Slow crack propagation in polyethylene under fatigue at controlled stress intensity

Fatigue tests were performed on circumferentially notched bars (CNB) of high density polyethylene in order to analyse the kinetics and mechanisms of crack propagation. Tests were performed at 80 °C in order to accelerate the processes. Unlike standard fatigue procedure in which the force amplitude is constant, the original system utilised in this work was capable of imposing a constant stress intensity amplitude, K^max, during the whole propagation range. This was made possible through the real-time monitoring of crack propagation, a(N), by means of a video-controlled technique. The results of the tests, for K^max in a range from 0.2 to 0.45 MPa m^1/2, show that stress intensity is the proper variable which controls crack propagation rate since, on the overall, crack speed is constant at constant K^max and no self-acceleration is observed unlike under force control. The empirical Paris law is verified under all conditions, with a stress intensity exponent close to 4. However, it is shown that constant crack speed is obtained only for K^max<0.25 MPa m^1/2, when propagation proceeds through the continuous stretching and breaking of microfibrils in the localised craze at the tip of the crack. By contrast, at larger K^max, it is observed that crack tip successively jumps across the extended crazed zone in which very coarse fibrils were previously stretched from voids nucleated in the plane of maximum normal stress at a long distance ahead of the crack tip.     

Fatigue crack propagation in,graphite fiber reinforced nylon 66

The rate of fatigue crack propagation in graphite fiber reinforced nylon 66 was measured. A model of the form å = β [K_max^1-γ ΔK^γ]^r was used to correlate the rate of crack propagation å with the maximum stress intensity K_max and the amplitude of the stress intensity ΔK experienced by the notched specimen during the fatigue test. The quantities β, γ and r were constant at fixed temperature and frequency of the test. It was also found that there exists both an upper and a lower threshold of stress intensity for the slow ropagation of damage during fatigue. The mechanism of crack propagation in the short graphite fiber reinforced nylon was found to be similar to the growth and fracture of crazes in thermoplastics. The propagation of damage at the crack tip is controlled by matrix deformation, cavitation, fiber breakage and fiber pullout. Damage can propagate in the absence of crack growth until a critical point is reached at which time the material fractures catastrophically.     

Instantaneous crack resistance during crack propagation in a viscoelastic solid

Instantaneous crack resistance values during the mode I stable crack propagation in poly(methyl methacrylate) (PMMA) were investigated with the aid of the sector area method at different test temperatures. The crack resistance during stable crack propagation is a gradually decreasing function of crack passage at all temperatures. The rate decreases as the test temperature decreases, down to ?30°C, irrespective of high initial crack resistance. The crack propagation velocity profiles, obtained through velocity gages, also show the decreasing function of crack passage. Both crack resistance R and its gradient with respect to the crack propagation velocity, dR/d?, become greater as the temperature decreases. R becomes greater as ? increases, contrary to the usual crack resistance behavior in metals.     

Mechanics of crack growth in epoxide resins

Experiments have been conducted employing tapereddouble-cantilever-beam joints with different epoxide adhesives. Depending on the adhesive employed, crack propagation occurred either (a) in a continuous stable manner with crack propagation velocities in the range 10^?4 to 5 m/s and values of the adhesive fracture energy, G_Ic, being almost independent of the crack velocity, or (b) intermittently in an unstable manner when the initial crack velocity was never less than about 20 m/s and, in some instances, rose to about 450 m/s; values of G_Ic (initiation) increased rapidly with increasing velocity. It is proposed that the amount of localized plastic deformation arising from shear yielding that occurs at the crack tip prior to crack propagation is controlling. Secondly, the longterm strength of stressed, structural adhesive joints has been investigated. The fracture of these joints over eight decades of time is uniquely described by a critical plastic zone size developed at the crack tip at failure.     

Slow crack growth in cellulose film

A simple photographic method for measuring the slow crack growth in transparent polymer film is described. Framing speeds up to 100 per second have been achieved. Experimental data for a cellulose film with centrally located initial cuts indicate that for 0 < t < 0.8t_b, the crack length c increases with time t according to where c₀ is the initial cut size, K a constant, and t_b the time to fracture.     

Internal stress in epoxide resin networks containing biphenyl structure

Bisphenol-A type and biphenyl type epoxide resins were cured with two types of aromatic diamines which have or do not have the carbon bridge between aromatic rings. Internal stress of these cured resins decreased with the introduction of a biphenyl structure into the networks. This reduction of the internal stress is attributed to the decrease of the elastic modulus in the glassy region caused by the low mobility of the biphenyl segment. On the other hand, the glass transition temperature increased with increasing the concentration of the biphenyl unit in the networks. These results revealed that the consistency of the decrease of the internal stress with the improvement of the heat resistance of the cured systems is possible by introducing the biphenyl structure to the epoxide resin networks.     

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.     

Slow stable crack growth in high density polyethylenes

The phenomenon of slow stable crack growth in polyethylene is investigated using notched specimens subject to constant load and the concepts of fracture mechanics. The effect of specimen geometry and dimension, the loading and the mode of loading on the applied stress intensity factor versus crack speed ($\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$) curves has been studied to demonstrate that K_c is the controlling stress parameter for crack growth under suitable conditions. $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves are obtained for a high density polyethylene homopolymer in distilled water and in a diluted detergent solution at four different temperatures. Results are also obtained for a much tougher medium density polyethylene copolymer whenever possible. Several mechanisms can be identified from the form of the $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ curves. Two, in particular, have been observed but not explained before: (i) crack growth with a time dependence of 0.25, and (ii) the high $\text{K}{}_{\mathrm{c}}\text{-}\text{a}\text{?}$ slopes for crack growth in a tough copolymer. With the help of scanning electron microscopic studies of the fracture surfaces, (i) is postulated to be due to diffusion controlled void growth process and (ii) is considered to be the result of crack tip blunting effects. From the temperature dependence of crack growth, the activation energy of the diffusion controlled crack growth process is found to coincide with that of the x-relaxation process in polyethylene implying that diffusion controlled crack growth may be related to the motion of main chains in the polymer.     

Fatigue crack growth in fiber reinforced plastics

Crack extension during fatigue loading is one of the primary causes of failure in engineering materials. While the fatigue crack resistance of homogeneous and even adhesive systems has received detailed study and characterization, relatively few and scattered results are available for fiber composites. One difficulty with obtaining such data for composites is their tendency to develop complex patterns of intra- and interlaminar damage which expand in a stable manner during fatigue. Such damage usually does not severely reduce the load carrying capacity of a structure but the complexity of the damage geometry has so far frustrated efforts to apply any unifying theories of growth. Measurement of the rate of macroscopic crack growth, through thickness crack extension, has been possible for certain composites and crack direction where the stable damage is constrained. These include cracks in 0°/90° laminates, woven fabric laminates, chopped strand mat laminates, sheet molding (SMC) materials, and short fiber reinforced thermoplastics. Macroscopic interlaminar cracks in continuous fiber systems have also received some recent attention. Fatigue crack growth in glass fiber composites for which most data are available, involves significant contributions from both static and cyclic load effects. A simple model for predicting fatigue crack growth rates from traditional S-N curve and fracture toughness data has proven useful for certain well behaved systems. Limited study has also been made of the effects of moisture and salt water on the fatigue crack growth rate.     

The dependence of rapid crack propagation in polyethylene pipes on the plane stress fracture energy of the resin

The critical temperature [CT] for rapid crack propagation [RCP] was measured in 11 polyethylene [PE] 200 mm diameter gas pipes each with different resins. The plane stress fracture energy [PSFE] in thin Charpy impact specimens of the resin was found to correlate with the CT. The higher the PSFE, the lower the CT. This result was related to the observation that the PSFE decreases as the temperature decreases. It was found that the impact energy of specimens from compression molded pipe that was remelted at 180°C gave a better correlation with the CT than specimens that were machined from the inner wall of the pipe. Consequently, it is now possible to predict the CT of a pipe by measuring the PSFE of the resin without making the pipe.     

环氧树脂混凝土在设备基础抢修中的应用

江苏安邦电化有限公司热电厂（心下简称我厂）^#1煤粉炉（生产能力35t／h）引风机基础系1987年投入使用，由于系统中的花岗岩砌制的水膜除尘器年久失修使水膜形成不连续，致使引风机叶轮吸附灰水．造成其动平衡失调产生振动，待运行人员发现该问题时引风机基础已受损。经检查，8颗地脚螺栓中1颗断裂、7颗出现不同程度的松动，但基础本体未出现结构性破坏的裂缝，仅四周的外粉刷层脱落。当时正值冬季热负荷高峰期，若不及时修复将影响企业对用户的供汽承诺。应厂部要求，我们迅速拿出了2套施工方案（见表1），经技术经济评价选择了方案2；其基础平面和剖面处理见图1、图2。     

The thermal degradation in vacuo of an amine-cured epoxide resin

The degradation in vacuo of the cured resin from stoichiometric amounts of the diglycidyl ether of bisphenol A and p,p′-diaminodiphenylmethane was studied at temperatures of 303.7 and 304.0 ± 0.2°C. The condensible products of the degradation at these temperatures were collected and the major components were isolated as pure compounds. These were identified as water, bisphenol A, 2-(benzofur-5-yl)-2-(p-hydroxyphenyl)propane, N,N,N′,N′-tetramethyl-p,p′-diaminodiphenylmethane, phenol, p-isopropylphenol, p-isopropenylphenol, N,N-dimethylaniline, N-methylaniline, N,N-dimethyl-p-toluidine, benzofuran, and N-methyl-p-toluidine. The presence of 2,2-bis(benzofur-5-yl)propane was suspected. The gaseous degradation products were identified as acetaldehyde, chloromethane, carbon dioxide, carbon monoxide, methane, ethane, ethene, and nitrogen. Tentative reaction mechanisms are proposed to account for the formation of the products identified. The possible nature of some of the minor degradation products is discussed. The results are related to those of previous studies of the degradation of the same and similar resin–hardener systems.     

Rapid crack propagation and arrest in polymers

Published results on dynamic fracture toughness vs crack velocity relations of polyester resin (Homalite-100), epoxy resin Araldite-B, modified epoxy resins and polycarbonate are reviewed. Commonality between the seemingly diversified experimental results as well as the existences of minimum dynamic fracture toughness, K_Im, and crack arrest stress intensity factor, K_Ia, as inherent material properties are discussed.     

Stability of crack propagation in epoxy resins

The propagation of cracks in epoxy resins has been studied using a linear elastic fracture mechanics approach and a double torsion testing geometry. Under constant crosshead displacement rate conditions cracks are found to propagate in an unstable ‘stick-slip’ manner at high temperatures and with low rates of testing whereas at lower temperatures and using higher rates of loading propagation is more stable and cracks propagate in a continuous manner. The presence of liquid water tends to cause a transition from stable to unstable propagation at room temperature. The influence of specimen geometry upon crack stability is also discussed.     

Phenomenological aspects of viscoelastic crack propagation

The strain rate dependence on dynamic crack propagation in viscoelastic solids was experimentally verified for PMMA by the velocity gauge method and fracture surface observation. It was found that the dependence was mainly due to the viscous term effect, and this was supported by the observation of deviation of the stress-strain curve from an elastic case. The elastic brittle crack propagation can be realized only in the high-strain rate case for PMMA.     

Bisphenol-A epoxy resin reinforced and toughened by hyperbranched epoxy resin

The study on toughening and reinforcing of bisphenol-A epoxy resin is one of important developmental direction in the field. This paper reports a one-pot synthesis of aromatic polyester hyperbranched epoxy resin HTDE-2, an effect of HTDE-2 content on the mechanical and thermal performance of the bisphenol-A (E51)/HTDE-2 hybrid resin in detail. Fourier transform infrared (FT-IR) spectrometer, scanning electronic microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical thermal analysis (DMA) and molecular simulation technology are used to study the structure of HTDE-2, performance and toughening and reinforcing mechanism of the HTDE-2/E51 hybrid resin. It has been shown that the content of HTDE-2 has an important effect on the performance of the hybrid resin, and the performance of the HTDE-2/E51 blends has maximum with the increase in HTDE-2 content. The impact strength and fracture toughness of the hybrid resin with 9 wt-% HTDE-2 are almost 3.088 and 1.749 times of E51 performance respectively, furthermore, the tensile and flexural strength can also be enhanced about 20.7% and 14.2%, respectively. The glass transition temperature and thermal degradation temperature, however, are found to decrease to some extent. __________ Translated from Journal of South China University of Technology (Natural Science Edition), 2006: 34(9): 90–94 [译自: 华南理工大学学报 (自然科学版)]     

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.