Static Crack Propagation Research Articles

Fracture stability and residual strength assessment of reinforced concrete beams

In conventional analysis and design procedures of reinforced concrete structures, the ability of concrete to resist tension is neglected. Under cyclic loading, the tension-softening behavior of concrete influences its residual strength and subsequent crack propagation. The stability and the residual strength of a cracked reinforced concrete member under fatigue loading, depends on a number of factors such as, reinforcement ratio, specimen size, grade of concrete, fracture properties, and on the tension-softening behavior of concrete. In this work, a method is proposed to assess the residual strength of reinforced concrete beams subjected to cyclic loading. The crack extension resistance based approach is used for determining the condition for unstable crack propagation. The effect of reinforcement is modeled as a closing force counteracting the effect of crack opening produced by the external moment. The effect of percentage reinforcement and specimen size on the failure of reinforced beams is studied. Finally, the residual strength of the beams are computed by including the softening behavior of concrete.

Effect of aggregate kind and graining on modelling of plain concrete under compression

Hardened concrete is a brittle composite material characterised by non-homogeneous internal structure. Under the influence of external loads composites destruction is caused by: discontinuities created by technological defects or local differences of mechanical properties of concrete components. In the vicinity of discontinuities local concentration of stresses occurs. They can cause (under compressive loading) defects initiation and their stable propagation, which leads to degradation of the whole material sample or structural element. The brittle fracture process occurs when the large defects start to growth in the unstable manner. The internal structure of the concrete is very complex due to changes of the number of defects and their shapes during the whole loading process up to the final failure. Cracks, which arose in the interfacial aggregate–mortar transition zone (ITZ) propagate mainly in cement matrix in the surrounding area of the concrete aggregate. Grain shapes cause: bending of cracks, wing initiation at tips of straight cracks and creation of defects with more complex shapes. The paper presents the method of mesomechanical modelling of compressed concrete. The subject of investigation is cement composites which are made on the basis of limestone (L) and gravel aggregates (G). The paper is particularly focused on an effect of grain diameters on the macroscopic response of the concrete. Crack dimensions, shapes and numbers are strictly related to the aggregate diameters of the concrete composite. The aggregate material strongly influences the mechanical properties of the ITZ and further overall mechanical response as well as crack propagation conditions. The presented theoretical predictions of different concretes and experimental investigations lead to the conclusion that the appropriate selection of grain diameter can improve structural properties of the concrete composites and enlarge their applications.

Effect of Mean Stress on Fatigue Strength of Non-Combustible Magnesium Alloy

Effect of mean stress on fatigue strength at N=107 of non-combustible magnesium alloy AMX602B(X=Ca) was investigated. Rotating bending fatigue test and tension-compression fatigue test were carried out on specimens with a small hole or crack. It was clarified that the fatigue strength at N=107 of the specimen with the small hole was about 30-150% higher than that of the specimen with the small crack within the range of σm=0~100MPa. This is the reason why the fatigue strength at N=107 of the specimen with the small hole can be not threshold condition for crack propagation but crack initiation. The fatigue strength at N=107 of the specimens with the small hole decreased within the range of σm=100~195MPa due to a static small crack initiating from the small hole at first loading. The effect of mean stress on fatigue strength at N=107 both oh the specimens with the small hole and crack could be estimated using modified Goodman diagram.

Crack-tip stresses and their effect on stress intensity factor for crack propagation

In this paper, analytical and numerical simulations of the crack-tip stresses are presented. Analytical calculations are performed utilizing modified Rice and elastic equations. 2D finite element analyses (FEA) are conducted in parallel using ANSYS. Results from both methods are compared and discussed in terms of the crack-tip compressive residual stresses. Then, the effect of the compressive residual stresses is quantified in terms of the stress intensity factor for crack propagation, K PR, using ‘clamping force’ method. The comparison between the calculated K PR values and those obtained experimentally by Lang demonstrates a fairly good agreement. Both the present results and Lang data independently support a two-parameter, K maxTH and Δ K TH, description of the threshold condition for fatigue crack propagation.

Ductile Crack Propagation Characteristics in Steel Thin Single Edge Notched Tension Specimens

Static and dynamic ductile crack propagation tests were carried out using thin single edge notched tension (SENT) specimens of carbon-manganese steel, each of which had a fatigue pre-crack or a sharp V-notch as a crack initiator. The crack tip opening angle (CTOA) was measured using digital images on the surface of the SENT specimens, and the critical values of CTOA for crack propagation decreased with increasing crack length while initial crack growth was still small. After the initial crack growth up to the distance of the specimen thickness, the critical CTOA remained almost constant. These tendencies were common in static and dynamic crack propagation specimens as well as fatigue pre-cracked and sharp V-notched specimens. There was no particular difference in the static crack propagation characteristics of both fatigue pre-cracked and sharp V-notched specimens. On average, it was observed that higher crack speeds affected the constant values of the critical CTOA by slightly reducing them. The constant CTOA tends to decrease with an increasing global constraint factor, and this suggests that the factor is insensitive to a crack starter, fatigue pre-crack or a sharp V-notch, but relatively sensitive to crack speed.

Explicit finite element modeling of static crack propagation in reinforced concrete

We propose a methodology to model complex fracture processes in reinforced concrete beams subjected to static loading. The discrete cohesive approach, accompanied by an insertion algorithm, is adopted and a modified dynamic relaxation method is chosen as an alternative solver. The concrete matrix and steel re-bars are modeled explicitly; the connection in between is represented by means of interface elements. Such elements allow for slip of re-bars and transmit forces to the matrix that may generate secondary cracking around the reinforcement. The methodology is validated against three-point bending tests on lightly reinforced concrete (LRC) beams.

Fine-crack mechanics in estimating fatigue resistance with stress concentrations

Conditions are considered for the initiation and growth of fatigue cracks in stress concentration zones, together with the conditions for crack edge opening during propagation. Calculation methods are presented: the limits to unrestricted fatigue resistance as regards crack formation and failure with allowance for the skewness in the nominal stress cycle; and working lives of specimens on the basis of the conditions for crack initiation and propagation.

Measurement and Analysis of Impact Test Data for X100 Pipeline Steel

Charpy upper shelf energy is widely used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. The measurement of this fracture criterion particularly for modern steels and its apportion into different components, i.e. fracture and non-related fracture energy, are of great importance for pipeline engineers. This paper presents the results of instrumented Charpy impact experiments on high-grade pipeline steel of grade X100. First, the instrumentation technique including the design and implementation of a strain gauge load-cell and the details of the data-recording scheme are reviewed. Next, the experimental data obtained from the Charpy impact machine so instrumented are presented and discussed. These include the test data from full and sub-sized Charpy V-notched specimens. The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The results showed that there was a significant drop in hammer velocity during the impact event. This resulted in a change in the fracture mode from dynamic to quasi-static which was more appreciable for full-size Charpy test samples. As a result, sub-sized specimens might be preferable for impact testing of this steel in order to guarantee the conditions of dynamic crack propagation in the specimen ligament. Accurate analysis of the instrumented impact test data showed that the ratio of crack initiation energy to propagation energy was around 30% for the X100 steel. It can be concluded that in impact testing of high-grade pipeline steel a significant portion of overall fracture energy is consumed in non-related fracture processes. This high fracture initiation energy should be accounted for if the current failure models are going to be used for toughness assessment of highstrength low-alloy gas pipeline steels.

Ｅｕｌｅｒｉａｎ有限要素法による亀裂進展解析

This paper describes static fracture analysis by an Eulerian finite element method. The Eulerian finite element method is attractive to simulate fracture because it can generate new free surface without special computational procedure. Although we have developed the Euerian finite element code, treated only dynamic fracture behavior in short time duration. One reason for this is that the previous Eulerian finite element code employs an explicit dynamic method to advance time. The present study extends the explicit dynamic Eulerian finite element method to implicit static Eulerian finite element method. In addition, static crack propagation is simulated by using the present static Eulerian finite element method based on fracture mechanics.

Assessment of fatigue strength of weld root in ship structure: an approximate procedure

The paper presents results of analysis of weld root-initiated crack growth conditions in a cruciform fillet-welded joint. The focus was on assessment of conditions for crack propagation in a boxing fillet-welded joint. An approximate procedure is developed which allows evaluating initial part of fatigue process in a boxing fillet-welded joint where the crack is controlled by the stress field emanated from the slit tip. The crack traversing the weld bead at the attachment ending is regarded as a design criterion. A comparison of simulated and experimental data is given which may be regarded favorable for the approach when the crack growth is controlled mostly by the elastic stress field in weld material.

CRACK PROPAGATION IN RECTANGULAR THIN‐WALLED PLATES WITH CONCENTRATORS SUBECTED TO CYCLING LOADING

The paper presents the results of experimental investigations of the crack propagation process in thin‐walled elements made from optically active materials (polycarbonate), making the visualization of the field possible isochromatics at the front of the crack. Influence of the orientation of principal directions was analyses An experimental investigation of thin‐walled plate with circular and rectangular cutouts subjected to cycling shear loading was made. Using the photo‐stress method, propagation of the crack and the field of isochromatics were recorded over time. In parallel, the areas of initiation of fatigue fractures were identified. The results presented in the paper constitute a fragment of a larger process of investigation of thin-walled structures at the stress pattern in conditions of fatigue crack propagation to failure.

R-curve behavior of alumina toughened with molybdenum and zirconia particles

Alumina matrix was added with metal molybdenum and different amounts of zirconia's particles. R-curves were measured during stable crack propagation and a piezo-spectroscopic technique has been used to assess the bridging stresses developed along the crack wake at the critical condition for crack propagation. The theoretical R-curves calculated from the average (microscopic) bridging stress distribution obtained by in situ Raman spectroscopy were in good agreement with the experimental data. If the molybdenum content is kept constant but the ZrO 2 amount is increased, the toughening due to t- m transformation increases, but at the same time the metal/ceramic interface is weakened by the presence of zirconia and the toughening due to metal particles bridging decreases. As a result of that, the presence of zirconia has only a limited beneficial influence on the overall toughness. The overall composite toughness is due to transformation and bridging toughening mechanisms, however, the two mechanisms do not have synergistic effects and crack bridging by molybdenum particles appears to be the dominant toughening mechanism.

EXPERIMENTAL INVESTIGATIONS IN CHISELLING SOLID CONCRETE

Experimental research was performed for analysing the chiselling process. The conditions of crack formation and crack propagation have been analysed. In the first step impact tests with a standard chisel shapes and with a pointed chisel were performed. These tests represented the basic investigations into the chisel load effects on mechanical processing of concrete. In the next step precise modifications were applied to the flat-chisel cutting edge with the aim of improving the cutting-edge geometry.

Thermal fatigue resistance of gas and plasma nitrided 41CrAlMo7 steel

The influence of gas and plasma nitriding on thermal fatigue resistance of a 41CrAlMo7 steel is considered. The role of compound and diffusion layers is discussed on the basis of the damage observed in service. The thick and porous compound layer, resulting from gas nitriding, is not very effective in preventing thermal crack nucleation and propagation up to the interface with the underlying diffusion layer. On the contrary, the thin and compact compound layer obtained by plasma nitriding is able to prevent crack nucleation. Overnitriding, i.e. excessive nitriding induced embrittlement, has to be avoided in order to obtain high crack arrest fracture toughness in the diffusion layer. This suggests the need to realize surface layers with optimized hardness. A simplified approach to predict the conditions for crack propagation in the diffusion layer as a function of the thermal loading and the properties of the nitride layer has been also proposed.

EXPERIMENTAL INVESTIGATIONS IN CHISELLING SOLID CONCRETE

Abstract Experimental research was performed for analysing the chiselling process. The conditions of crack formation and crack propagation have been analysed. In the first step impact tests with a standard chisel shapes and with a pointed chisel were performed. These tests represented the basic investigations into the chisel load effects on mechanical processing of concrete. In the next step precise modifications were applied to the flat-chisel cutting edge with the aim of improving the cutting-edge geometry.

Stress-driven morphological instability and catastrophic failure of microdevices

In microdevices, the competition between surface energy and elastic energy could lead at the phenomenon known as stress-driven morphological instability (MI), causing an increase of surface roughness with time. Several different mass transport mechanisms can trigger such a morphological alteration and operate simultaneously: surface and bulk diffusion, evaporation and condensation, chemical reactions. Unstable solids could eventually evolve towards crack-like surfaces thus altering mechanical, electrical and optical properties of the devices or even leading to catastrophic failures by supercritical crack propagation. In this work, a more general kinetic law is employed to estimate the onset of MI, considering the effect of the stress field on the atomic mobility. A more intuitive and straightforward approach is used to determine the stability conditions, where the rate of atomic mass motion is introduced as a stability parameter. The critical loads and wavelengths for the onset of MI, determined as a function of material parameters α and β, are compared with the limiting conditions for the supercritical crack propagation (SC) of a crack-like surface in order to asses if and under which situations catastrophic failures by SC can be observed. Two practical cases are investigated: fixed wavelength (Case I) and arbitrary rough surface with a fixed remote load (Case II). In Case I, absolute and relative threshold loads are found below which MI could never occur and a transitional wavelength over which MI would always lead to SC is introduced. In Case II, it is shown that dominant perturbation for MI would always lead to SC given enough time for the surface to evolve towards a crack-like profile. The influence of the material properties α and β on the critical parameters is also addressed.

939 接触部の端部と中央部におけるフレッティング疲労き裂の進展条件の検討

939 接触部の端部と中央部におけるフレッティング疲労き裂の進展条件の検討

The Geometric Compatibility Factor Determining Brittle Crack Propagation Across Boundary in Bainitic Low Alloy Steels

Boundary properties between two adjacent grains with different crystallographic orientation relation are important factors in deciding the mechanical properties of materials. in our previous reports, we introduced the geometric compatibility equation as a factor determining the brittle crack propagation condition into the adjacent grain in banitic low alloy steel as follows:M = cos α · cos θwhere α is the angle between crack normal in the cracked grain-I and normal to (001) plane in the adjacent grain-II and θ is the angle between normal to (001) plane in the grain II and loading axis.(see FIG. 1) Higher M gives greater possibility to the crack propagation. The geometric compatibility factor (M) could be easily calculated from the crystallographic data obtained from Electron Backscatter Diffraction (EBSD) analysis, and successfully predicted actual crack propagation plane. We also found that there was a critical M value to arrest the brittle crack propagation at the boundaries.

Fracture testing and finite element modeling of pure titanium

This paper presents the results of fracture experiments and corresponding finite element analyses (FEA) of pure titanium. This investigation was motivated by the desire to develop a J– R testing protocol and numerical procedures that are applicable to a titanium/titanium boride layered functionally graded material. Tensile tests and a two-dimensional axisymmetric finite element model were used to determine the plasticity data for the titanium. Crack growth experiments were conducted in three-point bending using single edge notched bend specimens. Three-dimensional FEA of crack growth initiation and two-dimensional FEA with automatic crack propagation were performed. Two crack propagation conditions based on experimental data were used: (a) crack length versus load-line displacement and (b) crack length versus crack mouth opening displacement. The subsequent predictions of the non-linear finite element models are in reasonable agreement with the measured value of J at initiation and with the rising J– R data during crack propagation.

Residual stresses and R-curve behavior of AlN/Mo composite

Abstract AlN matrix was added with different amounts of metal molybdenum particles. R-curves were measured during stable crack propagation and a piezo-spectroscopic technique has been used to assess both, the residual stresses in the AlN matrix and the bridging stresses developed along the crack wake (at the metal/matrix interface) at the critical condition for crack propagation. Thermal residual stresses were compressive for Mo fractions ⩽15%; they were ≈0 when 20 vol.% of Mo was added and tensile for fractions ⩾30 vol.%. These stresses affected the composites toughness in two ways: (i) compressive residual stresses in the AlN matrix slightly enhanced the critical stress intensity factor for crack initiation, KI0, of the composite, (ii) tensile residual stresses weakened the metal/ceramic interface, leading to an enbrittlement of the composite. For Mo⩽20%, metal particles bridged the crack whereas when 30 vol.% of Mo was present the metal/ceramic interface was weakened and became the most favorable site for fracture of the bridging sites. As a result the toughness of the composite was strongly reduced. The theoretical R-curves calculated from the average (microscopic) bridging stress distribution obtained by in situ Raman spectroscopy were in good agreement with the experimental data.