Multiple Cracking Behavior Research Articles

Interaction mechanism of radial collinear cracks on a high-speed train brake disc

This article focuses on the problem of multiple crack damage in brake discs. Using the extended finite element analysis method, the interaction mechanism between radially collinear multiple cracks in brake discs is analyzed, and the influence of crack spacing and length ratio on the propagation behavior of multiple cracks is further investigated. The results indicate that the interaction mechanism between multiple radial collinear cracks on the brake disc is complex, not only influenced by the crack spacing and crack length ratio but also related to the location of the cracks on the brake disc. The interaction of radial collinear cracks on the brake disc is mainly concentrated in the stress field at the vicinity of the adjacent tips between the cracks and has little effect on the crack opening displacement field. When the distance between cracks is less than half the length of the crack of interest, the two collinear radial cracks have a significant mutual influence and tend to merge into a longer crack. The length ratio between cracks has a significant impact on the stress transmission path around the cracks, and increasing the length ratio accelerates the crack-to-crack coalescence.

New interpretation model of the effect of short PE fibers in TRM on tensile behavior

An experimental investigation was conducted on the tensile behavior of textile reinforced mortar (TRM) composites containing different volumes of short polyethylene (PE) fibers in a cementitious matrix to improve the tensile strength and cracking mode of TRM. This investigation mainly aimed to study the action mechanism of short fibers and propose a reasonable textile-short fiber-matrix interface model. The short fiber content was varied, as was the matrix type and the number of grid layers. Ten groups of TRM tensile specimens subjected to the monotonic uniaxial tensile loading were fabricated and tested to examine their tensile characteristics. The results showed that the multiple-cracking behavior and failure mode of TRM were significantly improved by adding PE fibers into the matrix. The tensile strength was increased by 25.8–167.2 %. After the uniaxial tensile tests, the microscopic investigation of the interface between the textile and matrix was conducted on the tensile specimens using an environmental scanning electron microscope (ESEM). The significant influence of the short PE fibers on the improvement of the interface bonding between the textile and matrix was confirmed. More importantly, based on the ESEM results, a novel interface model was proposed to promote understanding of the action mechanism of short fibers.

Investigation on the interaction evolution and growth behavior of multiple cracks under fatigue loading

In this study, the cracks interaction is investigated on plates with different crack geometry parameters through finite element analysis and experiments. The presence of multiple cracks may lead to an enhancement or shielding effect on stress intensity factors (SIFs) depending on crack relative positions and sizes, thereby potentially accelerating or decelerating the crack propagation rate. When the crack tips of double parallel non-collinear cracks begin to overlap, the interaction between cracks transitions from an enhancement effect to a shielding effect, and the KII values reach their peak at this point, which are explained by the stress field distributions surrounding the cracks. For non-collinear double cracks with an inclined angle α, the shielding effect of inclined cracks becomes increasingly pronounced in comparison to horizontal cracks as the angle α increases. To verify the numerical results, fatigue crack propagation experiments were carried out on Al–Li alloy samples with double non-collinear cracks with different crack vertical distance. The experimental results show that growth direction of two non-collinear cracks underwent a transition when they overlapped, leading to load mode I changing into mixed mode of I + II. Based on both numerical simulations and experimental results, the material parameters C and m in Paris equation are modified, and then a new method for predicting the multiple cracks growth life is presented. By comparing with the experimental values, it was found that this method exhibits high accuracy and reliability.

Influence of FGM coating on the dynamic fracture behavior of multiple cracks in a homogeneous half-plane under in-plane loading

Influence of FGM coating on the dynamic fracture behavior of multiple cracks in a homogeneous half-plane under in-plane loading

Compressive and Tensile Behavior of High-Ductility Alkali-Activated Composites with Polyethylene Terephthalate Powder

Researchers have been engaged in the study of high-ductility concrete (HDC) due to its excellent ductility and cracking control ability. This study combines the concepts of HDC and alkali-activated composites (AAC) to develop high-ductility alkali-activated composites (HDAAC) using polyethylene terephthalate (PET) powder. Experimental investigations were conducted to assess the compressive and tensile properties of HDAAC, focusing on the impact of varying PET powder content (0%, 15%, 30%, and 45%) and fly ash/slag ratios (FA/GGBS, 6:4, 7:3, and 8:2). The results indicated that the compressive strength of HDAAC ranged from approximately 30 MPa to about 100 MPa, with the specimens maintaining good integrity after axial compression failure due to the bridging action of PE fibers. The replacement of quartz powder (QP) with PET powder slightly decreased the compressive strength and elastic modulus of HDAAC, albeit mitigating its brittleness under compression. An increase in GGBS content enhanced the compressive strength and elastic modulus of HDAAC due to the increased formation of the C-A-S-H reaction products, leading to reduced porosity and a denser microstructure. Under axial tension, HDAAC exhibited typical multiple-cracking behavior with significant pseudo-strain hardening. Increases in the PET content and FA/GGBS ratio resulted in finer cracks, indicating excellent crack control and deformation capabilities. The initial cracking strength, tensile strength, and ultimate tensile strain ranged from 3.0 MPa to 4.6 MPa, 4.2 MPa to 8.2 MPa, and 4.1% to 7.2%, respectively. Despite a decrease in the initial cracking strength and tensile strength with higher PET content, the ultimate tensile strain of HDAAC slightly increased. Observations under a scanning electron microscope revealed a distinct interfacial transition zone near the PET powder, leading to poor bonding with the alkali-activated matrix. In contrast, QP dissolved on the surface in highly alkaline environments, forming better interface properties. These variations in interface properties can be used to interpret the variations in the mechanical performance of HDAAC.

Crack propagation and failure mechanism of 3D printing engineered cementitious composites (3DP-ECC) under bending loads

Engineered cementitious composites (ECC), as a self-reinforcing material featuring the characteristic of multiple cracking and strain hardening, provides a feasible solution to overcome the dependence on steel reinforcement in 3D concrete printing (3DCP). Nevertheless, interlayers in 3D printing ECC (3DP-ECC) create potential paths for crack propagation under bending load, which is expected to result in different failure mechanisms for bent 3DP-ECC. Different from the conventional multiple cracking behaviors of cast ECC, the test results reveal that 3DP-ECC exhibits two new crack propagation modes depending on the interface bond strength. In the first mode, bending cracks deflect into the interlayer, leading to the splitting of two adjacent filaments, while the filaments in the upper part of the interface crack countinue to exhibit multiple cracking behavior and bear loads. In the second mode, the localized shearing cracks became dominant when the interlayer bond strength was excessively low. This study comprehensively discusses the bending performance and failure mechanisms of 3DP-ECC exhibiting different crack propagation modes. The findings are valuable for enhancing the ductility design of 3DP-ECC beam components, ultimately contributing to the improvement of 3D concrete printing technology.

The Strength and Fracture Characteristics of One-Part Strain-Hardening Green Alkali-Activated Engineered Composites.

Alkali-activated engineered composites (AAECs) are cement-free composites developed using alkali activation technology, which exhibit strain hardening and multiple micro-cracking like conventional engineered cementitious composites (ECCs). Such AAECs are developed in this study by incorporating 2% v/v polyvinyl alcohol (PVA) fibers into alkali-activated mortars (AAMs) produced using binary/ternary combinations of fly ash class C (FA-C), fly ash class F (FA-F), and ground-granulated blast furnace slag (GGBFS) with powder-form alkaline reagents and silica sand through a one-part mixing method under ambient curing conditions. The mechanical and microstructural characteristics of eight AAECs are investigated to characterize their strain-hardening performance based on existing (stress and energy indices) and newly developed tensile/flexural ductility indices. The binary (FA-C + GGBFS) AAECs obtained higher compressive strengths (between 48 MPa and 52 MPa) and ultrasonic pulse velocities (between 3358 m/s and 3947 m/s) than their ternary (FA-C + FA-F + GGBFS) counterparts. The ternary AAECs obtained a higher fracture energy than their binary counterparts. The AAECs incorporating reagent 2 (Ca(OH)2: Na2SO4 = 2.5:1) obtained a greater fracture energy and compressive strengths than their counterparts with reagent 1 (Ca(OH)2: Na2SiO3.5H2O = 1:2.5), due to additional C-S-H gel formation, which increased their energy absorption for crack propagation through superior multiple-cracking behavior. A lower fracture and crack-tip toughness facilitated the development of enhanced flexural strength characteristics with higher flexural strengths (ranging from 5.3 MPa to 11.3 MPa) and a higher energy ductility of the binary AAMs compared to their ternary counterparts. The tensile stress relaxation process was relatively gradual in the binary AAECs, owing to the formation of a more uniform combination of reaction products (C-S-H/C-A-S-H) rather than a blend of amorphous (N-C-A-S-H/N-A-S-H) and crystalline (C-A-S-H/C-S-H) binding phases in the case of the ternary AAECs. All the AAECs demonstrated tensile strain-hardening characteristics at 28 days, with significant improvements from 28% to 100% in the maximum bridging stresses for mixes incorporating 40% to 45% GGBFS at 365 days. This study confirmed the viability of producing green cement-free strain-hardening alkali-activated composites with powder-form reagents, with satisfactory mechanical characteristics under ambient conditions.

A comprehensive experimental study on the role of matrix structure on the multiple cracking behavior of PVA-ECC

In this study, the effect of W/B ratio and mineral additive types on the role of fiber–matrix synergy in PVA-ECC was comprehensively investigated with the aid of micro-mechanical design theory. The effects of rheological properties, failure cross-section fiber distribution, natural flaw size distributions on tensile performance were investigated, and the relationships among these parameters were researched. For maximum tensile stress, crack analysis were performed with DIC technique and crack width histograms were determined. Moreover, the relationships between tensile performance and compressive strength have been investigated. According to the results, it was determined that the combined effect of W/B ratio and mineral additive type in PVA-ECC had a significant effect on both the rheological and mechanical results, and their relationships were compared. Fiber distribution and flaw size distribution should also be considered together to explain the difference in tensile performance of different/same ECC mixtures. It has been determined that the current micromechanical theory PSH criteria are insufficient to provide saturated multiple crack behavior, and therefore need to be updated. In the investigation of crack pattern of ECC, crack width distribution rather than average crack width was found to be a more realistic approach.

RETRACTED: Effect of CaCO3 whiskers on tensile properties of ultra-high-performance engineered cementitious composites

Ultra-high-performance engineered cementitious composite (UHP-ECC) is expected to become an ideal construction material for its superior mechanical properties. However, the high cost of UHP-ECC, especially the use of expensive synthetic fibers, limiting its extensive engineering application. To address this, cheap calcium carbonate (CaCO3) whiskers were utilized to partially replace polyethylene (PE) fibers. The various volume fractions of PE fibers and CaCO3 whiskers were adapted to possess different tensile properties (ultimate tensile stress, strain capacity, crack number, and strain energy). The results indicated that the decrease in PE fiber content from 2% to 1.5% significantly deteriorated tensile properties of UHP-ECC (the decrease of 24.95% in tensile strength and 49.47% in strain capacity). However, the partial replacement of PE fibers by CaCO3 whiskers could effectively mitigate the performance degradation and improve the stable multiple cracking behavior. Especially, the tensile ductility of UHP-ECC with a combination of 1.75% PE fibers and 0.5% CaCO3 whiskers was 45.19% higher than that of UHP-ECC with mono 1.75% PE fibers, and also 102.5% higher than that of control group (2% PE fibers). Scanning electron microscopy images confirmed the synergistic effect of PE fibers and CaCO3 whiskers at the microscopic level. In according with micromechanics, the addition of CaCO3 whiskers could effectively improve the complementary energy of UHP-ECC by micro cracking arresting, which increased the pseudo strain hardening values and thus leading to the enhancement in tensile ductility of UHP-ECC. In addition, the hybrid fiber reinforcing index (RIv) had a positive correlation with the tensile properties of UHP-ECC. Therefore, a semi-empirical formula based on RIv could be adopted for the property prediction of UHP-ECC with different combination of PE fibers and CaCO3 whiskers.

Coalescence Behavior of Multiple Cracks in SUS304 Plate with under Creep Rupture Conditions

A creep test was conducted to clarify the coalescence behavior of cracks in a SUS304 flat plate with multiple cracks on different planes. The test temperature is 600 degrees. The shape of the specimen is a plate, and electrical discharge machined (EDM) notches were introduced on both sides of the central smooth part. Then, a fatigue pre-crack was generated at the tip of the notch. The position of the crack was expressed by the distance between the crack surfaces and the distance between the crack tips. As a result of the experiment, the cracks coalesced when the distance between the crack planes was shorter than about 3 mm. On the other hand, when the distance between the crack surfaces was about 7 mm, no crack coalescence was observed, and there was a change point in the crack coalescence behavior. It was found that the change point of the coalescence behavior of the crack obtained in the creep test satisfied codes for nuclear power generation facilities (JSME S NA1-2019). As a result of fracture surface observation, it was found that the crack growth from the fatigue pre-crack due to creep is due to grain boundary rupture regardless of the presence or absence of crack coalescence.

Fracture Strength and Crack Coalescence Behavior of SUS304 Plate with Multiple Cracks

Tensile tests were conducted at room and high temperature to clarify the fracture strength and crack coalescence behavior of multiple cracks in SUS304 flat plates. The shape of the specimen is a plate, and electrical discharge machined (EDM) notches were introduced on both sides of the central smooth part. Then, a fatigue pre-crack was generated at the tip of the notch. The position of the crack was expressed by the distance between the crack surfaces and the distance between the crack tips. As a result of the tensile test at room temperature, when the distance between the crack surfaces was shorter than about 8 mm, the cracks coalesced. On the other hand, when the distance between the crack surfaces was about 10 mm, no crack coalescence was observed, and there was a change point in the crack coalescence behavior. As a result of the high temperature test, crack coalescence was observed when the distance between the crack planes was about 8 mm, but the change point of the crack coalescence behavior is unknown because there is no data. It was found that the change points of the coalescence behavior of the cracks obtained in the experiment satisfy codes for nuclear power generation facilities (JSME S NA1-2019).

Development of engineered cementitious composites (ECC) using artificial fine aggregates

In this study, Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) using artificial fine aggregates [i.e., geopolymer aggregates (GPA) and cement-bonded aggregates (CBA)] were developed for the first time. The developed GPA-ECC and CBA-ECC showed a compressive strength over 120 MPa, and the GPA-ECC recorded the highest tensile strain capacity (9.0%) among the existing ambient-cured high-strength ECC in literature. Compared with fine silica sand ECC (FSS-ECC) as a control mix, GPA-ECC and CBA-ECC showed lower compressive and tensile strength, owing to their lower aggregate strengths. From digital image correlation analysis, a more saturated multiple cracking behavior was observed for GPA-ECC as compared to CBA-ECC and FSS-ECC. In addition, the use of artificial aggregates had marginal effect on the crack width distribution of high-strength ECC. The findings in this study demonstrate the feasibility of using artificial fine aggregates in ECC production and provide a new avenue to improve ductility and sustainability for ECC materials.

Probabilistic multiple cracking model of brittle-matrix composite based on a one-by-one crack tracing algorithm

The paper describes a probabilistic model capturing the multiple-cracking behavior of unidirectional brittle-matrix composites loaded in tension. The approach to modeling of composite fragmentation introduces two salient features that enhance both efficiency and flexibility compared to existing simulation methods: First, the algorithm identifies the emerging cracks one by one within a minimum number of load increments. Second, the crack-tracing algorithm is based on an abstract description of a crack bridge behavior. As a result, it is possible to combine the crack-tracing algorithm with a wide variety of crack bridge models, i.e. non-linear, deterministic or probabilistic. In this way, specific phenomena of bond behavior in different types of composites can be accounted for. The random nature of matrix cracking is reproduced using a random field simulation of the matrix strength. The model is verified by reproducing analytical results available for constant bond-slip law. The feasibility and robustness of the algorithm is demonstrated using an interactive web application that is directly executable from within a public github.com repository.

Development of basalt fiber engineered cementitious composites and its mechanical properties

Fiber plays a key role in the mechanical properties of Engineered Cementitious Composites (ECC), a new generation of fiber-reinforced concrete with excellent ductility and exceptional crack control capability. However, ECC loses its high ductility when exposed to fire, as the synthetic fibers typically used in ECC melt resulting in a loss of crack-bridging ability in elevated temperatures. In this study, the feasibility of using basalt fiber, an inorganic fiber with high-temperature resistance, to develop ECC is investigated experimentally. The results show that basalt fiber reinforced ECC (BF-ECC) exhibits a very unique strain-hardening and multiple-cracking behavior when compared with typical ECCs. The tensile stress–strain curve of BF-ECC is relatively smooth, with average crack width below 10 μm and crack spacing less than 3 mm. The unique features of BF-ECC are interpreted based on the bridging behavior of basalt fibers. This work paves the way for further developing ECC with high-temperature resistance.

Out-of-plane strengthening of unreinforced masonry walls using textile reinforced mortar added short polyvinyl alcohol fibers

Textile reinforced mortar (TRM) is a promising composite that exhibites typical multiple cracking behaviors. Short polyvinyl alcohol (PVA) fibers were added to a TRM matrix to increase its tensile strength and toughness and prevent the external matrix from detaching from the composite overlay in the experiments. Six groups of uniaxial tensile specimens were fabricated to investigate the tensile behaviors of the improved TRM composite. A group of unreinforced masonry (URM) walls and five groups of URM walls strengthened with the optimized composite were tested to study the out-of-plane behaviors. The results indicate that the ultimate strength of the composite significantly increased with the reinforcing ratio of carbon textiles. Detachment of the external matrix was not observed during all the tests. The deformation ability, bearing capacity and energy dissipation capacity of the strengthened specimens improved significantly. Finally, analytical models related to the failure modes of strengthened walls were proposed to predict the bearing capacity of strengthened walls.

Mode-I, Mode-II, and Mode-III Stress Intensity Factor Estimation of Regular- and Irregular-Shaped Surface Cracks in Circular Pipes

Stress intensity factor (SIF) of surface cracks (semi-circular, semi-elliptical, and irregular shapes) located on a circular pipe (hollow) has been determined using numerical method. The irregular-shaped crack geometry was considered in the present work in order to simulate the cracks that are generally developed during service condition. The interaction behavior of multiple cracks on SIF calculation has also been investigated. Crack aspect ratios [a/c; “a” is the crack depth and where “c” is semi-major axis of elliptical crack] ranging between 0.6 and 1.0 and crack depth ratios [(a/d); where “d” diameter of the pipe] ranging between 0.1 and 0.4 were considered. Irregular-shaped cracks were modeled by keeping the crack depth (a) constant. For a regular profile semi-elliptic crack [(a/c) = 0.6], higher SIF was observed at crack middle region [(S/S0) = 0]. In contrast, higher SIF was observed at crack surface region [(S/S0) = ± 1] for semi-circular crack. Asymmetric spreading of SIF was noticed because of added effect of mode-II and mode-III fracture. In addition, the leading mode of fracture at short crack depth [(a/d) ≤ 0.1] was mode-II (in-plane shear) and mode-III (out of plane shear) fractures whereas at deep crack depths [(a/d) ≥ 0.4], the leading mode of fracture was mode-I fracture which is crack opening mode. Marginal influence of crack aspect ratio was noticed at crack surface region [(S/S0) = ± 1] and the effect is significant at crack middle region [(S/S0) = 0]. For a constant crack depth, higher SIF was obtained at the middle region [(S/S0) = 0] compared to regular profile crack. When comparing the observations of regular semi-elliptic crack, higher influence of mode-II and mode-III fracture was observed for irregular-shaped semi-circular crack which may leads to additional crack growth. Thus, approximating the standard crack geometry may over/under predict the SIF values. It is also inferred that the presence of adjacent cracks significantly affects the SIF of middle crack due to interaction effect. The interaction behavior was observed to be more pronounced when multiple cracks are located at closer interval. The interaction effect reduces gradually when the distance between crack centers increases. The improved SIF solutions obtained with consideration of additional fracture modes will be useful during the residual life determination.

A numerical algorithm for multiple cracks propagation in concrete structure

AbstractMultiple cracks are extremely common in aged or partially damaged concrete structures. These cracks have a decisive influence on the safety of structures. In this study, a novel multiple‐crack‐length‐controlled method is presented for analyzing the cracking behavior of multiple cracks in concrete structures. This method introduces topological variables into the finite element calculation to identify the correct crack propagation pattern of multiple‐crack problems. The values of topological variables that are used to active and inactive cracks are continually updated by the maximum stress criterion during calculation. The mathematical formulation of the proposed method is constructed in the incremental form. Thus, it is easy to be implemented in the nonlinear problems and capable of considering the influences of stress history on the plastic behaviors. Furthermore, several numerical examples with different cracking modes are studied. The results show that the proposed method provides a robust, effective, and fast way to solve the multiple‐crack problems in concrete.

Greener engineered cementitious composite (ECC) – The use of pozzolanic fillers and unoiled PVA fibers

Engineered cementitious composites (ECC) with ductile strain-hardening and fine multiple cracking behaviors have been commonly accepted as an advanced and resilient alternative to conventional mortar and concrete. However, the conventional ECC has three issues: (1) uneconomical due to the high cement content and expensive polyvinyl alcohol (PVA) fibers; (2) environmentally unfriendly due to high cement content; (3) low workability due to very fine PVA fibers, all of which limit the practical application of ECC. Nevertheless, these problems of ECC can be resolved effectively by replacing cement partially with finer pozzolanic industrial solid wastes (ISWs) such as fly ash, silica fume and ground granulated blast furnace slag. These multi-sized substitutes of cement can improve the wet packing density and hence strength and workability of ECC, while the lower cement content can decrease the embodied carbon and contribute a greener ECC. Furthermore, an inexpensive type of unoiled PVA fiber is advocated herein for producing ECC. For verification, a test program was designed to investigate the compressive, tensile and flexural strength, as well as tensile strain and average crack width of ECC with two types of PVA fibers. It is evident that the use of unoiled fiber can preserve most of the technical performance indicators of ECC with conventional oiled fiber, which contributes to a more cost-effective and greener solution of cement composite.

Dynamic fracture development in a multiply cracked solid

We have been studying the collective, global behaviour of multiple cracks in solid materials and its connection with each individual, local crack motion. Here, by using a high-speed digital video camera, we examine fracture evolution in a two-dimensional, initially linear elastic rectangular polycarbonate specimen where sets of cracks are prepared and uniaxial tensile strain with a prescribed constant rate is externally applied. We trace local fracture development around every individual crack, and simultaneously, we obtain global stress-strain curves and physical properties like tensile strength of the specimens. By knowing the global characteristics, then, we concentrate our attention to more local, secondary and further fractures caused by the extending main or primary fracture and reversely its influence on the global nature of the multiply cracked specimens. For various different distribution patterns and values of density of initial cracks, we observe the dynamic evolution of the primary fracture and the fracture-induced waves, as well as the generation of the subsidiary fracture. We find that after a total split of the specimen into two by the propagation of the primary fracture, the secondary fracture may be initiated and propagated at a distance from the primary fracture. That is, fracture propagation may jump in multiply cracked solids even without additional external loading. Moreover, in our observations, the secondary fracture moves into the direction opposite to the primary one, and is once arrested and then resumes its propagation with some delay. The secondary and further fractures, or a cluster of fractures, are obviously not owing to additional external energy, but by dynamic waves produced upon enlargement of the primary fracture. The development of the secondary and further fractures in a specimen may be concealed in the global stress-strain relation but may rather appear in a global pattern of dynamic wave radiation such as doublet or a cluster of ruptures in the solid Earth, namely, earthquakes.

Effect of morphological parameters of natural sand on mechanical properties of engineered cementitious composites

Engineered Cementitious Composite (ECC) is an advanced construction material exhibiting tensile strain-hardening and multiple-cracking behavior. The tensile performance of ECC is known to be sensitive to the amount and size of sand used. Most studies on ECC have been performed with fine silica sand, while limited studies used coarser sand. However, the influence of morphological parameters of sand on the mechanical performance of ECC has not yet been explored, which is the focus of the present study. The morphological parameters (including particle roundness and sphericity) of sand were objectively determined using image analysis and computer algorithms, while the ECC compressive and tensile properties were experimentally measured. It was concluded that decreasing sand particle roundness and sphericity led to enhancements in compressive strength, ultimate tensile strength and tensile strain capacity of ECC. Gabbro sand, with the lowest roundness and sphericity among the four natural sand types studied, resulted in the best ECC mechanical properties. This natural sand can serve as an effective alternative to the typical manufactured fine silica sand used in ECC with reduced material cost and environmental impact. The findings of this study support the broader utilization of local natural sand in ECC in practical applications.