Main Damage Mechanisms Research Articles

Energy absorption and damage mechanisms in progressive crushing of corrugated NCF laminates: Fractographic analysis

To develop reliable and physically based models for the crash behaviour of composite laminates, a thorough understanding of the failure mechanisms is crucial. Compression tests of corrugated Non-Crimp Fabric (NCF) laminates, made of carbon fibre unidirectional (UD) fabric with a [0/90]3S stacking sequence and epoxy, have been performed to study the energy absorbing damage mechanisms. Samples from the specimens have been studied with optical microscopy and Scanning Electron Microscopy (SEM) to identify the mechanisms involved in the crushing process. The specimens tested fail partly in bending and partly in pure compression with a mode I delamination separating these two regions. In the region failing in pure compression, the main damage mechanisms are kink band formation and matrix cracking of transverse bundles, whereas in the part failing in bending mixed mode delaminations, intralaminar shear fracture of axial bundles and kink band formation through parts of bundles are identified.

Impact properties of novel corrosion resistant hybrid structures

The objectives of this study were to investigate the effect of impact energy and rubber thickness on the impact properties of layered steel/rubber/composite hybrid structures. Both stainless steel and mild steel based hybrid structures were investigated. The degree of damage, the failure modes, and the absorbed energy were studied.It was found that rubber between steel and composite layers absorbs the impact energy and decreases the interfacial and internal damage in the studied hybrid structure and in its components. The amount of the absorbed energy did not change substantially when comparing structures with and without rubber. However, the area of permanent damage showed a decrease of nearly 50% with the use of rubber when comparing a structure without rubber to a structure with 1.5mm rubber. In addition, it was observed that the area of the damage is linearly dependent on the impact energy. The main damage mechanisms found were delamination at the steel/rubber and composite/rubber interfaces and fibre/matrix debonding in the composite layer.

Microscale damage mechanisms and degradation of fiber-reinforced composites for wind energy applications: results of Danish–Chinese collaborative investigations

Recent research works in the area of experimental and computational analyses of microscale mechanisms of strength, damage and degradation of glass fiber polymer composites for wind energy applications, which were carried out in the framework of a series of Sino–Danish collaborative research projects, are summarized in this article. In a series of scanning electron microscopy in situ experimental studies of composite degradation under off-axis tensile, compressive and cyclic loadings as well as three-dimensional computational experiments based on micromechanics of composites and damage mechanics, typical damage mechanisms of wind turbine blade composites were clarified. It was demonstrated that the damage mechanisms in the composites strongly depend on the orientation angle of the applied loading with the fiber direction. The matrix cracking was observed to be the main damage mechanism for tensile axial (or slightly off-axis axial) loading; for all other cases (off-axis tensile, compressive and cyclic tensile loadings), the interface debonding and shear control the damage mechanisms.

Effect of strain rate on low cycle fatigue with hold time in 9Cr rotor steel

One of the main mechanisms of turbine rotor damage is cyclic thermal stress produced during transition conditions, such as warm-up and shutdown processes. In this paper, the cyclic damage with ramp rate is studied in terms of material properties. To evaluate thermal stress damage during the cyclic operation, experiments were carried out on low cycle fatigue with hold time and a range of strain rates for COST FB2, 9Cr ferritic steel. Initially, the experimental fatigue life was evaluated using the time fraction and ductility exhaustion methods. The two analysis methods are compared with real life cases of different hold times and strain rates, and the accuracy is discussed. Next, to predict the plastic deformation curve for low cycle fatigue with hold time more accurately, an attempt was made to simulate cyclic hysteresis loop behaviour numerically using a time dependent viscoplastic constitutive model, and this was compared with the experimental hysteresis loop curve for each cycle number.

Damage of Composite Materials

Composite materials are used extensively in aerospace and more recently in automotive applications. Their high mechanical performances and good fatigue durability offer definite advantages compared to more traditional materials. However, in comparison with metal, damage mechanisms are complex and variability of the mechanical properties due to anisotropy, heterogeneity and defects (due to materials but also processing technologies) more important.To be able to design composite parts, understanding the micro-mechanisms of damage is essential. The article presents the main fatigue damage mechanisms on thermoset and thermoplastic composites. A state of art is done on numerous works on composite fatigue and fractographic investigations. Some case studies of in-service failure are discussed.

Stress redistributions in unit cells of fibre-reinforced polymer composites with interface degradation

Fil: Godoy, Luis Augusto. Universidad Nacional de Cordoba. Facultad de Ciencias Exactas Fisicas y Naturales. Departamento de Estructuras; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas; Argentina

Is sheer thenardite attack impotent compared with cyclic conversion of thenardite–mirabilite mechanism in laboratory simulation tests?

The precipitation of mirabilite during the wetting stage of a cyclic total immersion test has widely been implicated as the main damage mechanism of sodium sulfate. We, however, hypothesized that the damage contribution from thenardite cannot be discounted in laboratory tests. To verify the supposition, a series of laboratory experiments was undertaken by subjecting three types of rocks to the sodium sulfate attack. To unambiguously demonstrate the potency of thenardite, in addition to cyclic total immersion tests, continuous partial immersion tests were also performed under different environmental conditions, which allowed either sheer thenardite precipitation or reversible thenardite–mirabilite conversion. Although rock breakdown occurred only during immersion, the results clearly indicated that thenardite alone could induce significant damage in both cyclic total immersion and continuous partial immersion tests. In most cases, tests that involved mirabilite precipitation, however, were more destructive than sheer thenardite tests, regardless of the type of salt supply technique. With the same duration and temperature of the immersion stage, the difference in drying temperature influenced the degree of damage, revealing the damage contribution from thenardite even in the tests with cyclic phase conversion. The reason why mirabilite is generally credited to be more destructive than thenardite and the effect of test duration on the damage potency of thenardite are also discussed.

String Current in Decananometer nand Flash Arrays: A Compact-Modeling Investigation

This paper presents a detailed compact-modeling investigation of the string current in decananometer nand Flash arrays. This investigation allows, first of all, to highlight the role of velocity saturation, low-field mobility, and drain-induced barrier lowering on the string current versus read voltage characteristics. Results are validated on a 41-nm technology for different positions of the selected cell along the nand string, different pass voltages, and different array background patterns. The effect of cycling on the string current is then investigated by means of postcycling bake experiments, showing that the impact of charge trapping/detrapping and interface state generation/annealing varies as a function of the read current level. Compact-modeling results display that, at low read currents, charge trapping/detrapping represents the main damage mechanism for the cells, while interface states come into play for read currents close to the string saturation level via mobility degradation.

Investigations into the damage mechanisms of form fixture hardening tools

In metal forming operations such as form fixture hardening, the interaction between the tools and the work-piece is strongly influenced by the tribological properties at the interface. Damage or excessive wear of the tools can be detrimental to the quality of the final component and it also has an impact on the process economy due to increased maintenance or more frequent replacement of tools. The objective of this study was to investigate the damage mechanisms encountered in real form fixture hardening tools in order to understand the causes of tool failure and ultimately to come up with possible solutions for this problem.Advanced techniques such as scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) were used for obtaining an in-depth understanding of the different phenomena involved in the failure of form fixture hardening tools. Two different tools having different hardness values and microstructures that had been used in production were analysed.The damage mechanisms found included abrasive and adhesive wear, material transfer, corrosion and mechanical and thermal fatigue. The main damage mechanism was found to be cracking caused by mechanical stresses on the surface. Although both tools presented similar types of damage, the severity was different and it was strongly influenced by the microstructure.

The influence of contact stress distribution and specific film thickness on the wear of spur gears during pitting tests

One of the main gear damage mechanisms is the formation of pitting and spalling on the tooth flank. Several factors have significant influence on the damage formation, such as: contact stress level; tooth profile type; relative contact speed; surface finish and lubrication conditions. This work comprehends the global observation of all such parameters and was carried out to explain the phenomena related to this wear mechanism. The wear test equipment uses the power recirculation principle and is commonly known as FZG test rig. The gears were made from AISI 8620 steel and had two types of surface finishing (by shaving or by milling). The wear experiments were performed with two torque stages: 135 N.m (running-in) and 302 N.m (steady-state), and two test temperatures: 60oC (running-in) and 90oC (steady-state). The wear level was determined by using image analysis. In order to calculate the specific film thickness and friction coefficient, the roughness of tooth flank was measured at each test stop. After the experiments were completed, it was possible to confirm that, for both manufacturing processes, the boundary lubrication regime was adopted at the tooth flank and the specific film thickness presents a different behavior when compared to addendum, pitch diameter and deddendum regions. The wear on the gear flanks depended on the lubricant film thickness and it was higher for the milled gears.

Failure locus of polypropylene nonwoven fabrics under in-plane biaxial deformation

The failure locus, the characteristics of the stress–strain curve and the damage localization patterns were analyzed in a polypropylene nonwoven fabric under in-plane biaxial deformation. The analysis was carried out by means of a homogenization model developed within the context of the finite element method. It provides the constitutive response for a mesodomain of the fabric corresponding to the area associated to a finite element and takes into account the main deformation and damage mechanisms experimentally observed. It was found that the failure locus in the stress space was accurately predicted by the Von Mises criterion and failure took place by the localization of damage into a crack perpendicular to the main loading axis.

Protective effects of melanocortins on short-term changes in a rat model of traumatic brain injury*.

Treatment for traumatic brain injury remains elusive despite compelling evidence from animal models for a variety of therapeutic targets. Melanocortins have established neuroprotective effects against experimental ischemic stroke. We investigated whether melanocortin treatment of traumatic brain injury induces neuroprotection and promotes functional recovery. Randomized experiment. Research laboratory at a university hospital. Male Sprague-Dawley rats (n = 215). Experimental rat model of diffuse traumatic brain injury, the impact-acceleration model. Brain tissue nitrites, phosphorylation level of extracellular signal-regulated kinases, and c-jun N-terminal kinases; and expression of active caspase-3, tumor necrosis factor-α, BAX, and Bcl-2 as well as serum levels of interleukin-6, high mobility group box-1, interleukin-10, and brain histologic damage were evaluated 24 or 48 hrs after the insult. Sensorimotor orientation and limb use were evaluated at day 7 and learning and memory at days 23-30 after injury. Posttraumatic treatment every 12 hrs with the melanocortin analog [Nle, D-Phe]-α-melanocyte-stimulating hormone (starting 3 or 6 hrs after injury) inhibited traumatic brain injury-induced upregulation of nitric oxide synthesis, phosphorylation level of extracellular signal-regulated kinases, phosphorylation level of c-jun N-terminal kinases, and active caspase-3; reduced expressions/levels of tumor necrosis factor-α, BAX, interleukin-6, and high mobility group box-1; and increased those of Bcl-2 and interleukin-10. These molecular changes were associated with a reduction in brain tissue damage, as highlighted by histopathological findings and improved functional recovery. Pretreatment with the melanocortin MC4 receptor antagonist HS024 abated the positive effects of [Nle, D-Phe]-α-melanocyte-stimulating hormone. Our data indicate that melanocortins protect against traumatic brain injury, in a broad time window and through activation of MC4 receptors, by counteracting the main traumatic brain injury-related mechanisms of damage. These findings could have major clinical implications.

Avaliação de vida residual de tubos de forno reformador

Este trabalho avalia a vida remanescente dos tubos catalisadores de um forno reformador devido ao fato do fim da vida de projeto de 100.000 horas estar próximo. O principal mecanismo de dano destes tubos é a fluência, por isso a análise seguiu roteiro da Part 10 do documento API-579/ASME FFS-1 (2007). O material dos tubos é uma liga HP modificada, para a qual existe escassez de dados de propriedades na literatura. Por isso, as propriedades de fluência necessárias para a avaliação foram determinadas experimentalmente utilizando correlação de dados de ensaios de tração a quente e fluência. Os resultados indicam que os tubos não necessitam ser substituídos de imediato e que sua vida residual é estimada em 22,3 anos operando na temperatura de projeto

Damage and Thixotropy in Asphalt Mixture and Binder Fatigue Tests

Fatigue cracking is considered one of the main damage mechanisms in asphalt pavement design. Design methods use fatigue laws obtained by laboratory testing of the materials involved. Typically, these tests consist of subjecting the asphalt mixture to cyclic loading until failure occurs. However, failure is associated not with specimen fracture (which is unusual), but with a slight decrease in the mechanical properties of the material, usually in the complex modulus. As a consequence, it is important to differentiate between real damage to the material and changes in its viscoelastic behavior and thixotropy. It is also crucial to account for the healing that occurs in asphalt material after rest periods. The above considerations are important in the fatigue testing of asphalt binders because these materials show pronounced viscoelastic behavior and thixotropy, especially when subjected to cyclic loading. This paper demonstrates that in many cases what is taken for fatigue failure during testing (i.e., a decrease in the complex modulus below half of its initial value) is actually thixotropy. Thus, the complex modulus can be recovered by reducing the loading or, as in this study, the strain applied. In contrast, asphalt mixtures experience irreversible damage, and depending on the asphalt binder, the thixotropic effects are more or less pronounced. This paper analyzes the failure criteria currently used in the fatigue testing of asphalt mixtures and binders and evaluates the parameters chosen, namely, complex modulus (G*) and phase angle (δ) to characterize asphalt binders (G*sin δ). A cyclic uniaxial tension–compression test under strain-controlled conditions was performed. Three test modalities were used: time sweeps (constant strain amplitude until total failure), increasing strain sweeps (increase in strain amplitude every 5,000 cycles), and up-and-down strain sweeps (alternating increases and decreases in strain amplitude).

Shock wave emission from a cloud of bubbles

Shock wave emission from a hemispherical cloud of bubbles, generated by short pulses of high-intensity focused ultrasound, was investigated by acoustic measurements and ultra-high-speed photography with framing rates of up to 200 million frames s−1 and exposure time of 5 ns. The maximum amplitude and the duration of the shock wave emitted during bubble cloud rebound increase with increasing the maximum radius of the cloud and, at equal maximum radius, are larger than the corresponding values obtained in the case of individual spherical bubbles. At a maximum radius of about 1300 μm, the shock pressure for a bubble cloud is a factor of three larger than the value measured in the case of individual spherical bubbles. In both cases, however, the shock pressure decays proportionally to r−1 with increasing distance r from the emission centre. The main damage mechanism of bubble clouds collapsing near a boundary material is the high pressure generated inside the cloud at the minimum volume. For a bubble cloud with a maximum radius of 760 μm, the maximum pressure at the bubble cloud, and thus the pressure acting on a nearby boundary material, is about 1.5 GPa. On the other hand, shock wave-induced mechanical effects on biological tissue are restricted to very small dimensions on a cellular or sub-cellular level. We discuss the implications of the results for the therapeutic applications of high-intensity focused ultrasound such as lithotripsy and histotripsy.

Numerical simulations of tungsten melt layer erosion caused by J×B force at TEXTOR

Tungsten, in the form of a macrobrush, is seen as one of the candidate materials for the ITER divertor. The melting of tungsten, melt motion and melt splashing are expected to be the main mechanisms of surface damage, which determines the lifetime of plasma-facing components. Experiments with long-time plasma action at the metallic surface in a strong magnetic field demonstrate that the J×B force generated by thermo-emission electrons dominates in the acceleration of the melt layer and leads to high target damage. In this paper, numerical simulations of tungsten target damage caused by the long-time plasma heat loads supporting the TEXTOR experiments are performed by the MEMOS code. The calculated tungsten target damage is in good agreement with the target damage observed in the TEXTOR experiments, which allows projections of surface damage to be made at ITER and DEMO conditions.

Damage investigation of the aged aluminium cable steel reinforced (ACSR) conductors in a high-voltage transmission line

In order to evaluate the damage of the 26-year-old ACSR conductors of the 100 kV Zhu–Xie transmission line in Tongling, Anhui province, China, the conductors across the #11 and #12 towers were studied via morphological observation, metallurgical analysis and individual wire tensile strength (IWTS) test, especially for the conductors under the vibration dampers and the strain clamps. The results show that the outer Al strands of the conductor under the vibration damper and the strain clamp are mainly subject to the scraping damage by the aluminum armour tape and the adhesion/abrasive wear injured by the aluminum armour tape and the other Al strands. As for the inner Al strands, the main damage mechanisms are of the adhesion/abrasive wear and the propagation of the fatigue cracks originated from the wear surface. For the steel core strands, only the zinc coating is partially destroyed, but the steel cores are survived. After 26 years running, the average IWTSs of the outer/inner Al strands decrease more or less, however, the IWTS of the steel core strands is almost not changed. The aged conductors are still in a good condition, but more intensive inspection and maintenance is suggested.

Tribological behavior of plasma sprayed Cr2O3–3%TiO2 coatings

► Tribological behaviors of plasma-sprayed conventional and nanostructured ceramic coatings using pin on disc type dry sliding and pot type slurry erosion test were investigated in the present work. ► The experimental results indicated that there were two main wear mechanisms, plastic smearing and adhesive tearing, in the worn coatings under dry sliding. ► The main mass removal mechanisms observed in all the coatings subjected to slurry erosion were the formation and propagation of brittle cracks resulting in the detachment of coating surface material. Tribological behaviors of plasma-sprayed conventional and nanostructured Cr 2 O 3 –3%TiO 2 ceramic coatings (i.e., CC3T and NC3T, respectively) using pin on disc type dry sliding and pot type slurry erosion test were investigated in the present work. The experimental results indicated that there were two main wear mechanisms, plastic smearing and adhesive tearing, in the worn coatings under dry sliding. Plastic smearing corresponded to a lower average friction coefficient and wear rate, while adhesive tearing corresponded to higher values. The erosive environment selected for the slurry erosion experiments include 10, 20 and 30% of SiO 2 slurry concentrations in water with particle size 75–106 μm. The main damage mechanism observed in all the coatings submitted to slurry erosion were the formation and propagation of brittle cracks resulting in the detachment of coating surface material. Microstructural investigation was also carried to investigate the wear and erosion mechanism of the coatings using FE-SEM and EDS analysis. Properties like microhardness and porosity were also investigated for these coatings. Tribological performance of NC3T was better as compared to CC3T as observed in the present work.

Effect of low-temperature plasma carbonitriding on the fretting behaviour of 316LVM medical grade austenitic stainless steels

Abstract 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.

Fatigue damage of ultrafine-grain copper in very-high cycle fatigue region

Ultrafine-grained copper produced by ECAP technique was tested in ultrasonic fatigue in the region of lives from 108 to 2 × 1010 cycles. The fatigue strength of ultrafine-grained copper is by a factor of 2 higher than that of conventional-grain copper. The occurrence of surface fatigue slip markings is rare. No grain coarsening was observed either in bulk or in areas underneath the surface slip markings. Localized collective grain-boundary slip of near-by oriented grains is considered to be the main damage mechanism leading to fatigue microcrack initiation either directly in the roots of surface intrusions or underneath the surface slip markings.