ndt-1 matrix c scan.pdf

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Ultrasonic evaluation of matrix damage in impacted composite laminates

F. Aymerich*, S. Meili

Dipartimento di Ingegneria Meccanica, Universita Degli Studi di Cagliari, Piazza dArmi, 09 123 Cagliari, Italy

Received 19 January 1999; received in revised form 5 October 1999; accepted 2 November 1999

Abstract

Conventional ultrasonic inspection methods are largely used for detection of delaminations in composite materials while only recently new

techniques have been proposed to identify matrix cracks in simple tension loaded coupon specimens. In this study delaminations and matrix

cracking caused by low-energy impacts on quasi-isotropic carbon/PEEK laminated plates are examined by means of different pulse-echo

techniques: conventional time-of-flight and amplitude C-scans at normal incidence are used to check for the presence of delaminations, while

backscattering C-scans (in which the transducer is set at an angle to the laminate plane) allow the detection of matrix cracks through the

laminate thickness. Selected results from full waveform ultrasonic analysis of impacted carbon/PEEK laminates are discussed and compared

with X-ray data in order to demonstrate the efficiency of the proposed inspection technique. 2000 Elsevier Science Ltd. All rights reserved.

Keywords: D. Non-destructive testing; D. Ultrasonics; Backscattering; B. Impact behavior

1. Introduction

The detection of load-induced disbonds or cracks in

composite structures is of primary importance when deter-

mining performance levels and serviceability of tested

components. The higher strength-to-weight ratio of lam-inated carbon-fiber composites as compared to metallic

structures is counterbalanced by a lower impact damage

tolerance mainly due to the layered and heterogeneous

configuration of laminates [13]. Composite laminates do

not allow significant energy dissipation by plastic defor-

mation, and this leads to weaker through-the-thickness

than in-plane mechanical properties for the structure.

Damage assessment cannot therefore disregard the occur-

rence of both low- and high-velocity impact loadings during

the structures life cycle. The latter lead to easy-to-detect

forms of damage, since high-speed impactors, interacting

with the material for a short time period, cause evidentexternal damage. The former though (which is likely to

occur during manufacturing, service and maintenance) can

bring about invisible front surface damage but significant

internal degradation, with inner damage spreading over a

wider area starting from the contact point; mechanical prop-

erties can thus be seriously lowered, leading to sudden and

unexpected failure of the component. For these reasons,

accurate non-destructive techniques are required to detect

and quantify damages resulting from low-velocity impacts

on composite laminates.

2. Damage assessment techniques

Impact damage in composite materials consists of differ-

ent fracture modes which combine giving rise to a quite

complex three-dimensional pattern [46]. Experiments

indicate that an impact energy threshold exists below

which no damage occurs; above that level matrix cracks

generated by shear or tensile flexural stresses around the

indentation area develop mainly in the intermediate and

backface layers. Matrix cracks are then followed by inter-

face delaminations growing from the crack tips; delamina-

tions occur between plies of different orientations and are

elongated along the fiber direction of the lower layer at that

interface, with the largest delaminations developingbetween layers with the highest orientation mismatch.

Delaminations appear in regular patterns producing

altogether a typical three-dimensional spiral staircase. As

the impact energy is further increased, superficial fiber frac-

tures initiate at the tensile side of the impacted sample and

may propagate through the remaining layers, leading to total

perforation of the laminate.

Due to the complex features of damage mechanisms,

more than one method is usually required for a complete

non-destructive evaluation of impact induced damage.

Advantages and disadvantages of different available

Composites: Part B 31 (2000) 16

1359-8368/00/$ - see front matter 2000 Elsevier Science Ltd. All rights reserved.

PII: S1359-8368(99) 00067-0

www.elsevier.com/locate/compositesb

* Corresponding author. Tel.: 39-070-6755707; fax: 39-070-

6755717.

E-mail address: [email protected] (F. Aymerich).


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techniques depend on the type of damage to be detected and

on the test conditions in which sophisticated laboratory

techniques can give highly accurate results, but may not

be able to assess the state of the structure under in-service

conditions.

Several inspection techniques (acoustic emission,

thermography, dye penetrant, stereo X-ray radiography,

ultrasonics), with different sensitivity levels, can be used

for non-destructive evaluation of composite materials

[710].

Acoustic emission involves the detection of energy

released by the material under stress during cracking events;

the method proves very efficient for monitoring structures

under service but a precise identification of size, shape and

location of flaws is still impossible, particularly in com-

posite materials, characterized by a distinct anisotropy.

Thermographic inspection, based on the analysis of ther-

mal patterns induced either by heating the specimen or by

applying a mechanical oscillatory load, is sensitive to

delamination-type defects but is not able to give informationon the through-thickness location of the flaw. Liquid pene-

trantstypically limited to surface examinations and then

with very limited applications to composite materialsare

used to infiltrate flaws of damaged components; after appli-

cation the excess dye is removed while the remaining

penetrant indicates the presence of surface cracks.

Penetrant-enhanced X-radiography, which utilizes a

radio-opaque liquid to infiltrate the examined area, can

easily detect matrix cracks and delaminations; single broken

fibers are below the limit of resolution, but localized paths

of broken fibers can be revealed due to their characteristic

jagged appearance. The main drawback of this technique isthat it can resolve only damage connected to the surface,

while internal defectsimpossible to fill with the dye

may remain undetected. When the exact through-the-thick-

ness position of the defect is required, stereoscopic X-radio-

graphy techniques can be adopted, in which two X-ray

images are obtained from two different angles and then

optically recombined to reconstruct a three-dimensional

view of the damage state. The interpretation of the resulting

stereoscopic image is however difficult, particularly in the

presence of numerous superposed damage planes, due to the

difficulty of precisely locating the different delaminated and

cracked layers.

Ultrasonic through-transmission or pulse-echo tech-niques rely on the use of high-frequency mechanical osci-

llations for the detection of damage mechanisms; by

measuring the signal amplitude and/or the time-of-flight of

the ultrasonic signal the location and size of the defects can

be estimated. The usually adopted normal incidence tech-

nique [1113] is most sensitive to flaws that lie parallel to

the surface (delaminations); on the contrary, matrix cracks,

lying perpendicularly to the surface, and fiber fracture paths

are difficult to detect because they do not offer a wide

enough reflecting surface as delaminations. A few workers

[1417] have shown that by orienting the transducer at an

angle to the tested surface, so as to acquire the energy back-

scattered from damage, transverse cracks running parallel to

the fiber direction can be detected in specimens with simple

lamination sequences loaded in tension. In this study it is

demonstrated how a combination of normal and oblique

incidence pulse-echo ultrasonic techniques can be used to

produce a highly detailed volumetric image of complex

damage states dominated by transverse matrix cracks and

delaminations, as those resulting from low-energy, low-

velocity impacts on composite laminates.

3. Experimental

The specimens used for impact tests were 90 90 mm2

square plates cut from 420 420 mm2 carbon/polyether-

etherketone (PEEK) panels (16-ply quasi-isotropic laminate

with 61% by volume of continuous AS4 fibers) supplied by

Fiberite Europe. PEEK is a thermoplastic resin, which

achieves a degree of crystallinity of about 33% after using

the manufacturers processing procedures. The lamination

sequence was 0=^ 45=902s with ply thickness of

0.125 mm and a total thickness of 2.2 mm.

Impact tests were performed on a purpose-built drop

weight impact testing machine, with specimens clamped

between two rings of 70 mm internal diameter. By varying

the falling mass and the drop height, different impact ener-

gies and velocities could be obtained. The impactor was

instrumented with a semiconductor strain-gage full bridge

bonded to the tup, provided with a hemispherical nose of

12.5 mm diameter. Impact and rebound velocities were

measured by an infrared sensor, which sees a three-

stripe flag attached to the impactor. In this studyresults are reported for 3.6 and 5 J impacts, each repre-

senting a particular state of damage which will be

completely described in terms of matrix cracks and

delaminations.

4. Ultrasonic testing procedure and results

The tested specimens, immersed in water, were scanned

at normal (to detect delaminations) and oblique (to identify

matrix cracks) incidence in pulse-echo mode by means of a

focussed broadband transducer (3.2 mm diameter, 18 mm

focal length) with a center frequency of 22 MHz. The test-ing device consists of a 0.025 mm resolution scanning

bridge, a 150 MHz Krautkramer HIS2 ultrasonic pulser/

receiver, and a 500 MHz Hewlett Packard 54520A digital

oscilloscope used for radio frequency echo signal acqui-

sition. A personal computer with in-house developed soft-

ware controls the scanning sequence and triggers the pulser/

receiver for emission of ultrasonic pulses and acquisition of

reflected echoes. During scanning the complete ultrasonic

waveform is digitized at each point, stored on the internal

buffer of the oscilloscope and, once the buffer is filled,

transferred to the computer hard disk. In this way a database

F. Aymerich, S. Meili / Composites: Part B 31 (2000) 162


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representing the three-dimensional internal structure of the

sample is built which allows post-processing of data for

reconstruction of damage on a ply-by-ply basis by selecting

the appropriate gate location and width.

Since delaminations are located parallel to the laminate

plane, they can be easily detected by normal incidence tests;

scans were performed focussing on the middle plane so as to

obtain a good lateral resolution within the specimen thick-

ness. Both amplitude C-scans showing delaminations at thedesired interface and time-of-flight C-scans displaying

damage depths were reconstructed by consulting the

acquired database. In order to limit the masking effect

brought about by delaminations close to the probe to deeper

damage, all the samples were examined from the two sides

and the information obtained recombined to a single image.

A typical C-scan image consisting of a 150 by 150 array,

with a spatial sampling step of 0.1 mm, requires an acqui-

sition time of about 15 min.

Matrix cracks, running parallel to the fibers, are virtually

impossible to detect with conventional normal incidence

techniques, due to the fact that they mainly lie in a plane

parallel to the path of the ultrasonic beam. When the trans-

ducer axis is oriented at an angle to the surface of the lami-

nate most of the beam energy is reflected (from the front

surface of the specimen or from inner delaminations) in

directions away from the transducer; the acquired echo is

in this case much weaker than that obtained at normal inci-

dence since it contains only low-level signals backscattered

by matrix cracks and, to a smaller extent, by fiber bundles.By adjusting the angle of incidence so as to maximize the

amplitude of the signal received from cracks, patterns of

multiple matrix cracks in different layers can be obtained

and mapped with an appropriate analysis of the acquired

data. In this study the transducer was attached to the vertical

z-axis of the scanning bridge through a rotatable head. The

probe direction was chosen to be normal to the fiber direc-

tion of the layer investigated and at an angle of 26 to the

normal, selected by rotating the ultrasonic transducer by

successive adjustments until the matrix cracks signal was

maximized.

F. Aymerich, S. Meili / Composites: Part B 31 (2000) 16 3

Fig. 1. Ply-by-ply amplitude C-scans of delaminations and matrix cracks in a 3.6 J impacted plate.


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5. Test results

The ultrasonically reconstructed damage pattern of a 3.6 J

impacted sample, displayed as amplitude C-scans on a ply-

by-ply basis, is shown in Fig. 1. Delaminations wereobtained by orienting the probe at a normal incidence and

setting a 40 ns software-based gate at the desired interface.

Matrix cracking was imaged from ultrasonic data acquired

at oblique incidence using a 50 ns width gate. The radio-

graphic image and the time of flight C-scan (with gray levels

corresponding to damage depths) of the same specimen are

shown in Fig. 2.

The ply-by-ply maps clearly show the characteristic two-

lobed shape of single delaminations, which combine to give

a staircase appearance closely dependent on the stacking

sequence. Moreover, by a comparison of Figs. 1 and 2, we

can observe that some delaminations, impossible to infiltrate

with the penetrant, remain entirely undetected by radio-graphic analysis.

As concerns matrix cracks, they mainly develop in the

backface layer at the tensile side and are easily resolved by

the ultrasonic backscattering technique (Fig. 1). If we

compare the ultrasonic images with the information result-

ing from radiographic analysis (Fig. 2) we can conclude that

the ultrasonic techniques adopted lead to a complete

characterization of the matrix damage for laminates

impacted at this energy level, with matrix cracks individu-

ally detected by backscattering procedures. A proper selec-

tion of gate settings in terms of width and location is

however essential: a narrow time gate produces a high-reso-

lution image of matrix cracks but also requires a careful

choice of its position if the depth of the defect is not

known in advance. The influence of gate width on the qual-

ity of backscatter imaging of matrix cracking is clearly

evident from the observation of the two scans of Fig. 3,

reconstructed by using respectively a 50 and a 300 ns gate.

Figs. 4 and 5 show the results of tests on a laminate

impacted at 5 J. Damage consists of a network of matrix

cracks distributed in the lower layers and delamination

areas coupled at adjacent interfaces; some fiber breakage,

undetected by ultrasonic analysis, develops as well in the 0

and 45 plies at the backface. Again the backscatter tech-

nique proves sensitive to the presence of matrix cracks, even

in the presence of a complicated damage scenario and with

different fractured layers. By the adoption of a sufficientlyshort gate, backscattering analyses produce very detailed

information on matrix fractures induced by impact, which

can be located and detected with a resolution higher than 3

cracks per mm.

6. Conclusions

Normal and oblique incidence ultrasonic techniques with

full waveform acquisition proved very sensitive to matrix

damage induced by low-velocity, low-energy impacts.

Traditional normal-incidence pulse-echo procedures canbe used to precisely characterize extension and through-

thickness location of delaminations. Oblique incidence

techniques provide a highly detailed description of matrix

cracking at various thickness levels in the laminate, on

condition that the entire backscattered echo is acquired

and the appropriate software-based gate is adopted to select

the required information at the desired depth.


Fig. 2. X-ray image (left) and time of flight C-scan (right) of damage in a

3.6 J impacted plate.

Fig. 3. Influence of gate width on the quality of crack imaging from backscattered echoes. (Top: 50 ns gate width; bottom: 300 ns gate width.)


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