Fracture toughness of glasses as measured by the SCF and SEPB methods

The fracture toughness, K_Ic, of six glasses was measured by the surface crack in flexure (SCF) and single-edged precracked beam (SEPB) methods. Results depended upon the loading rate as well as the test environment. Environmentally-assisted slow crack growth affects the results for tests done in air. Dry nitrogen testing is preferred. Crack healing may be a severe complicating factor with precracked flexure bar type specimens if the specimens are unloaded between the precracking and final fracture test. Success in K_Ic testing depends to a large degree on upon the ability to make good precracks.     

Controlled Precracking of Fracture Mechanics Specimens Using Contact Stresses

A simple method is proposed for introducing precracks into thin-plate fracture mechanics specimens, i.e., compact-tension and double-cantilever-beam specimens, using standard testing equipment and without performing complicated machining operations on the specimens. The surface of the specimen is first scored to a known length, using a scribe, and the specimen is then compressed between polymer blocks. Mismatch between the elastic properties of the polymer and the specimen results in an in-plane tensile stress in the vicinity of the scratch that causes a crack to initiate from the scratch and propagate through the specimen thickness. Provided that certain conditions are met, the crack arrests without significant growth beyond the initial scratch length. The result is a straight, through-thickness precrack of controllable length. Fracture toughness measurements made on glass specimens precracked using the proposed method are in good agreement with literature values for this material.     

Crack growth mechanism in some polyamides

Fracture toughness was measured with single-edge notched three-point bend specimens. The materials used were semicrystalline polymers, polyamide-66, polyamide-1010 and polyamide-610. Their fracture surface was investigated with the help of scanning electron microscopy. The crack growth mechanism is revealed and a model suggested. The stable crack growth feature of the fracture surface is the dimples. The stable crack growth includes: the crack tip blunting, the voids initiating, coalescing and extending, the fibrillated polymer rupturing and contracting, then the formation of dimples on the fracture surface. The unstable crack growth formed a fan cleavage zone. The crack growth passed along the boundaries of spherulitic crystals in which many subcracks have occurred. The mechanism of discontinuous subcrack propagation in ductile polyamide-610 was discovered. In the compressive zone of the bend specimen, the fracture mechanism is similar to the cleavage zone and exhibits shear failure response.     

Flat R-Curve from Stable Propagation of Indentation Cracks in Coarse-Grained Alumina

The fracture toughness of coarse-grained A1₂O₃, known for pronounced "Iong"-crack R-curve behavior, was studied in the "short"-crack regime utilizing the stable propagation of indentation cracks in bending. A combination of in situ microscopic crack growth observations and mechanical testing enabled measurement of crack extension curves. They reflect the contributions of residual indentation stress intensity and applied bending stress intensity on the total crack driving stress intensity and allow determination of the residual stress factor χ and the toughness K_R. The results indicate that χ depends on indentation load and A_R is surprisingly constant rather than increasing. To resolve the latter contradiction with long-crack R-curve behavior, combined short/long-crack fracture tests were performed with the same specimens. Starting with stable indentation crack growth and continuing with stable long-crack extension, the previous toughness results were confirmed, i.e., constant toughness from indentation cracks and increasing toughness from long cracks. The influence of crack-opening behavior on bridging-controlled R-curve toughening can qualitatively explain the observed discrepancies.     

An experimental examination of fracture criteria using brittle polystyrene

A four-point bend test was performed on injection-molded bar-shaped polystyrene specimens with notches of varying depths. The material investigated was found to be linear elastic and brittle. From the load–displacement curves, various fracture toughness parameters based on energy release rate theories and stress–intensity factors were calculated. The stress concentrations arising from the presence of finite size notches with finite root radii were also calculated. The local stress at crack initiation was found to be nearly constant for all specimens investigated while the fracture toughness parameters based on energy release rates were not constant. A distinct change in the crack propagation behavior was observed when the curvature of the stiffness vs. notch depth curve changed sign, clearly defining the onset of unstable crack growth.     

Mixed-Mode Fracture Toughness of Ceramic Materials

An experimental technique whereby pure mode I, mode II, and combined mode I-mode II fracture toughness values of ceramic materials can be determined using four-point bend specimens containing sharp, through-thickness precracks is discussed. In this method, notched and fatigue-precracked specimens of brittle solids are subjected to combined mode I-mode II and pure mode II fracture under asymmetric four-point bend loading and to pure mode I under symmetric bend loading. A detailed finite element analysis of the test specimen is performed to obtain stress intensity factor calibrations for a wide range of loading states. The effectiveness of this method to provide reproducible combined mode I-mode II fracture toughness values is demonstrated with experimental results obtained for a polycrystalline Al₂O₃. Multiaxial fracture mechanics of the Al₂O₃ ceramic in combined modes I, II, and III are also described in conjunction with the recent experimental study of Suresh and Tschegg (1987). While the mode II fracture toughness of the alumina ceramic is comparable to the mode I fracture toughness K _Ic, the mode III fracture initiation toughness is 2.3 times higher than K _Ic. The predictions of fracture toughness and crack path based on various mixed-mode fracture theories are critically examined in the context of experimental observations, and possible effects of fracture abrasion on the apparent mixed-mode fracture resistance are highlighted. The significance and implications of the experimental methods used in this study are evaluated in the light of available techniques for multiaxial fracture testing of brittle solids.     

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).     

Fracture Toughness Anisotropy and Toughening Mechanisms of a Hot-pressed Alumina Reinforced with Silicon Carbide Whiskers

The fracture toughness of a hot-pressed alumina and that of a hot-pressed alumina/SiC-whisker composite containing 33 vol% SiC whiskers were measured by four-point bending on single-edge precracked bend bars having sharp precracks created by "bridge indentation." Two batches of the composite were examined, one exhibiting a greater degree of whisker clustering than the other. The fracture toughness of the alumina was around 4 MN·m^-3/2 whereas that of the composite varied between 5 and 8 MN·m^-3/2 depending on microstructural uniformity and crack-propagation direction. Crack deflection in combination with a change in fracture from intergranular to transgranular fracture is proposed as an explanation of the superior fracture toughness of SiC-whisker-reinforced alumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack-propagation direction in identical crack planes. This effect could with good accuracy be described in terms of crack deflection for the composite with uniform whisker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack-growth direction was greater and could not entirely be explained by crack deflection.     

Fracture toughness evaluation of polytetrafluoroethylene

The objective of this preliminary work was to explore the fracture resistance of polytetrafluoroethylene (PTFE) (DuPont tradename Teflon) as part of materials characterization work related to the development of “reactive” material projectiles. Little mechanical property data is available on this material since it is commonly used only as a coating material with the dominant properties being its low friction coefficient and high application temperature. Additional end products of the “7C” derivative, however, includes sheet, gaskets, bearing pads, piston rings, and diaphragms. In this work, standard ASTM E1820 fracture toughness specimens were machined from a 14‐mm‐thick sheet of this material obtained from NSWC Dahlgren Laboratory. These specimens were tested at three test temperatures and four test rates to determine if fracture would occur in this material, and if so, how the fracture toughness depends on the test temperature and specimen loading rate. Standard axial tensile specimens were also tested at quasi‐static and elevated loading rates at temperatures from ambient to ?73°C. The major results are that while crack extension is difficult at ambient (20°C) temperature, for temperatures slightly below ambient, a rapid degradation of fracture resistance occurs. This reduction in fracture resistance is enhanced by rapid loading, and the material loses approximately 75% of its toughness (fracture energy absorption ability) at ?18°C if the crack opening loading rate of the C(T) specimen approaches 0.25 m/s. Further reductions in temperature or increases in the loading rate appear to result in a reduced rate of degradation of fracture toughness.     

Tearing instability in polypropylene

Polypropylene can fail by tearing instability when the elastic contraction is greater than the plastic extension due to crack growth. Tearing instability theory developed by Paris and co-workers describes the effect of specimen geometry on the ductile fracture properties of polypropylene. Crack growth in compact tension specimens was always stable, but the stability of crack growth in double edge notched and three point bend specimens depended on the specimen's dimensions.     

Fracture Toughness of a Toughened Silicon Nitride by ASTM C 1421

The fracture toughness of a sintered reaction-bonded silicon nitride was measured by the single-edge precracked beam and surface crack in flexure methods, which are two of the three complementary test methods in ASTM C 1421. Results were compared with chevron-notched beam results that were available from another source. Precracks ranged from tiny artificial flaws introduced by Knoop indentation to millimeter-long precracks in single-edge precracked beams. The fracture toughness values from the three methods were in good agreement at 5.6 MPa·m^1/2.     

Systematic investigations of fatigue crack growth behavior of a PE‐HD pipe grade in through‐thickness direction

Fatigue crack growth (FCG) experiments were performed on a commercial high‐density polyethylene (PE‐HD) pipe grade. To investigate the influence of different specimen types on FCG results, tests were conducted using compact tension (CT) specimens and cracked round bars (CRB). The effects of frequency and R‐ratio on FCG behavior were also studied. Furthermore, FCG tests were interrupted in the region of stable crack propagation. The crack front and the front of the process zone ahead of the crack were systematically characterized via microscopic methods in the thickness direction of the specimen. The experimental data are employed to study the mechanisms of process zone development and to determine the effective crack length by compliance relationships. This detailed information allows modeling FCG in PE‐HD at various positions in the thickness direction of the specimen. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103:1745–1758, 2007     

A Fracture Toughness Test Using the Ball‐on‐Three‐Balls Test

A method for fracture toughness measurement of ceramics using small disks and plates is presented. Similar to the surface‐crack‐in‐flexure (SCF) method a semielliptical surface crack is introduced centrally into one plane side of the specimen which is fractured in a ball‐on‐three‐balls test. Finite element simulations are used to evaluate the stress intensity factor (SIF) for this loading geometry for a range of crack sizes and crack geometries. Empirical formulae for the geometric function are provided for evaluation of the test. The effect of position uncertainties is investigated using FEM and experiments. Other sources which may contribute to the measurement error are identified and quantified, resulting in recommendations for the practical realization of the test. A determination of the fracture toughness within ±10% measurement uncertainty is possible with specimens larger than 8 mm in diameter and thicker than 0.5 mm. With larger specimens an uncertainty comparable to other fracture toughness tests can be achieved. For precise measurements it is important to position the crack within ±120 μm of the stress maximum, to know Poisson's ratio exactly and to test cracks that have the maximum SIF at their deepest point. A method how this can be achieved is presented.     

Crack-Shape Effects for Indentation Fracture Toughness Measurements

Various methods to measure fracture toughness using in-dentation precracks were compared using soda-lime glass as a test material. In situ measurements of crack size as a function of applied stress allow both the toughness K_c and the residual-stress factor χ to be independently determined. Analysis of the data showed that stress intensity factors based on classical half-penny crack shapes overestimate toughness values and produce an apparent R -curve effect. This is due to a constraint on crack shape imposed by primary lateral cracks in soda—lime glass. Models based on elliptical cracks were developed to account for the crack-shape effects.     

On the Vickers Indentation Fracture Toughness Test

The Vickers indentation fracture toughness test, or VIF, is addressed by considering its origins and the numerous equations that have been applied along with the technique to estimate the fracture resistance, or the K _{I c} of ceramics. Initiation and propagation of cracks during the VIF test are described and contrasted with the pre-cracking and crack growth for internationally standardized fracture toughness tests. It is concluded that the VIF test technique is fundamentally different than standard fracture toughness tests. The VIF test has a complex three-dimensional crack system with substantial deformation residual stresses and damage around the cracks. The VIF test relates to an ill-defined crack arrest condition as opposed to the rapid crack propagation of the standardized fracture toughness tests.
Previously published fracture toughness results employing the VIF technique are reviewed. These reveal serious discrepancies in reported VIF fracture toughness values. Finally, recent fracture resistance measurements by the VIF technique for the Standard Reference Material SRM 2100 are presented. These are compared with standardized test results for the same material. It is concluded that the VIF technique is not reliable as a fracture toughness test for ceramics or for other brittle materials. What the VIF actually measures in terms of fracture resistance cannot be readily defined. It is recommended that the VIF technique no longer be acceptable for the fracture toughness testing of ceramic materials.     

Ductile tearing instability in some engineering thermoplastic blends

The stability of ductile crack growth in two engineering thermoplastic blends is examined. The blends are a poly(phenylene oxide)/nylon 6,6 blend and a poly(butylene terephthalate)/polycarbonate blend. Fracture tests were performed with single-edge notched specimens in tension and three-point bending over a wide range of test speeds. Both larger radius notches and longer specimens were found to promote ductile tearing instability. This behavior is attributed to the higher driving force for crack growth produced by increased elastic energy storage before crack initiation. Over a certain range of test speeds, these factors lead to a novel effect of notch sharpness on toughness; a sharp notch gives rise to a higher fracture energy than does a blunt notch. The results are discussed in terms of the tearing modulus concept developed by Paris and co-workers.     

Indentation Studies on Y2O3-Stabilized ZrO2: II, Toughness Determination from Stable Growth of Indentation-Induced Cracks

Stable indentation cracks were grown in four-point bend tests to study the fracture toughness of two Y₂O₃-stabilized ZrO₂ ceramics containing 3 and 4 mol% Y₂O₃. By combining microscopic in situ stable crack growth observations at discrete stresses with crack profile measurements, the dependence of toughness on crack extension was determined from crack extension plots, which graphically separate the crack driving residual stress intensity and applied stress intensity factors. Both materials exhibit steeply rising R -curves, with a plateau toughness of 4.5 and 3.1 Mpa·m^1/2 for the 3- and 4-mol% materials, respectively. The magnitude of the plateau toughness reflects the fraction of tetragonal grains contributing to transformation toughening.     

Sliding mode interlaminar fracture toughness of interply hybrid composite materials

To measure the sliding mode interlaminar fracture toughness of interply hybrid composites, end notched flexure (ENF) specimens with three different types of stacking sequence have been utilized. Finite element analysis is applied to separate the contribution from different modes on the strain energy release rate. In addition, the methods of beam theory, compliance, and compliance calibration to calculate the G_C values are compared. The effects of interface friction, crack length, and specimen width are also discussed. The results show that the crack growth in the three types of specimens is dominated by the sliding mode and the Mode II interlaminar fracture toughness can be approximated. The compliance method is not recommended for hybrid ENF specimens and the effects of interface friction can be neglected. To get rid of the edge effect, the specimen width must be carefully chosen, while the fracture toughness does increase with the initial crack length.     

Effect of Precrack "Halos" on Fracture Toughness Determined by the Surface Crack in Flexure Method

The surface crack in flexure method, which is used to determine the fracture toughness of dense ceramics, necessitates the measurement of precrack sizes by fractographic examination. Stable crack extension may occur from flaws under ambient, room-temperature conditions, even in the relatively short time under load during fast fracture strength or fracture toughness testing. In this article, fractographic techniques are used to characterize evidence of stable crack extension, a "halo," around Knoop indentation surface cracks. Optical examination of the fracture surfaces of a high-purity Al₂O₃, an AlN, a glass-ceramic, and a MgF₂ reveal the presence of a halo around the periphery of each precrack. The halo in the AlN is merely an optical effect due to crack reorientation, whereas the halo in the MgF₂ is due to indentation-induced residual stresses initiating crack growth. However, for the Al₂O₃ and the glass-ceramic, environmentally assisted slow crack growth is the cause of the halo. In the latter two materials, this stable crack extension must be included as part of the critical crack size to determine the appropriate fracture toughness.     

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.