Assessment and Criticality of Defects and Damage

1
Project MMS 13
Assessment and Criticality of Defects and Damage
in Material Systems
M R L Gower, G D Sims NPL Materials Centre R J
Lee, M A Stone, S Frost AEA Technology,
Engineering Solutions
3rd MMS IAG 10th July 2003
2

• AGENDA
• 10.30 Welcome and introduction
• Introductions
• Review of minutes from 2nd IAG meeting
•  
• 1045 Project Progress
• MMS15 Martin Wall, Richard Lee, Gordon
  Bishop
• MMS13 Mike Gower, Richard Lee, Mark Stone
•  
• 1300 Lunch
•  

3

• AGENDA
• 1400 Related presentations from industrial
  partners
•  
• 1405 Fadhil Habib (BAE Systems)    'External
  threats to aircraft structures    
• 1420 Sam Luke (Mouchel)     'Installation and
  long term monitoring of West Mill Bridge
• 1435 John Cantrill (Vosper Thornycroft)  'NDT
  techniques used within VT Shipbuilding
• 1450 Bob Lewin (Rolls-Royce)    'Composite
  engine stators
• 1505 Roger Gregory (LTI) Inspection using
  shearography
•  
• 1520 Discussion
•  
• 1540 AOB
• 1545 Closure of meeting

4
Objectives

• To maximise UK industrial competitiveness in the
  composite materials industry, through
• Production of a comprehensive procedural guide
  for the detection, characterisation and
  assessment of the criticality of defects and
  damage in composite materials/structures
• Validated through several industrial case studies
  (ensuring full traceability of measurements)
  covering different material systems and
  applications
• Effective, widespread dissemination to UK
  industry using the latest electronic/web-based
  applications (e.g. CoDA, Smart Manuals, ADM
  Consortium)

5
Work Programme
Task 7 Case studies (NPL)
Task 6 Draft procedural guide (NPL)
INDUSTRY SUPPORT (range of sectors)
TASK 8 DISSEMINATION (NPL)
Task 2 Experimental testing (NPL material
data from MERL) Task 3 NDE assessment (AEAT
acoustic emission information from Brunel) Task 4
Defect characterisation (NPL) Task 5
Assessment of defect criticality (AEAT)
Task 1- Review (AEAT)
Other/past programme input CPD, DCC, VAMAS,
MMS4, MMS5, MMS15, IACFA, AMCAPSII
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Team Members

• NPL Materials Centre
• AEA Technology, Engineering Solutions
• Materials Engineering Research Laboratory (MERL)
  Ltd.
• Brunel University
• input from PhD Acoustic Emission as part of a
  Faraday Partnership Flagship Research Project
  (1998-2001)
• fast-track to latest information and research on
  acoustic emission - ideal technique for detecting
  damage initiation and growth

7
Schedule
8
Update on case studies

• A - Aerospace wing panels
• Carbon-fibre epoxy
• Subject to lightning strike
• Aims
• Most hazardous defect type?
• Accurately detected/characterised?
• Criticality issues - can it be tolerated, will it
  get worse, allowable sizes?

9
Case study A

• Main defect types
• Multi-level delaminations
• Ply damage
• Resin burn-out

Main damage restricted to top 8 plies (out of 32)
resin damage below this level?
10
Case study A

• Ultrasonic C-scan inspections performed by BAE
  Systems prior to impact
• Initial inspections have been performed on
  copper mesh panel using ThermoScope system
  (LOT Oriel Shayz Ikram)
• DSC tests planned on virgin material and from a
  depth lower than 8 ply level to assess whether
  resin has been damaged

11

• B - Thick marine laminate
• Resin infused glass/vinyl ester (UD, woven)
• Subject to demanding application, complex profile
  (e.g. attachments, cut outs etc)
• Main defect types
• Voidage
• Matrix cracking, delamination
• Aims
• Can defects be detected/characterised?
• Criticality issues - can it be tolerated, will it
  get worse, allowable sizes?
• Residual performance?

12

• C - Marine sandwich construction
• Glass-fibre/PP fabric skins (co-mingled), balsa
  core
• Subject to low-velocity impact
• Main defect types
• De-bond between skin and core (delamination)
• Skin delamination, damage to core
• Aims
• Generate damage tolerance data and perform NDT
  trials
• Can defects be detected/characterised?
• Criticality issues - can it be tolerated, will it
  get worse, allowable sizes?

13

• D - Overwrap bonded repair
• MMS6 (Classification and Assessment of Composite
  Materials Systems for use in Civil
  Infrastructure) study
• UD Ultra High Modulus carbon plates used for
  bridge strengthening
• Testing currently underway to determine most
  suitable test methods for characterising material
  properties
• Extra case studies can be completed (in
  confidence) with funding from provider

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Task 2 Experimental Testing

• Fully characterise 4 generic materials
• Related to materials used in case studies Task
  7
• Base data - for initial material selection/design
• Additional properties - required for modelling
  Tasks 5 and 7
• Modelling approaches based on LEFM fracture
  toughness data

15
Task 2 Experimental Testing

• Approach used similar to that set out in the
  Standard Qualification Plan produced in MMS2
• Comprehensive set of material tests
• for base material data
• for damage tolerance assessment and defect
  modelling
• Some data already exists for case study materials
• Verification of data
• Additional properties generated
• Not all tests required by all cases and
  additional tests required (e.g. Mixed mode I and
  II fracture toughness data, through thickness
  properties)
• Level of assessment determines which properties
  are required

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Task 4 Defect characterisation

• Classify defect types typically present in case
  studies
• Defects characterised with regard to
• type
• morphology/size
• types of material system/structure
• typical cause/loading condition
• location
• Pictorial catalogue displaying defect appearances
• Optical microscopy
• SEM

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Tasks 2 4 Experimental testing Defect
characterisation

• Delays in obtaining materials for experimental
  testing and defect characterisation
• Panels for two case study materials expected by
  end of July
• Test work started on case study D (MMS6)
• Test machines at NPL will be moving to the new
  building from 21st July

18
Review of damage tolerance test methods

• Recent work done at NPL linking defect
  characterisation with appropriate residual
  property test methods
• Standardisation of compression-after-impact (CAI)
• Review of impact resistance test methods (low
  velocity)
• Review of compression phase
• NPL proposed procedures
• Standardisation initiatives (e.g. VAMAS, ISO)
• Alternative residual property tests

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Damage tolerance issues

• Damage tolerance to non-catastrophic impacts is a
  major issue for all classes of composites
• Compression-after-impact (CAI) well established
  for thin aerospace panels failing under minor
  impact by delamination
• Deficiencies concern
• material property being assessed
• test conditions (geometries, impact energy)
• application to other material systems, thicker
  aerospace, sandwich structures, other residual
  strengths (i.e. tension, flexure, impact)
• method not standardised

20
Review of impact resistance test methods

• Large variation in
• specimen sizes
• support geometry
• clamping
• indentor (size but not shape)
• impact energy
• anticipated threat level
• energy to cause BVID (specified dent depth,
  delamination area)

21
Review of compression phase test methods

• Variety of specimen sizes inherent from impact
  resistance phase
• Compression support is fairly standard simply
  supported long edges, clamped short edges
• Some variation in loading rate (0.3 mm/min up to
  1.27 mm/min)

22
NPL proposed CAI procedure

• Impact resistance
• Based on the existing ISO 6603-2, Determination
  of multi-axial impact behaviour by the
  instrumented puncture test
• Instrumented tup more information at impact
• 100mm diameter circular support material
  behaviour cf. structural
• Specimen size of 140 x 140 mm representative of
  industry panel shape (4 cut from standard panel
  manufactured to ISO 1268)
• Hemispherical indentor (20 mm diameter)
• Better control over impact contact
• Representative of stone impact, tool drop

23
NPL proposed CAI procedure

• Impact resistance
• Lubricated better repeatability (no sticking)
• Unclamped
• Lower cost, difficulties at non-ambient
  temperatures
• Compression Phase
• Modified version of AITM 1.0010
• Energy ramped to find 0.5 mm dent depth level
• Specimen size 140 mm x 100 mm (cf. 150 mm x 100
  mm)
• 0.5 mm/min to failure
• Designed and built rig to take QMW, AITM 1.0010,
  modified AITM and sandwich panels (300 mm x 200
  mm x 10 mm)

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Active standardisation initiatives

• NPL is UK convenor for Impact Resistance work on
  TWA5 (Polymer Composites) of Versailles project
  on Advanced Materials And Standards (VAMAS)
• International pre-standards research aimed at
  providing technical base for drafting codes of
  practice and specifications for advanced
  materials
• In the USA SACMA RM-2 CAI test method is
  industry standard and the basis of organisational
  and national standards
• Committee D30 of ASTM International working on
  two ASTM standards through ASTM D30/MIL17/SAE P17
  triad
• Japanese Plastics Industry Federation (JPIF)
  proposal for New Work Item to ISOTC61/SC13
• JPIF willing to fund standardisation of CAI test

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Key features of JPIF proposal

• Similar to Boeing and prEN6038/AITM 1.0010
  methods
• Suitable for QI CFRP (UD, woven formats) fibre
  modulus of 200 GPa or higher
• Consists of 3 phases
• Impact
• Instrumented recommended
• Specimens (150 x 100 x 5 mm) supported and
  clamped over cut out (125 x 75 mm) in solid base
  plate
• 16 mm diameter hemispherical identor
• Mass of impactor 5-6 kg
• If BVID study 7 energy levels to find 0.3 mm
  dent depth
• Otherwise 6.67 J/mm and ultrasonic C-scan
  inspection

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Key features of JPIF proposal

• Inspection
• ultrasonic C-scan
• dent depth measurement
• Compression
• AITM 1.0010 jig
• Strain gauged
• 1 mm/min to failure
• NPL in favour of proposed method but would like
  to see closer agreement with the impact
  resistance test as specified in ISO 6602-3

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Impact damage characterisation of generic
composites

• 6 generic materials chosen for impact tests
• Injection moulded glass-fibre/nylon
• Chopped-strand-mat glass fibre/polyester
• Thermo-formed glass-fibre mat/polypropylene
• Glass-fibre/polyester pultrusion
• Resin injected/pre-pregged glass-fibre fabric
  epoxy
• Pre-pregged carbon-fibre/epoxy
• Ramp impact energy from low levels below damage
  initiation to much higher levels
• Examined types of impact damage using X-ray,
  ultrasonic C-scan, optical microscopy etc.

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Impact damage characterisation of generic
composites
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Impact damage characterisation of generic
composites
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Alternative residual property tests

• CAI suitable for CFRP but not other materials
• Tension-after-impact (TAI) test formulated with
  regard to the predominant damage types present in
  the glass fibre materials
• From impact panels, 140 mm x 50 mm strip cut with
  damage in central position in coupon
• Tested un-tabbed in tension at 2 mm/min
• Other residuals fatigue-after-impact, flexure
  etc.
• Subject of further work possibly in future
  Performance programme