Temperature and loading rate effects in the mode II interlaminar fracture behavior of carbon fiber reinforced PEEK

The combined effect of varying loading rate and test temperature on the mode II interlaminar fracture properties of AS4/carbon fiber reinforced PEEK has been investigated. End notch flexure tests have shown that this thermoplastic‐based composite system offers a very high value of interlaminar fracture toughness at room temperature. Increasing the test temperature leads to a reduction in the mode II interlaminar fracture toughness of the composite, with the value at 150°C being approximately one half of the room temperature value. In contrast, increasing the crosshead displacement rate has been shown to increase the value of G_IIc by up to 25%. A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry. Here, extensive plastic flow within the crack tip region was observed in all specimens. It is believed that the rate sensitivity of G_IIc reflects the rate‐dependent characteristics of the thermoplastic resin.     

High-Resolution Electron Microscopy of Silicon Carbide-Whisker-Reinforced Alumina Composite Interfaces in Specimens Subjected to Elevated Temperatures

Interfaces of silicon carbide-whisker-reinforced alumina (SiC( w )/Al₂O₃) composites were examined using high-resolution electron microscopy (HREM). HREM specimens were prepared from the bulk of samples that were previously tested for fracture toughness at 25°, 1000°, 1200°, or 1400°C, in ambient air. The test temperature history served as an independent variable. It was found that the as-received material did not possess a distinct interfacial layer and that the test temperature history (which included a 30°C/min heating and cooling rate, a 30-min soak prior to specimen loading, and a typical test duration of 5–10 min) did not appreciably change the interface thickness at any of the elevated test temperatures.     

Effect of glass fiber length on the creep and impact resistance of reinforced thermoplastics

Impact and flexural creep testing were conducted at temperatures between −22°F (−30°C) and 250°F (121°C) to evaluate and compare the end-use performance of continuous long glass fiber-reinforced thermoplastic sheet composites to that of short glass fiber-reinforced thermoplastics. The matrices studied consisted of amorphous (polycarbonate and acrylonitrile-butadiene-styrene) and semicrystalline (polypropylene) polymers. Data were obtained from both injection-molded specimens (short fibers), and from specimens machine-cut from compression-molded test panels (continuous long fibers). The creep results of this study demonstrated that continuous long fibers are more efficient than short fibers in reinforcing the thermoplastic matrices, resulting in enhanced load-bearing ability at elevated temperatures. The addition of continuous long glass fibers to the thermoplastic matrices led to a significant increase in the notched Izod impact strengths between the temperatures of −22°F (−30°C) and 77°F (25°C), and only slight improvement in the drop-weight impact strengths. The lack of correlation between notched Izod impact and drop-weight strengths is largely due to the difference in crack propagation and fracture initiation energies. Results of the Rheometrics instrumented impact test indicated a higher total fracture energy for the long glass-reinforced thermoplastic sheet composites than for the short glass-reinforced injection-molded thermoplastics. The decreased ease of crack propagation in thermoplastic sheet composites is associated with the high energy-absorbing mechanisms of fiber debonding and interply delamination. The results of this study point to the significant property improvement of continuous long fibers vs. short fibers. The creep strength of short fiber-reinforced thermoplastics are greatly affected by the nature of the stress transfer which in turn is influenced by the critical fiber length and temperature, which is not the case for the long fiber-reinforced thermoplastic sheet composites. Long fibers dramatically increase the impact resistance of thermoplastics. The retention of toughness at low temperatures coupled with elevated temperature performance greater than similar short glass fiber-reinforced thermoplastics effectively extends the capabilities of thermoplastic sheet composites at both temperature extremes.     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.     

Fatigue fracture behaviour of PEEK: 2. Effects of thickness and temperature

Experimental results on the effects of specimen thickness and environmental temperatures on fatigue fracture behaviour of poly(ether ether ketone) (PEEK) are reported. Low cycle fatigue experiments are conducted on injection moulded single-edge notched specimens of 1.57, 2.70 and 5.42 mm in thickness at ambient temperatures, and on specimens 2.70 mm thick at environmental temperatures of 39, 50, 63, 75 and 100°C. In all the thickness experiments and in the experiments with temperatures of 39 and 50°C, the crack tip profile is initially round. At long crack lengths the crack tip profile changes to a triangular shape. When the test temperature is 63, 75 and 100°C, the crack tip remains round throughout the fracture process. The crack tip angle is primarily dependent upon the test temperature. Examinations of the fracture surfaces and transverse sections indicate that in the thickest specimen, relatively rough fracture surfaces are observed and a few discontinuities (crazes or cracks) underneath the main crack path. Thus, crack propagates in a ‘brittle’ manner. In all other experiments both ‘brittle’ and ‘ductile’ modes of fracture are observed. The point of transition from ‘brittle’ to ‘ductile’ fracture is dependent upon the specimen thickness and test temperature. Fatigue striations are seen throughout the fracture surfaces. Correlation of the striations and the number of cycles indicates a one-cycle crack growth mode. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Crack opening displacements are independent of the specimen thickness and increase with rise in temperature. When crack growth rates are correlated with the elastic energy release rate, they are independent of specimen thickness and increase with increase in temperature.     

The role of crystalline phase on fracture and microstructure evolution of polytetrafluoroethylene (PTFE)

Polytetrafluoroethylene (PTFE) is a semi-crystalline polymer, which has been employed in a range of engineering applications due to its extremely low coefficient of friction, resistance to corrosion, and excellent electrical insulation properties. Despite failure-sensitive applications such as surgical implants, aerospace components, motor seals, and barriers for hazardous chemicals, the mechanisms of crack propagation in PTFE have received limited coverage in the literature. Moreover, PTFE exhibits complex crystalline phase behavior that includes four well-characterized phases with both local and long range order. Three crystalline structures (phases II, IV, and I) are observed at atmospheric pressure with transitions between them occurring at 19 and 30 °C. This observation provides a unique opportunity for investigation of the effects of a polymers crystalline phase on fracture and microstructure evolution. Moreover, due to the presence of three unique ambient pressure phases near room temperature, it is essential to develop an understanding of the effects of temperature-induced phase transitions on fracture mechanisms of PTFE to prevent failure over the normal range of operating temperatures. In this work, we present values for the J-integral fracture toughness of PTFE for a range of temperatures and loading rates employing the single specimen normalization technique. Crack propagation in PTFE is found to be strongly phase dependent with a brittle-to-ductile transition in the crack propagation behavior associated with the two room temperature phase transitions. Increases in fracture toughness are shown to result from the onset of stable fibril formation bridging the crack plane and increased plastic deformation. The stability of drawing fibrils is primarily determined by temperature and crystalline phase with additional dependence on loading rate and microstructure anisotropy. [LAUR-05-0004]     

Effect of Subcritical Crack Growth on Fracture Toughness and Work-of-Fracture Tests Using Chevron-Notched Specimens

A correlation between the plane strain stress intensity factor K_I , load, and crack extension has been analyzed for constant displacement and constant loading rate experiments, using chevron-notched, four-point-bend specimens. It is assumed that at the beginning of the experiment the chevron triangle tip is not ideally sharp. As loading continues, the crack initially moves with velocity v_t at K_I equal to a threshold value K_t . Maximum crack velocity is reached at K_I= K_IC , the fracture toughness. Depending on the type of material tested, a specific displacement or loading rate must be used to correlate the maximum load with K_Ic . An error in K_IC calculation is estimated if different displacement rates are applied. Repeated loading-unloading work-of-fracture (WOF) experiments generate values related to the resistance of the material to fracture initiation, K_t , only when the crack length approaches 100% of the specimen width. Values related to material's fracture toughness, K_IC are not generated in WOF tests.     

The effect of temperature and loading rate on the mode II interlaminar fracture properties of a carbon fiber reinforced phenolic

The combined effect of varying loading rate and test temperature on the mode II Interlaminar fracture properties of a carbon fiber reinforced phenolic resin has been investigated. End notch flexure tests at room temperature have shown that this composite offers a relatively modest value of G_IIc^NL at non‐linearity and that its interlaminar fracture toughness decreases with increasing loading rate. As the test temperature is increased, the quasistatic value of G_IIc^NL increases steadily and the reduction in G_IIc^NL with loading rate becomes less dramatic. At temperatures approaching the glass transition temperature of the phenolic matrix, the interlaminar fracture toughness of the composite begins to increase sharply with crosshead displacement rate. A more detailed understanding of the effect of varying the test conditions on the failure mechanisms occurring at the crack tip of these interlaminar fracture specimens has been achieved using the double end notch flexure (DENF) geometry.     

Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three-point bend bar test specimens. R -curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room-temperature " K _IC" fracture toughness values of 4.5 MPa · m^1/2 for the chevron-notch specimen and 3.9 MPa · m^1/2 for the straight-notch configuration were obtained. The critical stress intensity factor of the renotched chevron-notch specimen compared very closely with that of the straight-notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.     

Mechanical properties of zirconia-based ceramics asfunctions of temperature

Commercially available ceramics, MgO–ZrO₂, CeO₂–ZrO₂, and an in-house fabricated zirconia-toughened mullite were examined in this study for use as a structural component in diesel engines. The fast fracture strengths of these materials were measured by loading ASTM C-1161-B specimens in four-point flexure at 30 MPa/s and at 20, 200, 400, 600, and 850 °C. The dynamic fatigue or slow crack growth susceptibility was assessed at 20 and 850 °C by combining the fast fracture strengths with strength data obtained by testing the same specimens in four-point flexure at 0.30 and 0.003 MPa/s stressing rates, as specified in the ASTM C 1368 standard. Fracture toughness was measured following the ASTM C-1421 standard and using chevron notch specimens in three-point flexure at room and elevated temperatures. The strength of the zirconia-toughened mullite was invariant to increases in the temperature and decreases in the loading rate, while the MgO–ZrO₂ and CeO₂–ZrO₂ materials exhibited strength degradation as temperatures increased and the loading rates decreased. Temperature was observed to have the greatest influence on facture toughness. As temperatures increased, the fracture toughness values dramatically decreased for all the materials examined in this study. Improvements in the fracture toughness are needed most for these ceramic materials in order to meet the structural requirements and to develop a more durable and reliable diesel engine component.     

Crack Stability and Its Effect on Fracture Toughness of Hot-Pressed Silicon Nitride Beam Specimens

The effect of stable crack extension on fracture toughness test results was determined using single-edge precracked beam specimens. Crack growth stability was examined theoretically for bars loaded in three-point bending under displacement control. The calculations took into account the stiffness of both the specimen and the loading system. The results indicated that the stiffness of the testing system played a major role in crack growth stability. Accordingly, a test system and specimen dimensions were selected which would result in unstable or stable crack extension during the fracture toughness test, depending on the exact test conditions. Hot-pressed silicon nitride bend bars (NC132) were prepared with precracks of different lengths, resulting in specimens with different stiffnesses. The specimens with the shorter precracks and thus higher stiffness broke without stable crack extension, while those with longer cracks, and lower stiffness, broke after some stable crack extension. The fracture toughness values from the unstable tests were 10% higher than those from the stable tests. This difference, albeit small, is systematic and is not considered to be due to material or specimen-to-specimen variation. It is concluded that instability due to the stiffness of test system and specimen must be minimized to ensure some stable crack extension in a fracture toughness test of brittle materials in order to avoid inflated fracture toughness values.     

Influence on the delamination phenomenon of matrix type and thermal variations in unidirectional carbon‐fiber epoxy composites

This article analyzes the influence of temperature on the delamination phenomenon in two composites with the same carbon‐fiber reinforcement, but different epoxy matrices. Interlaminar crack initiation and propagation under Mode I with static and fatigue loading are experimentally assessed for different test temperatures: 20, 50, and 90°C. The materials under study are made of AS4 unidirectional carbon fibers and different matrices: one is made of a 3501‐6 epoxy matrix (AS4/3501‐6), whereas the other is made of an 8552 epoxy matrix modified to increase its toughness (AS4/8552). Both composites have a symmetric laminate configuration [0°]_16s. In the experimental program, double cantilever beam specimens were tested under static and fatigue loading. A fractographic study was also performed using a scanning electron microscope on samples obtained from specimens previously tested under static and fatigue loading. A comparison was made of the fracture surfaces at the different test temperatures and the experimental tests' results obtained. POLYM. COMPOS., 36:747–755, 2015. © 2014 Society of Plastics Engineers     

Elevated-Temperature Fracture Resistance of a Sintered α-Silicon Carbide

The fracture toughness of a dense, sintered commercial α-silicon carbide was determined for temperatures from 20° to 1400°C using both straight- and chevron-notched test specimens and also controlled-surface-microflaw specimens, all in three-point bending. The flexural strengths were also measured for the same range of temperatures and the trend is compared with that of the toughness. Measurements from this study are discussed and also compared with other results in the literature. Analysis reveals the importance of contrasting sharp crack and blunt crack techniques and also the need for addressing the microhardness indentation method separately. It is concluded that the fracture toughness of this silicon carbide is about 3 MPa · m½ and that the crack growth resistance is characterized by a flat R -curve behavior, both of which are independent of temperature from 20° to 1400°C.     

Determination of fracture toughness of poly (ethylene terephthalate) film by essential work of fracture and J integral measurements

Abstract

The plane stress fracture toughness of a semicrystalline poly (ethylene terephthalate) (PET) film of thickness 0·125 mm has been measured as a function of specimen size, specimen geometry, loading rate, and temperature using the essential work of fracture (EWF) approach. It was found that the specific essential work of fracture w _e was independent of specimen width, specimen gauge length, and loading rate, but was dependent upon specimen geometry and test temperature. Below the glass transition temperature (93°C), w _e for double edge notched tension (DENT) type specimens was temperature insensitive, but increased with temperature for single edge notched tension (SENT) type specimens. The w _e value for SENT specimens was consistently higher than for DENT specimens. Estimation of w _e via crack opening displacement was reasonable using the relationship w _e = σ_n e _0,y; estimations made via similar type equations were either too high or too low and were generally unsatisfactory. It was found that values of J integral obtained by power law regression and linear extrapolation of the J–R curves to zero crack growth were lower than w _e. The power law regression of the J–R curves with ?a taken as half the crack opening displacement value at maximum load gave J _c values which agreed reasonably well with w _e.     

A Fracture Toughness Test Using the Ball‐on‐Three‐Balls Test

A method for fracture toughness measurement of ceramics using small disks and plates is presented. Similar to the surface‐crack‐in‐flexure (SCF) method a semielliptical surface crack is introduced centrally into one plane side of the specimen which is fractured in a ball‐on‐three‐balls test. Finite element simulations are used to evaluate the stress intensity factor (SIF) for this loading geometry for a range of crack sizes and crack geometries. Empirical formulae for the geometric function are provided for evaluation of the test. The effect of position uncertainties is investigated using FEM and experiments. Other sources which may contribute to the measurement error are identified and quantified, resulting in recommendations for the practical realization of the test. A determination of the fracture toughness within ±10% measurement uncertainty is possible with specimens larger than 8 mm in diameter and thicker than 0.5 mm. With larger specimens an uncertainty comparable to other fracture toughness tests can be achieved. For precise measurements it is important to position the crack within ±120 μm of the stress maximum, to know Poisson's ratio exactly and to test cracks that have the maximum SIF at their deepest point. A method how this can be achieved is presented.     

Crack resistance behavior of polyvinylchloride

The effects of specimen dimension (thickness, width) and specimen configuration (SENB, CT), as well as test conditions (crosshead speed, temperature), over a range of crosshead speeds from 0.01 to 100 mm/min and temperatures from −40 to 60°C, on the crack growth resistance behavior were investigated in a commercial amorphous thermoplastic polyvinylchloride (PVC) using the J-integral and crack opening displacement (COD) concepts. With the combined application of these two different fracture mechanics parameters, more detailed information on fracture processes can be obtained. The relationship between deformation mechanisms and fracture toughness was discussed through a comparative analysis of J versus Δa and COD versus Δa resistance curves. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 1079–1090, 1997     

Temperature and Loading Rate Effects on the Fracture Behaviour of Adhesively Bonded GFRP Nylon-6,6

The combined effect of varying test temperature and loading rate on the Mode II fracture toughness of plasma-treated GFRP Nylon-6,6 composites bonded using a silica-reinforced epoxy adhesive has been studied. End notch flexure tests have shown that the adhesive system used in this study offers a wide range of fracture energies that are extremely sensitive to changes in temperature and loading rate. Increasing the test temperature resulted in a substantial reduction in the Mode II fracture toughness of the adhesive, with the value of G_IIc at 60°C being approximately one-half of the room temperature value. In contrast, increasing the crosshead displacement rate at a given temperature has been shown to increase the value of G_IIc by up to 250%. Compression tests performed on bulk adhesive specimens revealed similar trends in the value of [sgrave]_y with temperature and loading rate. In addition, it was found that the plasma treatment employed in this study resulted in stable crack propagation through the adhesive layer under all testing conditions.

A more detailed understanding of the effect of varying temperature and loading rate on the failure mechanisms occurring at the crack tip was achieved using the double end notch flexure (DENF) geometry, which was considered in tandem with the fracture surface morphologies. Here, changes in the degree of matrix shear yielding and particle-matrix debonding were used to explain the trends in [sgrave]_y and G_IIc.     

A Layered Alumina Composite Tested at High Temperature in Air

An oxidation-resistant interphase for layered alumina composites was prepared by aerosol spray deposition of submicrometer alumina powder. A model composite specimen was made by placing the interphase between thin layers of monolithic alumina. The composite sandwich was hot-pressed to control the interphase fracture resistance for successful crack deflection. Specimens were tested under four-point bending in air at two crosshead speeds at ambient temperature, 1000°C, and 1200°C. The fracture behavior was temperature dependent, with a higher work of fracture at higher temperatures. Interphase delamination and composite toughening behavior were very pronounced at all temperatures. At the highest temperature, the transition to multiple widely distributed cracks and increased crack deflection may be related to inelastic deformation in the alumina.     

Notch tip damage zone in biaxially oriented polypropylene at low temperature

It is well established that biaxial orientation produces large improvements in the mechanical properties of polypropylene; this study further shows that the large improvements in mechanical behavior are magnified especially below the glass transition temperature. In this study, the irreversible deformation behavior of polypropylene during sharp single-edge notch tension testing has been studied at two levels of biadial orientation at ?40°C. Unoriented polypropylene formed a narrow wedge-shaped damage zone that grows with increasing stress until catastrophic fracture occurs in a brittle manor. The damage zone consisted of many crazes that mainly grew perpendicular to the loading direction. The 50% oriented material initially developed a wedge-shaped damage zone that grew wider as loading increased. This resulted in a drop of the length-to-width ratio at high sample extensions. The specimen fractured with stable crack growth in a ductile manner, showing a large resistance to crack growth. The 80% oriented material had a circular damage zone that consisted of many delamination crases. These crases grew by splitting the specimen in the thickness direction. Stable crack growth dominated the final failure process with the 80% oriented material showing nearly three times the toughness of the unoriented material. © 1994 John Wiley & Sons, Inc.     

Thermal shock resistance of zircon/SiC composite

Abstract

In the present work, thermal shock tests of zircon/silicon carbide (30 wt-%) composite specimens were evaluated up to 1000°C and compared with pure zircon specimens at nearly the same porosity content. Results confirmed that the fracture strength of the quenched specimens was not affected with the increase in quenching temperatures by incorporating SiC particles, indicating resistance to crack propagation. On the other hand, the critical temperature difference ΔT, below which material maintains its original strength, was found higher in composite rather than pure zircon specimens. X-ray diffraction investigations showed that zircon on the surface layer of composite specimens decomposes and produces a specimen comprising core and shell.