Decrease In Residual Strength Research Articles

Study on mechanism of radial long cracks propagation on sliding surface of wheel brake discs

A long radial crack with a length of more than 60 mm was observed near the bolt hole of the brake disc of high-speed railway vehicles. It is necessary to study its causes and propagation mechanism. In this study, the gradient change of grain size was studied by microscopic observation of the brake disc after service. Through a variety of tensile tests, a relationship describing the gradient distribution of mechanical properties of materials was established. The radial crack propagation at the bolt hole of the friction surface was analyzed by using the extended finite element model (XFEM). The final crack lengths at the initial speeds of 250 km/h and 350 km/h are 32 mm and 43 mm, respectively. The reduction of material strength on the sliding surface and the stress concentration near the bolt hole make the crack propagate rapidly. The crack length increases with the decrease of residual strength. Cracks may continue to expand to the core through fatigue propagation, eventually leading to fatigue failure. It is proposed that the hot spot area around the bolt hole should be the key area for checking cracks during operation and maintenance.

Methodology for simulating the elastoplastic behavior in crack propagation problems using dual formulations of the boundary element method

Due to the application of cyclic loads, the existing crack length in a structure increases with time, resulting in greater stress concentration around the crack tip and therefore causing an increase in propagation velocity and a decrease in residual strength of the structure which, at a given time, becomes so low that the structure can no longer support the service loads. This paper presents a new approach to evaluate two-dimensional elastoplastic models in a crack propagation scenario, based on the boundary element method and its dual formulations. The methodology adopted consists of two stages: firstly, the simulation of the elastoplastic behavior, which considers the process of initial stresses and allows the treatment of several flow criteria, without taking into account the incompressibility of inelastic deformations. In this step, the domain integral due to non-homogeneous terms in the plastification region is transformed into a boundary integral by the Dual Reciprocity Method (DRM) using the polyharmonic splines functions due to its local behavior and, from the plastic calculus, the J-Integral is used to compute the SIF's. The second stage aims to simulate, in an incremental way, the crack propagation path using homemade software for crack modeling and analysis based on ElastoPlastic Fracture Mechanics (EPFM) and the Dual Boundary Element Method (DBEM). To validate the adopted methodology, as well as correctly simulate the mechanical behavior of cracks in the plastic regime of the material, three 2D models with straight and inclined cracks are used. Initially, the von Mises criterion for perfectly plastic materials is used and the numerical results compared with classic models from the literature, showing the effectiveness of the program in crack treatment and in the prediction of the propagation path, as well as the non-linear algorithm in the region plastification using the DRM.

Effect of high temperature on the compressive and flexural performance of fibrous concrete- an experimental investigation

Concrete is a versatile building material and is used for different structures including hazardous occupancy. The discharge of thermal load on reinforced structural elements is rare; but can occur as a result of fire breakout, post- earthquake effect or long-term temperature exposure. The thermal effects on concrete are of primary concern as they determine the integrity of structure after exposure to very high temperatures. From researches so far; the addition of fibres was found to improve the mechanical properties of concrete. So the aim of the present study is to evaluate the changes which the fibres can make in the residual properties of concrete after exposure to elevated temperatures. The study considers two types of synthetic fibres such as polypropylene fibres and glass fibres; added to concrete at various percentages by volume and subjected to different elevated temperatures. For the experimental study, the standard concrete specimens were prepared and exposed to varying temperatures from 100 to 800 °C. Then, the compressive strength and flexural strength of concrete samples were determined as per Indian standards. The parametric studies were carried out on the effect of types of fibres and fibre content for different elevated temperatures. The present study found that the fibrous concrete exhibit higher strength than ordinary concrete at temperatures up to 600 °C, despite the usual decrease in strength with increasing temperature. Moreover, a higher content of fibres in fibrous concrete enabled the concrete to withstand thermal effects even at high temperatures. The rate of decrease in residual strength was also observed to be lower in fibrous concrete compared to ordinary concrete. The study also found that incorporating higher content of glass fibre into concrete improved its residual mechanical properties. These results suggest that fibrous concrete could be a promising material for concrete elements under high-temperature applications.

Influence analysis on the shear behaviour and failure mode of grout-filled jointed rock using 3D DEM coupled with the cohesive zone model

Grouting is one of the primary methods used to reinforce the strength of fractured rock masses. In this study, a three-dimensional discrete element method (3D Co-DEM) grouted joint shear model, which consists of upper and lower intact rock blocks, 3D real joint surfaces and a grout-filled layer, was developed with the cohesive zone model. The 3D real joint surface was imported into the model using 3D scanning point cloud data, and the grout-filled layer was described by a cohesive zone model. Then, a numerical simulation algorithm was developed to implement the numerical direct shear test and grout-filled layer cracking simulation. Also, the parameters of the proposed method were calibrated, and the effectiveness of the cohesive zone model and direct shear numerical test were verified by comparison with analytical and experimental results. Finally, grouted jointed specimens with regular sawtooth were used to analyse the influencing factors of shear characteristics. Numerical tests showed that shear peak strength and residual strength decrease with increasing grout water-cement ratio but increase with increasing JRC. Specifically, with increasing sawtooth inclination angle, the shear peak strength first decreases and then increases, while the peak shear strength increases as the sawtooth height increases.

Damage tolerance mechanism of bioinspired hybridization helical composite in lobster homarus americanus

A typical twisted laminar structure was an essential feature for the exoskeleton of the lobster Homarus americanus to resist impact damage and penetration. In fact, two kinds of helical structures were observed in the exoskeleton with different pitch angles and ply thicknesses in the exocuticle and endocuticle. It is worth investigating the evolutionary mechanism of the hybrid helicoidal structure rather than a single helix. In this paper, two pitch angles and ply thickness were introduced to reproduce the hybridization helicoidal structures of crustacean ectoskeletons. Numerical investigations were carried out, including low-velocity impact and compression after impact (CAI). The results showed that the bionic architectures need to balance the protection function provided by the endocuticle and load-carrying performance provided by the exocuticle. In addition, the qualitative relationship between design parameters and performance was analysed. The decrease in angle and the increase in layer thickness will lead to an increase in energy dissipation during the impact process and a decrease in residual strength. The research results provide a reference for the design of hybrid bionic structures with multimechanism cooperation.

Quantifying the sensitivity of plate aspect ratio and edge boundary conditions on CFRP plate impact and post impact residual strength

Much literature exists on composite plate impact damage and residual strength post impact, but there is limited work which systematically examines how plate behaviour alters with plate aspect ratio and edge rotational restraint. Through simulation of composite stiffened plates under low velocity impact and Compression After Impact loading, the presented work demonstrates that laminate impact response and subsequent residual strength is highly sensitive to both plate aspect ratio and boundary conditions. Increasing edge restraint for low aspect ratio plates results in a moderate increase (up to 13.3%) in residual strength. Conversely, for higher aspect ratio plates (representative of aircraft stiffened panel components), the same change in edge restraint results in an equivalent decrease in residual strength (up to 13.0%). In particular, the interaction between impact induced damage and the onset of plate buckling observed in higher aspect ratio plates, and the subsequent effect on predicted residual strength, is not captured when examining low aspect ratio plates utilised in traditional material coupon testing practices.

RESEARCH OF THE INFLUENCE OF LEALED TECHNICAL LIGNOSULPHANATES ON THE PHYSICAL-MECHANICAL AND TECHNOLOGICAL PROPERTIES OF LIQUID-GLASS MIXTURES

The paper presents the results of a study of the effect of technical alkalized lignosulfonates on the physicomechanical and technological properties of liquid glass mixtures. As a result of the study, a composition was selected that achieved the lowest residual strength. Using a two-beam scanning electron microscope "Versa 3D", the structure of liquid glass films on a refractory quartz filler is examined with the aim of defects that provide a decrease in residual strength.

Acoustic fatigue properties investigation of plain weave C/SiC composite plate

In this paper, an acoustic fatigue model is proposed to obtain the residual strength of plain weave C/SiC composite plate subjected to acoustic loading. The constant amplitude normalized residual strength model and the S-N curves are used in new model, in which the probability density function of stress peaks are adopted to evaluate the effects of random loading on the degradation of residual strength. The degradation of residual strength in a time increment is calculated via an integration of the decrease of residual strength due to the stress peaks in the definitional domain of the peak probability density function. The evolutions of the residual strength under acoustic loading are determined via a double integral method. The residual strengths of C/SiC composite plate predicted by proposed model are in good agreement with the experimental results.

Effect of Cyclic Freezing-Thawing on the Shear Mechanical Characteristics of Nonpersistent Joints

The effects of freezing-thawing cycles and persistency differences have a significant impact on the shear mechanical properties of joints. In this paper, a series of joints direct shear tests were performed on freezing-thawing treated joints to investigate the effect of freezing-thawing cycles and the persistency on the shear strength deterioration of joints. Shear strength and residual strength decrease with the freezing-thawing cycle increase and joint persistency increase. Shear strength damage mainly generates in the initial stage of the freezing-thawing cycle, and the shear strength decreases slightly in the late freezing-thawing cycle stage. The freeze-thaw cycle has a minimal effect on the shear strength of joints with low persistency, yet has a great effect on joints with high persistency. The damage of joint roughness caused by freezing-thawing cycles increases with joint persistency increases. When the joint persistency is constant, the shear strength parameter decreases with the freezing-thawing cycle at first and then tends to be stable. Cohesion is the dominant factor that controls shear strength. When freezing-thawing cycles are constant, the friction angle decreases slowly with persistency at first and then decreases rapidly, and the friction angle is the dominant factor that controls shear strength.

Fatigue damage evolution in thick composite laminates: Combination of X-ray tomography, acoustic emission and digital image correlation

The main purpose of this study is to provide a thorough experimental investigation of fatigue damage mechanisms and evolution in thick carbon/epoxy laminate subjected to bending load. The use of X-ray computed tomography (CT) in this study has allowed the visualization of all damage present in the studied laminates, which made it possible to identify, quantify and locate them precisely and therefore, to identify the physical origin of residual strength decrease and acoustic emissions (AE). Furthermore, the results of the AE analysis have provided very valuable information about the nature and evolution of damage. However, the determination of the depth and size of internal damage was not possible with this technique. The displacement field measured by digital image correlation (DIC) made it possible to determine and monitor the strain field evolution during the experiments. The combination of the results of the three non-destructive techniques used in this work has allowed better characterization of fatigue damage evolution in the studied laminates, and provide a complete and accurate description of the different mechanisms involved during their damage process.

An investigation on residual strength and failure probability prediction for plain weave composite under random fatigue loading

The residual strength and the failure probability of the plain weave carbon-fiber reinforced epoxy composite subjected to random fatigue loading are investigated. According to the constant amplitude residual model and the experimental S-N curves, a new model is proposed, in which both the probability density function of stress peaks and the power spectrum density of stochastic processes are employed. The degradation of residual strength (dSr) in a time increment (dt) is calculated via an integration of the decrease of residual strength due to the stress peaks (dSrp) in the definitional domain of the peak probability density function. A linear damage accumulation of residual strength is used in this time increment. The evolutions of the residual strength with time are determined via a double integral method. The new model’s prediction results are in good agreement with the experiment results and it can give the curves of the variation of fatigue life with failure probability.

Effect of fatigue testing on the properties of Glass-Epoxy composites using the acoustic tool

This study presents the experimental results of the influence of mechanical fatigue on composite material. The plates of Glass fiber with SR 1500 epoxy resin with SD 2505 composite were realized by vacuum molding. Experimental tests were carried out on a standard hydraulic machine INSTRON 8516. The machine is interfaced with a dedicated computer for controlling and data acquisition. The fatigue tests were performed using sinusoidal type of waveform at a displacement control with frequency of 10 Hz. The evolution of Young’s modulus and strain based on fatigue gives us an idea about the resistance of the material. Degradation of mechanical properties was observed, and the results have showed that the Young’s modulus of plates undergo only minor changes. In fact, the residual stiffness and residual strength decrease when the cycle number of fatigue increase (100 to 50000 cycles), indicating that the studied composites have experienced some forms of mechanical damage.The mechanical tests were backed by Acoustic Emission Monitoring (AEM) during the load cycle, in order to understand the nature of the failure process in the composites such as fiber breakage, matrix crazing, matrix debonding and delamination etc.

Mechanical behavior of C/SiC composites under hypervelocity impact at different temperatures: Micro-structures, damage and mechanisms

To understand the mechanical behavior of C/SiC composites under hypervelocity impact at different temperatures, impact tests are conducted using a modified electrical gun, and corresponding numerical simulations are implemented. In addition, the decrease of residual strength induced by damage is also measured after impact tests. Tensile and shear failures occur in different regions of fracture, and the damage zone around the fracture is very confined (<5mm). Besides, the distribution of debris clouds shows “three zones” mode, forming with different mechanisms, and demonstrates that there is a high-energy powdering column in the centre zone. As for the diameter of penetration hole, it increases with impact velocity. At low temperature, with the change of mechanical properties and forming of new micro-cracks, the fracture is smoother, while the diameters of damage zone and penetration hole are smaller. Furthermore, the damage induced by hypervelocity impact significantly reduces the strength of C/SiC composites.

High temperature resistance of a very high volume fly ash cement paste

The physical and chemical changes taking place in a very high volume fly ash cement paste (FAN-4) following exposure to temperatures up to 1000°C are presented. Tests were repeated with commercially available CEM II/A-M Portland composite-cement (MS). FAN-4 pastes showed impressive residual strengths after heat exposure which increased dramatically when heated previously at 800 or 1000°C. This was in stark contrast to the MS paste, which showed a continual decrease in residual strength following heating and subsequent cooling. The increase in residual strength with FAN-4 paste coincided with a major shrinkage event, which was associated with sintering and the formation of the new mineral phases anorthite, gehlenite, wollastonite, diopside and albite. The MS cement formed generally non-hydraulic calcium silicate phases upon heating at ⩾800°C. The differences in phase formation were linked to different starting elemental compositions. The FAN-4 paste was considerably richer in Si, Al, Fe and alkalis but poorer in Ca. The elemental composition of the binder phase will be an important factor to take into account when determining the high-temperature performance of future cements and concretes, which are likely to contain significantly higher contents of supplementary cementitious materials than is presently the case.

High performance concrete under elevated temperatures

In this study, PP fibers and air entraining admixture (AEA) were used together in an high performance concrete (HPC) mix so as to create interconnected reservoirs in concrete and to improve fire performance of HPC. For this reason, nine mixes of HPC incorporating blast furnace slag with 0.24 water-to-binder ratio and various PP and AEA contents were produced. Specimens were cast in two different sizes in order to see the effect of size and 18 series of specimens were obtained. These series subjected to elevated temperatures (300°C, 600°C and 900°C) with a heating rate of 10°C/min and after air cooling, residual mass and compressive strength of specimens were determined. The heated specimens were observed both at macro and micro scales to investigate the color changes, cracking and spalling of HPC at various temperatures. Also, thermogravimetric analyses were performed on powder samples from each nine mixes. Results showed that addition of AEA diminished the decrease in residual strength but this result was found to be irregular after 300°C for thick specimens. The collaboration of AEA and PP fibers decreased the risk of spalling of HPC. Also, size of specimen was found to be important in deterioration of HPC.

Experimental research on instability mechanism of silty mudstone roofs under action of water and its application

Bearing capacity of silty mudstone roofs is likely to become weaker after reacting with water during excavation-and-support process, which has resulted in many roof collapse accidents in some mining areas. Based on the practical situation of one coal mine, we carried out experimental research on instability mechanism of silty mudstone roofs under action with water, including: (1) quantitative analysis on clay minerals, consisting of 15% illite, 19% montmorillonite and 12% illite/montmorillonite interstratified materials, who have high water absorption capacity and they are the internal cause of instability of silty mudstone roofs, (2) experiments of disintegration and expansion, which shows the completely disintegration time is 1.5-2h and expansion pressure is 0.87MPa and 3) experiment of complete stress-strain curves under natural condition as well as situation under action of water, which indicates that the peak strength decrease by 21.6% and 17.3% respectively when test block reacts with water in elastic and plastic stage, but residual strength decrease by 4.1%, 33.8%, 9.6% and 55.9% during elastic, plastic, strain-softening and residual stage under action of water. Overall, our study indicates that silty mudstone roofs contain high water absorption capacity minerals, which makes it disintegrate easily and expand strongly, another aspect is that its peak strength and residual strength decrease after reacting with water, which is the instability mechanism of silty mudstone roofs. In addition, we proposed a technology for controlling roofs stability, including water retention and improving support system stiffness. Field test indicates that a combined support system with high support system stiffness bolts can maintain the roof stability in this kind of coal drifts.

Thermal Shock Fracture Properties of Damage-Tolerant Advanced Pore-Free SiC

In the present study, quench tests were carried out to characterize the fracture behavior of advanced pore-free SiC with damage tolerance during thermal shock. The residual strength of specimens after the quench tests was also measured. It is found that the critical temperature difference and the decrease in residual strength of this material are larger than those of conventional SiC. This is because the fracture is dominated only by flaw size since the wake zone is not generated during thermal shock with high strain rate. The critical temperature difference as well as the crack propagation is calculated with the aid of the unified theory of thermal shock.

Effect of adding methods of metallic phase on microstructure and thermal shock resistance of Ni/(90NiFe 2O 4-10NiO)cermets

Ball mixing and electroless plating were respectively used as the adding methods of metallic phase to prepare Ni/(90NiFe 2O 4-10NiO) cermets for the inert anode in aluminum electrolysis. The microstructure and thermal shock resistance of cermet samples were studied. The results show that, for the samples prepared by ball mixing method, aggregation of metallic phase is found in either the green blocks or sintered samples and the extent of aggregation increases with the increase of metal content. For 6.5Ni/(90NiFe 2O 4-10NiO) cermets prepared with electroless plating method, the homogeneous and fine metallic particles are found in either the green compacts or sintered samples, but the relative density and thermal shock residual strength decrease by 3% and 28%–58% respectively, compared with samples prepared with ball mixing method.

Impact and compression after impact experimental study of a composite laminate with a cork thermal shield

The aim of this paper is to present an experimental study of impact and compression after impact (CAI) tests performed on composite laminate covered with a cork thermal shield (TS) intended for launchers fairing. Drop weight impact tests have been performed on composite laminate sheets with and without TS in order to study its effect on the impact damage. The results show the TS is a good mechanical protection towards impact as well as a good impact revealing material. Nevertheless, totally different damage morphology is obtained during the impact test with or without TS, and in particular at high impact energy, the delaminated area is larger with TS. Afterwards, CAI tests have been performed in order to evaluate the TS effect on the residual strength. The TS appears to increase the residual strength for a same impact energy, but at the same time, it presents a decrease in residual strength before observing delamination. In fact, during the impact tests with TS, invisible fibres’ breakages appear before delamination damage contrary to the impacts on the unshielded sheets.

Intralaminar fatigue behaviour of carbon fibre reinforced plastics

A new approach is discussed for characterizing the damage state of FRP with the appearance of micro cracking and its influence on the residual strength. For the quantitative analysis of micro cracking, X-ray refraction topography was used. Fatigue treatment of carbon fibre reinforced plastics (CFRP) laminates were performed to create defined damage states. With regard to real loading conditions of CFRP, studies on tube samples were performed to investigate two dimensional (tension–torsion) loads. A rheological material model was presented which described the increase of micro cracking in ±45° CFRP laminates with a continuum mechanical approach. The correlation between the results of the X-ray refraction topography, the damage parameter of the rheological model and the decrease of residual strength were shown. The inter fibre fracture surface of the investigated CFRP materials was determined with tension–torsion loaded tube specimens of 90°-laminate and a layer-wise strength analysis was done. Micro cracking occurs even at 50% inter-fibre fracture loading and can lead to a distinct strength reduction. The intralaminar fatigue effect itself seems to be an indication for the damage accumulation and the load history.