Advanced Ultrasonic System for High Performance Composite Laminate Nondestructive Evaluation

G. Caprino, R. Teti, V. Lopresto
I.L. Baciu, T. Segreto, A. Busco

University of Naples Federico II, Italy 

Introduction

• A quantitative non-destructive evaluation (NDE) of impact damaged composite laminates using a volumetric ultrasonic (UT) pulse-echo immersion testing system was carried out

• Non-crimp fabric (NCF) composite laminates made by resin infusion under flexible tooling (RIFT) technology were considered

• To verify the influence of diverse material design configurations, two different NCF laminate lay-ups were considered:

• biaxial

• quadriaxial

UT NDE System - Hardware Configuration

The UT system configuration for the NDE of NCF laminates is composed by the following hardware components:

• Digital oscilloscope for analog UT signal digitisation and transfer to PC via GPIB for UT signal post-processing 

• Oscilator/detector generating the electrical pulses for UT probe and receiving the returning pulses 

• Transmitter/receiver focused UT probe 

• Mechanical system consisting a 6-axis robotic arm for UT probe displacement

• PC for UT data acquisition and processing, and mechanical system displacement control

UT NDE System - Hardware Configuration

The block scheme of the UT NDE system is reported below

UT NDE System - Robotest© Software Package

The UT NDE system is run by an in-house custom made software code: Robotest©.

• The Robotest© software package allows for the generation of single or multiple UT images of any thickness portion of the internal structure of the material under examination by processing the UT volumetric file containing the complete UT waveforms detected during the FV-Scan

• The FV scan consists in the detection and digitization of the whole UT waveform for each position of the transducer during scanning

• UT data are organized in a volumetric file containing the whole set of complete digitized UT waveforms

• UT images corresponding to any segment of the UT signal, i.e. to any portion of the material thickness, can be obtained and analyzed

• The software also allows to retrieve the single UT waveform corresponding to any given in-plane location

UT NDE System - ROBOTEST© Software Package

UT image generation

• To obtain UT images, a typical UT waveform is retrieved from the volumetric UT file 

Time gate setting

• Then, a time gate is set on the waveform to identify the material portion(s) to be examined; the time gate can be subdivided into sub-gates (16 sub-gates in the example below)

Sub-gate images

• One image is generated for each sub-gate and each image represents the internal structure of the corresponding portion of laminate thickness

• In our case, the time gate was divided in four sub-gates to generate four UT images each corresponding to 1/4 of the laminate thickness (about 1 mm)

Materials

• Test material: NCF composite laminates, 4 mm in thickness, made by the RIFT technology  

• Hexcel HexPly®M36 resin matrix and Tenax HTS 5632 12k carbon fibres

• Polyester stitching yarn (50 dtex) was used for connection with stitch length 2.5 mm. The final fibre volume fraction was 62%

• Stacking sequence for biaxial laminates: [(+45°/-45°), (0°/90°), (+45°/-45°), (0°/90°), (90°/0°), (+45°/-45°), (90°/0°), (+45°/-45°)]_s

• Stacking sequence for quadriaxial laminates: [+45°/0°/-45°/90°, +45°/0°/-45°/90°, 90°/-45°/0°/+45°,90°/-45°/0°/+45°]

• In both cases, a final fiber aerial weight of 267 g/m2 per layer was obtained

Example of Impacted NCF Laminate Specimen

Quadriaxial NCF laminate impacted specimen

Impact Testing

Impact tests were carried out using:

• A falling weight machine with software for data recording and analysis 

• A cylindrical indenter with hemispherical nose was used for impact tests

   

Impact Testing II

• Rectangular specimens 100 mm x 150 mm were cut from the laminates and impacted with different impact energies

• A dedicated fixture system was used for NCF laminate specimen clamping

Impact Testing program

The Impact testing program for different impact energy values is reported in the table

Impact Test Conditions

Test conditions for drop weight low-velocity impact tests: preliminary tests allowed for the selection of suitable energy levels for continuing the test program

Ultrasonic Testing

• The specimens were subjected to UT volumetric scanning using the custom made software code Robotest© for FV-Scan UT NDE 

• Pulse-echo immersion volumetric UT scans were carried out using a focused (49.6 mm focal length), high frequency (15 MHz) transducer for maximum resolution

• The oscillator/detector was set up at 90 dB gain and medium damping

• The digital oscilloscope was set up at 1 V/div, 0.5 micros/div, and sampling frequency 100 MHz, resulting in 500 samplings detected for each UT waveform

• Each specimen was scanned over an area of 110 x 155 mm with scan step 1 mm. After UT NDE the delaminated area was measured

Mechanical Testing Results

• The delaminated area is plotted vs. impact energy, U. All experimental points approx. follow a straight line 

• Interestingly, the biaxial laminates show a smaller damaged area

• In comparison with classical laminates, a larger delaminated area was noted for the NCF material

UT testing results - image analysis

Quadriaxial laminate QHL3 IM 2: UT probe 15 MHz, impact energy 9 J

UT testing results - image analysis

Quadriaxial laminate BHL IM 2: UT probe 15 MHz, impact energy 9 J

UT image analysis

• From the UT images, it can be seen that the damage develops along the interfaces between layers having different orientations 

• Delamination extension increases with increasing distance (depth) from impact surface, resulting in the hat-shaped configuration of delamination damage

• The UT analysis also reveals an absence of delamination in a small zone directly below the impactor-material surface contact point

• The last UT image represents the in-plane projection of the damage developed at different depths in the whole laminate thickness

• By comparing the UT images for quadriaxial and for biaxial NCF specimens impacted under the same testing conditions, it can be seen that the internal damage is always larger for quadriaxial laminates

UT image analysis II

• UT images obtained from UT FV-scan of a classical quasi-isotropic CFRP laminate tested under similar conditions as the NCF laminates

• By comparing the UT image series of the classical and the NCF laminates, the latter shows a much more reduced impact induced delamination damage, confirming the great potential of NCF laminates when high impact resistance is a critical property