Influence of water and accelerated aging on the shear fracture properties of glass/epoxy composite

The degradation of interlaminar shear strength and shear fracture toughness of glass/epoxy composites due to uptake of distilled water and sea water has been studied. The composites were immersed in water for up to eight months at temperatures up to 70 °C. Unreinforced matrix resin samples were also immersed for periods up to 2 years. Sea water was absorbed less rapidly than distilled water. Weight gains below 1% did not influence the shear strength while higher weight gains reduced shear strength up to 25%. The loss in apparent interlaminar shear strength was uniquely related to specimen weight gain. Mode II fracture toughness, G _IIc, also decreased with increasing immersion time after an initial incubation period, but the accelerated tests were found to reduce G _IIc less than the room temperature tests at comparable weight gains.     

A practical testing approach to determine mode II fracture energy GIIF for concrete

Continuing the experiments on the double-edge notched specimens on which the mode II fracture toughness K _IIc of concrete was measured, a practical testing approach to determine mode II fracture energy G _IIF is studied using the same geometry.     

Cracked Brazilian disc specimen subjected to mode II deformation

The centrally cracked Brazilian disc specimen has been used by many researchers to study mode I and mode II brittle fracture in different materials. However, the experimental results obtained in the past from this specimen indicate that the fracture toughness ratio (K_IIc/K_Ic) is always significantly higher than the theoretical predictions. It is shown in this paper that the increase in the ratio K_IIc/K_Ic can be predicted if a modified maximum tangential stress (MTS) criterion is used. The modified criterion takes into account the effect of T-stress in addition to the conventional singular stresses. The fracture toughness ratio K_IIc/K_Ic is calculated for two brittle materials using the modified criterion and is compared with the relevant published experimental results obtained from fracture tests on the cracked Brazilian disc specimen. A very good agreement is shown to exist between the theoretical predictions and the experimental results.     

Mixed-mode high temperature toughness of silicon nitride

Both opening-mode and mixed-mode fracture toughness tests were carried out at 1200 and 1300 °C on a sinter/HIP grade of silicon nitride. Data for pure opening loading (K _Ic) agree well with other experiments on the same material, which showed that the toughness was lower at 1000 °C than at room temperature, but increased as temperature increased above 1000 °C. The ratio of K _IIc/K _Ic was sufficiently insensitive to temperature that it can be considered to be constant. Results are discussed in the context of mechanisms that have been proposed to explain fracture toughness in silicon nitride.     

Investigation of wood fracture toughness using mode II fracture (shearing)

The test results of fracture toughness for three wood species, such as pine, alder and birch are presented. Examination of fracture toughness is carried out using mode II fracture (shearing). Values of the stress intensity factor,K _IIc, are determined for the three main anatomic directions of wood. Microstructural tests of particular wood species, performed on specimens along the three main anatomic directions of wood, are discussed. Qualitative relationships are found to exist between the microstructure of wood and the obtained values of the stress intensity factor,K _IIc.     

Mode II fracture mechanics properties of wood measured by the asymmetric four-point bending test using a single-edge-notched specimen

Using a single-edge-notched specimen of spruce, an asymmetric four-point bending test was conducted to obtain the mode II fracture toughness G_IIc and critical stress intensity factor K_IIc, and the test method was numerically and experimentally analyzed. A three-point bend end-notched flexure test was also conducted and the results were compared with those of the asymmetric four-point bending tests. The crack length had a small influence on the load/loading-line displacement relationship in the asymmetric four-point bending test, so it was difficult to determine the value of G_IIc, which requires the measurement of loading-line displacement. In contrast, the value of K_IIc obtained by two tests was similar when the initial crack length ranged from 0.7 to 0.85 times the depth of the specimen. These results show that the asymmetric four-point bending test is a promising means of determining K_IIc.     

Experimental determination of KIIC of normal strength concrete

The double-edge notched specimen made of normal strength concrete has been subjected to compressive loads. It is shown that a shear crack develops at the notch tip. Visual observation and evaluation of displacement measurements have proven that this specimen allows pure mode II testing. It was possible to determine fracture toughness K_IIc. Recommendations for the testing are made.     

Experimental and numerical investigation of cracked chevron notched Brazilian disc specimen for fracture toughness testing of rock

The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to quantify mode I fracture toughness (K_Ic) of rock, and it has also been applied to mode II fracture toughness (K_IIc) testing in some research on the basis of some assumptions about the crack growth process in the specimen. However, the K_Ic value measured using the CCNBD specimen is usually conservative, and the assumptions made in the mode II test are rarely assessed. In this study, both laboratory experiments and numerical modeling are performed to study the modes I and II CCNBD tests, and an acoustic emission technique is used to monitor the fracture processes of the specimens. A large fracture process zone and a length of subcritical crack growth are found to be key factors affecting the K_Ic measurement using the CCNBD specimen. For the mode II CCNBD test, the crack growth process is actually quite different from the assumptions often made for determining the fracture toughness. The experimental and numerical results call for more attention on the realistic crack growth processes in rock fracture toughness specimens.     

Fracture of knitted randomly oriented short-fiber composite

Fracture behavior of a chopped-mat E-glass fiber reinforced hybrid resin composite knitted with continuous poly(ethylene terephthalate) (PET) fibers is investigated by a combined experimental and analytical study. In both opening-mode (mode-I) and shear-mode (mode-II) fracture studies of the composite, the macroscopic critical stress intensities or toughnesses of the material are found to be distinct along the warp and fill directions of the knitting PET fibers. Values of K _IIc ( ^F ) and K _IIc ( ^F ) are lower than those of K _Ic ( ^W ), owing to different fracture mechanisms involved. In the mixed-mode fracture of the composite, a failure envelope in K _I and K _II is constructed. The different mechanisms involved in opening-mode, mode-II and mixed-mode fracture are studied with SEM observations.     

Characterization of the paste-aggregate interfacial transition zone surface roughness and its relationship to the fracture toughness of concrete

Notched concrete beams containing varying amounts of pea gravel aggregate were tested under three-point bend, and their fracture toughness determined. The roughness of the region near the interface between the cement paste and the aggregate was evaluated by digitizing images from a confocal tandem scanning microscope. The average roughness of the paste was found to be related to the fracture parametersK _IC (critical stress intensity factor) and a _c (critical crack extension), as determined by the two-parameter fracture model. The roughness in the proximity of the paste-aggregate interface was generally higher than that of the paste far from the aggregate, and it decreased with the distance from the aggregate. This study indicates that aggregate particles increase the toughness of the cement paste portion of concrete, and that this is an important mechanism for toughening concrete.     

Fracture toughness of acrylic bone cements

Clinical experience has shown that fracture of PMMA-based bone cements is a significant factor in the failure of orthopaedic joint replacements. Earlier studies of the fracture toughness properties of bone cement have been limited to relatively large test specimens — ASTM standard test methods require the use of specimens with dimensions considerably larger that those associated with bone cement in clinical use. In this study, a miniature short-rod specimen was used to measure the fracture toughness (K _IC) or two bone cements (Simplex-P and Zimmer LVC). The dimension of our mini specimens approaches the cross-section of bone cements as usedin vivo. The short-rod elastic-plastic fracture toughness test method introduced by Barker was utilized to ascertain the effect of specimen preparation and ageing in distilled water on fracture toughness. Our study indicated that slow hand-mixed specimens possess comparable fracture toughness to centrifuged specimens. After ageing in water, however, centrifuged and slow hand-mixed specimens are more fracture resistant than specimens prepared by mixing the cement quickly. An optimum void content for the bone cements studied was suggested by the experimental results; for Simplex-P bone cement it appeared to be less than 1.6% whereas it was between 1.6 and 3.6% for Zimmer LVC cement. Simplex-P bone cement also showed superior fracture toughness compared to Zimmer LVC cement after storage in water for 60 days at 37° C.     

Critical values of stress intensity factor in mode II fracture of cementitious composites

The problem of mode II of brittle matrix composites is considered. After a short discussion of the present knowledge and a review of test results, the importance of fracture toughness in mode II is stressed. The test results presented concern both modes of fracture, and obtained values of stress intensity factorsK _Ic andK _IIc are discussed, taking into consideration the results of observations on SEM micrographs. It is suggested that the fracture toughness of concretelike materials should be expressed as a combination of mode I and II characteristics.     

Fracture toughness of geopolymeric concretes reinforced with basalt fibers

The purpose of this work was to investigate the influence of the volumetric fraction of the fibers on the fracture toughness of geopolymeric cement concretes reinforced with basalt fibers. The values of fracture toughness, critical stress intensity factor and critical crack mouth opening displacement were measured on 18 notched beams tested by three-point bending. The a₀/h (notch height/beam height) ratio was equal to 0.2 and the L₀/h (distance between the supports/beam height) ratio was equal to 3.According to the experimental results, geopolymeric concretes have better fracture properties than conventional Portland cement. They are also less sensitive to the presence of cracks.     

Fibre reinforcement and fracture response in geopolymeric mortars

The inorganic polymeric cement called geopolymer or PSS, has been studied in recent years as a binder for mortar and concrete. The present work reports the fracture toughness studies in mortars made of PSS cement matrix reinforced by wollastonite microfibers (Ca(SiO₃)). K_I‐curves for PSS cement composites were determined according to the superposition asymptotic assumption and compared with reference Portland cement (PC) composites. The maximum toughness gain occurs in both composite systems with V_f = 2%. For higher fibre volumes (3 and 5%), K_I values decrease, due to an increase in porosity. Microstructural analyses showed that toughening mechanisms, as debonding and pullout of the fibers, are more common in PSS cement composites than in the reference PC composites. The difference of toughness between PSS and PC cement (0% of fibers) is about 80%. This demonstrates the high performance of these geopolymeric materials.     

The fracture properties of glass polyalkenoate cements as a function of cement age

The fracture properties of two glass polyalkenoate cements based on a short chain-length and on a long chain-length poly (acrylic acid) have been studied as a function of the cement age. The stress intensity factor, K _I, increases with cement age for both cements. The un-notched fracture strength _f increases with cement age, largely as a result of an increase in the Young's modulus accompanying crosslinking of the polyacrylate chains by metallic ions. The toughness G _I remains approximately constant for the short chain-length cement, but reduces with cement age for the long chain-length cement. Analysis of the toughness data using a chain pull-out model leads to the conclusion that chains distant from the fracture plane are involved in fracture, and that the number of chains that take part in chain pull-out decreases as the crack opening displacement reduces with cement age.     

Theoretical analysis of the effect of weak sodium sulfate solutions on the durability of concrete

A theoretical analysis of the detrimental influence of weak sodium sulfate solutions (Na₂SO₄) on the durability of concrete is presented. It was conducted using a numerical model that takes into account the coupled transport of ions and liquid and the chemical equilibrium of solid phases within the (partially) saturated system. Numerous simulations were performed to investigate the influence of various parameters such as water/cement (w/c) ratio (0.45, 0.65 and 0.75), type of cement (CSA Type 10 and Type 50), sulfate concentration (0–30 mmol/l of SO₄) and the gradient in relative humidity across the material. All input data related to the properties of concrete were obtained by testing well-cured laboratory mixtures. Numerical results indicate that exposure to weak sulfate solutions can result in a significant reorganization of the microstructure of concrete. The penetration of sulfate ions into the material is not only at the origin of the precipitation of sulfate-bearing phases (such as ettringite and eventually gypsum) but also results in calcium hydroxide dissolution and C–S–H decalcification. Data also clearly emphasize the fact that w/c ratio remains the key parameter that controls the durability of concrete to sulfate attack.     

Mix design and strength of soil–cement concrete based on the effective water concept

Saturated surface-dry condition of fine aggregate has been well defined in mortar and concrete production as the dry mass percentage moisture content where aggregate particles are saturated. However in soil–cement concrete construction, no mix design based on the saturated surface-dry condition of soil was attempted partly because the control of soil moisture on site is difficult. We have proposed the definition and testing methods of the saturated surface-dry condition of soil in the previous papers. In this paper, we introduce the concept of effective unit water to the soil–cement concrete mix design and propose a mix design method for soil-sand-cement systems. Strength and density of the soil–cement concretes were determined and discussed. A number of mixes were proportioned with accurate control of composition (C/W, soil/sand ratio, free water, etc.) and consistency. It was found that strength can be determined by C/W for a wide variety of clayey soils in various soil/sand ratios except for a soil with very high content of organic material. A set of nomograms for C/W versus strength and soil/sand versus strength were presented for future proportioning.     

Effect of composition,environmental factors and cement-lime mortar coating on concrete carbonation

The paper describes the physicochemical processes of concrete carbonation and presents a simple mathematical model for the evolution of carbonation in time, applicable under constant relative humidity higher than 50%. The model is based on fundamental principles of chemical reaction engineering, and uses as parameters the ambient concentration of CO₂, the molar concentratrations of the carbonatable constituents, Ca(OH)₂ and CSH, in the concrete volume, and the effective diffusivity of CO₂ in carbonated concrete. The latter is given by an empirical function of the porosity of hardened cement paste and of relative humidity, derived from laboratory diffusion tests. The validity of the model for OPC or pozzolanic cement concretes and mortars is demonstrated by comparison of its predictions with accelerated carbonation test results obtained in an environment of controlled CO₂ concentration, humidity and temperature. The mathematical model is extended to cover the case of carbonation of the coating-concrete system, for concrete coated with a cement-lime mortar finish, applied either almost immediately after the end of concrete curing or with a delay of a certain time. Parametric studies are performed to show how the evolution of carbonation depth with time is affected by cement and concrete composition (water/cement or aggregate/cement ratio, percentage OPC or aggregate replacement by a pozzolan), environmental factors (relative humidity, ambient concentration of CO₂), the presence and the time of application of a lime-cement mortar coating and its composition (water/cement, aggregate/cement and lime/cement ratios of the mortar, percentage OPC or aggregate replacement by a pozzolan).     

Computational evaluation of flexural toughness of FRC and fracture properties of plain concrete

In this paper, a computational tool concerning the computation of flexural and fracture toughness of cement based composites is presented. Firstly, RILEM’s (Réunion Internationale des Laboratoires d’Essais de Matériaux) recommendations related to the analysis of FRC in three-point bend tests are discussed in their relevant aspects regarding the computational implementations. The determination of other mechanical properties such as the Young modulus has been added to the program. Taking this into account, a new formulation based on displacements is used. In the second part of the paper, the determination of fracture properties of concrete, such as the fracture energy, G _F , and the fracture toughness, K _IC ^S , is discussed regarding the computational strategies used in the implementations. Several features whereby anterior data can be reanalyzed, obtained from other standards and recommendations, have been incorporated into the program, therefore allowing comparative studies and back analysis activities.     

Properties of shrinkage-reducing admixture-modified pastes and mortar

A comprehensive study on the influence of a shrinkage-reducing admixture (SRA) on properties of ordinary Portland cement and shrinkage-compensating cement has been conducted. The properties investigated include: electrical resistivity, hydration heat rate, free shrinkage and restrained shrinkage cracking. It is found that the shrinkage-reducing admixture (SRA) can reduce the free shrinkage and postpone the occurrence of shrinkage crack significantly. The SRA has also an effect to retard the hydration process of Portland cement under normal condition. However, under semi adiabatic condition, it seems that SRA accelerates the hydration process. A new concept of the hardening ratio, ρ _H , is proposed in the study. It is indicated that the hardening ratio, ρ _H , and compressive strength development have very good linear relationship.