Main Damage Mechanisms Research Articles

Is homocysteine a pro-oxidant?

High plasma homocysteine concentrations have been found to be associated with atherosclerosis and thrombosis of arteries and deep veins. The oxidative damage mediated by hydrogen peroxide production during the metal-catalyzed oxidation of homocysteine is to date considered to be one of the major pathophysiological mechanisms for this association.In this work, a very sensitive and accurate method was employed to measure the effective production of H2O2 during homocysteine oxidation. Furthermore, the interaction of homocysteine with powerful oxidizing species (hypochlorite, peroxynitrite, ferrylmyoglobin) was evaluated in order to ascertain the putative pro-oxidant role of homocysteine.Our findings indicate that homocysteine does not produce H2O2 in a significant amount (1/4000 mole/mole ratio of H2O2 to homocysteine). Moreover, homocysteine strongly inhibits the oxidation of luminol and dihydrorhodamine by hypochlorite or peroxynitrite and rapidly reduces back ferrylmyoglobin, the oxidizing species, to metmyoglobin.All these results should, in our opinion, lead to a rethinking of the commonly held view that homocysteine oxidation is one of the main causative mechanisms of cardiovascular damage.

A homogenized microstructural approach to creep fracture

A two-dimensional nonlocal continuum model is proposed in this paper for creep damage in polycrystalline materials. Starting from previous micromechanical modeling, a heuristic homogenization approach is adopted to derive a theory for the macroscopic response. The model accounts for the main damage mechanisms (grain boundary sliding, nucleation, growth and coalescence of cavities along the grain boundaries) responsible for the creep fracture process. The resulting constitutive law takes into account the nonlocalities expressed through the gradients of the stresses and the damage variables.

Fatigue behaviour of aluminium matrix composites reinforced with continuous alumina fibres

Alumina (N610) reinforced pure Al (A9) or Al–2 wt-%Cu alloy (AU2) unidirectional (UD) composites which combine a highly ductile matrix with a strong interface bonding, present high static and dynamic mechanical performances. In the present paper, the fatigue behaviour of quasi­UD N610+A9 and N610+AU2 composites is investigated. Notwithstanding the presence of the transverse bundles, the longitudinal behaviour in tension and fatigue of these quasi-UD composites is nearly equivalent to that of the pure UD. Only in the case of the N610+AU2 quasi-UD composite, the fatigue limit is about 30% lower than that of the pure UD. Acoustic emission (AE) monitoring correlated with microfractography and microstuctural examinations has enabled identification of different stages in the evolution of damage and the association of these with damage and failure mechanisms. In fatigue, three main damage mechanisms are activated in a sequential and/or superposed mode during a three stage evolution.

Influence of Fibre-Matrix Adhesion on Mechanical Properties of Graphite/Epoxy Composites: III. Impact and Dynamic Mechanical Properties

An experimental investigation was carried out on the graphite/epoxy composites with two different fibre surface conditions to identify the effect of fibre-matrix adhesion on the impact and dynamic mechanical properties. The results obtained from Charpy and falling weight impact tests did not show a clear influence of fibre-matrix adhesion on the impact properties of the composites. This is attributed to the highly localised damage process of impact tests, in which the main damage mechanism is the fibre breakage and there are no extensive delamination and fibre pull-out mechanisms involved. The dynamic mechanical analysis (DMA) is a non-destructive method and it is different from the common methods for characterisation of fibre-matrix interfacial properties such as fibre pull-out and fibre fragmentation tests in which fibre-matrix interface debonding is produced. The results of DMA tests indicate the dynamic properties of the composites are not very sensitive to the change of fibre-matrix adhesion. During the DMA tests, there is no or little fibre-matrix debonding induced. Therefore, the DMA method may not be very applicable for evaluating the fibre-matrix interfacial adhesion in practical applications.

Prediction of the effective damage properties and failure properties of nonlinear anisotropic discontinuous reinforced composites

We studied the tensile behavior and damage of two different composites. An Al2080 reinforced with different volume fractions of silicon carbide particles and an SMC-R (polyester matrix with randomly oriented glass fibres). The main damage mechanism is particle failure (for the Metal Matrix Composites (MMC)) or fibre–matrix interface debonding (for the sheet moulding compound (SMC)). Moreover, for the AlSiC p composite, regions of the matrix adjacent to broken particles are sites with high hydrostatic tension and hence the nucleation of cavities is expected. Using the J integral and HRR modified solution it is possible to calculate the growth of these voids. Macroscopic failure is governed by a critical volume fraction of voids. In this paper, we propose a micro-mechanical approach to model the elastoplastic damageable composite behaviour of the composites. Moreover, in the case of the MMC, the proposed model is able to predict the failure strain of the composite as a function of the micro-structure. For the SMC, the elastic damageable behaviour law is introduced in a finite element code in order to predict the damage of a structure. In all cases, the simulated results are compared to experimental ones and show a fairly good agreement.

Scratch behavior and material property relationship in polymers

The scratch behaviors of a broad range of polymers are investigated to understand how the material characteristics of a polymer affect the scratch resistance. A constant load scratch test and a progressive load scratch test are chosen for the present study. A scratch model proposed by Hamilton and Goodman is applied to correlate the mechanics and material parameters during the scratch process. An attempt is made to correlate the scratch behavior and basic material properties, such as the Young's modulus, yield stress, and tensile strength. A correlation between the scratch behavior and material surface characteristics, such as the friction coefficient, scratch hardness, and elastic recovery, under the current scratch tests are also made. Scratch subsurface damage in polymers is studied using optical microscopy. It is shown that shear yielding is the main cause of the plastic flow scratch pattern, while tensile tear on the surface and shear-induced fracture on the subsurface are the main damage mechanisms found in the fracture scratch pattern. The main causes for the susceptibility of thermoplastic polyolefins to scratch damage are discussed. Approaches for making scratch-resistant polymers are also addressed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 47–59, 2001

Blunt-head trauma associated with widespread water-diffusion changes

Blunt-head trauma is a major cause of disability giving rise to neurological deficits, neuropsychological deficits, and epilepsy. Necropsy studies have identified surface contusion and lacerations, diffuse axonal injury, affecting predominantly central myelinated structures, and ischaemia as main mechanisms of damage.1 To localise damaged regions and measure the damage in vivo we used diffusion-tensor imaging (DTI). We investigated a 42-year-old man who had suffered from a severe blunt-head trauma in a road traffic accident with skull fracture and right frontotemporal extradural haemorrhage 9 years ago.

Hydroabrasive wear behaviour and damage mechanisms of different hard coatings

A broad range of different types of hard coatings were investigated under abrasive–erosive conditions. The slurry used in this investigation is composed of water and quartz sand (silica sand). Using different size distributions of the silica-sand particles, two-body and three-body abrasive wear were simulated. Supplementary microstructural investigations were performed to reveal the main damage mechanisms. In this work, different commercially available thermal, thermomechanical and diffusional coatings as well as coatings produced by physical vapour deposition (PVD) techniques, deposited on three different substrate materials, were investigated. A general result is that the wear behaviour of the different coatings seems to be governed primarily by the microstructure rather than by the coating process. Particular thermomechanical plasma- or detonation-sprayed coatings lead to a marked improvement of the wear resistance in comparison to the behaviour of the non-coated substrate. The wear behaviour of selected coating/substrate systems was ranked and discussed by taking into account the different microstructures and damage mechanisms.

Mechanical behavior of Al−Li/SiC composites: Part II. Cyclic deformation

The deformation and failure mechanisms under cyclic deformation in an 8090 Al-Li alloy reinforced with 15 vol pct SiC particles were studied and compared to those of the unreinforced alloy. The materials were tested under fully reversed cyclic deformation in the peak-aged and naturally aged conditions to obtain the cyclic response and the cyclic stress-strain curve. The peak-aged materials remained stable or showed slight cyclic softening, and the deformation mechanisms were not modified by the presence of the ceramic reinforcements: dislocations were trapped by the S′ precipitates and the stable response was produced by the mobile dislocations shuttling between the precipitates to accommodate the plastic strain without further hardening. The naturally aged materials exhibited cyclic hardening until failure, which was attributed to the interactions among dislocations. Strain localization and slip-band formation were observed in the naturally aged alloy at high cyclic strain amplitudes, whereas the corresponding composite presented homogeneous deformation. Fracture was initiated by grain-boundary delamination in the unreinforced materials, while progressive reinforcement fracture under cyclic deformation was the main damage mechanism in the composites. The influence of these deformation and damage processes in low-cycle fatigue life is discussed.

Multiscale modeling of the damaged plastic behavior and failure of Al/SiCp composites

Abstract We studied the tensile behavior and damage of an aluminium X2080 reinforced with different volume fractions of silicon carbide particles. The main damage mechanism is particle failure. Regions of the matrix adjacent to broken particles are sites with high hydrostatic tension and hence the nucleation of cavities is expected. Using J integral and HRR modified solution it is possible to calculate the growth of these voids. Macroscopic failure is governed by a critical volume fraction of voids. The originality of this work lies in the modeling of the composite using a micromechanical approach.

Fracture behaviour of a SiC-particle-reinforced aluminium alloy at high temperature

The fracture behaviour of a composite based on 7075 aluminium alloy was studied under uniaxial tensile loading in the temperature range 25–400 °C at a strain rate of 10 −3 s −1. The ductility of the composite was found to be much lower than that of the monolithic alloy at all temperatures, but both materials exhibited similar strength levels above 300 °C. Particle fracture was the main damage mechanism prior to final fracture at room temperature, while interface debonding together with inter-particle voids were dominant features in fracture at high temperature. Large particles and regions of clustered particles were found to be the locations prone to damage in the composite at both room and high temperatures. At room temperature, particle fracture was observed at clusters of particles as well as in large particles, whereas at high temperature voids nucleated in the matrix closely adjacent to particles and at particle ends. This can be attributed to the high local stress in these regions and the high probability of flaws in large particles. It was also found that the internal damage associated with particle clusters was in regions below the fracture surface and that the extent of damage in the reinforced alloy increased with increasing temperature.

Damage assessment in an Al/SiC composite during monotonic tensile tests using synchrotron X-ray microtomography

High resolution X-ray tomography is used to study the evolution of damage in an Al/SiC composite during monotonie tensile tests at room temperature. Two main damage mechanisms are observed when the plastic regime is reached: (i) The cracking of the matrix on brittle oxides resulting from the processing of the material; (ii) the cracking of SiC particles. The aspect ratio of the broken particles along the tensile direction is found to be high and the damage accumulation rate is different at the surface and in the bulk of the material. Those results are discussed with respect to the resolution of the imaging technique and to the strain level reached during the tests.

Notch fracture toughness of a cast duplex stainless steel: modelling of experimental scatter and size effect

A cast duplex stainless steel containing about 20% ferrite and embrittled at 400°C is studied. After ageing, the main damage mechanism is nucleation of cleavage cracks in the ferritic phase. This damage appears heterogeneously as millimetric clusters resulting in scatter and size effects in experimental results obtained for laboratory specimens (smooth and notched tensile bars, Charpy specimens). As the influence of the strain rate on the experimental results could be neglected, slow loading three point bending tests (instead of dynamic impact tests) on standard Charpy U or V-notch specimens were especially investigated. It was shown that the energy needed for crack propagation is almost constant, with low scatter. On the other hand, that needed for crack initiation depends on notch acuity and specimen thickness, with much more significant scatter. A model based on local damage rates measured by quantitative metallography and the use of the plastic criterion introduced by Gurson is proposed to predict fracture. Results of finite element calculations, integrating random damage nucleation, show that the model predicts both mean values and scatter of measured ductilities (smooth and notched tensile bars) and crack initiation energies (Charpy U-notch specimens). Size effects can also be accounted for.

Creep and fatigue interaction in a nickel-based alloy: Wang, X., Wang, D.N. and Kong, Q.P. Scr. Metall. Mater. (15 Feb. 1993) 28 (4), 401–404

An optimized low-temperature plasma carbonitriding process at 430 °C for 15 h was used to treat the surface of 316LVM medical grade austenitic stainless steel. By using a servo-hydraulic dynamic test machine, the fretting behaviour of both as-received and the surface-treated 316LVM plate samples against martensitic stainless steel balls was studied in the Ringer's solution at various displacement amplitudes. The experimental results demonstrate that the optimized low-temperature plasma surface treatment can produce a precipitate-free, C/N supersaturated S-phase layer. Such layer revealed a hardness of 5 times that of 316LVM substrate. The formation of S-phase can protect the 316LVM from corrosion in the Ringer's solution, and the hard surface-treated layer can significantly improve the fretting wear resistance of 316LVM material. When fretted in a mixed regime, a sharp drop of friction force was observed for untreated 316LVM after fretting for about 1000 cycles, which was mainly caused by the penetration of Ringer's solution into the contact area of the fretting pairs. As a comparison, no friction drop was observed through the fretting process on the surface-treated samples. Due to the high ratio of elastic modulus to hardness (E/H) of untreated sample, the main damage mechanism on untreated sample was adhesive and abrasive wear during the fretting process. However, because of the high hardness and low E/H ratio of the surface-treated sample, the main damage mechanism was mild abrasive wear. In conclusion, the low temperature plasma surface treatment can effectively improve the fretting wear properties of 316LVM stainless steel, which could pave the way to develop the high-performance, long-life body implants.

Plasticity of oxide dispersion strengthened ferritic alloys

Two 13%Cr oxide dispersion strengthened (ODS) ferritic alloys, DT and DY, exhibiting different oxide particle size distribution and a χ phase precipitation were studied. Their tensile properties have been tested from 20 to 700 o C. Experimental observations during room temperature tensile tests performed in a scanning electronic microscope have shown that the main damage mechanism consists in microcraking of the χ phase precipitates on grain boundaries. These alloys are high tensile and creep resistant between 500 and 700 o C. Their strongly stress-sensitive creep behaviour can be described by usual creep laws and incorporating a threshold stress below which the creep rate is negligible

Lipid peroxidation. Biopathological significance

Peroxidation of unsaturated lipids, initially studied in the chemestry of oil and fat rancidity, has become a problem of increasing interest in the biological field, because of its proposed role in a variety of pathological conditions. The general mechanism of the process, the formation of toxic aldehydes capable to react with protein and non protein thiols, and the overall effects in cellular membranes are reviewed. The possible implications of lipid peroxidation as one of the main mechanisms of cellular damage in both toxic injury and other pathological conditions are discussed.

Short-term degradation mechanisms of piping

The operation of PWR-type EDF plants has shown that components have been subjected to loadings higher than the design basis loads. For example, localized degradations on nuclear system pipes were found after relatively short times (10-10 4 hours). The main damage mechanisms involved are: • - erosion—cavitation arising from the type of hydraulic flow, • - vibrational fatigue arising from the flow or operation of mechanical • - corrosion fatigue occurring in some confined spaces (dead ends). This paper addresses these damage modes and the mitigating steps taken to cope with them, together with the initiatives taken for future reactors.

Interaction of XeCl laser radiation with polymers

An experimental investigation was made of the interaction of nanosecond ultraviolet radiation pulses with polyethylene, polytetrafluoroethylene, and polyethylene terephthalate. A study of ablation of polyethylene terephthalate showed that under certain conditions the rate of etching increased severalfold and the damage to the regions surrounding the irradiation spot decreased. Under these conditions the main polymer damage mechanism was two-photon dissociation of the molecules.

The effects of radiation on electronic devices and circuits / Wirkung von Strahlung auf elektronische Bauteile und Schaltungen

Radiation damage of semiconductor components constitutes a safety risk for the functioning of circuits when operated in a radiation-exposed environment. Despite the manifold dependences of the radiation damage, fundamental characteristic features are given so that sensitivity graduations of the active component spectrum can be established. A general description of the sensitivity and the most important parameter changes of devices is given by introducing the main damage mechanisms, as displacements of lattice atoms and ionization effects. Recently obtained results from experiments on the radiation susceptibility of charge coupled devices as well as on the influences of single component degradations on circuit behavior are presented.

Intraocular Nd:YAG laser surgery: laser-tissue interaction, damage range, and reduction of collateral effects

The damage mechanisms of intraocular Nd:YAG laser surgery and their respective damage ranges were investigated in vitro using bovine cornea specimens as a model tissue. The main damage mechanisms are plasma formation and expansion, emission of acoustic transients, and cavitation with jet formation. When a sequence of laser pulses is applied, the interaction of the acoustic transients with gas bubbles remaining from preceding laser exposures is also important. To distinguish the effects caused by the different physical mechanisms, laser pulses were aimed directly onto the corneal endothelium, through the cornea, and parallel to the cornea at various distances. Simultaneously, the cavitation bubble size was determined. The damage range of the acoustic transients produced by a 4 mJ laser pulse is several millimeters, when they can interact with small gas bubbles attached to the corneal endothelium. >