Problems in fracture mechanics characterization of rubber-modified glassy polymers,using double torsion

The double torsion fracture test was tentatively applied to some rubber-modified glassy polymers. By varying the specimen thickness, a substantial variation of the fracture toughness values was obtained, which cannot be attributed to variations of material's properties. Two sorts of problems were encountered, due to the large deformations attained during the test. Firstly, large deformations imply variations in the load moment arm since radii of load points are finite; secondly, system and/or material non-linearities, that are outside the scope of the classic theory of double torsion, may become significant. The application of a purely geometrical correction factor, to account for the large deformations, somewhat reduces, but does not eliminate, the observed thickness dependence of fracture toughness.     

Effect of small notch and absorbed hydrogen on the fatigue fracture in two-step stress test within fatigue limit diagram

It is usually regarded as a common understanding that fatigue failure would not occur if all stresses were kept within fatigue limit diagram. However, it was shown that fatigue failure occurred in some special cases of variable amplitude loading condition even when all stresses were kept within fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using two-step stress pattern for low alloy steel SCM440H. In the case of constant stress amplitude loading, non-propagating crack was formed only at low mean stress and not formed at high mean stress. However, in the case of two-step stress pattern in which the first step stress was chosen as  R  =−1 and the second step stress was with high mean stress, a non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred by the stresses within fatigue limit diagram. In this study, the effect of notch size and shape were examined. The effect of absorbed hydrogen was also investigated. Absorption of 0.3 ppm hydrogen caused more reduction of fatigue limit.     

Comments on “Thermal stresses in rubber-modified glassy polymers”

Journal of Materials Science -     

Effect of modifier concentration on the fracture behaviour of rubber-modified PMMA

Izod fracture surfaces of blends of PMMA with various amounts of a rubber modifier were studied using scanning electron microscopy. Attention was focused on modes of crack initiation and propagation and on the role of the modifier in the fracture process. It was found that the impact strength of this class of materials increased monotonically with an increase in modifier concentration, at least up to 40 wt% modifier. Unmodified PMMA was studied to provide a basis for understanding the morphological features on the fracture surfaces of the rubber-modified blends. It was confirmed that PMMA fractures through the formation and rupture of crazes. This phenomenon was also found to occur in blends containing 10 wt% modifier. However, blends with 20 wt% modifier crazed only in the later stages of the fracture process, when the crack speed had exceeded some critical value. No evidence of crazing was found in blends with 30 and 40 wt% modifier loadings, although extensive plastic deformation was observed on the fracture surfaces.     

Influence of notch severity on the impact fracture behavior of aluminum alloy 7055

In this paper, the conjoint influence of notch severity and test temperature on the impact behavior of an Al-Zn-Mg-Cu alloy 7055 in the T7751 microstructural condition is presented and discussed. Notch angles of 45°, 75° and 90° were chosen for a standard charpy impact test specimen containing two notches. For a given angle of the notch the increase in dynamic fracture toughness, with test temperature, is most significant for the least severe of the notches, i.e. 45°. At a given test temperature, the impact toughness of the T7751 microstructure decreased with an increase in notch severity. An increase in notch severity resulted in essentially Mode I dominated fracture at all test temperatures. The influence of localized mixed-mode loading is minimal for the alloy has low dynamic toughness. The impact fracture behavior of the alloy is discussed in light of alloy microstructure, mechanisms governing fracture and the deformation field ahead of a propagating crack.     

Influence of notch severity on thermomechanical fatigue life of a directionally solidified Ni‐base superalloy

The aim of this work is to understand the influence of notches under thermomechanical fatigue (TMF) in a directionally solidified Ni‐base superalloy. Experiments were performed utilizing linear out‐of‐phase and in‐phase TMF loadings on longitudinally oriented smooth and cylindrically notched specimens. Several notch severities were considered with elastic stress concentrations ranging from 1.3 to 3.0. The local response of the notched specimens was determined using the finite element method with a transversely isotropic viscoplastic constitutive model. Comparing the analysis to experiments, the locations observed for crack nucleation in the notch, which are offset from the notch root in directionally solidified alloys, are consistent with the maximum von Mises stress. Various local and nonlocal methods are evaluated to understand the life trends under out‐of‐phase TMF. The results show that a nonlocal invariant area‐averaging method is the best approach for collapsing the TMF lives of specimens with different notch severities.     

The influence of notch depth on the fracture mechanics properties of polymer concrete

This research deals with unreinforced and fiber-reinforced Polymer Concrete (PC). Polymer Concrete is usually composed of natural aggregates such as silica sand, binded together with a thermoset resin, such as unsaturated polyester. In this paper it is reported the use of a direct method to calculate two size independent fracture parameters, the critical stress intensity factor, K _Ic, and the critical crack tip opening displacement, CTOD_C, from experimental results of two different size single edge notched beams, 10 and 20 mm, subjected to three point bending under quasi-static loading condition. This method is called the Two Parameter Fracture Model (TPFM). Also the Fracture Energy, G _f, of the specimens were measured to verify its size dependency. Epoxy and polyester resin specimens were studied and epoxy resin specimens reinforced specimens with short carbon and glass fibers were considered.     

Effect of initial notch orientation on fracture toughness in fail-safe steel

A 0.4C–2Si–1Cr–1Mo steel bar with an ultrafine-elongated grain (UFEG) structures was produced by multi-pass warm caliber rolling. The test sample was machined from the rolled bar with 0°, 45°, and 90° rotation along the rolling direction, and a static three-point bending test was conducted at ambient temperature. The toughness anisotropy on the steel with UFEG structures were studied, including the crack propagation on the basis of the microstructural features. The strength and toughness decreased with an increase in the rotation angle along the rolling direction. The toughness decreased drastically, compared to the strength. The notch orientation dependence on toughness is due to differences in the spatial distribution of weak sites such as {100} cleavage planes and boundaries of elongated grains. For the toughness design in ultrafine-grained materials, it is essential to understand the spatial distribution of these weak sites as well as the grain size.     

Effect of notch on fatigue behaviour of a directionally solidified superalloy at high temperature

The effect of notch types and stress concentration factors (K_t) on low cycle fatigue life and cracking of the DZ125 directionally solidified superalloy has been experimentally investigated. Single‐edge notched specimens with V and U type geometries were tested at 850 °C with stress ratio R = 0.1. High temperature in situ optical method was used to observe crack initiation and short crack propagation. Scanning electron microscope observation of fracture was used to analyse the failure mechanism. The results reveal that fatigue resistance decreases with K_t increasing from 1.76 to 4.35. The ratcheting is found to be affected by both K_t and the nominal stress from the displacement–force curve. In situ observations indicate that the cracking does not occur at the notch apex but at the location where the max principal stress or Hill's stress is the highest. According to the scanning electron microscope observations, the failure of the notched specimens strongly depends on the anisotropy microstructures.     

The effect of initial notch shape of compact specimen on fatigue pre-cracking and fracture toughness testing

The circular notched compact specimens, along with standard specimens having straight or chevron notch are provided for fatigue and fracture toughness testings in order to study the crack observation capability during fatigue pre-cracking, skewness of the crack front, and the resulting fracture toughness K_Q. The test results indicated that circular notched specimens significantly facilitate the crack observation during fatigue testing as the cracks initiate on both surfaces of the specimen. No remarkable differences were observed on geometries of the fatigue crack front obtained and the resulting fracture toughness among these three types of specimen. The macroscopic observation of beach marks on the fracture surfaces revealed that, in the present material Ti-6Al-4V (ELI), the advance of only 1.3% of the whole crack length corresponded to the load level at which fracture toughness K_Q was evaluated.     

Effect of notch root radius on the initiation and propagation of fatigue cracks

Under the conditions of constant nominal applied stress, increasing notch root radius causes an increase in the number of cycles to initiate a fatigue crack at a notch root. An explanation of the effect is given in terms of the effective stress concentration factor of the notch. Data are presented which indicate that variations in notch root radius may cause changes in the crack growth rate during the initial stages of propagation from a notch.     

Effect of skin fracture on failure of a bilayer polymer structure

A thin skin of low tensile failure strain, if bonded to the tensile surface of an un-notched impact bend specimen of much tougher material, can change the global failure mode from ductile to brittle. A novel model of this well-known effect is developed and applied to results from impact tests on a tough core of polyamide-polyethylene blend, with a single skin of brittle EVOH. At a fixed crosshead speed, notched specimens of the blend become brittle at a relatively low temperature T _bt. Un-notched bilayer specimens continue to show skin fracture up to a considerably higher temperature T _fs; above this temperature they do not fail at all but below T _bt they too fail in a brittle manner. Within the temperature range from T _fs down to T _bt there is a transition from crack arrest, either at the skin/core interface or further into the core where a crack would not normally propagate, to brittle fracture. This brittle fracture temperature is predicted by modelling the process as a three-phase impact event. In the first phase, the striker bends the bilayer quasi-statically. The second phase begins with instantaneous fracture of the skin at its failure strain. The skin ends retract at finite speed, and a craze grows in the adjacent core material to accommodate the local strain singularity. The last phase is a striker-driven impact event similar to that in a notched bend specimen of the core material, except that the crack-tip craze already bears the adiabatic temperature distribution generated while it was driven open by skin retraction. The criterion for craze decohesion, and hence for a crack jump, is the same adiabatic decohesion criterion which accounts for the speed-dependence of impact fracture in notched monolayer specimens. Applied computationally, this model predicts whether a bilayer structure fails in a brittle way or whether cracks initiated in the skin are arrested, either temporarily or permanently, at the skin/core interface.     

Static fatigue in ceramic materials: influences of an intergranular glassy phase and fracture toughness

Static fatigue behaviour in various kinds of non-transforming ceramics has been investigated. It was found that static fatigue is closely related to the presence of a glassy phase between adjacent grains, as well as fracture toughness. Non-oxide ceramics, such as reaction-bonded silicon nitride which scarcely contains the glassy phase (group I), are insensitive to static fatigue, whereas non-transformation oxide ceramics, like alumina and non-oxide ceramics such as silicon nitride which contains the glassy phase (group II), are sensitive to static fatigue. However, static fatigue behaviour in the materials of group II also depends strongly on fracture toughness. K _IC. Namely, fatigue parameter n increases linearly as K _IC increases. From such a dependence the life time relation in the materials of group II is proposed as in terms of applied stress _S and K _IC.     

Evaluation of fatigue crack initiation life from a notch

Local strain at the notch-root and its effect on fatigue crack initiation was investigated in four metals by the real-time, fine-grid method. Special attention was focused on local notch-root strain behaviour until crack initiation. From the application of strain hysteresis at the notch root, the maximum strain under loading conditions during each cycle was investigated in detail. One of the main results was that the maximum strain value at the first cycle of the fatigue test coincided with that at crack initiation. Maximum strain defined from the cyclic strain changes at the notch root was proposed as one possible parameter for estimating fatigue crack initiation life. Based on the curvilinear relationship between maximum strain and number of cycles to crack initiation, a new life evaluation method for fatigue crack initiation is proposed. This approach differs fundamentally from the usual fracture mechanics method based on the stress intensity factor.