Fracture process zone in concrete tension specimen

doi:10.1016/S0013-7944(99)00111-3

Engineering Fracture Mechanics

Volume 65, Issues 2–3, 1 January 2000, Pages 111-131

https://doi.org/10.1016/S0013-7944(99)00111-3 Get rights and content

Abstract

Experiments were carried out with X-rays using contrast medium and three-dimensional (3D) Acoustic Emission (AE) techniques to investigate the behavior of the fracture process zone in concrete. The results show that, as the loading increases, a zone consisting of numerous microcracks accompanied by AE events develops ahead of the notch tip in the concrete compact tension specimen. The energy of each individual AE event was calculated and plotted on the (3D) map. The results obtained by the two methods were compared and related to provide a clear view of the fracture process zone of concrete.

Introduction

A general consensus exists related to the dependence of fracture behavior of concrete on a fracture process zone that develops around and ahead of the crack tip. However, a direct quantitative relationship between the process-zone characteristics and the measured toughness has not been established yet. It has recently been recognized that an important task in the future development of the fracture mechanics of concrete is to standardize the testing procedure. The length and the width of the fracture process zone are crucial in selecting the geometry and dimensions of the standard test specimen.

Then, what are the mechanisms to develop the fracture process zone and what happens in it? For the answer, some concepts about the fracture process zone are presented. One of them, published by Mihashi [1], considers that around the branches of the main crack there are closed microcracks (see Fig. 1) and in front of them opening microcracks exist. Another concept, published by Wittmann [2], is shown in Fig. 2. He separated an inner microcracking zone from a surrounding isolated microcracking zone. These concepts are valuable but do not define the fracture process zone completely. Therefore, it is necessary to obtain a reliable experimental evidence. There are significant discrepancies in existing results, probably resulting from the differences in the techniques employed, such as optical microscopy, electron microscopy, acoustic emission techniques, laser speckle techniques and so forth [3]. It is, therefore, very important to observe the internal fracture in concrete. Otsuka [4], [5], developed an inspection technique based on X-rays using contrast media, which can directly inspect internal cracks. Using this technique, the authors carried out some experiments that are reported elsewhere [6], [7], [8], [9]. Fig. 3 shows one of the results obtained where several microcracks are observed ahead of the notch tip. The authors insist that this microcrack zone corresponds to a fracture process zone or at least must be a main part of the fracture process zone. To confirm this point, the present research is conducted

This paper reports a further investigation of the fracture process zone using two methods: direct observation with the X-ray technique and indirect observation through an AE technique. The results obtained are compared and correlated to provide a clear view of the fracture process zone of concrete. To start this study, extensive and worthy works carried out by Shah and coworkers [10], Mihashi and coworkers [11], [12] and Ohtsu and coworkers [13] gave a lot of suggestion.

Section snippets

Specimens

Concrete containing high-early-strength Portland cement was used. The fine aggregate was a river sand and the coarse aggregate was crushed stone. The maximum aggregate size was 5, 10, 15 and 20 mm. All specimens were made from concrete with a specified cylinder strength of 20 MPa. Relatively low-strength concrete was used to accentuate effects of the fracture behavior of concrete.

Fig. 4 shows the configuration of the specimens used in the test. Four different-sized specimens, S-type, M-type,

Microcracks obtained by X-ray inspection

Fig. 8 shows relationships between load and crack opening displacement (COD) measured in S-type, M-type, and L-type specimens.

Fig. 9 shows results obtained by X-ray inspection in M-type specimen with a maximum aggregate size of 10 mm. X-ray inspections were made at points P1 (70% of peak load), P2 (peak load), P3 (70% of peak load), and P4 (30% of peak load) on the curve in Fig. 8. On the X-ray films, microcracks formed at the notch tip started to be observed at about 80% of peak load. Fig. 9(a)

Conclusions

Experiments were carried out by X-ray and AE techniques to investigate the behavior of the fracture process zone in concrete. From the results of these experiments, the following conclusions can be drawn.

1.
Microcracks in a complex configuration near the tip of the notch were observed by the X-ray technique with contrast medium. Locations of AE event sources were determined by the 3D AE technique.
2.
The energy of each individual AE event was calculated. It led to the fact that many of the events had

References (13)

H. Mihashi et al.
Influence of aggregate size on fracture process zone of concrete detected with 3D acoustic emission technique
Cem. & Concr. Res
(1991)
H. Mihashi
States of the art and a view of the fracture mechanics of concrete
Journal of JCI
(1987)
F.H. Wittmann
Fracture process zone and fracture energy. Fracture Mechanics of Concrete Structures
S. Mindes
Fracture process zone detection
K. Otsuka
X-ray technique with contrast medium to detect fine cracks in reinforced concrete
K. Otsuka
Detection of fine cracks in reinforced concrete through X-ray technique using contrast media
Proc. of JSCE
(1992)

There are more references available in the full text version of this article.

Cited by (123)

Acoustic emission for exploring loading rate impact on type I fracture mechanism of engineering cementitious composites after subjected to sub-high temperature
2024, Journal of Building Engineering
To investigate the crack extension mechanism of engineered cementitious composite (ECC) after sub-high temperature. This paper studies the mode I fracture properties of ECC after sub-high temperatures (20 °C, 100 °C, 200 °C) at three strain rates (2 × 10^-5s^-1,10^-4s^-1,10^-3s^-1), and monitors the damage process in real-time by acoustic emission (AE) technology. Crack evolution and failure modes of ECC during loading are investigated by AE signal parameters. The results show that compared with 20 °C, the fracture energy of ECC increases after 100 °C, and decreases after 200 °C. The damage characteristics of ECC after 100 °C are similar to those at room temperature. Due to the bridging effect of the PVA fibers, their internal microcracks spread around from the tip of the prefabricated cracks. The percentage of shear type cracks gradually increases. The damage types are dominated by matrix cracking and fiber pull-out. While the PVA fibers deteriorated after 200 °C, the damage within the ECC was more concentrated, and the width of the fracture process zone was reduced. The crack types were dominated by tensile cracks. The failure mode was dominated by fiber pull-off and matrix cracking. Construction of an AE energy-based damage model. The b-value can reflect the whole process of crack evolution from multi-cracks to main cracks within ECC.
Wavelet entropy-based damage characterization and material phase differentiation in concrete using acoustic emissions
2024, Engineering Failure Analysis
Damage evolution in concrete is a complex phenomenon involving micro-cracks that originate from different material phases (mortar matrix, ITZ and aggregate), and culminate into a major crack. The information about the individual damaged material phases can provide major insights into the global failure behaviour, cracking path, and fracture processes. The evolution of damage, damage mechanisms and the material phase of origin of micro-cracks is highly influenced by the type of concrete (Normal and high strength). In order to investigate the evolution of damage in concrete of varying strength, notched beam specimens with different water-to-cement ratios have been prepared and tested under $C M O D$ control. The evolution and growth of damage have been continuously monitored during testing through acoustic emission techniques. Based on acoustic emission results, it has been demonstrated that the damage progression in concrete can be better described as a function of the wavelet entropy of the AE events occurring during the entire loading duration, and therefore, an analytical model for the damage evolution has been proposed. The study reveals that, under the high-strength category, the level of deterioration reduces as the $w / c$ ratio rises, while in normal-strength concrete, a reverse trend is observed. Furthermore, a novel approach has been proposed that utilizes wavelet entropy and amplitude to differentiate amongst various damaged material phases with the help of an unsupervised clustering algorithm. Normal-strength concrete with a $w / c$ ratio of 0.4 has been used to demonstrate the proposed methodology and has been validated with the help of experiments performed on the mortar and the parent stone of aggregate. A higher occurrence of mode-I cracks compared to mode-II cracks across all material phases has been observed for the specimen. Additionally, ITZ and aggregate exhibit a greater proportion of tensile and shear cracking events than the matrix phase. In the end, a deep learning convolutional neural network (CNN) model is devised utilizing labelled scalogram images of the AE waveforms to discriminate between the various damaged material phases. A 10-fold training cross-validation of the dataset has been carried out to achieve a stable performance. The deep learning model performs well in discriminating the damaged material phases with high training, validation, and testing accuracy.
Characterization of crack-bridging and size effect on ultra-high performance fibre reinforced concrete under fatigue loading
2024, International Journal of Fatigue
The use of ultra-high performance fibre-reinforced concrete (UHPFRC) has become popular owing to its high strength, durability and toughness characteristics compared to conventional concrete. In this work, experimental investigations have been carried out to examine the fracture behaviour of UHPFRC beam specimens under static and fatigue loading with an emphasis on understanding the size effect. Experimental results of both monotonic and fatigue have been utilized to develop a mathematical formulation for predicting crack bridging degradation considering the fatigue loading history. Subsequently, the size effect behaviour on the crack-bridging strength of UHPFRC beams under fatigue loading conditions has been explored. An algorithm has been developed to predict crack propagation as a function of load cycles and to estimate the fatigue life corresponding to any load amplitude, accounting for the size effect. The crack-bridging strength has been observed to decrease with an increase in specimen size, lowering the fatigue life.
Fracture mode analysis of cementitious mortars by simultaneous application of 4D-XCT and acoustic emission technique
2024, Construction and Building Materials
Cementitious brittle construction materials are susceptible to fracturing due to their heterogeneous material composition and relatively weak bond between the aggregates and paste. Hence, enhanced methods of fracture analysis in these materials are an important subject of research. The acoustic emission technique (AET) is frequently used in the study of brittle construction materials, yet the random nature of cracking and the heterogeneity of materials strongly affect the AE results, and necessitates verification of AE data with complementary monitoring techniques. Therefore, in this work, a test setup is developed where fracture during Brazilian splitting tests is simultaneously monitored by AE sensors and X-ray computed tomography (XCT). The novel test setup is designed to incorporate AE monitoring into in-situ step-wise load tests of cementitious mortar samples inside an XCT scanner. Digital volume correlation (DVC) of the XCT scans allows in-depth analysis of the fracture mechanism and validation of the AE results. Using DVC the damaged zones with high shear and tensile strain values were identified in addition, the X-ray images were used to investigate the influence of heterogeneities in the overall failure modes. For the AE-based damage localization, three arrival time estimation methods are assessed, with the Akaike Information Criterion (AIC) showing a 50–75% improvement in locating AE sources within the cracked zone. In addition, two AE-based fracture mode analysis methods are compared with the DVC strain field plots. Using both average frequency and rise angle analysis (AF/RA) and peak frequency (PF) analysis, 80–100% AE events in the shear strain zone were able to be classified as shear mode. PF showed better performance in the early load stages while AF/RA showed consistent results throughout. In general, the shear and tensile crack mode classification of the AE events agreed well with the shear and tensile strain field plots.
An analytical approach for characterizing the fracture behaviour of ultra-high-performance fibre reinforced concrete
2024, Composite Structures
The application of ultra-high-performance fibre-reinforced concrete (UHPFRC) offers significant advantages in constructing durable and efficient structural elements due to its remarkable compressive and flexural strength and dense microstructure. However, accurately predicting the hardening/softening profile of UHPFRC in relation to post-cracking tensile behaviour poses challenges, particularly when considering the influence of specimen size. In this work, the post-cracking tensile behaviour has been characterized by considering beam specimens of varying sizes under centre point loading. The inverse analysis approach has been employed to extract relevant information. A polylinear tensile stress profile has been proposed to capture the different fracture mechanisms involved, and an analytical model has been developed to predict the relationship between stress and crack width in the post-cracking zone. Critical crack width and fracture process zone length have been calculated, revealing the substantial contributions of the micro-cracking and fibre-bridging zones to the cohesive zone development in UHPFRC. Furthermore, it has been observed that the microcracking zone diminishes as the specimen size increases, highlighting its significant role in the size effect observed in UHPFRC and similar materials. A modified size effect model has been proposed incorporating the influence of the microcracking zone observed in the UHPFRC composite.
A theoretical model for the prediction of fracture process zone in concrete under fatigue loading: Energy based approach
2024, Mechanics of Materials
Characterizing the fracture behaviour of concrete and accurately predicting its service life under fatigue loading poses a significant challenge due to its heterogeneous nature and complex fracture mechanisms. The proposed study focuses on developing an energy based theoretical formulation for predicting the evolution of the fracture process zone throughout repetitive loading cycles. A stiffness degradation approach has been adopted for developing the formulations for the critical energy dissipation and fully developed fracture process zone. Subsequently, the proposed analytical expressions have been calibrated and validated by performing experiments under centre point bending and using available experimental results in the literature. Experiments have been conducted on a centre-point bend beam with varying aggregate size in conjunction with the digital image correlation (DIC) technique to estimate the fracture characteristics. The influence of specimen size and heterogeneity has been discussed in the context of predicting the fracture behaviour, critical dissipated energy, and process zone length of plain concrete beam specimens under fatigue loading. The results indicate that the process zone length and critical energy release rate increase with an increase in specimen size, while the same decreases with an increase in aggregate size.

View all citing articles on Scopus

View full text

Fracture process zone in concrete tension specimen

Abstract

Introduction

Section snippets

Specimens

Microcracks obtained by X-ray inspection

Conclusions

Cem. & Concr. Res

States of the art and a view of the fracture mechanics of concrete

Journal of JCI

Fracture process zone and fracture energy. Fracture Mechanics of Concrete Structures

Fracture process zone detection

X-ray technique with contrast medium to detect fine cracks in reinforced concrete

Detection of fine cracks in reinforced concrete through X-ray technique using contrast media

Proc. of JSCE