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The Brittle Failure of Different Materials

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A special issue of Materials (ISSN 1996-1944).

Deadline for manuscript submissions: closed (30 June 2017) | Viewed by 59536

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Prof. Dr. Filippo Berto

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Department of Chemical Engineering, Materials and Environment, Sapienza University of Rome, 00184 Rome, Italy
Interests: fatigue and fracture behavior of materials; mechanical characterization; structural integrity of conventional and innovative materials
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Brittle or quasi-brittle fracture loading is a wide field of research, which involves many researchers devoted to investigate different aspects of the mechanics and physics of fracture. Materials usually treated include metal alloys, polymers, composites, rocks, ceramics. Brittle failure is not a phenomenon limited only to static loadings. It may also be related to the fatigue and failure under repeated loading cycles (mechanical or thermal). The material damage process is usually very complex because it involves the combined effects of loading, size and geometry, temperature and environment. The understanding of the phenomena tied to the dissipation of energy in various forms and the identification of microscopic properties and their interactions with macroscopic variables are the actual challenging topics. The Fracture Mechanics science emphasises material characterisation techniques and translation of specimen data to design.

We invite authors to submit original research and review articles that seek to define possible criteria against brittle and quasi-brittle failure under mixed mode loading, to present or discuss new sets of experimental data in combination with fracture assessment. Among the areas to be emphasized are: Case histories; material selection and structure design; sample calculations of practical design problems; material characterisation procedures; fatigue crack growth and corrosion; non-destructive testing and inspection; structural failure and ageing; failure prevention methodologies; and maintenance and repair.

Potential topics include, but are not limited to:

Continuum mechanics
Crack propagation under mixed mode loading
Criteria for fatigue and fracture assessment
Micromechanics
Nanomechanics
Energy absorption and dissipation
Local approaches based on strain energy density
Local approaches based on stress analysis
Scale effect
Singular stress field
Interface behavior of small and large bodies
Three-dimensional effects.

Prof. Filippo Berto
Guest Editor

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Keywords

Brittle fracture
Cracks
Strain energy density
Energy release rate

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Published Papers (9 papers)

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Research

6115 KiB

Open AccessArticle

Anisotropic Failure Strength of Shale with Increasing Confinement: Behaviors, Factors and Mechanism

by Cheng Cheng, Xiao Li and Haitao Qian

Materials 2017, 10(11), 1310; https://doi.org/10.3390/ma10111310 - 15 Nov 2017

Cited by 12 | Viewed by 4851

Abstract

Some studies reported that the anisotropic failure strength of shale will be weakened by increasing confinement. In this paper, it is found that there are various types of anisotropic strength behaviors. Four types of anisotropic strength ratio (

S A_{1}

) behaviors [...] Read more.

S A_{1}

) behaviors and three types of anisotropic strength difference (

S A_{2}

) behaviors have been classified based on laboratory experiments on nine groups of different shale samples. The cohesion

c_{w}

and friction angle

ϕ_{w}

of the weak planes are proven to be two dominant factors according to a series of bonded-particle discrete element modelling analyses. It is observed that shale is more prone to a slight increase of

S A_{1}

and significant increase of

S A_{2}

with increasing confinement for higher cohesion

c_{w}

and lower to medium friction angle

ϕ_{w}

. This study also investigated the mechanism of the anisotropic strength behaviors with increasing confinement. Owing to different contributions of

c_{w}

and

ϕ_{w}

under different confinements, different combinations of

c_{w}

and

ϕ_{w}

may have various types of influences on the minimum failure strength with the increasing confinement; therefore, different types of anisotropic behaviors occur for different shale specimens as the confinement increases. These findings are very important to understand the stability of wellbore and underground tunneling in the shale rock mass, and should be helpful for further studies on hydraulic fracture propagations in the shale reservoir. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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4916 KiB

Open AccessArticle

Compression Fracture of CFRP Laminates Containing Stress Intensifications

by Christian Leopold, Martin Schütt, Wilfried V. Liebig, Timo Philipkowski, Jonas Kürten, Karl Schulte and Bodo Fiedler

Materials 2017, 10(9), 1039; https://doi.org/10.3390/ma10091039 - 5 Sep 2017

Cited by 17 | Viewed by 8302

Abstract

For brittle fracture behaviour of carbon fibre reinforced plastics (CFRP) under compression, several approaches exist, which describe different mechanisms during failure, especially at stress intensifications. The failure process is not only initiated by the buckling fibres, but a shear driven fibre compressive failure beneficiaries or initiates the formation of fibres into a kink-band. Starting from this kink-band further damage can be detected, which leads to the final failure. The subject of this work is an experimental investigation on the influence of ply thickness and stacking sequence in quasi-isotropic CFRP laminates containing stress intensifications under compression loading. Different effects that influence the compression failure and the role the stacking sequence has on damage development and the resulting compressive strength are identified and discussed. The influence of stress intensifications is investigated in detail at a hole in open hole compression (OHC) tests. A proposed interrupted test approach allows identifying the mechanisms of damage initiation and propagation from the free edge of the hole by causing a distinct damage state and examine it at a precise instant of time during fracture process. Compression after impact (CAI) tests are executed in order to compare the OHC results to a different type of stress intensifications. Unnotched compression tests are carried out for comparison as a reference. With this approach, a more detailed description of the failure mechanisms during the sudden compression failure of CFRP is achieved. By microscopic examination of single plies from various specimens, the different effects that influence the compression failure are identified. First damage of fibres occurs always in 0°-ply. Fibre shear failure leads to local microbuckling and the formation and growth of a kink-band as final failure mechanisms. The formation of a kink-band and finally steady state kinking is shifted to higher compressive strains with decreasing ply thickness. Final failure mode in laminates with stress intensification depends on ply thickness. In thick or inner plies, damage initiates as shear failure and fibre buckling into the drilled hole. The kink-band orientation angle is changing with increasing strain. In outer or thin plies shear failure of single fibres is observed as first damage and the kink-band orientation angle is constant until final failure. Decreasing ply thickness increases the unnotched compressive strength. When stress intensifications are present, the position of the 0°-layer is critical for stability under compression and is thus more important than the ply thickness. Central 0°-layers show best results for OHC and CAI strength due to higher bending stiffness and better supporting effect of the adjacent layers. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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8673 KiB

Open AccessArticle

Grafting Modification of the Reactive Core-Shell Particles to Enhance the Toughening Ability of Polylactide

by Zhaokun Li, Shixin Song, Xuanchen Zhao, Xue Lv and Shulin Sun

Materials 2017, 10(8), 957; https://doi.org/10.3390/ma10080957 - 16 Aug 2017

Cited by 12 | Viewed by 5278

Abstract

In order to overcome the brittleness of polylactide (PLA), reactive core-shell particles (RCS) with polybutadiene as core and methyl methacrylate-co-styrene-co-glycidyl methacrylate as shell were prepared to toughen PLA. Tert-dodecyl mercaptan (TDDM) was used as chain transfer agent to modify the grafting properties (such as grafting degree, shell thickness, internal and external grafting) of the core-shell particles. The introduction of TDDM decreased the grafting degree, shell thickness and the T_g of the core phase. When the content of TDDM was lower than 1.15%, the RCS particles dispersed in the PLA matrix uniformly—otherwise, agglomeration took place. The addition of RCS particles induced a higher cold crystallization temperature and a lower melting temperature of PLA which indicated the decreased crystallization ability of PLA. Dynamic mechanical analysis (DMA) results proved the good miscibility between PLA and the RCS particles and the increase of TDDM in RCS induced higher storage modulus of PLA/RCS blends. Suitable TDDM addition improved the toughening ability of RCS particles for PLA. In the present research, PLA/RCS-T4 (RCS-T4: the reactive core-shell particles with 0.76 wt % TDDM addition) blends displayed much better impact strength than other blends due to the easier cavitation/debonding ability and good dispersion morphology of the RCS-T4 particles. When the RCS-T4 content was 25 wt %, the impact strength of PLA/RCS-T4 blend reached 768 J/m, which was more than 25 times that of the pure PLA. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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26171 KiB

Open AccessArticle

Investigation of the Quasi-Brittle Failure of Alashan Granite Viewed from Laboratory Experiments and Grain-Based Discrete Element Modeling

by Jian Zhou, Luqing Zhang, Duoxing Yang, Anika Braun and Zhenhua Han

Materials 2017, 10(7), 835; https://doi.org/10.3390/ma10070835 - 21 Jul 2017

Cited by 49 | Viewed by 6647

Abstract

Granite is a typical crystalline material, often used as a building material, but also a candidate host rock for the repository of high-level radioactive waste. The petrographic texture—including mineral constituents, grain shape, size, and distribution—controls the fracture initiation, propagation, and coalescence within granitic rocks. In this paper, experimental laboratory tests and numerical simulations of a grain-based approach in two-dimensional Particle Flow Code (PFC2D) were conducted on the mechanical strength and failure behavior of Alashan granite, in which the grain-like structure of granitic rock was considered. The microparameters for simulating Alashan granite were calibrated based on real laboratory strength values and strain-stress curves. The unconfined uniaxial compressive test and Brazilian indirect tensile test were performed using a grain-based approach to examine and discuss the influence of mineral grain size and distribution on the strength and patterns of microcracks in granitic rocks. The results show it is possible to reproduce the uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS) of Alashan granite using the grain-based approach in PFC2D, and the average mineral size has a positive relationship with the UCS and UTS. During the modeling, most of the generated microcracks were tensile cracks. Moreover, the ratio of the different types of generated microcracks is related to the average grain size. When the average grain size in numerical models is increased, the ratio of the number of intragrain tensile cracks to the number of intergrain tensile cracks increases, and the UCS of rock samples also increases with this ratio. However, the variation in grain size distribution does not have a significant influence on the likelihood of generated microcracks. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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4733 KiB

Open AccessArticle

Effects of the Crack Tip Constraint on the Fracture Assessment of an Al 5083-O Weldment for Low Temperature Applications

by Dong Hyun Moon, Jeong Yeol Park and Myung Hyun Kim

Materials 2017, 10(7), 815; https://doi.org/10.3390/ma10070815 - 18 Jul 2017

Cited by 14 | Viewed by 7233

Abstract

The constraint effect is the key issue in structural integrity assessments based on two parameter fracture mechanics (TPFM) to make a precise prediction of the load-bearing capacity of cracked structural components. In this study, a constraint-based failure assessment diagram (FAD) was used to assess the fracture behavior of an Al 5083-O weldment with various flaws at cryogenic temperature. The results were compared with those of BS 7910 Option 1 FAD, in terms of the maximum allowable stress. A series of fracture toughness tests were conducted with compact tension (CT) specimens at room and cryogenic temperatures. The

Q

parameter for the Al 5083-O weldment was evaluated to quantify the constraint level, which is the difference between the actual stress, and the Hutchinson-Rice-Rosengren (HRR) stress field near the crack tip. Nonlinear 3D finite element analysis was carried out to calculate the

Q

parameter at cryogenic temperature. Based on the experimental and numerical results, the influence of the constraint level correction on the allowable applied stress was investigated using a FAD methodology. The results showed that the constraint-based FAD procedure is essential to avoid an overly conservative allowable stress prediction in an Al 5083-O weldment with flaws. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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6684 KiB

Open AccessArticle

Failure Behavior of Granite Affected by Confinement and Water Pressure and Its Influence on the Seepage Behavior by Laboratory Experiments

by Cheng Cheng, Xiao Li, Shouding Li and Bo Zheng

Materials 2017, 10(7), 798; https://doi.org/10.3390/ma10070798 - 14 Jul 2017

Cited by 15 | Viewed by 5130

Abstract

Failure behavior of granite material is paramount for host rock stability of geological repositories for high-level waste (HLW) disposal. Failure behavior also affects the seepage behavior related to transportation of radionuclide. Few of the published studies gave a consistent analysis on how confinement and water pressure affect the failure behavior, which in turn influences the seepage behavior of the rock during the damage process. Based on a series of laboratory experiments on NRG01 granite samples cored from Alxa area, a candidate area for China’s HLW disposal, this paper presents some detailed observations and analyses for a better understanding on the failure mechanism and seepage behavior of the samples under different confinements and water pressure. The main findings of this study are as follows: (1) Strength reduction properties were found for the granite under water pressure. Besides, the complete axial stress–strain curves show more obvious yielding process in the pre-peak region and a more gradual stress drop in the post-peak region; (2) Shear fracturing pattern is more likely to form in the granite samples with the effect of water pressure, even under much lower confinements, than the predictions from the conventional triaxial compressive results; (3) Four stages of inflow rate curves are divided and the seepage behaviors are found to depend on the failure behavior affected by the confinement and water pressure. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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13873 KiB

Open AccessArticle

Numerical Studies on the Failure Process of Heterogeneous Brittle Rocks or Rock-Like Materials under Uniaxial Compression

by Songfeng Guo, Shengwen Qi, Yu Zou and Bowen Zheng

Materials 2017, 10(4), 378; https://doi.org/10.3390/ma10040378 - 1 Apr 2017

Cited by 36 | Viewed by 6100

Abstract

In rocks or rock-like materials, the constituents, e.g. quartz, calcite and biotite, as well as the microdefects have considerably different mechanical properties that make such materials heterogeneous at different degrees. The failure of materials subjected to external loads is a cracking process accompanied with stress redistribution due to material heterogeneity. However, the latter cannot be observed from the experiments in laboratory directly. In this study, the cracking and stress features during uniaxial compression process are numerically studied based on a presented approach. A plastic strain dependent strength model is implemented into the continuous numerical tool—Fast Lagrangian Analysis of Continua in three Dimensions (FLAC^3D), and the Gaussian statistical function is adopted to depict the heterogeneity of mechanical parameters including elastic modulus, friction angle, cohesion and tensile strength. The mean parameter μ and the coefficient of variance (h_cv, the ratio of mean parameter to standard deviation) in the function are used to define the mean value and heterogeneity degree of the parameters, respectively. The results show that this numerical approach can perfectly capture the general features of brittle materials including fracturing process, AE events as well as stress-strain curves. Furthermore, the local stress disturbance is analyzed and the crack initiation stress threshold is identified based on the AE events process and stress-strain curves. It is shown that the stress concentration always appears in the undamaged elements near the boundary of damaged sites. The peak stress and crack initiation stress are both heterogeneity dependent, i.e., a linear relation exists between the two stress thresholds and h_cv. The range of h_cv is suggested as 0.12 to 0.21 for most rocks. The stress concentration degree is represented by a stress concentration factor and found also heterogeneity dominant. Finally, it is found that there exists a consistent tendency between the local stress difference and the AE events process. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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14057 KiB

Open AccessArticle

Investigation of Quasi-Static Indentation Response of Inkjet Printed Sandwich Structures under Various Indenter Geometries

by Vishwesh Dikshit, Arun Prasanth Nagalingam, Yee Ling Yap, Swee Leong Sing, Wai Yee Yeong and Jun Wei

Materials 2017, 10(3), 290; https://doi.org/10.3390/ma10030290 - 14 Mar 2017

Cited by 43 | Viewed by 7978

Abstract

The objective of this investigation was to determine the quasi-static indentation response and failure mode in three-dimensional (3D) printed trapezoidal core structures, and to characterize the energy absorbed by the structures. In this work, the trapezoidal sandwich structure was designed in the following two ways. Firstly, the trapezoidal core along with its facesheet was 3D printed as a single element comprising a single material for both core and facesheet (type A); Secondly, the trapezoidal core along with facesheet was 3D printed, but with variation in facesheet materials (type B). Quasi-static indentation was carried out using three different indenters, namely standard hemispherical, conical, and flat indenters. Acoustic emission (AE) technique was used to capture brittle cracking in the specimens during indentation. The major failure modes were found to be brittle failure and quasi-brittle fractures. The measured indentation energy was at a maximum when using a conical indenter at 9.40 J and 9.66 J and was at a minimum when using a hemispherical indenter at 6.87 J and 8.82 J for type A and type B series specimens respectively. The observed maximum indenter displacements at failure were the effect of material variations and composite configurations in the facesheet. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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1624 KiB

Open AccessArticle

A Size and Boundary Effects Model for Quasi-Brittle Fracture

by Xiaofeng Gao, Georg Koval and Cyrille Chazallon

Materials 2016, 9(12), 1030; https://doi.org/10.3390/ma9121030 - 21 Dec 2016

Cited by 14 | Viewed by 5741

Abstract

The fracture behaviors of quasi-brittle materials are commonly specimen size (size effect) and crack size (boundary effect) dependent. In this study, a new failure model is developed for characterizing the size and boundary effects. The derivative of the energy release rate is firstly introduced to predict the nominal strength dominated by the strength mechanism. Combined with the energy criterion for the energy mechanism, an asymptotic model is developed to capture the effect of any crack size on the nominal strength, and its expression for geometrically similar specimens is also established, which is able to characterize the size effect. Detailed comparisons of the proposed model with the size effect law and the boundary effect model are performed, respectively. The nominal strength predictions based on the proposed model are validated with the experimental results of cracked three-point bending beam specimens made of concrete, of limestone and of hardened cement paste and compared with the model predictions given by the size effect law and the boundary effect model. Full article

(This article belongs to the Special Issue The Brittle Failure of Different Materials)

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