Fracture Toughness

A new mechanism of fracture toughness enhancement in nanocrystalline metals and ceramics is suggested. The mechanism represents the cooperative grain boundary (GB) sliding and stress-driven GB migration process near the tips of growing cracks. It is shown that this mechanism can increase the critical stress intensity factor for crack growth in nanocrystalline materials by a factor of three or more and thus considerably enhances the fracture toughness of such materials.

Flexural strength, crack-density evolution, work of fracture, and critical strain energy release rates were measured for wet and dry specimens of the Strombus gigas conch shell. This shell has a crossed-lamellar microarchitecture, which is layered at five distinct length scales and canbe considered a form of ceramic "plywood". The shell has a particularly high ceramic (mineral) content (99.9 wt%), yet achieves unusually good mechanical performance. Even though the strengths are modest (of the order 100 MPa), the laminated structure has a large strain to fracture, and a correspondingly large work of fracture, up to 13 kJ m-2. The large fracture resistance is correlated to the extensive microcracking that occurs along the numerous interfaces within the shell microstructure. Implications of this impressive work of fracture for design of brittle laminates are considered.

Twin wire arc-sprayed (TWAS) coating of commercially available SHS 7170-cored wire was obtained on Ti6AL4V alloy, and to improve its properties, it was further surface treated with high-power diode laser (HPDL). The cavitation erosion (CE) resistance of TWAS-coated samples was evaluated as per ASTM G-32-2003 and it was compared with laser-treated and untreated Ti6Al4V alloys. The CE resistance of TWAS-coated

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A detailed analytical and experimental investigation is presented to understand the dynamic fracture behavior of functionally graded materials (FGMs) under mode I and mixed mode loading conditions. Crack-tip stress, strain and displacement fields for a mixed mode crack propagating at an angle from the direction of property gradation were obtained through an asymptotic analysis coupled with a displacement potential approach. This was followed by a comprehensive series of experiments to gain further insight into the behavior of propagating cracks in FGMs. Dynamic photoelasticity coupled with high-speed photography was used to obtain crack tip velocities and dynamic stress fields around the propagating cracks. Birefringent coatings were used to conduct the photoelastic study due to the opaqueness of the FGMs. Dynamic fracture experiments were performed using different specimen geometries to develop a dynamic constitutive fracture relationship between the mode I dynamic stress intensity factor (K ID ) and crack-tip velocity ( ${\mathop a\limits^ \cdot }$ ) for FGMs with the crack moving in the direction of increasing fracture toughness. A similar ${\mathop a\limits^ \cdot }$ -K ID relation was also obtained for matrix material (polyester) for comparison purposes. The results obtained show that crack propagation velocities in FGMs were about 80% higher than the polyester matrix. Crack arrest toughness was found to be about 10% lower than the value of local fracture toughness in FGMs.

Ca 3 Co 4 O 9 thermoelectric (TE) ceramics have been processed using conventional sintering (CS), hotpressing (HP) and spark plasma sintering (SPS). Microstructure investigations have revealed that HP processing is effective for obtaining highly textured 349 ceramics, with a degree of crystallite orientation showing a texture index of 6 mrd 2 , and that SPS affords strong densified 349 ceramics, typically 98% of the theoretical X-ray density. Investigations of mechanical and thermal properties were undertaken for a reliable integration of these materials in practical devices. In detail, these properties were assessed by microindentation, nanoindentation and three point bending tests. Hardness, elastic modulus, strength and fracture toughness of the HP and SPS samples were drastically improved, compared with the CS specimen. The thermal expansions were measured and shown, for the samples processed by HP and SPS, different between the directions parallel and perpendicular to the stress axis. Thermoelectric properties were investigated from 5 to 850 K and a remarkable power factor value of 550 W m −1 K −2 was obtained at 850 K. Mechanical and TE properties were correlated to the microstructure and texture features.

Ti-6Al-4V ELI (extra low interstitials) grade alloy provides improved ductility and fracture toughness comparing to grade 5 Ti-6Al-4V alloy. In order to find the optimum superplastic forming and diffusion bonding (SPF/DB) condition, a series of tensile tests was carried out at the strain rate range of 10 −4 to 10 −2 s −1 and temperature range of 1073-1223 K. The maximum elongation of 1898% was obtained at the strain rate of 10 −3 s −1 at 850 • C. It was shown that the ELI grade alloy performs better than the grade 5 alloy in terms of the optimum superplastic condition for Ti-6Al-4V. Based on this result, diffusion bonding process of superplastic Ti-6Al-4V ELI sheet metals was developed. Bonding was completed by means of inert gas pressure applied in a bonding tool at high temperature. The microstructure of the bonding area was investigated and the bonding interface was microscopically undetectable. The evidence of nucleation of new grains and migration of grain boundaries at the interface proves the diffusion bonding process is successful. It is shown that the superplastic forming and diffusion bonding of Ti-6Al-4V ELI grade is possible at the temperature lower than those of conventional Ti-6Al-4V.

The API 5L X70 and X52 pipeline steel weld fracture toughness parameters are measured in a hydrogen environment and compared to the ones in air. The hydrogen environment is created by in situ hydrogen charging, using as an electrolyte a simulated soil solution, with three current densities, namely 1, 5 and 10 mA/cm 2. A specially designed electrolytic cell mounted onto a three-point bending arrangement is used and hydrogen charging is performed during the monotonic loading of the specimens. Ductility is measured in terms of the J 0 integral. In all cases a slight change in toughness was measured in terms of K Q. Reduction of ductility in the base metal is observed, which increases with increasing current density. A more complex phenomenon is observed in the heat affected zone metal, where a small reduction in ductility is observed for the two current densities (1 and 5 mA/ cm 2) and a larger reduction for the third case (10 mA/cm 2). Regarding microstructure of tested X70 and X52 base and HAZ metal, it is observed that the hydrogen degradation effect is enhanced in banded ferriteepearlite formations. The aforementioned procedure is used for calculating the fracture toughness parameters of a through-thickness pipeline crack.

This study involves the determination of the mechanical and fracture properties of polyamide 6 reinforced with short glass fibres (35 wt. %) at five different moisture contents (0, 1, 2, 3, and 6 wt. %, respectively). The mechanical characterisation consisted of tensile tests on plane specimens to determine the influence of moisture content on the material's mechanical parameters. The study of fracture properties was carried out by performing single-edge notch bending tests on specimens with the levels of moisture mentioned above in order to obtain the critical stress intensity factor, K Ic . The experimental results showed that the material fracture behaviour cannot be properly described following this methodology; for this reason, a more ambitious scope was established, consisting of determining the J-integral during stable propagation by combining the experimental force vs. displacement curves with the results of a finite element numerical model. The analysis clearly demonstrates the improvement in the material's toughness as humidity increases. The subsequent fractographic study performed through scanning electron microscopy allowed the influence of moisture content on the fracture micro-mechanisms developed during fracture and propagation to be determined.

It is well known that in a sandwich structure, the core plays an important role in enhancing the flexural rigidity and by controlling the failure mechanisms. If the core is made from foam, the strength of the core material and the debond strength at the core -skin interface almost entirely dictate the performance of structural sandwich composites especially under flexure. In this investigation attempts have been made to improve the performance of the sandwich by strengthening the core but partially sacrificing the debond fracture toughness of the sandwich construction. Strengthening of the core has been accomplished by infusing nanoparticles into the parent polymer of the core material when it was in the liquid stage. The core material is polyurethane foam made from polymeric isocyanate (Part A) and reacting with polyol (Part B). Spherical nanoparticles such as TiO 2 of about 29 nm in diameters were dispersed in Part A of liquid polyurethane through an ultrasonic cavitation process. The amount of nanoparticles infused into liquid foam varied from 1 to 3% by weight. Once Part A was doped with nanoparticles, it was mixed with Part B, and was cast in a rectangular mold to produce the nanophased polyurethane foam. The nanophased foam was then used with regular S-2 Glass fiber preforms and SC-15 epoxy to manufacture sandwich composites in a VARTM set up. Test coupons were then extracted from foam as well as from sandwich panels to conduct flexural and various other chemical tests. A parallel set of control panels were also made with neat polyurethane core materials. Thermogravimetric and SEM analyses have indicated that the decomposition temperature of the nanophased foams increases by about 27 8C and the cell size almost doubles with nanoparticle infusion. A significant improvement in flexural strength and stiffness has also been observed with 3% loading of TiO 2 nanoparticles. Debond fracture toughness parameters ðG c Þ were also determined for both categories of sandwich constructions, and it was seen that nanoparticle infusion reduces the value of G c by almost a factor of three. Despite this reduction, strength of nanophased sandwich increased by about 53% over the neat system. Details of manufacturing and analyses of test results are included in the paper. q

Fracture toughness of Al 2 O 3 platelet-reinforced hydroxyapatite (HAP) ceramics was investigated using the Vickers' indentation technique. The geometrical anisotropy of alumina platelets induces an anisotropic toughening. The ef®ciency of reinforcing mechanisms remains maximum for a crack propagating with an angular deviation inferior to 30 around the direction perpendicular to alumina disc faces. This is assumed to result from a crack de¯ection mechanism which induces a favorable contribution of mode II failure. A small effect of hydroxyapatite grain size becomes noticeable in the direction parallel to alumina disc faces. The toughening depends on the size and volume content of alumina platelets. Large size platelets provoke a spontaneous microcracking of the HAP matrix which is detrimental to the mechanical reliability, whereas small platelets lead to a strong toughening. The results relate to the intensity of thermoelastic residual stresses within the matrix around alumina inclusions.

Fibre reinforced composite materials are used extensively in stiffness critical, weight sensitive structures such as those found in aerospace and motor racing. They are characterized by high in-plane strength, stiffness and toughness and low density. The most widely used family of these materials is essentially two dimensional, characterized by relatively poor out of plane properties. As a consequence of low interlaminar toughness in particular, many possible applications are precluded and others severely compromised in performance per unit weight efficiency. Formula 1 racing represents the most advanced exploitation of composite materials both in terms of the percentage usage and complexity of application (1). In order to develop their products leading F1 teams work very closely with the major raw materials suppliers to expand the horizons of composites usage. The problems of interlaminar performance are discussed along with the techniques used to measure them and the fracture mechanics principles applied to improve them. A number of Formula 1 applications and developments are used to illustrate the effectiveness of the improved understanding of the interlaminar fracture behaviour of composites.

Epoxy/clay nanocomposites with a better exfoliated morphology have been successfully prepared using a so-called "slurry-compounding" process. The microstructures of the nanocomposites (epoxy/S-clays) were characterized by means of optical microscopy and transmission electron microscopy (TEM). It was found that clay was highly exfoliated and uniformly dispersed in the resulting nanocomposite. Characterizations of mechanical and fracture behaviors revealed that Young's modulus increases monotonically with increasing the clay concentration while the fracture toughness shows a maximum at 2.5 wt % of clay. No R-curve behavior was observed in these nanocomposites. The microdeformation and fracture mechanisms were investigated by studying the microstructure of arrested crack tips and the damage zone using TEM and scanning electron microscopy (SEM). The initiation and development of microcracks are the dominant microdeformation and fracture mechanisms in the epoxy/ S-clay nanocomposites. Most of the microcracks initiate between clay layers. The formation of a large number of microcracks and the increase in the fracture surface area due to crack deflection are the major toughening mechanisms.

Microstructure development and fracture toughness of Si3N4 composites were studied in the presence of seeds and Al2O3 + Y2O3 as sintering aids. The elongated β-Si3N4 seeds were introduced into two different α-Si3N4 matrix powders; one was the ultra fine powder matrix and the other was the coarse powder matrix. The amount of seeds varied from 0 to 6 wt%. The grain growth inhibition and the mechanism of toughening were discussed and correlated with microstructure. The maximum fracture toughness of 9.0 MPa m1/2 was obtained for ultra fine powder with 5 wt% seeds hot pressed at 1,700 °C for 6 h.

Thin-walled structural components are widely used in several engineering applications such as in aerospace, naval, nuclear power plant, pressure vessel, mechanical and civil fields. Since they are frequently characterised by a high slenderness, the safety assessment of such structural components requires to carefully consider the buckling collapse which can heavily limit their allowable bearing capacity. For very thin plates, buckling collapse can occur under compression, shear, or even under tension. In the present paper, the buckling and fracture collapse mechanisms in an elastic rectangular thin-plate with a central straight crack under shear loading are analysed. Different boundary conditions, crack length and orientation are considered. Through a parametric finite elements (FE) numerical analysis, the crack sensitivity of the collapse load of such a structural component is examined. The obtained results are discussed, and some interesting and useful conclusions are drawn. The collapse mechanism occurring earlier (buckling or fracture) is found by varying the fracture toughness of the material, and some failure-type maps depending on the geometrical parameters of the crack are determined.

There has been a growing requirement in the last years, particularly in the aerospace industry, for adhesives to withstand high temperatures. The most important factor to consider when studying the effect of temperature on adhesively-bonded joints is the variation of adhesive mechanical properties with temperature such as the stress-strain curve and the toughness. Adhesive strength and strain show temperature dependence, especially near the glass transition temperature (T g ) of the adhesive. Similarly, the fracture toughness is expected to show temperature dependence.

The load dependence of the Vickers microhardness on the as-grown (0 1 0) and (0 0 1), and cleaved (0 0 1) faces of cadmium tartrate pentahydrate (CTPH) single crystals has been investigated. The experimental results showed that, with an increase in the applied load, the microhardness of the as-grown (0 1 0) and (0 0 1) faces decreases, while that of the unheated and heated (0 0 1) cleavage faces decreases first up to a load of 2.5 N and then increases. Analysis of the experimental results revealed that: (1) radial crack length, indentation size and applied indentation load are mutually related, and these dependences related with fracture mechanics are the basis of Meyer's empirical law, (2) with increasing indentation load, changes in the mechanism of development of indentation cracks from radial cracks to lateral cracks and surface chipping of the material, followed by predominantly surface chipping of the material are responsible for indentation size effect in CTPH crystals, (3) proportional specimen resistance model and Meyer's law not only explain the indentation size effect but also can be used to determine load-independent hardness H * , and (4) there is no direct relationship between microhardness and fracture toughness of different CTPH samples, while the values of load-independent hardness H * , and brittleness indices b and B of CTPH crystals increase linearly with the Meyer constant A. Procedures are given to determine load-independent hardness H * from the transition values of load and corresponding indentation size.

Ca 3 ZrSi 2 O 9 (baghdadite) has become a major research focus within the biomaterial community due to its remarkable in-vitro and in-vivo bioactivity. Although baghdadite seems to exhibit interesting biological properties, as yet there has been no data published concerning its mechanical properties. This lack of knowledge hinders targeting this novel bioactive material towards potential applications. In this study we prepare dense Ca 3 ZrSi 2 O 9 bulk ceramics for the first time, allowing the evaluation of its mechanical properties including hardness, bending strength, Young's modulus, and fracture toughness. The preparation of baghdadite has been accomplished by a direct solid-state synthesis in combination with conventional sintering at 1350-1450 1C for 3 h. Our results show that samples sintered at 1400 1C exhibit the best mechanical properties, resulting in a bending strength, fracture toughness, and hardness of 98716 MPa, 1.370.1 MPa m 0.5 , and 7.970.2 GPa. With a comparable mechanical strength to hydroxyapatite, but with an increased fracture toughness by 30% and hardness by 13% baghdadite is highly suitable for potential applications in non-load bearing areas (e.g. coatings or filler materials).

Titanium and its alloys are extensively used in aerospace and mechanical application because of their high specific strength and high fracture toughness. On the other hand, titanium alloys often show low hardness, very low load bearing capacity and poor resistance to sliding wear, so that surface properties improvement is in many cases recommended, often by PVD processes.

This paper presents the effect of loading rate, (ELR), and direction of formation, (DF), of rigid polyurethane foams, (PUR 40 and PUR 140), on fracture toughness. Nominal densities of used foams in the experimental program were 140 kg/m 3 , (for ELS) and 40 kg/m 3 , (for DF), which is closed-cell rigid foams widely used for sandwich cores. Determination of fracture toughness for Mode I fracture of studied materials has made by three-point bending tests, (3PB), on specimens with notches, at room temperature (20 ± 2 ºC). All the specimens were cut from one and the same plate. The specimens were subjected to 3PB at a loading speed of 2 mm/min, except samples for determining the ELR where 2, 20, 200 and 400 mm/min loading speeds were used, and were taken into account the fact that the load must act exactly on the notch direction. All the specimens present brittle failure without plastic deformation.

Model diglycidyl ether of bisphenol-A based epoxy resins containing well-dispersed 15 nm block copolymer (BCP) nanoparticles were prepared to study the effect of matrix crosslink density on their fracture behavior. The crosslink density of the model epoxies was varied via the controlled epoxy thermoset technology and estimated experimentally. As expected, it was found that the fracture toughness of the BCP-toughened epoxy is strongly influenced by the crosslink density of the epoxy matrix, with higher toughenability for lower crosslink density epoxies. Key operative toughening mechanisms of the above model BCP-toughened epoxies were found to be nanoparticle cavitationinduced matrix shear banding for the low crosslink density epoxies. The toughening effect from BCP nanoparticles was also compared with core-shell rubber-toughened epoxies having different levels of crosslink density. The usefulness of the present findings for designing toughened thermosetting materials with desirable properties is discussed.

New high-strength, high-temperature Cu-Ni-Si alloys have been developed using additions of Cr, Zr, and/or Ti. These new alloys remain as precipitation hardenable as the base alloy, but the main strengthening phase may be different than Ni2Si (e.g., Cr2Ti). Substantial increases in mechanical strength were observed at both room and high temperature (773 K) when additions of Cr+Zr+Ti and Cr+Zr were made. Industrial testing of these alloys indicated a sevenfold increase in the lifetime of lateral blocks in continuous casting equipment of copper alloys.

The d e s i g n and c o n t i n u e d s a f e o p e r a t i o n o f n u c l e a r p r e s s u r e v e s s e l s demands t h a t t h e f r a c t u r e t o u g h n e s s o f r e a c t o r b e l t l i n e m a t e r i a l i s cont i n u o u s l y m o n i t o r e d f o r s i g n s o f d e g r a d a t i o n , which i n v a r i a b l y a r i s e s from n e u t r o n i r r a d i a t i o n e m b r i t t l e m e n t . A c c o r d i n g l y , r e a c t o r p r e s s u r e v e s s e l s u r v e i l l a n c e programs have been e s t a b l i s h e d whereby f r a c t u r e s p e cimens a r e p l a c e d in the r e a c t o r c o r e t o be p e r i o d i c a l l y removed f o r t e s ting d u r i n g t h e l i f e o f the v e s s e l [1]. Due to l i m i t a t i o n s in r e a c t o r space and the expense o f i r r a d i a t i n g s u f f i c i e n t numbers o f s u r v e i l l a n c e s p e c i m e n s , the v a s t m a j o r i t y o f such specimens a r e o f C h a r p y -s i z e dimens i o n s ( i . e . , 10 mm s q u a r e V-notch bend b a r s ) . U n f o r t u n a t e l y f o r t h e c l a s s o f s t e e l u t i l i z e d f o r n u c l e a r p r e s s u r e v e s s e l c o n s t r u c t i o n a t t h e o p e r at i n g t e m p e r a t u r e s in q u e s t i o n s , measurement o f a " v a l i d " f r a c t u r e t o u g h -

The effect of different precracking methods on the results of linear elastic K Ic fracture toughness testing with medium-density polyethylene (MDPE) was investigated. Cryogenic conditions were imposed in order to obtain valid K Ic values from specimens of suitable size. Most conservative K Ic values were obtained by slow pressing a fresh razor blade at the notch root of the specimen. Due to the low deformation level imposed on the crack tip region, the slow pressing razor blade technique also produced less scatter in fracture toughness results. It has been shown that the slow stable crack growth preceding catastrophic brittle failure during K Ic tests in MDPE under cryogenic conditions should not be disregarded as it has relevant physical meaning and may affect the fracture toughness results.

The fracture and fatigue behaviour of prototype automotive pistons produced in an aluminium alloy matrix composite in industrial conditions has been studied. Fracture toughness increased when the testing temperature rose from 20°to 75°C and kept near constant up to 250°C, when a significantly lower value was recorded. A change in the failure operating mechanism, which can explain this trend, was observed by analysing the fracture surfaces in the scanning electron microscope. Room temperature fatigue tests performed with R=0.1 stress ratio led to an average value of the Paris law exponent higher than those reported in aluminium alloys but low for an industrially produced brittle composite. A higher exponent and a much larger scattering were observed in those fatigue tests carried out under R=0.5 stress ratio.

The main purpose of the present paper is to investigate the effect of crosshead speed, specimen thickness, and welding on the fracture toughness. The material of the investigated pipe is a high density polyethylene (HDPE), which is commonly used in natural gas piping systems. The welding technique used in this study is butt-fusion (BF) welding technique. The crosshead speed ranged from 5 to 500 mm/min and specimen thickness ranged from 9 to 45 mm for both welded and unwelded specimens at room temperature, T a ¼ 20 C. Curved three point bend (CTPB) specimens were used to determine K Q . Furthermore, the results of fracture toughness, K Q , will be compared with the plane-strain fracture toughness, J IC , for welded and unwelded specimens. The experimental results revealed that K Q increases with increasing the crosshead speed, while K Q decreases as the specimen thickness increases. The investigation reveals that the apparent fracture toughness, K Q , for HDPE pipe of unwelded specimen is greater than that of corresponding value for welded specimen. The same trend was observed for the plane-strain fracture toughness, J IC . At lower crosshead speeds there is a minimum deviation in K Q between welded and unwelded specimens, while the deviation becomes larger with increasing crosshead speed.

In the present work, AL-alloy containing 12% silicon (LM 13) matrix nano composites were fabricated in sand moulds by using copper end blocks of copper end chill thickness 10 &15 nm with cryogenic effect . The size of the reinforcement (NanoZro 2 ) ranges from 50-80nm being added ranges from 3 to 15 wt % in steps of 3 wt % . Cryogenically solidified Nano Metal Matrix Composites were compressed by using hydraulic compression machine. Specimens were prepared according to ASTM standards and tested for their strength, hardness and fracture toughness. Micro structural studies of the fabricated Nano Composites indicate that there is uniform distributions of reinforcements in the matrix materials (LM 13). An increasing trend of hardness, UTS & fracture toughness has been observed. The best results have been obtained at 12 wt %. The results were further justified by comparing two copper end chill thickness 10 &15 mm. Finally the Volumetric Heat Capacity of the cryo-chill is identified as an important parameter which affects mechanical properties.

Ultra high molecular weight polyethylene (UHMWPE) is a semicrystalline polymer that has been used for over four decades as a bearing surface in total joint replacements. The mechanical properties and wear properties of UHMWPE are of interest with respect to the in vivo performance of UHMWPE joint replacement components. The mechanical properties of the polymer are dependent on both its crystalline and amorphous phases. Altering either phase (i.e., changing overall crystallinity, crystalline morphology, or crosslinking the amorphous phase) can affect the mechanical behavior of the material. There is also evidence that the morphology of UHMWPE, and, hence, its mechanical properties evolve with loading. UHMWPE has also been shown to be susceptible to oxidative degradation following gamma radiation sterilization with subsequent loss of mechanical properties. Contemporary UHMWPE sterilization methods have been developed to reduce or eliminate oxidative degradation. Also, crosslinking of UHMWPE has been pursued to improve the wear resistance of UHMWPE joint components. The 1 st generation of highly crosslinked UHMWPEs have resulted in clinically reduced wear; however, the mechanical properties of these materials, such as ductility and fracture toughness, are reduced when compared to the virgin material. Therefore, a 2 nd generation of highly crosslinked UHMWPEs are being introduced to preserve the wear resistance of the 1 st generation while also seeking to provide oxidative stability and improved mechanical properties.

The standard ISO 1.2738 medium-carbon low-alloy steel has long been used to fabricate plastic molds for injection molding of large automotive components, such as bumpers and dashboards. These molds are usually machined from large pre-hardened steel blooms. Due to the bloom size, the heat treatment yields mixed microstructures, continuously varying from surface to core. Negative events (such as microcracks due to improper weld bed deposition or incomplete extraction of already formed plastic objects) or too large thermal/mechanical stresses can conceivably cause mold failure during service due to the low fracture toughness and fatigue resistance typically encountered in large slack quenched and tempered ISO 1.2738 steel blooms. Alternative steel grades, including both non-standard microalloyed steels, designed for the same production process, and precipitation hardening steels, have recently been proposed by steelworks. However, the fracture toughness and the fatigue properties of these steels, and hence their response during the service, are not well known. Results of an experimental campaign to assess the fracture toughness and fatigue properties, as well as the basic mechanical properties, of a microalloyed and a precipitation hardening plastic mold steel blooms are presented and commented, also in respect to the results previously obtained by two commercial ISO 1.2738 ones. Experimental results show that these steels generally exhibit low fracture toughness values; in the traditional quenched and tempered bloom steels the brittleness may be caused both by the presence of mixed microstructures and by grain boundaries segregation, while in the precipitation hardened one the brittleness probably stems from the precipitation phenomena. This study suggests that microalloyed and precipitation hardening steels may be used to produce large plastic mold, yet the fracture toughness still remains the most critical property.

A steel cannon barrel was tested and analyzed in order to predict its lifetime. Mechanical properties, fracture toughness and fatigue behavior were determined experimentally. The cannon barrel was assumed to have a smooth inner surface. On that basis, ®nite element analyses were carried out in order to determine the stress intensity factor as a function of crack length for the case in which the cannon barrel contains two symmetrically located cracks. The fatigue data was assumed to follow the Paris±Erdogan law. A statistical Monte Carlo analysis for the fatigue life of the cannon barrel was carried out with many of the parameters assumed to be random variables. #

The aim of this study is to characterize the fracture behavior of biodegradable poly(lactic acid) (PLA). Especially, the effects of crystallinity and loading-rate on the fracture behavior are emphasized. Annealing was performed to control the crystallinity of the PLA samples prepared, and then their fracture toughness values were measured under quasi-static and impact loading conditions. The results showed that the quasi-static fracture toughness of PLA decreases with increase of crystallinity; on the other hand, the impact fracture toughness tends to increase with crystallinity. The crack growth behaviors of the PLA specimens having different crystallinity were also observed by polarizing and scanning electron microscopies. The microscopic results exhibited that under quasi-static loading, disappearance of multiple crazes in the crack-tip region results in the decrease of the fracture toughness with crystallinity. On the contrary, under impact loading, the increase of the fracture toughness with crystallinity is considered to be related to the increase of fibril formation. q

MP35N (35% Co, 35% Ni, 20% Cr, and 10% Mo) is a superalloy that exhibits a remarkable combination of high strength and high fracture toughness. This alloy derives its strength mainly from two processes -cold work and a subsequent aging heat treatment. This study examines the influence of the amount of cold-work, imposed in the drawing and the rolling processes, and the subsequent aging heat treatment on the strength and ductility (which is also an indicator of toughness) of this alloy. It was found that high levels of cold-work followed by the aging heat treatments resulted in a drastic drop in the ductility of the alloy. This drastic loss of ductility was correlated with appearance of macro-scale shear bands in the samples. The physical origin of these localized shear bands is investigated in this paper. Furthermore, this study established a processing window for MP35N (in cold-drawing and cold-rolling processes) where the aging heat treatment provides the desired increase in the strength without adversely influencing the ductility of the alloy. (R.D. Doherty) 0921-5093/99/$ -see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 -5 0 9 3 ( 9 9 ) 0 0 5 0 2 -X

Three-directional carbon fiber reinforced silicon carbide composites were fabricated using liquid silicon infiltration. Siliconization was carried out at nine different conditions to achieve best mechanical properties. Flexural strength, tensile strength and fracture toughness of the composites were determined at each condition. Flexural and tensile strengths increase with siliconization time and temperature. At 1650 • C and 120 min mechanical properties are found to be the best. Arithmetic mean average flexural and tensile strength values were found to be 177 and 90 MPa, respectively. The flexural strength data were also analyzed by the Weibull modulus approach; it was found to be the highest (13.28) for composites siliconized at 1550 • C, 120 min and the least (7.01) at 1450 • C, 10 min. The fracture toughness of the composite blocks was found be in the range 5-6 MPa √ m. The strength data were also correlated with chemical composition of the composites and the microstructure obtained during processing.

A potential problem with high-intensity lights might be failure of polymer chains to grow and cross-link in a desired fashion, thereby affecting the structure and properties of the polymers formed. The purpose of this study was to evaluate mechanical properties of resin composites polymerized using four different light-curing units.

The mechanical behaviour of different Mg-PSZ ceramics is studied. Results of their edge fracture (EF) and single edge V-notch beam (SEVNB) tests are discussed. These inelastic ceramics exhibit nonlinear relations between the fracture load and the distance from the extreme point on the chip scar to the specimen edge. They also possess nonlinearly rising R-lines. It is established that the data points plotted in the EF base diagram fall below the baseline (lower barrier to the onset of fracture). By projecting these data points onto the baseline, fracture toughness values close to those of the matrix are determined. The limitations of conventional procedures for evaluating the mechanical behaviour of these ceramics were found out. It has been demonstrated that the EF test method can be quite adequate for enhancing the reliability of comparative fracture resistance estimates. #

Epoxy-nanocomposite resins filled with 12-nm spherical silica particles were investigated for their thermal and mechanical properties as a function of silica loading. The nanoparticles were easily dispersed with minimal aggregation for loadings up to 25 wt% as determined using transmission electron microscopy (TEM) and ultra-small-angle X-ray scattering (USAXS). A proportional decrease in cure temperatures and glass transition temperature (for loadings of 10 wt% and above) was observed with increased silica loading. The morphology determined by USAXS is consistent with a zone around the silica particles from which neighboring particles are excluded. The ''exclusion zone'' extends to 10Â the particle diameter. For samples with loadings less than 10 wt%, increases of 25% in tensile modulus and 30% in fracture toughness were obtained. More highly loaded samples continued to increase in modulus, but decreased in strength and fracture toughness. Overall, the addition of nanosilica is shown as a promising method for property enhancement of aerospace epoxy composite resins.