Proportional Limit Stress Research Articles

Static bending strength performances of cross-laminated woods made with five species

Thirty types of three-ply parallel- and cross-laminated woods were prepared from five species, and their static bending strength performance were investigated. The modulus of elasticity (MOE), proportional limit stress, and modulus of rupture (MOR) perpendicular to the grain were increased by cross-laminating, and the extent of the increase increased with decreasing density of the species. The measured values of MOE parallel and perpendi-cular to the grain of parallel-laminated woods and perpendicular to the grain of face laminae of cross-laminated woods were approximately equal to those calculated from true MOEs of individual laminae. However, the MOE parallel to the grain of face laminae of cross-laminated woods was much lower than the calculated MOE owing to the effect of the deflection caused by shear force on the MOE. The percentage of deflection caused by shear force versus total deflection (Y s) showed high values, from 16.1% (buna) to 40.5% (sugi), and it decreased linearly with increasing shear modulus in the cross section of the core. In addition, there was an extremely high positive correlation between the MOR and the measured MOE parallel to the grain of face laminae of cross-laminated woods. The MOR was also highly dependent on the shear modulus in cross section of the core.

Mechanical Properties of High Purity SiC Fiber-Reinforced CVI-SiC Matrix Composites

Mechanical properties of silicon carbide composites reinforced with highly crystalline fibers and fabricated by the chemical vapor infiltration method were evaluated. Materials used were SiC/SiC composites reinforced with unidirectional Hi-Nicalon Type-S fibers and unidirectional Tyranno SA fibers with various fiber/matrix interphase. Also, SiC/SiC composites reinforced with plain weave Tyranno SA fibers with carbon or multilayers of silicon carbide and carbon interphase were evaluated. In-plane tensile, transthickness tensile and interlaminar shear properties were evaluated by the in-plane tensile test, the transthickness tensile test, the diametral compression test and the compression test of double-notched specimens.The elastic modulus and proportional limit stress were improved by using high purity silicon carbide fibers. The in-plane tensile properties were insensitive to carbon interphase thickness for a range of thicknesses between 30 and 230 nm. It was found that the in-plane tensile strength of composites containing multilayers of silicon carbide and carbon coating of fibers and fiber bundles was superior to that of composites with carbon alone. Transthickness tensile strength and shear strength of high purity silicon carbide composites were successfully evaluated.

Effect of Carbon and Silicon Carbide/Carbon Interlayers on the Mechanical Behavior of Tyranno‐SA‐Fiber‐Reinforced Silicon Carbide‐Matrix Composites

Several CVI‐SiC/SiC composites were fabricated and the mechanical properties were investigated using unloading–reloading tensile tests. The composites were reinforced with a new Tyranno‐SA fiber (2‐D, plain‐woven). Various carbon and SiC/C layers were deposited as fiber/matrix interlayers by the isothermal CVI process. The Tyranno‐SA/SiC composites exhibited high proportional limit stress (∼120 MPa) and relatively small strain‐to‐failure. The tensile stress/strain curves exhibited features corresponding to strong interfacial shear and sliding resistance, and indicated failures of all the composites before matrix‐cracking saturation was achieved. Fiber/matrix debonding and relatively short fiber pullouts were observed on the fracture surfaces. The ultimate tensile strength displayed an increasing trend with increasing carbon layer thickness up to 100 nm. Further improvement of the mechanical properties of Tyranno‐SA/SiC composites is expected with more suitable interlayer structures.

Processing optimization and mechanical evaluation of hot pressed 2D Tyranno-SA/SiC composites

Abstract Non-coated and carbon-coated 2D woven Tyranno SA/SiC composites were fabricated by hot pressing (HP) via liquid phase sintering (LPS). Various preparation conditions were optimized using non-coated fiber fabrics and mechanical evaluation was further conducted for the composites using carbon coated fiber fabrics. Results have indicated that intra-bundle infiltration of the fiber preforms during polymer impregnation and pyrolysis (PIP) pretreatment would affect the density of the composites. Modification of the polymer (polycarbosilane—PCS) to filler (SiC with sintering additives) ratio could promote the densification process, especially the intra-bundle matrix formation. Using non-coated fiber preforms, at 1750 °C, 15MPa, suitable polymer precursor and filler content [PCS/(PCS+SiC)=50%] was beneficial for obtaining composites with higher density. Applying carbon coated fiber fabrics and adopting the optimized polymer to filler ratio for pretreatment, composites with non-catastrophic fracture behavior could be obtained. Meanwhile, carbon coating can well protect fiber from damaging. At 1780 °C under 20 MPa, higher density of 2.82 g/cm3 could be reached. This composite exhibited a relatively strong fiber/matrix bonding and the improved mechanical properties such as ultimate strength, proportional limit stress and elastic modulus, either in bending or tensile tests.

Preparation of SiC/SiC Composites by Hot Pressing, Using Tyranno‐SA Fiber as Reinforcement

The development of advanced Tyranno SA SiC fiber with a near‐stoichiometric composition and a well‐crystallized microstructure has made it possible to prepare SiC/SiC composites even under harsh conditions. To assess the reinforcing effectiveness of Tyranno SA fiber at high temperature under pressure, unidirectional SiC/SiC composites were prepared by hot pressing, using pyrolytic carbon (PyC)‐coated Tyranno SA fiber as a reinforcement and nanopowder SiC with sintering additives for matrix formation. The effects of sintering conditions on the microstructural evolution and mechanical properties of the composites were characterized. As the sintering temperature increased (from 1720° to 1780°C) and the sintering pressure increased (from 15 to 20 MPa), the density of the composites gradually increased. Simultaneously, the elastic modulus, the proportional limit stress, and the strength, under both bend and tensile tests, also improved. At lower temperature and/or pressure, long fiber pullout was a predominant fracture behavior, indicating relatively weak fiber/matrix bonding. However, at high temperature and/or pressure, short fiber pullout became a main fracture characteristic, indicating relatively strong fiber/matrix bonding. These phenomena were also confirmed by the characteristics of the hysteresis loops derived from the stress–strain curves produced by a tensile test with unloading–reloading cycles. In the present investigation, the reinforcement of Tyranno SA fiber is effective for providing noncatastrophic fracture behavior to composites.

Mechanical properties of several advanced Tyranno-SA fiber-reinforced CVI-SiC matrix composites

A recently developed SiC fiber, Tyranno-SA (2D plain-woven), was used as the reinforcement in several SiC/SiC composites. The composites were fabricated by chemical vapor infiltration (CVI) process. The mechanical properties and fracture behaviors were investigated using three-point bending test. The Tyranno-SA fiber possesses rough fiber surface with pure SiC surface chemistry, which may result in strong fiber/matrix bonding and fiber sliding resistance. Various pyrolytic carbon (PyC) and SiC/PyC interlayer coatings were applied in the composites to modify the mechanical properties of the interface. The interlayers were deposited by isothermal CVI process. The test results revealed a close PyC layer dependence of the strength of the composites. The ultimate flexural strength (UFS) increased with the increasing of the PyC layer thickness up to 100 nm, and then, kept at similar level till 200 nm. The Tyranno-SA/SiC composites exhibited relatively high proportional limit stresses due mainly to the large Young's modulus of the fiber. Fiber pullouts were observed at the fracture surfaces of all the interlayered composites. Attractive promising was exhibited on further improvement of the mechanical properties of the composites through further improvement of the interfacial properties and the matrix densification process.

Hi-Nicalon<SUP>TM</SUP> Fiber-Reinforced CVI-SiC Matrix Composites: II Interfacial Shear Strength and Its Effects on the Flexural Properties

The interfacial shear strengths (ISSs) of a series of 2D Hi-Nicalon/SiC composites with various pyrolitic carbon (PyC) or PyC-SiC multiple fiber/matrix interlayers were investigated using the single fiber pushout tests. The influence of the obtained ISS on the proportional limit stress (PLS) of the materials upon bending was discussed based on the experimental results and a thoretical model calculation. The ISS showed close PyC layer thickness dependence. The ISS decreased quickly from 505 MPa to ∼ 100 MPa with increasing the PyC layer thickness up to ∼ 200 nm, beyond which slight decrease of the ISS occurred till 760 nm of the PyC layer. The ISS showed significant influence on the PLS. Good agreement between the model calculations and the experimental results was obtained, when correlating the PLS to the ISS. The comparison between the model calculation and the experimental results may be indicative on further efforts on further improvement of the mechanical performance of SiC/SiC composites.

Measurement of bending properties of wood by compression bending tests

We measured Young's modulus, proportional limit stress, and bending strength by the compression bending test and examined the applicability of the testing method by comparing it with conventional bending test methods. Long columns of todomatsu (Japanese fir,Abies sachalinensis Fr. Schmidt) with various length/thickness ratios were the specimens. A compressive load was axially applied to the specimen supported with pin ends. Young's modulus, the proportional limit stress, and the bending strength were obtained from the load-loading point displacement and load-strains at the outer surfaces until the occurrence of bending failure. Four-point bending tests were also conducted, and the bending properties obtained were compared with the corresponding properties obtained by the compression bending tests. Based on the experimental results, we believe that when the stress-strain relation is measured by the load-loading point displacement relation using specimens whose length/thickness ratio is large enough, the bending properties can be obtained properly using the compression bending test.

Determination of creep-induced residual stress in fiber-reinforced glass–ceramic matrix composites by X-ray diffraction

Residual strains were measured in the matrix of laminated SiC fiber-reinforced BMAS glass–ceramic matrix composites using X-ray diffraction. Strains were determined from the (8 2 4) plane reflections of barium osumilite phase using Cu K α radiation and applying the sin 2 ψ measurement method. The stresses in the longitudinal and transverse directions of the 0° outer ply of the as-received and creep-conditioned specimens were compared. The longitudinal residual stress in the matrix of the as-received specimen was −71 MPa. The residual stresses in the longitudinal direction of the creep-conditioned specimens were −124 and −191 MPa for the specimens that were tensile crept at 1100°C for 12 h under 75 MPa and for 30 h under 90 MPa, respectively. The increase in the compressive residual stress in the matrix of the crept specimens was attributed to the effectiveness of high-temperature creep conditioning. The measured increase in the residual stress in the matrix of the composite specimens was in agreement with the increase in the proportional limit stress obtained from the stress–strain curves.

Predicting ultimate tensile and bending strengths of three finger-jointed tropical African hardwoods using acoustic emissions

SYNOPSIS The acoustic emissions behaviour of finger-joints from three tropical African hardwoods, Obeche (Triplochiton scleroxylon), Makore (Tieghemella heckelii) and Moabi (Baillonella toxisperma) were examined with a view to establishing their potential usefulness for non-destructively predicting ultimate tensile and bending strengths. Stress at first acoustic emission event-count as well as the cumulative event-count at 80 percent of mean failure stress and the cumulative event-count at 80 percent of mean proportional limit stress for all specimens were separately correlated to finger-joint strengths. The correlation suggested that all three acoustic emission properties could be used to non-destructively predict the ultimate tensile and bending strengths of finger-joints from the three hardwoods. Correlation coefficients obtained for the prediction models developed were, generally, good and statistically significant (a = 0.05). Stress at first event-count seemed best correlated to ultimate tensile strength of finger-joints from the three species, whilst cumulative event-count at 80 percent of mean failure stress was best correlated to modulus of rupture. A logarithmic regression function seemed to fit the regression of acoustic emission event-counts on ultimate tensile strength and modulus of rupture better than a linear function. The acoustic emission technique seems applicable as a non-destructive testing method for predicting the tensile and bending strengths of finger-joints from the three tropical African hardwoods.

Creep‐Induced Residual Stress Strengthening in a Nicalon‐Fiber‐Reinforced BMAS‐Glass‐Ceramic‐Matrix Composite

The feasibility of inducing a compressive residual stress in the matrix of a Nicalon‐fiber‐reinforced BMAS‐glass‐ceramic‐matrix composite through a creep‐load transfer treatment was studied. Specimens were crept at 1100°C under constant tensile load to cause load transfer from the matrix to the fibers, then cooled under load. Upon removal of the load at room temperature, the matrix was put into compression by the elastic recovery of the fibers. This compressive residual stress in the matrix increased the room‐temperature proportional limit stress of the composite. The increase in the proportional limit stress was found to be dependent upon the applied creep stress, with an increase in creep stress resulting in an increase in the proportional limit stress. Acoustic emission results showed that the onset of significant matrix cracking correlated closely to the proportional limit stress. Changes in the state of residual stress in the matrix were supported by X‐ray diffraction results. Fracture surfaces of all specimens exhibited fiber pullout behavior, indicating that the creep‐load transfer process did not embrittle the fiber/matrix interface.

Fabrication and Mechanical Properties of Reaction-Bonded Carbon Fiber/Si/SiC Composites.

Two types of low-density carbon/carbon preforms which had different fiber volume fractions and fiber orientations, i.e., a carbon woven fabric (≈55vol%)/carbon and a chopped carbon fiber (≈40vol%)/carbon composites, were reaction-bonded with a silicon melt at 1750°C in vacuum to fabricate dense carbon fiber/Si/SiC composites. The reaction-bonding process increased the density to -2.1g/cm3 from 1.6 and 1.15g/cm3 for carbon woven and chopped carbon preforms, respectively. All of the composites fractured with extensive fiber pullout. The higher the density, the higher the stiffness and proportional limit stress. The carbon woven fabric/Si/SiC composites with a density of 2.06g/cm3 exhibited a -120MPa ultimate strength and a -80MPa proportional limit in bending test.

The influence of loading frequency on the high-temperature fatigue behavior of a Nicalon-fabric-reinforced polymer-derived ceramic-matrix composite

Fiber-reinforced ceramic-matrix composites are under development for high-temperature structural applications. These applications involve fatigue loading under a wide range of frequencies. To date, high-temperature fatigue experiments have typically been performed at loading frequencies of 10 Hz or lower. At higher frequencies, a strong effect of loading frequency on fatigue life has been demonstrated for certain CMC`s tested at room temperature. The fatigue life of CMC`s with weak fiber-matrix interfaces typically decreases as the loading frequency increases. This decrease is attributed to frictional heating and frequency dependent interface and fiber damage. More recently, it has been shown that the room temperature fatigue life of a Nicalon-fabric-reinforced composite with a strong interface (SYLRAMIC{trademark}) appears to be independent of loading frequency. The high-temperature low-frequency fatigue behavior of the SYLRAMIC composite has also been investigated. For a fatigue peak stress {sigma}{sub peak} above a proportional limit stress of 70 MPa, the number of cycles to failure N{sub f} decreased with an increase in {sigma}{sub peak}. The material endured more than 10{sup 6} cycles for {sigma}{sub peak} below 70 MPa. In this paper, the influence of loading frequency on the high-temperature fatigue behavior of the SYLRAMIC composite is reported. It will be shown that themore » fatigue limit is unaffected by the loading frequency, that the number of fatigue cycles to failure N{sub f} increases with an increase in frequency, and that the time to failure t{sub f} decreases with an increase in frequency.« less

High‐Temperature Tensile Behavior of a Boron Nitride‐Coated Silicon Carbide‐Fiber Glass‐Ceramic Composite

Tensile properties of a cross‐ply glass‐ceramic composite were investigated by conducting fracture, creep, and fatigue experiments at both room temperature and high temperatures in air. The composite consisted of a barium magnesium aluminosilicate (BMAS) glass‐ceramic matrix reinforced with SiC fibers with a SiC/BN coating. The material exhibited retention of most tensile properties up to 1200°C. Monotonic tensile fracture tests produced ultimate strengths of 230–300 MPa with failure strains of ∼1%, and no degradation in ultimate strength was observed at 1100° and 1200°C. In creep experiments at 1100°C, nominal steady‐state creep rates in the 10−9 s−1 range were established after a period of transient creep. Tensile stress rupture experiments at 1100° and 1200°C lasted longer than one year at stress levels above the corresponding proportional limit stresses for those temperatures. Tensile fatigue experiments were conducted in which the maximum applied stress was slightly greater than the proportional limit stress of the matrix, and, in these experiments, the composite survived 105 cycles without fracture at temperatures up to 1200°C. Microscopic damage mechanisms were investigated by TEM, and microstructural observations of tested samples were correlated with the mechanical response. The SiC/ BN fiber coatings effectively inhibited diffusion and reaction at the interface during high‐temperature testing. The BN layer also provided a weak interfacial bond that resulted in damage‐tolerant fracture behavior. However, oxidation of near‐surface SiC fibers occurred during prolonged exposure at high temperatures, and limited oxidation at fiber interfaces was observed when samples were dynamically loaded above the proportional limit stress, creating micro‐cracks along which oxygen could diffuse into the interior of the composite.

Mechanical behaviour of saturated wood under compression

The mechanical behaviour of three species of hardwoods soaked in different swelling liquids, compressed at high rates of strain, was investigated using a split Hopkinson pressure bar system. Variations in elastic moduli, proportional limit and maximum stress with respect to the treatments were studied. It was found that the saturated specimens could be as stiff as the dry ones. This result was explained by the behaviour of the liquid present in the large cavities of the wood, i.e. the lumens of the cells, which must be different from that observed at low rates of strain. At large rates of strain, this liquid cannot flow out of the pores and must behave like a solid; therefore the structure of the material is reinforced and, as a consequence, the softening effect of the soaking agent can be masked.

Adaptations of immature trabecular bone to exercise and augmented dietary protein

Exercise and diet synergistically influence bone, but it remains unclear whether augmenting dietary protein intake during moderate exercise has a beneficial or negative effect on immature bone mechanical integrity. Thus, we examined lumbar vertebral bodies (L6) and femoral necks (FN) in trained and untrained rats fed either a recommended protein (15%) or higher protein (30%) diet. Male Wistar rats (8 wk old) were assigned to one of two exercise groups (high protein exercise [HPE], recommended protein exercise [RPE], run 3 d.wk-1 on a motor-driven treadmill at approximately 80% of their maximum oxygen capacity) or to one of two sedentary caged-control groups (high protein control [HPC], recommended protein control [RPC]. After 8 wk, in the HPE group, FN maximum normal stress was significantly greater than all other groups, and FN maximum load and energy at maximum load (per unit body mass) were significantly greater than the sedentary control groups. L6 stress at the proportional limit and initial-maximum stress did not differ among groups, but L6 percent ash was significantly greater in the HPE and RPC groups. Thus, coupling high dietary protein with moderate exercise can produce positive effects on immature rat femoral neck mechanical properties and structure.

Frequency Dependence of Fatigue Life and Internal Heating of a Fiber‐Reinforced/Ceramic‐Matrix Composite

The influence of loading frequency on the fatigue life and internal (frictional) heating of unidirectional SiC‐fiber/ calcium aluminosilicate‐matrix composites was investigated at room temperature. Specimens were subjected to tension–tension fatigue at sinusoidal loading frequencies from 25 to 350 Hz and maximum fatigue stresses of 180 to 240 MPa. The key findings of the study were that (1) fatigue life decreased sharply as the loading frequency was increased, (2) for all loading frequencies, fatigue failures occurred at stress levels that were significantly below the monotonic proportional limit stress of ∼285 MPa, and (3) pronounced internal heating occurred during fatigue, with the surface temperature of the fatigue specimens increasing by 160 K during 350‐Hz fatigue at a peak stress of 240 MPa.

High fat-sucrose diet effects on femoral neck geometry and biomechanics

Diets high in fat and sucrose may affect calcium metabolism, by decreasing calcium absorption in the gastrointestinal tract and decreasing calcium reabsorption in distal renal tubules, but information about the effects of high fat-sucrose diet on immature-bone material and structural properties is lacking. The present study examined the effects of a high fat-sucrose diet on the geometrical and biomechanical properties of the femoral neck in rapidly-growing rats. Sprague-Dawley female rats were assigned randomly to a high fat-sucrose diet group or a control-diet group for 10 weeks. Cantilever-bending tests to failure were conducted, and geometrical and material properties were calculated from fracture-surface cross-sections and were correlated with mechanical data. Geometrically, the high fat-sucrose diet decreased the relative size of the femoral neck cortical shell and increased the trabecular core. The femoral necks of rats fed the high fat-sucrose diet had signiflcanctly lower structural rigidity, load at proportional limit, maximum load, energy at proportional limit, total energy, normal stress at proportional limit and maximum normal stress than the controls.

Silicon carbide fibre-reinforced glassceramic composite tensile behaviour at elevated temperature

The tensile performance of 0/90 cross-ply composite of SiC fibre-reinforced glass-ceramic is examined as a function of temperature, environment and stressing conditions. Tension, fatigue and stress rupture tests were all performed in both air and argon atmospheres up to a maximum temperature of 1300° C. It is shown that, in argon, composite ultimate strength is retained up to 1300° C. In air, strength loss occurs at elevated temperatures with an associated change in failure mode. Prolonged loading in fatigue or under constant stress in air at high temperature causes further strength loss which appears limited by the proportional limit stress of the composite.

Structural properties of spinal nerve roots: Biomechanics

The biomechanics of spinal nerve roots obtained from normal and nerve-crushed mice were evaluated. Photographs and longitudinal force measurements were taken as nerve roots were elongated through mechanical failure. Proportional limit stress and strain as well as the apparent modulus were calculated from photographic and force measurements to characterize nerve root strength, elasticity, and stiffness, respectively. Resulting mechanical data were indicative of an extremely weak material. Comparisons of nerve and nerve root mechanical properties revealed major differences. While nerve root elasticity was comparable to nerve, nerve root strength was only 10% that of nerve and root stiffness was only 20% of nerve values. Differences in nerve and root mechanics are attributed to the large discrepancies in relative amounts of connective tissue. Also in sharp contrast with peripheral nerve, unilateral nerve crush produced no significant alterations in root mechanics. Comparisons of nerve and nerve root strengths suggested possible pathways for dissipation of peripherally applied forces through epineurial and dural structures.