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Linhas areas InteligentesCOMPOSITE-NDT

DAMAGE EVALUATION

AND NDT

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DAMAGE TO COMPOSITE STRUCTURE

GENERALWhen damage is discovered on a composite structuralpart, and before any further repair work, a completeinvestigation of the affected area is to be carried-out. Theinvestigation of damage is done using the related chaptersand pages of the SRM (Structural Repair Manual) in order

to determine whether the damage is repairable or not, andwhat is the applicable type of repair.

DAMAGE DETECTIONDamage can be discovered during scheduled inspectionsrequired by the maintenance program, or in un-scheduledinspections when the part has been subjected to accidental

damage.

DAMAGE EVALUATIONA complete inspection of the damaged area or componentwill give the required information concerning the extent andthe type of damage. According to the type, extent and the

importance of the affected zone, the determination of thedamage acceptance level can be conducted.

ACCEPTANCE LEVELIn order to define the applicable repair type and itsassociated limits (time and size), it is necessary todetermine first, whether the damage is allowable,

repairable or not repairable.

The acceptance level of damage is determined usingthe graphics and instructions contained in the affectedcomponent related SRM chapter (52 thru 57) andallowable damage section of the Structural RepairManual.

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MPD

SRM

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DAMAGE CLASSIFICATION

For composite surfaces, as per the Structural RepairManual, damage is split into two main categories: skin notperforated and skin perforated damage.

Skin not-perforated damage includes:

abrasion, scratches, gouges, nicks, debonding, delamination, dents

Skin perforated damage includes lightning strike holes accidental impact of foreign object and needsinvestigation for delamination moisture contamination.

EXTENT OF DAMAGEClose visual and nondestructive testing methods such astap-testing, ultrasonic and X-Rays are used to determinethe size of the damages. For delaminated/debonded areadetermination, a minimum inspection area is defined. Incase of indication, the inspection area must be extended

until the limits of the affected zone are located. Theacceptance level of damage is determined using thegraphics and instructions contained in the affected

component related SRM chapter (52 thru 57) andallowable damage section of the Structural RepairManual.

SURFACE ZONESAs damage is not of the same significance in each area

of the component, each composite surface of theaircraft is divided into zones of different structuralimportance. For each component the correspondingzones are given in the related SRM chapter (e.g. spoilerSRM chapter 57) section allowable damage

ALLOWABLE DAMAGE

For each of the defined zones, a graphic is to be usedto determine allowable damage limits, recommendedrepair types and repair associated limitations. Damagetype and dimensions as well as initial thicknesses haveto be known to select and work with these graphics.Visual inspection is the principal method for damage

detection. Delamination or disbonding can be caused byan impact, an abnormal loading or an undetectedmanufacturing defect.

NOTE:Such damages is not always visible on the materialsurface. The compression strength of the component is

affected and water or fluid ingress is very likely to occur.

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SCRATCHES IGOUGESA scratch is due to contact with a sharp object, only

surface fibers are affected. While a gauge is wider anddeeper than a scratch, several plies are affected, but theedges of gauge are generally smooth.For scratches, ingeneral only a surface restoration is necessary to preventany water or fluid ingress.Gouges have consequences on the structural strength and

have to be repaired by removing the damaged plies andperforming a hand lay-up.

ABRASIONAbrasion is a surface damage due to scuffing, rubbing orscraping of the component. Fibers are not damaged andmechanical performances are not affected. Abrasion

damage is repaired by restoration of the surface protection,in order to avoid any water or fluid ingress.

CORROSIONGalvanic corrosion may occur when an aluminium alloypart is in direct contact with carbon fiber surface in

presence of a corrosive environment. In that case it is thealuminium alloy part which corrodes (fitting, lightning strikeprotection straps, .) and which has to be replaced orrepaired if possible.

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LIGHTNING STRIKE DAMAGE

Carbon Fiber is a conductive material while Glass orAramid fibers are non- conductive materials. The effect of alightning strike will not be the same. For non-conductivematerials (Glass, Aramid) a large part of the component, ifnot completely protected, may be blown-out because bothskins are affected and the core generally vaporised due to

the extreme heat.Damage on Carbon fiber structures will be less significant(spots, small holes, or charring).

EROSION

Erosion could affects all the leading edge surfaces,especially when initial surface protection system has firstbeen damaged.Erosion, when undetected or unrepairedmay generate composite deterioration, the component maybe completely perforated and water or fluid ingress arelikely to occur. Restore the protection in the area, install

additional protection if necessary.

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WATER ABSORPTIONAny detected moisture has to be dried to avoid further

damage.During any repair procedure, make sure that repair partsare completely dry, in order to avoid any materialdelamination during heat application.Water ingress insandwich structures is due to porosities in the skin. Itreduces performances and increases the weight of the

affected structure.Water absorption is a phenomenon dueto resin properties. The absorption stops when the resin issaturated.

CHEMICAL DEGRADATIONChemical degradation, principally affects the resin and isgenerally due to accidental contact with agressive chemical

liquids or products.ln case of chemical degradationdetection, the whole contaminated area must be repaired.

DENT IDEPRESSIONA depression or a dent is a deformation of the area in thedirection of thickness. It may be caused by impact ordepression.This type of defect requires further NDTinvestigations to detect delamination or debonding. Onsandwich structure the honeycomb is generally damagedand requires a repair.

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NON DESTRUCTIVE INSPECTION (NDI) TECHNIQUES

A variety of NDI techniques are available as inspectiontools for documenting manufacturing and service-relateddefects in composites. However, as in metallic structures,no single nondestructive inspection method can locate andisolate all defects. The equipment and techniques utilizedin the nondestructive inspection of composite structures

ranges in complexity from the use of a coin tap test to theuse of lasers in holographic equipment. A list of NDItechniques, proven to detect defects in composites, ispresented below:

Tap testing Visual or optical inspection

Bondtester or Resonator Penetrant Thermography Holography Acoustic Emission Ultrasonics Radiography

Among the listed techniques, the most commonlyused methods are ultrasonics (pulse echo or throughtransmission) and radiography (xray).

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TAP TESTING

Tap testing is widely used for a quick evaluation of anyaccessible aircraft surfaces to detect presence of disbonds.The tap testing procedure consists of lightly tapping thesurface of the part with a coin, light hammer, or othersuitable objects. The acoustic response is compared withthat at a known good area. A flat or dead response is

considered unacceptable. The acoustic response of a goodpart can vary dramatically with changes in geometry, inwhich case a standard of some sort is required. The entirearea of interest must be tapped. The method is limited tofinding relatively shallow defects. In a honeycombstructure, for example, the far side bondline cannot beevaluated. Thus, two-side access is required for a

complete inspection of honeycomb structures. The methodis portable, but no records are produced. The method isalso highly dependent on the inspectors subjectiveinterpretation of the test response.

VISUAL OR OPTICAL INSPECTION

Visual or optical inspection procedure are used for a quickassessment of the aircraft surface conditions for presenceof flaws or damage.

Visual inspection is inexpensive, easy, and fast. However,

if the inspected material is not transparent, like composites,it is only capable of finding flaws that are evident on thevisible surface.

Internal flaws in composites, such as delaminations,disbonds and matrix crazing are not detectable.In addition, tight surface cracks and edge delaminationsmay not be detectable. Visual aids such as mirrors,borescopes and magnifiers are portable, and may beused to facilitate detection. Visual inspection results

may be recorded in the form of photographs, if desired.The surface to be inspected should be clean and free ofconditions that may mask or obscure defects.

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ULTRASONICS (CONT)

the back surface of the composite part. The attenuation ofthe reflected pulse is influenced by the presence of internaldefects in the part, while the time delay of the reflectedpulse is related to the depthwise location of defects in thecomposite part.

Composite parts suspected of containing defects that areopen to the outer surfaces should be handled with care. Inthese situations, water may infiltrate into the part throughthese defects, and make it difficult to detect the flaws. And,water contamination of a honeycomb core in sandwichconstructions may lead to serious corrosion problems.Reference standards are required to calibrate the

ultrasonic test equipment, and the inspectors should beexperienced in operating them.

RADIOGRAPHY

X-ray inspection technique is widely used for quick and lowcost inspection of composite structures. X-ray equipmentcan be adopted to handle from small to relatively largeparts.

X-rays are a form of electromagnetic radiation like light.They are produced when electrons, traveling at a high

speed, collide with matter or change direction. In X-raytubes, the electrons are accelerated by a difference ofpotential between the cathode (source of electrons) andthe target.

Gamma rays are similar to X-rays in their characteristics,and are emitted from the disintegrating nuclei ofradioactive substances like radium and cobalt 60.

Both X-rays and gamma rays have extremely shortwavelengths that provide them the ability to penetrate

materials that absorb or reflect ordinary light.Radiography is a non-destructive test method that usesX-rays or gamma rays.

A radiograph is a photographic record produced by thepassage of these rays through a test object onto a film.The radiation proceeds in straight lines to the object;

some of the rays pass through the object and the othersare absorbed. The amount transmitted depends on thenature of the material and its thickness.

The presence of a void in the material, for example,causes more radiation to pass through the sectioncontaining the void than through the surrounding region.

A radiograph is like a shadow picture- -the darkerregions on the film represent the more penetrable partsof the test specimen, and the lighter regions are moreopaque to the radiation.

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SRM-5

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PREPARATION BEFORE REPAIR

Before any repair action, it is necessary to make a correctsurface preparation of the repair area. This will ensure themaximum bonding strength and durability.

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REMOVAL OF SURFACE PROTECTION

All the surface protection (including paint and primer) is tobe removed using mechanical methods only. Chemicalstrippers may cause deterioration of the resin.

WARNING:

Do not use chemical strippersDo not erode laminates

NOTEDo not apply water directly to the surface, wet the cloth

first.

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CLEANING AND DEGREASING

The repair surface needs to be cleaned and degreased, inorder to ensure the good bonding of the repair.

NOTE : Do not apply cleaning agent directly on the repairsurface, wet the cloth before.

WARNING : Cleaning agents are dangerous.

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WATER BREAK TEST

The correct cleaning and degreasing of the repair surfaceis to be checked by a water break test.The test consists of spraying a thin layer of demineralizedwater on the repair surface, and checking that the waterremains in a thin continous coating.

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WET LAY-UP

The wet lay-up consists of applying several layers of fabricmaterial impregnated with resin onto repair surface, inorder to restore the damaged skin.

Previous to the lay-up operation the fabric material is handimpregnated with resin or adhesive. Repair layers (plies)are up on the surface following a specified sequence andthe cured at Room Temperature (RT).

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APPLICABLE WET LAY-UP SEQUENCES

There are two main options which be used for the lay-upoperation. The applicable one is generally specified in therepair instructions.

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VACUUM BAGGING

The technique of vacuum bagging is used in advancedcomposite repair for several reasons:

To use atmospheric pressure to consolidate the layers inthe laminated repair To remove trapped air and gases produced during curingTo hold heater blankets, thermocouples and other

materials in place during the curing cycle To produce the desired surface finish on the repair

There is no universally accepted method of vacuumbagging, aircraft manufacturers have a variety of methods,but you should be aware of the purpose of each of the

materials used. If you understand why each layer is usedyou will not have to remember how to bag up a repair foreach type of aircraft.

BAGGING PRINCIPLES

Two main principles must be obeyed when vacuumbagging a repair:

The air and gases inside a vacuum bag must have aneasy path to the vacuum pump over the entire repairsurface

The amount of resin lost from the repair plies must bekept to a minimum

Additionally it is necessary to ensure that none of thebagging materials adhere to the repair and that theheater blanket is protected from resin contamination.

The figure below shows a typical vacuum baggingsequence specified by Boeing.

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INSULATIONInsulation in the form of breather fabric or glass cloth is

normally placed on top of the heater blanket to reduceheat losses and to minimise the effects of draughts onthe repair. It is often recommended that extra insulationon the outside of the vacuum bag is used to even outtemperature variations on the repair surface.

BAGGING FILM

This is a nylon film which is sealed over the repairmaterials so that a vacuum can be applied. It is availablein a variety of temperature resistance and elongation.The higher temperature resistant films normally stretchless.

BAG SEATING TAPEAlso known as bagging tape, extruded sealing compoundThis material is a high temperature mastic type of tapeand is used to seal the vacuum bag to the surface of theitem under repair. It is available in a number oftemperature ranges.

CAUL PLATEAlso known as a pressure plate, pressure intensifier.

This is a thin metal plate which is place between thebreather fabric and the heater blanket. Its purpose is

To help to even out the temperature over the entire repair To help to even out the pressure over the whole repair To improve the flatness of the surface of the repair.

Caul plates are suitable for flat or single curvature panels,but not for complex shapes, where special tooling shouldbe used instead.In general thin caul plates (

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VACUUM BREECH UNITSAlso known as through bag units, vacuum probes, sniffers.

These are specially designed connectors which passthrough the vacuum bag and make a seal, so that the pipeto the vacuum pump can be connected. There are severaldesigns.

Two breech units are normally used on a repair. One to

remove the air from the vacuum bag and one to record thelevel of vacuum inside the bag. It is important that thebreech units are placed on opposite sides of the repair, areplaced directly on to breather fabric, and are not placeddirectly over any wires or heater blankets.

THERMOCOUPLESAlso known as T/Cs, temperature probes.

A thermocoup]e consists of two wires, made from specialalloys, which are welded together at one end. When thewelded portion is heated it produces a small electricalcurrent. By connecting the thermocouple to a hot bonder

they can be used to record and control the temperature ofa repair.

Thermocouples come in a variety of types, depending onthe alloys used and are distinguished by letters:J type, K type etc.

The most commonly used thermocouples for hot bondingare J type.

A number of thermocouples should be place around therepair to record and control the temperature accuratelyduring curing. They should be placed near to, but not on,the repair. Boeing recommend the use of up to eightthermocouples for a repair.

Bad thermocouple connections are the most common

cause of problems during the cure of a repair.

FLASH BREAKER TAPEAlso known as flash tape, pressure sensitive tape, hightemperature tape, PS tape.

This is a high temperature tape which can be used for alarge number of functions inside the vacuum bag. It willnot be affected by high temperatures. It is the only typeof tape which should be used on the surface of theheater blanket.

COMPOSITE NDT

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THERMOCOUPLE MAPPING

Each cure cycle contains at least one ramp stage anddwell (soak) stage. The temperatures must be carefullycontrolled during these stages and maintained betweenpredefined limits. Most modern hot bonders allow you todefine upper and lower tolerance limits for each stage ofthe cure cycle and will sound an alarm or shut down if thelimits are exceeded.

In ideal conditions all points beneath a heater blanket willheat up at the same rate and maintain the same dwelltemperature, but in reality there will be a variation oftemperatures across the component. A variety of factorscan lead to these variations, the most common being:

Variations in thickness of the component Ribs, bosses etc. External draughts Inaccuracy of thermocouples Cold or hot areas within the heater blanket Leaks within the vacuum bag

Lack of insulation in some areas of the repair

There is a trend for manufacturers to call for a number ofthermocouples to be placed around the repair to monitorthese variations during repair of their components. Boeingrepair documents, for instance, requires the continuousmonitor and control of eight thermocouples during metal tometal bonded repairs.

CONTROL OF VARIATIONS IN TEMPERATURE

In most cases a repair will be heated by a single heaterblanket using a number of strategically placedthermocouples. Since the whole area of the blanket iscontrolled together there is no facility to increase ordecrease the heat input to different areas of the repair.

Control of local temperatures can only be achieved by

increasing or decreasing heat loss from the area byaddition or removal of insulation. Thus an accurate mapof the placing of the thermocouples in a repair isessential so that when a thermocouple is recording ahigh or low temperature the exact location can beidentified.

Always make a habit of recording the positions andnumbers of the thermocouples in a repair. This can bedone on a separate map or on the vacuum bag itself.

COMPOSITE-NDT

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VACUUM BAGGING

The technique of vacuum bagging is used in advancedcomposite repair for several reasons:

To use atmospheric pressure to consolidate the layers inthe laminated repair To remove trapped air and gases produced during curingTo hold heater blankets, thermocouples and other

materials in place during the curing cycle To produce the desired surface finish on the repair

There is no universally accepted method of vacuumbagging, aircraft manufacturers have a variety of methods,but you should be aware of the purpose of each of the

materials used. If you understand why each layer is usedyou will not have to remember how to bag up a repair foreach type of aircraft.

BAGGING PRINCIPLES

Two main principles must be obeyed when vacuumbagging a repair:

The air and gases inside a vacuum bag must have aneasy path to the vacuum pump over the entire repairsurface

The amount of resin lost from the repair plies must bekept to a minimum

Additionally it is necessary to ensure that none of thebagging materials adhere to the repair and that theheater blanket is protected from resin contamination.

The figure below shows a typical vacuum baggingsequence specified by Boeing.

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