Cryogenic Fracture Research Articles

Electron backscattered diffraction study of cleavage fracture in pure iron

EBSD studies were performed to explore the potential of trace analysis to determine the crystallography of cleavage fracture plane and planar defects that form during cryogenic fracture of pure iron. Single- and double-surface trace analysis revealed that cleavage occurred along the cube {1 0 0} planes. In addition, using EBSD we were able to identify the planar defects that formed during fracture to be twins on {1 1 2} planes. The combination of trace analysis and orientation mapping was able to uniquely identify the specific variants of the {1 1 2} planes associated with each set of twins. The twin/matrix interface was mostly scalloped suggesting that twin formation could be the result of twinning on multiple twin planes.

Specimen size effects in the cryogenic fracture toughness testing of Fe–12Cr–12Ni–10Mn–0.24N stainless steel

This paper examines the effects of test specimen size on the cryogenic fracture toughness properties of a nitrogen-strengthened austenitic stainless steel for superconducting magnet structures in fusion energy systems. Single-specimen J-integral tests were performed on CT (compact tension) specimens with and without side-grooves in liquid helium at 4 K. The aspect ratio (specimen width to thickness) and thickness were varied. A three-dimensional finite element analysis was also conducted to investigate the effects of specimen thickness and side-groove on the through thickness distributions of the J-integral values. The results of the finite element analysis are used to supplement the experimental data.

A Novel Polyamide 12 Polymerization System and its Use for a LCM-process to Produce CFRP

The aim of this study was to investigate the scope of the recently developed polymerization molding system for polyamide 12 (PA12), especially with respect to the formation of carbon fiber-reinforced PA12 composites. Initial screening of PA12 composite formation was performed by means of an internal mixer in order to identify the suitable type of surface-treated carbon fiber (CF). The content of residual lauryllactam (LL) monomer reflected the influence of the CF-treatment on the polymerization molding involving anionic in situ polymerization of LL. The cryogenic fracture surfaces were analysed by scanning electron microscopy (SEM) in order to evaluate the adhesion quality between the components. Finally, a bench-scale polymerization molding process was established successfully for the fabrication of multi-axial laminates. Macromechanical tests and dynamic mechanical thermo-analysis (DMTA) indicated that the performance of the new polymerization molding process is very similar to that of state-of-the-art PA12/CF composites.

Cryogenic Damage and Fracture Behaviors of G-11 Woven Glass-Epoxy Laminates

We discuss the cryogenic damage and fracture behaviors of G-11 woven glass-epoxy laminates. In conjunction with the cryogenic fracture toughness test, a finite element analysis was conducted to predict the fracture and deformation for models of the compact tension (CT) specimens. Effective elastic moduli were determined under the assumption of uniform strain inside the representative volume element. Hoffman’s criterion was selected as the criterion for fiber-dominated failure, and matrix cracking was detected by the maximum strain criterion. The virtual crack extension method was adopted to calculate strain energy release rate which leads to determination of stress intensity factor. In order to verify the model, correlations between experimental and analytical results were made, in terms of the load-displacement response and the extent of damage growth. Reasonable agreements between the calculations and the experimental data were achieved.

Fracture behavior of thick-section weldment in Fe-12Cr-12Ni-10Mn-0.24N stainless steel at liquid helium temperature

Type JJ1, a nitrogen-strengthened austenitic stainless steel (Fe-12Cr-12Ni-10Mn-0.24N), is emerging as the preferred structural material for superconducting fusion magnet casings to operate at liquid helium temperature (4 K). This article describes an examination of the cryogenic fracture behavior of thick-section (200 mm) weldment in JJ1 forged stainless-steel plate. The compact tension geometry was used for the elastic-plastic fracture toughness tests at 4 K. The through-thickness fracture toughness was investigated with particular attention to the influence of the nitrogen and inclusion contents. The magnitude of the 4-K fracture toughness decreased with increasing inclusion content. In addition to fracture toughness data, temperature rise near the crack tip during fracture was measured. Significant adiabatic heating occurred, and internal temperature rises up to 52 K were reported.

Small punch testing for determining the cryogenic fracture properties of 304 and 316 austenitic stainless steels in a high magnetic field

This paper examines the effect of magnetic field on the fracture properties of austenitic stainless steels at liquid helium temperature (4 K). Small punch tests were performed on cold-rolled 304 and 316 austenitic stainless steels. Previously proposed correlation for small punch and elastic–plastic fracture toughness test methods was applied to predict a small punch test-based fracture toughness from equivalent fracture strain.

Experimental and Analytical Evaluation of the Notched Tensile Fracture of CFRP-Woven Laminates at Low Temperatures

This paper describes an experimental and analytical study on the cryogenic fracture behavior of CFRP-woven laminates under tension with a sharp notch. Single-Edge Notched Tension (SENT) tests were performed at room temperature, liquid nitrogen temperature (77 K), and liquid helium temperature (4 K). During the SENT tests, AE method was implemented to assist in identifying the occurrence of damage which forms from a notch. The growth of crack from notch tip was characterized by microscopical observation and AE response. A finite element analysis was also carried out to calculate the strain energy release rate and stress intensity factor for SENT specimens. The results of the finite element analysis are used to supplement the experimental data. An example is given to show how the acoustic emission data obtained on SENT specimens can be used to nondestructively test the fracture properties of CFRP-woven laminates at low temperatures.

Influence of Thermal Aging on Cryogenic Fracture Toughness of JN 1 Steel for Superconducting Magnet Structures of Fusion Reactors

Elastic-plastic fracture toughness tests were conducted at 77K and 4K to examine an influence of thermal aging on fracture properties of an austenitic stainless steel JN 1 at cryogenic temperature, which is a candidate material for superconducting magnet structures of fusion reactors. The tests were applied to solution-treated material and materials aged at 923-1073 K for 5 h. The experimental results showed that the fracture toughness JIC measured on the thermally aged materials were smaller than that on the solution-treated material and decreased with increasing aging temperature. This decrease in fracture toughness due to thermal aging was caused by precipitation and coarsening of grain boundary M23C6 carbides. Additionally, small punch (SP) tests were conducted at cryogenic temperatures to investigate the relationship between the obtained JIC and SP properties. It was found that the equivalent fracture strain eqf in SP specimens was linearly correlated with JIC irrespective of testing temperature. This correlation suggests that the SP testing technique provides a useful tool for evaluating JIC at cryogenic temperature.

Microfibrillar Reinforced Composites from PET/LDPE Blends: Morphology and Mechanical Properties

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight (wt) ratio of PET/LDPE equal to 30/70 and 50/50, respectively. Dogbone samples with MFC structure were characterized by means of light microscopy (LM), scanning (SEM), and transmission (TEM) electron microscopy, and mechanical testing. The SEM observations of cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils while LM and TEM on thin slices indicate on orientation in machine direction. By means of TEM on stained ultrathin slices, one observes the formation of transcrystalline layers of the LDPE matrix on the surface of the PET microfibrils where the crystalline lamellae are parallel to each other and are placed perpendicularly to the fibril surfaces. By this transcrystallization and the relatively high aspect ratio of the fibrils (at least 50), the impressive mechanical properties of the injection‐molded samples can be explained. The elastic modulus is about 10 times higher than that of LDPE and about three times higher than LDPE reinforced with glass spheres, approaching the modulus of LDPE reinforced with 30 wt% short‐glass fibers (GF). The tensile strength is at least two times higher than that of LDPE or of LDPE reinforced with glass spheres, approaching that of LDPE reinforced with 30 wt% GF.

201 Fe-12Cr-12Ni-10Mn-0.24Nステンレス鋼の極低温破壊特性と試験片寸法効果(材料力学I)

201 Fe-12Cr-12Ni-10Mn-0.24Nステンレス鋼の極低温破壊特性と試験片寸法効果(材料力学I)

Small Punch Testing for Determining the Cryogenic Fracture Properties of 304 and 316 Austenitic Stainless Steels in a High Magnetic Field

Small Punch Testing for Determining the Cryogenic Fracture Properties of 304 and 316 Austenitic Stainless Steels in a High Magnetic Field

Collapse pressures of biodegradable stents

Biodegradable stent prototypes were produced from poly l-lactic acid polymers with different molecular weights. The effects of molecular weight, drug incorporation and stent design on the collapse pressure of the stents were evaluated. While molecular weights did not show a significant effect on the collapse pressure of the stents, drug incorporation at high percentage decreased the collapse pressure of the stents substantially. Cryogenic fracture surfaces showed significant drug agglomeration as the concentration increased. The design of the stent was also found to a have significant effect on the collapse pressure. The stent produced from the same material has a higher collapse pressure when the load bearing surface area is increased.

Cryogenic fracture and adiabatic heating of austenitic stainless steels for superconducting fusion magnets

This paper discusses the fracture and the adiabatic heating of austenitic stainless steels that may be used in cryogenic structures for the superconducting magnets of magnetic fusion reactors. Elastic–plastic fracture toughness tests were performed on compact specimens in liquid helium at 4 K. Adiabatic heating was detected by measuring the internal temperature. The effect of test specimen size and side grooving on the cryogenic fracture toughness, and the temperature rise during dynamic crack growth are examined. A size independent toughness parameter results in the case of side-grooved specimens. The effect of inclusion content on the fracture mechanics parameters is also discussed.

LDPE-based thermoplastic elastomers containing ground tire rubber with and without dynamic curing

Attempts were made to use a ground tire rubber (GTR) fraction (particle size: 0.4–0.7 mm) to produce thermoplastic elastomers (TPEs) composed of low-density polyethylene (LDPE), fresh rubber and GTR with and without dynamic curing. GTR has been thermomechanically decomposed in the presence (GTR PM) and absence of processing oil (GTR M) and its decomposition was followed by the increase in the acetone-soluble fraction. The composition of the TPEs was constant, viz. LDPE: rubber:GTR=50:25:25 wt.%. As fresh gums styrene/butadiene (SBR), natural (NR) and ethylene/propylene/diene (EPDM) rubbers were used in blends with GTR R (reference, not treated), GTR M and GTR PM at 1:1 ratio. Dynamic vulcanisation occurred either by sulfur or by peroxide curatives. Based on the thermomechanical and mechanical properties of the resulting TPEs, the best performance was achieved by recipes containing GTR M and EPDM after dynamic vulcanisation with sulfur. This was traced to the dual compatibilizer effect of the EPDM. The ethylene segments of the EPDM are compatible with the LDPE, whereas the surface of the partially decomposed GTR may be involved in both molecular entangling and interphase sulfur crosslinking mechanisms which all yield a better interfacial bonding between the GTR particle and the matrix. The phase structure of the TPEs were assessed by dynamic-mechanical thermal analysis (DMTA) and fractography (etched cryogenic fracture surface) and discussed.

Direct electron microscopic observation of transcrystalline layers in microfibrillar reinforced polymer-polymer composites

Microfibrillar reinforced composites (MFC) comprising an isotropic matrix from a lower melting polymer, i.e., low density polyethylene (LDPE), reinforced by microfibrils of a higher melting polymer, recycled from bottles, i.e., poly(ethylene terephthalate) (PET), were processed under industrially relevant conditions via injection molding in a weight ratio of PET/LDPE = 50/50. Dog bone samples with MFC structure were characterized by means of scanning (SEM) and transmission (TEM) electron microscopy. SEM observations on cryogenic fracture surfaces show an isotropic LDPE matrix reinforced by more or less randomly distributed PET microfibrils. By means of TEM on stained ultrathin slices one observes the formation of transcrystalline layers of LDPE matrix on the surface of the PET microfibrils. In these layers the crystalline lamellae are aligned parallel to each other and are placed perpendicularly to the fibril surfaces. This is in contrast to the bulk matrix where the lamellae are quasi-randomly arranged.

Mechanical Properties of 32Mn-7Cr-0.6Mo-0.3N Austenitic Steel for Cryogenic Applications

The tensile, impact, and fracture toughness tests from ambient temperature to 77 K were carried out on 32Mn-7Cr-0.6Mo-0.3N austenitic steel. The fracture surfaces and the phase constitution were analyzed using scanning electron microscopy and x-ray diffraction. The results show that the relation between yield strength and temperature is σ0.2 · 300 + 1392.4 exp (−0.0106T). The 77 K yield strength is 883 MPa · m1/2 and the KJ0.05 value is about 236 MPa · m1/2. The cryogenic intergranular fracture is fully suppressed. The 77 K fracture surfaces exhibit a tough character composed of many dimples and few small quasi-cleavage facets. The results of x-ray analysis show that the austenite phase of the steel is stable even under cryogenic deformation conditions.

Blends containing core-shell impact modifiers Part 1. Structure and tensile deformation mechanisms (IUPAC Technical Report)

Two impact modifiers, based respectively on polybutadiene (PB) and poly(butyl acrylate-co-styrene) (PBA), are compared in blends with four glassy polymers: polycarbonate (PC), poly(methyl methacrylate) (PMMA), poly(styrene-co-acrylonitrile) (PSAN), and poly(vinyl chloride) (PVC). Dynamic mechanical tests show glass transitions at about -80 °C in PB and -15 °C in PBA. Both modifiers have grafted PMMA shells, which are seen in the transmission electron microscope (TEM) to be about 10 nm thick. The two-stage PB particles have 200-nm-diameter polybutadiene cores, whereas the three-stage PBA particles have 260-nm-diameter PMMA cores, with 20-nm thick PBA rubber inner shells. Under tension, the PB particles cavitate to form single voids on reaching a critical volume strain, and subsequently offer little resistance to dilatation. By contrast, tensile tests performed in situ in the TEM show that the PBA shells form fibrils that are anchored to the rigid core, and act as constraints on further dilatation: the stresses developed in the PBA fibrils can be sufficient to draw fibrils from both the PMMA core and the PSAN matrix. There is evidence that the PMMA shells can debond from the matrix both in cryogenic fracture and in fatigue at 23 °C. Tensile dilatometry shows that the PB particles cavitate at higher strains than the PBA particles, but that the PB particles then cause a rapid volume increase, leading to a low strain at break. By contrast, the PBA particles produce a more controlled dilatation, and higher strains to break. Later papers in this series treat the mechanical and rheological behavior of these blends in more detail.

Plasticized starch–cellulose interactions in polysaccharide composites

This paper is focused on the interactions between leafwood cellulose fibres and a plasticized wheat starch matrix. Different plasticized starch (TPS)-based composites have been elaborated. LDPE-based composites are used as reference materials (no fibre–matrix interactions). After extrusion and injection moulding, the properties of the different composites are analysed. Mechanical properties (tensile tests), thermo-mechanical properties (DMTA) and morphology (SEM) are evaluated. DMTA analysis shows for TPS composites a strong evolution of the main relaxation temperature, which can be linked to the existence of cellulose–starch interactions resulting in a decrease of starch chain mobility. This phenomenon is consistent with the evolution of mechanical behaviour. SEM observations correlate this hypothesis. After cryogenic fracture, TPS composites present fibres, which are embedded in the matrix. On the composites, reinforcing effects have been observed according to the evolution of fibre length and fibre content.

Creep damage mechanisms in polyethylene gas pipes

This study deals with the creep damaging processes of two polyethylene (PE) resins. One is a ductile material while the other is a brittle one. Scanning electron microscopic (SEM) observations as well as chemical analysis are used to identify elementary process involved in the crack initiation and propagation. Of the two considered resins, only the second exhibits a lifetime controlled by slow crack growth (SCG). It is shown that catalytic residues act as initiating agents for the damage. Nevertheless, the presence of such particles in an extruded resin does not lead to a creep damage in every case. Intraspherulitic micro-cracks (mirror zones) can then propagate. Propagation takes place between the lamellae, seemingly through a disentanglement of tie molecules connecting the lamellae. The orientation of the micro-cracks is perpendicular to the largest principal stress. Cryogenic fracture surfaces indicate that the mirror zone gives rise to discontinuous crack growth bands (DCGB). In a pre-cracked specimen, only the DCGB stage takes place in the crack tip.

Cryogenic fracture toughness evaluation for austenitic stainless steels by means of unloading compliance method

Most research to date concerning the cryogenic toughness of austenitic stainless steels has concentrated on the base metal and weld metal in weldments. The most severe problem faced on the conventional austenitic stainless steel is the thermal aging degradation such as sensitization and carbide induced embrittlement. In this paper, we investigate the cryogenic toughness degradation which can be occurred for austenitic stainless in welding. The test materials are austenitic stainless JN1, JJ1 and JK2 steels, which are materials recently developed for use in nuclear fusion apparatus at cryogenic temperature. The small punch (SP) test was conducted to detect similar isothermally aging condition with material degradation occurred in service welding. The single-specimen unloading compliance method was used to determine toughness degradation caused by thermal aging for austenitic stainless steels. In addition, we have investigated size effect on fracture toughness by using 20% side-grooved 0.5TCT specimens.