As initiation of significant deformation begins close to ultimate load, there is with a safety factor of 1. NOTE A separate verification can be necessary for environmental factors or other unusual failure modes, e.
The different graphs relate to different insert diameter di and to different core types. By potential load redistribution, a reduced factor of safety can be considered. An increase in hi is advised especially if rupture of the potting underneath the insert is anticipated. This is taken into account by neglecting the load contribution of the upper face sheet. As discussed in In this case the border of the insert hole is bonded with resin during potting [See also: The diameter D of the foot of the attached part needs to be at least as large as the typical potting diameter, i.
CFRP, can differ from that of metallic face sheets, [See also: This part of the load, Qc, depends on the stiffness of the core with respect to the load direction. The total capacity also depends upon whether each face sheet is loaded equally, i. The diameter of the foot of the attached part should be at least as large as the typical potting diameter.
In addition, the effect of several inserts close to each other, their relative direction of loads as well as the effect of their distance from free boundaries taken into account in an approximate manner. KD dimpling coefficient; which depends on the plate geometry, boundary conditions and type of loading.
The analysis of test data gave a value of KD of 2. Partially potted. Under such conditions it is not clear if the advantages of the design can be maintained with very thin CFRP face sheets. Where it cannot be avoided, e.
The insert body, and any fasteners used, should be capable of transferring the loads into the two face sheets, i. The allowable is therefore dependent upon the weakest face sheet, [See: In the case of a moment plus a shear load, the moment adds to the shear on one side and subtract from the shear on the other face. Depending upon the relative strengths of the two face sheets and the loading conditions, the added shear load can increase or decrease the permitted moment.
Under the conditions given in This expression does not take into account coupling of the potting on the lower face sheet and becomes more conservative for a fully potted insert. Owing to the complexity of CFRP face sheets, bending moments should be carried by a couple of inserts. If this is not possible, the diameter D of the foot of the attached part should be at least as large as the potting diameter, i. Although minimum, average and typical values are stated, minimum values are applied.
Owing to the deformation or installation of the connecting bolt, torsional loads cannot always be totally avoided. Thereby QSS becomes the shear permissible load for the calculated face sheet. MSS bending permissible load, [See: TSS torsion permissible load, [See: Bending and torsion loads need to be covered by adequate design, e. The edge of a sandwich panel can be either free, i.
If two inserts are placed close to each other that their effective potting radii are almost together, then the two inserts behave as one. PSS initial capability of insert, [See: This results in a combined stress distribution, i.
QSS initial shear load capability of insert, [See: NOTE In this case, these two inserts act as one. In the case of several adjacent inserts, the situation is more complex. CFRP face sheets have a brittle failure behaviour, sometimes with a damage tolerance of an equivalent hole of between 3 mm and 5 mm.
A CFRP panel can not be adequately designed without reflecting such effects. They are highly relevant with respect to the correct superposition of local stress failure criteria which are valid for CFRP. Owing to the complicated nature of CFRP composites, it can be concluded that tests are necessary to obtain reliable data for edge coefficients.
PSS1 initial capability of insert 1, without influence of insert 2. This is illustrated by an example. The reduced capability is determined by Eqn.
PSSi initial capability of intermediate insert. Therefore, the group effect can be considered relevant for sandwich panels with CFRP face sheets.
The global stress of the panel, the stress concentration effect of the insert hole and the local stresses induced by the insert pin load act in combination. CFRP face sheets have brittle failure behaviour, sometimes with a damage tolerance of an equivalent hole of between 3 mm and 5 mm. A CFRP sandwich panel cannot be adequately designed without reflecting such effects. They are highly relevant with respect to the correct superposition of local stress failure criteria, which are valid for CFRP.
Owing to the complicated nature of the CFRP composites, it can be concluded that tests are necessary to obtain reliable data for group effects. This results in local oversizing of the structure and fixing elements, e. General comments regarding the determination insert stiffness for sandwich panels having composite face sheets are also given.
The results are not very accurate because many assumptions were made to obtain a stiffness of the insert alone, i. The failure of the insert element itself is not experienced provided that there is an adequate fillet radius of the lower flange connection, which is defined in most standards.
Failures of the face sheets have never been experienced in spacecraft insert panel design. The strength of the potting body under operational condition is influenced by elevated temperatures, [See also: Cycling bending or torsion of inserts needs to be excluded in design, e. NOTE 2 If significant cyclic torsion or bending cannot be avoided, verification is based on detailed tests. NOTE 1 These values are for preliminary design and need to be supported by testing. These spectra are also presented as a distribution.
N is the accumulated number of times a certain load level is exceeded. As the amplitude FA i. Special attention is necessary if the total sequence of load history is formed by different spectra with significantly different mean loads. Sometimes these variations form the major part of the load history with respect to fatigue damage, e. Such variations of mean loads should be considered as additional cycles.
After a certain damage accumulation, the item fails under the applied peak load. As the mean stress ratio R, [See: A scatter factor of 4 should be used.
These enable a first estimate of the fatigue life in an early design phase, but need supporting tests. An area that needs consideration is the interface between the composite face sheet and the adhesive bond to the core. Tests have shown that the insert capability even increases with decreasing temperature. This should be avoided when coupled with simultaneous mechanical loading of the insert. The influence needs careful investigation if the effect of elevated temperatures is covered by standard procedures.
An extensive investigation using test and analysis is considered necessary in order to provide a simplified, approximate procedure able to handle elevated temperatures. It also includes links to appropriate information. The inserts are then fitted into their respective bore holes by the bonding tabs, so that they are the same with respect to the face sheet. If the distance between the fitted insert and the face sheet at the bottom of the hole is less than 3 mm, an appropriate amount of insert potting compound is applied to the bottom of the insert prior to its insertion.
This enables the loose face sheet to be supported after the insert is fitted. Otherwise cells underneath the insert do not fill properly. The vent tube can remain in the potting or be removed before curing. NOTE If the vent tube remains in the potting, it should be made from an acceptable material for space use. Mixing potting compound, [See: A strand of potting compound about 6 mm long appearing from the vent hole indicates that the bore cavity and the surrounding honeycomb cells are well filled.
Hence, additional venting is necessary, [See: The cure cycle used largely depends on the chemical formulation of the resin used for potting and whether the resin or assembly can be cured at elevated temperature without causing damage, e. This is why proper manufacturing processes, and their control, are essential.
Final machining processes can be stipulated as part of the overall processing, rather than solely as corrective actions.
NOTE 2 The face sheet cannot be damaged during final machining. The reference sample is produced at the same time as the manufactured assembly and undergoes all the same processes, e.
This is a destructive mechanical test that tears the insert out of the reference sample, [See also: Number of samples The number of reference samples, n needed depends on their criticality, i. The level of proof loading, i. Filling of the insert injection and venting holes with potting compound. NOTE All inspection criteria are checked, documented and compared with the requirements to establish if potted inserts are acceptable or unacceptable.
The repair process used depends on whether the core and face sheets are damaged or not. A suitable method is the injection of liquid resin into each cell of the core through small holes drilled in the face sheets.
For inserts near the edge of a sandwich panel, the resin can be injected from the side of the panel. This is reflected in lower than expected insert capabilities. It can occur if the control procedures for the bore hole are inadequate, and cells are not open to the flow of injected potting compound, [See also: NOTE 1 If the bore hole is too large, then the shear load capability decreases. NOTE 1 The manufacture of sandwich panel should meet the relevant standards, e.
This in turn affects the insert strength capability. NOTE Lubricants or coolants cannot be used during any drilling or cutting processes. All drilling processes should avoid damage to the surrounding core. The insert location is fixed by a centre bore of 6 mm or 8 mm diameter. The face sheet disc is then pulled off the core with a pair of tweezers.
Cutting of honeycomb core A punching tube maintained perpendicular to the sandwich plate is used to cut a cylindrical part of the core to the necessary depth. NOTE 1 The bore hole depth exceeds the insert height by 3 mm to 4 mm. After machining, any bent or dislocated core cells are removed or straightened so that the insert potting compound can flow undisturbed into all open honeycomb cells. Then, a reamer is used to extend the bore hole to the nominal insert diameter. Conical holes arise due to the reduced supporting effect of the bonded face sheet on the core at increasing hole depth.
In addition to the precautions taken to avoid deformation and damage to the core, extreme care is exercised not to damage the face sheets, either locally around the machining site or globally, e.
Although it is difficult to avoid such distortions, e. The usual approach for a laminate is to provide rigid support behind it during machining. Whilst the backface of sandwich panels with very thin CFRP face sheets can be easily supported, the front face sheet relies on the support offered by the core.
Depending on the position, a small machined hole can fall intracell or at the position of a honeycomb cell wall. Larger holes, as needed for inserts, cross several cell walls and intracell spaces, consequently the support provided varies. Normal laboratory conditions are applicable during manufacturing. Degreasing of inserts Before potting, inserts are degreased in a perchlorethylene steam bath.
Identification No. Alternatively, an acetone bath can be used, but frequent changing of the contaminated liquid is necessary. NOTE 1 The time between degreasing and potting cannot exceed 8 hours. The specific gravity of the resin mixture is typically 0. Therefore control of the mixed potting compound viscosity just before its use is necessary to aid potting reproducibility. A typical viscosity range is between 45 Pa.
Viscosity is also the parameter which determines usable pot life, which is stipulated as part of process control activities. This avoids any moisture present affecting the cure or resulting properties. Potting compounds are processed with the aid of a compressed air cartridge, e. The injection pressure appears to have no effect on the distribution of the potting compound, although the size and shape of the injection tool needs to be optimised.
The mixed resin undergoes an exothermic reaction during cure. This is accelerated by the presence of low thermal conductivity fillers, e.
Deviation from the viscosity needed can be corrected by further addition of microballoons or pure resin mixture. Some variation within properties can therefore be expected between the two resin systems, [See: 7. UVV Variations between batches of material are not uncommon. Storage conditions can affect the final properties of cured resins, [See: This can also extend to consumables used in the processing. These can be applied as alternatives. Hardness A hardness test on each batch of mixed resin and hardener provides a cure check on the potting compound.
Material characteristics The requirements stated in materials specifications are checked. Tests are conducted to recognised standards, e. These tests are conducted in accordance to recognised standards, e.
ASTM or equivalents. Consumables It is increasingly common for consumables used in composite processing, e. This is especially true where structural adhesive bonding is used as an assembly process, e. Destructive tests are applied to reference samples. When load is acting on the insert, the sandwich plate is pressed against one for pure tension or two for tension and compression aluminium plates which both have a central hole, 70 mm in diameter, to ensure a sufficient free area around the insert.
Loading is stopped after the maximum load has been reached, i. This enables a steady improvement in potting technology. The bending moment is applied by means of a rigid cantilever beam, linking to the crosshead of the test machine. The distance at which the crosshead links to the cantilever beam is adjustable. The thread connection and the contact area between the beam and the insert are precisely aligned.
Thus a defined torque can be applied. This socket, and the corresponding insert screw, can be selected according to insert type, e. The manufacture control procedures applied to laminates are therefore appropriate to face sheets on sandwich panels. After the face sheets are bonded to the core, the sandwich panel is then subjected to further inspection, e. NOTE Permissible values cited within this handbook are based on a correctly performed potting process without significant failures.
Safety equipment is maintained, is made available and used by personnel during the various manufacturing stages. To enable free flow of potting compound under the insert, the core cells beneath the insert are open, i. This leads to a strength reduction below the minimum permissible values, [See: NOTE 1 This can occur if a blunt or damaged punching tube is used or the bore hole drilling and reaming process is insufficient. NOTE 2 To avoid contamination, lubricants or cooling fluid cannot be used.
NOTE Process control is very important because it detects most failure types. NOTE Compliance with the standard means that minimum core strength values can be applied. Geometric effects on insert static strength, [See: Insert dynamic tests, [See: The deflection is also measured and the deflection behaviour recorded as a curve. One minute is a typical loading time and failures usually occur after about a minute.
This makes it easier to judge fracture conditions when samples are subsequently examined. It is also possible to measure the ultimate load in other directions, e. The test is performed in a series of sequential steps: 1. The temperature and humidity measurement system of the test room is activated. The measured temperature and humidity values are stored in a computer.
The test is performed at room temperature. Strap plates are clamped first to the lower grip and then to the upper grip of the testing machine. The other testing fixture is clamped to the lower grip of the testing machine. The lower grip is then raised to enable the connection between the strap plate and the lower testing fixture by means of another shear pin. A visual inspection is performed to verify the alignment of the specimen with the testing machine.
At the beginning of the test, slippage occurs between the insert flange and the strap plates. Slippage is considered to occur when a constant or decreasing value of the load is registered while the stroke is still increasing.
The load at which slippage occurs is noted. The failure load is recorded, i. The failure mode is determined and the specimen photographed from both sides. The test is performed in a torsion testing machine. Another task is the control and determination of the deflection behaviour during loading, to determine the point load cycle number at which core damage first occurs. This point is detectable by an increase of the deflection amplitude at constant load amplitude. For endurance tests, i. The load and deflection amplitudes are recorded during the dynamic test.
For high and medium numbers of load cycles, records are taken at chosen intervals. They show increased deflection caused by the damage growth during loading. The load amplitude is kept constant.
The purpose of the test is to determine the static residual strength of samples subjected to different numbers of dynamic load cycles. This adhesive layer can be relatively thin. Only a small amount of resin is needed to ensure good contact with the surrounding cell walls of the honeycomb core.
However, the full length of the extremely stiff carbon fibre tube actively contributes to the shear load transfer into the much softer honeycomb core, because the tube always goes through the whole sandwich thickness. The inner radius of the tube is identical with the bore hole radius of the face sheets. After curing of the adhesive, the carbon fibre tube is ready for fitting of one or two metallic insert caps.
Two basic types have been developed. The smaller metallic cap on the top side serves only as a guiding element for the screw. The standard diameter of these collars is 11mm, corresponding to the inner diameter of the carbon fibre tube insert. When the assembled cap is bonded into the carbon fibre tube insert, the 0. The length of the Type 2 cap determines the size of the bonded area between the carbon fibre sleeve on the cap and the carbon fibre tube in the sandwich.
Good adhesive bonding can be readily achieved between the two CFRP components. Consequently the insert density per area of the sandwich can be higher. This is significantly lighter than comparable potted inserts for sandwich thicknesses up to 50 mm. Special expertise and training regarding the proper injection of potting compound is not necessary. This can play a part in reducing costs.
Similar developments, which are currently the subject of evaluation exercises, will be included in future handbook revisions. NOTE This applies to tensile permissible loads only. Links to each of the individual graphs can then be selected.
NOTE 1 Not all available core and insert combinations are plotted, so the graphs illustrate the behaviour of common insert and sandwich panel combinations. Any errors in either the original graphs, or resulting from redrawing, mean that the graphs cannot to be used for design purposes without further verification.
If the insert height hi or the potting height hp coincide with the core height c, then the insert is truly fully potted. This is described in C. The shear stress distribution in the core of a circular sandwich panel with a rigid, central and clamped insert subjected to normal load is given by, [See also: See: C. NOTE This type of failure only occurs under tensile load. From Eqn. This is due to tensile rupture of the bond between the core and the upper face sheet near the insert.
Under these circumstances, the insert capability is reduced. Correlation coefficient Standard deviation CC 1 G 0. Much of this results from local changes in the sandwich panel. The individual face sheets of the sandwich panel tend to bend about their own neutral planes rather than about the neutral plane of the sandwich panel.
There is, however, one very important exception to this. In the case of sandwich panels with inserts loaded normal to the plane of the sandwich panel tensile or compressive loading , the active failure mechanism is nearly always shear rupture of the honeycomb core at the interface between the potting and the honeycomb; especially by shear rupture of the undoubled core foils.
NOTE The lower of the two insert strength predictions is used. If Pcrit Pp crit, then the partially potted insert behaves like a true partially potted insert. In addition to the above design calculations, it is necessary to ensure that tensile rupture does not occur in the potting resin underneath the insert or underneath the potting.
In the modelling, it is assumed that the insert of radius bi is an infinitely rigid body, but that both the potting compound and the honeycomb core are deformable in shear. Assuming further that the thickness of the core is much larger than the thickness of the face sheets, i. The approximations imposed in the derivation of Eqn.
The results obtained using Eqn. Partially potted inserts A partially potted insert, i. For this case, the core underneath the potting is subjected to tensile or compressive stresses. However, up to a certain value of d, failure still occurs due to shear rupture of the core next to the potting, and the static load carrying capability Pcrit still increases linearly with d; as predicted in by Eqn.
NOTE Ps It is assumed that the upper and lower face sheets, despite the possible differences in material properties and thicknesses, carry loads of equal magnitude. Load part carried by shear stresses in the core around the potting. Pn Load part carried by normal stresses in the honeycomb core underneath the potting material.
Theoretically, the two failure modes do not occur simultaneously, i. PsR Load part carried by shear stresses in the core around the potting over the height of the insert.
PnR Load part carried by normal stresses in the potting resin underneath the insert. Real potting radius, [See: 7. Compressive load: Additional criteria D.
The reason for this is that tensile rupture of the bond between the core and the upper face sheet adjacent to the insert is induced. Pcrit is given by Eqn. NOTE D. NOTE The validity of the equations is independent of the boundary conditions specified along the outer radius of the considered sandwich plate, provided that the sandwich plate radius is large enough.
Usually, the results obtained are of sufficient accuracy if the radius of the analysed sandwich plate is twice the potting radius, i. For such cases it is necessary to adopt more refined modelling. ESA Contract No.
Thomsen, Aalborg University, DK. NOTE E. Thickness 45 mm Perf. Thickness 80 mm Compressive strength: Stabilised: 3. Type Dia.
Peak load: Max. Between a face sheet thickness of 1. Effect of core thickness: A thicker core, i. Edge inserts and metallic fittings integrated within sandwich panels are not included. A worked example of the insert verification process is given for a box mounted on a sandwich panel using potted inserts at the four corners, [See: F.
Some of the originals supplied are of a poor quality. Inserts were specially designed to withstand the loads. Axial loads are often the more critical. Inserts, in particular fully potted types, are used to transmit shear loads. During the ASAP 4 structure optimisation, margins of safety were evaluated by measurement of interference and edge coefficients.
In this DLR Braunschweig developed and built a complex lightweight structure based on carbon fibre sandwich plates with several hundreds of inserts.
The full length of the extremely stiff carbon fibre tube actively contributes to the shear load transfer into the much softer honeycomb core, because the tube always passes through the whole sandwich thickness. In the course of the verification process of the ROSETTA Lander, the qualification of the new insert design was performed with regard to the specific mission requirements and the specific structural configuration.
However, a qualification of the insert design under more general conditions remained to be done. The influence of the sandwich parameters on the static and dynamic strength of the inserts under different load cases was systematically investigated.
The influence of thermal conditioning before testing, edge effects and the effect of the insert size were also considered. This specification, in conjunction ams-h the general requirements wms-h ams-h heat treatment covered ams-h Ams-hestablishes the requirements for annealing, stress relieving, and ams-h heat treatment of austenitic corrosion-resistant steel parts. A collection of more than recipes formulated to optimize brain health, boost memory, improve ams-h, sharpen the ams-h nervous system, and more.
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