Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - TICOAJO (TItanium COmposite Adhesive JOints)

Teaser

In Europe the nearby state-of-the-art HLFC is achieved by extracting the (turbulent) boundary layer by perforated suction surfaces. First trials using micro-perforated titanium suction surfaces are being performed at nose sections of wings and tail air foils. A promising...

Summary

In Europe the nearby state-of-the-art HLFC is achieved by extracting the (turbulent) boundary layer by perforated suction surfaces. First trials using micro-perforated titanium suction surfaces are being performed at nose sections of wings and tail air foils. A promising structural solution is the combination of a micro-drilled outer titanium surface adhesively bonded with an inner composite (segmented) structure.

The critical topic dealt within this proposal is exactly this titanium-composite adhesive joint hence, TICOAJO. Failure of this union must be investigated for certification and characterization of the adhesive properties is needed for future numerical simulation purposes.

The main objectives of TICOAJO are:
-find cost effective industrial manufacturing process of titanium-composite joints,
-extensively test these joints; static, fatigue and at high strain rates,
-investigate environmental influences; CTD, ambient and HTW
-characterize the adhesive properties for future numerical simulation purposes
-predict damage growth behaviour on sub-component level
-experimentally verify the validity of this prediction by an representative test

Work performed

The TicoAjo project has started in February 2017 with the kick off meeting at NLR in Marknesse. From earlier calculations performed on the titanium to composite test coupons some concerns were raised about the thickness of the material which would result in plasticity during testing and wrong results of the fracture toughness. This showed that combined with the thermal effects of the two materials, the tests and data extraction would not be straight forward or trivial. Detailed calculations were performed and from this the use of aluminium backing material on the baseline titanium and composite material was preferred. After some iteration and waiting for suppliers the FM300 and FM94 adhesives were chosen. Unfortunately this material ordering did lead to a large delay.

Using the optimal pre-treatment results from the TUD and the optimal coupon design as calculated by PATRAS, the complete series of union1 to union4 was manufacturing and tested statically. Static DCB and ENF testing have been performed at NLR and UPATRAS on the titanium/composite DCB samples. The results have been evaluated and showed that during the DCB tests for the thermoset CFRP union types an unstable damage growth going from the material interface, to inside the CFRP laminate. Although this indicates a strong interface resulting from excellent pre-treatment, this behaviour limits the calculation of only the initiation fracture toughness values. This issue needs to be addressed for fatigue, dynamic testing and the demonstrator.

The results achieved so far were presented during a TicoAjo session at a very wide audience the ECCM18 conference in June, Greece. On the demonstrator level for the leading edge the thermal stress could be of concern. The material situation needs to be monitored but the issues appears to be mostly mitigated.

Final results

The approach for evaluating the titanium and CFRP joint (test setup and data reduction) is novel and beyond state of the art. The evaluation using testing and the test-data evaluation is not trivial because of the effects of the backing structure and the thermal residual stress in the structure.
-Novel pre-treatment procedure for titanium CFRP adhesive joints
-New hybrid test setup using backing beams for low yield materials
-Data reduction to determine the fracture toughness including temperature effects.