Dynamice fracture in a semicristalline polymer: an analysis of the fracture surface

The fracture behaviour of a specific material, a semi-crystalline biobased polymer, was here studied. Dynamic fracture tests on strip band specimens were carried out. Fracture surfaces were observed at different scales by optical and electron microscopy to describe cracking scenarios. Crack initiation, propagation and arrest zones were described. Three distinct zones are highlighted in the initiation and propagation zone: a zone with conical markings, a mist zone and a hackle zone. The conical mark zone shows a variation in the size and density of the conical marks along the propagation path. This is synonymous with local speed variation. Microcracks at the origin of the conical marks in the initiation zone seem to develop from the nucleus of the spherulites. In the propagation zone with complex roughness, the direction of the microcracks and their cracking planes are highly variable. Their propagation directions are disturbed by the heterogeneities of the material. They branch or bifurcate at the level of the spherulites. In the arrest zone, the microcracks developed upstream continue to propagate in different directions. The surface created is increasingly smoother as the energy release rate decreases. It is shown that the local velocity of the crack varies in contrast to the macroscopic speed.

Experimental and numerical study of the interaction between dynamically loaded cracks and pre-existing flaws in edge loaded PMMA specimens

Dynamic fracture tests are carried out for four groups of hole-containing edge loaded specimens. The crack growth velocity, crack path, and dynamic toughness are extracted from the experiments using high-speed photography and digital image correlation. The importance of the interaction between the in-coming stress wave and the pre-existing hole is revealed and analyzed. A micromechanical damage model is calibrated to the experimental data from two of the specimens' designs and evaluated for its predictive capabilities using the other experimental configurations. The studied model is shown to be in reasonable agreement with the experimental data, and its limits are discussed

Study on Fracture Mode Mixity Under Impact Loading Based on SHTB

Mode mixity plays an essential role in the criteria of mixed mode fracture. In this paper, a novel approach for precisely controlling mode mixity under dynamic loading is proposed. Numerical simulation of all fracture mode (AFM) specimen and modified compact tension shear (MCTS) specimen loaded by split Hopkinson tension bar (SHTB) apparatus is carried out. With a constraint on MCTS specimen on the direction perpendicular to the incident bar, the dynamic stress intensity factor (DSIF) ratio of mode I to mode II remains constant during the loading process. When the constraint is absent, the DSIF ratio varies due to the vibration of clamps and specimen. The DSIF of MCTS specimen under different loading angles is also studied, and the ratio [Formula: see text] approximately equals the tangent of loading angle, which is also proven in experiments. Moreover, numerical results indicate that the influence of the shape of clamps is significantly reduced by applying a constraint on the specimen. It is concluded that AFM specimen is not suitable for dynamic fracture tests owing to over complicated clamps.

Viscoelastic Effect on the Fracture Toughness of GFRP: Experimental Approach

The dynamic fracture tests were carried out for a glass fiber reinforced plastic specimen with a crack and dynamic fracture toughness was evaluated by examination of cracking at an initial slit root. Before the crack initiated at the slit root, a whitened damage zone was created surrounding the slit tip. The damage zone consists of micro cracking in the matrix, debonding between a fiber and the matrix, and fracture of the fiber. The comparison of the dynamic fracture toughness and the static fracture toughness value shows that the former is around 12 MPa√m and apparently higher than the later, which is 7 MPa√m. To understand those experimental results and mechanics of the damage zone, a dynamic debonding test was carried out and dynamic bonding strength was estimated as around 70 MPa.

Dynamic Fracture Initiation: A Comparison of Two Experimental Methods

This paper presents a comparison of dynamic fracture initiation results obtained from two independent experimental methods: (1) the instrumented version of the precracked Charpy test and (2) a new dynamic fracture test developed at Brown University (BU). Dynamic fracture tests were conducted using both methods on 4340 steel at room temperature and on 1018 cold-rolled steel (CRS) at temperatures over the range −157 to 107°C. This range covered temperatures on the lower shelf, the transition regime, and the upper shelf of the transition temperature curve. Stress intensity rates of K˙I = 2.2 × 105 MN-m−3/2/s and K˙I = 2.2 × 106 MN-m−3/2/s were used in the instrumented precracked Charpy and BU tests, respectively. In the analysis of test results, both elastic (KIc) and elastic-plastic (J-integral) fracture parameters were computed. Results of the two test methods were found to be generally in good agreement, except that some differences were observed between 1018 CRS test results on the upper shelf. Possible reasons for these differences are discussed.