Design with new Materials


Hot structures made of ceramic composite materials with continuous fibres in a SiC matrix require adequate joining methods. For this purpose, the special fastener design made in laminated CMC has been developed. The corresponding patented solution is an exemplar of the advanced design approach based on the system engineering.

 

Striving to work out the problem on the basis of conventional monolithic ceramics was less promising right from the beginning. This kind of ceramic materials is extremely sensitive to stress concentration and very small flaws. Most of these defects, however, can be overcome by the CMC materials. The current CMC materials, for example, have the tensile strength capacity of 300 MPa with the fracture strain up to 1% and their behavior under the load is non-linear. As a result, they are able to resist the negative effect of the stress concentration in the component. However, the conventional design approach to achieve the fastening elements from CMC material is not promising. The adjustment of the fibre layers in the position favourable for the load carrying capacity of the fastener, due to small dimensions very restricted. Thus, a fully new design approach for an adequate resolution of the problem was necessary. The main-strategy of the solution is based on an optimal utilisation of the advantages of laminated CMC-Structures, while the typical weaknesses are to be largely rejected.

Ceramic bolt

Zu diesem For this purpose, the bolt is cut from a laminated CMC material plate whose thickness is slightly below the core diameter of the threaded part. In this way, the continuously cut threads (5) are interrupted at the both sides where the connection of threads to the shank is being limited to laminate matrix bonding and consequently would be considerably reduced in strength. By this solution this much weaker part of the material for the threads is rejected avoiding possible troubles.

 

The threaded portions are directly joined to the bolt shaft by the laminated structure reinforced with the additional 45 layers (3 '). For this reason, the flank angle of 90 (4) was selected for the threaded profile. The fibre laminates thus take up the position parallel to the thread flanks, which also maximizes their load bearing capacity. This solution contributes significantly to the improved load bearing capacity of the threaded portion of the joint.

Analysis of the thread profile

Prior to manufacture, the shape and dimensions of the fastening parts were optimized numerically. The numerical design optimization is a very important step to achieve the highest properties of the connection.

The nut of this connection, cut from the same plate with laminated layers parallel to the contact surface has a standard continuous screw threads.

TEST RESULTS

After successful manufacture of the fastening components, different tests have been performed. The results are shown in this Table.

 

The specimens after fracture show the fracture mode (fracture at the first carrying thread), which is typical for a well-balanced design of a fastener.

 

PARAMETER

DESCRIPTION

DATA/RESULTS

Material:

CVMC (C/SiC Gradient-CVI-Produktion)

MAN-Technologie AG

Fastener geometry:

Outer diameter and pitch

8 x 2 mm 

Testing conditions:

Tensile test at room temperature.

1 MPa/s.

 

Bolt loaded by nut

 

Properties achieved:

Minimum strength of the threaded part of the screw

230 MPa

 

Typical shear strength of the threaded profile

140 MPa

Failure mode:

Fracture at the first carrying thread

Loaded threads without distortion (easy disassembly after a break)

Cyclic shear test:

At 70% static strength

20000 cyc. without failure

Testing within structure (160 bolts in connection)

Thermal transient and static load

Without failure at 1600C°C

Summarizing one can say that the developed CMC-joining is available to the structure-engineer for an efficient and reliable application in all structures where the application of CMC-Materials, because of high temperature, required low weight and high strength, is necessary. It is obvious that the presented solution is also completely compatible with the CMC-components of the large structure.

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