Institute of Fundamental Technological Research

Nonlinear properties of metal matrix composites (MMCs) are studied. The research combines results of loading-unloading tensile tests, microstructural observations and numerical predictions by means of micromechanical mean-field models. AA2124/SiC metal matrix composites with SiC particles, produced by the Aerospace Metal Composites Ltd. (AMC) are investigated. The aluminum matrix is reinforced with 17% and 25% of SiC particles. The best conditions to evaluate the current elastic stiffness modulus have been assessed. Tensile tests were carried out with consecutive unloading loops to obtain actual tensile modulus and study degradation of elastic properties of the composites. The microstructure examination by scanning electron microscopy (SEM) showed a variety of phenomena occurring during composite deformation and possible sources of elastic stiffness reduction and damage evolution have been indicated. Two micromechanical approaches, the incremental Mori–Tanaka (MT) and self-consistent (SC) schemes, are applied to estimate effective properties of the composites. The standard formulations are extended to take into account elasto-plasticity and damage development in the metal phase. The method of direct linearization performed for the tangent or secant stiffness moduli is formulated. Predictions of both approaches are compared with experimental results of tensile tests in the elastic–plastic regime. The question is addressed how to perform the micromechanical modelling if the actual stress–strain curve of metal matrix is unknown.

metal matrix composites, tension with unloadings, damage, microstructure, non-linear effective properties

Metal matrix composites (MMCs) with aluminium alloy AA2124 matrix reinforced with 17 and 25% of SiC were investigated. Symmetric tension-compression fatigue tests at ambient temperature and tensile creep tests at elevated temperature were carried out. Specimens were subjected to fatigue or creep loading conditions. Tests were stopped after given number of cycles or strain values respectively. Electronic Speckle Pattern Interferometry (ESPI) was used in order to detect damage under fatigue and creep conditions. Strain fields were observed on specimens’ surfaces to localise strain concentrations that would indicate areas of damage development.

Creep, Damage, ESPI (Electronic Speckle Pattern Interferometry), Fatigue, MMC (Metal Matrix Composite)

The results of comparative examinations of mechanical behaviour during fatigue loads and microstructure assessment before and after fatigue tests were presented. Composites of aluminium matrix and SiC reinforcement manufactured using the KoBo method were investigated. The combinations of two kinds of fatigue damage mechanisms were observed. The first one governed by cyclic plasticity and related to inelastic strain amplitude changes and the second one expressed in a form of ratcheting based on changes in mean inelastic strain. The higher SiC content the less influence of the fatigue damage mechanisms on material behaviour was observed. Attempts have been made to evaluate an appropriate fatigue damage parameter. However, it still needs further improvements.

fatigue, damage, metal matrix composites, microstructure

An influence of the chemical composition, porosity and ageing on mechanical behaviour of light, multifunctional aluminium alloys (AlSi8Cu3 and AlSi7MgCu0.5) subjected to creep and low cycle fatigue (LCF) was investigated. The materials were tested to verify their applicability as the cylinder heads in car engines. During creep tests, a strain response of the materials was observed under a range of the step-increased stresses and different temperatures. The LCF tests were carried out under strain control in three blocks of 100 cycles each with a constant strain amplitude. The results of creep and LCF tests were analysed with regard to chemical composition, type of porosity and ageing of the materials tested. An influence of porosity on the creep resistance and lifetime was considered. The results of the LCF tests were compared for the materials in the as-received state and after ageing. An experimental evaluation of cyclic behaviour because of the LCF was carried out to check whether the hardening or softening effects can be observed in the materials. Taking into account the various history of loading, a stress response of the materials was investigated.

creep, cylinder heads, fatigue, light alloys

A consolidation of powders using the KOBO method at elevated temperature was elaborated for production of Al/SiC metal matrix composite (MMC). The observations of the mean strain and inelastic strain range during the force controlled high cycle fatigue (HCF) tests identified the ratcheting mechanism combined with mechanism characterized by cyclic plasticity. Damage parameters were calculated on the basis of strain signal acquisition during fatigue tests. Fatigue properties, creep resistance and life time of Al/SiC MMC were gradually improved with the increase of the SiC particles content. The microstructural observations of the material before and after tests were also performed.

MMC, KOBO method, fatigue, creep, monotonic loading, microstructure, damage

The AlSi8Cu3 and AlSi7MgCu0.5 cast aluminium alloy s of different composition and heat treatment were investigated to verify their applicability as cylinder heads in the car engines [1]. Creep tests under the step-increased stresses at different temperatures, and low cycle fatigue (LCF) tests for a range of strain amplitudes and temperatures were carried out. The results exhibit a significant influence of the heat treatment on the mechanical properties of the AlSi8Cu3 and AlSi7MgCu0.5. An interesting fact is that the properties strongly depend on the type of quenching. Lower creep resistance (higher strain rates) and lower stress response during fatigue tests were observed for the air quenched materials in comparison to those in the water quenched. Cyclic hardening/softening were also observed during the LCF tests due to the heat treatment applied. The mechanical properties determined during the tests can be used to identify new constitutive equations and to verify existing numerical models.

creep, low cycle fatigue, aluminum alloys, heat treatment

The paper presents experimental results of creep and low cycle fatigue (LCF) tests carried out on the as-received cast aluminium alloys with different chemical composition and porosity. The test programmes contain creep investigations under step-increased stresses at different temperatures, and cyclic plasticity under different strain amplitudes and temperatures

creep, low cycle fatigue, aluminium alloys

Fatigue tests of AA2124 based metal matrix composites (MMCs) with SiC particles reinforcement were carriedout. The MMCs were manufactured using powder metallurgy. Hysteresis loops were observed during subsequent cycles. Cyclic plasticity and ratcheting were identified as main fatigue damage mechanisms. Inelastic strain amplitude and mean inelastic strain were calculated to be used as fatigue damage indicators. Ultrasonic measurements during fatigue tests were performed but they did not allow to detect damage during fatigue test.

metal matrix composites, fatigue, damage

Creep tests under a range of step-increasing tensile stresses were carried out on the AlMg/SiC composites produced by means of the KOBO method. The content of reinforced phase was equal to 2.5, 5, 7.5 and 10%. The AlMg/SiC creep characteristics were compared to those of the Al/SiC composites. Structural assessments of materials in the as-received state and after creep were carried out.

metal-ceramic composites, KOBO method, creep, silicon carbide (SiC)

Fatigue tests and microstructural observations were carried out to perform damage analysis of Al/SiC and AlMg/SiC metal matrix composites ( MMCs ). The materials were manufactured using a KoBo method. Ratcheting was the dominant damage mechanism for Al/SiC and damage parameter on the basis of mean inelastic strain was calculated. For AlMg/SiC, in most cases, cyclic softening followed by hardening during subsequent cycles were noticed. Microstructural observations identified existence of SiC concentrations which influenced the volumetric fraction of defects after fatigue tests

metal matrix composites, fatigue damage, microstructure, KoBo method