**5.6 Influence of phase shifts / strain compensation**

336 Recent Trends in Processing and Degradation of Aluminium Alloys

curves follow a common progression and only some slight lifetime-decreasing effects are

The typically start-stop-operation of a motor vehicle as well as the alternating fired and nonfired operation causes dwell times. To study this effect out-of-phase TMF tests with four different dwell times at the particular maximum temperatures were conducted (8, 24, 144

Whereas with the alloy AlCuBiPb, a higher dwell time always causes a lifetime decreasing effect. This is not the case with the alloy AlSi7MgCu0.5, as figure 6 (left) shows, where the strain values are scaled between the minimum and maximum in this range. The TMF strainlife curve for the dwell time of *tD3*=144 s is quite steep for higher strain values (and therefore temperatures). A point of intersection of the curves for the lower dwell times (8 s and 24 s) is visible at about 1000 cycles. This phenomenon is explained with pronounced softening effects in the first few cycles that occur due to the high aging tendency at the high dwell time and temperature level. The capacious over-aging at this level can mainly be seen in the highly plastic parts, what results in an upward movement of the total strain-life curve. For that reason the high over-aging at the dwell time of 144 s shows that a high mechanical strain amplitude can be endured for a longer time compared to the smaller dwell times. If the maximum temperatures are low, this effect turns around at the dimensioning level for cylinder heads (about 5000 cycles) and a lifetime decreasing effect is visible with a higher

Two extreme LCF tests were conducted at the mean value for the TMF maximum temperatures. A LCF test series at a constant higher temperature of 250°C and one tested at room temperature, but pre-aged at 250°C for 500 hours. Figure 6 (right) shows that these two LCF strain-life curves span the TMF range for all dwell times of the materials investigated, if the mechanical strain (and not the thermal strain) is considered. The comparison of the LCF hysteresis loops at 250°C with comparable mechanically strained TMF hysteresis loops for all dwell times also shows a good accordance. Moreover the cyclic deformation behaviour according to Ramberg-Osgood shows the best accordance of the LCF-250°C curve with the TMF curves. This investigation shows that the macroscopic

behaviour is comparable if the aging status is similar (Riedler et al., 2004).

 **AlSi7MgCu0.5 TMF-OP - LCF**

**TMF tD1**

1,E+01

1,E-01 1,E+00 1,E+01 1,E+02 **Strain** ε **mecha,t at Nf/2 [‰]**

 **LCF - T=25°C / 250°C / 25°C - pre-aged 500h at 250°C, TMF-OP - tD = 8 / 24 / 144 s Stress-strain curves acc. Ramberg-Osgood** 

 **AlCuBiPb, R=-1**

1,E+02

**Stress** σ**a at N f/2 [N/mm 2**

Fig. 6. Influence of TMF dwell-time and LCF-pre-aging and constant elevated temperature

on the lifetime (on the left) and on the cyclic deformation behaviour (on the right)

**]**

1,E+03

LCF 25°C LCF pre-aged LCF 250°C OP-TMF tD=8s OP-TMF tD=24s OP-TMF tD=144s

**LCF 25°C / preaged 500 h at 250°C**

1,E+02 1,E+03 1,E+04 **Number of cycles to failure Nf [-]**

ascertainable at pulsating executed TMF test series.

**5.5 Influence of dwell-time** 

and 864 s).

dwell time.

1,E+00

**LCF 250°C**

**TMF tD3**

**TMF tD2**

εref,min

**Strain amplitude** ε

 **mecha,t at N f/2** 

**[‰]**

1,E+01

εref,max

In order to investigate the influence of stiffness and phase shifts, different TMF conditions were tested. Besides the ideal OP-TMF situation (ε*t,mech*=ε*th*, *KTM*=1.0), two overcompensated conditions of the thermal strain (*KTM*=1.5 and 2.0), a 75% compliance (*KTM*=0.75, which is near to the real circumstances in cylinder heads) and an ideal in-phase-TMF situation (*KTM*=-1.0) were tested.

When the local strains are taken into account at rigid clamped specimens, all OP-TMF results can be drawn together in a common strain vs. cycles to failure diagram. Because of creep damage at higher temperatures the IP-TMF lifetime is shorter than the OP-TMF lifetime, as figure 10 shows.
