Semi-brittle Material Research Articles

Flexural behavior of RC continuous beams having hybrid reinforcement of steel and GFRP bars

Fiber reinforced polymers (FRP) have been considered to be the alternative material to steel in reinforced concrete structures (RC) with the advantages of corrosion resistance, non-conductivity, and high strength-to-weight ratio. FRP-RC beam is featured with higher deformability and larger crack width. However, there is no recognizable ductility in FRP-RC members. Due to the lack of the ductility of FRP-RC beam, it was suggested using hybrid reinforcement of steel and FRP to achieve a preferable strength and ductility. The flexural behavior of the concrete beams reinforced with hybrid reinforcement was investigated through series of six continuous beams. The positive moment GFRP reinforcement ratios of 0.75% and 1.25% were used and the steel reinforcement ratios were ranged to determine the optimum percentage to work efficiently with GFRP. The experimental results showed that increasing GFRP positive moment reinforcement ratio to be more than 1.4 [Formula: see text] increased the flexural capacity of the section but with less ductile behavior. However, increasing the steel positive moment reinforcement ratio leaded to the increase of both the flexural capacity and the ductile behavior in a condition that the steel positive moment reinforcement ratio was [Formula: see text]. It was shown that the flexural capacity predication of the hybrid section using the semi brittle elastic material method to present the de-bonding of moment that occurred at the failure stage had a good accuracy with the experimental results.

Effect of a single overload on the cyclic R-curve behaviour of a γ-TiAl TNM alloy

The effect of single overloads on the threshold of stress intensity range of a semi-brittle material, a γ-titanium aluminide, was analysed. This material class exhibits a pronounced R-curve behaviour, which is mainly caused by the formation of shear ligaments, crack deflection and branching. Samples with short cracks on deep sharp notches were subjected to overloads of 80 % of the fracture toughness. Contrary to ductile materials, the experiments revealed no significant increase in the long-crack threshold, however a pronounced increase in the slope of the R-curve for the threshold of stress intensity range.

Flexural Performance of Cement-Treated Sand Reinforced with Geogrids for Use as Sub-Bases of Pavement and Railway Structures.

Cement-treated sand (CTS) exhibits undesirable brittle behavior after the applied stress reaches its peak strength. This research investigates the flexural behavior of CTS that is reinforced with uniaxial geogrid (CTSG). A total of 6% cement content was mixed with sand. Uniaxial geogrids with three different strengths were utilized to create the CTSG samples. The number of reinforcement layers, including single and double reinforcements, was studied. The image processing method was applied to analyze the surface cracks in the specimens. The results show that the geogrid type and the number of reinforcement layers affect the flexural behavior of the CTSG. Geogrid reinforcement changed the behavior of the CTS from a brittle material to a semi-brittle or ductile material because the residual tensile stresses were carried by the geogrids. The high-strength geogrid with a double reinforcement layer proved to be most effective in enhancing the peak strength and toughness with improvement ratios of 1.80 and 11.7, respectively. Single and double reinforcement layers with all geogrid types can reduce surface cracks with average crack reduction ratios of 64% and 83%, respectively. The CTSG can be successfully used as a sub-base layer to increase flexural performance and the lifetime of pavement and railway structures.

Mechanisms of Anisotropy in Salt Rock Upon Microcrack Propagation

Salt rock is a polycrystalline material of interest for geostorage because of its low permeability and potential to self-heal by pressure solution at favorable stress and temperature conditions. It is often assumed that microcrack propagation and healing lead to isotropic stiffness changes. The goal of this study is to check this assumption and to gain a fundamental understanding of the mechanisms that control the accumulation of damage and irreversible deformation. Cyclic axial loading tests are performed under a confining pressure of 1 MPa on synthetic salt rock generated by thermal consolidation. The stress–strain curves and the microstructure images taken at key stages of the cycles reveal the formation of a complex system of sliding and wing microcracks, the orientation of which is loading dependent. We interpret the mechanisms that control the coupled evolution of crack families by a discrete wing crack elastoplastic damage (DWCPD) model. Crack propagation is controlled by Mode I and Mode II fracture mechanics criteria. Sliding “main” cracks grow if a cohesive frictional criterion is met, while the wing cracks propagate in tension. Displacement jumps at crack faces are related to the deformation of the rock representative elementary volume (REV). The DWCPD model can capture the nonlinear stress–strain relationship and the degradation of stiffness during the loading cycles. Simulations show that microcracks occur following two stages: (1) wing cracks initiate and main cracks do not propagate; (2) wing cracks and main cracks then propagate simultaneously. Higher friction at the crack faces leads to higher strength. With a larger cohesion, salt rock strength increases, damage development is delayed and exhibits a stick-slip evolution. At higher confinement, the initiation of wing cracks is delayed, which results in an increase of strength. The damage rate is higher in specimens that are damaged prior to compression than in the ones that are not. The proposed DWCPD model can be extended to any polycrystalline semi-brittle material, and can be applied to understand the formation of crack patterns in geostorage facilities.

Discrete Element Method Modelling of the Diametral Compression of Starch Agglomerates.

Starch agglomerates are widely applied in the pharmaceutical, agricultural, and food industries. The formation of potato starch tablets and their diametral compression were simulated numerically and verified in a laboratory experiment to analyse the microscopic mechanisms of the compaction and the origins of their breakage strength. Discrete element method (DEM) simulations were performed using EDEM software. Samples comprised of 120,000 spherical particles with radii normally distributed in the range of 5–36 μm were compacted in a cylindrical die with a diameter of 2.5 cm. The linear elastic–plastic constitutive contact model with a parallel bonded-particle model (BPM) was used to model the diametral compression. DEM simulations indicated that the BPM, together with the linear elastic–plastic contact model, could describe the brittle, semi-brittle, or ductile breakage mode, depending on the ratio of the strength to Young’s modulus of the bond and the bond-to-contact elasticity ratio. Experiments confirmed the findings of the DEM simulations and indicated that potato starch (PS) agglomerates can behave as a brittle, semi-brittle, or ductile material, depending on the applied binder. The PS agglomerates without any additives behaved as a semi-brittle material. The addition of 5% of ground sugar resulted in the brittle breakage mode. The addition of 5% gluten resulted in the ductile breakage mode.

Determination of fracture toughness parameters of concrete using compact pressure test

Fractures start with the formation of cracks and occur as the cracks spread in building materials. This causes significant damage to the buildings. Fracture mechanics is a science that investigates crack behavior, crack analysis and what to do for prevention of cracks. In the present study, concrete fracture toughness parameters were investigated by compact pressure test. In this context, research and a series of experiments were conducted on fractures, types of fractures, fracture mechanics, and compact pressure test. In the present empirical study, 5 unnotched and 15 notched, a total of 20 samples were used. 40 mm notches were created on notched samples. Cube splitting experiments were conducted using 10 mm wide strips on unnotched samples. For Type-I, Type-II and Type-III that were used in the calculation of the fracture toughness parameters of the said samples, fracture loads, tensile values and fractured sample details are presented. Since the concrete is a semi-brittle material, compact pressure samples were analyzed according to the principles of linear elastic fracture mechanics. It was concluded that the standard deviation for the critical fracture parameter (KIc) of the sample, which was naturally more stable than the present study, was smaller and as the structure size increased, the fracture toughness value increased as well.

Investigation of the effect of chamfer size on the behaviour of hybrid joints made by adhesive bonding and riveting

The paper reports the results of a numerical study performed on: (a) purely adhesive joints and (b) new hybrid single lap joints with a variable adherend thickness in the lap region. The variable thickness creates chamfer defined by a geometric parameter ch which has a very positive influence on the mechanical response of the joint. The novelty in this paper is the investigation of the effect of chamfer size on the behaviour of hybrid joints made by 2 simple techniques: adhesive bonding and riveting. In particular, 10 types of chamfer geometries are considered, each causing a different stiffness of the adherends being joined. As a result, the strength of the connection is increased and its weight reduced, which is of vital importance in aircraft constructions.The adherends and rivets are assumed to be made of aluminium, i.e., an elastic-plastic material, and subjected to gradual degradation due to tension. The adhesive layer is modelled as a semi-brittle material with progressive degradation using cohesive elements. Following the creation of 3D finite element models, the samples are subjected to quasi-static uniaxial deformation (nonlinear analysis with ABAQUS/Explicit).The numerical results lead to the conclusion that the variable geometry, i.e., chamfering, has a very positive effect. At the maximum chamfer length equal to 10 mm, the increase in the maximum force was about 32.8% compared to the model without chamfer.

Characterization of the fracture toughness of micro-sized tungsten single crystal notched specimens

Fracture experiments using micrometer-sized notched cantilevers were conducted to investigate the possibility of determining fracture mechanical parameters for the semi-brittle material tungsten. The experiments were also used to improve the understanding of semi-brittle fracture processes for which single crystalline tungsten serves as a model material. Due to the large plastic zone in relation to the micrometer sample size, linear elastic fracture mechanics is inapplicable and elastic-plastic fracture mechanics has to be applied. Conditional fracture toughness values J Q were calculated from corrected force vs. displacement diagrams. Crack growth was accessible by direct observation of in-situ experiments as well as with the help of unloading compliances. As a further tool, fracture toughness can be determined via crack tip opening displacement. The micro samples behave more ductile and exhibit higher fracture toughness values compared to macro-sized single crystals and fail by stable crack propagation.

Erosion Wear Resistance of TiAlCrN Coating on Stainless Steel Impacted by Solid Particles

TiAlCrN coating was deposited on a stainless steel using unbalanced magnetron sputtering technique. The erosion wear resistance of the TiAlCrN coating was investigated by comparison with that of the TiN coating deposited by filtered vacuum arc deposition. SEM was used for observing the surface morphologies of coatings both the un-eroded and eroded. Scratch test was used to evaluate the adhesion of the coatings. The erosion wear was tested with a gas blast apparatus at room temperature. In the test, the coatings were impacted at an impingement angle of 90° by angular SiC solid particles with an average diameter of 124um. The maximum depth of the erosion scar measured by the optical profiler was used to evaluate the erosion wear loss of the coatings. TiAlCrN coating proved to have much lower erosion rate than TiN coating. Unlike TiN coating, the TiAlCrN coating behaved like semi-brittle material.

Erosion Wear Behavior of TiAlCrN Coating on Hard Metal Impacted by Solid Particles

The erosion wear resistance of TiAlCrN coating on a hard metal deposited using unbalanced magnetron sputtering technique was investigated by comparison with that of the TiN coating deposited by filtered vacuum arc deposition. SEM was used for observing the surface morphologies of coatings both the un-eroded and eroded. Scratch test was used to evaluate the adhesion of the coatings. The erosion wear was tested with a gas blast apparatus at room temperature. In the test, the coatings were impacted at an impingement angle of 90° by angular SiC solid particles with an average diameter of 124um. The maximum depth of the erosion scar measured by the optical profiler was used to evaluate the erosion wear loss of the coatings. TiAlCrN coating proved to have much lower erosion rate than TiN coating. Different from the typical brittle erosion behavior of TiN coating, the TiAlCrN coating behaved like a semi-brittle material.

Tensile deformation of anisotropic porous copper with directional pores

The tensile deformation of anisotropic porous copper with unidirectionally oriented cylindrical pores was investigated by an acoustic emission method. In the loadings parallel and perpendicular to the orientation direction of the pores, many cracks are formed after yielding and they strongly affect the deformation. The formed cracks rapidly grow and connect with each other near the peak stress of the stress–strain curve, thereby leading to final fracture. Crack formation is easier under perpendicular loading than under parallel loading, because high stress concentration and stress triaxiality occurs around the pores. As a result, the strength and elongation for perpendicular loading are much smaller than those for parallel loading. Furthermore, in the case of perpendicular loading, the localized deformation around pores drastically decreases the plastic Poisson's ratio. These results indicate that a porous copper macroscopically behaves as a semibrittle material under perpendicular loading, while the porous copper exhibits ductility under parallel loading.

Interface Effects on the Quasi-Static and Impact Toughness of Discontinuously Reinforced Aluminum Laminates

Trilayer laminates consisting of two layers of aluminum alloy 7093 surrounding one layer of 7093/SiC/15p were produced via two roll-bonding techniques as well as by adhesive bonding. The effects of systematic changes in interface characteristics (i.e., weak bond via roll bonding with a thin ductile interlayer material, stronger bond via roll bonding without a thin ductile interlayer, and strongest bond via adhesive bonding with a semibrittle material) on the subsequent laminate toughness was studied. The fracture resistance of the laminates and the constituent materials was examined via instrumented Charpy notched impact testing in the crack-arrester orientation as well as by fracture-toughness testing of bend bars tested in the crack-divider and the crack-arrester orientations. The notched impact resistance of the trilayer crack-arrester laminates was found to be greater than both monolithic 7093/SiC/15p and 7093 samples of similar global thickness. The laminated structure promoted crack arrest, deflection, and large-scale deformation of the unreinforced layers, producing R-curve behavior. The tendency for interface delamination was predicted and confirmed based on recent mechanics-based analyses. The trilayer laminate structures tested in the crack-divider orientation exhibited a greater R-curve than either of the 7093/SiC/15p or 7093 samples tested at similar global thickness. Both types of roll-bonded laminates (i.e., stronger interface and weak interface containing a thin metal interlayer) exhibited a greater enhancement in Charpy impact toughness and mode I fracture toughness than did the adhesively bonded (i.e., semibrittle interface) laminates. These relative improvements in toughness were rationalized by estimating the contributions to energy absorption by the delamination and crack bridging in these systems and by the effects of the interface type on these processes. These results are generally relevant to the performance of these materials under impact and under certain blast loading and penetration situations.

Atomistic analysis of discontinuous deformation during cutting processes

The aim of this work was to analyze the influence of the micro-structural properties on the nanometer level material removal process. For this purpose atomistic models for a ductile and a semi-brittle metal were set up and molecular dynamics simulations were performed. The ductile material showed a discontinuous cyclic process for the chosen orientation, where chip formation and pre-deformation zone growth alternated at a 90 ps period. For the semi-brittle material, the process is governed by a strong localization of the deformation to the contact area and a cyclic change of the chip cross-section at a period of about 120 ps.

A coupled elastoplastic damage model for semi-brittle materials and extension to unsaturated conditions

In this paper, a coupled elastoplastic damage model is proposed for semi-brittle materials. This model is applied to a specific semi-brittle sedimentary rock material. A brief account of experimental investigations is presented in the first part. The data obtained show an important plastic deformation coupled with stress-induced damage corresponding to initiation and growth of microcracks. Influences of mineral compositions and water content on the mechanical behaviour are also investigated. Based on these experimental evidences, the general formulation of the model is presented in the second part of the paper. The effective elastic properties of isotropic damaged material are determined based on relevant considerations from micromechanics. Damage evolution law and plastic damage coupling are described by using the framework of irreversible thermodynamics. A non-associated plastic flow rule is used. The model is extended to partially saturated conditions in order to study coupled hydromechanical behaviours in drying–wetting processes. Comparisons between numerical simulations and test data are performed for various loading paths. It is shown that the proposed model is able to describe the main features of mechanical behaviour observed in this class of materials.

Segment linkage during evolution of intracontinental rift systems: insights from analogue modelling

Abstract On the basis of the effective scale-independence of brittle structures, from microcracks to regional fault systems, we have used analogue centrifuge models to provide insights into the initiation and evolution of continental active rifts, with a single dilational fracture segment representing a prototype rift segment. In the models, which are physically and dynamically scaled, a semi-brittle compound material, with flexural rigidity, was devised to simulate the lithosphere as a single layer. This is justified by the fact that, at the largest scale, the lithosphere behaves as a single viscoelastic, flexurally rigid unit. Silicon polymers of two different viscosities and densities represent the asthenosphere. A parallelepiped of lower viscosity and lower density material is embedded within and near the base of the second polymer that fills up most of the model box. The former is activated as a plume-like diapir that rises, spreads laterally and ultimately generates extensional stress in the semi-brittle layer on top. In terms of model fracture distributions, both narrow and wide failure modes were achieved, analogous to narrow and wide modes of rifting. The processes of fracture initiation, propagation and coalescence are the same for each mode. During the early stages of model runs, fracture initiation is more important than the propagation of existing segments in relieving stress, although the latter dominates during later stages. The key parameters of overlap, offset, obliquity and propagation angle, defined and illustrated, are used to distinguish different ways in which pairs of fractures coalesce. We recognize three distinct types of coalescence (type 1, type 2 and type 3) involving both tip-to-tip and tip-to-sidewall linkages. These are described and both graphically and statistically discriminated. Whether narrow or wide mode, an intracontinental rift system consists of a number of discrete fault-bound rift segments. Similar types of interactions to those in the models have been identified between pairs of rift segments from the Cenozoic Baikal and East African rift systems, and the Mesozoic to early Tertiary Central African rift system. The factors that control the type of coalescence between natural rift segments would appear to be precisely those that govern linkages of fracture segments in the models.

Experimental Investigation into Deformation of Semi-Brittle Materials. Communication II: Strengthening and Loosening under Complex Loading

The deformation of semi-brittle material is investigated under conditions of complex loading. Measurement of three principal strains made it possible to determine the influence of the stress state type and complex additional loading on the character of elastoplastic deformation, as well as material loosening.

The anisotropic nature of local crack stability in BCC crystals

In a semi-brittle material, dislocation emission from the crack tip and from near tip sources controls the local elastic-plastic tip stress field. As such, this local dislocation emission, which is connected to the intrinsic constraint of crystal plasticity, might also lead to anisotropic features of both the effective surface energy, γ eff , and the local driving force, G local . The major objective of the current study is to elucidate some of the local anisotropic aspects involved in the cleavage fracture mode of body centered cubic (BCC) crystals. Special emphasis is given to the effective surface energy recognized here as an anisotropic entity expressed by [γ eff]φ, λ (here φ, λ designate the specific cleavage plane and the crack growth direction, respectively). These local views become possible through an extensive research activity in hydrogen induced subcritical growth of Fe-3%Si crystals. The experimental program included internal and external hydrogen interactions for different crystal orientations. Attention was given to the study of internal flaw/slow flaw growth in addition to sharp crack extension in prefatigued single edge notched specimens. A detailed phenomenological and theoretical analysis which is related to the local fracture parameters is described, with some application to the current hydrogen interaction problem as well. This case of quasi-static slow crack growth on a cleavage plane enabled the identification of the anisotropic features mainly by: extensive crack tip morphology investigation, the shapes of growing crack fronts or voids, a modified elastic-plastic dislocation simulation at the crack tip and further distinction between the microcrack initiation and propagation stages. Analysis of the above elements as based on experimental results indicates that the crack tip stability and the crack front shape are related to the dislocations and/or slip plane arrangements with anisotropic control of crack initiation and arrest. It is emphasized that crack growth itself need not be anisotropic and, in fact, may be more isotropic.