Delamination growth mechanisms in woven glass fiber reinforced polymer composites under Mode II fatigue loading at cryogenic temperatures

The purpose of this research is to characterize the cryogenic delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates subjected to Mode II fatigue loading. Mode II fatigue delamination tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using the four-point bend end-notched flexure (4ENF) test method, and the delamination growth rate data for the woven GFRP laminates were obtained. The energy release rate range was determined by the finite element method. Microscopic examinations of the specimen sections and fracture surfaces were also carried out. The present results are discussed to obtain an understanding of the fatigue delamination growth mechanisms in the woven GFRP laminates under Mode II loading at cryogenic temperatures.     

Cryogenic delamination growth in woven glass/epoxy composite laminates under mixed-mode I/II fatigue loading

This paper investigates the fatigue delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode I/II conditions at cryogenic temperatures. Fatigue delamination tests were performed with the mixed-mode bending (MMB) test apparatus at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), in order to obtain the delamination growth rate as a function of the range of the energy release rate, and the dependence of the delamination growth behavior on the temperature and the mixed-mode ratio of mode I and mode II was examined. The energy release rate was evaluated using three-dimensional finite element analysis. The fractographic examinations by scanning electron microscopy (SEM) were also carried out to assess the mixed-mode fatigue delamination growth mechanisms in the woven GFRP laminates at cryogenic temperatures.     

Mode I fatigue delamination growth in GFRP woven laminates at low temperatures

The cryogenic fatigue delamination behavior of glass fiber reinforced polymer woven laminates under Mode I loading has been investigated experimentally and numerically. Fatigue delamination tests were conducted using double cantilever beam specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). Fracture surface examination using scanning electron microscopy revealed delamination mechanisms under fatigue loading. A finite element analysis was also employed to calculate the J-integral range and damage distributions. The effects of temperature and loading condition on the fatigue delamination growth rates were discussed.     

Fatigue delamination growth in woven glass/epoxy composite laminates under mixed-mode II/III loading conditions at cryogenic temperatures

We investigate the cryogenic delamination growth behavior in woven glass fiber reinforced polymer (GFRP) composite laminates under mixed-mode II/III fatigue loading. Fatigue delamination tests were conducted with six-point bending plate (6PBP) specimens at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K), and the delamination growth rate data for various mixed-mode ratios of Modes II and III were obtained. The energy release rate was evaluated using the three-dimensional finite element method. In addition, the fatigue delamination growth mechanisms were characterized by scanning electron microscopic observations of the specimen fracture surfaces.     

Analysis of mixed-mode interlaminar fracture and damage behavior of GFRP woven laminates at cryogenic temperatures

The objective of this work is to investigate the interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates loaded in a mixed-mode bending (MMB) apparatus at cryogenic temperatures. The finite element analysis (FEA) is used to determine the mixed-mode interlaminar fracture toughness of MMB specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A FEA coupled with damage is also employed to study the damage distributions within the MMB specimen and to examine the effect of damage on the mixed-mode energy release rate. The technique presented can be efficiently used for characterization of mixed-mode interlaminar fracture and damage behavior of woven laminate specimens at cryogenic temperatures.     

Cryogenic Interlaminar Fracture Properties of Woven Glass/Epoxy Composite Laminates Under Mixed-Mode I/III Loading Conditions

We characterize the combined Mode I and Mode III delamination fracture behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. The eight-point bending plate (8PBP) tests were conducted at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) using a new test fixture. A three-dimensional finite element analysis was also performed to calculate the energy release rate distribution along the delamination front, and the delamination fracture toughnesses were evaluated for various mixed-mode I/III ratios. Furthermore, the microscopic examinations of the fracture surfaces were carried out with scanning electron microscopy (SEM), and the mixed-mode I/III delamination fracture mechanisms in the woven GFRP laminates at cryogenic temperatures were assessed. The fracture properties were then correlated with the observed characteristics.     

Cryogenic fatigue behavior of plain weave glass/epoxy composite laminates under tension-tension cycling

This paper focuses on understanding the tension-tension fatigue behavior of woven glass fiber reinforced polymer laminates at cryogenic temperatures. Tension-tension fatigue tests at frequencies of 4 and 10 Hz with a stress ratio of 0.1 were conducted at room temperature, 77 and 4 K. The fatigue stress versus cycles to failure (S-N) relationships and fatigue limits for 10⁶ cycles were obtained. Fractured specimens tested under fatigue tests were also examined with optical microscope.     

Interlaminar shear properties of composite insulation systems for fusion magnets at cryogenic temperatures

This paper reports the cryogenic interlaminar shear properties of composite insulation systems for the superconducting magnet coils in the International Thermonuclear Experimental Reactor (ITER). Short beam shear tests were performed at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) on three insulation systems consisting of woven glass fiber reinforced plastic (GFRP) composites with different polymer resins and polyimide films, and the dependence of their apparent interlaminar shear strength on the temperature and the polymer resins was discussed. A detailed observation of failed specimens was made to verify the failure mechanisms. Insulation systems subjected to gamma irradiation at room temperature were also considered, and the effect of irradiation on the cryogenic interlaminar shear properties was examined.     

Effects of temperature on delamination growth in a carbon/epoxy composite under fatigue loading

This paper presents a study of delamination growth in HTA/6376C carbon fibre/epoxy laminates. Tests were conducted under Mode I, Mode II and mixed-mode static and fatigue loading at both ambient conditions and elevated temperature. The results show that the strain energy release rate threshold values for delamination growth under fatigue loading are significantly lower than the critical energy release rates in static tests. At elevated temperature, the threshold values in the fatigue loading were only about 10% of the critical values in the static tests. A fractographic analysis of the delamination growth revealed that the fracture surfaces generated at elevated temperature generally were similar to the fracture surfaces generated at room temperature. Nevertheless, some differences in morphology of the fracture surfaces were observed, and their effect on the static and fatigue delamination growth is discussed in detail.     

Analysis of mode II interlaminar fracture and damage behavior in end notched flexure testing of GFRP woven laminates at cryogenic temperatures

Summary Interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates at cryogenic temperatures is investigated for end notched flexure (ENF) pure Mode II configuration. The corrected beam theory (CBT) and finite element analysis (FEA) are used to calculate the Mode II interlaminar fracture toughness of ENF specimen at room temperature (RT), liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). A FEA coupled with damage is also employed to study the damage distributions within the specimen and to examine the effect of damage on the Mode II energy release rate. The numerical results show that damage occurs at the matrix and causes a decrease in the energy release rate. The technique presented can be efficiently used for the characterization of cryogenic Mode II interlaminar fracture and damage behavior of woven laminate ENF specimens.     

Acoustic emission and fracture behavior of GFRP woven laminates at cryogenic temperatures

This paper describes an experimental and analytical study on fracture and damage behavior of GFRP woven laminates at cryogenic temperatures. CT (compact tension) tests were carried out at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K) to evaluate the critical values of the fracture mechanics parameters. During the CT tests, AE (acoustic emission) method was implemented. AE signals can identify the critical load at which gross failure occurs. A FEA (finite element analysis) was also applied to calculate the fracture mechanics parameters. The failure criteria (Hoffman criterion and maximum strain criterion) or the damage variable based on the continuum damage mechanics was incorporated into the model to interpret the experimental measurements and to study the damage distributions within the specimen. Several methods of calculating J-integral are discussed.     

Mixed-mode interlaminar fracture and damage characterization in woven fabric-reinforced glass/epoxy composite laminates at cryogenic temperatures using the finite element and improved test methods

The present research examines experimentally and analytically the mixed-mode interlaminar fracture and damage behavior of glass fiber reinforced polymer (GFRP) woven laminates at cryogenic temperatures. The mixed-mode bending (MMB) tests were performed with the improved test apparatus, at room temperature, liquid nitrogen temperature (77 K) and liquid helium temperature (4 K). The energy release rates at the onset of delamination crack propagation were evaluated for the woven GFRP specimens using both beam theory and finite element analysis. The fracture surfaces were also examined to verify the fracture mechanisms. In addition, the initiation and growth of damage in the specimens were predicted by a damage analysis, and the damage effect on the mixed-mode interlaminar fracture properties at cryogenic temperatures was explored.     

Mode III Interlaminar Fracture Behavior of Glass Fiber Reinforced Polymer Woven Laminates at 293 to 4 K

This paper deals with mode III delamination properties of glass fiber reinforced polymer woven laminates at room temperature (293 K), liquid nitrogen temperature (77 K), gas helium temperature (20 K), and liquid helium temperature (4 K). In order to evaluate these properties, the Split Cantilever Beam (SCB) fracture test is performed. The load is applied to a test specimen through a set of identical grips in order to reduce (in some degree) the mode II loading at the free edges. A three-dimensional finite element analysis is used to study the stress and strain state of the specimens and to interpret the experimental measurements. The strain energy release rate is calculated by using the virtual crack closure technique. It is found that the strain energy release rate is dominated by the mode III component. A non-uniform distribution of the strain energy release rate along the delamination front is obtained with mode III component having maximum at the center of the delamination front, while mode II component increases towards the free edges. The strain energy release rate is also determined using the crack closure technique. A finite element analysis is also carried out to calculate the stress intensity factors for the SCB specimens. The fracture surfaces are examined by scanning electron microscopy to identify the fracture mechanisms. The most important conclusion from the present study is that at temperature lowering from 293 to 20 K the mode III fracture toughness increases, further cooling to 4 K produces a toughness decrease.     

Failure criteria for mixed mode delamination in glass fibre epoxy composites

Critical strain energy release rate of glass/epoxy laminates using the virtual crack closure technique for mode I, mode II, mixed-mode I + II and mode III were determined. Mode I, mode II, mode III and mixed-mode I + II fracture toughness were obtained using the double cantilever beam test, the end notch flexure test, the edge crack torsion test and the mixed-mode bending test respectively. Results were analysed through the most widely used criteria to predict delamination propagation under mixed-mode loading: the Power Law and the Benzeggagh and Kenane criteria. Mixed-mode fracture toughness results seem to represent the data with reasonable accuracy.     

Tension-tension fatigue behavior of GFRP woven laminates at low temperatures

This paper describes an experimental study on the fatigue damage behavior of GFRP woven laminates in terms of stiffness degradation and residual strength under cyclic loading at low temperatures. Uniaxial, load-controlled, tension-tension fatigue tests were conducted at room and low temperatures. The applied stress versus cycles to failure (S-N) relationships and fatigue limits were obtained for the GFRP woven laminates and the microcrack evolution due to fatigue loading was characterized using optical microscopy. Temperatures were also measured using a thermocouple embedded in the center of the specimens.     

Cryogenic through-thickness tensile characterization of plain woven glass/epoxy composite laminates using cross specimens: Experimental test and finite element analysis

This paper investigates the through-thickness tensile behavior of woven glass fiber reinforced polymer (GFRP) composite laminates at cryogenic temperatures. Tensile tests were carried out with cross specimens at room temperature and liquid nitrogen temperature (77 K), and the through-thickness elastic and strength properties of the woven GFRP laminates were evaluated. The failure characteristics of the woven GFRP laminates were also studied by optical and laser scanning microscopy observations. A three-dimensional finite element analysis was performed to calculate the stress distributions in the cross specimens, and the failure conditions of the specimens were examined. It is found that the cross specimen is suitable for the cryogenic through-thickness tensile characterization of laminated composite materials. In addition, the through-thickness Young's modulus of the woven GFRP composite laminates is dominated by the properties of the matrix polymer in the given temperature, while the tensile strength is characterized by both, the fiber to matrix interface energy and the cohesion energy of the matrix polymer.     

Influence of the matrix constituent on mode I and mode II delamination toughness in fiber-reinforced polymer composites under cyclic fatigue

Mode I and mode II fracture behaviour under static and dynamic loading was analyzed in two composites made up of the same reinforcement though embedded in two different matrices. Specifically, the delamination energy under static and dynamic loading was obtained for both materials and both fracture modes, i.e. the number of cycles necessary for the onset of fatigue delamination. Subsequently, the crack growth rate (delamination rate) was obtained for different percentages of the critical energy rate. The main goal of the study was to ascertain the influence of the matrix on the behaviour of the laminate under fatigue loading.From the experimental results for the onset of delamination, similar fatigue behaviour was observed at a low number of cycles for both matrices and both fracture modes, while in fatigue at a high number of cycles, a higher fatigue limit was obtained in the composite with the modified resin (higher toughness) for both fracture modes. From the point of view of crack growth rate, both materials behaved similarly for different levels of stress under fatigue and the two fracture modes for small crack lengths (initial growth zone < 5 mm), although the growth rate increased for large crack lengths. This behaviour was the same in both loading modes.     

Analytical and finite element investigations of shear/compression test fixtures

Insulation systems are critical components of the international thermonuclear experimental reactor (ITER). They must meet the super conducting magnets design requirements, including mechanical strength under combined shear and compressive stresses at cryogenic temperatures. Past cryogenic magnet systems often relied on woven glass/epoxy materials for insulation. An important point is to find a reliable shear/compression test method for these materials. The present work investigates a commonly used shear/compression setup and aims at measuring the reliability of the obtained test results. Therefore, the stress and failure analysis is performed analytically and numerically using the finite element method. The model is based on woven glass fiber reinforced materials which are subjected to combined shear and compressive stresses as well as to thermal loading, that results from cooling from 293 K to the test temperature of 77 K. A short analytical section shows the problems of common failure criteria which are used to describe the interaction of the shear and compression stresses. The numerical—finite element—section is based on three-dimensional linear elastic finite element models under thermo-mechanical loading. The locations of high stress gradients are investigated using an average stress criterion. Three different model geometries (15°, 45°, and 70°) are analyzed and finally compared with respect to their reliability.     

A vibration free cryostat using pulse tube cryocooler

This paper introduces a new vibration free cryostat cooled by liquid helium and a 4 K pulse tube cryocooler. The cryogenic device mounts on the sample cooling station which is cooled by liquid helium. The boil off helium is recondensed by the pulse tube cryocooler, thus the cryostat maintains zero boil off. There is no mechanical contact between the cryogenic part of the cryocooler and the sample cooling station. A bellows is used to isolate the vibration which could transfer from the cryocooler flange to the cryostat flange at the room temperature. Any vibrations generated by the operation of the cryocooler are almost entirely isolated from the cryogenic device. The cryostat provides a cooling capacity of 0.65 W at 4.21 K on the sample cooling station while maintaining a vapor pressure of 102 kPa. The sample cooling station has a very stable temperature with oscillations of less than ±3 mK during all the operations. A cryogenic microwave oscillator has been successfully cooled and operated with the cryostat.     

Microscale experimental investigation of failure mechanisms in off-axis woven laminates at elevated temperatures

Off-axis woven laminates fabricated from carbon fiber and a high glass transition temperature thermosetting resin were subjected to tensile static and fatigue loading at temperatures ranging from room temperature up to 205 °C. The damage mechanism prevalent to these specimens was investigated by post-mortem examination using a scanning electron microscope. During most of their life fatigue specimens had accumulated minimal damage which consisted of matrix cracks, transverse bundle cracks and intra-ply delamination. Just before failure fiber bundles began to straighten out and rotate towards the loading direction. This behavior led to large elongation and necking of the specimens before fracture. Overall, the matrix-dominated material behavior and fiber reorientation due to the off-axis configuration had a far greater influence on the fracture morphology than the gradual accumulation of damage due to fatigue loading. It was also found that damage formation was strongly influenced by the type of applied loading and the test temperature.