**10. References**

344 Recent Trends in Processing and Degradation of Aluminium Alloys

**Simulated lifetime [-]**

Both energy criteria and the model according to Neu-Sehitoglu were used for fatigue life modelling. In contrast to empirical approaches this model distinguishes between different damage mechanisms. The adequate method is chosen according to the type of application; in doing so, it is important to indicate the model limits. Energy criteria seem to represent the best compromise between accuracy and complexity in their application. Major differences

Practical application to cylinder heads shows that the fatigue life, calculated on the basis of the dissipated plastic energy, largely depends on the chosen state of ageing. The solution to

The commercial fatigue lifetime prediction software FEMFAT (FEMFAT Manual, 2005) features, since version 6.5, a module for calculating the damage under thermo-mechanical

With this model it is possible to calculate the local damage portions caused by fatigue, oxidation and creep. The calculation is based on shear strains, which are determined by a critical plane method and are therefore also applicable for multiaxial loading. The predominant part of the overall damage is caused by pure fatigue. The portion of oxidation damage amounts to some 10% in the regions of maximum loading. The fatigue life computation by means of FEMFAT-Sehitoglu provides realistic results concerning the

TMF energy criteria is a suitable tool for TMF lifetime assessment of aluminium, provided the limitations of the application are known. They are representative for the cyclic material behaviour and good damage indicators, since they are associated with the macroscopic crack initiation. The damage rate model of Sehitoglu is powerful to describe more influences, albeit with the major disadvantage being the need of an extensive data basis for

Fig. 11. Quality of the TMF fatigue life calculation using the Sehitoglu damage model

**8. Comparative consideration and application to components** 

occur if the damage mechanisms involved are changing.

this problem is an ageing-dependent cumulative damage model.

loading according to the damage rate model by Neu-Sehitoglu.

**Experimental lifetime**

critical areas and fatigue lives.

**9. Conclusion** 

 **[-]**


**15** 

*1Greece* 

*2United Kingdom* 

**Deformation Characteristics of Aluminium** 

**Composites for Structural Applications** 

*1Department of Materials Science and Engineering, University of Ioannina,* 

*2Faculty of Arts, Computing, Engineering and Sciences, Sheffield Hallam University,* 

Silicon carbide (SiC) particulate-reinforced aluminium matrix composites (AMC) are attractive engineering materials for a variety of structural applications, due to their superior strength, stiffness, low cycle fatigue and corrosion fatigue behaviour, creep and wear resistance, compared to the aluminium monolithic alloys. An important feature of the microstructure in the Al/SiC composite system is the increased amount of thermal residual stresses, compared to unreinforced alloys, which are developed due to mismatch in thermal expansion coefficients of matrix and reinforcement phases. The introduction of the reinforcement plays a key role in both the mechanical and thermal ageing behaviour of the composite material. Micro-compositional changes which occur during the thermomechanical forming process of these materials can cause substantial changes in mechanical

The satisfactory performance of aluminium matrix composites depends critically on their integrity, the heart of which is the quality of the matrix/particle reinforcement interface. The nature of the interface depends in turn on the processing of the AMC component. At the micro-level, the development of local concentration gradients around the reinforcement can be very different to the nominal conditions. The latter is due to the aluminium alloy matrix attempt to deform during processing. This plays a crucial role in the micro-structural events

The strength of particulate-reinforced composites also depends on the size of the particles, interparticle spacing, and the volume fraction of the reinforcement [1]. The microstructure and mechanical properties of these materials can be altered by thermo-mechanical treatment as well as by varying the reinforcement volume fraction. The strengthening of monolithic metallic material is carried out by alloying and supersaturating, to an extent, that on suitable heat treatment the excess alloying additions precipitates out (ageing). To study the deformation behaviour of precipitate hardened alloy or particulate reinforced metal matrix composites the interaction of dislocation with the reinforcing particles is much more dependent on the particle size, spacing and density than on the composition [2]. Furthermore, when a particle is introduced in a matrix, an additional barrier to the movement of dislocation is created and the dislocation must behave either by cutting

properties, such as ductility, fracture toughness and stress corrosion resistance.

of segregation and precipitation at the matrix-reinforcement interface.

through the particles or by taking a path around the obstacles [3].

**1. Introduction** 

Theodore E. Matikas1 and Syed T. Hasan2

