Fracture toughness and crack instability in tough polymers under plane strain conditions

The plane strain fracture toughness and fracture mechanisms of several tough engineering plastics have been studied and compared with poly(methyl methacrylate) (PMMA), a relatively brittle polymer. The tough polymers all are observed to form a multiple craze zone at the crack tip, which is shown to be the primary source of plane strain fracture toughness in these materials. The multiple craze zone is retained during slow crack growth but is metastable, and at a critical stress intensity and associated crack velocity, the system passes through a transition to a greatly accelerated single craze mode of unstable propagation.     

Orientation and strain cycle effects on the impact performance of polyethylene

The effects of orientation by plastic strain on the impact fracture resistance of a pipe‐grade polyethylene have been investigated. Isotropic samples of bulk polymer were subjected, by plane‐strain compression, to uniform Hencky strains of up to ±40%. In some samples this strain was reversed to restore the original dimensions. Impact bend specimens were prepared from samples oriented either normal to or within the fracture plane. Plane‐strain fracture resistance and transition temperature were measured at 1 m/s by using the ISO 17281 method, and plane stress fracture resistance was measured by using the Reversed Charpy test. Orientation within the fracture plane by plastic compression across it compromises the relatively high plane‐stress toughness of this material and increases the brittle‐tough transition temperature, while the opposite is true of plastic extension. Reversion from a state of adverse orientation, by completing a strain cycle, only partially restores the fracture resistance of the isotropic polymer. POLYM. ENG. SCI., 45:596–605, 2005. © 2005 Society of Plastics Engineers     

Designing against fracture in brittle plastics

The fracture toughness of a variety of sharply notched tension, bending and rotating disc specimens of PMMA is examined using linear fracture mechanics. It is observed that rapid fracture with a brittle glassy appearance usually follows a period of slow crack growth, denoted by fan shaped markings of local ductility, though still brittle overall. In this near brittle regime the fracture toughness is sensitive to strain rate so that high values of effective surface energy are easily induced by rapid testing or notch bluntness. At impact rates the toughness increases again. For design purposes, in the absence of environmental effects, the onset of slow cracking and rapid (glassy) fracture, can be associated with fracture toughness K_1c of about 800 Ibf/in^3/2 (90 kg/cm^3/2) and 1600 Ibf/in^3/2 (180 kg/cm^3/2) respectively. Detailed studies have not been made on other materials but a guide to the levels of notch toughness and notch brittle temperatures are given for several plastics.     

Correlation of Tensile Properties of Tough Amorphous Polymers with Internal Friction

Abstract

The ultimate stress-strain behavior of five tough amorphous polymers was studied at temperatures from 4.2 to 300°K using an Instron tensile tester which was adapted for cryogenic measurements. The polymers were found to fail by one of three modes depending upon test temperature and sample pretreatment condition. The transition from a general ductile behavior to brittle fracture was accompanied by a maximum in toughness which could be correlated with the γ transition in these polymers. At still lower temperatures there was a change in the brittle failure which correlated with the magnitude of the internal friction intensity of δ = 0.007 – 0.020. This transition in brittle fracture mode was characterized by a maximum in the brittle fracture stress. It is proposed that the brittle fracture at very low temperature occurs at abnormally low stresses due to stress concentrations factors which can not be relieved since molecular mobility becomes greatly restricted under these cryogenic conditions.     

Characterizing environment-dependent fracture mechanisms of ceramic matrix composites via digital image correlation

Here, we unveil a methodology for a novel assessment of the fracture mechanics of SiC/SiC ceramic matrix composites enabled by in situ stereoscopic digital image correlation to quantify in-process flexural strain and crack opening displacement measurements. This technique isolates individual cracks on the composite surface as discontinuities in the spatial displacement field and correlates key fracture characteristics with the flexural strain of composite specimens during coupled four-point bend / hermeticity testing. Fracture was observed along the specimen length, originating at the tensile underside and propagating around the circumference of the tubular specimens with generally uniform spacing. Multiple specimens were also tested after heat treatments to 1200°C in open air, in vacuum, and in helium for 48 h to evaluate the environmental effects on the fracture mechanisms of SiC/SiC composites, which revealed degradation of flexural properties after treatment in open air resulting in brittle failure. Indentation-based fracture toughness measurements were performed, which confirmed a 25% reduction in toughness after open air heat treatment relative to the other heat treatments. This assessment indicated that significant oxidation may occur within the composites from these heat treatments and suggested that further protection of the composites may be necessary for high-temperature applications.     

Comparison of the sandwiched beam (SB) and opposite roller loading (ORL) techniques for the pre-cracking of brittle materials

The “sandwiched beam” (SB) and “opposite roller loading” (ORL) methodologies suitable to introduce sharp through-thickness cracks in brittle materials are critically reviewed and compared in this work. In both cases a sharp crack is obtained in a notched specimen by means of a suitable loading. In the SB technique the specimen is placed between two support bars and bent in a 3- or 4-point configuration. The ORL procedure is based on the symmetrical loading by four rollers which induces a local tensile stress. Results show that both techniques are successfully usable on brittle materials: in both cases suitable specimens are obtained for fracture toughness measurements. The crack length can be reasonably controlled and varies in a wide range. The SB procedure typically provides cracks with α≌0.5, while shorter cracks are obtained by the ORL technique. Fracture toughness is measured on specimens prepared using the two techniques. The obtained values result in good agreement with literature data.     

Toughness of high‐density polyethylene in plane‐strain fracture

Under uniaxial tension, high‐density polyethylene (HDPE) often fractures in a ductile manner. The fracture involves extensive necking that is known to occur in the plane‐stress condition. However, for large‐scale HDPE products like polyethylene pipe, crack can grow rapidly in a brittle manner. This type of fracture is known to be in the plane‐strain condition, which has much lower toughness than that in the plane‐stress condition. In this study, we extended the concept of essential work of fracture (EWF) to the plane‐strain condition and evaluated the fracture toughness of extruded HDPE plate. The results show that the specific work of fracture in the plane‐strain condition is also a linear function of the specimen ligament length. Therefore, the corresponding EWF value can be determined in a similar manner, that is, through linear extrapolation to zero ligament length. For HDPE, the study found that the plane‐strain toughness is about one order of magnitude smaller than the plane‐stress counterpart. Interestingly, the plane‐strain EWF value is very close to the estimated toughness of pressurized polyethylene pipe, which was reported in the literature, predicted using numerical simulation based on data from the standard small‐scale steady‐state (S4) test. POLYM. ENG. SCI. 46:1428–1432, 2006. © 2006 Society of Plastics Engineers.     

Self‐reinforcement of high‐density polyethylene/low‐density polyethylene prepared by oscillating packing injection molding under low pressure

This article reports the toughness improvement of high‐density polyethylene (HDPE) by low‐density polyethylene (LDPE) in oscillating packing injection molding, whereas tensile strength and modulus are greatly enhanced by oscillating packing at the same time. Compared with self‐reinforced pure HDPE, the tensile strength of HDPE/LDPE (80/20 wt %) keeps at the same level, and toughness increases. Multilayer structure on the fracture surface of self‐reinforced HDPE/LDPE specimens can be observed by scanning electron microscope. The central layer of the fracture surface breaks in a ductile manner, whereas the break of shear layer is somewhat brittle. The strength and modulus increase is due to the high orientation of macromolecules along the flow direction, refined crystallization, and shish‐kebab crystals. Differential scanning calorimetry and wide‐angle X‐ray diffraction find cocrystallization occurs between HDPE and LDPE. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 799–804, 1999     

High-Strength Alumina/Alumina:Calcium-Hexaluminate Layer Composites

The strength degradation of alumina/alumina:calcium-hexaluminate/alumina trilayers, after damage from Hertzian contacts, is evaluated. Relative to the monolithic alumina and alumina:calcium-hexaluminate constituent layer materials, the trilayers show markedly improved strength retention in the damaged state at high contact loads. The outer, fine-grained alumina layers are classically brittle, characterized by cone cracking, whereas the inner alumina:calcium hexaluminate layer is essentially quasi-plastic, with a well-defined "yield" zone that consists of distributed microdamage. The improved strength behavior of the trilayer composite is rationalized in terms of a synergistic interaction between the contact-induced deformation modes in the two layers, with each mode partially ne-gating the effectiveness of the other as a source of failure. This result offers the prospect of hybrid structures with hard outer layers, to provide wear resistance, and soft, tough underlayers, to inhibit brittle fracture.     

The mechanical performance of cross-plied composites

The mechanical performance of fiber glass epoxy, cross-plied laminates having either ductile or brittle matrices, was evaluated in tension. The laminates with ductile matrices have higher initial strength, slower crack propagation in the transverse layers and higher ultimate stress and strain. In both the ductile and brittle systems, the laminae have higher strength, stiffness and toughness than equivalent unidirectional composites. This improved performance results from the interaction between the perpendicular layers which gives them additional stiffness due to shear and transverse coupling effects and also increases the resistance of each individual layer to crack propagation and plastic flow.     

The coated inclusion effect on the mode of fracture of encapsulated particulates

The fracture toughness of hard plates, reinforced with brittle inclusions, can be altered by introducing in these composites an intermediate thin layer made of a deformable phase. This was proved in this study, where cracked PMMA plates including bonded rubbery encapsulations inside their matrices were subjected to static or dynamic tensile loadings. The improved toughness of these composite plates at high strain rates, comparative to the respective toughness of homogeneous plates, indicated the beneficial role of the rubber encapsulation in the failure of these composite plates. While studying the fracture behavior of the plates at different strain rates, it was observed that the propagating crack, when approaching the inclusion, follows an interfacial path along either the external or the internal surface of the soft coating or propagates and stops inside the core inclusion, depending on the applied strain rate. Throughout the experimental analysis the method of dynamic caustics was used for the evaluation of the stress intensity factor and crack velocity. Interesting results were obtained by studying the failure characteristics of these composite plates, as well as the variation of fracture toughness and the stress intensity factors depending on crack velocities. By considering that the plates examined constitute a satisfactory model for the representative element of the particulate material, the results found are useful because they provide an insight into the behavior of such particulates.     

Plane strain fracture toughness of polyethylene pipe materials

The plane strain fracture toughness of medium density polyethylene pipe materials has been investigated over a range of test temperatures and rates. Conditions are defined under which valid fracture toughness values can be obtained; at higher temperatures the material is notch-insensitive. Fracture surface morphology is described, and features are compared with predictions from the Dugdale model. The toughness derives from a band of fibrillar, drawn morphology associated with crack initiation or slow growth. The plane strain fracture toughness correlates with percent crystallinity according to the same relationship whether the crystallinity is varied by thermal treatment, comonomer content, or molecular weight.     

Fracture studies on a polyacetal—new thermoplastic polyurethane blend

Studies based on fracture mechanics have been made on polyacetal toughened with a synthesized thermoplastic polyurethane. The effects of elongation rate and defects like holes and notches on tensile properties have been investigated. Using three-point bending specimens, fracture mechanics parameters such as strain energy release rate, G, Rice's contour integral, J, and fracture toughness, K have also been determined under plain stress conditions. The mechanical loss from the hysteresis curves, and ductile, brittle fractography using scanning electron microscopy have also been studied.     

Fracture Resistance Measurement Method for in situ Observation of Crack Mechanisms

A special test fixture has been developed for fracture mechanical testing of brittle materials inside an environmental scanning electron microscope. The fixture loads a double cantilever beam specimen with pure bending moments and provides stable crack growth. Crack growth is detected by in situ observation and acoustic emission. As an example, crack growth in a cubic-phase yttria-stabilized zirconia is detected easily by in situ observation of the crack-tip region. Many fracture toughness measurements are obtained for each specimen, giving high confidence in the measured fracture toughness value. In situ observation is useful for the study of toughening mechanisms and subcritical crack-growth behavior and to sort out erroneous measurements (e.g., due to crack branching).     

Experimental approach to mechanical property variability through a high‐density polyethylene gas pipe wall

An experimental investigation was designed to establish the distribution of mechanical properties throughout a high‐density polyethylene (HDPE) gas pipe wall. The proposed approach used a continuous and uniform filament that was automatically machined from the pipe on a precision lathe at a very low cutting speed and an optimal depth of cut to minimize heating and structural disturbances. Typical engineering stress–strain curves, in every layer, were obtained on a testing machine especially designed for polymers, and they were statistically analyzed. The stress–strain behavior of HDPE pipe material could basically be divided into three distinctive zones, the second of which remained important. The average stress level illustrating cold drawing for a given layer was almost constant throughout the pipe wall. The measured stresses and moduli correlated very well with the pipe thickness, and they increased from the outer layers toward the inner layers. This was explained by the crystallinity evolution because the pipe production process was based on a convective water‐cooling system with a temperature gradient, which generated residual stresses. Computed statistical stress–strain correlations at yielding, the onset of cold drawing, and fracture points revealed acceptable linear relations for an error level of p ≤ 0.05. On the other hand, an increasing linear correlation characterized the relationship of the yield stress and elastic modulus. This result was confirmed by literature for standard specimens, prepared by compression molding, that did not represent an actual pipe structure with respect to an extrusion thermomechanical history. Such an approach to mechanical property variability within an HDPE pipe wall highlighted the complexity of the hierarchical structure behavior in terms of stress–strain and long‐term brittle failure. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 272–281, 2005     

A New Method for Precracking Beam for Fracture Toughness Experiments

A simple and versatile precracking method using a triangular notch as a crack starter in limited bending was developed, which is suitable for both brittle ceramics and quasi-plastic materials that are difficult to precrack by the conventional bridge-indentation technique. Slow growth of large crack in brittle or quasi-brittle ceramics was controlled and observed in situ in this way. The precracking tests performed on various ceramics exhibited high reliability and feasibility. The precracked specimens were subsequently used to measure the fracture toughness, and the resultant data showed that the fracture toughness determined by using the precracked specimens reflected the minimum value of the toughness measured in single edge-notched beam (SENB) tests.     

Fracture Behavior of the Space Shuttle Thermal Protection System

Stable crack-growth and fracture-toughness experiments were conducted using precracked specimens machined from LI-900 reusable surface insulation (RSI) tiles of the space shuttle thermal protection system (TPS) at room temperature. Similar fracture experiments were conducted on fracture specimens with preexisting cracks at the interface of the tile and the strain isolation pad (SIP). Stable crack growth was not observed in the LI-900 tile fracture specimens which had a fracture toughness of 12.0 kPaμm. The intermittent subcritical crack growth at the tile-pad interface of the fracture specimens was attributed to successive local pull-outs due to tensile overload in the LI-900 tile and cannot be characterized by linear elastic fracture mechanics. No subcritical interfacial crack growth was observed in the fracture specimens with densified LI-900 tiles where brittle fracture initiated at an interior point away from the densification.     

Accurate measurement of fracture toughness of free standing diamond films by three-point bending tests with sharp pre-cracked specimens

The three-point bending test was used for the first time for evaluating plane strain fracture toughness, K_IC, of free-standing pre-cracked diamond film specimens. A novel approach to introduce sharp cracks into diamond films was presented. This approach, with a success rate of approximately 80%, allowed the initiation and growth of sharp cracks at the top of the notch in diamond films being observed and controlled in situ under a scanning electron microscope (SEM). This made a more accurate and reliable measurement of K_IC in diamond films possible. Fracture toughness values obtained with the sharp pre-cracked specimens were compared with those of simply laser-notched ones. Results showed that with pre-cracked specimens, the measured fracture toughness would be approximately 15% lower than that of laser-notched specimens with notch widths of 0.1–0.12 mm. The method provided for accurate determination of plane strain fracture toughness which may help to clarify the discrepancies and to understand the mechanical properties of CVD diamond films.