Inspection of Carbon Fibre – Titanium – Carbon Fibre Stepped-Lap Joint

The optimal combined use of composites and metallic material is essential for the performance of modern tactical aircrafts. At the same time, the resulting structures include complex metal-to-composite joints that may develop failures during use. The inspection of such structures is often highly demanding. The present paper discusses the complex inspection of stepped-lap joint between carbon fibre wing panel and titanium attachment of tactical aircraft. The joint has been found to be susceptible to disbonding both in the outer and inner titanium to carbon fibre interfaces. Due to the multi-material multi-layer structure, the inspection is particularly demanding, especially for the inner interface. Successful inspection can be developed by careful analysis of the ultrasonic signal. A flawless joint on the outer interface will result in phase change at the interface, that can be seen on the RF signal. A disbond prohibits this phase change and thus can be detected by noting the absense of this phase change. Defects bigger than 4 mm can be detected using this effect. Disbond on the outer surface will prohibit sound from traveling to the inner surface. Consequently, disbond on outer surface can be detected by noting a decreased amplitude in the inner-interface echo. A disbond on the inner surface will, likewise, prohibit sound from going through the interface and thus increase reflection from that interface. Consequently, a disbond on the inner surface can be detected by noting an increase in the amplitude of the inner-interface echo. Thus disbonds on both inner and outer surfaces can be detected by monitoring possible decrease or increase in the inner surface echo indicating disbond in the outer or inner surface, respectively.Peer reviewe

Hydrogen-induced delayed cracking in TRIP-aided lean-alloyed ferritic-austenitic stainless steels

Abstract Susceptibility of three lean-alloyed ferritic-austenitic stainless steels to hydrogen-induced delayed cracking was examined, concentrating on internal hydrogen contained in the materials after production operations. The aim was to study the role of strain-induced austenite to martensite transformation in the delayed cracking susceptibility. According to the conducted deep drawing tests and constant load tensile testing, the studied materials seem not to be particularly susceptible to delayed cracking. Delayed cracks were only occasionally initiated in two of the materials at high local stress levels. However, if a delayed crack initiated in a highly stressed location, strain-induced martensite transformation decreased the crack arrest tendency of the austenite phase in a duplex microstructure. According to electron microscopy examination and electron backscattering diffraction analysis, the fracture mode was predominantly cleavage, and cracks propagated along the body-centered cubic (BCC) phases ferrite and α’-martensite. The BCC crystal structure enables fast diffusion of hydrogen to the crack tip area. No delayed cracking was observed in the stainless steel that had high austenite stability. Thus, it can be concluded that the presence of α’-martensite increases the hydrogen-induced cracking susceptibility