- Apr 15, 2010
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1 Composites Inspection and Repair Steve Kane Adjunct BCC Aerospace Technology In conjunction with SpaceTEC
2 Topics Introduction Composites Review Characteristics Materials Fabrication Techniques Composites Safety Shop Safety Inspection Damage Identification Repair Techniques
3 Outline Introduction Composites Inspection and Repair Day 1 PowerPoint Lecture: Composites Characteristics, Materials, Fabrication Techniques, Composites Safety, Shop Safety, Inspection, Damage Identification, Repair Techniques Lunch Lab Project 1 Composite Sandwich Construction Day 2 Review, Project Assessment Lab Project 2 Damage Inducement, Core Repair, Scarf Repair Technique Lunch Exam, Discussion, Certificate
4 Introduction Purpose: Inspection and Repair Workshop Follow-on to Introduction to Composites Provide knowledge of and a practical application for inspection of composites and repair of damages Intended to be advisory in nature Repair data provided by OEM manufacturers, qualified engineers, and/or regulatory agencies must be consulted in any inspection and/or repair method
5 Composites Review
6 Definition of Composites From Introduction to Composites: What are Composites? Two or more materials working together Each contributes its own structural properties Each retains its unique identity Two major components: Reinforcing Fiber ceramic strong but brittle Matrix (glue) plastic tough yet flexible
7 How They Work How do they work? Individually: Fiber reinforcement strong, stiff, but brittle Matrix tough, elastic, but lacks rigidity + = Together = strength, stiffness and toughness, a superior material!!
8 Types Three Basic Types: Natural occurs in nature Wood Artificial produced from naturally occurring materials to improve on natural Adobe Plywood Concrete Synthetic produced from artificial materials to improve on artificial Fiberglass Graphite (Carbon) Fiber Aramid (Kevlar)
9 Characteristics, cont. Specific Strength:
10 Advantages, cont. Specific Stiffness:
11 Advantages Advantages of Composites: Do not corrode Smooth surfaces relatively easy to achieve Many manufacturing methods available: Hand layup, Vacuum bagging, Resin Transfer Molding (RTM), Pultrusion, Filament winding, Resin infusion High strength/stiffness-to-weight ratios possible 4-10 times that of metals Structures can be tailored to meet specific applications Orienting fibers to carry specific loads
12 Limitations What are their limitations? Fiber reinforcement prone to cracking Brittle nature means little or no elasticity Sudden failure at yield point Matrix mitigates brittleness Works as stop-drill would in aluminum Not much chance all cracks will line up Shear forces bog down Bicycle Analogy Solid ground moves freely Softer ground more resistance
13 Disadvantages Disadvantages: Labor intensive Special training needed Raw materials expensive Manufacturing equipment expensive Health and safety concerns for materials Cannot be recycled Little or no warning before failure Some materials may not be compatible with metals Galvanic Series Carbon (noble) and aluminum (active)
14 Weave Patterns Plain Weave One over and one under every other yarn Provides fabric stability The least pliable, least strong weave Basket Weave Similar to plain but two warp yarns over and under More pliable and stronger than plain Crowfoot (Four-Harness Satin) More pliable, easier to form on compound curves Three yarns over and one yarn under
15 Weave Patterns Five-Harness Satin (5HS) Similar to Crowfoot but one more filling yarn, 4 over and 1 under More pliable than crowfoot Eight-Harness Satin (8HS) Similar to 5-harness except one yarn floats over seven and under one Very pliable weave and has good drape Great for compound curve surfaces Most expensive weave
16 Weave Patterns
17 Weave Patterns
18 Textile Terminology Warp Direction Parallel to the Long direction of the roll Also defined as parallel to the selvage edge Fill Direction 90 degrees to the Warp direction Selvage edge to selvage edge Sometimes called Weft direction Selvage Edge Tightly woven edge to prevent edge raveling Parallel to warp threads Bias A 45 degree angle to the warp threads Fabric can be stretched along the bias but seldom along warp
19 Textile Terminology
20 Textile Terminology
21 Fiber Orientation Advanced Composites are by design Orientation of fibers is proportional to properties The more fibers in a given direction, the stronger and stiffer Unidirectional (0) degrees for tension, compression, or bending Bidirectional (+/- 45s) degrees for shear Control fiber angles +/- 2 degrees Sandwich Construction Enhances performance by placing the load carrying fibers on the outside of the part
22 Common Layup Terms Symmetry A laminate in which all of the ply orientations are symmetrical about the mid-plane of the laminate
23 Common Layup Terms Balance A Balanced laminate has equal numbers of + and angled plies
24 Common Layup Terms Quasi- Isotropic A laminate laid up with an equal number of plies at 0, +45, -45, and 90 angles
25 Common Layup Terms Nesting vs. Stacking Placing plies so the fibers of one ply align with the yarns of the adjacent ply Only possible with harness-satin weaves
26 Shop Safety
27 Environment Working Environment: Good housekeeping directly impacts safety Keep area neat & orderly Properly dispose of mixing containers Keep fabric remnants swept up Wipe up spills, keep tools clean Do not block access to safety equipment
28 Personal Safety Personal safety while sanding or drilling: Respirators must be worn Wear shop coat to minimize particles entering pores of skin Use eye protection Always shower at the end of the day after working with composites
29 Shop Safety Compressed air in the shop area Moisture is your enemy! Check moisture traps often Air Tools Flexible lines can take on a life of their own if unsecured Disconnect tools from air supply before changing cutters, sanding discs and drills Point the exhaust away from other people Never blow surfaces with compressed air! Causes projectiles Can cause delaminations Use a brush or vacuum for cleaning parts, machines, and work tables
30 Tool Safety Tool Safety Use Eye Protection Safety glasses with side shields are a must Wear dust mask when cutting, drilling, sanding NIOSH-rated Cutting When Cutting: Keep hands, fingers away from cutting surfaces Razor Knives Very sharp, use caution
31 Tool Safety Tool Safety, cont. Drilling When Drilling: Back up materials Dont use your hands! Always know what is behind Never force drills! Will cause breakouts on other side Can disbond laminate-to-core interface Use high speed, low pressure Let the bit do the work!
32 Tool Safety Tool Safety, cont. Sanding When Sanding: Wear eye protection/dust masks Work in a suitable area: Down-draft tables Exhaust systems Clean up debris: Clean up before tracking around Wash hands before eating or using the restroom!
33 Sandwich Structures Sandwich structures: Combination of strong, thin skins and a relatively light core material Very efficient structures with high stiffness-to- weight ratios Also called honeycomb Chief purpose of core: Passes shear forces between the skin surfaces Allows substantially improved structural properties in thicker sections with only slight increase in weight
34 Sandwich Structures Typical sandwich structures:
35 Sandwich Structures Disadvantages of sandwich construction: Sandwich structures have thin skins that can be easily damaged by even minor impacts. Susceptible to moisture intrusion: Can cause unintentional weight gain Freezing may cause disbonds (if subjected to lower temperatures of higher altitudes) If core becomes contaminated with oil, fuel or hydraulic fluid, it is virtually impossible to remove completely and must be replaced
36 Sandwich Structures Honeycomb Form most commonly used in aerospace Made from Nomex (aramid paper), fiberglass, or aluminum Fire retardant, flexible and lightweight Offers best strength-to-weight ratio Cell shape Most common is hexagon, known as Hex Core Suitable for flat panels Difficult to curve
37 Sandwich Structures
38 Sandwich Structures Manufactured honeycomb core: Core direction is important: L is ribbon, strong direction W is weak direction
39 Sandwich Structures Over-expanded during manufacture A flattened hexagon Easily curved in ribbon (L) direction
40 Sandwich Structures Capable of compound curves Used for radomes and nose cones More expensive than Hex or Ox-Core
41 Sandwich Structures Foam Core Offers higher density than honeycomb Greater crush resistance Bonds to skin are less strong Must be cut and shaped to fit Common types Polystyrene Polyurethane Polyvinyl chloride (PVC)
42 Foam Core Polystyrene Foam Used extensively in sail and surf board manufacture Light (40kg/m3 ), inexpensive, easy to sand characteristics Low mechanical properties Rarely employed in high performance component construction Cannot be used with polyester resin systems Will be dissolved by the styrene present in the resin
43 Foam Core Polyurethane Foam Moderate mechanical properties Foam surface at the resin/core interface tends to deteriorate with age Leads to skin delamination Can readily be cut and machined to required shapes or profiles Structural applications limited to formers to create frames or stringers for stiffening components Used in lightly loaded sandwich panels Thermal insulation
44 Foam Core Polyvinyl Chloride (PVC) Foam Closed-cell construction Good mechanical properties One of the most commonly used core materials for high performance sandwich structures Good solvent resistance Will work with most available matrix materials Can be formed in compound shapes at elevated temperatures in vacuum mold
45 Foam Core Syntactic Foam Mixture of microspheres (miniature glass spheres), epoxy resin Mixture ratio controls strength and density Can use high-strength microspheres, toughened resins Can become heat-resistant with addition of high temperature resins Used primarily as a non-structural filler material Can be used in foam core repairs
46 Wood Core Balsa Wood Excellent structural core material Low cost, easy to use Can have moisture problems Can burn in a fire Used less in aviation due to FAA flammability requirements Used primarily in marine construction Inherent floatability
47 Shaping Cutting Foam: Hot Wire Cutting Easiest method for foam cores Care must be taken with smoke Band saw Can be used with miter for angled surfaces Razor knife Hand finish, trim work
48 Shaping Sanding: Lightweight material is easy to sand, shape Can use belt sanders, hand sanders Foam core materials can be sanded with another piece of like foam
49 Machining Composites For drilling, cutting, sanding or grinding of composite materials Do not use cutting fluids as fibers may absorb them Do not use same cutting tools on graphite and Kevlar Drilling and countersinking Delamination, fracture and breakout are types of failures
50 Machining Composites Delamination Peeling away of the bottom layer as the force of drill pushes the layers apart Fracture Occurs when a crack forms along one of the layers due to the force of the drill Breakout Occurs when the bottom layer splinters as drill completes the hole Separation Occurs when gap opens between layers as The drill passes through successive layers
51 Machining Composites The material being drilled should be backed with wood whenever possible to prevent these problems! When exiting the back side of a hole with a drill, very light or no pressure should be used. dont push drill through, Let It Cut! This will prevent delaminations.
52 Machining Composites Carbide drill bits will work on all types of composites. They also last longer than standard steel drills. Drill motor speed is important High speed works best Do not use excessive force Dagger or spade drills can be used. They reduce the tendency of the fibers to break rather than be cut
53 Machining Composites Hole Saws Use special Diamond Dust cutting edges Band Saws Special Diamond Dust or carbide blades 12 To 14 TPI
54 Machining Composites Fasteners Composi-lok Must be made of titanium or corrosion resistant steel Aluminum fasteners must not be used because of tendency to corrode the aluminum
55 Inspection and Damage Identification
56 Inspection Why Composites Inspection? Much larger percentage of Composites in the world today Need to detect discontinuities that may lead to premature failure Boeing 787 fuselage section
57 Composite Damage Composites fail in a different manner than metals: Catastrophically, with little or no warning! American Airlines Flight 587; Airbus A300-605R, November 12, 2001, on a flight from New Yorks JFK airport to Santo Domingo, Dominican Republic
58 Composite Damage Airbus A-300-308 aircraft experienced control difficulties shortly after takeoff. After returning to the origin airport, it was discovered that the majority of the Airbus' rudder had torn away from the vertical stabilizer.
59 Composite Damage Space Shuttle RCC Panel Testing, Southwest Research Institute (SwRI), San Antonio, TX May, 2003 Before Simulated Foam Strike: After Simulated Foam Strike: 1.7 lb. Bipod Ramp
60 Composite Inspection Two Classifications of Inspection Nondestructive Inspection Visual Ultrasonic Infrared Shearography Thermography Destructive Testing Coupon testing
61 Visual Simplest nondestructive technique for inspecting laminates Identifies surface imperfections: Impact damage (scuffing, chipping, surface cracking, or crazing) Near-surface delaminations (appear as bulges) Severe disbonding (damage appears white) With access to back side, illumination will make internal defects such as delaminations visible as dark or grey areas Main tool for visual inspections: Good light source Low incident-angle illumination
62 Visual Visual Guidelines: 1. Become familiarized by examining the applicable diagram/drawing. 2. If necessary, remove surface coat around damaged area. 3. Examine tactily by running hands over surface of suspect damage area to feel for surface imperfections and anomalies. 4. Dimple and dent damage is similar in appearance to hail damage on a metal surface. 5. Delamination and disbonding are more difficult to detect: Sometimes it is possible to feel this type of damage by pressing on the area. May feel soft and movement between the separated layers may be detected.
63 Visual Visual Guidelines, cont.: 6. Use a back light to reveal internal defects and delaminations. Examine exposed laminate for stress whitening. 7. If possible, the backside of the suspected area should be examined. 8. A borescope can be a helpful tool for examining interior areas. Interior surfaces are usually not painted and damage to glass-fabric structures will show up as a white area. 9. Use a Sharpie to mark suspect areas to facilitate a coin tap test. Note: Paint will generally crack before damage occurs in a laminate, therefore cracked paint does not indicate the extent of the damage, only that damage may have occurred.
64 Acoustic Manual Coin Tap Test Primitive Most common method for hidden damage Hearing-based, manual test practiced widely in the aircraft industry Used to determine laminate damage in a composite Acoustic sounds are produced when a small metal object is tapped on a surface Looking for clear, sharp sound Dull thud indicates a void or delamination
65 CATT Computer-Aided Tap Test (CATT) Automated: Quantitative and imaging capabilities added Removes "human factor" responsible for variation Magnetic cam-action cart provides equally-spaced uniform taps Simple encoding method gives imaging capability. Effective for both composite and metal honeycomb structures. Quantitative inspection results in the form of images that can be archived electronically. David K. Hsu, [email protected] http://faculty.washington.edu/scottcs/NSF/CNDE_14.pdf
66 Ultrasonic Three main methods of ultrasonic testing: Pulse Echo: High frequency sound waves are introduced into a material and are reflected back from surfaces or flaws. Through Transmission High frequency sound waves are introduced into a material by a transmitter on one side and detected by a receiver on the other. Pitch/Catch High frequency ultrasonic energy is transmitted at any angle to the surface of a material and received as reflected energy returning at the reflected angle.
67 Pulse-Echo Reflected sound energy is displayed versus time, and the inspector can visualize a cross section of the specimen showing the depth of features that reflect f sound. initial pulse crack back surface echo echo flaw Specimen 0 2 4 6 8 10 Oscilloscope, or flaw detector screen
68 Pulse-Echo Advantages: Can be performed with access to one side only Can detect disbonds and delaminations deeper inside f structure than tap testing Can give information about defect depth, down to which ply in many cases Limitations: Requires rather expensive portable equipment and a well- trained operator Difficult to cover large areas in a reasonable time More suitable for small areas Does not work well with core materials
69 Through Transmission This method is used for nondestructive testing of multi-layered laminates. f Transmitting Transducer Couplant Sound waves Material Receiving Transducer
70 Pitch - Catch Used primarily for cylindrical tubes and other nonlinear parallel sided surfaces, the pitch-catch method can determine depths of flaws in material as well as detect the location in the X-Y plane. f
71 Shearography Nondestructive inspection technique using interferometry to measure the phase difference between light waves traversing different paths Laminate disbonds Voids Basic Principles of Shearography http://www.shearography.com/
72 Thermography Nondestructive inspection technique using infrared light and an infrared camera to detect discontinuities Video image is taken as parts are heated and cooled Uses thermal differences to gather information about the part Core to laminate bonds Ply delamination 3-Dimensional Temperature Profile of an Impact Damage Zone on a Composite Pressure Vessel as Viewed by Infrared (IR) Thermography. Nondestructive Evaluation (NDE) of Composite Structures NASA-White Sands Test Facility http://www.wstf.nasa.gov/Enable/Composites/NDE.htm
73 Radiography High frequency, short wavelength radiant energy Excellent for metallic inclusions Effective for finding water trapped in honeycomb Not good for delaminations parallel to the plane of the X-ray image Expensive equipment and training Safety requirements
74 Damage Classification
75 Types of Damage Damage classifications: Cosmetic damage - a defect on the outer surface that does not involve the structural reinforcement (fibers). Usually only chips or scratches Impact damage - occurs if struck by a foreign object or from careless handling during transportation or storage. Penetrating - includes fractures and penetration through the laminate. Non-penetrating - includes abrasions, delaminations, surface impact, or gouges through one laminate surface.
76 Types of Damage, cont. Damage classifications, cont.: Solid laminate damage - structural or non-structural damage which extends beneath the surface protection and affects the solid laminate structure underneath. Three types: Delamination - separation of layers of material in a laminate. Cracks - can occur in advanced composite structures, just as in metallic ones. Hole Damage - may occur from improper drilling techniques, over torqueing fasteners, or as a result of pull-through.
77 Types of Damage, cont. Damage classifications, cont.: Secondary bond damage - occurs between two pre- cured components. Could be structural or non-structural in nature. Sandwich structure damage - damage to composite sandwich structures. Three types: Laminate only - damage to one side of the sandwich structure only with no core damage. Can be identified using the solid laminate damage classifications. Laminate and core - damage to one side of the sandwich construction and damage to the core. No damage to the opposite laminate surface. Sandwich penetration - damage to both sides of the sandwich construction. Both laminate surfaces are punctured and the foam core is exposed.
78 Repair Techniques
79 Preparation Once extent of damage is known: Prepare Environment: Make sure the repair area has adequate ventilation. The area must be completely enclosed. Control temperature and humidity. Min temp: 60 F (16 C); relative humidity > than 46% Max temp: 75 F (24 C); relative humidity < than 71% Isolate area from machining or other processes that generate dust, oil vapors, or other contaminants. Isolate adjacent surfaces that may come in contact with adhesives prior to application.
80 Repairs To ensure a sound repair: Know the ply lay-up information such as ply count and warp clocks If you dont know the ply count, measure the thickness (one ply is generally between 0.008 and 0.010 inch) and divide by 0.009 Remove surface contaminants with solvent Remove surface coatings (paint/gel coat) mechanically. NEVER USE PAINT STRIPPERS!
81 Repairs, cont. Removal of core damage If damage has occurred to core material, it must be removed first, using a hole saw Step-cutting (Scarfing) To accomplish the proper step-cuts in the laminate, each successive layer of fiber must be removed. Great care must be exercised to avoid further damage Cleaning All Repairs Must Be Cleaned After Sanding. Use Vacuum And Solvent. DO NOT USE COMPRESSED AIR!
82 Repairs, cont. Sanding technique using vacuum source http://www.cirrusdesign.com/downloads/pdf/mc/mc20060801.pdf
83 Summary Weve talked about: What Composites Are Characteristics, Advantages and Disadvantages Materials Composites and Shop Safety Composite Damage, Inspection, and Repair Any Questions?
84 Acknowledgements: Maria Clinton and Gary Eisenberg, Antelope Valley Community College, Lancaster, CA David K. Hsu, [email protected] Joe Escobar, Composites: Tips for working on Cirrus composite structures. www.amtonline.com/publication/article.jsp Ultrasonic Testing of Aerospace Materials http://www.nasa.gov/offices/oce/llis/0765.html Lee T. Ostrom, Ph.D., CSP, CPE, Cheryl A. Wilhelmsen, MS; Detectability of Dents in Composite Materials, http://www.airlines.org/NR/rdonlyres/A6DADE79-6D14- 40F8-8403-BB1AA43B699A/0/Wed1130a_LeeOstrom.pdfLoad More