The tensile stress may be applied externally or may simply be a consequence of residual, or moldedin, stresses. Since variance of any of these factors can change the environmental stress crack resistance of the container, the test results are representative only of a given test performed under defined conditions in the laboratory. Ldpe is more likely to deform under tensile stress. Hdpe when the precrack was opened by razor blade and by fatigue. Fatigue fracture in polyethylene thus the stresses and work of deformation, are fixed by the three quantities x, y and c crack length, and the equivalent.
Plastics will exhibit environmental stress cracking when exposed to an aggressive chemical environment if and only if a tensile stress is present. Stresscontrolled tensiontension fatigue tests were performed at 21. When force is applied to plastic, it travels along the polymer chains causing molecules to strain to remain in contact. The listed values are measured in pounds per square inch. Experimental study of stress cracking in high density polyethylene pipes a thesis submitted to the faculty of drexel university by jingyu zhang. An experimental analysis for determining the fatigue strength of hdpe100. Mdpe is often used for gas pipes, bags and packaging film. Hdpe sheets stressrelieved s r these sheets have gone through an additional annealing process to reduce stress. Because of its low fatigue resistance, an important part of design for pvc pipe is an evaluation of fatigue life as given in awwa c90007. While crystallinity provides strength, it also can lead to cracking. The ratio of the minimum cyclic stress and the maximum cyclic stress, i. The name fatigue is based on the concept that a material becomes tired and fails at a stress level below the nominal strength of the material.
New standard fortifies testing of corrugated hdpe pipe materials. In the case of failure involving fracture, the determination of the failure mode involves identifying how the crack initiated and how it subsequently extended. Study on the mechanical properties of stay cable hdpe. Environment stress cracking of hdpe products is quite common due to its semi crystalline nature which gets attacked due to chemicals present in the environment. With force and time, crazing or stress cracking occurs. The causes of cracks in pvc pipes scott knickelbine pin.
The amount of crystalline content in the container depends on the rate of cooling. This study aims to compare different statistical methods and distributions, in order to give convenient modeling of tensile and fatigue test results of commercially available polyethylene compression molded sheets. Since fatigue lines were observed on the fracture surface, fatigue tests were adopted to estimate the lifetime of the duct under thermal cyclic loading in the field. This is because stress from different sources increases during the summertime. Pehd fatigue damage accumulation under variable loading based. Esc is the consequence of a combination of mechanical stress and exposure to an aggressive chemical. Fatigue life prediction and damage modelling of highdensity.
Experimental study of stress cracking in high density polyethylene. Some performance evaluation tests that once took only days to run now take months with todays. Environmental stress cracking esc is one of the most common causes of unexpected brittle. Degradation and failure of some polymers polyethylene and. It is associated with the phenomenon of crazing and solvent plasticization of the polymer. This study focuses on the phenomenon of cracking formation in hdpe sheathing at fatigue stress levels corresponding to those experienced by real hdpe bridge cable tubes. Escr problems get more pronounced in the summer, because of several factors related to increased heat and humidity. The polymer chains eventually creep toward the pull and deform. Astm d2561 17 standard test method for environmental stress. Fatigue failure of polyethylene electrofusion joints. Recent results from environmental stress cracking of pe pipe and exposed polyamide pa66 to.
Slow crack propagation in polyethylene under fatigue at controlled. Unfortunately, the large majority of hdpe fatigue studies concerns the crack propagation, which assumes that stress concentration and crack already exists. Yield and fatigue behavior of polypropylene and polyamide. It was demonstrated that a significant improvement in the stress cracking resistance of hdpe based materials can be achieved through blending with other types of polyethylene. It should be noted that surface temperatures in hdpe pipe exposed to direct sunlight exceed 150.
Technical note stress crack resistance for geomembrane products the stress crack resistance scr of polyethylene pe geomembranes has been the subject of study since the introduction of the materials over 20 years ago. Plastic failure through environmental stress cracking. Moldedin stress can be another contributor to stresscracking. Standard test method for environmental stresscrack. Jan 18, 2011 hence, an important design factor for pe pipes became commonly known as environmental stress cracking resistance escr, which led to the development of mdpe. Mdpe exhibited excellent scg and stress crack resistance, and dominated pe pipe manufacture for 30 years until the development of bimodal hdpe pe100.
Environmental stress cracking esc is not the result of the continual battle that the plastics industry has in convincing environmentalists and politicians that plastics can be the solution to environmental problems and not the cause. It is a special case of stress corrosion cracking in that it combines a corrosion process and applied stress on the materials. Geomembranes made from high density polyethylene hope have a high percent crystallinity and are therefore of concern with regard to stress cracking. Fatigue corrosion is generally transgranular and does not show branching, which is characteristic of scc.
Hdpe pipe heat fusion joining for strong, lifetime leak free joints flexible, fatigue resistant, light weight and impact resistant for easy, low cost installation resistance to environmental stress cracking resistant to surges and durable performance under temperature extremes. Hdpe is colored black to offer maximum resistance to stress cracking which is a common failure mode for this product. Since variance of any of these factors can change the environmental stresscrack resistance of the container, the test results are representative only of a given test performed under defined conditions in. Mdpe has good shock absorption and is more resistant to notching and stress cracking than hdpe.
For specific cases in which the effect of an environment on polyethylene has not been documented, testing is required to identify any possible environmental stress cracking effect. However, in most of pipeline parts, fatigue may occur without any previously formed crack. Evaluating environmental stress cracking of medical plastics. Aug 01, 2008 molded in stress can be another contributor to stress cracking. Evaluating environmental stress cracking of medical. Enhancing the environmental stress crack resistance of hdpe. Although the information and statements herein are believed to be accurate, no guarantee of their accuracy is made. The tensile stress may be applied externally or may simply be a consequence of residual, or molded in, stresses. This concept may contribute to environmental stress crazingcracking but is generally believed to be a minor effect in consideration of the overall mechanism particularly where severe stress cracking occurs.
Measuring resistance to slow crack growth scg is of utmost importance when seeking the highest durability of hdpe high density polyethylene materials for corrugated pipe used in highway and bridge construction. During the summer months, stresscracking of hdpe containers reaches its peak. Hdpe is also nontoxic and nonstaining and meets fda and usda certification for food. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused by tensile stresses less than its shortterm mechanical strength. Effect of aging on the stress crack resistance of an hdpe. Environmental stress cracking of polypropylene springerlink.
The nature of polyethylene pipe failure plasticstoday. Hdpe geomembranes are prone to stress cracking when subject to tensile stress less than their shortterm mechanical yield strength. Polyethylene pipe, electrofusion, fatigue, ultrasonic analysis. Premature cracking can be caused by topload stress, internal pressure, moldedin stress, and tail swing. This study aims to compare different statistical methods and distributions, in order to give convenient modeling of tensile and fatigue test results of commercially available. Razor blades were prepared with two different radius of curvature, while the fatigue precracking had the radius of curvature close to zero, as demanded by fracture mechanics concept.
Investigations on environmental stress cracking resistance. Realtime control of propagation is based on the current value of stress intensity, k, considering that the assumptions of linear elastic fracture mechanics lefm. Environmental stress crack resistance of polyethylene. Statistical analysis of hdpe fatigue lifetime springerlink. It is important to note that solvents can also affect polyethylene, but in an entirely different manner than environmental stress cracking agents. In this case, fatigue produces accumulated material damage. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which in turn is caused by relaxation of tie molecules. How to prevent stress cracking what is stress cracking.
Fatigue testing of hdpe used in cables was conducted to examine the mechanism of hdpe fatigue cracking and patterns of fatigue deterioration of cables were discovered. Fatigue corrosion is similar to scc, except that the stresses are cyclic and it can occur in any environment. Environmental stress cracking resistance of polyethylene. Polyethylene flooring vs pvc and pp flooring materials. Todays approach to stress corrosion cracking in polyethylene coatings of steel pipes for standard applications in water and gas distribution systems merely considers the coating types featuring the low peeling resistance of the early editions of din 30670, i. The study was made possible by targeted localization of coating holidays using various methods of cathodic corrosion protection ccp and the following subsequent examinations of the damages and especially the change of.
Diagnose it before it ruins your parts residual stress is an inherent result of plastics manufacturing processes and occurs when molten polymer is cooled and shaped. Geomembrane cracking starts with small cracks formed by slow crack growth. Fracture of highdensity polyethylene used for bleach bottles. Stress corrosion cracking of polyethylene practical. Environmental stress crack resistance service temperatures up to 60c 140f is highly resistant to a wide variety of rather reactive cracking of escr is carried out on molded specimens 18 in. No matter how much experience you have working with plastics, sometimes a direct comparison of physical properties is the best way to determine which material is best for your applicationproject. Because parts made with vistamaxx enhanced hdpe are tougher than those made with pure hdpe, they are less likely to fail through environmental stress cracking. Because the mechanisms of failure for solvent attack and environmental stress cracking are so different, the means of optimizing material parameters are different as well. The black color increases pipe temperature from radiant heat. Longitudinal cracking was observed in hdpe ducts and it was resulted by circumferential stress inducing from temperature cycles in the field and residual stress in the pipe.
Introduction polyethylene pe has been used in the water industry since the early 1950s 1. Stress cracking sc is recognized as one of the major concerns for high density polyethylene hdpe pipes. Atsm d638 this document lists both visible and uv light transmission characteristics. Pvc pipe can undergo material fatigue and cracking when exposed to significant vibration for an extended time. Basic understanding of stress cracking in hdpe products. Environmental stress cracking esc is one of the most common causes of unexpected brittle failure of thermoplastic especially amorphous polymers known at present.
This is usually ascertained using a number of visualbased techniques, such as stereomicroscopy. These products can interact both chemically and physically with the polymer constituting these containers, leading to a decrease in the performance of the material and undermining the structural integrity of the component. Nevertheless, current practice of fatigue test is limited to material rating purpose only. This can happen when pvc pipe is rigidly joined to a pump or other vibrating piece of equipment. Environmental stress cracking esc is the premature cracking of a polymer due to the combined action of a stress and a fluid. With the improvement of industry standards, the products developed from low density pe ldpe to. Studies have shown that hdpe pipes are not susceptible to fatigue under typical municipal field conditions. Shortterm fatigue testing is one approach to predicting longterm creep performance of. In addition, side grooved machined samples were evaluated with razor blade precrack opening. Because of this, environmental stress cracking and creep rupture are parallel failure mechanisms. Highdensity polyethylene hdpe can be blowmolded and used for the production of bottles for aggressive products. The median rank, the maximum likelihood and the kolmogorovsmirnov fitting are compared for. While crystallinity provides strength, it also can lead to.
Hdpe highdensity polyethylene sheets offer excellent impact resistance, light weight, low moisture absorption, and high tensile strength. A materials resistance against this type of failure is called environmental stress cracking resistance escr. Stress controlled tensiontension fatigue tests were performed at 21. Fatigue cracking results from cyclic stresses that are below the ultimate tensile stress, or even the yield stress of the material. It is commonly used for both rigid containers and plastic films.
When a polymer is stressed in air to just below its yield point, stress cracking can occur after period of time. Yield and fatigue behavior of polypropylene and polyamide6. Experimental study of stress cracking in high density. Experimental analysis the main experimental means were. Pdf slow crack propagation in polyethylene under fatigue at. Dynamic fatigue 19 the application of a cyclic shortterm stress resulting in small increments of damage resulting in initiation and gradual propagation of a brittle crack which fails at some point in a catastrophic manner fatigue failure is identifiable by striations showing incremental crack growth. Technical note stress crack resistance for geomembrane products. This type of failure takes a longtime and varies depending on the geomembrane resin polyethylene resin and additives. In general, the mechanism of environmental stress cracking in polyethylene involves the. The stress cracking is more severe with detergent, bleach, motor oil and some industrial chemical packaging. According to astm d883, stress cracking is defined as an external or internal crack in a plastic caused by tensile stresses less than its shortterm mechanical.
The addition of a surfaceactive chemical, an environmental effect, accelerates this type of failure, thus this mechanism is termed environmental stress cracking esc. The empirical paris law is verified under all conditions, with a stress intensity exponent close to 4. Hdpe, like steel, is a semicrystalline material, with no crystals in the melted state. Pdf degradation and failure of some polymers polyethylene. Degradation and failure of some polymers polyethylene and polyamide for industrial applications.
Conversely, the circumferential cracking in corrugated hdpe pipes was caused by longitudinal stress from bending and residual stress of the pipe. Fatigue lifetime of hdpe structures such as pipes is recognized to show a large scatter. Environmental stress cracking can take place through continuous crack growth or via a stepwise progressive mechanism of cracking and arrest, depending on the molecular structure of the plastic, the composition of the chemical agent, the level of stress, and other environmental conditions. Environmental stress cracking is accelerated due to higher temperatures, cyclic loading, increased stress concentrations, and fatigue. Hence, an important design factor for pe pipes became commonly known as environmental stress cracking resistance escr, which led to the development of mdpe. Grace hsuan, jy zhang and wk wong department of civil, architectural and environmental engineering, drexel university, philadelphia, usa abstract the 100year stress crack resistance scr of corrugated hdpe pipes was evaluated using a 600 mm diameter pipe. Sc is a brittle failure that occurs at a stress level lower than the shortterm. Technical note stress crack resistance for geomembrane. This paper describes various forms of stress corrosion cracking in polyethylene and discusses them on the basis of practical examples of polyethylene steel pipe coatings. Environmental stress cracking in polyethylene takes place because of interlamellar failure, which. It is well known from the pipe and cable insulation industries that pe can stress crack over long periods of time.