Crack Paths 2012

Figure 3: Universal fixture for fatigue tests

The Wölher curves (with crack following or at final rupture) have been performed during constant stress

amplitude, stress at different temperatures to understand the influence of this parameter (R=0,1, f=30Hz,

T= -30, 20, 120°C).

2) Preparation samples

Mocks-ups were sampled on a complete heat-exchanger and cut with a band-saw to obtain a single tube

and a header part. In order to follow the initiation of the crack (when the tube is leaking), we filled the

tubes with a coloured liquid and monitored the fatigue test with a webcam (fig 4).

Figure 4: Following of the crack

For microscope observation or EBSDanalyses, all samples have been embedded and polished up to OPS.

In order to observe the microstructure, an HBF4(tetrafluoroboric acid) attack was perform on samples.

Results

Figure 5-a presents Wölher curves at room temperature, -30°C and 120°C for a complete rupture (tube

completely torn-off of the header). First of all, fatigue test on mock-ups allowed us to obtain a very high

reproducibility of the result (0,9

temperature in comparison with those at room temperature or at 120°C.

This phenomenomis observed on aluminium fatigue samples [1] and linked to the mechanical property

modifications as a function of temperature (decreasing of the Young modulus and yield strength with an

increase in temperature).

Sametype of results was observed at rupture initiation (Fig 5-b).

499