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     

Fatigue crack propagation behavior of polyether ether ketone under biaxial loading

Polyether ether ketone (PEEK) has become a promising material in total joint replacement. However, it still faces the risk of fatigue fracture during service. In this paper, the effects of biaxial stress ratio λ, cyclic stress ratio R, and load phase difference θ on fatigue crack propagation (FCG) behavior of PEEK are investigated. In the case of vertical cracks, results show that the FCG rate of PEEK increases with the R value, while decreases with the increase of λ value. Furthermore, the effective stress intensity factor range ΔK_eff can uniformly describe the biaxial FCG behavior at different cyclic stress ratios. In the case of 45° slant cracks, compared with mode-I intensity factor range ΔK_I, the energy release rate range ΔG is more accurate for describing the FCG behavior under various load phase differences. In addition, the investigation on the 45° crack propagation path shows that a bifurcated Y-shaped crack appears under 180° load phase difference, while no bifurcated crack appears under 90° load phase difference and uniaxial loading. Three different methods are used to predict the crack propagation path. The comparison results show that the maximum circumferential stress (MTS) criterion can well predict the crack propagation path under out-of-phase biaxial loading and uniaxial loading.     

Retardation of Fatigue Crack Growth in Ceramics by Glassy Ligaments: A Rationalization

In high-temperature fatigue crack growth (FCG) experiments on ceramic materials containing amorphous grain boundary phases, the crack growth rates under cyclic loads were observed to be lower than those predicted solely on the basis of crack growth velocities measured under static loads. In this paper, a rationalization was offered for such a behavior by means of a phenomenological glass-bridging model which takes the relaxation behavior of glass into account. In ceramics which exhibit subcritical crack growth through cavitation ahead of the crack tip, the maximum stress intensity factor of the fatigue cycle required to initiate FCG was observed to be always greater than or equal to the threshold stress intensity factor for crack growth under sustained far-field loads. This trend was also explained with the aid of the glass-bridging model and invoking the equivalence between bridging and damage zones. The elevated temperature FCG behavior of nitride-based ceramics which exhibit grain bridging in the wake during crack propagation was discussed and contrasted with oxide-based ceramics which show glass bridging.     

Effect of hot humid environmental exposure on fatigue crack growth of adhesive-bonded aluminum A356 joints

In this study, the effect of hot humid environmental exposure prior to and after adhesive curing on the fatigue crack growth (FCG) behavior of adhesive-bonded aluminum A356 joints was investigated by using single-edge notch bend (SENB) specimen. The results showed that while the exposure after adhesive curing had a significant effect on the FCG resistance of the joints, the exposure of the assembled joints prior to adhesive curing had a relatively small influence. In addition, the effect of load ratio R on the FCG behavior of the exposed joints was studied. The results showed that the FCG of the exposed joints is more sensitive to load ratio R than that of the unexposed joints.     

The effects of frequency on fatigue threshold and crack propagation rate in modified and unmodified polyvinyl chloride

The cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC‐M) and without (PVC‐U) chlorinated polyethylene (CPE) impact modifier, was studied. The effect of impact modifier upon fatigue crack growth rate and threshold was evaluated at frequencies of 1, 7, and 20 Hz. It was shown that the addition of CPE lowered the threshold stress intensity factor amplitude for crack growth (ΔK_th) of PVC‐M compared to that of PVC‐U at lower frequencies, and that the effect became more pronounced at lower frequency. At lower stress intensity factor amplitudes (below ΔK = 1 MPa·m^1/2), there was a slight difference between the crack growth rates of U‐ and M‐PVC. The crack advance mechanism is investigated by microscopic observation of the crack tip process zone. Although the zone is relatively large in PVC‐M, associated with higher toughness, it did not improve the fatigue crack growth resistance significantly. Fracture surface observations reveal a higher density of fibrils on the fatigued surface of PVC‐M with the density, relative to that observed in PVC‐U, reducing with frequency. It is therefore hypothesized that accelerated fibril failure is a mechanism of fatigue. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Fatigue behavior of bonded composite repairs

In this study, recent efforts into the research and development of composite repairs bonded to defective aircraft structures are discussed. The fatigue crack growth (FCG) behavior of pre-cracked Al 7075/T6 substrates with bonded composite patches was investigated experimentally and analytically. Boron-epoxy patches with 2-, 4- and 6-plies were installed on Al substrates with single-side-crack. Tension-tension fatigue tests were also conducted on Al substrates to establish their fatigue behavior for comparing with the repaired specimens. A considerable increase in the fatigue life and a decrease in the stress intensity factor (SIF) were observed as the number of plies increased. An analytical model, based on Rose's analytical solution and Paris' power law, was developed to predict the FCG behavior of the repaired substrates. The analytical and experimental results are found to be in good agreement.     

Fatigue crack propagation in mica-filled polyolefins

Edge notched samples of polypropylene (PP) and high-density polyethylene (HDPE) containing different mica concentrations were tested in mode I tensile loading. Crack growth was approximated by a non-linear regression of exponential form using statistical software (SAS). Characterization of fatigue crack propagation (FCP) was made using the Paris-Erdogan law. The crack front in PP was preceded by a wide plastic zone in which craze developed, leading to a discontinuous crack growth. Using spline functions, a margin between maximum and minimum FCP rates, recorded during the crack progression, is presented along with the average FCP rates. It is shown that mica-reinforced PP samples exhibit higher FCP rates than unfilled PP. In HDPE, mica reduces FCP rates resulting in a higher resistance to fatigue crack propagation. Effect of test frequency is presented for unfilled polymers and 10 percent mica concentration by weight in both matrices. An increase in the test frequency has no significant effect on FCP rates for both raw and mica-reinforced PP. Unfilled and mica-filled HDPE show noticeable decrease in FCP rates with increasing frequency.     

A crack layer approach to fatigue crack propagation in high density polyethylene

Fatigue crack propagation (FCP) in high density polyethylene (HDPE) is observed to occur with an accompanying layer of damage ahead of the crack tip. The crack layer theory, which accounts for the presence of both the damage and the main crack, is applied to the problem. It is observed that the kinetic behavior of HDPE under fatigue consists of three regions: initial acceleration, constant crack speed (“deceleration”), and reacceleration to failure. Within the first two regions, crack propagation appears “brittle,” while in the third region “ductile” behavior is manifested. Ultimate failure occurs via massive yielding of the unbroken ligament. Two damage mechanisms are found to be responsible for HDPE failure: formation of fibrillated voids and yielding. Both mechanisms are present throughout the entire lifetime of the crack, but the former dominates the “brittle” crack propagation region, while the latter is more prominent in the “ductile.” Throughout the analysis the resistance moment R_t is approximated as the total volume of transformed material associated with crack advance. Crack layer analysis produces a satisfactory fit of the experimental data and yields a specific enthalpy of damage, γ^*, value in the 1–2 cal/g range.     

Effects of loading history on fatigue crack growth in high density polyethylene and toughened poly(methyl methacrylate)

Fatigue crack growth measurements are made on two grades of high density polyethylene (HDPE), of density 940 and 958 kg/m³, and on one transparent grade of toughened poly(methyl methacrylate) (PMMA). In both grades of HDPE, prolonged cycling is necessary to initiate a crack from a razor notch. Relationships between crack growth and stress intensity factor K are studied in all three materials. The results show that Paris plots of log (da/dn) against log ΔK are linear provided that: (a) the loading pattern applied to the specimen remains constant; and (b) growth has taken place for a sufficient period to eliminate the effects of previous loading history. However, da/dn is strongly dependent upon the ratio of minimum to maximum stress and upon the stress amplitude during previous cycles. Overloads cause accelerated growth which is followed by crack arrest when the stress amplitude is restored to its original level. The use of a transparent polymer enables a more detailed study of crack growth phenomena to be made during fatigue testing.     

The effects of water and frequency on fatigue crack growth rate in modified and unmodified polyvinyl chloride

A study of the influence of water environments on the cyclic fatigue crack behavior of polyvinyl chloride (PVC), with (PVC‐M) and without (PVC‐U) chlorinated polyethylene (CPE) impact modifier was undertaken and compared with corresponding results in air. Two frequencies of 1 and 7 Hz were applied to assess the influence of frequency on the fatigue behavior; a higher fatigue resistance and threshold were obtained with increasing frequency. This trend is more significant in water. However, in this environment, the fatigue resistance deteriorated under conditions of higher stress intensity factor amplitude (ΔK) and frequency. The fatigue properties of PVC‐U are the most affected by the presence of water, particularly at low frequency and higher ΔK. Examination of the fracture surface showed the interaction of water molecules and the PVC matrix with the formation of (1) a nodular structure, close to the fatigue threshold and (2) plasticized structures at high ΔK, which are associated with a greater threshold value and fatigue resistance. The absorption of the water retarded the fibrillation of craze and caused crack blunting effects. Water functions as a plasticizer, particularly at high ΔK, through the formation of the plasticized structures. Results are compared with those observed from an in‐service failure. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Fatigue crack growth in PE-HD pipe grades

Abstract

In this work, fatigue crack growth (FCG) experiments were used to evaluate the long term performance of pipe grade polyethylene (PE). The parameters for the ranking tests were defined based on basic investigations of the influence of test frequency, R-ratio (F _min/F _max) and specimen configuration on FCG in PE at room temperature. Eight different commercial pipe grades (PE-80 and PE-100) were included in the investigations and two different specimen configurations [Compact Type (CT) and Cracked Round Bar (CRB)] were used. A procedure was established to initiate quasi-brittle cracks that led to failure within 60 h. Crack-growth initiation times were determined from crack-opening displacement data and crack growth kinetics were analysed using linear elastic fracture mechanics (LEFM). A ranking of the materials investigated by their FCG behaviour was possible and compared with results from Full Notch Creep Tests (FNCT).     

Fatigue and fracture characteristics of a fine-grained (Mg,Y)–PSZ zirconia ceramic

The fatigue and fracture characteristics of a partially-stabilized fine-grained zirconia with spinel additions, (Mg,Y)–PSZ, were studied. Fracture toughness, crack growth resistance curves and fatigue crack growth (FCG) behavior, under both sustained and cyclic loading, were evaluated. Mechanical fatigue effects were clearly evidenced by (1) remarkable crack growth rate differences under cyclic and static loading and (2) significant loading ratio effects. Comparing the cyclic and the static FCG behavior allows to deduce a higher cyclic fatigue sensitivity of the fine-grained (Mg,Y)–PSZ with respect to a commercial peak-aged Mg–PSZ used as a reference material. By in situ observation of crack extension under cyclic loading, the fatigue mechanisms could be resolved. Mechanical degradation of bridging ligaments, as already known for coarse-grained Mg–PSZ, is one source of cyclic fatigue. An additional source attributed to the particle dispersed microstructure of the (Mg,Y)–PSZ is the interaction between crack faces and hard spinel particles. The sensitivity of (Mg,Y)–PSZ and Mg–PSZ to cyclic fatigue is discussed in terms of the respective microstructures, prevalence and operativity of distinct mechanical fatigue mechanisms.     

Static fatigue and compressive stress generation in an aged crack

In classic experiments by Michalske and others, it was found that cracks aged statically below the fatigue limit acquired a temporary strength increase compared to the non‐aged crack. In our previous publication we observed that cracks growing near the fatigue limit exhibited a time dependent slowing down of crack growth. Both of these phenomena are related to a toughening of the crack tip that we attribute to a water‐assisted surface stress relaxation mechanism. To test this hypothesis, the K‐v crack growth curves have been measured using the double cantilever beam (DCB) experimental technique for two commercial glasses, a sodium aluminosilicate, and a potassium aluminosilicate, both of which exhibit clear fatigue limits in air. Using polarimetry, it is shown that the stress state near an unloaded but previously aged crack tip is opposite in sign to the stress state near the tip of a crack held in Mode I loading. These results clearly indicate that a stress relaxation mechanism is occurring at the crack tip.     

Kinetics of fatigue and creep crack propagation in PVC pipe

The kinetics of creep and fatigue crack growth in PVC pipe were studied in order to develop a methodology for predicting long‐term creep fracture from short‐term fatigue tests. Fatigue and creep crack propagation followed the conventional Paris law formulations with the same power 2.7: da/dt = A_fΔK^2.7_I and da/dt = BK^2.7_I, respectively. The activation energy for creep crack propagation, obtained from the temperature dependence of the Paris law prefactor, allowed extrapolation of high temperature creep fracture to low temperature creep crack growth rates. The activation energy for fatigue crack propagation was much lower than that for creep. Therefore, fatigue and creep could not be directly correlated by using the prefactor in the covenentional Paris law formulations. Furthermore, a unique value of the Paris law prefactor did not describe frequency and R‐ratio (amplitude) effects in fatigue crack propagation. Nevertheless, conformity of crack growth rates measured under all conditions to the same Paris law power suggested that correlation should be sought in alternative formulations of the crack growth rate.     

Fatigue crack growth behavior and mechanism of closed‐cell PVC foam

The applicability of linear‐elastic fracture mechanics parameters (ΔK and K_max), elastic–plastic fracture mechanics parameter (ΔJ), and time‐dependent fracture mechanics parameter (C*) to characterize fatigue crack growth (FCG) rate of closed‐cell polyvinyl chloride foam was investigated in the present work. The effect of stress ratios (R = 0.1 and 0.4) on FCGs was observed when the ΔK, K_max and ΔJ were used as fracture mechanics parameters. As a fracture mechanics parameter that combines ΔK and K_max, the K* successfully characterized FCGs (da/dN) at R = 0.1 and 0.4. While, a time‐dependent fracture mechanics parameter (C*) successfully correlated da/dt of creep crack growth (CCG) test, but it failed to correlate da/dt of FCG tests. The FCGs at both R = 0.1 and 0.4 were cyclic dependent, while the CCG was time dependent. For cyclic‐dependent crack growth, the interaction between polymer‐chain scission and small scale crack‐tip blunting was the main mechanism, whereas the interaction between polymer‐chain pullout and large scale crack‐tip blunting dominated fracture process for time‐dependent crack growth. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Temperature and frequency effects on fatigue crack growth in acrylonitrile–butadiene–styrene (ABS)

Fatigue crack growth (FCG) in a commercial-grade acrylonitrile–butadiene–styrene (ABS) over the temperature and frequency ranges of 10–70°C and 0.01–10 Hz was studied. A model for the effects of temperature and frequency on the FCG rate was refined. The refined model is shown to accurately predict FCG rates in ABS. Three different types of fatigue fracture surfaces have been found. The first type is characterized by discontinuous growth bands; the second, by a rather smooth surface; and the last, by a rough surface relative to the second. The transition between the first and second types was found to be dependent on temperature and frequency as well, whereas the transition between the second and last types was found to be only dependent on temperature. These findings are discussed in relation to crazing. The apparent activation energy (ΔH_th) was evaluated for both the first and second types to be 19.22 kJ/mol. © 1995 John Wiley & Sons, Inc.     

Modeling of the fracture mechanism of HDPE subjected to environmental stress crack resistance test

Environmental stress crack resistance (ESCR) is a commonly used test to characterize cracking failure of high‐density polyethylene in applications such as wires, cables, blow molded containers, and other rigid packaging applications. From a resin design standpoint, it is important to understand the mechanism of environmental stress cracking especially in the case of materials with significantly different ESCR values. Currently, two standard ESCR tests, ASTM D1693 and ASTM F2136, are commonly accepted to measure environmental stress crack resistance of HDPE. An accurate observation of ESC is important to understand the fracture mechanism of samples. In this study, the ESCR performance of six HDPE samples was determined per ASTM D1693. The failed specimens were further characterized by scanning electron microscopy and fractographic methodology to investigate the failure mechanism. HDPE resins with low ESCR values had crack surfaces characterized by shorter and fewer fibrils. A new empirical model to predict polymer ESCR using tie chain concentration with different integration range, and water vapor transmission rate, to characterize detergent diffusion in the crack, was developed. The proposed empirical parameter improves the prediction of ESCR. The ability to predict ESCR performance from resin properties is a beneficial tool for new product development. POLYM. ENG. SCI., 2009. © 2009 Society of Plastics Engineers     

Overload effect on the fatigue crack propagation of PC/ABS alloy

The effect of single overload within an otherwise constant amplitude loading sequence on the fatigue crack propagation (FCP) behavior of the alloy of polycarbonate and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length of the specimen. Optical and scanning electron microscopes are used to observe the features of crack surface and the process of crack tip deformation. The overload waveform has slight effect, while the overload ratio has great effect on the crack growth retardation. A small crack increment is produced during overloading. The crack growth rate reduces quickly, and then increases gradually until it reaches the steady crack growth rate level when the loading recovers to normal constant amplitude fatigue loads. Porous or dimple features govern the fatigue crack surfaces.     

Fatigue crack growth and craze-induced crack tip shielding in polycarbonate

This paper expounds a revised characterisation of the elastic stresses ahead of a crack tip in polycarbonate which takes account of the elastic–plastic boundary stresses induced by the presence of the crazed region that surrounds a crack. The advanced experimental techniques used in this work have provided insights into fractography, identification of the crazed region and location of the crack tip position (using confocal laser scanning microscopy and scanning electron microscopy). In addition, the four-parameter model of crack tip stresses has led to modified definitions for crack tip stress intensity factors which explicitly account for craze-induced shielding effects on the fatigue crack growth rate in polycarbonate. The model is generic and offers the potential for increased understanding of fatigue crack growth in polycarbonate.     

A fatigue damage model for composite materials

Composite materials using polymer resins as matrices possess viscoelastic properties such that the fatigue behavior of the composite could be changed by different stress levels, stress ratio, stress frequency, or temperature. Based on a physical phenomenon of damage growth, this work develops a fatigue damage model including the nonlinear effects of stress ratio and stress frequency on the damage processes for carbon/epoxy composites. A damage index is defined and used to confirm the damage evolution behavior, and a series of fatigue tests of unidirectional specimens under monotonous loading and two‐stress level loading are conducted to test the proposed fatigue damage model. The results reveal that the proposed model could reasonably predict the fatigue life of composite materials under complicated loading conditions and it also includes the sequence effect of cyclic block loadings. POLYM. COMPOS., 2009. © 2008 Society of Plastics Engineers