A Technique for the Measurement of Adhesive Fracture Energy by the Blister Method

The adhesive fracture energy, G_1c, of a model adhesive/adherend system, consisting of poly(methyl-methacrylate) plates bonded with a cyanoacrylate adhesive, has been evaluated using the Tapered Double Cantilever Beam and Blister test geometries. A refined Blister testing technique is described which, using relatively large diameter test plates [200mm], is capable of arresting the initial propagation of the-invariably less than naturally sharp-starter crack. This allows us to three subsequent G_c determinations for the same specimen from starter cracks of natural sharpness.

Adhesive fracture energy values determined for the model system using TDCB test pieces, 0.110±0.017 kJm^-2, were in good agreement with those obtained for Blister specimens in which arrested cracks had been repropagated, 0.119±0.013 kJm^-2. As is generally observed, values calculated from the initial propagation of starter cracks were somewhat higher for the TDCB specimens, 0.140±0.045 kJm^-2. Corresponding values for the Blister tests were significantly higher with more scatter, 0.194±0.074 kJm^-2.

It is concluded that meaningful G_1C data are only obtained from the Blister test if values obtained from the propagation of the initial starter crack are discarded. No matter how carefully prepared, these flaws will be less than naturally sharp. The precracking technique described here detects crack growth and, by releasing the hydraulic pressure driving it, arrests a propagating crack before catastrophic failure can take place.     

Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics

A fracture-mechanical theory is presented for crack propagation in brittle ceramics subjected to thermal shock. The criteria of crack stability are derived for a brittle solid uniformly cooled with triaxially constrained external boundaries. Thermal stress crack instability occurs between two values of critical crack length. For short initial crack length, crack propagation occurs kinetically, with the total area of crack propagation proportional to the factor S_t² (1-2 v )/EG, where S_t is tensile strength, v is Poisson's ratio, E is Young's modulus, and G is surface fracture energy. Under these conditions the newly formed crack is subcritical and requires a finite increase in temperature difference before propagation will proceed. For long initial crack length, crack propagation occurs in a quasi-static manner and can be minimized by maximizing the thermal stress crack stability parameter R_st= [G/α² E ]^1/2, where α is the coefficient of thermal expansion. For heterogeneous brittle solids, such as porous refractories, the concept of an "effective flaw length" is introduced and illustrated on the basis of experimental data in the literature. The relative change in strength of a brittle solid as a function of increasing severity of thermal shock is estimated. Good qualitative agreement with literature data is found.     

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.     

Simulation of crack propagation in Alumina particle-dispersed SiC composites

Addition of alumina particles to silicon carbide results in strongly improved toughness values. In order to come to a better understanding of this phenomenon, crack propagation is simulated for a 20 vol% alumina particles-dispersed silicon carbide composite material using the Body Force Method. Special emphasis is paid to the influence of graded compositions. Numerically obtained crack paths are compared to crack paths generated experimentally by Vickers indentations. Moreover, mechanical properties of the investigated material were measured experimentally. Microstructural toughness variations as well as the direction of crack propagation are found to be strongly influenced by residual stresses due to the mismatch between thermal expansion coefficients of alumina and silicon carbide and by the actual crack location. According to tensile residual stresses in the radial direction cracks approaching a particle are deviated circumferentially in the matrix around the particle. Moreover, the failure behavior of cracks propagating into a zone of increasing or decreasing volume fraction of alumina particles is found to behave differently as residual stress fields superimpose in the case of particle clustering. ©.     

微型定容燃烧腔内C2~C4烷烃/空气火焰传播

在直径35 mm、高度2 mm光学可视的定容燃烧腔内,实验研究了常温常压静止乙烷/空气、丙烷/空气和正丁烷/空气预混气在燃烧腔中心由电火花点燃后向外传播的火焰传播特性。结果表明：3种燃料空气混合气可形成火焰传播的当量比范围不同,范围由大到小排序为乙烷>丙烷>正丁烷;3种燃料均存在由光滑火焰面向褶皱火焰面转变的传播形态;在微型定容燃烧腔内,3种燃料的火焰传播速度均低于常规尺度下定容燃烧弹内火焰传播速度,且火焰传播速度随半径增加而减小;随着当量比增加,火焰锋面容易出现褶皱和断裂现象,在高当量比情况下,火焰传播会出现短暂停滞。     

Cyclic Fatigue Crack Propagation in Alumina under Direct Tension—Compression Loading

Cyclically induced crack propagation occurs in alumina subjected to direct tension—compression loading. The crack increment per cycle (da/dN) has a power-law dependence on the peak stress intensity factor (K_max). Cyclic crack growth can occur at lower values of K_max than are required to produce static fatigue effects. Subcritical crack-growth behavior was found to be dependent on specimen geometry: it is suggested that direct compressive loads and crack length are both factors that affect cyclic fatigue behavior, and that the use of K alone to characterize fatigue crack growth in ceramics may be questionable.     

Fracture toughness characterization of a PC/ABS blend under different strain rates by various J-integral methods

A published, nonconventional J-integral method, based on the hysteresis energy and the ASTM E813 methods, has been employed to test the fracture toughness of a polycarbonate (PC)/acrylonitrile-butadiene-styrene (ABS) blend. The critical J values (J_Ic) at crosshead speeds ranging from 0.5 to 20 mm/min obtained from the hysteresis energy method are ∼10 to 20% higher than those obtained from the E813–81 method and ∼50 to 70% lower than those obtained from the E813–87 method. However, the hysteresis energy method results in comparable J_Ic values with a modified ASTM E813–87 method when the 0.2 mm offset line is replaced with a 0.1 mm offset line. The critical displacements in terms of the onset of crack initiation, determined from the plots of hysteresis energy vs. displacement, hysteresis ratio vs. displacement, and the true crack growth length vs. displacement, are fairly close in value. This indicates the critical crack initiation and the corresponding J_Ic obtained from this hysteresis energy method indeed represent the actual physical event of the onset of crack initiation.     

Subcritical Crack Propagation in 3Y-TZP Ceramics: Static and Cyclic Fatigue

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y₂O₃ (3Y-TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10^-12-10^-3 m/s in several environments, including air, water (in the temperature range of 3°-85°C), secondary vacuum (10^-5 mbar), and silicon oil. Analysis of the experimental results-three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs-shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static-fatigue/cyclic-fatigue sequences show that both stress corrosion by water and pure cyclic-fatigue effects are operative under alternative stresses.     

A theory for the flexural strength of steel fiber reinforced concrete

A theory is presented to predict the flexural tensile strength of concrete reinforced with short, discontinuous steel fibers randomly oriented and uniformly dispersed in a cement-based matrix. The theory is based on a dual criterion of crack control and composite mechanics. The first crack in the fibrous composite occurs due to bond slip. The fracture process consists of progressive debonding of fibers during which slow crack propagation occurs. Final failure occurs due to unstable crack propagation when fibers pull out and the interfacial shear stress reaches the ultimate bond strength. The theory is supported by test data on fiber reinforced concrete, mortar and paste.     

Impact damage in a composite material

Damage development in plates of a glass fiber-reinforced polypropylene is recorded, using short pulse photography, during instrumented falling weight tests using the excess energy approach. It is seen that the damage is progressive throughout the test but its initiation cannot be detected either by freeze-frame photography or visual observation. Specimens are therefore subjected to low-energy impact followed by microscopic observation of the tensile face. It is found that the initial damage mechanism is cracking of the matrix at the fiber-matrix interface, the crack propagating along the fiber. The finite element method is used to estimate the stress distributions at damage initiation in 4-ply and 8-ply samples cut from unidirectional and cross-ply plaques. The computed results indicate that initial damage occurs when the transverse tensile stress reaches a critical value.     

Fracture behavior of exposed HDPE specimens with an embrittled layer due to UV-exposure

The applicability of fracture mechanics was studied on UV-exposed HDPE Charpy specimens. The failure distribution of the stress at failure measured in three-points bending tests showed a bifurcation in failure processes. At high stresses yielding occurs, resulting in large strain at failures. At lower stresses crack propagation causes low strain at failures. Due to the bifurcation in failure processes the ductile-brittle transition temperature of exposed specimens is hard to determine. Specimens notched after exposure showed a decrease in the critical stress intensity values. The effective notch depth of exposed specimens was found to be larger than the thickness of the oxidized layer measured by FT-IR and density measurements.     

Study on the long-term behaviour of glass fibre in the tensile stress field

Glass fibre is a random network structure composed of [SiO₄] tetrahedra. The structure contains a large number of defects, which act as crack initiation points. Under tensile stress, cracks undergo crack initiation, stable propagation, failure propagation, and fracture, and the stress that begins after unstable propagation is called the critical fracture stress. When the stress is less than the critical value, the crack is subject to the force of chemical bonds during the crack propagation process, and crack arrest occurs. When the stress is greater than the critical value, the glass fibre will undergo destructive fracture. In this paper, long-term tensile tests were carried out on glass fibre and a glass fibre composite under different constant tensile stress conditions. The fracture times of the glass fibre and glass fibre composite under different tensile stresses were obtained, the critical fracture stress of glass fibre was inferred, and the fracture mechanism was explained.     

Acoustic emission during fracture of short glass fiber reinforced poly(vinyl chloride)

The fracture mechanics approach and acoustic emission (AE) analysis have been coupled in an investigation of slow crack growth of short fiber reinforced poly(vinyl chloride). It was found that AE occurs during less than one percent of the time of crack growth suggesting that so-called continuous crack propagation is based on discontinuous microscopic damage. The time dependence of various AE parameters exhibits a good linear relationship with crack speed. For a unit of newly created fracture surface, a constant amount of acoustic energy is released independent of crack speed. At very low speeds, below 3·10⁻³ mm/s, a change in mechanism from pure matrix related to fiber related crack growth is observed, which is also reflected by a change in the amplitude distribution of the AE events.     

Cyclic Fatigue Crack Propagation Behavior of 9Ce-TZP Ceramics with Different Grain Size

Cyclic fatigue crack growth behavior has been investigated in 9 mol% Ce-TZP ceramics with grain sizes varying from 1.1 to 3.0 μm. To ascertain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double-cantilever-beam specimens in two conditions: (a) with a sharp precrack without preexisting t - m transformation and (b) with a sharp crack after R -curve measurements, i.e., with preformed t - m transformation in the crack region. Fatigue crack propagation occurs at applied stress intensity factor values as low as about 40% of the K _I,∞ values measured in the R -curves. The size and shape of the t - m transformation zones are found to be different for specimens obtained in monotonic loading R -curve measurements and in cyclic fatigue tests. For the specimens without preexisting t - m transformation the overall crack growth behavior can be described by the Paris power law relation: d a /d N = AδK^m _I with m values of 15 for the 1.1-μm grain size and between 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a "V"shape d a /d N versus Δ K _I relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding effects.     

On the essential work of fracture method: Energy partitioning of the fracture process in iPP films

Summary The fracture properties of an iPP are investigated by the EWF method. A separation between crack initiation and propagation fracture parameters is done by splitting the total energy of the load-displacement curves in two. The influence of the DDENT specimen height and the test rate on these different parameters is studied, obtaining that varying the height has no influence in the range 40 to 80mm, but changing the crosshead speed (2 to 100mm/min) has an effect on the fracture parameters. It is interesting to note that the “Initiation Specific Essential Work” (w_e ^I) seems not to be sensible to the stress-state transition. Received: 20 October 1998/Revised version: 1 December 1998/Accepted: 7 December 1998     

Determination of Fracture Toughness at High Temperatures after Subcritical Crack Extension

As a consequence of R -curve behavior, ceramic materials may exhibit increased fracture toughness ( K _Ic) following slow crack extention. In this investigation, the effect of crack propagation on fracture toughness is studied in static bending tests. For the calculation of stress intensity factors ( K _I) the stress distribution must be known at the moment of fracture. As a consequence of creep, this stress distribution must deviate from the linear distribution. The corresponding stress intensity factors are computed using the fracture mechanical weight function. Experimental results for fracture toughness are communicated for a 2.5%-MgO-doped hot-pressed Si₃N₄ at 1300°.     

PE100管及其热熔接头的蠕变裂纹扩展行为

聚乙烯(PE)管性能优异,广泛应用于城市水及燃气供应系统。PE管的主要破坏形式是长期静压载荷下的慢速裂纹扩展失效。在蠕变条件下,采用光滑试样和裂纹圆棒试样对PE100管及其热熔接头进行了测试,得到了基于蠕变断裂参数C*的蠕变裂纹扩展动力学关系式。利用扫描电子显微镜(SEM)分析了裂纹圆棒试样的断面形貌,对比分析结果发现,蠕变裂纹扩展失效模式对应的最大应力为15.05 MPa,热熔接头熔合面分布有约11个/mm^2、直径范围为1~5μm的微气孔,热熔接头断裂微纤平均长度比母材约小20%~45%。当热熔对接时,熔合面存在的微气孔以及系带分子的浅渗透是导致PE100热熔接头蠕变裂纹扩展抗力降低的主要原因。     

An innovative model coupling TGO growth and crack propagation for the failure assessment of lamellar structured thermal barrier coatings

The failure of plasma-sprayed thermal barrier coating (TBC) is often caused by the coating spallation due to crack propagation. In this study, a new model with stacking lamellae is developed based on the cross-section micrograph to explore crack propagation behavior within the ceramic top coat (TC) during isothermal cycling. The dynamic growth process of thermally grown oxide (TGO) is simulated via material properties change step by step. The stress profiles in the lamellar model are first evaluated, and the pore and lamellar interface crack effects on the stress state are further explored. Then, the successive crack growth, linkage, and ultimate coating spallation process is simulated. The results show that the stress intensity in TC enhances with thermal cycling. Large stress concentration always occurs near the pore and lamellar interface crack, which can result in the incipient crack growth. Moreover, the lamellar interface crack also changes the stress distribution within the TC and at the TC/bond coat interface. The multiple crack propagation upon temperature cycling is explored, and the possible coalescence mechanism is proposed. The lamellar crack steadily propagates at the early stage. The crack length sharply increases before the occurrence of coating spallation. The simulated coat spalling path is in line with the experimental result. Therefore, the new lamellar model developed in this work is beneficial to further reveal coating failure mechanism and predict coating lifetime.     

Comparison of fast fracture properties of thermoplastics

The fast fracture behavior of polyethylene, polycarbonate and poly(methyl methacrylate) are compared for two different thicknesses of material. The aim was to make fracture measurements under precisely defined steady load conditions after the material has reached a stress relaxed state. It was essential that the experimental technique provided good isolation between crack initiation and crack propagation. This was achieved in small rectangular shaped specimens by adding a tongue to one side extending the intended crack path. A fast crack was initiated in the tongue to propagate into the main section after it had been loaded and allowed to stress relax. When necessary the tongue was embrittled by cooling so a crack can be initiated with minimum force. This new method obtains the threshold load to just maintain crack propagation and also the increase of crack velocity with load above this threshold up to the limiting condition.     

Improvement of the Adhesive Fracture Toughness of Bonded Aluminum Joints Using E-Glass Fibers at Cryogenic Temperature

Fracture toughness and crack resistance of aluminum adhesive joints were measured at the cryogenic temperature of ?150°C, with respect to the orientation and volume fraction of the E-glass fibers in the epoxy adhesive. Cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed using two different test rates of 1.67 × 10^?2 and 8.33 × 10^?4 mm/s to observe the crack propagation trends. From the experiments, it was found that the DCB joints bonded with the epoxy adhesive reinforced with E-glass fibers not only showed a stable crack propagation with a low crack propagation speed, but also higher fracture toughness and crack resistance than those of the DCB joints bonded with the unreinforced epoxy adhesive at a cryogenic temperature of ?150°C.