Browsing by Author "Marquis, Gary, Prof., Aalto University, Department of Mechanical Engineering, Finland"

Filter results by typing the first few letters
Now showing 1 - 2 of 2
  • Mechanical modeling of particulate reinforced metal matrix composites
    (2016) Nafar Dastgerdi, Jairan
     School of Engineering | Doctoral dissertation (article-based)
    To predict and optimize the mechanical properties of new class of advanced composites, real engineering situations and appropriate assumptions should be considered. Moreover, a profound understanding of the relationship between real microstructure of composites and their mechanical properties is necessary. This study is concerned with the debonding damage and particle distribution effects of reinforcements on overall mechanical properties of particulate reinforced light weight metal matrix composites. A micromechanical model is developed to take into account debonding of reinforcements, particle size and the elastoplasticity by means of incremental damage theory. Reinforcement/matrix interfacial debonding phenomena is characterized by a cohesive zone model implemented in finite element method. The results show that the assumption of fully bonded particles in composites under loading is incomplete and the influence of cohesive energy at interface is considerable. In the case of a lower cohesive energy value, there are potential sites for debonding  and the stress-strain relation of damaged composite deviates from that of the perfect composite at a lower stress level. Mechanical properties of particulate reinforced composites are highly dependent on the real microstructure of the composite and spatial distribution of reinforcement particles. A new micromechanical model based on defining clustering parameters is presented to take account of effects of the randomness of the particles. For composite with clustering defect, the clustered regions would start yielding at a higher macroscopic stress during uniaxial tension. The finite element simulation based on the real morphology shows that the plastic flow on the matrix inside the cluster is inhibited. Due to the plastic flow constraint, there is a great tendency towards debonding and crack initiation around the perimeter of a cluster. Experimental findings show that there is a strong relationship between damage formation and the local volume fraction of reinforcements. Moreover, effects of microstructure and particle clustering on fatigue properties and crack initiation and propagation of novel amorphous particles reinforced Mg-composites are investigated. The experimental results show that the crack growth in particulate reinforced composites is a highly localized phenomenon influenced primarily by the distribution and microstructure of particles near the vicinity of the crack tip. The rate of the crack growth through the clustered region was significantly higher than through the matrix or through a region of well-dispersed particles. It is shown that composites with more uniform particle distribution possess a superior fatigue resistance and fatigue limit.
  • A study on effectiveness limitations of high-frequency mechanical impact
    (2016) Mikkola, Eeva
     School of Engineering | Doctoral dissertation (monograph)
    High-frequency mechanical impact (HFMI) has recently emerged as an efficient and user-friendly method for improving the fatigue strength of welded steel structures. Further benefit from the treatment can be obtained by using high-strength steels, as the level of improvement depends on steel strength. As a result, HFMI offers potential for high-strength lightweight design of welded structures. This is of interest in industries like shipbuilding and bridge construction, where welding is a widely used joining method. However, for standardizing HFMI as a weld toe improvement method, the limitations of the technology need to be understood. HFMI-treatment introduces compressive residual stresses at the weld toe, improves the local geometry and strain hardens the treated surface region. However, the effectiveness of the treatment is considered to rely on the existence of compressive residual stresses. This means that high mean stresses and high peak loads during variable amplitude loading, which might relax these stresses, can limit the benefit from HFMI. Therefore, this work focuses on the effectiveness limitations of HFMI under different loading histories. The aim is to understand the influence of different loading histories on fatigue improvement and to evaluate proposed fatigue assessment guidelines for HFMI-treated joints. First, available fatigue data was analysed statistically to investigate the effects of high mean stresses and variable amplitude loading with respect to expected fatigue strength improvement. To study the behaviour of residual stresses further, the local material behaviour at the weld toe needs to be known. This was determined by strain-controlled fatigue tests of thin HFMI-treated steel sheet specimens. The resulting data was used in simulations of local stress-strain response at an HFMI-treated transverse attachment to investigate the effects of stress ratio and peak loads on fatigue damage. Finally, allowable peak stresses in HFMI-treated welded joints were discussed based on the numerical results and available experimental data. The statistical analysis showed that the proposed fatigue assessment guidelines fit the current high mean stress and variable amplitude loading data. However, in some cases the proposed allowable stress limits were too conservative. This occurred even when residual stress relaxation was expected based on previous measurements and the current numerical results. The simulations indicated fatigue improvement due to weld toe geometry improvement and strain hardening as the reason. A maximum stress range of 1.2 times yield strength for negative stress ratios and maximum stress ratio of 0.7 were suggested as new design limits. However, further experimental and numerical work is recommended to confirm these limits.