Unstable Fracture Behavior of Rubber Toughened Poly(Styrene-co-Acrylonitrile)

A linear elastic fracture mechanics (LEFM) approach was used to study fracture characteristics of ABS materials. The effects of crack (ligament) length and rubber content on the microscopic deformations taking place at the front of crack tip and in the bulk of the specimens were investigated. The results of fractography studies showed that, in addition to rubber content, the microscopic deformations are influenced by crack length. For some materials this manifests itself as a change in macroscopic response. The ligament length dependent behavior was increased for the samples with higher rubber contents. The results also showed that, although the elastic behavior with unstable crack growth is the dominant micromechanism of deformation, stable crack propagation still occurred in some compositions. All the fracture parameters, including fracture toughness, fracture energy, plastic zone size, and crack tip opening, increased with rubber content. The changes in microscopic and, as a consequence, in the macroscopic deformation behavior of a given specimen with ligament length were attributed to changes in yield stress of the sample and maximum stress on the ligament.     

Dynamic caustics and its applications

For the study of elastodynamic problems of propagating cracks it is necessary to evaluate the dynamic stress intensity factor K^d_I which depends on the form of expressions for the stress components existing at the running crack tip at any instant of the propagation of the crack and the corresponding dynamic mechanical and optical properties of the material of the specimen under identical loading conditions. In this paper the distortion of the form of the corresponding reflected caustic from the lateral faces of a dynamically loaded transparent and optically inert specimen containing a transverse crack running under constant velocity was studied on the basis of complex potential elasticity theory and the influence of this form on the value of the dynamic stress intensity factor was given. The method was applied to the study of a propagating Mode I crack in a PMMA specimen under various propagation velocities and the corresponding dynamic stress intensity factor K^d_I evaluated. Also, crack propagation behaviour of notched composites in dynamic loading modes are reviewed and evaluated. A relatively large data base using metal-epoxy particulates, rubber-toughened poly(methyl methacrylate), and Sandwich plates are given. In all cases, a combination of high-speed photography and the optical method of dynamic caustics has been used. Results on the dynamic crack propagation mode, fracture toughness and crack propagation velocities of several rubber-modified composite models are presented. The composite models studied include specimens with one and/or two ‘complex’ two-stage inclusions, i.e. PMMA round inclusions surrounded by concentric rubber rings, one and/or press-fifting inclusions without rubber interface, all under dynamic loading. In all cases both qualitative and quantitative results were obtained. Also, results on crack propagation mode, crack propagation velocity, stress intensity factors and on the influence of the sandwich phases on crack propagation mode are presented.     

Atomistic scale fracture behaviours in hierarchically nanotwinned metals

In the present study, a series of large-scale molecular dynamics simulations have been performed to investigate the atomistic scale fracture behaviours along the boundaries of primary twins in Cu with hierarchically nanotwinned structures (HTS), and compare their fracture behaviours with those in monolithic twins. The results indicate that crack propagation along [1?1?2] on the twin plane in monolithic nanotwins is brittle cleavage and fracture, resulting in low crack resistance and fracture toughness. However, the crack resistance along the boundaries of primary twins in HTS is much higher, and a smaller spacing of secondary twins (λ ₂) leads to even higher fracture toughness. With large λ ₂, the crack growth is achieved by void nucleation, growth and coalescence. However, considerable plastic deformation and enhanced fracture toughness in HTS could be achieved by the crack blunting and by the extensive dislocation accommodation ahead of the crack tip when λ ₂ is small.     

Characterization of the fracture toughness of micro-sized tungsten single crystal notched specimens

Fracture experiments using micrometer-sized notched cantilevers were conducted to investigate the possibility of determining fracture mechanical parameters for the semi-brittle material tungsten. The experiments were also used to improve the understanding of semi-brittle fracture processes for which single crystalline tungsten serves as a model material. Due to the large plastic zone in relation to the micrometer sample size, linear elastic fracture mechanics is inapplicable and elastic-plastic fracture mechanics has to be applied. Conditional fracture toughness values J _Q were calculated from corrected force vs. displacement diagrams. Crack growth was accessible by direct observation of in-situ experiments as well as with the help of unloading compliances. As a further tool, fracture toughness can be determined via crack tip opening displacement. The micro samples behave more ductile and exhibit higher fracture toughness values compared to macro-sized single crystals and fail by stable crack propagation.     

Hot cracks in rubber: origin of the giant toughness of rubberlike materials

We study crack propagation in rubberlike materials and show that the nonuniform temperature distribution which occurs in the vicinity of the crack tip has a profound influence on the crack propagation, and may strongly enhance the crack propagation energy G(v) for high crack velocities v. At very low crack-tip velocities, the heat produced at the crack tip can diffuse away, but already at moderate crack-tip velocities a very large temperature increase occurs close to the crack tip resulting in a "hot-crack" propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt and may result in unstable crack motion, e.g., stick-slip motion or catastrophic failure.     

Crack motion in viscoelastic solids: The role of the flash temperature

We present a simple theory of crack propagation in viscoelastic solids. We calculate the energy per unit area, G(v), to propagate a crack, as a function of the crack tip velocity v. Our study includes the non-uniform temperature distribution (flash temperature) in the vicinity of the crack tip, which has a profound influence on G(v). At very low crack tip velocities, the heat produced at the crack tip can diffuse away, resulting in very small temperature increase: in this “low-speed” regime the flash temperature effect is unimportant. However, because of the low heat conductivity of rubber-like materials, already at moderate crack tip velocities a very large temperature increase (of order of 1000 K) can occur close to the crack tip. We show that this will drastically affect the viscoelastic energy dissipation close to the crack tip, resulting in a “hot-crack” propagation regime. The transition between the low-speed regime and the hot-crack regime is very abrupt, which may result in unstable crack motion, e.g. stick-slip motion or catastrophic failure, as observed in some experiments. In addition, the high crack tip temperature may result in significant thermal decomposition within the heated region, resulting in a liquid-like region in the vicinity of the crack tip. This may explain the change in surface morphology (from rough to smooth surfaces) which is observed as the crack tip velocity is increased above the instability threshold.     

Organic fiber reinforced plastics based on complex hybrid matrices including polysulfone and carbon nanotubes as modifiers of epoxy resins

Organic Fiber Reinforced Plastics (OFRP) based on aramid fibers are as a rule used in constructions working under extremal conditions. In view of this, the possibility of increasing the resistance of OFRP to destruction by modifying matrices with thermoplastic polymers and carbon nanotubes (CNTs) offers much promise. In this work, we present the results obtained in a study of the properties of OFRP based on Rusar fibers and epoxy matrices containing either CNTs or a thermoplastic (PSK-1 polysulfone) or both these components simultaneously. The data obtained substantiate the possibility of using epoxypolysulfone matrices for the preparation of wound composites. This modification noticeably increases crack and impact resistance of OFRP based on aramid fibers without decreasing the glass transition temperature, as when matrices are plasticized by rubber and active diluents. The strongest effect of polysulfone introduced into an epoxy matrix is observed at a large (20 wt %) content of PSK-1. The modification of epoxypolysulfone matrices by CNTs also increases the shear strength of OFRP and almost does not change the fracture toughness and compression strength. The introduction of CNTs into epoxy matrices is less effective and can increase crack growth resistance of OFRP by approximately 30% only at a large (1%) content of CNTs. Small CNT admixtures (0.3–0.6%) do not influence the fracture toughness. Possible mechanisms of the changes observed are considered.     

Why is nacre strong? II. Remaining mechanical weakness for cracks propagating along the sheets

In our previous paper (Eur. Phys. J. E 4, 121 (2001)) we proposed a coarse-grained elastic energy for nacre, or stratified structure of hard and soft layers found in certain seashells . We then analyzed a crack running perpendicular to the layers and suggested one possible reason for the enhanced toughness of this substance. In the present paper, we consider a crack running parallel to the layers. We propose a new term added to the previous elastic energy, which is associated with the bending of layers. We show that there are two regimes for the parallel-fracture solution of this elastic energy; near the fracture tip the deformation field is governed by a parabolic differential equation while the field away from the tip follows the usual elliptic equation. Analytical results show that the fracture tip is lenticular, as suggested in a paper on a smectic liquid crystal (P.G. de Gennes, Europhys. Lett. 13, 709 (1990)). On the contrary, away from the tip, the stress and deformation distribution recover the usual singular behaviors ( and 1/, respectively, where x is the distance from the tip). This indicates there is no enhancement in toughness in the case of parallel fracture. Received 16 November 2001     

In situ observation of interfacial debonding process induced by matrix crack propagation in two-dimensional unidirectional model composites

Interfacial debonding behavior is studied for unidirectional fiber reinforced composites from both experimental and analytical viewpoints. A new type of two-dimensional unidirectional model composite is prepared using 10 boron fibers and transparent epoxy resin with two levels of interfacial strength. In situ observation of the internal mesoscopic fracture process is carried out using the single edge notched specimen under static loading. The matrix crack propagation, the interfacial debonding growth and the interaction between them are directly observed in detail. As a result, the interfacial debonding is clearly accelerated in specimens with weakly bonded fibers in comparison with those with strongly bonded fibers. Secondary, three-dimensional finite element analysis is carried out in order to reproduce the interfacial debonding behavior. The experimentally observed relation between the mesoscopic fracture process and the applied load is given as the boundary condition. We successfully evaluate the mode II interfacial debonding toughness and the effect of interfacial frictional shear stress on the apparent mode II energy release rate separately by employing the present model composite in combination with the finite element analysis. The true mode II interfacial debonding toughness for weaker interface is about 0.4 times as high as that for a stronger interface. The effect of the interfacial frictional shear stress on the apparent mode II energy release rate for the weak interface is about 0.07 times as high as that for the strong interface. The interfacial frictional shear stress and the coefficient of friction for weak interface are calculated as 0.25 and 0.4 times as high as those for strong interface, respectively.     

Study on the Morphological and Mechanical Properties of Nylon 6/ABS/Nano-SiO2 Composites

The mechanical properties and morphology of the composites of nylon 6, acrylonitrile-butadiene-styrene (ABS) rubber, and nano-SiO₂ particles were examined as a function of the nano-SiO₂ content. A mixture with separation and encapsulation microstructures existed in the nylon 6/ABS/nano-SiO₂ at lower nano-SiO₂ content, and ABS and nano-SiO₂ improved the toughness synergistically, while obvious agglomeration appeared at higher nano-SiO₂ content and the impact strength decreased. Moreover, the addition of nano-SiO₂ particles also affected the dispersion of the rubber phase, resulting in the appearance of smaller rubber particles. The deformation and toughening mechanisms of the composites were also investigated; they resulted from rubber voiding, crack forking, and plastic deformation of the matrix.     

Fullerene–epoxy nanocomposites-enhanced mechanical properties at low nanofiller loading

In this study, we characterized the mechanical properties of fullerence (C₆₀) epoxy nanocomposites at various weight fractions of fullerene additives in the epoxy matrix. The mechanical properties measured were the Young’s modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material’s resistance to fatigue crack propagation. All of the above properties of the epoxy polymer were significantly enhanced by the fullerene additives at relatively low nanofiller loading fractions (~0.1 to 1% of the epoxy matrix weight). By contrast, other forms of nanoparticle fillers such as silica, alumina, and titania nanoparticles require up to an order of magnitude higher weight fraction to achieve comparable enhancement in properties.     

Effect of the shape of filler particles on the strength of a polymer composite

When metallic or polymeric powder fillers are introduced into poly(2,6-dimethyl-1,4-phenyleneoxide) (PDPO), the strength of the filled composites is found to decrease, while when glass or polymeric fiber fragments are introduced into PDPO, the effect is reverse. The fracture activation energy of the composites is shown to coincide with the fracture activation energy of PDPO. The variation of the strength is caused by the dependence of structure-sensitive parameter γ of the matrix on the concentration and shape of filler particles. On the one hand, filler particles are stress concentrators and their introduction decreases the strength. This effect prevails in the composites filled by spherical particles. On the other hand, when anisotropic particles are introduced, matrix molecules are arranged parallel to their axis, which decreases γ and increases the strength. This effect is predominant in the composites filled by fiber fragments.     

A model of a belt chamber cracking based on crack profiles calculations

Abstract

The problem of a Belt chamber matrix cracking is presented. The influence of crack surface quality on the effective values of near crack tip stress is discussed. It is shown that under working conditions of the vessel, the existing shear friction between upper and lower crack surfaces caused by crack surface roughness can prevent the crack surface sliding displacement. Therefore, the control variable for matrix cracking is the value of stress intensity factor K_I corresponding to normal node of loading only. The calculations are performed by finite element method within the range of linear elastic fracture mechanics.     

利用声发射技术监测颗粒填充聚合物材料的裂纹扩展过程

 利用声发射技术监测了颗粒填充聚合物材料含单边缺口试样承受三点弯曲载荷时裂纹尖端形成损伤并断裂的全过程,明显地区分了裂纹尖端起裂和扩展的不同阶段,有效地识别了颗粒填充聚合物材料的破坏模式。研究表明,在承载状态下,裂纹尖端损伤起始和扩展分为3个阶段,且裂纹起裂至快速扩展存在一个演变过程;结合SEM观察结果,判定该材料的断裂模式以颗粒与基材的界面分离为主。     

Self-oscillating regime of crack propagation induced by a local phase transition at its tip

We report an analytical study of propagation of a straight crack with a stress-induced local phase transition at the tip. We obtain its contribution to the dynamic fracture energy in explicit form and demonstrate that it nonmonotonically depends upon the crack tip velocity. We show that its descending part gives rise to the instability of the steady propagation regime. We obtain the dynamic phase diagram and indicate those domains where self-oscillating regimes of the crack motion take place.     

Effect of surface treatment condition on interfacial degradation behavior in GFRP under acidic conditions

Interfacial degradation behavior of E-glass cloth reinforced vinyl ester resin under acidic conditions has been investigated. Specimens with different surface treatment conditions were prepared. Mode I fracture toughness tests were performed using DCB specimen, and the effect of surface treatment condition and immersion time on the crack propagation behavior is discussed. The crack propagation behavior changes as a function of the condition of the silane coupling agent and the immersion time due to the degradation of the interphase. A technique is proposed to evaluate the interfacial property. The change of fracture toughness of interphase and resin as a function of immersion time is studied by the crack propagation behavior and the fracture toughness of interphase and resin evaluated by this technique. The fracture toughness of interphase decreases rapidly with immersion in acidic solution.     

Mode II brittle fracture assessment using an energy based criterion

In this paper the minimum strain energy density criterion is modified to predict the values of mode II fracture toughness reported in the literature for several brittle and quasi-brittle materials. The experimental results are all related to mode II fracture tests performed on the semicircular bend specimen. The modified mode II fracture criterion takes into account the effect of T-stress (in addition to the singular terms of stresses/strains) when calculating the strain energy density factor at a very small critical distance from the crack tip. It is shown that the proposed criterion provides significantly better predictions for mode II fracture toughness compared with the classical minimum strain energy density criterion.     

化学钢化光学窗口玻璃强度分析与检测

以脆性材料的断裂力学为基础,根据化学钢化光学窗口玻璃表面应力分布状态,分析了化学钢化对光学窗口玻璃强度的影响。将Weibull模型与玻璃微裂纹生长理论相结合,分析并提出了化学钢化光学窗口玻璃的强度检验和寿命预测方法,为化学钢化光学窗口玻璃的强度设计和检验提供指导。     

Fracture Micromechanisms of Polypropylene/Polycarbonate/Poly(styrene-b-(ethylene-co-butylene)-b-styrene) (PP/ PC/SEBS) Ternary Blends: The Effects of SEBS Content

Ternary blends of polypropylene/polycarbonate/poly(styrene-b-(ethylene-co-butylene)-b-styrene) (PP/PC/SEBS) with varying SEBS contents were produced via melt blending in a co-rotating twin-screw extruder. The phase morphology of the resulting ternary blends and its relationship with bending and impact behaviors were studied. Transmission optical microscopy (TOM) of the crack tip damage zone and scanning electron microscopy (SEM) of impact fractured surfaces were performed to characterize the fracture mechanism. With increasing SEBS content in the PP/PC/SEBS ternary blends, the number of PC/SEBS core-shell particles increased and the size of the core-shell particles enlarged. It was shown that with an SEBS content of 5%, the crack initiation resistance decreased and then was almost unchanged with further increase of SEBS content, while resistance to crack growth increased continuously with increasing of SEBS content. Preliminary analysis of the micromechanical deformation suggested that the high impact toughness observed for samples containing 20 and 30 wt% of SEBS could be attributed to cavitation of the rubbery shell and, consequently, shear yielding of the matrix. This plastic deformation absorbed a tremendous amount of energy. Due to low interfacial adhesion between PC particles and PP matrix in samples containing 5 and 10 wt% of SEBS, debonding occurred too early, so the occurrence of matrix shear yielding was delayed and resulted in premature interfacial failure and, hence, rapid crack propagation.