Causing Unbond
CHAPTER 6 6 QUALITY QUALITY CONTROL CONTROL
CHAPTER 6 6 QUALITY QUALITY CONTROL CONTROL
6.1 INTRODUCTION 6.1 INTRODUCTION
Quality control is required to
Quality control is required to ensure that the sandwich structure will mensure that the sandwich structure will meet the requirements to achieveeet the requirements to achieve its design purpose.
its design purpose. It typically includes inspection of each batch It typically includes inspection of each batch of the core, facings, of the core, facings, and adhesives toand adhesives to ensure that they meet
ensure that they meet certain minimum requirements. certain minimum requirements. It also encompasses monitoIt also encompasses monitoring of the process,ring of the process, including verification that the correct materials are used during assembly, and that the appropriate cure including verification that the correct materials are used during assembly, and that the appropriate cure cycle is followed.
cycle is followed. Finally, Finally, the finished sandwich structure is inspectthe finished sandwich structure is inspected for dimensional tolerance and non-ed for dimensional tolerance and non-conformities.
conformities.
Many of the issues and procedures associated with quality control in solid composite laminates also Many of the issues and procedures associated with quality control in solid composite laminates also apply to sandwich structures;
apply to sandwich structures; see Vsee Volume 3, Chapter 3. olume 3, Chapter 3. In addition, many In addition, many nondestructive inspectionnondestructive inspection methods suitable for sandwich stru
methods suitable for sandwich structures are discussed in Vctures are discussed in Volume 3, Section 8.3.1. olume 3, Section 8.3.1. This chapter will fo-This chapter will fo-cus on items unique to sand
cus on items unique to sandwich structures.wich structures.
6.2
6.2 MA MATERIAL TERIAL PROCUR PROCUREMENT QUALITY EMENT QUALITY ASSURANCE PROCEDURES ASSURANCE PROCEDURES
6.2.1
6.2.1 Specifications Specifications and and documentationdocumentation
The specification for materials, fabrication processes, and material testing techniques must ensure The specification for materials, fabrication processes, and material testing techniques must ensure compliance with the engineering requirements.
compliance with the engineering requirements.
Chapter 2 in this volume describes acceptance test methods for characterizing core materials, Chapter 2 in this volume describes acceptance test methods for characterizing core materials, fac-ings, and the bond between them.
ings, and the bond between them. Section 3.2.1 of VSection 3.2.1 of Volume 3 provides a brief discussion of requirementolume 3 provides a brief discussion of requirementss for specifications and documentation.
for specifications and documentation. Sections 3.3 and 3.4 of VSections 3.3 and 3.4 of Volume 3 provide information on variableolume 3 provide information on variable statistical sampling plans that control the frequency and extent of material property verification testing to statistical sampling plans that control the frequency and extent of material property verification testing to achieve targeted quality levels.
achieve targeted quality levels.
6.2.2
6.2.2 Receiving Receiving inspectioninspection
The composite user’s material specifications typically define inspection requirements on incoming The composite user’s material specifications typically define inspection requirements on incoming materials to ensure that they meet engineering requirement
materials to ensure that they meet engineering requirements. s. A generA general discussion of this topic is foundal discussion of this topic is found in Volume 3, S
in Volume 3, Section 3.2.2. ection 3.2.2. Additional details of sampling plans that contAdditional details of sampling plans that control the frequency and extent ofrol the frequency and extent of material property verification testing to achieve targeted quality levels can be found in material property verification testing to achieve targeted quality levels can be found in ANSI/ASQC-Z1.4-1993 and ANSI/ASQC-Z1.9-ANSI/ASQC-Z1.4-1993 (References 6.2.2 (a)
1993 and ANSI/ASQC-Z1.9-1993 (References 6.2.2 (a) and (b)).and (b)).
Honeycomb core is typically characterized by the material from which it is made (e.g., aluminum, Honeycomb core is typically characterized by the material from which it is made (e.g., aluminum, Nomex
Nomex ® ® , Korex, Korex ® ® ), the density, the cell size, and the cell configuration (e.g., hexagonal, overexpanded,), the density, the cell size, and the cell configuration (e.g., hexagonal, overexpanded, flex).
flex).
Acceptance test requirement
Acceptance test requirements may vary from user to users may vary from user to user. . A typical example of acceptanA typical example of acceptance test re-ce test re-quirements for honeycomb core is shown in T
quirements for honeycomb core is shown in Table 6.2.2. able 6.2.2. Cores with heavier densities (in the 12-16 pcfCores with heavier densities (in the 12-16 pcf range) may have different requirements or
range) may have different requirements or test methods.test methods.
Visual inspection checks the core for nonconformities such as distorted cells, broken or buckled cell Visual inspection checks the core for nonconformities such as distorted cells, broken or buckled cell walls, node bond separations, p
walls, node bond separations, partially bonded nodes, and fuzzy artially bonded nodes, and fuzzy or chipped cell edges. or chipped cell edges. An example of aAn example of a piece of honeycomb core with distorted cells is shown in Figure 6.2.2.
piece of honeycomb core with distorted cells is shown in Figure 6.2.2. Other tests which may also be re-Other tests which may also be re-quired include water migration between cells,
quired include water migration between cells, electrical transmission testing, and a flexibilitelectrical transmission testing, and a flexibility test.y test.
Perishable materials such as adhesives and prepregs typically require revalidation testing to allow Perishable materials such as adhesives and prepregs typically require revalidation testing to allow additional use of the material
additional use of the material after expiration of the normal storage or out tafter expiration of the normal storage or out time life (see Volume 3, Sectionime life (see Volume 3, Section
Volume
Volume 6, Chapter 6, Chapter 6 6 Quality ControlQuality Control 3.2.2).
3.2.2). Core is generally stable at room teCore is generally stable at room temperature and, therefomperature and, therefore, does not require revalidation testing.re, does not require revalidation testing.
It may
It may, however, require special handlin, however, require special handling and storage to g and storage to prevent moisture uptake.prevent moisture uptake.
TABLE 6.2.2
TABLE 6.2.2 Typical acceptance tests required for honeycomb core.Typical acceptance tests required for honeycomb core.
TESTING REQUIRED TESTING REQUIRED
PROPERTY PRODUCTION
PROPERTY PRODUCTION
ACCEPTANCE ACCEPTANCE (Manufacturer) (1) (Manufacturer) (1)
PRODUCTION PRODUCTION ACCEPTANCE ACCEPTANCE
(User) (1) (User) (1)
ASTM STANDARD ASTM STANDARD (References 6.2.2(c) - (f)) (References 6.2.2(c) - (f)) Visual
Visual and and Dimensional Dimensional XX Dimensional
Dimensional Stability Stability (2) (2) X X D D 67726772 Cell
Cell Size Size X X XX
Density
Density X X X X C C 271271
Bare
Bare Compressive Compressive Strength Strength (3) (3) X X X X C C 365365 Sta
Stabilized bilized CompressiveCompressive Strength and Modulus (3) Strength and Modulus (3)
X
X X X C C 365365
Shear
Shear Strength Strength and and Modulus Modulus X X X X C C 273273 (1)
(1) Manufacturer is Manufacturer is defined as the defined as the manufacturer of manufacturer of the core the core material. material. User is defined User is defined as theas the sandwich part fabricator
sandwich part fabricator. . Production acceptance tProduction acceptance tests are defined as tests are defined as tests performed byests performed by the supplier or user for initi
the supplier or user for initial acceptance.al acceptance.
(2)
(2) TTests for dimension changes after specimen is heated and then cooled to room tempera-ests for dimension changes after specimen is heated and then cooled to room tempera-ture.
ture.
(3) Tests may be required under standard laboratory conditions, at elevated temperature, (3) Tests may be required under standard laboratory conditions, at elevated temperature,
and/or following immersion in
and/or following immersion in water or other fluids.water or other fluids.
NOTE:
NOTE: TTable 6.2.2 needs direction of testing (ribbon or transverse) specified.able 6.2.2 needs direction of testing (ribbon or transverse) specified.
FIGURE 6.2.2
FIGURE 6.2.2 Distorted cells in honeycomb core.Distorted cells in honeycomb core.
Volume
Volume 6, Chapter 6, Chapter 6 6 Quality ControlQuality Control
6.3
6.3 PA PART F RT FABRICATION ABRICATION VERIFICATION VERIFICATION
6.3.1
6.3.1 Process Process verificationverification
Volume 3, Section 3.3.1 discusses material control, material storage and handling, tooling, facilities Volume 3, Section 3.3.1 discusses material control, material storage and handling, tooling, facilities and equipment, in-process control, part cure, and process control specimens.
and equipment, in-process control, part cure, and process control specimens.
Nonmetallic honeycomb core may require special storage conditions, since moisture absorption tends Nonmetallic honeycomb core may require special storage conditions, since moisture absorption tends to reduce its compressive stren
to reduce its compressive strength. gth. The core may be air-dried before machining, to help improve itThe core may be air-dried before machining, to help improve its sta-s sta-bility.
bility. Once machined it may be dried at an elevated temperature Once machined it may be dried at an elevated temperature and sealed in an airtight container untiland sealed in an airtight container until it is used.
it is used.
The cure cycle for sandwich panels is frequently different from that for solid laminates made from the The cure cycle for sandwich panels is frequently different from that for solid laminates made from the same composite material.
same composite material. A lower-pressurA lower-pressure cure is typically used for sandwich structures with nonmetal-e cure is typically used for sandwich structures with nonmetal-lic honeycomb core than for solid laminates, since higher pressures can cause the core to shift during lic honeycomb core than for solid laminates, since higher pressures can cause the core to shift during cure, especially in areas of ramps, and excessive pressure might even cause crushing damage to the cure, especially in areas of ramps, and excessive pressure might even cause crushing damage to the core.
core. In addition, many manuIn addition, many manufacturers use a modified tfacturers use a modified temperature profile for sandwich emperature profile for sandwich structures withstructures with composite facesheets, which includes a constant temperature hold during the ramp-up to the final cure composite facesheets, which includes a constant temperature hold during the ramp-up to the final cure temperature.
temperature.
Many manufacturers use process control specimens to verify that the parts meet engineering Many manufacturers use process control specimens to verify that the parts meet engineering re-quirements.
quirements. These specimens may be tThese specimens may be taken from excess areas removed froaken from excess areas removed from the part itself, or a tm the part itself, or a testest panel laid up and cured on the t
panel laid up and cured on the tool with the part. ool with the part. The mechanical and physical tests performed The mechanical and physical tests performed for solidfor solid laminates, such as strength, modulus, fiber volume, and glass transition temperature, may be performed laminates, such as strength, modulus, fiber volume, and glass transition temperature, may be performed on the facesheet
on the facesheets. s. In addition, the prIn addition, the process control specimens may ocess control specimens may be subjected to tbe subjected to tests specific toests specific to sandwich parts, such as flatwise tension to check
sandwich parts, such as flatwise tension to check the core-to-facesheet bond.the core-to-facesheet bond.
6.3.2
6.3.2 Nondestructive Nondestructive inspectioninspection
Volume 3, Section 3.3.2 discusses common nondestructive inspection (NDI) techniques including Volume 3, Section 3.3.2 discusses common nondestructive inspection (NDI) techniques including vis-ual, ultrasonic, and X-ray inspect
ual, ultrasonic, and X-ray inspection. ion. In addition, many NDI In addition, many NDI methods suitable for honeymethods suitable for honeycomb core struc-comb core struc-tures are discussed in Volume 3, Section 8.3.1.
tures are discussed in Volume 3, Section 8.3.1.
Ultrasonic inspection is most commonly used for nondestructive inspection (NDI) of Ultrasonic inspection is most commonly used for nondestructive inspection (NDI) of compos-ite/honeycomb sandwich structures.
ite/honeycomb sandwich structures. A throughA through-transmission C-scan can be used to inspect both facings,-transmission C-scan can be used to inspect both facings, to detect crushing or other damage in the core, and to check the bond lines between the facings and to detect crushing or other damage in the core, and to check the bond lines between the facings and core.
core. A pulse echo A pulse echo A-scan can be used to detect porosity or other noA-scan can be used to detect porosity or other nonconformities in each facing sepa-nconformities in each facing sepa-rately
rately. . Inspection in the region of core splices is espInspection in the region of core splices is especially difficult.ecially difficult.
Ultrasonic NDI generally requires calibration standards specifically fabricated for sandwich structures.
Ultrasonic NDI generally requires calibration standards specifically fabricated for sandwich structures.
These standards may incorporat
These standards may incorporate defects such as core crushing e defects such as core crushing and core-to-facesheet disbonding. and core-to-facesheet disbonding. Ref- Ref-erence 6.3.2 discusses construction variables that should be taken into account when developing the erence 6.3.2 discusses construction variables that should be taken into account when developing the standards.
standards.
6.3.3
6.3.3 Destructive Destructive teststests
Volume 3, Section 3.3.3 discusses types of tests, and some guidance on types of parts to be tested Volume 3, Section 3.3.3 discusses types of tests, and some guidance on types of parts to be tested and frequency of testing.
and frequency of testing. First article destructive testing of honeycomb core partFirst article destructive testing of honeycomb core parts can be particularly use-s can be particularly use-ful to verify that the process produces good core to facesheet fillets and acceptable quality in the core ful to verify that the process produces good core to facesheet fillets and acceptable quality in the core ramp regions.
ramp regions. Both of these features arBoth of these features are difficult to inspect using NDI methoe difficult to inspect using NDI methods.ds.
Volume
Volume 6, Chapter 6, Chapter 6 6 Quality ControlQuality Control
6.4
6.4 ST STA ATISTICAL TISTICAL PROCESS CONTROL PROCESS CONTROL
Volume 3, Section 3.4 discusses quality tools, control charts, process capability, process feedback Volume 3, Section 3.4 discusses quality tools, control charts, process capability, process feedback adjustment, and design of
adjustment, and design of experiments. experiments. That discussion applies equally to sandwiches aThat discussion applies equally to sandwiches and solid lami-nd solid lami-nates.
nates.
6.5
6.5 MANAGING CHANGE IN MANAGING CHANGE IN MA MATERIALS A TERIALS AND PROCESSES ND PROCESSES
Volume 3, Section 3.5 discusses qualification of
Volume 3, Section 3.5 discusses qualification of new materials or processes, divergence and new materials or processes, divergence and risk, andrisk, and production readiness.
production readiness.
Volume
Volume 6, Chapter 6, Chapter 6 6 Quality ControlQuality Control
REFERENCES REFERENCES 6.2.2(a)
6.2.2(a) ANSI/ASQC ANSI/ASQC Z1.4-1993,Z1.4-1993, Sampling Procedures and Sampling Procedures and TTables by Attributes ables by Attributes ..
6.2.2(b)
6.2.2(b) ANSI/ASQC ANSI/ASQC Z1.9-1993,Z1.9-1993, Sampling Procedures and Tables for Inspection by Variables forSampling Procedures and Tables for Inspection by Variables for Percent Nonconforming
Percent Nonconforming ..
6.2.2(c)
6.2.2(c) ASTM ASTM C C 271, 271, “Standard “Standard TTest est Method Method for for Density Density of of Sandwich Sandwich Core Core Materials,” Materials,” ASTM,ASTM, Philadelphia, PA, 1999.
Philadelphia, PA, 1999.
6.2.2(d)
6.2.2(d) ASTM ASTM C C 273, 273, “Standard “Standard TTest est Method Method for for Shear Shear Properties Properties of of Sandwich Sandwich Core Core Materials,”Materials,”
ASTM, Philadelphia, PA, 2000.
ASTM, Philadelphia, PA, 2000.
6.2.2(e)
6.2.2(e) ASTM ASTM C C 365, 365, “Standard “Standard TTest est Method Method for for Flatwise Flatwise Compressive Compressive Properties Properties of of SandwichSandwich Cores,” ASTM, Philadelphia, PA, 2000.
Cores,” ASTM, Philadelphia, PA, 2000.
6.2.2(f)
6.2.2(f) ASTM D ASTM D 6772, 6772, “St“Standard andard TTest est Method Method for for Dimensional Dimensional Stability Stability of of Sandwich Sandwich Core Core Materi- Materi-als,” ASTM, Philadelphia,
als,” ASTM, Philadelphia, PA, 2002.PA, 2002.
6.3.2
6.3.2 SAE SAE ARP ARP 5606, 5606, “Aerospace “Aerospace Recommended Recommended Practice Practice for for Composite Composite Honeycomb Honeycomb NDI NDI Ref- Ref-erence St
erence Standards,” andards,” SAE International, Warrendale, PSAE International, Warrendale, PA, 2001.A, 2001.
Volume
Volume 6, Chapter 6, Chapter 7 7 SupportabilitySupportability