Interpretation of results of full notch creep test and comparison with notched pipe test

Abstract

An investigation of two tests commonly used to determine resistance to slow crack growth in PE pipes and materials is detailed, in order to gain a greater understanding of the mechanisms involved and to resolve differences in results observed. The full notch creep test (FNCT) is carried out on small notched bars machined from sheet or pipe loaded to create high constraint at the notch tip. The notched pipe test (NPT) is a pressure test on pipe containing external machined notches. In this test, it has been observed that the use of more flexible materials allows deformation in the crack tip region and contributes to slow crack growth resistance via crack tip blunting. Good pipe performance can be achieved by selecting materials with high inherent slow crack growth resistance or by combining inherent resistance with blunting mechanisms promoted by a lower density material. It is concluded that the FNCT test, while useful for an indication of inherent slow crack growth resistance, cannot be used to predict pipe performance for a range of materials, and therefore is unsuitable as a reference test for a pipe product specification. The NPT test remains the benchmark test for pipe performance and is referenced by many specifications.     

Notched Fracture Behavior of an Oxide/Oxide Ceramic-Matrix Composite

The fracture behavior of an oxide/oxide ceramic-matrix composite, alumina/alumina-silica (Nextel610/AS), was investigated at 23° and 950°C using a single edge notched specimen geometry with clamped ends. Crack growth and damage progression were monitored during the tests using optical microscopy, ultrasonic C-scans, and crack mouth opening displacement. The net section strength of Nextel610/AS was less than the unnotched ultimate tensile strength. The failure mode was nonbrittle with considerable nonlinear deformation prior to and after the peak load at 23° and 950°C. The effect of temperature on the notched strength was significant. Net section failure stress decreased 50% when temperature was increased from 23° to 950°C. Observations of damage progression indicated that the reduction in notch strength with temperature was associated with self-similar crack growth at 950°C. Ultrasonic C-scans were found to be an effective method of monitoring damage progression. Ultrasonic attenuation ahead of the notch tip was correlated with surface matrix cracks and exposed fiber lengths on the fracture surface.     

Crack propagation in biaxially oriented polypropylene at low temperature

The crack growth behavior of polypropylene biaxially oriented by cross-rolling was studied at low temperature. Single edge notch testing produced a stable tearing type of crack growth in both 50% and 80% biaxially oriented polypropylene at ?40°C, in contrast to the brittle fracture of unoriented polypropylene. The crack growth in the two oriented materials began slowly and accelerated to a constant rate that was higher in the 80% oriented material than in the 50% oriented material. The main difference between the crack growth behavior of the two was the longer period of initial slow growth in the case of 80% orientation. This period of slow growth corresponded to crack growth through the notch tip damage zone. Residual strength diagrams were used to present the crack growth data obtained when the stress state was intermediate between plane stress and plane strain. Fractography revealed large differences among the fracture surfaces of the three materials with the unoriented polypropylene showing a grainy appearance from the brittle fracture. The two oriented materials showed considerable ductility. The 50% oriented material showed many voids in the fracture surface, indicating that voiding during the fracture process contributed significantly to the toughness improvement. The 80% oriented polypropylene showed delamination crazing on the fracture surface with layered material and fibrils bridging the crazes.     

Effect of short chain branching on the viscoelastic behavior during fatigue fracture of medium density ethylene copolymers

A method to determine viscoelastic changes in medium density polyethylene (MDPE) pipe specimens associated with the crack tip during fatigue crack initiation (FCI) and propagation (FCP) experiments is described. The load-displacement curves are analyzed to obtain the phase angle, δ. Changes in δ are related to the number of cycles of crack initiation of three different MDPE copolymers: hexene (H), butene (B), and methyl pentene (MP) copolymers. These changes are related to craze formation and growth at the notch tip, leading to crack initiation and to the irreversible work, W_i, expended on them. Within a given material, step wise increments in δ distinguish the onset of crack initiation and the brittle-to-ductile transition in crack growth. The magnitudes of tan δ and W_i are noted to be in quantitative agreement with the resistance of the three copolymers to FCI and brittle propagation that rank in the order: isobutyl (MP) > ethyl (B) > butyl (H). Similar crystallinity of the three copolymers insinuates a hypothesis that variance in the nature of chain entanglements associated with the respective branch type might be accountable for the observed differences in viscoelastic character. The final stage of failure by ductile tearing is dominated by large scale plastic flow that seemingly overshadows the material differences governing time dependent brittle fracture.     

Fracture toughness of acrylonitrile-butadiene-styrene by J-integral methods

The fracture toughness of acrylonitrile-butadiene-styrene (ABS) was determined by three J-integral methods, ASTM E813-81, E813-87, and by hysteresis. The critical J values (J_1c) obtained are fairly independent of the specimen thickness, ranging from 10 to 15 mm. ASTM E813-81 and hysteresis methods result in comparable J_1c values, whereas the ASTM E813-87 was ～40% to 50% higher. The critical displacement determined from the plots of hysteresis (energy or ratio) and the true crack grow length vs. displacement are close. This indicates the critical displacement determined by the hysteresis method is indeed the displacement at onset of crack initiation, and the corresponding J_1c represents a physical event of crack initiation. The elastic storage energy. The input energy minus the hysteresis energy, is the most important factor in determining the onset of crack initiation. The critical elastic storage energy (at the beginning of crack growth) was found close to the J_1c obtained from the E813-81 or the hysteresis method.     

Mode-l Fracture Behaviour of Adhesive Joints. Part II. Stress Analysis and Constraint Parameters

The constraint effect on the fracture behaviour of a rubber-modified epoxy was investigated using compact tension (CT) adhesive joints. An elastic-plastic finite element analysis was conducted to evaluate the stress distribution ahead of the crack tip in the bulk adhesive and adhesive joints of different bond thickness. The models with sharp and finite radius crack tips were evaluated in the analyses. The constraint effect of adherends on the stress triaxiality ahead of the crack tip in the adhesive joints were discussed. The constraint parameters were investigated using the J-Q theory and the J-CTOD relationship. It was found that as the adhesive thickness was increased, the stress triaxiality ahead of the crack tip was relieved by the remarkable deformation of the adhesive material. Similarly, the crack tip constraint was reduced with increasing bond thickness so that the fracture energy increased towards the value of the bulk adhesive. A higher constraint was associated with a lower fracture energy and vice versa. Furthermore, the J-integral did not have a unique relationship with the crack-tip opening displacement (CTOD) for different adhesive bond thickness, as this depends on the constraint around the crack tip. The results of this study will help improve reliability assessment of adhesive joints in engineering applications.     

Numerical analysis of notch-tip fields in rubber-modified epoxies

In this work, the near-tip fields in notched specimens of pressure-sensitive nonporous and porous materials are investigated by finite element analysis. The specimen geometry and material properties are adopted from the corresponding experiments on rubber-modified epoxies. The Drucker-Prager yield criterion is first used to describe the yielding of nonporous materials. The yielding behavior of porous materials is based on a generalized Gurson yield criterion. The yield criterion for porous materials accounts for both the matrix material pressure sensitivity and the macroscopic pressure sensitivity due to porosity. Modifications are made on the yield criterion under negative mean stresses in order to account for the specific loading and geometry of the specimen. The computational results are compared with observed experimental cavitation zones and intense shear zones near the notch tip in specimens. Moreover, the near-tip fields and crack initiation sites ahead of the notch tip related to the volume fraction of rubber particles are investigated. The computational results suggest that the lowering of the mean stress ahead of the tip in rubber-modified epoxies with higher volume fractions of rubber changes the fracture mode from being controlled by high mean stresses at the elastic-plastic boundary to being controlled by large plastic strains closer to the notch tip.     

Fatigue crack growth in polyethylene

Fatigue crack propagation (FCP) rates are studied in 6 mm thick specimens of high density polyethylene (HDPE) containing razor notches, Centrally-notched plates and single-edg notched bars are subjected to sinusoidal tension-compressio or tension-zero cycling at 0.5 or 2.0 Hz under load control a room temperature; crack growth is monitored using a travelling microscope. After many thousands of cycles with no observable damage at the tip of the razor notch, a craze like zone begins to form. This zone grows slowly until it reaches the length characteristic of a mature crack at the same ΔK. Crack growth proper then begins. The number of cycles to initiate crack growth falls linearly with increasing ΔK at the razor notch Subsequent crack growth is determined both by the current value of ΔK and by loading history. When ΔK is increasing, FCP rates follow a standard Paris law curve. However, reduced, FCP rates are observed following an overload.     

Measurement of cohesive zone parameters in tough polyethylene

The initiation, growth and final failure of the craze ahead of a crack tip generally controls the onset of crack growth in polyethylene, particularly in slow crack growth. In this research, circumferentially deep notched tensile specimens are used to analyze the evolution and failure of crazes in polyethylene under plane strain conditions. An experimental method is used wherein the traction‐separation behavior of the craze structure is measured directly in‐situ. Results yield a cohesive zone type analysis in which a two parameter traction curve, containing within it all information pertaining to local decohesion, fully describes the separation of the interface. Experimentally measured rate dependent trends in the work of separation (I) provide good discrimination between different grades of tough polyethylene at both high and quasi‐static test speeds, and highlight the exceptional long term behavior of one particular grade. The method also allows for the quantification of the long term behavior of each grade by more traditional stress‐time analysis.     

Strain rate dependence of failure processes in polycarbonate and nylon

A study has been made of two types of failure, namely, monotonic fractures using Charpy-type specimens and fatigue crack propagation using rectangular plates containing an initial central notch. The work was conducted on an amorphous polymer (polycarbonate) and a semicrystalline polymer (nylon N 6.6). Monotonic tests were performed in an Instron testing machine between 0.002 and 20 in./min, and in a Charpy testing machine between 2000 and 11800 in./min. The cyclic tests (under constant K conditions) were carried out at frequencies that ranged from 0.1 to 20 Hz. A model for the relationship between the cyclic rate of crack growth and appropriate LEFM parameters, previously described, has now been converted into cyclic strain energy transformations. In computing the strain energy, the value of the time-dependent modulus of the material was used; and under cyclic loading conditions the glassy (short time) value was employed. The authors have proposed that the modulus measurements obtained at very low temperatures, where the viscous response of the material is highly restricted, will approximate the glassy value, E_g, found by conducting relaxation measurement tests at different temperatures down to ?197°C. Within the range of tests conducted, the fracture toughness values of both PC and N 6.6 apparently decrease with increase in loading rate. Fatigue crack growth in both materials is influenced by loading frequency and cyclic waveform. These variations may be related to the magnitude of the viscous energy loss and hence to the available energy for crack extension per cycle.     

Polycarbonate and co-continuous polycarbonate/ABS blends: influence of notch radius

The influence of notch tip radius in the range of 1-0.002 mm was studied on polycarbonate (PC) and co-continuous PC/acrylonitrile-butadiene-styrene (ABS). Co-continuous PC/ABS blend was obtained by mixing PC and ABS containing 15% polybutadiene (PB) in a twin screw extruder. PC and PC/ABS specimens were injection moulded into test bars. A notch was milled-in, with notch tip radius of 1, 0.5, 0.25 and 0.1 mm. Very sharp notches with a radius of 0.015-0.002 mm were obtained with an Excimer LAZER. The specimens were tested by single edge notch tensile tests at 1 m/s (apparent strain rate 28.5 s⁻¹) and at different temperatures (−60 to 130 °C). Initiation and propagation phases of the fracture process were monitored and the brittle-ductile transition temperature (T_bd) determined. It appeared that the amount of deformation in the initiation phase of fracture was extremely sensitive to notch tip radius. Temperature measurements of the deformation zone showed that the size of the deformation zone decreased with decreasing notch radius. The T_bd of PC increased rapidly with decreasing notch radius, until the glass transition temperature was approached. Remarkably, for PC the notch sensitivity was strongest around the standard notch tip radius of 0.25 mm. This means that a small deviation of this standard notch leads to large deviations in the results. The PC/ABS blend was much less sensitive to notch tip radius and the T_bd was almost constant. Thus the sensitivity of PC to sharp defects can be neutralised by adding ABS.     

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.     

Fatigue behavior of medium-density polyethylene pipes

A round bar specimen and a square bar specimen cut out from medium-density polyethylene pipes with a notch were made and a fatigue test was conducted to cause a brittle fracture. The initiation and growth of a craze and crack at the tip of a notch was observed. In the range where loading cycles are few and displacement of the specimen does not increase, the craze prior to crack initiation occurs. Also, the effect of frequency was investigated. The pure creep failure and the fatigue failure at low frequency were compared. The lower the frequency, the smaller the reciprocal of the actual loading time T_f becomes. It is also found that this tensile fatigue test is a useful test method to assure the quality of pipes.     

Influence of viscosity and capillary force on the relationship between crack toughness and crack velocity for an acrylonitrile-butadiene-styrene plastic in environmental stress cracking liquids

The dependence of crack propagation energy R of an acrylonitrile-butadiene-styrene (ABS) resin on crack velocity was characterized in air and in several organic liquids. In the liquids an abrupt transition in R from a level like that in air down to a level characteristic of the liquid occurred with decreasing crack velocity. The velocity at the transition varied strongly with liquid viscosity. A simple model of each craze at the crack tip as a set of pipes through which liquid flows, driven by capillary force and retarded by viscous drag, serves to predict the transition velocity.     

A comparison of the crack tip damage zone for fracture of Hexcel F185 neat resin and T6T145/F185 composite

Hexcel F185 neat resin and T6T145/F185 graphite fiber-reinforced composite were subjected to Mode I loading in the compact tension (CT) geometry (fibers parallel to the crack) and the energy per unit area of crack extension, J_IC, determined to be 8100 and 1600 J/m² respectively. In-situ fracture studies using scanning electron microscopy on a CT-type specimen of F185 showed extensive microcracking in a damage zone ahead of the crack tip, which was similar to the microcracking observed in the whitened area ahead of the crack tip in the macroscopic CT specimens. A simple calculation using a rule of mixtures approach suggests that the diminished size of the damage zone and the presence of rigid fibers in the damage zone in the composite are not a sufficient explanation for the significantly lower delamination toughness of the composite compared to the neat resin. From this it may be inferred that the strain to failure locally in the damage zone ahead of the crack in the composite may also be lower than that which can be tolerated in the neat resin. Evidence for this idea comes from the observation that microcrack coalescence seems to occur preferentially at the fiber/resin interface.     

A fatigue crack initiation map for polycarbonate

The mechanisms of fatigue crack initiation for various stress levels and thicknesses have been determined for single-edge notched specimens of polycarbonate and used to assemble a map. Three basic fatigue crack initiation mechanisms were identified and named as cooperative ductile (the damage zone formed ahead of crack consisting of yielded material), solo-crack brittle (very little damage zone development), and cooperative brittle (identified as a cloud of microcracks or crazes that developed at the notch tip). With a given applied stress and within the same failure mechanism, the values of the number of cycles to crack initiation decrease with increase in thickness. The transition from cooperative ductile to solo-crack brittle initiation mechanisms is sudden with increasing thickness. Transition from cooperative ductile to cooperative brittle with decreasing stress was less well defined. Regions where combinations of mechanisms were observed are also identified in the map. © 1993 John Wiley & Sons, Inc.     

Fracture of fiber reinforced composites

Characteristics of the fracture of fiber reinforced plastic composites are described in terms of the elastic stress distribution at the crack tip, the mechanism of crack tip damage, and the modes and conditions of final fracture. The three-dimensional, stress field at the tip of a sharp crack in a laminate is presented and contrasted to traditional two-dimensional models. The response of the material in the form of inter- and intraply damage formation and growth under increasing load is characterized, and its effect in blunting the main crack is examined. The final fracture conditions, which may range from quasi-brittle to notch insensitive, are discussed and related to the damage zone extension. Observed and anticipated effects of various material and geometric parameters are also discussed.     

Crack growth angle prediction of an internal crack under mixed mode load for unfilled elastomer using the strain energy density factor

This article investigates the prediction of the crack growth angle of an existing internal crack under mixed mode loading at the crack tip for an unfilled ethylene propylene diene terpolymer rubber (EPDM). For the realization of mixed mode loading, the cracks of the uniaxial loaded specimens were oriented with different angles to the loading direction. The energy density factor was used as a potential criterion for determining the crack growth angle. The determination of the strain energy density factor was carried out simulatively in Abaqus. The second-order Ogden model was used to describe the rubber-like material behavior. The relative local minimum of the strain energy density factor provides the possible growth angle. The experimental investigations show that the initial cracks grow orthogonally to the loading direction for the different crack orientation angles. For the crack orientation angle parallel to the load direction, the crack growth was observed because the strong stretching of the specimen caused strong necking in the crack region. The crack growth for the remaining crack orientation angles were induced due to shear loading at the crack tip. The predictive angle of different crack orientation angles shows very good accordance to the measured crack growth angles.     

Evaluation of fiber-matrix interphasial toughness in unidirectional composites

Interphasial toughness was evaluated for polyester and epoxy unidirectional glass fiber reinforced composites using single edge-notched specimens. This was accomplished via a calculation of an energy term based on J-integral which accounted for the energy required to form a longitudinal split crack ahead of the original notch. It was found that the fiber-matrix interphasial fracture resistance of the epoxy composite is substantially greater than that of the polyester composite. This technique may be an excellent way of evaluating the interphasial quality in unidirectional composite laminates.