experimentation that in-situ dielectric measurement can be correlated to changes in the rheological viscosity during the flexible diaphragm resin transfer moulding process.


Dielectrometry may also be useful for the measurement of the moisture content of composites.


Soleimani [52b] has described a volumetric image reconstruction technique for electrical capacitance tomography (ECT).


Electric [19]

The electrical resistivity of commercial fibre reinforced composites varies in value by fifteen decades from the high resistivities of aramid/epoxy to the conducting carbon fibre materials. These properties have been used non-destructively to monitor laminate thickness, fibre orientation, moisture content and crack growth. The current-voltage characteristics have been used to monitor the curing reaction of the thermosetting resin (ion-graphing, phaseometry and the recent AGC sensor) and to follow thermal degradation.


Harper [52c] has developed a robust, inexpensive electronic sensor which not only indicates the arrival of the resin flow front, but also provides an indication of the state of cure. The AGC (arrival, gel, cure) sensor is a galvanic cell in which the resin is the electrolyte. The output from the cell may be used as a stop-go criteria indicated by LEDs or as a graph of voltage against time.


Corona discharge [18]

The principle of the corona discharge NDT method is that an electric field of high intensity imposed across a void contained in a dielectric material will ionise the gasses in the void. The electrons are accelerated towards the void wall and may be detected either by the minute pulse of current which occurs in the secondary coil of the transformer, or by the electromagnetic radiation resulting from the collision. Some indication of the size and shape of the void can be obtained by studying the characteristics of the corona pulse. The method was used until 1968, but no subsequent reports of the technique have been published.

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