QC protocols and Acceptance criteria
Acceptance criteria are used to define the allowable values
(typical minimum)that should be attained from either on-site or off-site QC
testing. For the application of composite materials for strengthening flexural
structures QC testing covers three categories relating to:
• state-of-cure
• mechanical performance
• adhesion (long-term durability).
The QC protocols for carrying-out the QC testing can be accessed here. The acceptance values will vary according to the materials being used and are therefore included within the classification documentation.
QC testing performed on-site is generally undertaken by the (sub) contractor. Off-site testing is always carried out by an independent test house. The acceptance definitions are based upon the short-term values listed in the classification documentation.
If any values fail the acceptance criteria the non-conformance steps should be followed.
On-site QC acceptance criteria
On-site testing is generally undertaken by the (sub) contractor. The related on-site testing falls into two categories:
1. Hardness test (state-of-cure)
The acceptance value for determining the state-of-cure is equal to the short term value – (4*SD).
Note: QC mean is rounded down to nearest integer and standard deviations (SD) are rounded up to nearest 0.5 of an integer.
Example: application of a structural adhesive - based upon the material specified in the classification scheme.
State-of-cure (hardness)
The mean of a batch (minimum of ten specimens) of hardness data, QCmean,
should be equal to or exceed the acceptance value, where;
QCmean >= 76 Shore D ( 82 – 4*(1.5) = 76 )
measured after 24 hours
or
QCmean >= 82Shore D ( 84 – 4*(0.5) = 82 ) measured
after 7 days
Note: 1 batch equals 50kg of adhesive usage, or, if a new adhesive batch code
is used
2. Pull-off test (strength of substrate or adhesive and failure mode)
The acceptance value for pull-off strength is equal to the short term value – (4*SD)
The acceptance criterion for adhesion is based upon a minimum percentage (75%) of the dominant failure mode resulting from the specimens used to provide the classification short term values.
Note: QC mean is rounded down to nearest integer and standard deviations (SD) are rounded up to nearest 0.5 of an integer.
Example: steel dolly bonded to a cast-iron substrate - based upon the material specified in the classification scheme.
Strength
The mean of a batch (minimum of three specimens) of QC pull-off tests, QCmean
, should be equal to or exceed the acceptance value, where;
QCmean >= 10 MPa ( 16– 4*(1.5) = 10 )
Adhesion
The mean failure mode of a batch (minimum of three specimens) of QC pull-off
tests, QCmean , should be equal to or exceed the acceptance
value , where;
QCmean >= 75 % P ( 75% cohesive failure in the Primer
)
Note: 1 batch equals 50kg of adhesive usage, or, if a new adhesive batch code
is used
Off-site
QC acceptance criteria (independent testing)
QC testing performed on specimens cured on-site but tested off-site by an independent test house. All specimens should have been left to cure on site for a minimum of 5 days.
These are divided into three categories:
3. Determination of the adhesive/resin’s glass transition temperature (State-of-cure)
The acceptance value for the glass transition temperature of the adhesive or resin is equal to the short term value ± 5°C.
Note: QC mean is rounded down to nearest integer and standard deviations (SD) are rounded up to nearest 0.5 of an integer.
Example: application of a structural adhesive - based upon the material specified in the classification scheme.
Glass transition temperature (Tg)
The mean of a batch (minimum of three specimens) of QC pull-off tests, QCmean
, should be within the acceptance limits;
55°C =< QCmean <= 65°C ( 60 ± 5°C
)
Note: 1 batch equals 50kg of adhesive usage, or, if a new adhesive batch code
is used
4. Determination of the adhesive’s mechanical performance (tensile or flexural properties)
The acceptance value for bulk tensile modulus (stiffness) is equal to the short term value ± (4*SD)
The acceptance value for bulk tensile strength (stiffness) is equal to the short term value – (4*SD)
Example: application of a structural adhesive - based upon the material specified in the classification scheme.
Stiffness
The mean of a batch (minimum of three specimens) of QC pull-off tests, QCmean,
should be within the limits;
3 GPa =< QCmean <= 14 GPa ( 9 ± 4*(1.5)
and rounded down)
Strength
The mean of a batch (minimum of three specimens) of QC pull-off tests, QCmean,
should be equal to or exceed the acceptance value, where;
QCmean >= 26 MPa ( 34 – 4*(4.0) = 26 )
Note: 1 batch equals 50kg of adhesive usage, or, if a new adhesive batch code
is used
5. Evaluation of the adhesive and composite material system (Lap shear strength and failure mode)
The acceptance value for lap shear strength is equal to the short term value – (4*SD)
The acceptance criterion for adhesion is based upon a minimum percentage of the dominant failure mode resulting from the specimens used to provide the classification short term values.
Note: QC mean is rounded down to nearest integer and standard deviations (SD) are rounded up to nearest 0.5 of an integer.
Example: bonded CFRP pultrusion using a structural epoxy adhesive - based upon the material specified in the classification scheme.
Strength
The mean of a batch (minimum of three specimens) of QC pull-off tests, QCmean
, should be equal to or exceed the acceptance value, where;
QCmean >= 8 MPa ( 10 – 4*(0.5) = 8 )
Adhesion
The mean failure mode of a batch (minimum of three specimens) of QC pull-off
tests, QCmean , should be equal to or exceed the acceptance
value, where;
QCmean >= 75 % A ( 75% cohesive
failure in the Adhesive )
Note: 1 batch equals 50kg of adhesive usage, or, if a new adhesive batch code
is used
If any values fail the acceptance criteria the non-conformance steps should be followed.
Non-conformance of QC specimens to acceptance criteria
In the case where QC batch test results do not pass some, or any, of the acceptance
criteria the following steps have been identified and subsequent actions proposed.
Failure to meet acceptance critieria
In the case where any batch fails a specific acceptance criterion relating
to expressions 1 to 5, the following actions should be taken:
Step 1: Check the mechanical performance of other QC tests taken from the
same batch. If all others pass the relevant criteria the strengthening is
deemed acceptable. If another QC test, taken from the same batch, fails go
to step 2.
Step 2: The designer must re-evaluate the strengthening to take account of
the reduced performance properties obtained from site. If the new design values
are deemed sufficient the strengthening is accepted. If the design values
are not acceptable go to step 3.
Step 3: Carry out some form of remediation, which might include the installation
of additional plates, monitoring of suspect areas of strengthening, additional
destructive QC tests on existing strengthening, or additional measures to
improve adhesive cure (e.g. forced heating). In severe cases the defective
strengthening materials may have to be removed and the strengthening repeated.
Go to step 4.
Step 4: Perform QC tests due to any remediation works and, where applicable,
repeat QC acceptance procedures.
Failure to meet adhesion criteria
If the average key mode of failure for each batch is significantly different
from that of the control specimens long term monitoring of the relevant strengthened
areas is recommended. In this case the following actions should be taken:
Step 1: Where there are low numbers, or isolated cases, of an apparent change
in failure mode, visual inspection is recommended over an agreed period and
frequency.
Step 2: Where there are large numbers of cases where an apparent change in
failure mode is exhibited, the application of additional strengthening is
recommended in order to allow selective Destructive Testing (e.g. pull-off
testing) of the original strengthening over an agreed period and frequency
with respect to the suspect batches.
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