Particle Discontinuous Deformation Analysis of Static and Dynamic Crack Propagation in Brittle Material

Crack propagation in brittle material is not only crucial for structural safety evaluation,but also has a wideranging impact on material design,damage assessment,resource extraction,and scientific research.A thorough investigation into the behavior of crack propagation contributes to a better understanding and control of the properties of brittle materials,thereby enhancing the reliability and safety of both materials and structures.As an implicit discrete elementmethod,the Discontinuous Deformation Analysis(DDA)has gained significant attention for its developments and applications in recent years.Among these developments,the particle DDA equipped with the bonded particle model is a powerful tool for predicting the whole process of material from continuity to failure.The primary objective of this research is to develop and utilize the particle DDAtomodel and understand the complex behavior of cracks in brittle materials under both static and dynamic loadings.The particle DDA is applied to several classical crack propagation problems,including the crack branching,compact tensile test,Kalthoff impact experiment,and tensile test of a rectangular plate with a hole.The evolutions of cracks under various stress or geometrical conditions are carefully investigated.The simulated results are compared with the experiments and other numerical results.It is found that the crack propagation patterns,including crack branching and the formation of secondary cracks,can be well reproduced.The results show that the particle DDA is a qualified method for crack propagation problems,providing valuable insights into the fracture mechanism of brittle materials.

APPLICATION OF DIAMOND TWIST DRILL FOR DRILLING HARD-BRITTLE MATERIALS

A new conception, which combines the advantages of both twist drill and diamond grit, is proposed to develop a new tool for drilling hard brittle materials. The manufacturing process of the drill is introduced, and drilling experiments are carried out by using of the drill developed. As a result, not only it can drill holes with a high efficiency, but also a good quality of hole inlet and outlet can be obtained for such materials as glasses, marble, granite, Al 2O 3, etc. The prospect in practical application of the technique developed is also discussed.

Research on Ultrasonic Vibration Grinding of the Hard and Brittle Materials

It is well known that grinding techniques are main methods to machine hard and brittle materials such as engineering ceramics. But the conventional grinding has many shortcomings such as poorer surface finish, quicker wear and tear of grinding tools, lower efficiency and so on. Ultrasonic vibration grinding （UVG） which combines ultrasonic machining and grinding emerged as a developing and promising technique in recent years. In this paper, experimental studies on UVG were conducted on several kinds of hard and brittle material by altering processing parameters such as vibration frequency and its amplitude, diamond abrasive grit size, cutting depth, feeding speed and rotary speed of tools. The experimental results show that alteration in any of above mentioned parameters will bring effects on the processed surface finish of these materials. Of them, the diamond abrasive grit size has the greatest. Moreover, conventional grinding experiments were also carried out on these materials. By comparison, it was found that the UVG is superior to the conventional method in terms of the ground surface quality, the working efficiency and the wear rate of tools.

QUASI-MICROMECHANICAL CONSTITUTIVE THEORY FOR BRITTLE DAMAGED MATERIALS UNDER TENSION

One of fundamental but difficult problems in damage mechanics isthe formulation of the ef- fective constitutive relation ofmicrocrack-weakened brittle o quasi-brittle materials under complexloading, especially when microcrack interaction is taken intoaccount. The combination of phenomenological and mi- cromechanicaldamage mechanics is a promising approach to construction andapplicable damage model with a firm physical foundation.

MICROMECHANICAL MODELLING OF STRAIN SOFTENING IN MICROCRACK-WEAKENED QUASI-BRITTLE MATERIALS

By using the concept of domain of microcrack growth(DMG),the micromechanisms of damage in quasi-brittle materials subjected to triaxial either tensile or compressive loading are investigated and the complete strew-strain relation including four stages is obtained from micromechanical analysis.The regime of pre-peak nonlinear hardening corresponds to the distributed damage,i.e.the stable propagation of microcracks.After the attainment of the ultimate strength of load-bearing capacity, some microcracks experience the second unstable growth and the distributed damage is transmitted to the localization of damage.These analyses improve our understanding of the hardening and softening behaviors of quasi-brittle materials.

Experimental study on 3D internal penny-shaped crack propagation in brittle materials under uniaxial compression

Fractures are widely present in geomaterials of civil engineering and deep underground engineering.Given that geomaterials are usually brittle,the fractures can significantly affect the evaluation of underground engineering construction safety and the early warning of rock failure.However,the crack initiation and propagation in brittle materials under composite loading remain unknown so far.In this study,a three-dimensional internal laser-engraved cracking technique was applied to produce internal cracks without causing damage to the surfaces.The uniaxial compression tests were performed on a brittle material with internal cracks to investigate the propagation of these internal cracks at different dip angles under compression and shear.The test results show that the wing crack propagation mainly occurs in the specimen with an inclined internal crack,which is a mixed-ModeⅠ–Ⅱ–Ⅲfracture;in contrast,ModeⅠfracture is present in the specimen with a vertical internal crack.The fractography characteristics of ModeⅢfracture display a lance-like pattern.The fracture mechanism in the brittle material under compression is that the internal wing cracks propagate to the ends of the whole sample and cause the final failure.The initial deflection angle of the wing crack is determined by the participation ratio of stress intensity factors KII to KI at the tip of the internal crack.

3-D NUMERICAL SIMULATION OF FRACTURE PROCESSES IN HETEROGENEOUS BRITTLE MATERIALS

By using the lattice model combined with finite element methods andstatistical techniques, a numerical approach is developed to establish mechanical models ofthree-dimensional heterogeneous brittle materials. A special numerical code is introduced, in whicha lattice model and statistical approaches are used to simulate the initial heterogeneity ofmaterial properties. The size of displacement-load step is adap-tively determined so that only fewelements would fail in each load step. When the tensile principal strain in an element exceeds theultimate strain of this element, the element is considered broken and its Young's modulus is set tobe very low. Some important behaviors of heterogeneous brittle materials are indicated using thiscode. Load-displacement curves and figures of three-dimensional fracture patterns are alsonumerically obtained, which are similar to those observed in laboratory tests.

A Novel Method for Measuring Flexure Strength of Brittle Materials

A novel test method to measure flexure strength for silicon slices and other brittle materials was suggested by using simple supported circular slice samples and steel ball impact loading. The maximal tensile stress expression under concentrating impact load for center of simple supported circular plate was derived based on kinetic energy theorem and small deflection theory of thin plate. The instrument used in the method is simple, the test results are accurate, and the test does not need large scale loading apparatus. An experimental correct method was presented for test results deviated from small deflection limit when the slices were thinner or the strength of slice was higher.

The Stress Analysis for Notched Specimen of Brittle Material under Plane Dead Loading

The stress distribution of notched specimen of brittle material under a plane pressure was studied using a photoelastic meth- od,When elastic deformation appeared inside the specimen,the force transferred by dowel part was triangular transverse force and frictional force on the upper surface of the sample.The quantity of the frictional force was about 31 percent of transverse force.The stress inside the sample was linear along the central cross section of the sample and there was maximum tensile stress σ_y at the tip of the notch.Basing on shearing stress deviation method,the tensile stress σ_y,σ_x and shearing stress τ_(xy) at the cen- tral sections and four adjacent cross sections were calculated.The result pointed out that σ_x and τ_(xy) were smaller than σ_y.There- fore,σ_y was the main factor for crack formation and propagation.

Methods for Detection of Subsurface Damage:A Review

Subsurface damage is easily induced in machining of hard and brittle materials because of their particular mechani?cal and physical properties. It is detrimental to the strength,performance and lifetime of a machined part. To manu?facture a high quality part,it is necessary to detect and remove the machining induced subsurface damage by the subsequent processes. However,subsurface damage is often covered with a smearing layer generated in a machining process,it is rather di cult to directly observe and detect by optical microscopy. An e cient detection of subsur?face damage directly leads to quality improvement and time saving for machining of hard and brittle materials. This paper presents a review of the methods for detection of subsurface damage,both destructive and non?destructive. Although more reliable,destructive methods are typically time?consuming and confined to local damage infor?mation. Non?destructive methods usually su er from uncertainty factors,but may provide global information on subsurface damage distribution. These methods are promising because they can provide a capacity of rapid scan and detection of subsurface damage in spatial distribution.

A NEW DAMAGE MODEL FOR MICROCRACK-WEAKENED BRITTLE SOLIDS

In the present paper, a micromechanically based damage model for microcrack-weakened solids is developed. The concept of the domain of microcrack growth (DMG) is defined and used to describe the damage state and the anisotropic properties of brittle materials. After choosing an appropriate fracture criterion of microcrack, we obtain the analytical expression of DMG under a monotonically in- creasing proportional plane stress. Under a complex loading path, the evolution equation of DMG and the overall effective compliance tensor of damaged materials are given.

Research on Cutting Force of Ultrasonic Diamond Wire Saw

Based on impulse and vibration machining theories,a mathematical model of cutting force for the electroplated diamond ultrasonic wire saw was established using superposition principle.The differences between the cutting forces with and without ultrasonic effect were analyzed theoretically and experimentally.The results indicate that the cutting force of diamond wire increases along with the spindle speed decrease and the lateral pressure increase.The force in ultrasonic vibration cutting is about 20% to 30% less than that in conventional cutting.Also,the cutting trajectory of single diamond grit in sawing process is simulated,and the reason that the ultrasonic vibration can reduce the cutting force is explained further.

High Efficiency ELID Grinding of Garnet Ferrite

Hard and brittle materials such as ferrite, optical glass and ceramics have been widely used in many fields because of their good characteristics and still gain more attentions. However, it is difficult to machine and get good surface quality. Some parts made of these materials have large machining allowances and need to be produced with large batch, but the machining efficiency is very low with usual grinding method. So it is of great importance to research the high efficiency grinding technology of hard and brittle materials. Electrolytic in-process dressing (ELID) grinding is a new grinding technology which has been adopted to the ultra-precision machining of hard and brittle materials. With the function of in-process dressing of metal bond diamond and CBN wheel, ELID grinding has the ability to keep the sharpness of the wheel surface and is widely used in fine abrasive grinding, but it also has the potentialities to high efficiency grinding. In this paper, the mechanism of ELID grinding and its grinding performance are analyzed, then the cast iron bond diamond wheels and ELID grinding device are used on a surface grinder to research the feasibility of ELID grinding to high efficiency grinding. To make comparison, the garnet ferrite (YAG) work piece has been machined in plunge grinding both by ELID grinding and by the resin bond diamond wheel. The grinding force and surface quality are tested and analyzed. It has been found that the grinding force of the cast iron bond diamond wheel with ELID grinding is apparently smaller than that of the resin bond diamond wheel. Under the same conditions, it is about 2/5～3/5 as the force using the resin bond diamond wheel. So with the same grinder and machining conditions, ELID grinding can machine work piece with greater depth of cut. Because of the smaller grinding force, it is also beneficial to avoid the edge collapse of the work piece and keep the integrity of the grinding surface. This experiment shows that the grinding efficiency can be highly improved and the surface quality be ensured by applying ELID grinding technology and adopting large grinding depth. The results indicate that the ELID grinding technology can be effectively used in the high efficiency machining of garnet ferrite and other hard and brittle materials.

Femtosecond laser shockwave peening ablation in liquids for hierarchical micro/nanostructuring of brittle silicon and its biological application

This paper presents a new technique,termed femtosecond laser shock peening ablation in liquids(fs-LSPAL),which can realize simultaneous crack micro/nanomanufacturing and hierarchical micro/nanolaser ablation,giving rise to the formation of diverse multiscale hierarchical structures,such as macroporous ratcheted structures and enéchelon microfringes decorated with parabolic nanoripples.Through analysis of surface morphologies,many phenomena have been confirmed to take place during fs-LSPAL,including enéchelon cracks,nanostriation,ripple densification,crack branching,and selective formation of high spatial frequency laser-induced periodic surface structures of 100–200 nm in period.At a high laser power of 700 mW,fs-LSPAL at scanning speeds of 0.2 mm s^-1 and 1 mm s^-1 enables the generation of height-fluctuated and height-homogeneous hierarchical structures,respectively.The height-fluctuated structures can be used to induce‘colony’aggregates of embryonic EB3 stem cells.At 200 mW,fs-LSPAL at 1 mm s^-1 is capable of producing homogeneous tilt macroporous structures with cracked structures interleaved among them,which are the synergistic effects of bubble-induced light refraction/reflection ablation and cracks.As shown in this paper,the conventional laser ablation technique integrated with its self-driven unconventional cracking under extreme conditions expands the horizons of extreme manufacturing and offers more opportunities for complex surface structuring,which can potentially be used for biological applications.

A review on glass welding by ultra-short laser pulses

Glass welding by ultra-short pulsed(USP)lasers is a piece of technology that offers high strength joints with hermetic sealing.The joints are typically formed in glass that is transparent to the laser by exploiting nonlinear absorption effects that occur under extreme conditions.Though the temperature reached during the process is on the order of a few 1000°C,the heat affected zone(HAZ)is confined to only tens of micrometers.It is this controlled confinement of the HAZ during the joining process that makes this technology so appealing to a multitude of applications because it allows the foregoing of a subsequent tempering step that is typically essential in other glass joining techniques,thus making it possible to effectively join highly heat sensitive components.In this work,we give an overview on the process,development and applications of glass welding by USP lasers.

Femtosecond laser-induced periodic surface structures on hard and brittle materials

Hard and brittle materials have high hardness,excellent optical stability,chemical stability,and high thermal stability.Hence,they have huge application potential in various fields,such as optical components,substrate materials,and quantum information,especially under harsh conditions,such as high temperatures and high pressures.Femtosecond laser direct writing technology has greatly promoted the development of femtosecond laser-induced periodic surface structure(Fs-LIPSS or LIPSS by a femtosecond laser)applications of hard and brittle materials due to its high precision,controllability,and three-dimensional processing ability.Thus far,LIPSSs have been widely used in material surface treatment,optoelectronic devices,and micromechanics.However,a consensus has not been reached regarding the formation mechanism of LIPSSs on hard and brittle materials.In this paper,three widely accepted LIPSS formation mechanisms are introduced,and the characteristics and applications of LIPSSs on diamonds,silicon,silicon carbide,and fused silica surfaces in recent years are summarized.In addition,the application prospects and challenges of LIPSSs on hard and brittle materials by a femtosecond laser are discussed.

Development of Diamond-like Carbon Fibre Wheel

A unique diamond-like carbon (DLC) grinding wheel was developed, in which the DLC fibres were made by rolling Al sheets coated with DLC films and aligned no rmally to the grinding wheel surface by laminating Al sheets together with DLC fibres. In this paper, the formation process of DLC fibres and the fabrication process of a DLC fibre wheel were investigated. Many grinding experiments were also carried out on a precision NC plane milling machine using a newly developed DLC wheel. Grinding of specimens of silicon wafers, optical glasses, quartz, granites and hardened die steel SKD11 demonstrated the capabilities of nanometer surface finish. A smooth surface with a roughness value of Ra2.5 nm (Ry26 nm) was achieved.

Using High-Performance Computing for the Modeling of High-Speed Dynamics

Since computers and software have spread into all fields of industry, extensive efforts are currently made in order to improve the safety by applying certain numerical solutions. For many engineering problems involving shock and impact, there is no single ideal numerical method that can reproduce the various regimes of a problem. An approach wherein different techniques may be applied within a single numerical analysis can provide the ＂best＂ solution in terms of accuracy and efficiency. This paper presents a set of numerical simulations of ballistic tests which analyze the effects of soda lime glass laminates. The goal is to find an appropriate solver technique for simulating brittle materials and thereby improve bullet-proof glass to meet current challenges. To have the correct material model available is not enough. In this work, the main solver technologies are compared to create a perfect simulation model for soda lime glass laminates. The calculation should match ballistic trials and be used as the basis for further studies. These numerical simulations are performed with the nonlinear dynamic analysis computer code ANSYS AUTODYN.

Thermal Stress in HFEF Hot Cell Windows Due to an In-Cell Metal Fire

This work investigates an accident during the pyrochemical extraction of Uranium and Plutonium from PWR spent fuel in an argon atmosphere hot cell. In the accident, the heavy metals (U and Pu) being extracted are accidently exposed to air from a leaky instrument penetration which goes through the cell walls. The extracted pin size pieces of U and Pu metal readily burn when exposed to air. Technicians perform the electrochemical extraction using manipulators through a 4 foot thick hot cell concrete wall which protects them from the radioactivity of the spent fuel. Four foot thick windows placed in the wall allow the technicians to visually control the manipulators. These windows would be exposed to the heat of the metal fire. This analysis determines if the thermal stress caused by the fire would crack the windows and if the heat would degrade the window seals allowing radioactivity to escape from the cell.

Fracturing damage process in dynamic split experiments of a brittle glass

In this study, the 3-dimensional discrete element method is firstly introduced to explain the fracturing damage process of the dynamic split experiment of a special brittle glass ZnS. The corresponding dynamic split experiment is also performed by using the split Hopkinson pressure bar. Then the numerical results correspond closely to those obtained by experiments, and the fracturing damage mode shows that the sample under high strain rate loading would crack along vertical diameter in the band region between two loading edges, which differs from the static damage mode. Furthermore, by comparing a group of contrast numerical tests, the numerical results prove that loading area upon the top side of samples would influence the fracture mode of dynamic split experiments, which indicates that the narrow loading plane is better.