Designing with New Materials

Designing with new Materials

Hot structures made of ceramic composite materials with continuous fibres in a SiC matrix require adequate joining methods. To this end, the special fastener design made in laminated CMC has been developed. The corresponding solution is an example of the sophisticated design approach.

Striving to solve the problem on the basis of conventional monolithic ceramics has lead to less promising results. This kind of ceramic materials is extremely sensitive to stress concentration and very small flaws. The CMC materials can overcome most of these shortcomings. The current CMC materials, for example, allow the tensile strength capacity of 300 MPa combined with the fracture strain up to 1 % und are because of that well candidates for such application. However, the conventional design approach to achieve the fastening elements from CMC material was not so successful. The positioning of the fibres' lay-up in the for the fastener carrying capacity favourable position was, based on the applied forms and manufacture technology, due to small dimensions very restricted. Therefore, a fully new design approach for an adequate solution of the problem was necessary. The main-strategy of the solution is based on an optimal application of the advantages of laminated CMC-Structures, while the typical weaknesses are most extensive eliminated.

Bolt design

For these purposes the bolt is cut from the laminated CMC plate material. The plate thickness is slightly below the core diameter of the threaded part. In this way the continuously cut thread (5) is interrupted at both sides where the connection of the threads to the shaft is limited to laminate matrix bonding and, therefore with the significantly reduced strength. By this solution the weak part of the thread is rejected and cannot lead to further damage by crashing and milling within the threads in contact.

The thread profile is strengthened by the angle of 90° (4). The threads are directly connected to the shaft through the laminated structure strengthened by additional * 45 plies (3'). In this way the laminate fibres take parallel position to the thread flanks. This novel solution, significantly contribute to the increased carrying capacity of the connection.

Nut

The nut of this connection, cut from the same plate with laminated layers parallel to the contact surface has endless screw thread.

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, which is typical for a well-balanced design of a fastener.
PARAMETER	DESCRIPTION	DATA/RESULTS
Material:	CMC (C/SiC gradient CVI Production)	MAN-Technologie AG
Fastener geometry:	Outer diameter	8 mm
	Pitch	2 mm
Testing conditions:	Tensile test at room temperature	1 MPa/s.
	Bolt loaded by nut
Achieved properties:	Mean load at fracture	8 kN
	Minimum strength of the threaded section	230 MPa
	Mean shear strength of the thread profile	140 MPa
Failure mode:	Fracture at first carrying thread
	Loaded threads without distortion (easy disassembly after fracture)
Cyclic shear test:	At 70% static strength	20000 cyc. without failure
Testing with structure	Thermal transient and static load - 160 bolts in connections	Without failure at 1600°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.

The main aim of the Consultation and Analysis, demonstrated by the example, is to offer clients, which want to introduce advanced methods and products in their programs, a comprehensive know-how, expertise and the newest relevant information for these purposes.

P.Agatonovic@t-online.de