Investigations of fracture instability in crack growth for several metals-Part I: Experimental results

The tests were performed using three-points bending specimens (3PB specimens) and compact specimens (CT specimens), an extensometer fixture surrounding the CT specimens was used for displacement measurements. Stable, unstable crack growth and local unstable crack growth were observed, plus two types of unstable crack growth in CT specimens were observed from load-displacement curves measured by the extensometer fixture. The crack extensions were measured by key-curve method, and J-resistance curves, tearing modulus curves and local crack-opening angle curves for CT specimens were determined. The tearing modulus parameter and local crack-opening angles for prediction of crack stable growth and fracture instability were employed. Tearing instability criterion gave the incorrect conclusion for the evaluations of fracture instability in the paper, but the analysis of local crack-opening angles can give some rough estimations. At last, fractography for CT specimens was described.Part II appeared in Vol. 68, no. 4 (1994/1995).     

An experimental study on the failure of carbon/epoxy single lap riveted joints after thermal exposure

The bearing strength of a blind riveted single lap joint of a carbon/epoxy composite after thermal exposure was studied. Considering the maximum service temperature based on material specifications and glass transition temperature, six types of specimens were prepared and tested in accordance with ASTM D5961. Failure loads of specimens exposed to a 177 °C environment were above 90% compared to sound specimens. Failure loads of 77% were obtained for specimens exposed to a 232 °C environment for 4 h. Bearing and rivet pulling-through failure modes were observed in all specimens. Delamination caused by degradation of the matrix property was found in specimens exposed to temperature higher than 204 °C.     

Effects of machining parameters on fatigue behavior of machined threaded test specimens

The effects of machining parameters on the fatigue strength of fine-machined threaded specimens were investigated by comparing with the endurance limits of circumferentially notched specimens with the same profile. A four-point rotary bending fatigue test machine was used to obtain constant bending moment and pure alternating stress along the thread. All specimens were machined from SAE 4340 steel, the typical material used for deep well oil drilling pipes. Notched specimens were fine-machined according to ASTM standards, while the threaded specimens were machined under the optimized cutting conditions, which maximize tool life and geometric precision, as well as under selected modified conditions. Endurance limits of all specimens after 2 × 10⁶ cycles were experimentally determined and S–N curves were plotted for 90% reliability for all experiments. The effects of cutting force, radial feed, tool wear, and two thread cutting methods on the fatigue strengths of the threaded specimens were determined. Experimental results show that the fatigue strengths of threaded specimens lie within a large range, depending on machining conditions, as compared to circumferentially notched specimens. The most influential factors on the fatigue strength of threaded specimens are tool wear and cutting velocity, while the effects of cutting method and radial feed are less significant.     

Influence of synthesis methods of the PTCR effect in semiconductive BaTiO3

The molten salt synthesis method was used for the synthesis of semiconductive Ba_0.9Sr_0.1TiO₃ doped with Sb(0.1–0.4 mol%) and the electrical properties of the specimens prepared by molten salt synthesis were compared with those of specimens prepared by the conventional calcining of mixed oxides method. The molten salt synthesis specimens showed greater effects on the positive temperature coefficient of resistivity in their resistivity-temperature characteristics and larger resistivities at room temperature and larger current variations in current-time characteristics than the specimens by calcining of mixed oxides. These differences in electrical properties are explained by the differences in microstructures of the two specimens and the existence of K ions in the molten salt synthesis specimens. The grain size and size distribution in the molten salt synthesis specimens were smaller and more homogeneous than those of the specimens by calcining of mixed oxides at the same sintering temperature.     

Effect of fabric types on the impact behavior of cement based composites in flexure

Two different fabric types were used to investigate the effect of the fabric types on the static and impact behavior of fabric reinforced cement based composites by using three point bending tests for various drop heights of hammer and position of the specimens on the supports. For each fabric type, 18 specimens with dimensions of 50 mm × 150 mm × 12 mm were produced with the pultrusion process. The vertical specimens have more stiffness, less ultimate deflection and higher load carrying capacity than the horizontal specimens for same drop heights. However, the horizontal specimens subjected to impact loads have higher stresses than the vertical specimens due to the section properties. The tests showed that polyvinyl alcohol (PVA) fabric reinforced cement based composites carried higher impact loads, were stiffer and had less deflection than other composites. At the drop heights over 100 mm, the impact strength of the horizontal specimens sharply decreased, while that of the vertical specimens was remained same.     

Influence of ultraviolet radiation on the low velocity impact response of laminated beams

In this study, an E-glass fiber reinforced epoxy composite laminate is subjected to ultraviolet radiation (UV) for up to 48 h. The intensity of the UV radiation is 1.5 J/cm²/min. A total of 39 specimens are prepared and tested, in which three specimens are used as control and 36 specimens are conditioned. For the conditioned specimens, 18 specimens are exposed to air and 18 specimens are immersed in a water base while subjected to UV radiation. Low velocity impact tests are conducted on the conditioned specimens and control specimens using instrumented drop-weight impact machine. Compression after impact (CAI) tests are conducted using a MTS machine to determine the residual load carrying capacity of the impact damaged specimens. Microscopic observation is conducted to aid in the analysis of the damage mechanism. The test results show that UV radiation alone has a significant effect on reducing the residual load carrying capacity of impact damaged laminated beams. The presence of water enhances the damage effect of UV radiation.     

Static and Dynamic Energy-Absorption Capacity of Graphite-Epoxy Tubular Specimens

This article examines the energy-absorption capacity of 50k and 12k carbon fiber composite tubular specimens crushed axially, in both quasi-static and dynamic fashions. Round and square tubular specimens with - 45° and - 45°/0° fiber orientation schemes were studied. The fundamental issue was to compare the energy-absorption capacity of the lower-cost 50k material with the 12k material, and to examine the influence of specimen geometry, fiber orientation schemes, and dynamic effects on the energyabsorption characteristics. Specimens made from the 50k material absorbed less energy than similar specimens made with the 12k material, and the load ratios were generally higher for the 50k specimens. The square specimens tended to have lower values of specific energy absorption (SEA) than the circular specimens. In addition, the crush modes were somewhat different and the load ratios were generally higher for the square specimens than for the circular specimens. Changing the fiber orientation schemes did not have much of an effect on the SEA, nor on the crush modes, but the presence of 0° fibers led to higher load ratios for the 50k specimens. The specimens tested dynamically had lower SEAs and higher load ratios than similar specimens that were tested statically.     

Investigation of the dynamic Young's modulus and vibration damping for cryomilled NiAl–AlN composites

Measurements were made of the dynamic Young's modulus (stiffness) and damping for NiAl specimens containing varying amounts of aluminium nitride and prepared by cryomilling. Five specimens of NiAl–AlN were measured at room temperature, each specimen having a different percentage of AlN in the range 1%–30%. Further measurements were made on the 1% and 30% AlN specimens for the temperature range 22–495°C. At room temperature, the Young's modulus of the NiAl–AlN specimens decreased linearly with increasing percentage of AlN. The temperature dependence of Young's modulus for the 1% and 30% AlN specimens was found to be linear. These results were compared with those of typical nickel-based superalloys for a similar temperature range. The damping for all of the NiAl–AlN specimens was of the order of that for other intermetallic compounds (10-3–10-4). The study opened up the possibility of NiAl–AlN being a less-expensive and more easily produced alternative to nickel-based superalloys for high-temperature, high-stiffness applications. © 1998 Kluwer Academic Publishers     

Double cantilever beam testing of repaired carbon fibre composites

Low-temperature cure adhesive systems have been evaluated for repair of composites. Pre-cracked DCB fracture toughness specimens were repaired using an ethylcyanoacrylate (ECA) system and an epoxy-based system cured with 4,4′-methylene-bis(aminocyclohexane) (PACM), and the post-repair fracture toughness measured. The results showed that both groups of repaired specimens exhibited higher G_Ic than the original specimens. However, the ECA-repaired specimens showed a greater tendency to fracture in a “stick-slip” fashion compared to specimens repaired with epoxy.     

Correlation of fracture behavior in high pressure pipelines with axial flaws using constraint designed test specimens--Part I: Plane-strain analyses

This work presents a numerical investigation of crack-tip constraint for SE(T) specimens and axially surface cracked pipes using plane-strain, nonlinear computations. The primary objective is to gain some understanding of the potential applicability of constraint designed fracture specimens in defect assessments of pressurized pipelines and cylindrical vessels. The present study builds upon the J-Q approach using plane-strain solutions to characterize effects of constraint on cleavage fracture behavior for the analyzed fracture specimens and cracked pipes. Under increased loading, each cracked configuration follows a characteristic J-Q trajectory which enables comparison of the corresponding driving force curve in the present context. A key outcome of this investigation is that toughness data measured using SE(T) specimens appear more applicable for cleavage fracture predictions of pressurized pipelines and cylindrical vessels than standard, deep notch fracture specimens under bend loading. The results provide a strong support for use of constraint-designed SE(T) specimens in fracture assessments of pressurized pipes and cylindrical vessels.     

Hydrogen embrittlement susceptibility of laser-hardened 4140 steel

Slow strain rate tensile (SSRT) tests were performed to investigate the susceptibility to hydrogen embrittlement of laser-hardened AISI 4140 specimens in air, gaseous hydrogen and saturated H₂S solution. Experimental results indicated that round bar specimens with two parallel hardened bands on opposite sides along the loading axis (i.e. the PH specimens), exhibited a huge reduction in tensile ductility for all test environments. While circular-hardened (CH) specimens with 1 mm hardened depth and 6 mm wide within the gauge length were resistant to gaseous hydrogen embrittlement. However, fully hardened CH specimens became susceptible to hydrogen embrittlement for testing in air at a lower strain rate. The strength of CH specimens increased with decreasing the depth of hardened zones in a saturated H₂S solution. The premature failure of hardened zones in a susceptible environment caused the formation of brittle intergranular fracture and the decrease in tensile ductility.     

Accumulation of time-dependent strain during dwell-fatigue experiments of iBN-Sylramic melt infiltrated SiC/SiC composites with and without holes

A 2 h tensile load cycle is repeated for specimens without holes and specimens with a central hole under the following conditions: (i) a stress ratio of 0.05, (ii) various stress levels, (iii) 815 °C and 1204 °C, (iv) different times up to 600 h and (v) different hole diameters. An increase in the accumulation of time-dependent strain with the increase in time and stress level is observed for specimens with and without holes. All specimens recovered part of their time-dependent strain at the minimum stress level.Residual strain to failure for specimens without holes showed a reduction with the increase in the amount of strain accumulated during the experiment. A curve fitting procedure of this experimental data, as well as data from archived literature, pointed at the possibility of the existence of a similar finite amount of strain to failure of all specimens tested at both temperatures.     

Effect of Mode-Mixity on the Fracture Toughness of Ti-6Al-4V/FM-5 Adhesive Joints

Using several different fracture tests, the fracture toughness of a chromic acid anodized titanium (Ti-6A-4V)/polyimide (FM-5) adhesive system was evaluated. Mode I, mode II, and mixed mode (I and II) tests were conducted using double cantilever beam (DCB), end notch flexure (ENF), and mixed mode flexure (MMF) geometries. Interfacial type failures were observed in the ENF and MMF specimens as a result of the mode II loading inherent in these tests. Pure mode I loading, as is the case with symmetric DCB specimens, resulted in cohesive failures with a fracture energy around 2500 J/m² on as-received specimens. The asymmetric DCB specimens had fracture energy values around 2000 J/m², the MMF specimens close to 1970 J/m², and ENF specimens around 1300 J/m². All the above measurements were made on as-bonded (unaged) specimens. Titanium/FM-5 bonds supplied by the Boeing Company were then aged in one of three different environments for 2 and 6 months respectively. The environments included: 177^degC in air and 2 psia, and 204^degC in air. Following the aging, DCB, ENF, and MMF tests were conducted on the specimens. The results showed that aging in all three environments resulted in decreases in fracture energy for the above specimen testing configurations. The largest drop (20 percent) in fracture toughness was noted in specimens aged for 6 months in air at 204^degC. An unusual finding from this study, in contrast to what other researchers have seen on other systems, was that increasing mode II loading resulted in significant reductions in toughness. Crack path selection and interaction with the woven glass scrim within the bonded specimens may be responsible for the lower mode II fracture energies. From the tests conducted, failure envelopes were developed to predict failure energy and type for use in designing structural joints. This revised version was published online in July 2006 with corrections to the Cover Date.     

Influence of oxygenation time on crack growth in titanium alloy under cyclic bending

We present the results of fatigue crack-growth tests for Ti–6Al–4V titanium alloy. Plane specimens 4 mm in thickness with a unilateral notch were tested. The analysis of crack growth was carried out for specimens without and after oxygenation process for 2 and 4 h. The fatigue crack growth rate was determined. It follows from the test results that the fatigue crack growth in the specimens without thermochemical treatment differs from the fatigue crack growth in the specimens after oxygenation. It has been also found that the applied surface treatment increases the fatigue life of the considered alloy.     

Dynamic splitting tensile properties of concrete and cement mortar

To investigate the dynamic tensile behaviours of concrete and cement mortar, a 50‐mm split Hopkinson pressure bar was applied on Brazilian disc specimens for dynamic tensile experiments, in which strain rate varied from 10^?5 to 20 s^?1. The high‐speed camera testing technique was used to capture the dynamic fractured process of the specimens at relative high strain rate. The experimental results revealed that the dynamic tensile strength of concrete specimens has a stronger strain rate effect than that of cement mortar specimens. Then three typical failure patterns of the specimens were confirmed in dynamic experiments. In addition, one‐parameter semi‐empirical relation between dynamic tensile strength and strain rate was established. Finally, the limitation of dynamic splitting experiments on Brazilian disc specimens was discussed in detail at high strain rate, in which the crack initiates from the contact point between the incident bar and specimens rather than the centre of the specimens.     

Estimation of fracture toughness of steel using chevron notched round bar specimens

A generalized methodology has been outlined in this paper for estimating the minimum normalized stress intensity factor (Y*_min) of chevron notched round bar specimens, subjected to three‐point bend loading. Using such specimens, a series of fracture toughness tests have been carried out for the first time on two steels. The major inferences drawn from this investigation are: (i) reproducible fracture toughness values can be achieved using chevron notched rod specimens of identical configuration and (ii) the estimated magnitudes of fracture toughness obtained by using chevron notched rod specimens are in good agreement with those achieved by using chevron notched rectangular bar specimens of the same material.     

Tensile properties of iron-based P/M steels with ferrite + martensite microstructure

The effect of intercritical heat treatments on the tensile properties of iron-based P/M steels was investigated. For this purpose, atomized iron powder (Ancorsteel 1000) was admixed with 0.3 wt.% graphite powder. Tensile test specimens were cold pressed at 700 MPa and sintered at 1120 °C for 30 min under pure argon gas atmosphere. After sintering, ∼20% pearlite volume fraction in a ferrite matrix was obtained. To produce coarse ferrite + martensite microstructures, the sintered specimens were intercritically annealed at 724 and 760 °C and quenched in water. To obtain fine ferrite + martensite microstructures, the sintered specimens were first austenitized at 890 °C and water-quenched to produce a fully martensitic structure. These specimens were then intercritically annealed at 724 and 760 °C and re-quenched. After the intercritical annealing at 724 and 760 °C and quenching, martensite volume fractions were ∼ 18% and 43%, respectively, in both the coarse- and fine-grained specimens. Although the intercritically annealed specimens exhibited higher yield and tensile strength than the as-sintered specimens, their elongation values were lower. Specimens with a fine ferrite + martensite microstructure showed high yield and tensile strength and ductility in comparison to specimens with a coarse ferrite + martensite microstructure. The strength values of specimens increased with increasing martensite volume fraction.     

Fracture toughness of acrylic bone cements

Clinical experience has shown that fracture of PMMA-based bone cements is a significant factor in the failure of orthopaedic joint replacements. Earlier studies of the fracture toughness properties of bone cement have been limited to relatively large test specimens — ASTM standard test methods require the use of specimens with dimensions considerably larger that those associated with bone cement in clinical use. In this study, a miniature short-rod specimen was used to measure the fracture toughness (K _IC) or two bone cements (Simplex-P and Zimmer LVC). The dimension of our mini specimens approaches the cross-section of bone cements as usedin vivo. The short-rod elastic-plastic fracture toughness test method introduced by Barker was utilized to ascertain the effect of specimen preparation and ageing in distilled water on fracture toughness. Our study indicated that slow hand-mixed specimens possess comparable fracture toughness to centrifuged specimens. After ageing in water, however, centrifuged and slow hand-mixed specimens are more fracture resistant than specimens prepared by mixing the cement quickly. An optimum void content for the bone cements studied was suggested by the experimental results; for Simplex-P bone cement it appeared to be less than 1.6% whereas it was between 1.6 and 3.6% for Zimmer LVC cement. Simplex-P bone cement also showed superior fracture toughness compared to Zimmer LVC cement after storage in water for 60 days at 37° C.     

Effect of fire exposure on cracking,spalling and residual strength of fly ash geopolymer concrete

Fly ash based geopolymer is an emerging alternative binder to cement for making concrete. The cracking, spalling and residual strength behaviours of geopolymer concrete were studied in order to understand its fire endurance, which is essential for its use as a building material. Fly ash based geopolymer and ordinary portland cement (OPC) concrete cylinder specimens were exposed to fires at different temperatures up to 1000 °C, with a heating rate of that given in the International Standards Organization (ISO) 834 standard. Compressive strength of the concretes varied in the range of 39–58 MPa. After the fire exposures, the geopolymer concrete specimens were found to suffer less damage in terms of cracking than the OPC concrete specimens. The OPC concrete cylinders suffered severe spalling for 800 and 1000 °C exposures, while there was no spalling in the geopolymer concrete specimens. The geopolymer concrete specimens generally retained higher strength than the OPC concrete specimens. The Scanning Electron Microscope (SEM) images of geopolymer concrete showed continued densification of the microstructure with the increase of fire temperature. The strength loss in the geopolymer concrete specimens was mainly because of the difference between the thermal expansions of geopolymer matrix and the aggregates.     

An experimental study on the effect of tow variations on compressive characteristics of plain weave carbon/epoxy composites under compressions

The effect of tow deformation on the static and fatigue characteristics of fabric composites under compression was investigated by experimental approach. Sheared specimens made of plain weave carbon/epoxy prepregs were prepared using a picture frame rig and the shear angles were 0°, 16°, 26°, 34°, 46°. To verify the effect of the tow variations of the fabric composites on compressive characteristics, the unidirectional composite specimens composed of the same fibre and matrix system with the same stacking sequences as the fabric composites were prepared for comparison. The static compressive test results showed that the static compressive strength of sheared fabric specimens was lower than that of the unidirectional specimens with the same stacking angle. On the other hand, the fatigue test results showed that fatigue life of sheared fabric specimens was higher than that of the unidirectional specimens for mild shear deformation cases. It was proved that these results were fully affected by the tow deformation caused by the shear deformation of the fabric specimens. The compression–compression fatigue behaviours of sheared fabric specimens were verified by appropriate unit-cell models.