Mechanics of Transformation-Toughening in Brittle Materials

Particles which undergo a stress-induced martensitic transformation are known to toughen certain brittle materials. The enhanced toughness can be considered to originate from the residual strain fields which develop following transformation and tend to limit the crack opening. The increased toughness can estimated from the crack-tip stress-intensity change induced by the transformation of a volume of material near the crack tip. It is found that the initial zone, prior to crackgrowth, provides no change in stress intensity. As the crack grows, the zone (associated with a positive transformation strain) induces a stress-intensity reduction that rises to a maximum level after some crack propagation. The influence of particle-size distribution on the stress-intensity reduction is also discussed.     

A Unique Failure Criterion for Characterizing the Fracture of Propellants

The problem of crack growth in a nitrocellulose/nitroglycerin propellant has been studied employing a continuum fracture mechanics approach. Values of the stress-intensity factor, K_c, at the onset of crack propagation have been ascertained and found to be dependent upon both the thickness of the specimen and the test temperature. By proposing that the value of K_c arises from the sum of a plane-strain and a plane-stress component a unique failure criterion, which is constant over a wide temperature range, has been identified. Namely, that crack propagation in the propellant occurs when the plane-stress plastic-zone at the crack tip attains a critical size.     

聚乙烯压力管道裂纹快速扩展性及止裂性测试

聚乙烯压力管道在低温时易出现裂纹快速扩展，其破坏一般都具有突发性，难以预防。本文采用小尺寸稳态试验方法，测试了在不同温度下，裂纹扩展长度随压强的变化情况，同时也测试了在不同压强下，裂纹扩展长度随温度的变化情况。通过计算分析，确定了裂纹快速扩展的临界压强和临界温度。给出了影响该试验结果的因素如温度、压力、介质、裂纹扩展速度及裂纹尖端的流体减压等。并通过试验测试了焊缝以及套筒对塑料管道开裂的影响。最后给出了塑料管道在实际运行中的一些具体的止裂措施。     

Fracture Resistance of SiC-Fiber-Reinforced Si3N4 Composites at Ambient and Elevated Temperatures

The crack growth behavior in unidirectional SiC-fiber-rein-forced Si₃N₄-matrix composites fabricated in our laboratories was investigated as a function of fiber volume fraction and temperature. Both the stress-intensity factor and an energy approach were adopted in the characterization of the crack growth behavior. Crack resistance increased with crack extension ( R -curve or T -curve) as a result of bridging effects associated with the intact fibers. Large-scale bridging was observed, and was considered in the determination of the R -curves. Temperature and fiber volume fraction affected the crack propagation behavior. At room temperature a single crack was initiated at the notch tip; it then branched and delaminated upon further loading. In contrast, at 1200°C, little crack branching was observed. Increasing fiber volume fraction increased the degree of crack branching. Temperature and fiber volume fraction also affected the R -curve behavior. Raising the temperature to 1200°C did not significantly degrade the room-temperature R -curve effect. Increasing the fiber volume fraction from 14% to 29% substantially enhanced the toughening effect and the R -curve behavior.     

Fatigue Crack Propagation in Ceria-Partially-Stabilized Zirconia (Ce-TZP)-Alumina Composites

Fatigue crack propagation rates in tension-tension load cycling were measured in ZrO₂-12 mol% CeO₂-10 wt% Al₂O₃ ceramics using precracked and annealed compact tension specimens. The fatigue crack growth behavior was examined for Ce-TZPs of different transformation yield stresses obtained by sintering for 2 h at temperatures of 1500°C (type A), 1475°C (type B), 1450°C (type C), and 1425°C (type D). The threshold stress-intensity range, ΔK_th, for initiation of fatigue crack propagation increased systematically with decreasing transformation yield stress obtained with increasing sintering temperature. However, the critical stress-intensity range for fast fracture, ΔK_c, as well as the stress-intensity exponent in a power-law correlation (log (da/d N ) vs log ΔK) were relatively insensitive to the transformation yield stress. The fatigue crack growth behavior was also strongly influenced by the history of crack shielding via the development of the crack-tip transformation zones. In particular, the threshold stess-intensity range, Δ K _th, increased with increasing size of the transformation zone formed in prior quasi-static loading. Crack growth rates under sustained peak loads were also measured and found to be significantly lower and occurred at higher peak stress intensities as compared to the fatigue crack growth rates. Calculations of crack shielding from the transformation zones indicated that the enhanced crack growth susceptibility of Ce-TZP ceramics in fatigue is not due to reduced zone shielding. Alternate mechanisms that can lead to reduced crack shielding in tension-tension cyclic loading and result in higher crack-growth rates are explored.     

Slow crack propagation in polyethylene under fatigue at controlled stress intensity

Fatigue tests were performed on circumferentially notched bars (CNB) of high density polyethylene in order to analyse the kinetics and mechanisms of crack propagation. Tests were performed at 80 °C in order to accelerate the processes. Unlike standard fatigue procedure in which the force amplitude is constant, the original system utilised in this work was capable of imposing a constant stress intensity amplitude, K^max, during the whole propagation range. This was made possible through the real-time monitoring of crack propagation, a(N), by means of a video-controlled technique. The results of the tests, for K^max in a range from 0.2 to 0.45 MPa m^1/2, show that stress intensity is the proper variable which controls crack propagation rate since, on the overall, crack speed is constant at constant K^max and no self-acceleration is observed unlike under force control. The empirical Paris law is verified under all conditions, with a stress intensity exponent close to 4. However, it is shown that constant crack speed is obtained only for K^max<0.25 MPa m^1/2, when propagation proceeds through the continuous stretching and breaking of microfibrils in the localised craze at the tip of the crack. By contrast, at larger K^max, it is observed that crack tip successively jumps across the extended crazed zone in which very coarse fibrils were previously stretched from voids nucleated in the plane of maximum normal stress at a long distance ahead of the crack tip.     

Real-Time Observation of a Non-Equilibrium Liquid Condensate Confined at Tensile Crack Tips in Oxide Glasses

As crack propagation in oxide materials at low crack velocities is partly determined by chemical corrosion, proper knowledge of the crack tip chemistry is crucial for understanding fracture in these materials. Such knowledge can be obtained only from in situ studies because the processes that occur in the highly confined environment of the crack tip are very different from those that take place at free surfaces, or that can be traced post mortem . We report on the occurrence of a hydrous liquid condensate between the two fracture surfaces in the vicinity of the tip of tensile cracks in silica. Observations are performed in real time by means of atomic force microscopy (AFM) at continuously controlled crack velocities in the regime of stress corrosion. Condensate formation and changes in the extent and the shape are demonstrated for a wide range of macroscopic humidities at different crack speeds. Its liquid character is confirmed by the study of AFM phase-contrast data. It is believed that this evidence of a nanoscale liquid hydrous phase at the crack tip will provide novel insights into the chemistry of failure of oxide materials.     

Stress distribution in a 2024-T3 aluminum plate with a circular notch,repaired by a graphite/epoxy composite patch

The aim of this study is to analyze by finite element method the single- and double-sided composite patch repairs designed to reduce the concentration of the stresses at circular notches and cracks. The results show that there is a considerable reduction in the asymptotic value of the stress-intensity factors and the normal stresses at the crack tip. The use of a double-sided patch suppresses the bending effect due to the eccentricity of the patch on one side only and reduces the shear stresses in adhesive.     

Strength and Toughness of a Ceramic Reinforced with Metal Wires

Composites consisting of fine stainless steel fibers 6, 12, or 25 μm in diameter in a matrix of wustite were prepared by hot-pressing. The fracture stress (σ _F ) of unnotched beams and the critical stress-intensity factor ( K _1C) for crack initiation in notched beams were measured; both increased linearly with the volume fraction of fiber. These improvements were attributed partially to the action of yielding fibers which bridged the crack surfaces; plastic deformation of fibers at the crack tip was thought to play a role. Large increases (largest for the 25-μm fibers) in the total work of fracture (γ _F ) were attributed to the work done in fracturing crack-bridging fibers. For long cracks bridged by fibers, over a substantial fraction of the crack, the critical applied stress intensity factor for further crack propagation increased strongly with increasing bridged crack length. The form of this increase could be accounted for by considering the negative stress intensity factor resulting from the closure forces exerted on the crack by the bridging fibers.
Based on a thesis submitted by J. G. Zwissler for the Ph.D. degree at Northwestern University, Evanston, III., June 1976.
Supported by the U. S. Office of Scientific Research, Office of Aerospace Research, Grant No. AF–AFOSR–73–2431 and by a fellowship from the International Nickel Co.     

In-situ characterization on crack propagation behavior of SiCf/SiC composites during monotonic tensile loading

The microstructure and crack propagation path of 2.5D SiC_f/SiC composites were observed by synchrotron radiation x-ray computed micro tomography (SR-μCT) equipped with in-situ tensile device. The results showed that the pore morphologies of the SiC_f/SiC composites are mainly divided into three types in three-dimension space: interconnected pores, isolated pores and micro pores in fiber bundles. The crack initiation occurred at the root of the defects under in-situ tensile load and the crack was perpendicular, parallel to the stress axis or mixed mode to propagate. At the interface scale between fiber and matrix, the crack deflection will be controlled by physical parameters such as fracture energy release rate and the modulus of elasticity. At the fiber bundle scale, the crack is easy to shear propagate along the interface between weft and warp fiber bundles due to the existence of the mechanical bonding and residual tensile stress.     

In situ TEM observation of crack propagation in LiTaO3 particle toughened Al2O3 ceramics

Direct observation of crack propagation in LiTaO₃/Al₂O₃ composite ceramics was carried out using in situ transmission electron microscopy (TEM). Domain switching induced by crack propagation, crack deflection and branching at domain boundaries and ripples similar to the contrasts of 180° domains at the microcrack tip inside LiTaO₃ grains were detected evidently. Domain switching, crack deflection, branching and energy dissipation resulting from the formation of contrasts similar to the 180° domains at the microcrack tip, were proposed as the toughening mechanisms in LiTaO₃/Al₂O₃ ceramics.     

Failure analysis of carbon black filled styrene butadiene rubber under fatigue loading conditions

Abstract

The present paper deals with the fatigue crack growth in a carbon black-filled styrene butadiene rubber (CB-SBR) under fully relaxing loading conditions. More precisely, it is devoted to the determination of the scenario of crack growth. For that purpose, an original ‘microcutting’ technique, previously applied by the authors on natural rubber (NR), is used to observe microscopic phenomena involved in fatigue crack growth thanks to scanning electron microscopy (SEM). Results show that the crack tip grows following a tearing line by generating ligaments; it explains the differences between fatigue responses of crystallisable and non-crystallisable rubbers during crack propagation. So, contrary to crystallisable elastomers such as NR, the microstructure of SBR is similar at crack tip and in the bulk material, and the crack tip does not resist crack propagation. Moreover, the morphology of fracture surfaces only depends on particles encountered by the fatigue crack during its propagation.     

Mechanics of crack growth in epoxide resins

Experiments have been conducted employing tapereddouble-cantilever-beam joints with different epoxide adhesives. Depending on the adhesive employed, crack propagation occurred either (a) in a continuous stable manner with crack propagation velocities in the range 10^?4 to 5 m/s and values of the adhesive fracture energy, G_Ic, being almost independent of the crack velocity, or (b) intermittently in an unstable manner when the initial crack velocity was never less than about 20 m/s and, in some instances, rose to about 450 m/s; values of G_Ic (initiation) increased rapidly with increasing velocity. It is proposed that the amount of localized plastic deformation arising from shear yielding that occurs at the crack tip prior to crack propagation is controlling. Secondly, the longterm strength of stressed, structural adhesive joints has been investigated. The fracture of these joints over eight decades of time is uniquely described by a critical plastic zone size developed at the crack tip at failure.     

Assessment of viscoelastic crack bridging toughening in refractory materials

Viscoelastic bridges can be formed in refractory ceramics while cooling from high temperatures. Such bridges can shield crack tips, thus reducing the effective crack tip stress intensity factors leading to higher resistance to creep and thermal shock. The extent to which the crack tip stress intensity is reduced can be estimated from fracture mechanics models that include experimental measurement of crack bridging and microstructural parameters. In this paper a novel approach is proposed for the assessment of the effective crack bridging toughening from combining destructive and non-destructive test methods. Fracture toughness values were determined applying chevron notched specimen technique and surface damage of the specimen was monitored by image analysis. Different cordierite–mullite compositions characterized by different microstructure morphologies and crack propagation behaviour were investigated. A brief discussion about the correlation between thermo-mechanical properties, microstructure, crack propagation behaviour and thermal shock resistance is presented. Moreover, an empirical model able to determine the presence and effectiveness of the viscoelastic crack bridging ligaments acting in the microstructure under thermal shock conditions and their degradation with increasing thermal shock cycles from parameters measured at room temperature is presented.     

拉弯复合应力下焊接头表面裂纹疲劳扩展规律的研究

通过对16MnR钢对焊的弓形试极疲劳裂纹扩展试验，研究了在拉弯复合应力下对焊接头表面裂纹疲劳扩展规律。采用Newman－Raju公式分析了有限宽板在拉弯复合应力下表面裂纹的应力强度因子的计算。试验研究表明在拉弯复合应力下当a／t≤0．8时表面裂纹疲劳扩展规律仍可用Paris公式来描述，并且c向和a向的Paris系数之间仍存在着C_c＝0．9~nC_a的关系。     

Peak stress intensity dictates fatigue crack propagation in UHMWPE

The majority of total joint replacements employs ultra-high molecular weight polyethylene (UHMWPE) for one of the bearing components. These bearings may fail due to the stresses generated in the joint during use, and fatigue failure of the device may occur due to extended or repeated loading of the implant. One method of analysis for fatigue failure is the application of fracture mechanics to predict the growth of cracks in the component. Traditional analyses use the linear elastic stress intensity factor K to describe the stresses near a loaded crack. For many materials, such as metals, it is the range of stress intensity, ΔK, that determines the rate of crack propagation for fatigue analysis. This work shows that crack propagation in UHMWPE correlates to the maximum stress intensity, K_max, experienced during cyclic loading. This K_max dependence is expected due to the viscoelastic nature of the material and the absence of crazing or other cyclic load dependent crack tip phenomena. Such a dependence on a non-cyclic component of the stress allows cracks to propagate under load with little or no fluctuating stresses. Consequently, traditional fatigue analyses, which depend on the range of the stress to predict failure, are not always accurate for this material. For example, significant static stresses that develop near stress concentrations in the component locking mechanisms of orthopedic implants make such locations likely candidates for premature failure due the inherent underestimate of crack growth obtained from conventional fatigue analyses.     

Finite Element Techniques Applied to Cracks Interacting with Selected Singularities

The finite-element method for computing the extensional stress-intensity factor, K ₁, for cracks approaching selected singularities of varied geometry is described. Stress-intensity factors are generated using both displacement and J -integral techniques, and numerical results are compared to those obtained experimentally in a photoelastic investigation. The selected singularities considered are a colinear crack, a circular penetration, and a notched circular penetration. Results indicate that singularities greatly influence the crack-tip stress-intensity factor as the crack approaches the singularity. In addition, the degree of influence can be regulated by varying the overall geometry of the singularity. Local changes in singularity geometry have little effect on the stress-intensity factor for the cases investigated.     

Relating the generation of dynamic strain energy to crack propagation in polymers

With many fracture research studies, there is a need to compare frequently experimental findings and modeling simulations. One example is when studying the relationships between crack velocity and dynamic strain energy generated in polymers. It helps in these cases if the same computer can perform the modeling simulations as used to capture and process experimental data. Personal computer (PC)–size machines are often used to capture experimental data, and a fracture simulation model has been devised that can be hosted on a personal computer. In this study, the model is used with the frozen tongue experimental technique to research steady state crack propagation in medium‐density polyethylene (MDPE). This is to study the generation of dynamic strain energy in the material and the factors affecting its availability to the crack tip. This includes the effects of changes in the material's properties immediately about the crack tip, the conditions that determine the threshold to just maintain crack propagation, and the conditions that determine the relationship between crack velocity and strain energy up to the limiting crack velocity.     

The acoustic emission analysis of the crack processes in alumina

The crack processes in single-phase alumina specimens have been investigated by acoustic emission (AE) analysis with regard to the increase of the crack resistance. During loading of a notched specimen, individual AE signals are observed at first, which are probably due to the generation of microcracks in a process zone around the notch. At higher loads signal clusters are found, which should be due to the coalescence of microcracks. By these coalescence events the main crack is formed. At macroscopic crack propagation most AE events are located within the crack tip zone. However, up to about 20% of all events are located within the crack flank zone behind the crack tip. Thus, it can be concluded that there is an energy dissipation at the crack tip at beginning of the loading, which determines the starting value of the crack resistance. At macroscopic crack propagation crack flank interactions contribute to the increase of crack resistance, too. However, it cannot be decided from AE if the contribution of the process zone at the crack tip or of the crack flanks in the wake of the crack tip play the major role in increasing the crack resistance.