Cracked Round Bar Specimens Research Articles

Development of mechanochemical degradation induced slow crack growth model for high density polyethylene in cracked round bar specimen

Development of mechanochemical degradation induced slow crack growth model for high density polyethylene in cracked round bar specimen

Novel test system and test procedure for fatigue crack growth testing with cracked round bar (CRB) specimens

Abstract A novel test system for fatigue crack growth (FCG) testing with cracked round bar (CRB) specimens was developed and implemented, utilizing a commercially available linear-torsion electro-dynamic test machine, several specifically designed test fixtures and devices, and a unique test and data reduction procedure. The test arrangement includes two different cyclic testing devices, which allow for tests at adjustable test temperatures and under predefined environmental conditions with variable gaseous or liquid environmental media. The test system and procedure allow for a quasi-automatic determination of crack growth rates via a self-designed optical crack length measurement device and tests air or other transparent gaseous or liquid environmental media. To account for the frequently observed asymmetric crack growth in CRB specimens, a special technique has been established, in which the entire CRB specimen is rotated in the test machine to enable measurements of crack lengths at various circumferential positions. To demonstrate the potential of the test system, high-density polyethylene (PE-HD) was investigated at an elevated temperature of 80 °C and in different environmental media (air, non-chlorinated water and chlorinated water with 5 mg/l free chlorine). When using the crack length mean value derived from crack length measurements from various circumferential positions, the fatigue crack growth (FCG) resistance of PE-HD at 80 °C in air was found to be independent of the degree of crack growth asymmetry. As to the environmental influence, chlorinated water with a chlorine content of 5 mg/l free chlorine was found to represent a more critical environmental condition than non-chlorinated water. Fatigue crack growth curves in air exhibited a significantly slower slope than the other two environments, leading to a crossover of the FCG curves with a tendency for higher and lower crack growth rates, respectively, at low and high stress intensity factor levels.

Global and Local Aging in Differently Stabilized Polypropylenes Exposed to Hot Chlorinated Water with and without Superimposed Mechanical-Environmental Loads.

The influence of chlorinated water on the global and local aging behavior of polypropylene (PP) was investigated for three differently stabilized PP grades consisting of the same PP base polymer. While one of the PP grades contained only a processing stabilizer (PP-S0), the other two were modified with a primary phenolic antioxidant (PP-S1) and a combination of a primary phenolic antioxidant and a hindered amine stabilizer (PP-S3). To study global aging effects, micro-sized specimens were pre-exposed to chlorinated water (5 mg/L free chlorine) at 60 °C for up to 750 h. Over the entire exposure period, significant material aging was detected by monitoring a continuous decrease in stabilizer content, oxidation induction temperature, mean molar mass, and mechanical strain at break. In terms of aging resistance and ultimate mechanical performance, PP-S1 was found to outperform the other two material formulations under these test conditions. Moreover, superimposed mechanical-environmental fatigue tests with cracked round bar specimens were carried out with the three PP grades in non-chlorinated (0 mg/L free chlorine) and chlorinated (5 mg/L free chlorine) water at 80 °C and 95 °C to study local crack tip aging effects. While the fatigue crack growth resistance substantially deteriorated for all three materials in chlorinated water, a significantly stronger effect was found for the higher temperature, with crack growth rates at a given stress intensity factor range in chlorinated water being ca. 30 to 50 times faster than in non-chlorinated water, depending on the material. Molar mass measurements of material samples taken from various positions of the tested CRB specimens provided clear evidence of enhanced local crack tip aging due to the chlorinated water environment.

Chlorinated Water Induced Aging of Pipe Grade Polypropylene Random Copolymers.

Polypropylene random copolymers (PP-R) are common materials for pressurized hot water pipes. In many pipe systems, potable water is disinfected by chlorine to prevent waterborne diseases. This paper deals with hot chlorinated water induced aging of two PP-R grades with varying morphology. One material had a conventional monoclinic α crystal form (PP-Rα), whereas the other was explicitly beta-nucleated resulting in a trigonal β crystal form with a fine spherulite structure (PP-Rβ). Micro-sized specimens with a thickness of 100 µm were used for aging experiments at 60 °C in chlorinated water with 5 mg/L free chlorine, and aging indicators were monitored for exposure times of up to 2000 h. On the other hand, superimposed mechanical-environmental tests were carried out by using cracked round bar specimens with a diameter of 14 mm to determine the fatigue crack growth (FCG) resistance of both PP-R grades at 60 °C in non-chlorinated and chlorinated water. PP-Rβ was found to outperform PP-Rα with an about 30% higher time-to-embrittlement value of 2000 h. Furthermore, PP-Rβ exhibited an enhanced FCG resistance in both non-chlorinated and chlorinated water. The effect of chlorine content on the deterioration of the FCG resistances was significantly more pronounced for PP-Rα.

The Effect of Residual Stress on the Process of Crack Growth Rate Determination in Polymer Pipes

Accelerated tests of the polyethylene pipes are necessary for the pipe lifetime calculations. Accepted methodology for prediction of slow crack grow rate in these materials is fatigue testing of CRB (cracked round bar) specimens. This paper deals with the FEM modelling of the crack propagation during the CRB test under the influence of residual stresses. The crack growth is described based on the stress intensity factor and Paris-Erdogan law. The purpose of this research is to determine, if the residual stress influences crack behavior during CRB test.

Evaluation of fatigue crack behavior in electron beam irradiated polyethylene pipes

A cracked round bar (CRB) fatigue test was employed to determine the slow crack growth (SCG) behavior of samples from high density polyethylene (HDPE) pipes using PE4710 resin. The structure property relationships of fatigue failure of polyethylene CRB specimens which have undergone various degree of electron beam (EB) irradiation were investigated by observing fatigue failure strength and the corresponding fracture surface morphology. Tensile test of these HDPE specimens showed improvements in modulus and yield strength while the failure strain decreased with increasing EB irradiation. The CRB fatigue test of HDPE pipe showed remarkable effect of EB irradiation on number of cycles to failure. The slopes of the stress-cycles to failure curve were similar for 0–100kGy; however, significantly higher slope was observed for 500kGy EB irradiated pipe. Also, the cycle to fatigue failure was seen to decrease as with EB irradiation in the high stress range, ∆σ=(16MPa to 10.8MPa); however, 500kGy EB irradiated samples showed longer cycles to failure than the un-irradiated specimens at the stress range below 9.9MPa and the corresponding initial stress intensity factor (∆KI,0)=0.712MPam1/2. The fracture surface morphology indicated that the cross-linked network in 500kGy EB irradiated PE pipe can endure low dynamic load more effectively than the parent pipe.

Determination of fracture toughness and fatigue crack growth rate using circumferentially cracked round bar specimens of Al2014T651

Fracture toughness (KIC) and fatigue crack growth rate (FCGR) are the important material properties in fracture mechanics. ASTM-E399 and ASTM-E647 are the standards for determination of KIC and FCGR of metallic materials. These standards recommend the use of compact tension (CT) or single edge notched bend (SENB) test specimens. Literature review indicates that CT or SENB specimens are complex in nature, difficult to manufacture, require typical fixtures for loading during experimentation and the test procedures using CT or SENB are time consuming and cumbersome. An alternate specimen geometry which can overcome the above said drawbacks is needed by the industry which can be used as standard test specimen geometry. This research work explains use of circumferentially cracked round bar (CCRB) specimens of high strength Al2014T651 alloy for determination of KIC and FCGR.The pre-cracked round bar specimen was loaded in tensile in a universal testing machine and pulled till failure. Using suitable stress intensity factor equations the fracture toughness can be calculated. In case of crack growth test, the pre-cracked round bar specimen is allowed to rotate under fatigue load. The ratio of length of crack propagated to the number of cycles to failure was the crack growth rate. The SEM analysis of fractured surfaces was also done.The results are comparable with the values reported in the literature obtained by using standard test specimens. There are numerous advantages of using round bar specimen in KIC and FCGR tests. It is concluded that, the methodology of determination of fracture toughness and fatigue crack growth rate using CCRB specimens is relatively simple, reliable, fast and economical. CCRB specimen may be recommended as a standard test specimen for fracture toughness as well as crack growth tests.

Characterisation of quasi-brittle fatigue crack growth in pipe grade polypropylene block copolymer

The aim of this work was to characterise quasi-brittle fatigue crack growth in a pipe grade polypropylene block copolymer. Cyclic cracked round bar specimen tests have been identified as a possible way to induce cyclic quasi-brittle crack growth within a reasonable amount of time. To ensure quasi-brittle failures, fracture surfaces have been examined using scanning electron microscopy. For further acceleration of the tests, higher temperatures with adapted load ranges have been applied. For 50 °C and 80 °C, a decrease in testing times of approx. 60% and 80% was achieved without significant changes in failure behaviour (e.g. cyclic crack opening displacement (Δδ) curves or fracture surfaces) of the tested specimens. Relationships between applied load and testing times, as well as between crack initiation and failure have been examined. A change in ratio of crack initiation to total failure time was observed at higher temperatures. This has to be considered in future tests, especially when comparing different types of polypropylene at temperatures, differing from the real application.

Investigation of slow crack growth initiation in polyethylene pipe grades with accelerated cyclic tests

A model for crack initiation in CRB specimens for a high density polyethylene (PE-HD) pipe grade was developed for possible implementation in a lifetime assessment procedure. Additional, an analysis of the crack tip process zone was performed in order to get a better understanding for the mechanisms of the initiation of slow crack growth. Using concepts of linear elastic fracture mechanics (LEFMs), cyclic tests on Cracked Round Bar (CRB) specimens at a temperature of 23°C were carried out at different R-ratios. The tests, which were performed at two different Stress Intensity Factors (KI,max), were stopped after different cycle numbers prior crack initiation. Using a hot melt adhesive, the crack tips of the CRB specimens were frozen in a loaded condition, in order to prevent a collapse of craze fibrils in the process zone. After a suitable specimen preparation with microtome, optical microscopy and scanning electron microscopy (SEM) were used to observe the geometry of the process zone at the crack tip and to measure the dimensions of the process zone deformation. Depending on the cycle numbers the results showed an increase of the length of the process zone. The measured process zone geometries were extrapolated to an R-ratio of 1 which represents static loading conditions and compared to the Dugdale-Model, which predicts the process zone size at a crack tip.

Investigation of the Slow Crack Growth Behavior of Static and Cyclic Loaded Specimens of Polyethylene by 2D and 3D Optical Fracture Surface Analysis

AbstractSummary: A fracture surface investigation was conducted to study the applicability of cracked round bar (CRB) specimens for an accelerated extrapolation concept for a lifetime assessment of polyethylene (PE) pipes. Scanning electron microscopy and topography metrology with InfiniteFocus were used to study the slow crack growth behavior in CRB specimens at different loading conditions. The results confirm the compliance of the CRB test with the requirements of linear elastic fracture mechanics.

Determination of Plane-Strain Fracture Toughness Using Cracked Round Bar Specimens and their Applicability for Reliability Assessment Calculations

The research work aimed to present the usability of the cracked round bar specimen type for the determination of the linear-elastic plain strain fracture toughness values on steels; and to demonstrate the applicability of the testing results for the reliability assessment of structural elements having cracks or crack like defects. Micro-alloyed structural steel and heat resistant steel were tested at elevated temperatures, at 260°C and 410°C, respectively, using small diameter specimens. Four different methods (equations) were used for the evaluation of the tests, and the determined fracture toughness values were compared with each other. Reliability assessment calculations were realised on two structural element models, critical crack sizes and safety factors were determined for all cases

Cyclic crack growth tests with CRB specimens for the evaluation of the long-term performance of PE pipe grades

It is well known that resistance against slow crack growth is important for the lifetime of pressurized polyethylene (PE) pipes. Thus, several methods have been proposed in recent years to evaluate the long-term performance of PE using fracture mechanics. It is generally believed that this leads to results more quickly compared to internal pressure tests. In the presented research work, a method was implemented using fatigue loading of cracked round bar (CRB) specimens to characterize crack growth resistance. The method was applied to five commercially available PE pipe materials and the results were compared with the full notch creep test (FNCT). The same ranking was found with both methods, but it was obvious that fatigue crack growth (FCG) experiments were faster by up to two orders of magnitude, especially when characterizing modern (bimodal) PE types.

Mechanics of deformation and ductile tearing in cracked round bar specimens

This paper presents observations and analyses of the deformation and early stage of ductile tearing in cracked round bar (CRB) fracture specimens. Fracture surface topography analysis (FRASTA), stability analysis and simple kinematic analysis are applied to CRB specimens of HY-130 steel and pressure vessel steel welds. The J–resistance curve for HY-130 steel is constructed over the first 250 μm or so of crack extension and bounds on the amount of crack extension preceding the point of maximum load are established. Further, the influence of crack growth on J Ic defined at maximum load is assessed and it is argued that, for small CRB specimens, this definition is consistent with that of standard testing methods. It is shown that crack mouth and crack tip opening displacement values can be readily estimated from global displacement measurements with extensometers. It is also shown that bending of the CRB specimen due to an eccentric ligament has a relatively small effect (<5%) on the toughness value of tough alloys. These results point the way for future research and should prove useful in establishing more reliable, simple, testing practices and in developing validated data analysis methods for CRB fracture tests.