Monotonic Fracture Test Research Articles

Fracture investigation at 300oC on carbon-steel and austenitic stainless-steel pipes of NPPs under cyclic loads

The Leak-Before-Break (LBB) of high enthalpy piping of Indian NPPs require fracture stability demonstration of pipelines with postulated cracks under design basis earthquake loads. Earlier large number of cyclic fracture and corresponding monotonic fracture tests on large sized pipe components were conducted at room temperature. In current program, monotonic/cyclic fracture experiments are extended to reactor operating temperature (300 oC). The aim of this study is to demonstrate the structural integrity of SA333Gr6 piping and Stainless Steel (SS) 304LN welds under monotonic/cyclic loading which simulates extreme earthquake events and operating temperatures. Total six pipes/pipe welds of carbon steel (8-inch NB ~219 mm outer diameter and 19 mm thick) and SS304LN (12-inch NB~324 mm and outer diameter ~25 mm thick) with through-wall cracks have been tested under four-point bending. Two monotonic fracture tests have been conducted at 300 oC under monotonic loadings. Different experimental results like LLD (Load Line Displacement), Total Load, CMOD (crack mouth opening displacement), Crack Growth values have been obtained. Based on this data the load carrying capacity are obtained for both 8-inch and 12-inch cracked pipes. The cyclic fracture tests have also been conducted at 300 oC under fully reversing cyclic loading on four pipes. Three 8-inch SA333Gr6 pipes have been tested under different magnitude of peak load values. Similarly, one 12-inch SS pipe is also tested under cyclic loading. For carbon steel pipes, the reduction in load capacity is slightly higher under cyclic loadings at 300 oC compared to that at room temperature. However, for stainless steel pipes the load reduction factor at 300 oC is in good agreement with that at room temperature.

Case study: Characterization of short-term field aged asphalt binders in cold mix asphalt pavement

In recent years, rejuvenators have been used in cold asphalt mixtures to assist sustainability efforts. However, most studies on this topic have focused on laboratory prepared samples only. This study characterized the short-term field asphalt binders that were extracted from the surface layer of a field project that utilized cold mix with 100% reclaimed asphalt pavement (RAP) and rejuvenator in Florida, USA. Rheological testing was performed on the extracted RAP binders with and without rejuvenator with respect to high and intermediate temperature performance grading (PG), multiple stress creep recovery (MSCR), frequency sweep, and monotonic fracture tests. Results showed that there was no significant difference in rutting resistance between extracted rejuvenated RAP binder and extracted RAP binder. The rejuvenated RAP binder had lower stiffness and better relaxation properties than the RAP binder suggesting it can soften recycled binders. Furthermore, the rejuvenated RAP binder exhibited higher fatigue life and higher failure strain than the RAP only binder, suggesting that the addition of rejuvenator in the 100% RAP can restore the functionality and flexibility of recycled binders in a short-term field aging period.

Prediction of fatigue crack growth behavior of chemically stabilized materials using simple monotonic fracture test integrated with computational cohesive zone modeling

Mechanical behavior and fatigue damage characteristics of chemically stabilized soils under cyclic loading conditions are complex and require better understanding due to its large demand in many critical infrastructures including roadways. Fatigue tests are usually not easy to conduct and time-consuming, and the challenges increase when materials become more complex due to chemical stabilization. This study proposed a simple monotonic fracture test that is integrated with computational fracture modeling to identify fracture characteristics for predicting fatigue damage behavior of complex materials such as chemically stabilized soils. Toward that end, an extrinsic inelastic cohesive zone model (CZM) was used. The fracture parameters of the CZM were first obtained from a monotonic fracture test that is integrated with its model simulation. With the fracture parameters, the same CZM was used to predict fatigue behavior and ultimate failure. The simulation results indicate that the predicted fatigue behavior and performance are comparable to the fatigue test results. With limitations remained for further improvements, the proposed experimental-computational approach incorporated with the inelastic fracture model such as the CZM in this study presented its benefits for predicting time-consuming and labor-intensive fatigue behavior of complex materials.

Cyclic-tearing behaviour and J–R curves of Indian NPP pipes under displacement-controlled cyclic loading

Currently the ductile fracture assessment of cracked pipes, under earthquake loading, is based on the monotonic J–R curve where the earthquake load is considered as a one-time applied load monotonically increasing to its maximum magnitude. The cyclic nature of earthquake load and associated cyclic-tearing damage (or cyclic J–R curve) is not explicitly considered. A series of cyclic-tearing and monotonic fracture tests is performed on circumferential through-wall-cracked pipes and pipe welds (five material categories) which are used in Indian Nuclear Power Plants (NPPs) piping system. The fracture behaviour of these pipes under cyclic loading vis-à-vis monotonic loading has been studied. The cyclic J–R curves of all material categories are significantly lower than the monotonic J–R curves and found strongly dependent on the applied load-history. Hence, cyclic J–R curve shall be evaluated for the load-history which is consistent with plant site (anticipated seismic loading), piping system layout and the crack location. The maximum moment observed during cyclic loading test is lower by a small to moderate amount than that obtained in the monotonic fracture tests on identical pipe. A relation has been obtained for cracked pipes which assesses if large crack growth/tearing would take place under displacement-controlled cyclic loading.

Characterising mode I/mode II fatigue delamination growth in unidirectional fibre reinforced polymer laminates

Abstract In this paper, fatigue life estimation for delamination growth of laminated fibre reinforced polymer (FRP) composite structures in mode I and mode II based on fracture mechanics is presented. The proposed method was applied to delamination of glass/epoxy laminated composite. Both the threshold energy release rate (Gth) and the delamination propagation based on Paris’ law were studied. The double cantilever beam (DCB) specimen for mode I and 3 points End-Notched Flexure (3ENF) specimen for mode II were used for monotonic fracture tests and the resistance GIR and GIIR as a function of delamination length were determined. For DCB tests, the fatigue onset life test was conducted and the threshold energy release rate, GIth, was found for the subcritical region. Constant amplitude, displacement controlled cyclic fatigue test for both modes was conducted and the delamination crack growth rate (da/dN) as a function of maximum cyclic energy release rate GImax and GIImax for DCB and 3ENF specimens were determined, respectively. From curve fitting to the experimental data, the Paris’ law material constants C and m for mode I and mode II were obtained. Finally, the SEM fractography studies of delaminated surfaces of 3ENF static and fatigued specimens have been done and the different features observed on these surfaces were discussed.

Pure shear fracture study in a brittle graphite material containing a U-notch

A disc-type specimen containing a central bean-shaped slit, called the U-notched Brazilian disc specimen, was utilized to measure experimentally the mode II notch fracture toughness of a type of commercial polycrystalline graphite. First, 24 U-notched Brazilian disc graphite specimens were fabricated for four different notch tip radii, and the monotonic fracture tests were performed under pure mode I and pure mode II loading conditions; 12 for each loading mode. The recorded fracture loads were converted to the corresponding notch fracture toughness values, and the ratio mode II notch fracture toughness/mode I notch fracture toughness was experimentally determined. Then, assuming that the experimental values of mode I notch fracture toughness are known, the mode II notch fracture toughnesses were theoretically predicted by means of two brittle fracture criteria, namely the existing U-notched maximum tangential stress and the newly developed U-notched mean stress. A very good agreement was found to exist between the theoretical and the experimental results. Also, found in this research was that both the criteria could predict well the experimentally measured fracture initiation angles.

A Laboratory Study to Predict the Rutting and Fatigue Behavior of Asphalt Concrete Using the Indirect Tensile Test

Abstract Rutting, fatigue, moisture susceptibility, and thermal cracking are the primary distresses of asphalt pavement. Currently several test methods are used to predict the rutting and fatigue behavior of asphalt concrete. This study uses the indirect tensile (IDT) test to evaluate both the rutting and fatigue behavior of asphalt. Recycled concrete aggregate (RCA) and virgin aggregate are blended at different percentages (0 %, 20 %, 40 %, 60 %, 80 %, and 100 %) to produce mixes with a variety of fatigue and rutting performance. The IDT rutting test was performed by running high temperature IDT flow time and strength tests. In addition, flow number tests were performed using an asphalt mixture performance tester (AMPT). A strong correlation is observed between the high temperature IDT flow time/strength and flow number from AMPT. The test results show that IDT tests can be used to predict the rutting behavior of asphalt concrete at high temperatures. To characterize fatigue, cyclic IDT and monotonic fracture energy tests were performed at intermediate temperatures. At intermediate temperatures, good correlation is found between the fatigue life obtained from cyclic IDT test results and the fracture energy obtained from monotonic fracture test results. Based on the laboratory test results, the IDT test can be used to evaluate both the fatigue and rutting behavior of asphalt concrete. Considering that IDT testing has been used to characterize thermal cracking and moisture susceptibility, the IDT test has the potential to serve as a single performance test for fatigue, rutting, thermal cracking and moisture damage. Validation of the findings with more materials and field performance are recommended.