Strain Crack Research Articles

Experimental Investigation of Bending Behavior of Reinforced Ultra-High-Performance Concrete Decks

Ultra-high-performance Concrete (UHPC) has a wide range of potential civil engineering applications, owing to its excellent mechanical properties and durability. To investigate the bending behavior of reinforced UHPC components, nine reinforced UHPC deck specimens were tested by conducting four-point bending experiments. The crack development, load-carrying capacity, and strain distribution were observed, and the tests results revealed that the introduction of steel fibers into UHPC obviously improved the structure’s toughness and crack resistance. Based on the tests results, a theoretical formula was derived for calculating the bending stiffness of the UHPC decks during the entire loading process, which was divided into three representative stages based on the initial cracking and yielding. The results for the load-deflection response obtained by the formulas are in good agreement with the experimental results.

Strengthening of 2D-C/SiC composite through creep with low stress and high temperature

ABSTRACTTo improve the resistance to matrix cracking of 2D-C/SiC composite, the composite was treated at certain creep conditions, which involve relative lower stress, higher temperature. The creep strain and matrix cracking stress (σmc) were used to determine the creep strengthening effect. Moreover, the stress redistribution model was applied to explain the stress transfer from the matrix to fibre, and therefore the creep strengthening mechanisms. The results indicate that 2D-C/SiC can be strengthened by creep at certain conditions. The ability to resist the creep strain and σmc can be improved by certain creep treatments with creep temperature of 1400°C, lower stress and longer time. The strengthening effect is resulted from the stress redistribution during the creep. The tensile residual stress is relaxed in the matrix and transferred to the fibre. The stress redistribution leads to compressive stress in the matrix and takes the advantage of superior creep resistance of carbon fibres.

Strain visualization of growing short fatigue cracks in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint: Intergranular cracking and crack closure

Physically short fatigue crack growth behavior in the heat-affected zone of a Ni–Cr–Mo–V steel welded joint was investigated in-situ in SEM to reveal effects of crack closure and tortuous crack growth path on crack-tip straining behavior with the help of high resolution digital image correlation technique. The crack growth path and associated fatigue damage at microstructural scale was characterized by electron backscatter diffraction and electron channeling contrast imaging techniques. Results showed that the short crack growth was influenced by both local microstructure and global strength gradient. The short crack could deflect intergranularly when meeting with one large grain due to misfit deformation and strain localization. The crack closure, which was accompanied during loading and unloading in terms of crack surface contact, shielded the real crack-tip and manifested itself well in both compressive strain at crack wake and strain development at crack-tip. It is indicated that strain localization at lathy boundaries and formation of sub-grains was responsible for the fatigue of short cracks.

APPLICABILITY OF ENERGY FLOW VECTOR IN THE MECHANICS OF DESTRUCTION

When studying the crack resistance of elements of welded structures, the energy criterion of failure is often used. The energy criterion is successfully applied to cracks propagating without a break, but when a crack strains at an angle to its original direction, the application of this criterion is constrained by the absence of a sufficiently simple method for calculating the intensity of the released energy of elastic deformations. G.P. Cherepanov and other researchers propose to calculate it using the concept of the energy flux vector. The proposed assumption allows to correctly calculate the values of its projection, but in fact, after the «fracture» of the crack, the form of the stress field in the vicinity of the peak sharply changes, which is expressed in a change in the stress intensity factors. As a result, the conclusions obtained are in satisfactory agreement with experiment and other fracture criteria only at small values. The example in the article shows that the intensity of the released energy does not have a vector property. Therefore, in the general case, one cannot use the concept of the energy flux vector.

Study of crack dynamic propagation behavior of fine-grained concrete under static loading

Under static loading, crack propagation processes are a dynamic process due to very high propagation speeds. For fine-grained concrete material under static loading, the crack propagation speed can reach 723.7 m/s, thus the crack propagation process under static loading actually can be considered as a dynamic issue, not a static issue. Crack propagations with a rapid speed are extremely unfavorable to the stability of concrete structures. Therefore, it is of significance to study the dynamic fracturing of concrete structure under static loading. Three-point bending (TPB) tests were conducted by using concrete specimens. A novel test system coupled with crack propagation gauges (CPGs) and strain gauges (SGs) was utilized to measure the crack speed and the initiation loads. The experimental results show that crack propagation velocities are not a constant and the propagation toughness of fine-grained concrete varies with the crack tip displacement. The initiation toughness is higher than the propagation toughness.

Dynamic localized shear failure influenced by changing rates in brittle solids containing initial microcracks

Dynamic microcracks growth has a great influence on macroscopic dynamic mechanical properties under impact loadings in brittle solids containing numerous initial microcracks. Confining pressure plays an important part in shear failure along with shear bands in brittle solids. However, the theoretical micro-macro relationship between shear failure and microcracks growth subjected to impact loadings is rarely proposed. Furthermore, evaluations of changing rates-dependent dynamic mechanical properties have a great meaning for engineering applications. In this study, a micro-macro dynamic localized shear failure model is proposed to explain the effects of changing rates on stress-deformation constitutive curves. A dramatic dynamic failure criterion is proposed by calculating zero crack velocity (ZCV) under constant strain rate during continuous deformation. The formulation of this micro-macro model is based on Ashby and Sammis's crack model, dynamic crack fracture propagation toughness law, a suggested physical correlation of the microcrack and deformation rates, and a deformation path function of changing rates. This suggested rate correlation of microcrack and deformation explains a unified correspondence of crack velocity, axial strain rate and shear velocity with increasing crack length, axial strain and shear displacement, which is established by unifying crack damage and deformation damage. The deformation path function of changing rates describes the step variation of crack velocity, strain rate and shear velocity with ascending crack length, axial strain, and shear displacement, respectively. Effects of parameters in this path function of changing rates on axial stress-crack curve, axial stress-strain, and shear stress-displacement curve are discussed. Rationality of the proposed model is verified by comparing with the published results. The theoretical results will provide an important help for evaluating the mechanical behaviors of applied brittle solids materials in engineering.

Statistical bias indicators for the long-term displacement of steel-concrete composite beams

Steel-concrete composite beams are widely employed in constructions and their performance at the serviceability stage is of concern among practitioners and design regulations. In this context, an accurate evaluation of long-term deflections via various rheological concrete models is needed. In this work, the performance and predict capability of some concrete creep and shrinkage models ACI, CEB, B3, FIB and GL2000 are ascertained, and compared by using statistical bias indicators. Ten steel-concrete composite beams with existing experimental and numerical results are then modeled for this purpose. The proposed modeling technique uses the finite element method, where the concrete slab and steel beam are modeled with shell finite elements. Concrete is considered as an aging viscoelastic material and cracking is treated with the common smeared approach. The results show that when the experimental ultimate shrinkage strain is used for calibration, all studied rheological models predict nearly similar deflections, which agree with the experimental data. In contrast, significance differences are encountered for some models, when none calibration is made prior to. A value between twenty and thirty times the cracking strain is recommended for the ultimate tensile strain in the tension stiffening model. Also, increasing the relative humidity and decreasing the ambient temperature can lead to a substantial reduction of slab cracking for beams under negative flexure. Finally, there is not a unique rheological model that clearly excels in all scenarios.

The effect of annealing and anisotropic behaviour on tensile and fatigue properties of AA 8090

Purpose: This paper aims to assess microstructures and mechanical properties of annealed and un-annealed Al-Li alloys (AA8090) and provide valid information regarding influence of anisotropy on tensile properties, fatigue lives. Design/methodology/approach: The methodology included investigating the influence of annealing on grain size, tensile strength and fatigue lives of AA8090. Optical microscope, scanning electron microscope and X-ray diffraction were utilized to analyse the crystallographic texture. Findings: The results showed that the un-annealed alloy exhibited much finer grain structure in three directions, namely longitudinal (L) rolling direction, L-45° and Long transverse (LT) combined with stronger crystallographic texture. Regarding to mechanical properties, un-annealed alloy presented superior tensile strength with strong anisotropic behaviour. L and LT grains direction showed highest tensile strength value of 550 MPa and L-45° showed lowest tensile strength value of 420 MPa. Results of fatigue test revealed that annealed Al-Li alloy has lower fatigue lives with high influence of test direction on fatigue properties. Higher variation in fatigue life to failure links with un-annealed alloy over annealed alloy. Examination of fractured surface showed that the morphology of fractured surface is a mixture of ductile and brittle fractures in both annealed and un-annealed alloys with more brittle behaviour in un-annealed alloy. Research limitations/implications: The main challenge of this work is the determination of the test direction (test angle in respect to rolling direction), which is necessary to provide correct information regarding mechanical properties. Further study of low cycle fatigue can be done in future, which will be an excellent indication to mechanical properties of this alloy since it provides more understanding to the behaviour of the material and better comprehend crack propagation and strain stress concentration. Practical implications: AA8090 alloy is an important candidate for aerospace and aircraft industries. Influence of annealing heat treatment and rolling direction on mechanical properties of AA8090 alloy provides more accurate information to the manufacturers who deal with this alloy. Originality/value: This study is affording a significant information regarding the effect of annealing and anisotropic behaviour on mechanical properties of AA8090. To our knowledge, there are few reports that study this combination of factors on AA8090 alloy.

In-Depth Understanding of Fatigue Micromechanisms in Cemented Carbides: Implications for Optimal Microstructural Tailoring

The fatigue mechanics and mechanisms of cemented carbides (composites usually referred to as hardmetals) are reviewed. The influence of microstructure on strength lessening and subcritical crack growth for these ceramic-metal materials when subjected to cyclic loads are highlighted. The simultaneous role of the ductile metallic binder as a toughening and fatigue-susceptible agent for hardmetals results in a tradeoff between properties measured under monotonic and cyclic loading: fracture strength and toughness on one hand, as compared to fatigue strength and crack growth resistance on the other one. Toughness/fatigue–microstructure correlations are analyzed and rationalized on the basis of specific crack–microstructure interactions, documented by the effective implementation of advanced characterization techniques. As a result, it is concluded that the fatigue sensitivity of cemented carbides may be reduced if either toughening mechanisms beyond ductile ligament bridging, such as crack deflection, are operative, or strain localization within the binder is suppressed. In this regard, grades exhibiting metallic binders of a complex chemical nature and/or distinct microstructural assemblages are proposed as options for effective microstructural tailoring of these materials.

Study on microcrack growth-based dynamic compressive mechanical properties in brittle rocks

The dynamic microcrack growth has a great influence on the macroscopic dynamic mechanical properties of brittle rocks under dynamic compressive loadings. However, the relationships between dynamic microcrack growth and macroscopic dynamic mechanical properties are rarely studied. Based on the microcrack growth-induced stress-strain constitutive relationship, the relationship between quasi-static and dynamic fracture toughness, the relationship between crack velocity and strain rate, the relationship between strain rate and dynamic fracture toughness, a micromechanics-based stress-strain constitutive model is proposed. Furthermore, the relationship between crack velocity and strain rate is derived by the time derivative of equation describing the relationship between crack length and axial strain. The relationship between strain rate and dynamic fracture toughness is obtained by coupling the suggested relationship between crack velocity and strain rate, and the crack velocity and fracture toughness. Effects of strain rate on stress-strain relation and dynamic compressive strength are studied. The reasonability of the proposed dynamic constitutive model is verified by the experimental results. Sensitivities of initial damage, confining pressure, parameters m, e0 and R on stress-strain relation, dynamic compressive strength and dynamic elastic modulus are discussed. These studies will provide a certain theoretical help for analyzing the stability of brittle surrounding rocks in deep undergrounding engineering.

A reassessment of the multiaxial ductility C* creep crack growth equation based on the strain energy integral of the HRR singular field terms

A revaluation of the upper bound plane strain multiaxial C* crack growth rates is undertaken for a range of materials. The analysis assumes that the multiaxial ductility factor can be approximated by taking the integrated dimensionless deviatoric and hydrostatic strain energy density of the HRR singular fields. Using the equivalency approximation between the plastic and elastic strain energy, a damage mechanics approach is used to determine the crack tip triaxial constraint. The new multiaxial factor was found to be independent of the power law behaviour, and for plane stress HRR fields gave a multiaxial factor of close to one. The new factor enables estimates of the upper bound crack rates under plane strain in the Nikbin Smith Webster equation to be undertaken. Creep crack growth data from nine materials were used to assess the new approximation for upper bound crack growth rates. The results suggest the current upper bound plane strain crack growth rates could be decreased from the current thirty times to three times the plane stress results for a range of materials.

Experimental investigation on the failure mechanism of a rock landslide controlled by a steep-gentle discontinuity pair

A type of rock landslide is very common in practical engineering, whose stability is mainly controlled by the rock bridge between the steep tensile crack at the crest and the low-inclination weak discontinuities at the toe (namely, ligament is the term for the locking section in the slope). To obtain a deeper understanding into the failure process of this kind of landslide, twenty-four physical slope models containing a steep-gentle discontinuity pair (a steep crack in the upper part and a low-inclination discontinuity in the lower part) were tested by applying vertical loads at the crests. The results indicate that the inclination angle of the ligament (θ) has great influence on the failure and stability of this type of rock slope. With the change of θ, three failure patterns (five subtypes) concerning the tested slopes can be observed, i.e., tensile failure of the ligament (Type 1), tension-shear failure of the ligament (Type 2) and two-stage failure of the main body (Type 3). The failure process of each failure mode presents five stages in terms of crack development, vertical load, horizontal/vertical displacements and strains in the ligaments. The specific range of the ligament angle between different failure patterns is summarized. The discussion on the failure resistances and ductility of different failure patterns, and the guiding significances of the experimental findings to the stability evaluation and the reinforcement were conducted.

Reinforcing Efficiency of Micro and Macro Continuous Polypropylene Fibers in Cementitious Composites

The effect of the microstructure of hydrophilic polypropylene (PP) fibers in the distribution of cracking associated with the strengthening and toughening mechanism of cement-based composites under tensile loading was studied. Using a filament winding system, continuous cement-based PP fiber composites were manufactured. The automated manufacturing system allows alignment of the fiber yarns in the longitudinal direction at various fiber contents. Composites with surface-modified hydrophilic macro-synthetic continuous polypropylene fibers and monofilament yarns with different diameters and surface structures were used. Samples were characterized using the tensile first cracking strength, post-crack stiffness, ultimate strength, and strain capacity. A range of volume fractions of 1–4% by volume of fibers was used, resulting in tensile first cracking strength in the range of 1–7 MPa, an ultimate strength of up to 22 MPa, and a strain capacity of 6%. The reinforcing efficiency based on crack spacing and width was documented as a function of the applied strain using digital image correlation (DIC). Quantitative analysis of crack width and spacing showed the sequential formation and gradual intermittent opening of several active and passive cracks as the key parameters in the toughening mechanism. Results are correlated with the tensile response and stiffness degradation. The mechanical properties, as well as crack spacing and composite stiffness, were significantly affected by the microstructure and dosage of continuous fibers.

An Integrated Numerical Method to Predict the Strain Sensing Behavior of Flexible Conductive Fibers under Electric–Thermal Conditions

Flexible conductive textiles have attracted an increasing amount of attention due to being flexible and stretchable with multi-functions, in addition to the strain sensing function. In this paper, an integrated numerical method is established to predict the strain sensing behavior of PPy-coated flexible conductive fibers. Based on our previous investigation on the strain sensing behavior of the PPy-coated elastic conductive fibers, it has been known that the main strain sensing mechanisms of the conductive fibers are micro-cracks opening under stretching and closing under unloading. Therefore, in the numerical approach, extended finite element method is used to simulate the multi-crack initiation and propagation. Then, by establishing a representative element model including a single crack, the resistance variation with the increase in crack width/depth can be obtained using the electric–thermal package in ABAQUS software. Combining the relationship between crack width and strain, the relationship between fractional increment in resistance (\( \Delta R /R_{0} \)) and the strain can be obtained. The prediction of the fractional increment in resistance (\( \Delta R /R_{0} \)) versus strain is consistent with the experimental results of PPy-coated Lycra fibers. The new integrated method is further verified by experimental results of PPy-coated XLA fibers, showing that the integrated numerical approach can capture the complicated strain sensing mechanisms of PPy-coated elastic fibers and predict the strain sensing behavior of flexible conductive fibers. This approach will make it possible to shorten the period, and reduce the cost in R&D of the PPy-coated electrically conductive fibers, comparing with the corresponding experimental work.

Numerical and experimental investigation of flexural performance on pre-stressed concrete structures using electromechanical admittance

Abstract This paper presented a numerical and experimental investigation of the performance of flexure-critical pre-stressed concrete (PSC) structures using the electro-mechanical admittance (EMA) of piezoceramic transducers (PZT). In the numerical analysis, three-dimensional finite element modeling was developed for a PSC beam subjected to four-point bending loads. The focus of the numerical analysis was the accurate detection of concrete cracking, and its growth using EMA signatures obtained from a multi-physics harmonic analysis of the electro-mechanical coupled fields. In the experimental study, two PSC beams with different grouting conditions were subjected to four-point bending tests until failure, and were simultaneously monitored by five surface-mounted PZTs. The crack observations, strain measurements, and load–deflection curves were recorded as references for the qualitative and quantitative assessments of the PSC structural flexural performance by the extraction of EMA signature characteristics, root mean square deviation (RMSD) and its rate indices. The experimental results revealed that the proposed method offered a higher sensitivity in comparison with conventional methods. The EMA characteristics could detect concrete cracking and reinforcement yielding in advance; the RMSD rate was more sensitive in the prediction of uploading and failure in the specimens; and the structural performance states such as crack initiation, propagation and penetration, yield and failure were effectively determined using RMSD characteristics. The promising findings in this study provided a potential method for the assessment of flexure-critical PSC structural performances.

Experimental and numerical modeling study of solidification cracking in Alloy 52M filler metal in the cast pin tear test

Finite element analysis (FEA) model of the solidification process in the cast pin tear test (CPTT) was developed using the commercial software ProCAST™. Modeling predictions of the solidification and strain accumulation sequences and of the cracking sensitive region location were validated using parallel CPTT testing of Alloy 52M filler metal. Predicted pin length dependences of the hot tearing indicator (HTI) and the effective plastic strain (EPS) were in good correlation with the experimentally determined zero to 100% CPTT cracking curve in Alloy 52 M. Sensitivity study with the developed model identified material properties and test parameters with strong influence on the accuracy of modeling predictions. Parallel computational modeling and CPTT experimentation was performed to quantify the critical strain for solidification cracking in Alloy 52M filler metal. This study demonstrated that numerical modeling of weldability tests can be used for quantification of material-specific properties related to cracking susceptibility in engineering alloys.

Influence of Strain Rate and Waveshape on Environmentally-Assisted Cracking during Low-Cycle Fatigue of a 304L Austenitic Stainless Steel in a PWR Water Environment

In this paper, the low cycle fatigue resistance of a 304L austenitic stainless steel in a simulated pressurized water reactor (PWR) primary water environment has been investigated by paying a special attention to the interplay between environmentally-assisted cracking mechanisms, strain rate, and loading waveshape. More precisely, one of the prime interests of this research work is related to the consideration of complex waveshape signals that are more representative of solicitations encountered by real components. A detailed analysis of stress-strain relation, surface damage, and crack growth provides a preliminary ranking of the severity of complex, variable strain rate signals with respect to triangular, constant strain-rate signals associated with environmental effects in air or in PWR water. Furthermore, as the fatigue lives in PWR water environment are mainly controlled by crack propagation, the crack growth rates derived from striation spacing measurement and estimated from interrupted tests have been carefully examined and analyzed using the strain intensity factor range ΔKε. It is confirmed that the most severe signal with regards to fatigue life also induces the highest crack growth enhancement. Additionally two characteristic parameters, namely a threshold strain εth* and a time T*, corresponding to the duration of the effective exposure of the open cracks to PWR environment have been introduced. It is shown that the T* parameter properly accounts for the differences in environmentally-assisted growth rates as a function of waveshape.

Effects of self-healing on tensile behavior and air permeability of high strain hardening UHPC

High strain hardening UHPC was expected to be self-healing due to its multiple micro-cracks in tension and the low water-to-binder ratio. The direct tensile tests with the acoustic emission (AE) analysis method and air permeability tests were carried out on high strain hardening UHPC in order to investigate the self-healing behavior under three re-curing conditions (water, 80%RH and 45%RH). Moreover, the cyclic tensile loading was applied to create more severe damage and two re-curing schemes (water and steam) were used to evaluate the self-healing effects. Experimental results show that the direct tensile stress-strain curves of high strain hardening UHPC re-cured in water and 80%RH conditions have the recovery of first cracking strain. New crystals were observed in the micro-cracks after self-healing, whose strength was proved to be lower than that of the intact part. The high strain hardening UHPC can recover 40%-47% of the first cracking strain after 28 days of water re-curing, where new damages characterized by AE sources number were found to generate inside the UHPC matrix. Air permeability test shows a sensitive response to the damage created by the tensile deformation. Steam re-curing method shows more effective and less time consuming for self-healing than water re-curing method.

Indices to Determine the Reliability of Rocks under Fatigue Load Based on Strain Energy Method

Rock is a complicated material which includes randomly distributed grains and cracks. The reliability of rocks under fatigue load is very important during the construction and operation of rock engineering. In this paper, we studied the deformation and failure process of red sandstone under fatigue load in a laboratory based on a new division method of strain energy types. The traditional elastic strain energy density is divided into two categories: grain strain energy density and crack strain energy density. We find that the proportion of the grain strain energy density to total strain energy density can be used as an indicator of rock yield and the proportion of the crack strain energy density to total strain energy density can be used as an indicator of rock failure. Subsequently, through extensive literature research, we found that such a phenomenon is widespread. We also find the proportion of grain strain energy density to total strain energy density when yielding is affected by rock types and elastic modulus. The proportion of crack strain energy density to total strain energy density in the pre-peak stage is stable and not affected by rock types and elastic modulus, which is about 0.04~0.13. These findings should be very helpful for rock stable state judging in rock engineering.

Effects of beam–column geometry and eccentricity on seismic behaviour of RC beam–column knee joints

The seismic behaviour of reinforced concrete beam–column knee joints is more complex than the conventional interior and exterior joints due to significantly different closing and opening force actions in knee joints. The strong-column weak-beam design principle is not strictly applicable due to the roof level location of knee joints resulting in weaker columns in terms of their flexural capacity. The general behaviour of knee joints and the effect of eccentricity of the beam axis with the column centre line is not well known due to the lack of experimental test results. Four full-scale knee joint sub-assemblages were tested under reversed cyclic loading to study the effects of the eccentricity of beam axis to column centre line and the ratio of beam–column flexural capacity. Details of the cyclic performance of the tested knee joints in closing and opening actions in terms of shear strength, hysteretic behaviour, joint cracking and reinforcement strain are presented. The test results indicated a relatively weak opening behaviour of knee joints in general. The weakening effect of eccentricity was observed strikingly in opening actions. Stronger columns were effective in improving the opening capacity.