**4. Concluding remarks**

The influence of processing conditions in the micromechanical behaviour of Al/SiC composites has been discussed. Two different manufacturing processes (cast and rolled), three reinforcement percentages (20%, 30%, 31%) and three processing states (as received, HT-1, T6 heat treated) have been compared.

The importance of processing conditions in the micro-structural events of segregation and precipitation has been depicted at the micro/nano level using microhardness measurements and nano-scale phase identification of the matrix-reinforcement interface, and the developments of strengthening mechanisms in the composite have been identified. The HT-1 heat treatment condition clearly showed an increase in the microhardness, due to β precipitates as well as other phases and oxides formed in the composite. T6 heat treatment showed the highest microhardness values due to formation of β precipitates, which contribute to strengthening of the interface.

Microhardness and tensile testing results show that the composite micro-mechanical behaviour is influenced by certain factors. In the absence of precipitates (as received state) or in the case of dispersed precipitates (aluminium matrix) the dominant parameters influencing the micromechanical behaviour of the composite are the reinforcement percentage, the interparticle distance and the mean size of particulates. However, when precipitates are concentrated in the areas close to the interface (T6 condition) these precipitates contribute to the strengthening of the composite material.

The thermographic examination of the materials show that heat treated composite samples exhibit regular crack propagation behaviour. Stress concentration, due to the presence of particle reinforcements, produced controlled crack growth and higher stresses, which were

*Model Plotted on Original Load Lifetime Plot*

Aluminium Honeycomb Sandwich Panel Metallic Foam Sandwich

Fatigue Model Data

Panel

1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 *Number of Cycles to Failure*

Fig. 27. Showing Fatigue Data for Aluminium Honeycomb, Metallic Foam Sandwich Panel

The influence of processing conditions in the micromechanical behaviour of Al/SiC composites has been discussed. Two different manufacturing processes (cast and rolled), three reinforcement percentages (20%, 30%, 31%) and three processing states (as received,

The importance of processing conditions in the micro-structural events of segregation and precipitation has been depicted at the micro/nano level using microhardness measurements and nano-scale phase identification of the matrix-reinforcement interface, and the developments of strengthening mechanisms in the composite have been identified. The HT-1 heat treatment condition clearly showed an increase in the microhardness, due to β precipitates as well as other phases and oxides formed in the composite. T6 heat treatment showed the highest microhardness values due to formation of β precipitates, which

Microhardness and tensile testing results show that the composite micro-mechanical behaviour is influenced by certain factors. In the absence of precipitates (as received state) or in the case of dispersed precipitates (aluminium matrix) the dominant parameters influencing the micromechanical behaviour of the composite are the reinforcement percentage, the interparticle distance and the mean size of particulates. However, when precipitates are concentrated in the areas close to the interface (T6 condition) these

The thermographic examination of the materials show that heat treated composite samples exhibit regular crack propagation behaviour. Stress concentration, due to the presence of particle reinforcements, produced controlled crack growth and higher stresses, which were

precipitates contribute to the strengthening of the composite material.

0.00

0.20

0.40

0.60

*Load (KN)*

and Calculated Model Data

**4. Concluding remarks** 

HT-1, T6 heat treated) have been compared.

contribute to strengthening of the interface.

0.80

1.00

1.20

related to regular energy release by the material during fracture, indicative of brittle fracture behaviour. On the other hand, the large plastic deformation of the aluminium alloy can be associated with the absence of stress-peaks in conjunction with the monotonic temperature rise for a large part of the temperature / time curve prior to the specimen failure.

A model has been applied to predict the interfacial fracture strength of aluminium in the presence of silicon segregation. This model considers the interfacial energy caused by segregation of impurities at the interface and uses Griffith crack-type arguments to forecast the energy change in terms of the coincidence site stress describing the interface and the formation energies of impurities at the interface. Based on Griffith's approach, the fracture toughness of the interface was expressed in terms of interfacial critical strain energy release rate and elastic modulus. The interface fracture toughness was determined as a function of the macroscopic fracture toughness and mechanical properties of the composite using two different approaches, a toughening mechanism model based on crack deflection and interface cracking and a stress transfer model. The model shows success in making prediction possible of trends in relation to segregation and interfacial fracture strength behaviour in SiC particle-reinforced aluminium matrix composites. The model developed here can be used to predict possible trends in relation to segregation and the interfacial fracture strength behaviour in metal matrix composites. The results obtained conclude that the role of precipitation and segregation on the mechanical properties of Al/SiCp composites is crucial, affecting overall mechanical behaviour.

The tension-tension fatigue properties of Al/SiC composites as a function of heat treatment have been discussed as well as the associated damage development mechanisms. The composites exhibited endurance limits ranging from 70% to 85% of their UTS. The T6 composites performed significantly better in absolute values but their fatigue limit fell to the 70% of their ultimate tensile strength. This behaviour is linked to the microstructure and the good matrix-particulate interfacial properties. In the case of the HT1 condition, the weak interfacial strength led to particle/matrix debonding. In the T1 condition the fatigue behaviour is similar to the HT1 condition although the quasi static tensile tests revealed a less ductile nature.

The crack growth behaviour of particulate-reinforced metal matrix composites was also investigated. Aluminium A359 reinforced with 31% of SiC particles subjected to two different thermal treatments, as well as wrought aluminium 2xxx series specimens, have been examined using thermographic mapping. Heat treated composites, and especially those samples subjected to T6 aged condition, exhibited different behaviour of crack propagation rate and stress intensity factor range than the as-received composite specimens. Furthermore, the composite specimens exhibited different fatigue crack growth rate characteristics than the base aluminium alloy samples. It becomes evident that the path of fatigue crack growth depends on the heat treatment conditions, where crack propagation relies on strengthening mechanisms, such as precipitation hardening. The microstructure of the interphase region was also found to play a significant role in the crack growth behaviour of particulate-reinforced composites. In this sense, T6 heat treated Al/SiCp composite samples exhibits better interphase bonding behaviour than the other composite systems.

The fatigue crack growth curves reveal an approximately linear, or Paris law region, fitting the function da/dN = C ΔK. Crack growth rate vs. stress intensity range curves have been obtained using lock-in thermography. These results are in agreement with crack growth rate measurements using the conventional compliance method and calculations based on the

Deformation Characteristics of Aluminium Composites for Structural Applications 383

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Paris law. It becomes, therefore, evident that lock-in thermography has a great potential for evaluating nondestructively the fracture behaviour of metallic composite materials.

Finally, cyclic deformation data reveals that metallic foam sandwich panel samples do not produce consistent results with acceptable repeatability of results but by using calculated crack propagation life data and experimental data for both aluminium honeycomb and metallic foam sandwich panels a method of analysis has been proposed to predict fatigue life of metallic foam sandwich panels.
