Manufacturing Aspects of Fabrication of Composite Reference Standard for NDT Ultrasonic Inspection

Pranab Biswal 1, B.N.Srinivasa Reddy 2, Pratim. M. Baruah 3

Senior Manager (NDT) 1, Chief Manager (NDT) 2, Deputy Manager (QA) 3

pranab.biswal@hal-india.com 1, srinivasareddybn@gmail.com 2, pratim.m.baruah@gmail.com 3 Aerospace Composites Division, Hindustan Aeronautics Limited, Bangalore-560037

Abstract This paper provides a brief review of the manufacturing aspects considered during development of a composite reference standard used for system standardization during ultrasonic testing of composite components. Most of the calibration / test blocks available in the market are in accordance with national and international standards but are metallic in nature (i.e Aluminum, Steel etc.). These standards cater only to the inspection requirements of metallic materials. Whereas in the composite manufacturing industry/environment, it is not appropriate to use metallic test blocks as the properties are completely different. For validating NDT techniques, characterizing defects and standardizing test equipment parameters with respect to composite components, a dedicated composite reference / calibration standard is required. The composite reference standard should be representative of the design configuration and the dimensional complexity of the part and should focus on parameters like skin thickness, material type, surface finish, bonding adhesive, internal structure and fabrication process. The reference standard should also contain induced artificial discontinuities which would produce a similar response to that produced by typical defects like voids, de-lamination, de-bonds and inclusions (FOD) in composites. Documenting the manufacturing aspects will standardize the manufacturing process and provide reference data during any new development, modification and periodic assessment.. Key Words: Composite, Defect, De-lamination, FOD, Manufacturing process, NDT, Reference standard, Ultrasonic Testing, 1. Introduction Increased use of composite in aerospace has increased the importance of NDT methods which are capable of identifying flaws in composites. The composite manufacturing process makes the composite parts susceptible to different types of defects which mainly occur due to: human error, improper manufacturing process, improper materials etc. Hence there is a great need of establishing NDT techniques capable of detecting manufacturing defects to consistently maintain the quality of the products. NDT also plays a key role in gathering information about structural properties and service life of aerospace components. Ultrasonic testing is extensively used in composite inspection because of its ability to detect defects such as: delaminations, disbonds, inclusion (FOD), porosity, and voids. In order to evaluate the capability of defect detection and assessment of defect severity, it is essential to develop a test method to standardize the ultrasonic system and it can be achieved by making use of reference standards. Hence composite reference standards, as they are often called, are required to be developed to ensure that the test equipment is able to correctly detect the signals for a particular setup and procedure. Results obtained during reference standard testing shall give information on choice of technique based on the following: inspected material, thickness range, object geometry, required damage detection, the best sensitivity.

2. Typical defects in composite (A) Resin rich: - Local resin content is higher than the average of laminate due to improper lay-up, compaction or curing, (B) Resin starvation: - Local resin content is lower than the average of laminate due to improper flow of resin, (C) Delamination: - Separation between two or more layers

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in a laminate because of contamination or improper adhesion during processing, (D) Disbond: - Separation between the bonded joints caused by either contamination or improper adhesion during processing e.g disbond between laminate to core, sheets and core or sheet-to-sheet, (E) Void: - Presence of air or gas trapped inside the laminate during curing of the component. It is of measurable size, (F) Porosity: - Presence of small air or gas bubbles inside the laminate caused by volatiles. It is of non-measurable size, (G) Inclusion (FOD): - Any undesirable materials, which are inadvertently left in the bonding area of a composite structure.

This paper emphasis on defects like: Delamination, Disbonding & Inclusion (FOD)

3. Ultrasonic inspection of composite parts Ultrasonic inspection is the most valuable technique for inspection of composite parts. Ultrasonic operates on the principle of transmitted and reflected sound waves. As the ultrasonic beam passes through the composite, it is attenuated or lost due to scattering, absorption, and beam spreading. This loss or attenuation is usually expressed in decibels (dB). Thicker laminates will attenuate more sound than thinner laminates. An ultrasonic wave traveling through a composite laminate that encounters a defect such as delamination or porosity will reflect some of the energy at the interface while the remainder of the energy passes through the defect. The more severe the defect, the greater the energy reflected and lesser the energy transmitted.

Through transmission ultrasonic technique is one of the two most common methods used to inspect composite laminates and assemblies. This method is based on two aligned transducers; one is a transmitter and the other is a receiver and part is coupled between them. If the part contains a defect, such as porosity or a delamination, some (or all) of the sound will be either absorbed or scattered, so that some (or all) of the sound is not received by the receiving transducer. Through transmission method is excellent at detecting porosity, disbonds, delaminations, and some types of inclusions (FOD). However, this method cannot detect all types of foreign object inclusions and does not provide data in terms of depth of defect.

Since through transmission method is not capable of detecting all types of foreign object inclusions and the depth of defects, pulse echo ultrasonic inspection is frequently used in conjunction with through transmission ultrasonic technique to inspect parts. Pulse echo method is based on presentation of reflected sound from within the part or the back-wall surface using a single probe acting as both transmitter and receiver. Sound is reflected from back-wall surface or from the discontinuity due to impedance mismatch and presented as signal amplitude along the vertical axis at the corresponding time of flight / distance along horizontal axis.

4. Composite reference standard salient features Selection of a reference standard depends upon the technique of testing, type of material to be inspected, form type of the discontinuity to be detected and other specification requirements. Sometimes it is preferable or required to prepare a reference standard from a piece of the same material as that of the component to be tested. The advantage of such a reference standard is that the test object and the standard will have the same composition, manufacturing history, surface condition and geometry. Here are specified some of the salient features required for a composite reference standard:

4.1 The reference standard shall represent the design configuration and complexity of the component to be inspected with respect to component type (laminate or sandwich), component

thicknesses, type of material, surface finish ( tool side & bag side or both tool side), secondary bonding medium, internal structure (fittings), fabrication & curing processes.

4.2 The Reference standard(s) shall contain artificial discrepancies as per the minimum defect dimensions stated in the Acceptance Criteria and which would produce a response for the normally encountered defects like voids, delamination, disbond, inclusions (FOD), voids, and porosity. The defect shape may be triangular or circular or square / rectangular. Records shall identify that standards are free from natural material discontinuities that would make the reference standard unsatisfactory for use.

4.3 The reference standard shall represent full range of part geometry (i.e minimum to maximum thickness). Multiple standards shall be used to represent the full range of defect sizes, if not possible to demonstrate in a single reference standard.

4.4 Ultrasonic reference standard shall have near surface and far surface reference defects to establish near surface resolution and far surface resolution and shall contain defects to assure sensitivity requirements (i.e minimum acceptable defect size) as per the acceptance criteria. 5. Manufacturing Aspects of Fabrication of Composite Reference Standard

The following points shall be considered during fabrication of composite reference standard:

5.1 Identify the Type of Reference Standard

First step in fabrication of any composite reference standard is to identify the type of reference standard required and its applicability. The reference standard shall represent the design configuration and complexity of the component to be inspected. The selection of the reference standard type should consider the following points:

(i) Its configuration should be Laminate composite or Sandwich composite (ii) Its configuration should either be a replica of the component type or standard panel type (iii) Its applicable range with respect to part geometry (i.e. to coverage minimum to maximum

thickness) (iv) Its material type (exactly the same material as per part geometry or any other equivalent

material) (v) Its size for operator convenience (i.e. if it’s a replica of component: Full component or

partially component and if panel type: length, width and number of steps) (vi) Its surface finish required ( i.e. one tool surface & other bag surface or both tool surface)

Figure-1 specifies C-scan color plot of some typically reference standards type:

Component Type Panel Type Laminate Type Sandwich Type

Figure-1

5.2 Identify the Type of Artificial Discontinuity

The Reference standard(s) should contain artificial inclusions which would be capable to produce a response similar to that produced by typical defects like voids, delamination, disbond, inclusions (FOD), voids, and porosity. The artificial discontinuity should create sufficient acoustic impedance mismatch so that sound can be reflected from the discontinuity surface and can be detected. The artificial discontinuity (i.e Inclusion or FOD) shall be selected such that it either creates a solid-air interface to reflect large amount of sound or shall create a higher attenuation zone.

To identify the right type of artificial inclusions, a number of composite panels need to be fabricated by simulating various FODs - of various thicknesses at different depth levels. Multiple iterations of ultrasonic testing are then carried out to establish the relationship between ultrasonic attenuation level and different types of inclusions.

The following steps can be followed for selecting the artificial discontinuity with respect to ultrasonic attenuation level. (i) Listing out some of the FOD items can be used as inclusion in composite reference standard as shown in Figure-2

1. Release Film 2. Bagging Film 3. Prepreg Cutting Blade 4.Prepreg Protective Film-BD

5. Brass sheet 6. Tooltech Tape 7. Flash Breaker 8.Prepreg Protective Film-UD

Figure-2 (ii) Fabricating an NDT panel including various FOD items at different ply level, to simulate the defect conditions as shown in Figure-3

A typical Carbon & Glass Panel with FODs nearer to Tool Side & Bag Side during fabrication

Figure-3

(iii) Establishing relation between ultrasonic attenuation level with various types FODs in Composite by ultrasonic A-Scan & C-Scan as shown in Figure-4 & Figure-5

Ultrasonic A-Scan PE Method (5.0 MHz) Ultrasonic A-Scan TTU Method (5.0 MHz)

Figure-4

Ultrasonic C-Scan TTU Method

(1.0 MHz) Ultrasonic C-Scan TTU Method

(2.25 MHz) Ultrasonic C-Scan TTU Method

(5.0 MHz)

Figure-5

5.3 Defining size and location of artificial discontinuity

The reference standard(s) must contain artificial defects / discontinuity of dimensions that meet the requirement of Acceptance Criteria.

The reference Standard(s) shall: (i) Have discontinuities at all possible locations covering the full range by having the required sizes and types of inclusions placed in radii, at bond lines, at chamfers and in flat areas, (ii) Incorporate inclusions at minimum 3 locations for laminate construction i.e. minimum, midrange and maximum thickness of the representing parts and at 2 locations for sandwich construction, i.e between core and top skin & between core and bottom skin of the representing parts, (iii) Have reference defects to assure sensitivity requirements (i.e maximum acceptable defect size or minimum unacceptable defect size dimension) as applicable to the acceptance criteria, (iv) Contain defects of various shapes i.e triangular, circular, elliptical, square, and rectangular, (v) Have near surface and far surface reference defects to accurately establish near surface resolution and far surface resolution as applicable to the acceptance criteria or better.

5.4 Preparing detail drawings / sketch of the reference standard

A detailed drawing for the reference standard should be prepared on which following information would be specified:

(i) Type of reference standard and its identification number, (ii) Applicable material list with cure ply thickness of each ply & honeycomb core thickness, (iii) Complete dimensions in terms of length, width & thickness, (iv) Number of ply for each thickness configuration and applicable ply number, (v) Applicable Lay-up plies orientation, (vi) Applicable manufacturing curing cycle, (vii) Information regarding types, sizes & depth location of artificial defect to be incorporated, (viii) Dimensions between two artificial defects and dimension with respect to edge of panel, (ix) Applicable Tool side & Bag side of the panel, (x) Excess material in all the sides for final trimming.

Figure-6 specifies typically Drawings of a reference standard

Figure-6

5.5 Preparing the detailed manufacturing process plan / lay-up scheme

A detailed manufacturing process plan should be prepared with respect to the drawing of the reference standard and it would include the following:

(i) A detailed route sheet / scheduling sheet, which shall define various work centers and various manufacturing & inspection operations as applicable like: issue of raw material, tool preparation, material cutting and ply identification, ply lay-up, intermediate compaction, vacuum bagging, curing, demoulding, final trimming / machining, visual & dimensional check, NDT checks and final tagging / accepting.

(ii) A detailed lay-up scheme specifying ply number, ply orientation, ply dimension, ply material, defect number, defect material, defect dimension, compaction schedule etc. in addition to the route / scheduling sheet.

(iii) A detailed manufacturing curing cycle specifying: Heating rate, Cooling rate, Dwell temperature, Dwell Time, applicable vacuum, applicable pressure etc. & all other supporting documents like: various process work instructions, copy of drawings, Copies of Do’s & Don’ts etc.

5.6 Preparing detail defect dimension report before lay-up

As it is quite a challenge to create artificial discontinuities to the exact size as per drawing requirements, it is advisable that the before manufacturing of reference standard all the artificial discrepancy dimensions shall be measured and recorded, so that during ultrasonic scanning of the reference standard the dimensions shall be verified for its accuracy and reference standard shall be certified or validated. It is also necessary to verify the defect / discrepancy dimensions during system standardization of the equipment. The defect dimensions report shall reflect all defect dimensions in 00 Direction & 900 directions along with the thickness of the defect.

Below Figure-7 (Table-1) specifies a typically lay-up scheme & Figure-7 (Table-2) specifies a typically defect dimensions report:

Table-1 Table-2

Figure-7

5.7 Documenting & certifying the detail manufacturing process of reference standard

During manufacturing phase of the reference standard, it is the responsibility of NDT personnel & QA personnel to ensure all the process documents are verified and certified for its compliance. The following should be documented and certified during fabrication stage: (i) Material Details: Lot Number, Sub-lot / roll Number, Shelf life Expiry, Out life as applicable, (ii) Cutting of all the plies as per lay-up scheme or drawings and identification of the same with ply number, (iii) Tool / Template condition, its identification and tool preparation for lay-up, (iv) Layout marking (orientation, reference points, dimension scaling in X & Y direction), (v) Lay-up of the plies and artificial discontinuity as per lay-up scheme, (vi) Intermediate compaction of the plies with sufficient vacuum if applicable, (vii) Final vacuum bagging and leak check, (viii) Curing details from cure chart (i.e Temp, Dwell time, Heating & cooling rate, pressure, vacuum etc.), (ix) Demoulding of the part after curing, (x) Final trimming and edge sealing as applicable, (xi) Destructive testing of coupons to certify curing process if applicable.

5.8 Preparing the detailed Visual & Ultrasonic scan report

One of the important phases in the fabrication of an ultrasonic reference standard is to prepare a detailed Non-destructive inspection report for the standard. The NDT report shall contain: Visual, Ultrasonic A-Scan & Ultrasonic C-Scan as applicable.

Visual inspection should be carried out on the reference standard to verify the standard is free from edge delamination, surface irregularity, marks (or dents), wrinkles, resin rich, resin starvation etc which might affect the inspection result.

Ultrasonic A-Scan shall be carried out to confirm the ultrasonic response from the artificial discontinuity with respect to Defect size, Defect depth location and delectability. Ultrasonic A-Scan pulse echo method shall be carried out from tool surface to confirm that the standard complies with the drawing requirements. For near surface resolution Lucite shoe or delay line transducer can be used if required. Attenuation in laminate reference standard shall not vary more than 2 to 3 dB across regions of same thickness.

Based on the availability of facility and complexity of the reference standard, Ultrasonic C-Scan shall be carried out to verify the ultrasonic response from the artificial discontinuity with respect to defect size and the ability of being detected. Whenever there is any difficulty in interpreting the C-Scan color plot due to intermixing of colors, C-Scan grey plot shall be preferred to confirm the discontinuity.

Figure-8 specifies typically A-Scan & C-Scan report of a reference standard:

Reference Standard

Photo

Ultrasonic A-Scan reporting of artificial discontinuity with detail dimension & location of defect

C-Scan Grey Plot C-Scan Color Plot A-Scan echo response

Figure-8

5.9 Preparing the detailed dimensional report & identification of the reference standard

The detailed dimensional report of the reference standard should be prepared as a part of the inspection report and should be maintained for the reference standard. The detailed dimensional should be prepared after the final trimming operation so that the reference standard is of the exact size. The dimensional report should reflect the reference standard dimension as well as defect dimension and its position from reference points.

If the reference standard is not identified with a suitable identification number during fabrication, then the identification shall be done either by painting on the surface, a hanging metal tag by drilling a hole on it or sticking any other suitable name plate by wet lay-up method. The identification shall be placed only at locations where no artificial discontinuity place. Figure-9 specifies typically dimension report of a sandwich reference standard:

Figure-9

5.10 Validating the reference standard for use

Finally the reference standard should be validated and certified by NDT Level-3 personnel. Validation of reference standard can be performed in various ways to verify sensitivity, resolution (Near surface & Far surface), minimum unacceptable defect size as per quality grades and compliance of the reference standard with drawing requirements. The validation can be performed with one or more NDT techniques as specified below

(i) Ultrasonic A-Scan Pulse-Echo Method to validate Ultrasonic A-Scan Through-Transmission method or vice versa

(ii) Ultrasonic C-Scan Method to validate Ultrasonic A-Scan Method (iii) Radiographic CT-Scan or any other NDT Method to validate Ultrasonic C-Scan Method

Figure-10 & Figure-11 specifies typically method of validating a reference standard:

Verification of Sensitivity & Resolution on a reference standard by Ultrasonic C-Scan

Verification of Near Surface & Far Surface Resolution on a reference standard by Ultrasonic C-Scan

Verification of a Stepped Laminate Thickness calibration reference standard by Ultrasonic C-Scan

Figure-10

Scout View Higher attenuation indication at

De-lamination Zone

Typical CT-Scan Result of defect area Typical C-Scan Result of defect area Validation of Ultrasonic C-Scan Results By Radiographic CT-Scan

Figure-11 6. Conclusion

The responsibility of the fabrication of composite reference standards solely lies with the manufacturer and NDT personnel involved in the process. During fabrication of composite reference standards, several factors shall be considered starting from design, material selection, manufacturing processes like lay-up and curing, NDT inspection methods, minimum defect size, defect location at various depths, defect types etc. to achieve a good NDT reference standard. All the necessary data with respect to the manufacturing process needs to be generated by the composite manufacturing organization by introducing various artificial inclusions of various sizes at various depths to simulate the actual defect occurrence during manufacturing. Documenting all the manufacturing aspects will standardize the reference standards and validate the composite manufacturing process during any new development, modification and periodic evaluation.

7. Reference 1. AMS CACRC Commercial Aircraft Composite Repair Committee, “Composite Honeycomb NDI

Reference Standards”, SAE Aerospace Recommended Practice ARP 5606, Dec 2011 2. AMS CACRC Commercial Aircraft Composite Repair Committee, Composite Repair NDT/NDI

Handbook," SAE Aerospace Recommended Practice ARP 5089, Nov. 2011 3. Yolken H. Thomas and Matzkanin George A., “Nondestructive Evaluation of Advanced Fiber

Reinforced Polymer Matrix Composites”, February 2009 4. Roach Dennis & Dorrell Larry, “Development of Composite Honeycomb and Solid Laminate

Reference Standards to Aid Aircraft Inspections”, NDT.net - March 1999, Vol. 4 No. 3, 5. Galella Dave, “FAA Inspection Administration Research Activities for Composite Materials”,

Composite Damage Tolerance & Maintenance Workshop, ATO-P, July 2006 6. Oster Reinhold, “Non-destructive testing methodologies on helicopter fiber composite

components challenges today and in the future”, World Conference on Nondestructive Testing, April 2012

7. Martin R, “Ageing of Composites", Woodhead Publishing Ltd, 2008 8. HSU David K., “Nondestructive Inspection of Composite Structures: Methods and Practice”,

World Conference on Nondestructive Testing, Oct 2008 9. Gower, M., Sims, G., Lee, R., Frost, S. and Wall, M., Measurement Good Practice Guide No. 78

“Assessment and Criticality of Defects and Damage In Material Systems," National Physical Laboratory, Teddington, Middlesex, United Kingdom, June 2005.