Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

Experimental approach for microscale mechanical characterization of polymeric structured materials obtained by additive manufacturing

In this paper, an original experimental method is developed for local strain characterization at the surface of additively manufactured polymeric materials. The process used herein is material extrusion. This experimental method is based on the use of microscopic speckle pattern deposited at the surface of micro single edge notched specimen (μ_SENT) made of acrylonitrile butadiene styrene (ABS). Two configurations of filament orientation were used for the specimen manufacturing. Images of the μ_SENT specimen surface were recorded during in-situ tensile test. The quantitative analysis of images was made by digital image correlation (DIC). The evolutions of the local strain heterogeneities and the crack tip are evidenced on the kinematic fields. It is shown that the crack propagates in the low resistance path which is the interface area between filaments. It is also evidenced that the intersection of perpendicular filaments in two adjacent layers blocks crack growth. The local strain evolutions at the surface of the specimen are compared to the macroscopic response of the material. The method developed herein allows the determination of the materials mechanical properties. The identification of the crack tip location using digital image correlation (DIC) and J-integral calculation lead to plot the J-R curve. The J-R curves comparison of the two specimen configurations shows that the fracture toughness is directly related to the material structure.     

Measurement of rapid crack propagation in pressure pipes: A static S4 approach

A method for creating rapid crack propagation in pressurized pipes under slow static loading using modified S4 apparatus is described. In the development of the method a complexity involved with dynamic loading in the S4 test (ISO 13477) is eliminated by the use of a displacement controlled static loading machine. The experimental system consisted of an universal testing machine, a low compliance wedge loading device, notch tip quenching apparatus and a pipe specimen where a through thickness hole is drilled to accommodate the wedge loading device. The pipe specimen is made in such a way that a section containing a hole is free from the internal pressure, while the rest of the specimen is made to carry the internal pressure which would eventually drive the unstable crack along the pipe axis. The idea of such rapid crack initiation under static loading was derived from the concept of time-temperature equivalence, where impact loading may in part be simulated by lowering the temperature at the site of rapid crack initiation. The details of the method for rapid crack propagation under static loading are described and the correlation of the results to rapid crack propagation obtained by ISO 13477 is illustrated. The two methods were shown to compare quite well in terms of critical pressure determination and the details regarding normalized rapid crack length versus the internal pressure curve as well as the crack propagation pattern.     

应变速率和电位对应力腐蚀裂纹扩展的影响

以滑移-溶解-再钝化模型为基础,推导出应力腐蚀裂纹扩展速率与裂尖应变速率和电位之间的理论公式.计算表明,在裂纹扩展速率与裂尖应变速率的关系曲线中有两个特征区域.裂纹扩展速率在区域I随裂尖应变速率增加而增大,而在区域II不随裂尖应变速率的改变而变化.用慢应变速率拉伸技术（SSRT）测量了304L不锈钢的裂纹增长速率.当电位控制在区域II的阳极区时,理论计算的裂纹扩展速率与实验得到的结果比较吻合.     

Micromechanics of fatigue crack propagation in glassy thermoplastics

By the aid of the optical interference method the size of the craze zone at the crack tip has been measured during fatigue crack propagation (FCP) in two glassy thermoplastics thus giving a basis to re-examine proposed models. In contrast to previous assumptions it has been found, that in PMMA of high molecular weight crack propagation occurs only during a short interval of the loading cycle when the fibrils are stretched most severely and it is not limited by crack tip blunting; between the dimensions of the craze zone and the crack advance per cycle which is also reflected by markings on the fracture surface no simple correlation has been found. In PVC first the craze grows continuously during many loading cycles up to its final size and then the crack propagates by a jump separating the craze zone only partly. Thus at all stress intensity levels investigated the length of the final craze zone has been found to be distinctly larger than the jump spacing on the fracture surface. By aid of SEM-photography it is shown that in PVC during FCP cracking occurs by separation of fibrils instead of void coalescence.     

Photoelastic analysis of matrix crack-tilted fiber bundle interaction

Matrix crack-tilted fiber bundle interaction was explored using photoelasticity. First, the isochromatic fringe patterns near the matrix crack tip, either shielded by a tilted fiber bundle or crossed by a broken fiber bundle, were observed. Then, the stress intensity factors of cracks at varying distances from the tilted fiber bundle were extracted from the isochromatic fringe patterns. Finally, finite element simulation was conducted in ABAQUS software to verify the experimental results, and the difference between photoelasticity measurement and FEM simulation were discussed. The results show that the mode I stress intensity factor of the crack near a tilted fiber bundle increases with the increase of crack length and decreases with the increase of the Young's modulus of the fiber bundle. However, the mode II stress intensity factor, which clearly increases as crack length increased and, as opposed to mode I, increases as the Young's modulus of the fiber bundle increased.     

Comparison of caustics and the strain gage method for measuring mode I stress intensity factor of PMMA material

Both caustics and strain gage methods are shown to be effective in the determination of mode I stress intensity factor (SIF) at the crack tip. However, there is a lack of investigation with regard to the quantitative comparison of the two methods. In this paper, dynamic three-point bending tests were performed on polymethyl methacrylate (PMMA) specimens using the caustic and the strain gage methods simultaneously. The SIFs calculated by these two methods were analyzed and compared. The results show that the deviations of the SIF values between caustics and the strain gage method are slightly more significant when the orientation of the gage is obtuse rather than acute. Furthermore, this deviation increases with the gage mounted closer to the edge of the specimen when the orientation is acute. In addition, the whole cracking process could be clearly recorded by the caustic method, while the time-evolution of SIF can be roughly obtained with a limited number of strain gages during the crack propagation.     

Effect of Fiber Volume Fraction on the Flexural Properties of Unidirectional Carbon Fiber/Epoxy Composites

In this study, the effect of fiber content on the flexural property of continuous carbon fiber/epoxy composites was investigated. Samples with four different fiber volume fractions, 50, 60, 70, and 80 vol.%, were fabricated. For comparisons, cast epoxy resin was also prepared. It was observed that the flexural strength and modulus of this material are enhanced with increasing fiber volume fractions in the range of 50–70 vol.%. Results show that the carbon fiber/epoxy composites possess the largest fracture force and displacement when the fiber volume fraction is 70 vol.%, which is mainly attributed to the effective stress transfer of fibers. This can restrict crack tip propagation and blunt the crack tip, then consume abundant deformation energy and result in an increase of fracture work. On the other hand, poor flexural property was observed when the sample with high fiber volume fraction (80 vol.%) was tested. Three different types of failure modes were observed according to the fiber content.     

Ex situ and in situ characterization of stress corrosion cracking of nickel-base alloys at high temperature

Recent ex situ and in situ characterization of the major factors influencing primary water stress corrosion cracking (PWSCC) and outside diameter stress corrosion cracking (ODSCC) of nickel-base alloys of Alloy 600 and Alloy 690 as the structural materials in pressurized water reactors (PWRs) was introduced to understand cracking mechanisms. In the primary water environment of PWRs, the effect of stress on PWSCC was analyzed using the in situ direct-current potential drop method combined with electrochemical noise measurement. The compositional and microstructural changes around a crack tip including the oxide film formed during crack propagation were evaluated by using ex situ microscopic methods with energy-dispersive spectroscopy (EDS). The equivalent local strain was also evaluated by using the electron backscatter diffraction method as ex situ technique. With the aid of ex situ and in situ characterization, it was proposed that the intergranular precipitation of Cr oxides ahead of the crack tip by O penetration along the grain boundary that make the “oxidized grain boundary” can significantly increase the susceptibility to PWSCC propagation of the alloys. In the secondary water environment of PWRs, the properties of oxide film formed on the surface of the alloys were characterized as a function of the Pb content as impurity by using in situ methods of potentiodynamic polarization measurement and electrochemical impedance spectroscopy and ex situ techniques of EDS and X-ray photoelectron spectroscopy. From these ex situ and in situ characterizations, the degradation model of the passivity of the surface oxide film by Pb incorporation was proposed to understand the ODSCC mechanism.     

Mechanisms and micromechanics of fatigue crack propagation in glassy thermoplastics

An iterative approach is used to estimate, from interference optics measurements, the variation of refractive index and, hence, extension ratio along the length of a craze at the tip of a fatigue crack. The finite element method is used to compute craze surface stress distributions which are found to be similar to those obtained for static loading. High extension ratios, in the range 6 to 8 for retarded fatigue crack growth in poly(vinylchloride), are attained in the craze fibrils at the crack tip before crack jump occurs. The craze thickens primarily by surface drawing during the early stages of its growth but in the later stages the fibril creep mechanism predominates. The critical fibril extension ratio is not reached in a single cycle, as in normal fatigue crack propagation, and crack jump does not occur until, typically, after several hundreds of cycles during which the fibrils accumulate considerable damage.Presented in part at the 7th Int. Conference Deformation, Yield and Fracture of Polymers, Cambridge, UK, 11–14 April 1988.     

有机玻璃断裂韧性断面形态组份结构关系的研究

测定了有机玻璃的抗裂纹增长因子K_(1C)与材料组份、拉力机夹头速度的关系。发现增塑剂和交联剂对抗裂纹性能的影响,可从聚合物分子运动能力对裂纹端点塑性屈服过程的影响说明。在对断面形态、断面层厚度、折光指数以及K_(1C)值与拉力机夹头速度关系研究之后,认为用“撕布”模式和次级断裂模式分别解释慢裂纹、快裂纹扩展过程中的实验现象是合适的。     

Crack propagation from a preexisting flaw at a notch root

We study crack kinking from a preexisting crack initiated at a notch root. This makes it necessary to evaluate the stress intensity factors at the tip of the initial crack, as a function of the “stress intensity factor of the notch” (the multiplicative coefficient of the singular stress field at the notch root in the absence of the crack), the length of the crack, the aperture angle of the notch and the angle between its bisecting line and the direction of the crack. Applying Goldstein and Salganik's well-known principle of local symmetry yields then the prediction of the kink angle of the crack extension. It is found that although the notch is always predominantly loaded in “mode I”, this angle is generally not large enough for the crack tip to get closer to the bisecting line of the notch.     

Stepwise fatigue crack propagation in polyethylene resins of different molecular structure

Stepwise fatigue crack propagation in a range of polyethylene resins, some of which are candidates for use in pipes for natural gas distribution, was studied. Examination of the effect of molding conditions on fatigue crack propagation in a pipe resin indicated that fast cooling under pressure produced specimens with the same crack resistance as specimens taken from a pipe extruded from this resin. The mechanism of stepwise crack propagation in fatigue was the same as reported previously for creep loading. Observations of the region ahead of the arrested crack revealed a complex damage zone that consisted of a thick membrane at the crack tip followed by a main craze with subsidiary shear crazes that emerged from the crack tip at an angle to the main craze. The effects of molecular parameters, such as molecular weight, comonomer content, and branch distribution, on the kinetics of fatigue crack propagation were examined. Correlation of creep and fatigue crack resistance made it possible to relate fatigue fracture toughness to molecular parameters by invoking concepts of craze fibril stability developed for creep. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2355–2369, 1998     

连续碳纤维增强的聚芳醚酮复合材料的层间破坏

用双悬臂梁和端开口弯曲试件分别研究了连续碳纤维增强的聚芳醚酮复合材料(CF/PEK-C)的Ⅰ型和Ⅱ型的层间破坏。CF/PEK-C的Ⅰ型层破坏的线弹性断裂判据G_(Ⅰc)和弹塑性断裂判据J_(Ⅰc)分别为0.69KJ/m~2且与裂纹长度无关。CF/PEK-C的Ⅱ型层间破坏的稳定性,与裂纹和半距之比α/L有关。当α/L小于0.7时,表现为不稳定的Ⅱ型层间破坏的断裂韧性G_(Ⅱc)为1.62KJ/m~2。当α/L大于0.7时,则为稳定的Ⅱ型层间破坏。此时的G_(Ⅱc)与临界点的选择有关。由亚临界点和0.95点法得出的G_(Ⅱc)值分别为1.73和2.74KJ.M~2。     

Fatigue crack propagation mechanisms in poly(methyl methacrylate) by in situ observation with a scanning laser microscope

Crack propagation tests were performed on an amorphous polymer, poly(methyl methacrylate), to investigate fatigue crack propagation mechanisms. A scanning laser microscope with a newly developed tensile testing machine was used to observe in situ crack propagation in compact‐type specimens. A crack usually propagated within the craze located at the crack tip under both static and cyclic loading conditions. When a crack stably propagated into the craze under static loading conditions, bright bands composed of the broken craze were observed at the edges along the crack wakes. However, there were successive ridges and valleys in place of bright bands along the crack wakes under cyclic loading conditions. When stable fatigue cracks were propagated at the loading half‐cycle in each cycle, new craze fragments appeared that were similar to the bright bands under static loading. However, the thickness of these fragments decreased in the following loading cycle, and a new valley was formed. This suggested that the valleys were formed by the contact between the fracture surfaces near the crack tip during unloading. Fatigue crack propagation is thought to be due to fibrils weakened by crack closure between fracture surfaces. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 3103–3113, 2001     

Influence of empty hole on crack running in PMMA plate under dynamic loading

The influence of original prefabricated empty hole on dynamic crack propagation was studied. The dynamic behavior of crack propagation due to the impacting of drop hammer on the defective PMMA medium of manufactured hole defect (different extent of left shift, respectively, for L = 110 mm, L = 111.25 mm and L = 112.5 mm) was conducted using the test system of digital laser dynamic caustics (DLDC). The stress intensity factor and velocity at the running crack tip were analyzed. Moreover, the recently developed distinct lattice spring model (DLSM) was validated using the experimental data. It was found that the numerical modeling can reproduce the experimentally observed phenomena. Combining the experimental and numerical results, it can be concluded that the influence of empty hole defects of brittle materials on dynamic fracturing is great.     

Mechanism of fracture in glassy polymers. III. Direct observation of the craze ahead of the propagating crack in poly(methyl methacrylate) and polystyrene

Observation of optical interference fringes at the tip of a crack in a glassy polymer allows the construction of the configurations of the crack tip and the craze that precedes it. The craze extends 25 μ beyond the crack tip in poly(methyl methacrylate) and 550 μ beyond the tip in the polystyrene studied. The craze at the crack tip in PMMA may be seen to deform elastically as much as 100% under stress before crack propagation recommences. Such deformation is estimated to account for as much as 40% of the nominal Griffith energy of crack propagation.     

Durability study of flexible bonded joints: The effect of sustained loads in mode I fracture tests

Adhesives in bonded structures are exposed both to external loads and environmental conditions; durability studies are currently needed to assess their service lifetime. Conditioning strategies differ in considering external load conditions (such as stressed or not stressed) for the durability analysis of double cantilever beam (DCB) bonded joints. Different test procedures such as ASTM D3762 (wedge testing) or ISO-25217 (DCB testing) exist to characterise the evolution of the fracture strength and toughness found in bonded joints. These methods depend on crack-length measurements, however, and achieving an accurate visual determination may be difficult due to the large fracture process zones (FPZs) that develop in the adhesive layer, especially in flexible or degraded bonded joints. To compensate, crack-length-independent data-reduction methods such as the compliance-based beam method (CBBM) or the J-integral method can be used, but experimental research is lacking on the suitability of these methods in ageing tests. A lack of consensus also exists in testing methodologies to evaluate the durability of bonded joints, especially when examining flexible bonded joints. The present work evaluates the influence of damage on fracture toughness within flexible bonded joints exposed to service conditions. Wedge tests and DCB tests are conducted using DCB specimens bonded with a flexible structural adhesive, proving that the degradation of flexible bonded joints exposed to environmental conditions is significantly accelerated when external loads act on them. The findings show that crack length estimation is affected due to environmental effects and thus, that crack-length-dependent test methods are not applicable in ageing tests.