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DNV-OS-C501 Composite Components
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SECTION 7
Joints and InterfacesSec.7
A. General
Sec.7
A 100 Introduction
Sec.7 A
101 Joints and interfaces are special sections or components of
a structure. They can in principle be analysed an tested the same
way as a structure or component. However, some special considerations
are described in the following sections.
Sec.7 A
102 Requirements for joints and interfaces are based on achieving
the same level of reliability as the structure of which it is part.
Sec.7 A
103 If metal components are part of a joint or interface, the metal
components shall be designed according to relevant standards for
such components. This standard does not cover metal components.Sec.7
A 200 Joints
Sec.7 A
201 Joints are defined here as load bearing connections between
structures, components or parts.
Sec.7 A
202 Three basic types of joints are considered in this standard:
| — | laminated
joints, i.e. joints fabricated from the same constituent
materials as the laminates that are joined, such as e.g. over-laminations,
lap joints, scarf joints etc. |
| — | adhesive joints,
i.e. joints between laminates, cores or between laminates and other
materials e.g. metals |
| — | mechanical joints,
i.e. joints including fasteners, e.g. bolted connections. |
Sec.7
A 300 Interfaces
Sec.7 A
301 Interfaces are defined here as the area or region where different
structures, components or parts meet each other. All joints have
interfaces.
Sec.7 A
302 If the interface shall transfer loads it also has the function
of a joint. All requirements for joints apply to such an interface.
Sec.7 A
303 A typical interface is the area where the surface of a load bearing
structure and a liner meet.Sec.7
A 400 Thermal properties
Sec.7 A
401 The effects of thermal stresses and strains and displacements
shall be considered for all joints and interfaces.Sec.7
A 500 Examples
Sec.7 A
501 Examples of good practise shall be evaluated with great care.
The examples are usually given for certain load and environmental
conditions, without stating those explicitly. The qualification
and analysis requirements of this standard shall also be applied
joints based on good practise. See also B400 on how
experience can be utilised.Sec.7
B. Joints
Sec.7
B 100 Analysis and testing
Sec.7 B
101 The same design rules as applied for the rest of the structure
shall be applied to joints, as relevant.
Sec.7 B
102 Joints are usually difficult to evaluate, because they have complicated
stress fields and the material properties at the interfaces are
difficult to determine.
Sec.7 B
103 Joints may be designed according to three different approaches:| — | An analytical approach, i.e.
the stress/strain levels at all relevant parts of the joint
including the interface are determined by means of a stress analyses
(e.g. a FEM-analyses) and compared with the relevant data on the
mechanical strength. |
| — | Design by qualification testing only, i.e. full scale
or scaled down samples of the joint are tested under relevant conditions
such that the characteristic strength of the complete joint can
be determined. |
| — | A combination of an analytical approach and testing,
i.e. the same approach specified in section
10 C for updating in combination with full scale component
testing. |
Sec.7 B
104 The options in Table B1 may be used for the different types
of joints:Sec.7 B
| Table B1 Design approaches
for different categories of joints |
| Type of joint | Analytical
approach | Qualification testing | Analyses combined with
testing
(updating) |
| Laminated joint | x | x | x |
| Adhesive joint | | x | x |
| Mechanical joint | | x | x |
Sec.7 B
105 The level of all stress (strain) components in all relevant areas
of the joint, including stress concentrations, shall be determined
according to the same procedures as specified for the rest of the
structure. Special emphasis shall be put on possible stress concentrations
and local yielding in the joint. It shall be recognised that the
stress concentrations in the real structure may be different than
determined through the analyses due to e.g. simplifications made,
effects of FEM-meshing etc.
Sec.7 B
106 An analytical analysis is sufficient, if the stress field
can be determined with sufficient accuracy, i.e., all stress concentrations
are well characterised and a load model factor gSd can be clearly defined. In
all other cases experimental testing according to section 10 shall
be carried out to confirm the analysis.
Sec.7 B
107 If the material properties, especially of the interface cannot
be determined with sufficient accuracy, experimental testing according
to section 10 shall be carried out.
Sec.7 B
108 Long term performance of a joint may be determined based on
long term materials data, if a clear link between the material properties
and joint performance can be established. The requirements of 102
and 103 also apply for long term performance.
Sec.7 B
109 The load cases should be analysed with great care for joints.
Relatively small loads in unfavourable directions can do great harm
to a jointed connection. Especially loads due to unintended handling,
like bending, stepping on a joint etc. should not be forgotten.
Sec.7 B
110 Joints may be analysed by testing alone as described in section 10 B.
Sec.7 B
111 The most practical approach is likely to use a combination
of analysis and testing. Since a large conservative bias may be
necessary in the analysis to account for the many uncertainties
in a joint design it is recommended to use the updating procedures
of section 10 C400 to obtain a
better utilisation of the joint. The purpose of this approach is
to update the predicted resistance of the joint with the results
from a limited number of tests in a manner consistent with the reliability approach
of the standard.Sec.7
B 200 Qualification of analysis method
for other load conditions or joints
Sec.7 B
201 If an analysis method predicts the tested response and strength
of a joint based on basic independently determined material properties
according to section 10 C, the
analysis works well for the tested load conditions.The same analysis method may be used:
| — | for the same joint under different
load conditions, if the other load conditions do not introduce new
stress concentrations in the analysis |
| — | for a joint that is similar to an already qualified
joint, if all local stress concentration points are similar to the
already qualified joint and all material properties are known independently. |
Sec.7 B
202 Local stress concentrations are similar if the local geometry
of the two joints and the resulting stress fields at these local
points can be scaled by the same factor.
Sec.7 B
203 An analysis method that predicts the test results properly but
not entirely based on independently obtained materials data can
only be used for other load conditions or joint geometry if it can
be demonstrated that the material values that were not obtained
by independent measurements can also be applied for the new conditions.Sec.7
B 300 Multiple failure modes
Sec.7 B
301 Most joint designs can fail by various failure modes. All possible
failure modes shall be carefully identified and analysed. See section 10 D.Sec.7
B 400 Evaluation of in-service experience
Sec.7 B
401 In service experience may be used as experimental evidence
that a joint functions well.
Sec.7 B
402 This evidence shall only be used if the load and environmental
conditions of the in-service experience can be clearly defined and
if they match or are conservative for the new application.
Sec.7 B
403 Material properties of the joints to be compared should be
similar. The analysis method should be able to address all differences
between the joints according to B100 and
200.Sec.7
C. Specific joints
Sec.7
C 100 Laminated joints
Sec.7 C
101 Laminated joints rely on the strength of the interface for load
transfer. The interface has resin dominated strength properties.
Defects in the interface tend to be more critical than defects in
the interface of plies of laminate, because the joint interface
is the only and critical load path.
Sec.7 C
102 The strength of the joint may be different from the through
thickness matrix properties of the laminate, because the joint may
be a resin rich layer and the joint may be applied to an already
cured surface instead of a wet on wet connection. (see manufacturing).
The strength of the joint should be documented.
Sec.7 C
103 Laminated joints are very sensitive to peel conditions. Peel
stresses should be avoided.
Sec.7 C
104 For the interface between the joining laminates the matrix
design rules given in section 5 apply. The resistance of the interface
shall be determined with the same level of confidence as specified
in section 4 A600. It shall be
recognised that the resistance of the interface between the laminates
may not be the same as the corresponding resistance parameter of
the joining laminates. Resin rich layers may even have to be analysed
by different failure criteria, e.g., the yield criterion in section 6 F.
Sec.7 C
105 The laminates themselves, including possible over-laminations,
shall be analysed like regular laminates.Sec.7
C 200 Adhesive Joints
Sec.7 C
201 All issues related to laminated joints also apply to adhesive
joints.
Sec.7 C
202 Geometrical details should be clearly specified, especially
at points of stress concentrations like the edges of the joints.
Sec.7 C
203 The relationship between all elastic constants of both substrates
and the adhesive should be carefully considered. Mismatches may
introduce stresses or strains that can cause failure of the joint.
Sec.7 C
204 Thermal stresses should be considered.
Sec.7 C
205 Long term performance of adhesive should be established with
great care. The long term performance is not only influenced by
properties of the substrate, the adhesive and the interface, but
also by the surface preparation and application method.
Sec.7 C
206 Relevant long term data shall be established exactly for the
combination of materials, geometry, surface preparation and fabrication
procedures used in the joint.
Sec.7 C
207 An adhesive joint may also introduce local through thickness
stresses in the composite laminate that can lead to failure inside
the laminate in the joint region.Sec.7
C 300 Mechanical joints
Sec.7 C
301 Mechanical joints are often very sensitive to geometrical tolerances.
Sec.7 C
302 Creep of the materials shall be considered.
Sec.7 C
303 The pretension of bolted connections shall be chosen by considering
possible creep of the material under the bolt.
Sec.7 C
304 It is preferred to design the joint in a way that its performance
is independent of the matrix. This way matrix cracking or degradation
of matrix properties are not important for the performance of the
joint.Sec.7
C 400 Joints in sandwich structures
Sec.7 C
401 All aspects related to laminated, adhesive and mechanical
joint apply also to sandwich structures.
Sec.7 C
402 Sandwich structures have internal joints between core and
skin and between cores. These joints are usually evaluated independently,
but their properties are treated as an internal part of the sandwich
system. Often the core properties are modified to incorporate the
joint properties.
Sec.7 C
403 When two sandwich structures are joined complicated stress
fields may result inside the sandwich structure. Stresses inside
the core can be very different near a joint compared to the typical
shear stresses in a panel.
Sec.7 C
404 A large sandwich plate can be well described by core shear
properties obtained from large test specimens. In the neighbourhood
of joints local variations in properties of the core may become
critical.Sec.7
D. Interfaces
Sec.7
D 100 General
Sec.7 D
101 If loads shall be transferred across an interface all aspects
related to joints shall be considered.
Sec.7 D
102 If interfaces only touch each other friction and wear should
be considered according to section 6
M.
Sec.7 D
103 Fluids may accumulate between interfaces. They may accumulate
in voids or debonded areas and/or break the bond of the
interface. The effect of such fluids should be analysed. Possible
rapid decompression of gases should be considered section 6 L.
Sec.7 D
104 Liners that do not carry any structural loads shall have a high
enough strain to failure or yield that they can follow all possible
movements of the interface. Yielding of liners should be avoided,
since yielding can cause local thinning or introduce permanent stresses
after yield. If yielding cannot be avoided it shall be analysed
carefully. The effect of local yielding on the surrounding structure
shall also be considered.
Sec.7 D
105 If one substrate may crack (e.g. have matrix cracks), but the
other shall not crack, it shall be shown that cracks cannot propagate
from one substrate across the interface into the other substrate.
Possible debonding of the interface due to the high stresses at
the crack tip should also be considered.
Sec.7 D
106 It is recommended to demonstrate by experiments that cracks
cannot propagate across the interface from one substrate to the
other. It should be shown that by stretching or bending both substrates
and their interface that no cracks forms in the one substrate even
if the other substrate has the maximum expected crack density.Guidance note:
This is a typical situation for pressure vessels with liners.
The load bearing laminate may have some matrix cracks, but the liner shall
not crack to keep the vessel tight. Local debonding of the liner
may be acceptable if the liner will not collapse due to its own
weight, negative internal pressure or other effects.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
Guidance note:
A weak bond between the substrates is beneficial to prevent crack
growth across the interface. However, it means that debonding may
happen easily.---e-n-d---o-f---G-u-i-d-a-n-c-e---n-o-t-e---
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