Tensile and fatigue assessments of brazed stainless steel joints using digital image correlation

Abstract

Abstract. For mechanical tests of brazed stainless steel joints, the local deformation behaviour within the small area of the brazing seam is a major concern, because local strains cannot be detected with standard mechanical extensometers. The current study allows a fundamental comprehension of the gauge length influence on the strain measurements of brazed joints with smooth and notch-containing surfaces, under quasistatic and cyclic loadings. Therefore, the optical measurement technique of digital image correlation (DIC) is used within tensile and fatigue tests of brazed AISI 304L/BAu-4 joints in an as-received and pre-corroded condition. A triggered image acquisition of the DIC system is successfully applied to evaluate the local ratcheting fatigue behaviour in the area of the brazing seam at a frequency of 10 Hz. The gauge length influence, analysed in the range of 0.5 to 12.5 mm, is more pronounced with increasing tensile and fatigue stresses and is significantly enhanced for notch-containing surfaces. Instrumented load increase tests with strain, electrical, magnetic and temperature measuring techniques have proven to be appropriate to estimate fatigue properties of the brazed joints with a deviation of 4%. Fatigue and corrosion fatigue damage mechanisms are evaluated by using scanning electron microscopy with secondary and back-scattered electron detectors.

Fatigue and Corrosion Fatigue Behaviour of Brazed Stainless Steel Joints AISI 304L/BAu-4 in Synthetic Exhaust Gas Condensate

“…The S-N curve for the brazed joints is described with the fit line σ max = S D ∙ (N D /N f ) 1/k according to Basquin, with the coefficient of determination R 2 = 0.9018. Thereby, the fatigue strength of 397 MPa at 2 × 10 6 cycles and the gradient k = 12.4 were calculated [8]. The results of the base material [5] were positioned within the scatter range of the experimental data of the brazed joints.…”

“…The pre-corrosion was performed for up to six weeks with periods of semi-immersion, drying and a steam phase and a weekly heating for 5 h at 400 °C. The alternating corrosion exposure mainly leads to a local circumferential corrosive attack in the diffusion zones (Figure 1c) [8].…”

“…Figure 3a shows the experimental setup with thermal imaging cameras (thermoIMAGER TIM 400, Micro-Epsilon Messtechnik GmbH, Ortenburg, Deutschland), a feritscope (Fischerscope MMS, Helmut Fischer GmbH, Sindelfingen, Deutschland), a crack growth monitor (CGM-7, Matelect LTD, Harefield, UK) and a DIC system (Q400, Limess GmbH, Krefeld, Deutschland). For the DIC analyses [8,24,25], a speckle pattern was painted on the surface of the specimens using an airbrush system.…”

“…Brazed joints with gold-base filler metals show a great corrosion resistance and high mechanical strength at room temperature [6,7,8]. In this study, the fatigue behaviour of brazed AISI 304L/BAu-4 joints was analysed and compared to the properties of the austenitic base material.…”

“…The pre-corrosion was executed for up to six weeks, as defined in [23]. First results of tensile and fatigue tests of the brazed AISI 304L/BAu-4 joints in the as-received condition and after pre-corrosions, obtained by means of the digital image correlation (DIC) technique, were published in earlier studies by the authors [8,24,25]. Parts of the fractographic analyses were used for the current study to compare damage mechanisms generated by corrosion fatigue loading and fatigue loading before and after a long-term pre-corrosion.…”