Fracture characteristics of vinylester resin under impact fatigue

The impact fatigue behavior of a vinylester resin was studied with a pendulum‐type repeated‐impact tester especially designed and fabricated for the determination of single‐impact and repeated‐impact strengths. A well‐defined energy–endurance impact fatigue curve was obtained with a progressive endurance at values of the impact energy below the critical value, with the endurance limit set at an energy level of 31 N mm, 17.4% of the single‐impact energy. The nature of the crack propagation was investigated for a single impact as well as high, medium, and low impact energy levels with progressively longer endurance. The fracture characteristics varied with the impact energy imparted and the number of cycles endured. The rate of lip growth was high at the higher impact energy levels with a lower number of endurance cycles and low at the lower impact energy levels with longer endurance; the repeated impacts created large and small compressive zones through the bending of specimens with the development of long and short lips, respectively. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 1995–2001, 2002     

In situ observation of damage nucleation in graphite and carbon/carbon composites

The flexure strength and the fracture toughness at 300 K and 77 K were measured in two isotropic polycrystalline graphites with very different microstructure and in one carbon/carbon composite. In addition, the micromechanisms of damage initiation at the notch tip were examined in situ during the fracture tests through a long focal distance microscope. It was found that the mechanical response of carbon-based materials was insensitive to the effect of cryogenic temperatures. In graphite with coarse microstructure, cracks appeared at very low stresses in various points of an ample region surrounding the notch tip, and damage progressed by their stable crack growth and link up. On the contrary, damage was localized at the notch root in graphite with a fine microstructure. High stresses were necessary to nucleate a single crack, which grew unstably, leading to immediate specimen failure. Damage in carbon/carbon composites was nucleated in the form of matrix cracks around the notch tip, but fiber yarns impeded the crack propagation until the load had increased significantly. This process was repeated several times, leading to a serrated load-deflection curve and to a marked increase in the overall fracture resistance.     

Energy Principle of Elastic-Plastic Fracture and Its Application to the Fracture Mechanics of a Polycrystalline Graphite

Utilizing loading-unloading procedures on the basis of nonlinear energy principles, an empirical method for evaluating the nonlinear fracture mechanics parameters, i.e. the nonlinear energy toughness G_c , the crack growth resistance R , the J_c value, and the plastic energy dissipation rate φ _p , was established. These parameters were experimentally determined for an isotropic polycrystalline graphite enabling the elastic-plastic fracture mechanics of graphite to he addressed. The graphite exhibits a typical elastic-plastic fracture with ×38% of the total fracture energy consumed as plastic energy. It was concluded that the widely used assumption of the applicability of linear elastic fracture mechanics to polycrystalline graphites can lead to erroneous results if the fracture tests are conducted with the usual specimen size. The proposed experimental method for evaluating elastic-plastic fracture parameters is potentially very effective for studying various nonlinear fractures in other ceramic materials.     

Acoustic emission from polygranular graphites

Acoustic emission (AE) was monitored from six industrial graphites of widely different grain size subjected to three-point bend loading. The AE is characteristic of graphite type and microstructure. The development of AE with applied load is shown to be associated with the micromechanical events that cause nonlinear stress-strain behaviour in graphites, and postfracture AE is indicative of the crack propagation mode at fracture. For a given graphite, the total number of AE and the proportion of AE events of large amplitude both increase with load. For different graphites, the total number of AE at fracture and the proportion of AE events of small amplitude tend to increase with grain size. The changing pattern of AE with load and graphite type is explained by ascribing AE events of small amplitude mainly to easy cleavage of favourably oriented basal planes. AE events of large amplitude are ascribed mainly to extension of microcracks into less favourably oriented basal planes and into the binder phase. The lower proportion of AE events of small amplitude found for fine-grained graphites is related to the effects of size reduction of filler particles by grinding.     

On the fracture toughness of polycrystalline alumina measured by SEPB method

Fracture toughness (K_IC) of a polycrystalline alumina was evaluated using a single edge precracked beam (SEPB) method. A Knoop indentation-induced microcrack was introduced into a bend bar specimen, and then a sharp pop-in precrack was developed by applying the bridge loading technique. The precrack length (a/W) was varied by changing the indentation load and/or the support groove width of the anvil. The precracked specimens were fractured by three-point bending under a cross-head speed of 0·5 mm/min at room temperature. K_IC values of a polycrystalline alumina were dependent on precrack length for a/W<0·35. The dependence was discussed in terms of residual stress around the indentation-induced crack and crack mouth opening displacement (CMOD).     

Fatigue peeling at rubber interfaces

Abstract

A fatigue peeling test has been developed to evaluate the failure of rubber to rubber interfaces under cyclic loading. Results obtained through this method have been compared to those of a typical fatigue crack growth experiment. The results show that the trends between these two failure modes are similar with the peeling necessary to drive the crack being slightly higher than the strain energy release rate at the same crack growth rate. Cyclic and time dependent contributions to the fatigue crack growth behaviour have been calculated using this test for an styrene–butadiene rubber compound and the results appear to be consistent with previous work although the origin of the cyclic contribution remains uncertain. The influence of pressure at the interface during vulcanisation has also been investigated and it has been observed that the fatigue peel behaviour is proportional to the surface area of contact developed during the curing cycle.     

Fracture behavior of vinylester resin matrix composites reinforced with alkali‐treated jute fibers

Jute fibers were treated with 5% NaOH solution for 4 and 8 h, respectively, to study the mechanical and impact fatigue properties of jute‐reinforced vinylester resin matrix composites. Mechanical properties were enhanced in case of fiber composites treated for 4 h, where improved interfacial bonding (as evident from scanning electron microscopy [SEM]) and increased fiber strength properties contributed effectively in load transfer from the matrix to the fiber; but their superior mechanical property was not retained with fatigue, as they showed poor impact fatigue behavior. The fracture surfaces produced under a three‐point bend test and repeated impact loading were examined under SEM to study the nature of failure in the composites. In case of untreated fiber composites, interfacial debonding and extensive fiber pullout were observed, which lowered the mechanical property of the composites but improved their impact fatigue behavior. In composites treated for 4 h under repeated impact loading, interfacial debonding occurred, followed by fiber breakage, producing a sawlike structure at the fracture surface, which lowered the fatigue resistance property of the composites. The composites with fibers treated with alkali for 8 h showed maximum impact fatigue resistance. Here, interfacial debonding was at a minimum, and the fibers, being much stronger and stiffer owing to their increased crystallinity, suffered catastrophic fracture along with some microfibrillar pullout (as evident from the SEM micrographs), absorbing a lot of energy in the process, which increased the fatigue resistance property of the composites. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2588–2593, 2002     

Mechanical behavior of ceramic-metal joint under quasi-static and dynamic four point bending: Microstructures,damage and mechanisms

This paper studies the mechanical behavior of Alumina ceramic-Kovar joint under quasi-static and dynamic four-point bending (FPB). The joint is fabricated by molybdenum-manganese (Mo-Mn) metallization method with extra additions. The bend strength of the joint is improved by glass phase migration. Electronic universal testing machine and modified split Hopkinson pressure bar (SHPB) are employed to realize the loading process. The microstructure of the joint is investigated by scanning electron microscope (SEM) and the chemical composition is determined by energy dispersive spectrometer (EDS). Digital image correlation (DIC) technique is used to determine the displacement contours and ultra-high speed camera is used to monitor the deformation and crack evolution around the joint. It is found that the specimen will slip due to the different Young's modulus of base material. The dynamic bend strength of the joint is lower than the quasi-static bend strength. The failure mechanism of the ceramic-metal joint is mainly intergranular failure for the dynamic bend but mixed transgranular/intergranular failure for the quasi-static case. The crack starts from the inherent voids inside the ceramic and then expands along the metallization band between solder and ceramic.     

Fatigue crack growth in ferroelectrics under electrical loading

Fatigue crack growth is investigated in the polycrystalline ferroelectrics PZT-5H and PLZT 8/65/35 under electrical loading. The fatigue cracks exhibit features such as bifurcation and tunnelling, followed by crack arrest. The nature of the resulting damage differs in the two materials: in PZT-5H a narrow zone of intergranular cracks propagates across the specimen, wedging the crack surfaces apart. In PLZT 8/65/35 a broad microcracked band spreads across the specimen. The rate of crack growth is found to correlate well with the amplitude of electric displacement.     

Failure Modes in Ceramic-Based Layer Structures: A Basis for Materials Design of Dental Crowns

A research program on failure modes induced by spherical indenters in brittle layer structures bonded to polymeric substrates, in simulation of occlusal function in all-ceramic dental crowns, is surveyed. Tests are made on model flat and curved layers bonded onto a dentin-like polymer base, in bilayer (ceramic/polymer) and trilayer (ceramic/ceramic/polymer) configurations. All-transparent systems using glass as a porcelain-like outer or veneer layer and sapphire as a stiff and strong core support layer enable in situ observation of the entire evolution of fracture modes in the brittle layers, from initiation through to failure. With the fracture modes identified, tests are readily extended to systems with opaque polycrystalline dental core ceramics, notably alumina and zirconia. A variety of principal failure modes is identified: outer and inner cone cracks developing in the near-contact region at the top surface; radial cracks developing at the bottom surface along the loading axis; margin cracks from the edges of dome-like structures. All of these modes are exacerbated in cyclic loading by time-cumulative slow crack growth, but inner cones are subject to especially severe mechanical fatigue from hydraulic pumping of water into the crack fissures. Conditions under which each mode may be expected to dominate, particularly in relation to geometrical variables (layer thickness, contact radius) and relative material properties, are outlined. Clinical issues such as crown geometry, overload versus fatigue failure, role of residual stresses in fabrication, etc. are addressed.     

Stable Crack Growth in Polycrystalline Magnesia under Monotonic and Cyclic Loads

The quasi-static, stable growth of cracks in polycrystalline magnesia under various loading conditions (cyclic compression, monotonic tension, and tension–compression fatigue) was studied. Crack length was monitored continuously through a conductive film attached to the specimen surface. The fracture surfaces and the crack path were examined by scanning electron microscopy. The different fracture mechanisms responsible for the observed stable crack growth in each loading condition are discussed.     

Impact fatigue of a polycarbonate/acrylonitrile-butadiene-styrene blend

This study analyzes the impact properties of a polycarbonate/acrylanitrile-butadiene-styrene (PC/ABS) blend. The specimens were prepared under various injection molding conditions, including filling time, melting temperature, and mold temperature. Impact tests were performed with a Dynatup drop weight impact tester at different impact energies (10, 15, 20, 25 J). The fracture mechanism was examined with a scanning electron microscopy. The results indicated that the load-time history of the PC/ABS blend has approximately a sinusoidal form in impact. The best injection molding conditions are a filling time of 12 s, a melting temperature of 260°C and a mold temperature of 80°C. In this case, the specimen shows the highest energy absorbed in single impact, together with the highest impact number in impact fatigue. The impact number and the accumulation energy seem to follow an exponential curve as the impact energy decreases. The PC/ABS blend material clearly exhibited ductile fracture with a continuous reduction in strength by viscoplastic deformation. The higher the impact number, the higher the accumulation energy. The accumulation energy of impact fatigue with impact energy 10 J is about 35–45 times greater than the energy absorbed in single impact. Tearing, shear fracture, and plastic deformation are the major fracture mechanisms of the PC/ABS blend matrix in single impact and repeated impact conditions.     

Graphite—the most fascinating nuclear material

A review is made of the behaviour of graphite subjected to irradiation in nuclear reactors with particular emphasis on the dimensional changes of graphite crystallites and their relationship to the macroscopic dimensional changes of polycrystalline graphites. The theory of the formation of the lattice defects responsible for these changes is examined and deficiencies in present knowledge highlighted.     

Compressive fatigue damage and failure mechanism of fiber reinforced cementitious material with high ductility

The fiber reinforced cementitious material with high ductility has potential use in particular environments and structures that undergo repeated or fatigue loads. In this study, a series of monotonic and fatigue tests were performed to investigate the compressive fatigue behavior of this material. It is found that the fatigue life of this material is higher than that of plain concrete and steel fiber reinforced concrete under the same stress level. In addition, the failure deformation of fiber reinforced cementitious material with high ductility under fatigue load was larger than the monotonic envelope, while the envelope coincides with the monotonic loading curve for concrete or fiber reinforced concrete. The failure surface and damage process were investigated and a new failure mode of polyvinyl alcohol fiber with crushed end was discovered. The fatigue failure surface could be divided into three regions, including fatigue source region, transition region and crack extension region.     

Dimensionless Load—Displacement Relation and Its Application to Crack Propagation Problems

The dimensionless load (     ) and load-point displacement (û) are introduced to discuss crack propagation problems. The     −ũ relation for the case of equilibrium crack propagation in any linear elastic material is demonstrated to lie on a single universal fracture curve independent of the material. The concept of the universal     −ũ relation is extended to yield a direct and simple form of the dimensionless total energy (     _eq (α)) which is applicable to many types of instability problems for crack extension, when the shape factor ( Y ) of the specimen is known as a function of crack length ( a ). In addition, by applying the deviation of the experimentally observed     —ũ curve and the theoretical universal fracture curve, the evaluation of the nonlinear fracture resistance parameter of a polycrystalline graphite material, as an example, was demonstrated.     

全钢子午线轮胎疲劳破坏现象仿真与优化设计研究

基于有限元的轮胎疲劳寿命评价方法,建立295/80R22.5全钢子午线轮胎的有限元模型。使用ABAQUS隐式分析方法获得轮胎的应变、应力以及应变能密度等基本参数后,根据轮胎实际使用中的破坏情况选择影响疲劳寿命的评价指标。结果表明:高速耐久工况下,应选择应变能密度和帘线张力作为影响疲劳寿命的评价指标,而且适当增加带束胶厚度能有效提高轮胎的疲劳寿命;低速重载工况下,应当选择帘线和橡胶间的剪应力(层间剪应力)作为影响疲劳寿命的评价指标,且适当降低胎体帘布层和补强层的高度能有效提高轮胎的疲劳寿命。     

Fracture Mechanics of Titanium/Bioactive Glass-Ceramic Particulate Composites

Composite bioactive glass-ceramics reinforced with 30 vol% Ti particles were produced by warm-forging and in situ crystallization. The bend strength of the composite was 87 ± 7 MPa. A fracture mechanical study in air and Ringer's bioactive solution determined crack velocity vs stress intensity factor and the effect of stressing rate on the bend strength. Time to failure of 10 years is predicted for the composite loaded at 10 MPa in Ringer's solution or 50 MPa in air. A disappointing fatigue performance of the relatively tough metal/ceramic composite illustrates that satisfactory short-time fracture resistance does not guarantee lifetime under fatigue conditions (long-term fracture resistance).     

R-Curve Behavior of a Polycrystalline Graphite: Microcracking and Grain Bridging in the Wake Region

The contributions of nonlinear fracture processes both in the microcracking frontal process zone and in the following wake region and of grain bridging to crack-growth resistance parameters are discussed in terms of the R-curve behavior of an isotropic polycrystalline graphite. The R-curve behavior of the graphite is characterized by rapidly increasing values at the initial stage of crack extension (Δa≤1 to 2 mm) followed by a steady-state plateaulike region and then a distinct decrease when the primary crack tip approaches the end surface of the test specimen. Scanning electron microscopy of fracture mechanics specimens revealed a dominant role of grain bridging in the following wake regions on the rising R-curve behavior and confirmed the significant size effect of the large-scale microcracking process zone on the falling R-curve behavior. The stress-derived fracture toughness (K_R) and the energy fracture toughness (R_c) are discussed in relation to the micro-cracking residual strain.     

Statistical Analysis of Bending Strengths for Brittle Solids: A Multiaxial Fracture Problem

A general approach for the statistical analysis of brittle fracture under tensile multiaxial stress states was evaluated for a fracture criterion based on recent concepts of noncoplanar crack extension. The resultant failure expressions were applied to bend tests and used to interpret experimental results obtained with porcelain cylinders. Implications for the interpretation of bending tests and some important trends in the failiire strength with specimen size are discussed.