Showing papers on "Flexural strength published in 1987"

Experimental Characterization of Advanced Composite Materials

Abstract: INTRODUCTION Background Laminate Orientation Code Influences of Material Orthotropy on Experimental Characterization Typical Unidirectional Composite Properties References ANALYSIS OF COMPOSITE MATERIALS Constitutive Relations Micromechanics Laminated Plate Theory St. Venant's Principle and End Effects in Composites Lamina Strength Analysis Laminate Strength Analysis Fracture Mechanics Concepts Strength of Composite Laminates Containing Holes References PROCESSING OF COMPOSITE LAMINATES Processing of Thermoset Composites Autoclave Processing of Thermoplastic Composites Determination of Volume Fractions of Fibers, Resin and Voids References TEST SPECIMEN PREPARATION, TEST EQUIPMENT, STRAIN AND DEFORMATION MEASUREMENTS Cutting the Composite Laminate Tabbing Materials Tab Bonding Suggested Tab Bonding Procedures Hinge Attachment for DCB and MMB Specimens Specimen Conditioning Strain and Displacement Measurements Testing Machines References LAMINA TENSILE RESPONSE The Need for Lamina Testing Introduction to Tensile Testing Primary Concerns Specimen Configurations and Test Procedures Data Reduction References LAMINA COMPRESSIVE RESPONSE Shear-Loading Test Methods End-Loading Test Methods Combined Loading Compression (CLC) Test Methods Compression Test Procedures Failure Modes General Data Reduction Backing out Unidirectional Lamina Strength from a Test of a Cross-Ply Laminate Summary of Compression Test Methods References LAMINA SHEAR RESPONSE Iosipescu Shear Test Method (ASTM D 5379) Two-Rail Shear Test Method (ASTM D 4255) Three-Rail Shear Test Method (ASTM D 4255) [+-45]ns Tension Shear Test Method (ASTM D 3518) Short Beam Shear Test Method (ASTM D 2344) Summary References LAMINA FLEXURAL RESPONSE Testing Configuration Three-Versus Four-Point Loading Specimen Preparation and Flexural Test Procedures Data Reduction References LAMINA OFF-AXIS TENSILE RESPONSE Deformation and Stress in Unconstrained Specimens Influence of End Constraint Off-Axis Tensile Strength Test Procedure Data Reduction References LAMINA THERMOELASTIC RESPONSE Temperature Gage Sensing System Temperature Compensation Measurement of Thermal Expansion Data Reduction References LAMINATE MECHANICAL RESPONSE Data Reduction for Stiffness Properties Laminate Strength Analysis Test Specimen Preparation Test Procedures Data Reduction Example of a Typical Analysis: Axial Tensile Response of a Laminate References LAMINATE THERMOELASTIC RESPONSE Preparation of Test Specimens and Measurement of Thermal Expansion Data Reduction Analysis of Thermoelastic Response References OPEN-HOLE TENSILE AND COMPRESSIVE STRENGTHS OF LAMINATES Point and Average Stress Criteria Test Specimen Preparation Tensile Test Procedure and Data Reduction Standardized Open-Hole Tension Test Method Standardized Open-Hole Compression Test Methods References CHARACTERIZATION OF DELAMINATION FAILURE Double Cantilever Beam (DCB) Test End-Notched Flexure (ENF) Test Four-Point Bend ENF (4ENF) Test Mixed-Mode Bending (MMB) Test Edge-Cracked Torsion (ECT) Test References Appendix A: Compliance and Stiffness Transformations and Matrix Operations Appendix B: Preparation of Panels and Test Specimens Appendix C: Sample Laboratory Report Appendix D: Unit Conversions Index

Strengthening of Oxide Ceramics by Transformation‐Induced Stress

Abstract: Alumina-zirconia composites were fabricated by isostatic pressing and sintering of powder mixtures in such a way that bar-shaped specimens consisted of three layers. The outer layers contained A12O3 and unstabilized ZrO2 while the central layer contained A12O3 and partially stabilized ZrO2 (with 2 mol% Y2O3). When cooled from the sintering temperature, some of the zirconia in the outer layers transformed to the monoclinic form while zirconia in the central layer was retained in the tetragonal form. The transformation of zirconia in the outer layers led to the establishment of surface compressive stresses and balancing tensile stresses in the bulk. The existence of surface compressive stresses was verified by a strain gauge technique and bending strength measurements on samples with varying thickness of the outer layers. The layered composites exhibited greater strength in comparison with monolithic Al2O3-ZrO2 specimens. Further, variation of strength in bending with outer layer thickness (for a fixed total thickness) indicated that failure occurred from internal flaws. Scanning electron microscopy of fracture surfaces revealed that strength-limiting flaws were voids located in the central layer near the interface separating the central and the outer layers.

High-strength ceramics through colloidal control to remove defects

Abstract: Although the theoretical tensile strength of alumina is ∼46 GPa (ref 1), conventional powder processing rarely gives products better than 05 GPa because large defects, ∼50–100 µm in size, are invariably trapped during powder compaction2,3 Thin-film, fibrous, melt-processed and vapour-deposited ceramics can achieve higher strengths of several GPa (refs 4–6), but such processes are much more expensive than simple sintering of powder compacts Here we show how colloidal control of powders to remove agglomerates can be used to give high-strength sintered ceramics: for example, a commercial alumina powder can be improved from 037 to 104 GPa in bend strength

Microstructure and Mechanical Properties of Mullite Prepared by the Sol‐Gel Method

Abstract: Mullite (3Al2O3·2SiO2) of stoichiometric composition was prepared by mixing boehmite sol and silica dispersion and gelling at a pH of 3. Complete mullitization takes place at or above 1300°C. Ultrafine mullite powder prepared by calcining gel at 1400°C and attrition milling could be sintered to >98% (theoretical density) at 1650°C for 1.5 h. The flexural strength of the sintered body at room temperature was 405 MPa and 350 MPa at 1300°C. Only traces of a secondary phase were observed along the grain boundary.

Compression of oceanic lithosphere: An analysis of intraplate deformation in the Central Indian Basin

Abstract: The development of intraplate structure in the Central Indian Basin is examined using models in which deformation is due to flexural buckling and the hydrodynamic growth of instabilities. Comparison of the models reveal that in a strong viscous lithosphere, deformation of the layer occurs by flexural folding at a wavelength which corresponds to the flexural buckling theory; in a lithosphere of intermediate strength, the layer deforms by folding characterized by thickening which localizes beneath topographic heights; and in a relatively weak lithosphere, the layer incurs an even greater amount of localized thickening and deforms in the symmetric or pinch-and-swell mode by inverse boudinage. It is noted that the models in which the layer folds either flexurally or with periodic thickening correspond with observed depth distribution of seismicity in the Central Indian Basin and with experimental rock rheological data.

Carbon/graphite fiber reinforced poly(methyl methacrylate): properties under dry and wet conditions.

Abstract: The flexural properties of poly(methyl methacrylate) (PMMA) reinforced with carbon/graphite (C/G) fibers with three different surface treatments were investigated by transverse bend testing after dry and wet storage. The fibers used were (1) commercially available fibers, (2) cleaned fibers, and (3) cleaned and sized fibers. The coating agents of commercial unidirectional and braided C/G fibers as well as impurities on C/G fibers for medical uses were characterized by means of high-performance liquid chromatography (HPLC). The agar overlay technique was used to assess the cytotoxicity of leachable elements from different fibers and processed composites. Composites with both unidirectional and braided tubular C/G fibers were investigated after storage in water. Fracture stress and flexural modulus decreased when "commercial" fibers were used as reinforcing material. Composites with cleaned and sized fibers gave only minor differences in flexural properties after dry and wet storage. By means of SEM micrographs the adhesion behavior of unsized C/G fibers, epoxy sized fibers, cleaned fibers, and cleaned and sized fibers were assessed. After water storage a substantial part of the cleaned fibers adhered to the matrix material. The adhesion capacity of the other fibers was reduced since the water absorption caused separation of fiber and matrix.

An experimental study of synthetic fibre reinforced cementitious composites

Abstract: Fibre reinforcement is one of the effective ways of improving the properties of concrete. However, current studios on fibre -reinforced concrete (FRC) have focused mainly on reinforcements with steel and glass fibres. Thin paper reports on an experimental programme on the properties of various synthetic fibre reinforced cementitious composites and the properties of the reinforcing fibres. Acrylic, polyester, and aramid fibres were tested in uniaxial tension, both in their original state as we!! as after ageing in nerO*nL Samples of these fibres were found to lose varying amounts of strength with time, depending on the ageing temperature. Two different test methods were used to measure the fibre-cement interfacial bond strength. The tensile properties of concrete reinforced with acrylic, nylon, and aramid fibres, in the form of random distribution or unioxial alignment, were studied by means of three different tests: compact tension, flexural, and splitting tensile tests. The properties of concrete, particularly that of apparent ductility, were found to be greatly improved by the inclusion of such fibre reinforcement.

Thermal shock behavior of an alumina-SiC whisker composite

Abstract: Thermal shock testing of an alumina-20 vol% SiC whisker composite showed no decrease in flexural strength with temperature differences up to 900°C. Alumina, on the other hand, normally shows a significant decrease inflexural strength with a temperature change of >400°C. The improvement in the thermal shock resistance of the composite is believed attributable to the increased fracture toughness of this material.

The influence of cement/wood ratio and cement type on bending strength and dimensional stability of wood-cement composite panels'

Abstract: This study examined the influence of decreasing cement/wood ratios from 3.0 to 1.5 at 0.5 increments on flexural and dimensional stability properties of cement-bonded composite panels. In addition, two types of Portland cement (I and III) were employed to assess if a difference exists in properties over time between the two types. Cure periods were reduced from 28 to 14 days to investigate whether significant reductions occur in these properties. Results indicate that modulus of rupture increases as the cement/wood proportion is lowered. A cement/wood ratio of 2.0 was found to demonstrate optimum bending strength. Modulus of elasticity, however, increased linearly with greater cement/wood ratios. Generally, wood-cement panels made in this study exhibited high dimensional stability when exposed to a 24-hour water soak. No significant differences were observed between the Lehigh cement types used in this study believed to be due to compound composition similarities. In most cases, reducing cure periods from 28 to 14 days had little influence on board properties.

Fiber-reinforced SiC composites with improved mechanical properties

Abstract: Continued development of ceramic-ceramic composites has produced SiC-reinforced SiC composites with enhanced physical and mechanical properties. Composites have been produced with exceptional strain tolerance (strain gage derived) and fracture toughness values which are significantly higher than those of nonreinforced SiC. Flexural strengths at both room temperature and at elevated temperatures also are presented.

The influence of postcuring on the fracture properties of photo‐cured dimethacrylate based dental composite resin

Abstract: The dependence of the fracture behavior of photocured dimethacrylate-based composite resins on the matrix crosslink density (varied by postcuring at various temperatures) was investigated. In general, the fracture toughness (KIc) was increased by postcuring as has been observed for epoxy-amine networks. The flexural and diametral tensile strength was also raised by postcuring. Calculation of the inherent flaw size (ao) from these properties produced conflicting dependencies on the state of cure. It is suggested that improved wear resistance and incisal edge strength of dental composite resin restoratives may be achieved by increased degree of cure.

Structure and mechanical properties of ion implanted ceramics

Abstract: The microstructure of ion implanted ceramics depends upon both implantation and material parameters. Generally, high fluences, low substrate temperatures, and covalent bonding promote amorphization of the implanted zone. Conditions which give damaged but crystalline near-surface layers cause increases in hardness, apparent fracture toughness, flexural strength, and wear resistance. Amorphous surfaces have lower hardness but may exhibit higher wear resistance. Both conditions produce a residual surface stress that is an important factor in determining the surface mechanical properties. Examples of these changes are given for α-Al 2 O 3 , α-SiC, and TiB 2 .

Errors Associated with Flexure Testing of Brittle Materials

Abstract: : Requirements for accurate bend-testing of four-point and three-point beams of rectangular cross-section are outlined. The so-called simple beam theory assumptions are examined to yield beam geometry ratios that will result in minimum error when utilizing elasticity theory. Factors that give rise to additional errors when determining bend strength are examined, such as: wedging stress, contact stress, load mislocation, beam twisting, friction at beam contact points, contact point tangency shift, and neglect of corner radii or chamfer in the stress determination. Also included are the appropriate Weibull strength relationships and an estimate of errors in the determination of the Weibull parameters based on sample size. Such analyses and results provide guidance for the accurate determination of flexure strength of brittle materials within the linear elastic regime. Error tables resulting from these analyses are presented.

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

Abstract: 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.

Bending Behavior of Steel-Fiber Concrete Beams

Abstract: An analytical and experimental investigation into the moment-curvature M-¢ and load-deflection P-o characteristics of steel-fiber concrete (SFC) beams is reported. Explicit expressions are given for the moduli of elasticity and the tensile and compressive stress-strain behavior of the composite. In flexure, the composite is treated as a himodulus material, and its M-¢ and P-o relationships in the elastic and cracked range are derived. Analytically predicted curves are found to agree well with those obtained from experiments conducted on two series of mixes. This approach serves as a useful tool to predict flexural strength and to quantify the toughness of the material using available indexes.

Permissible crack widths in steel fibre reinforced marine concrete

Abstract: The paper presents some results from a continuing study of the marine durability of steel fibre reinforced concrete. The overall aim of the investigation is to develop the material for marine applications. The results reported here pertain to pre-cracked specimens of steel fibre reinforced concrete which were exposed to wet-dry cycles of marine spray in the laboratory simulating tidal zone conditions of exposure. Two types of concrete mixes were used in the investigation—one with standard concrete constituents and OPC and the second replacing about 26% of cement with pfa. The cement content of the mixes was 590 and 435 kg m−3, respectively. Fibre reinforcement was provided by means of low carbon steel fibres and melt extract steel fibres at a vf l/d ratio of 100 and 147. Prism specimens were manufactured and these were precracked to induce cracks of width ranging between 0.03 and 1.73 mm. After cracking, both sealed and unsealed specimens were exposed to laboratory marine spray cycles using sea water. Some control specimens were cured in the laboratory air throughout. Tests were carried out after 650 marine cycles (450 days) and 1450 marine cycles (900 days). Based on data on flexural strength, energy absorption capacity, stiffness and state of corrosion of the fibres, recommendations are made regarding suitable permissible crack widths for the design of steel fibre reinforced concrete for marine applications. The results indicate that a permissible crack width of 0.2 mm is satisfactory for concrete reinforced with melt extract fibres. A smaller value is recommended for concrete reinforced with low carbon steel fibres. Complete healing of open cracks of small widths is observed under exposure to marine cycles.

Improved Impulse-Frequency Response Techniques for Measurement of Dynamic Mechanical Properties of Composite Materials

Abstract: The dynamic mechanical properties of composite material specimens are rapidly determined by using two new computer-aided impulse techniques. Small beam specimens are excited in either flexural or extensional vibration by an electromagnetic hammer with a force transducer in its tip, while specimen response is measured with an eddy current probe or accelerometer. A desktop computer/Fast Fourier Transform analyzer system is then used for rapid data acquisition and computation of the complex modulus by curve-fitting to the frequency response function.