Thermo-mechanical fatigue fracture

Fig. 1 Bright-field image of solution-treated AC2B alloy after thermo-mechanical fatigue test

 Thermo-mechanical fatigue fracture occurs when the material is exposed to thermal cycling under confinement, due to the generation of thermal stress or such stress plus external force (explanation 1,2).We have evaluated cast aluminum materials for heat engine pistons and cylinder heads, and cast irons for exhaust manifolds. Among them, we have evaluated the morphology of DAS, porosity and eutectic silicon grains (Paper 1,2),and the effects of surface cold hardening (Paper 3,4); and compared out-of-phase and in-phase thermo-mechanical fatigue and low-cycle fatigue at constant temperatures(Paper 5).We also conducted a multi-step simulation to investigate the damage behaviors of silicon grains in cast aluminum (Paper 2).It was also experimentally shown that Sr treatment reduced the size of the silicon grains, controlled damage, and improved the thermo-mechanical fatigue properties of the cast aluminum (Paper 1).Surface cold hardening improved the thermo-mechanical fatigue life because the treatment produced a relatively stable tissue and reduced the defects on the surface(Paper 3,4).Interestingly, it was also found that thermo-mechanical fatigue loading of AC2B cast aluminum caused θ'-Al2Cu to precipitate perpendicularly to the direction of compression at high temperatures (Paper 6).Figure 1 is a bright-field image of the precipitates on a specimen that was subjected to a thermo-mechanical fatigue test at εmech=0.5% and 50~250°C immediately after solution treatment, and fractured after the test. The orientation of the precipitates was parallel to the loading direction at in-phase, and perpendicular at out-of-phase. This was due to the stress aging caused by the large negative misfit strain along the direction perpendicular to the face of the plate-shaped precipitates. We have also reported a method for extending thermo-mechanical fatigue life by actively exploiting this phenomenon (Paper 7).

Fig. 2 Improving thermo-mechanical fatigue resistance by smart coating

As shown in Fig. 2, the in-phase thermo-mechanical fatigue properties can be improved by applying gentle thermo-mechanical load at out-of-phase, so that the preferential orientation of the precipitates becomes perpendicular to the loading direction. In the figure,σ0.2denotes the difference between the 0.2% strengths along and perpendicular to the loading direction. The symbols in the figure denote various pretreatments (discribed in detail Paper 7).The treatments prevented the precipitates from lining parallel to the loading direction during in-phase thermo-mechanical fatigue, and extended the life significantly.

Fig. 3 Effects of precipitate orientation control pretreatments. Both (a) the in-phase thermo-mechanical fatigue life, and (b) the anisotropy of static strength (Δσ0.2 on the Y axis is the difference between the 0.2% strengths along and perpendicular to the loading direction) can be expressed in percentages of precipitates oriented along the loading direction.

 We have also developed and published a smart coating method involving a completely new idea (Fig. 3). The method uses the large in-phase thermo-mechanical fatigue of crack openings. Coating materials melt at close to the highest temperature, impregnate the produced cracks, solidify when the temperature drops, and close the cracks. This significantly reduces the force that drives crack propagation, and prevents the generated thermo-mechanical fatigue cracks from propagating.

Review paper

  1. 戸田裕之,小林正和:材料の熱疲労破壊,軽金属,Vol.59,No.6,2009,312-319
  2. 戸田裕之,小林俊郎,疲労試験,鋳造工学,Vol.76,No.6,2004,548-552

Research paper

  1. 金曽誠,西戸誠志,小林俊郎,戸田裕之,AC4CHアルミニウム合金鋳物における熱機械疲労特性および機械的性質に及ぼすミクロ組織の影響,鋳造工学,Vol.74,No.11,2002.699-705
  2. H. Toda, J. Katano, T. Kobayashi, T. Akahori and M. Niinomi, Assessment of thermo-mechanical fatigue behaviors of cast Al-Si alloys by experiments and multi-step numerical simulation, Materials Transactions, Vol.46, No.1, 2005, 111-117(日本鋳造工学会小林賞受賞)
  3. 戸田裕之、福永哲也、小林正和、上杉健太朗、小林俊郎、山田 徹、大場義夫、柿崎みな子,表面冷間加工処理を施したAC4CHアルミニウム合金鋳物の熱機械疲労特性,軽金属,Vol.58,No.6,2008,236-241
  4. 戸田裕之、小林正和、新原智晴、山田徹、大場義夫、柿崎みな子,表面冷間加工処理を施したAC4Bアルミニウム合金鋳物の熱機械疲労特性,軽金属,Vol.60,No.4,2010,190-191
  5. L. Qian, Z.G. Wang, H. Toda and T. Kobayashi, Effect of reinforcement volume fraction on the thermo-mechanical fatigue behavior of SiCw/6061 Al composites, Materials Science & Engineering. A, Vol.1A357, No.1-2, 2003, 240-247
  6. M. Toyoda, H. Toda, H. Ikuno, T. Kobayashi, M. Kobayashi, K. Matsuda, Preferential orientation of precipitates during thermo-mechanical cyclic loading in an aluminum alloy, Scripta Materialia, Vol.56, 2007, 377-380
  7. H. Toda, T. Fukunaga and M. Kobayashi Improvement of thermomechanical fatigue life in an age-hardened aluminum alloy, Scripta Materialia, Vol. 60, 2009, 385-387
  8. H. Toda, M. Toyoda, T. Kobayashi, T. Akahori and M. Niinomi, Feasibility study on smart coating for failure prevention under thermo-mechanical fatigue loading, Journal of Intelligent Material Systems and Structures, Vol. 17, 2006, 1099-1103