Decrease In Fracture Strength Research Articles

The effect of silica nanoparticles on the shock adiabatic relation and tensile strength in polyurethane

This work exploded the effect of silica nanoparticles on shock adiabatic relation and tensile fracture in polyurethane based on atomistic simulations. It is found that the non-uniform interface structures introduced by nanoparticles can increase the shock impedance, which also leading to increased twisting of polyurethane chains and the generation of local hotspots near the nanoparticles. The interface structures near the nanoparticle consists of polyurethane chains with a high gyration radius, and the average gyration radius increases approximately linearly with the increase in nanoparticle content. At lower shock velocity, the potential energy increment initially increases and then decreases with the increase in nanoparticle content. Bond analysis reveal that flexible segments dominate the bending and twisting deformations of polyurethane, while nanoparticles enhance the corresponding deformation degrees. Furthermore, nanoparticles can serve as void nucleation sites for early-stage tensile damage, resulting in a decrease in fracture strength. Differing from metals, the relationship between tensile strength and fracture temperature follows a more linear law. Nanoparticles inhibit the later growth and coalescence of voids by increasing the temperature and steric hindrance. The density and size distribution characteristics of voids are consistent with the changes in potential energy during the shock compression process. Unlike classical spallation, the curled polyurethane chains in the tensile region gradually orient along the shock direction, evolving into a void-fiber structure.

High-temperature mechanical properties and fracture mechanism of A6B2O17 (A= Hf, Zr; B= Ta, Nb) high-entropy ceramics

A6B2O17 (A= Hf, Zr; B= Ta, Nb) high-entropy ceramics are prepared by a solid reaction method, and the temperature-dependent mechanical properties are in situ determined by a high-temperature three-point bending combined with the digital image correlation method. The results show that (Hf1/2Zr1/2)6(Ta2/3Nb1/3)2O17 (H1Z1T2N1) ceramics exhibit superior high-temperature mechanical properties. As the temperature increases from 25 °C to 1200 °C, the fracture toughness and fracture strength decrease from 3.19 MPa·m1/2 to 2.37 MPa·m1/2 and from 97.38 MPa to 72.48 MPa, respectively. The H1Z1T2N1 ceramics still maintain impressive mechanical properties at high temperatures due to the moderate grain size, low porosity and intergranular fracture. Furthermore, the higher entropy (9.61 R J/mol·K) of this ceramic leads to a significant high entropy effect, and the lattice distortion effects cause high-density dislocation regions, inducing a dislocation toughening mechanism.

Effects of expanded graphite content on the performance of MgO‐C refractories

AbstractExpanded graphite was introduced into a MgO‐C refractory to suppress its thermal expansion and thus enhance thermal shock resistance. The refractory was prepared by mixing MgO powders (particle size = 75 μm and 1 mm), antioxidant and curing agents, flake and expanded graphite, and a novolak‐type phenolic resin at 50°C. This was followed by aging at 20°C for 24 h, compacting by uniaxial pressing, curing at 210°C for 5 h, and heat treatment at 1500°C. With an increase in expanded graphite content from 0 to 4 wt.%, the bulk density decreased, apparent density remained unchanged, and apparent porosity increased. The gaps created in the vicinity of MgO particles because of this increase in apparent porosity buffered the thermal expansion of MgO. This increased resistance to thermal shock up to 1500°C. However, the increase in expanded graphite content also had a detrimental effect reflected in the decrease in fracture strength and increase in the residual strain after repeated thermal shock. This contradiction indicates that composition optimization is important for the practical performance enhancement of MgO‐C bricks. The optimum content of the expanded graphite was determined as 2 wt.%.

Anisotropic crystal orientations dependent mechanical properties and fracture mechanisms in zinc blende ZnTe nanowires.

The orientations of crystal growth significantly affect the operating characteristics of elastic and inelastic deformation in semiconductor nanowires (NWs). This work uses molecular dynamics simulation to extensively investigate the orientation-dependent mechanical properties and fracture mechanisms of zinc blende ZnTe NWs. Three different crystal orientations, including [100], [110], and [111], coupled with temperatures (100 to 600 K) on the fracture stress and elastic modulus, are thoroughly studied. In comparison to the [110] and [100] orientations, the [111]-oriented ZnTe NW exhibits a high fracture stress. The percentage decrease in fracture strength exhibits a pronounced variation with increasing temperature, with the highest magnitude observed in the [100] direction and the lowest magnitude observed in the [110] direction. The elastic modulus dropped by the largest percentage in the [111] direction as compared to the [100] direction. Most notably, the [110]-directed ZnTe NW deforms unusually as the strain rate increases, making it more sensitive to strain rate than other orientations. The strong strain rate sensitivity results from the unusual short-range and long-range order crystals appearing due to dislocation slipping and partial twinning. Moreover, the {111} plane is the principal cleavage plane for all orientations, creating a dislocation slipping mechanism at room temperature. The {100} plane becomes active and acts as another fundamental cleavage plane at increasing temperatures. This in-depth analysis paves the way for advancing efficient and reliable ZnTe NWs-based nanodevices and nanomechanical systems.

Micro aggregate and pozzolanic reactivity of fly ash: effect on fracture properties

ABSTRACT The performance of fly ash blended concrete is influenced by the pozzolanic and/or micro-aggregate effect of fly ash particles based on the replacement levels. Dominance of either of this effect greatly influences the mechanical and fracture properties of fly ash blended concrete. In view of this, in the present study, 0%, 20%, 30%, 35%, 40%, 50%, 60% and 70% of fly ash is replaced for cement to investigate the dominance of pozzolanic or micro aggregate effect. The results obtained from the study indicate the reduced strength with increased replacement of fly ash. Similarly, the peak load at fracture decreases with the increased replacement of fly ash. Further, it is concluded that with the increased curing age the compressive strength increases and the fracture strength decreases. Furthermore, the decreased content of calcium hydroxide (CH) with increased fly ash content as obtained from thermo gravimetric analysis (TGA) confirms the presence of excessive fly ash that can contribute as filler material demonstrating the micro-aggregate effect.

Zirconia fixed dental prostheses fabricated by 3D gel deposition show higher fracture strength than conventionally milled counterparts

Zirconia restorations, which are fabricated by additive 3D gel deposition and do not require glazing like conventional restorations, were introduced as “self-glazed” zirconia restorations into dentistry. This in vitro investigation characterized the surface layer, microstructure and the fracture and aging behavior of “self-glazed” zirconia (Y-TZPSG) three-unit fixed dental prostheses (FDP) and compared them to conventionally CAD/CAM milled and glazed controls (Y-TZPC-FDPs). For this purpose, the FDPs were analyzed by (focused ion beam) scanning electron microscopy, laserscanning microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and a dynamic and static loading test. For the latter, half of the samples of each material group (n = 16) was subjected to 5 million cycles of thermocyclic loading (98N) in an aqueous environment in a chewing simulator. Afterwards, all FDPs were loaded to fracture. Y-TZPSG-FDPs demonstrated a comparable elemental composition but higher surface microstructural homogeneity and fracture strength compared to Y-TZPC-FDPs. Microstructural flaws within the FDPs’ surfaces were identified as fracture origins. The high fracture strength of the Y-TZPSG-FDPs was attributed to a finer-grained microstructure with fewer surface flaws compared to the Y-TZPC-FDPs which showed numerous flaws in the glaze overlayer. A decrease in fracture strength after dynamic loading from 5165N to 4507N was observed for the Y-TZPSG-FDPs, however, fracture strength remained statistically significantly above the one measured for Y-TZPC-FDPs (before chewing simulation: 1923N; after: 2041N). Within the limits of this investigation, it can therefore be concluded that Y-TZPSG appears to be stable for clinical application suggesting further investigations to prove clinical applicability.

Compression and shear fracture behavior of single rice paddy under effect of husking operation

Husking is an important process to determine rice quality. Husking ratio and fracture behavior are key points during paddy husking, which are affected by rubber roller line speed, rice moisture content and drying temperature. Based on husking process, the intrinsic property and mechanical-structural damage behavior of three kinds typical rice paddy were tested under different working condition and analyzed by texture analyzer with in-situ observation, optical and electron microscope. Results showed that three kinds rice paddy have different fracture forms and show obvious plastic deformation at the rice grain center under compression, with similar fracture energy and fracture strength. Shear force exerted by PU led to a change in the fracture form of rice grains, showing crack and fracture in the short axis direction. Degree of shear fracture was greater for rice paddy at high speed. In addition, with the increase of soaking time, the moisture content of rice grains increased along with the internal generation of hygroscopic cracks, which led to the decrease of fracture strength and fracture energy. During the drying process, the glassy transformation of the rice grains will reduce their quality, and as the temperature rises, the hardness increases and cavities are created inside.

Modeling of ion exchange in glass considering large viscoelastic deformation and mechano–electrochemical coupling

AbstractThis work proposes a theoretical description of ion exchange in glass involving mechano–electrochemical coupling, distinct from the previous models. The generalized Maxwell viscoelastic constitutive relation and large‐deformation formulae are employed, which is indispensable for thin glass panels widely used in consumer electronics. With the theory, two‐dimensional numerical simulations based on axisymmetric and plan‐strain models are conducted. The numerical results signify lateral deformation in a double‐side ion‐exchanged glass and bending caused by single‐side or electric‐field‐assisted ion exchange. With the decrease in glass thickness, the increase in time, or applied electric potential, these deformations become more significant, leading to the relief of in‐plane stresses in the ion‐exchanged layer and the increase in stresses in other parts. This could lead to a remarkable decrease in fracture strength or cracking of an ultrathin glass panel. As the stress and diffusion are coupled, the decrease in stress in the ion‐exchanged layer leads to the increase in the depth of layer of invading ions. These numerical results clearly show the necessity to incorporate the mechano–electrochemical coupling and large deformation in modeling ion exchange, which is uninvolved in previous theoretical models. Moreover, the proposed theoretical model is directly extendable to three‐dimensional scenarios, such as curved or foldable ultrathin glass panels.

In situ measurements of the high-temperature mechanical properties of ZrO2-doped YTaO4 ceramic by three-point bending combined with a digital image correlation method

ZrO2-doped YTaO4 ceramics with different ZrO2 contents (Y1-xTa1-xZr2xO4 ceramics, 2x = 0, 0.1, 0.15, 0.2) are prepared by a solid-state reaction method. The high-temperature mechanical properties are determined by an in situ high-temperature three-point bending combined with the digital image correlation (DIC) method. The results show that when the temperature rises from 25 °C to 800 °C, the temperature dependence of the elastic modulus, fracture toughness and fracture strength with a ZrO2 doping content of 10% mol is relatively small. The elastic modulus, fracture toughness and fracture strength decrease from 105.4 GPa to 96.1 GPa, from 2.6 MPa m1/2 to 2.3 MPa m1/2 and from 73.8 MPa to 64.8 MPa, respectively. Transgranular fracture is the main fracture mode. When the crack propagates through the twin band, a phase transformation and a typical domain change occur. Furthermore, the crack propagating through the twin band at room temperature (25 °C) is more tortuous than that at 800 °C, and more energy needs to be consumed. This illustrates that the fracture toughness and fracture strength at room temperature (25 °C) are higher than those at high temperature.

Temperature- and Defect-Induced Uniaxial Tensile Mechanical Behaviors and the Fracture Mechanism of Two-Dimensional Silicon Germanide.

Recently, monolayer silicon germanide (SiGe), a newly explored buckled honeycomb configuration of silicon and germanium, is predicted to be a promising nanomaterial for next-generation nanoelectromechanical systems (NEMS) due to its intriguing electronic, optical, and piezoelectric properties. In the NEMS applications, the structure is subjected to uniaxial tensile mechanical loading, and the investigation of the mechanical behaviors is of fundamental importance to ensure structural stability. Here, we systematically explored the uniaxial tensile mechanical properties of 2D-SiGe through molecular dynamics simulations. The effects of temperature ranges from 300 to 1000 K and vacancy defects, for instance, point and bi vacancy, for both armchair and zigzag orientations of 2D-SiGe were investigated. In addition, the influence of system areas and strain rates on the stress–strain performance of 2D-SiGe has also been studied. With the increase in temperature and vacancy concentration, the mechanical properties of 2D-SiGe show decreasing behavior for both orientations and the armchair chirality shows superior mechanical strength to the zigzag direction due to its bonding characteristics. A phase transformation-induced second linearly elastic region was observed at large deformation strain, leading to an anomalous stress–strain behavior in the zigzag direction. At 300 K temperature, we obtained a fracture stress of ∼94.83 GPa and an elastic modulus of ∼388.7 GPa along the armchair direction, which are about ∼3.17 and ∼2.83% higher than the zigzag-oriented fracture strength and elastic modulus. Moreover, because of the strong regularity interruption effect, the point vacancy shows the largest decrease in fracture strength, elastic modulus, and fracture strain compared to the bi vacancy defects for both armchair and zigzag orientations. Area and strain rate investigations reveal that 2D-SiGe is less susceptible to the system area and strain rate. These findings provide a deep insight into controlling the tensile mechanical behavior of 2D-SiGe for its applications in next-generation NEMS and nanodevices.

A molecular dynamics study of the mechanical properties of the graphene/hexagonal boron nitride planar heterojunction for RRAM

Due to their high flexibility and precise interface modulation, resistive random access memory (RRAM) devices composed of graphene/hexagonal boron nitride (Gr/h-BN) planar heterojunctions are regarded as a possible solution and have gained a substantial amount of attention. Molecular dynamics simulations with a modified Tersoff potential are conducted to investigate the mechanical properties of three planar heterojunction RRAM molecular models (Gr/h-BN_V1/V2/V3), which are established according to the interface type and atomic position. Among them, the Gr/h-BN_V1 model exhibits the best mechanical properties, including the fracture strength, fracture strain, and Young's modulus, at all simulation temperatures. The extensive simulations reveal the effects of the temperature, aspect ratio, single-vacancy (SV) and Stone-Wales (SW) defects on the tensile strength of Gr/h-BN RRAM models. As the temperature increases from 10 K to 500 K, the fracture strengths of the Gr/h-BN_V1, Gr/h-BN_V2 and Gr/h-BN_V3 models decrease by 12.6%, 59.6% and 12.0%, respectively. It can be observed that when the aspect ratio is 2.83, the comprehensive mechanical properties of the Gr/ h-BN_V1 model are superior to those of the other models. A 6.7% concentration of single atomic defects in the SV_C, SV_B and SV_N models causes decreases in the fracture strength of 28.0 GPa, 33.9 GPa and 22.6 GPa, respectively, indicating that the mechanical properties are more sensitive to the introduction of boron atomic vacancies than carbon and nitrogen atomic vacancies. At all defect concentrations, the decrease in fracture strength and fracture strain caused by SW defects in the SW_CN model is minimized, while the demarcation point of the relative influence of the defect concentration on the tensile strength is 16% in the SW_CB and SW_CC models. Our present work provides useful insights into the application of highly flexible and stretchable Gr/h-BN planar heterojunctions for RRAM devices.

Improving flame retardancy of in-situ silica-epoxy nanocomposites cured with aliphatic hardener: Combined effect of DOPO-based flame-retardant and melamine

Silica-epoxy nanocomposites were prepared via an “in-situ” sol-gel synthesis process and a phosphorus (P) flame-retardant i.e. 6H-dibenz[c,e][1,2]oxaphosphorin,6-[(1-oxido-2,6,7-trioxa-1-phosphabicyclo[2.2.2]oct-4-yl)methoxy]-, 6-oxide (DP) and melamine (Mel) were further added to the matrix to improve its fire performance. The main components of epoxy resin were bisphenol A diglycidyl ether (DGEBA) and isophorone diamine (IPDA) hardener. The addition of DP as well as silica alone into the epoxy system stopped the melt dripping phenomena in the vertical fire test (UL 94), however, the addition of melamine was crucial for achieving the highest fire classification (UL 94-V0 rating). The presence of DP and Mel in the silica-epoxy nanocomposite promoted a large reduction (ranging from 53% up to 80%) in the heat release rate (HRR) and a delay (up to 31%) in the ignition time in the cone calorimetry experiments. Improved fire performance of the epoxy system was attributed to i) a condensed phase activity of silica, DP and melamine to form a protective thermal barrier during combustion and ii) a minor gas phase flame inhibition activity of DOPO component of DP. The mechanical characterization of the epoxy nanocomposites through tensile tests showed that the addition of DP increases the stiffness of the epoxy resin, resulting in a strong increase of Young modulus (up to 32%) and in a slight decrease of fracture strength, elongation at break and toughness. An increased glass transition temperature (up to 8%) of the epoxy system possibly due to hydrogen bonds and polar interactions of DP with the matrix was also observed.

Influence of Different Damage Patterns of the Stem Taper on Fixation and Fracture Strength of Ceramic Ball Heads for Total Hip Replacement.

Background Modularity finds frequent application in total hip replacement, allowing a preferable individual configuration and a simplified revision by retaining the femoral stem and replacing the prosthetic head. However, micromotions within the interface between the head and the stem taper can arise, resulting in the release of wear debris and corrosion products. The aim of our experimental study was to evaluate the influence of different taper damages on the fixation and fracture stability of ceramic femoral heads, after static and dynamic implant loading. Methods Ceramic ball heads (36 mm diameter) and 12/14 stem tapers made of titanium with various mild damage patterns (intact, scratched, and truncated) were tested. The heads were assembled on the taper with a quasistatic load of 2 kN and separated into a static and a dynamic group afterwards. The dynamic group (n = 18) was loaded over 1.5 million gait cycles in a hip wear simulator (ISO 14242-1). In contrast, the static group (n = 18) was not mechanically loaded after assembly. To determine the taper stability, all heads of the dynamic and static groups were either pulled off (ASTM 2009) or turned off (ISO 7206-16). A head fracture test (ISO 7206-10) was also performed. Subsequent to the fixation stability tests, the taper surface was visually evaluated in terms of any signs of wear or corrosion after the dynamic loading. Results In 10 of the 18 cases, discoloration of the taper was determined after the dynamic loading and subsequent cleaning, indicating the first signs of corrosion. Pull-off forces as well as turn-off moments were increased between 23% and 54% after the dynamic loading compared to the unloaded tapers. No significant influence of taper damage was determined in terms of taper fixation strength. However, the taper damage led to a decrease in fracture strength by approximately 20% (scratched) and 40% (truncated), respectively. Conclusion The results suggest that careful handling and accurate manufacturing of the stem taper are crucial for the ceramic head fracture strength, even though a mild damage showed no significant influence on taper stability. Moreover, our data indicate that a further seating of the prosthetic head may occur during daily activities, when the resulting hip force increases the assembly load.

Pseudo-ductile fracture of 3D printed alumina triply periodic minimal surface structures

Additive manufacturing enables the fabrication of periodic ceramic lattices with controllable micro-architectures. Many studies reported their catastrophic brittle fracture behaviour. However, ceramic lattices may fail by a layer-by-layer pseudo-ductile fracture mode, by controlling micro-architectures and porosities. Moreover, their fracture behaviour can be optimised by introducing strut/wall thickness gradients. This paper investigates the fracture behaviour and the fracture mode transition of ceramic triply periodic minimal surface (TPMS) structures. Alumina TPMS structures with relative densities of 0.14-0.37 are fabricated by ceramic stereolithography. Quasi-static compression tests validate a transition density range for non-graded samples: low (<0.21) and moderate (>0.25) relative density samples show layer-by-layer pseudo-ductile and catastrophic brittle fracture modes, respectively. The pseudo-ductile failure mode increases the energy absorption performance, enabling load-bearing capacity for a compressive strain up to 50%. With appropriate thickness gradients, graded structures exhibit significant increase of energy absorption without a decrease of fracture strength compared to their non-graded counterparts.

Structure mediation and ductility enhancement of poly(l-lactide) by random copolymer poly(d-lactide-co-ε-caprolactone)

Abstract Two types of random copolymer poly(d-lactide-co-ε-caprolactone) (PDLA-r-PCL) were added into poly(l-lactide) (PLLA) matrix by melt blending. The structure and property of PLLA/PDLA-r-PCL blends were investigated by thermal gravimetric analysis, differential scanning calorimetry, scanning electron microscopy, wide-angle X-ray diffraction, and mechanical measurement. The results suggested that PDLA-r-PCL had little effect on the thermal property of PLLA. PDLA-r-PCL uniformly dispersed in PLLA matrix, which contributed to the improvement of stretching properties. During stretching at 25°C, with the increased content of PDLA-r-PCL, the elongation at break of PLLA increased and the strength decreased. The degree of decrease in fracture strength was related to the molecular structure of PDLA-r-PCL. When a small amount of stereocomplex crystals (SC) were formed in PLLA/PDLA-r-PCL blends, the strength was maintained or slightly enhanced even though the elongation at break of the blends was significantly improved by soft chains of PCL. It might be caused by the synergistic effects of SC crystals and plasticization of PCL chains.

Mechanical Properties of Advanced Gas-Cooled Reactor Stainless Steel Cladding After Irradiation

The production of helium bubbles in advanced gas-cooled reactor (AGR) cladding could represent a significant hazard for both the mechanical stability and long-term storage of such materials. However, the high radioactivity of AGR cladding after operation presents a significant barrier to the scientific study of the mechanical properties of helium incorporation, said cladding typically being analyzed in industrial hot cells. An alternative non-active approach is to implant He2+ into unused AGR cladding material via an accelerator. Here, a feasibility study of such a process, using sequential implantations of helium in AGR cladding steel with decreasing energy is carried out to mimic the buildup of He (e.g., 50 appm) that would occur for in-reactor AGR clad in layers of the order of 10 µm in depth, is described. The implanted sample is subsequently analyzed by scanning electron microscopy, nanoindentation, atomic force and ultrasonic force microscopies. As expected, the irradiated zones were affected by implantation damage (< 1 dpa). Nonetheless, such zones undergo only nanoscopic swelling and a small hardness increase (~ 10%), with no appreciable decrease in fracture strength. Thus, for this fluence and applied conditions, the integrity of the steel cladding is retained despite He2+ implantation.

Fracture strength of silicon wafers sawn by fixed diamond wire saw

A larger breakage ratio occurs with the decrease of wafer thickness due to the decrease of fracture strength for as sawn silicon wafers, which is a severe problem to limit the production yield of silicon wafers. It is necessary to understand the fracture behavior of as swan silicon wafers for increasing the wafer yield. In this paper, a predictive model for wafer fracture strength is proposed according to the linear-elastic fracture mechanics. The simulation results are comparable with the experimental results from references, indicating the correctness of this proposed model. Besides that, influences of surface crack parameters on the fracture strength of silicon wafer are discussed. Results indicate that the fracture strength of silicon wafer changes less when the surface crack inclination angle between the crack plane and saw mark is between 0 and 25°. However, the fracture strength increases with the increase of the surface crack inclination angle when the angle is larger than 25°. This proposed model can predict the strength distribution of silicon wafers, and it is very helpful to understand the fracture behavior of silicon wafers.

強度ミスマッチ継手の脆性破壊強度に対するき裂先端の応力上昇および塑性ひずみ集中の影響

In order to investigate the influence of strength mismatching on brittle fracture strength for large heat-input welded joints, a series of tension tests was conducted using surface notched specimens. Furthermore, notched round-bar tension tests were carried out for simulated bond material (homogeneous material) which was base metal heat-treated on the same condition as the weld bond. Thereby, cleavage fracture strength of the bond material without the influence of strength mismatching was obtained. The point of fracture initiation was identified by SEM observation and, the critical maximum principal stress at the point (σ1c) was calculated by FE analysis for each specimen. σ1c values of surface notched specimens were found to be distributed in the range of 900-1,400N/mm². This result indicates that the strength mismatching does not significantly affect σ1c. In addition, distribution of σ1c for the notched round-bar specimens is in good agreement with that for surface notched specimens. This indicates that the cleavage fracture of the weld bond is a dominant fracture on the welded joints. Peak values of maximum principal stress σ1ᵖ and equivalent plastic strain epᵖ increase with increase in the strength mismatching ratio defined as the ratio of yield strength of weld metal to that of HAZ (σYWM/σYHAZ). The increase in σ1ᵖ causes the decrease in fracture strength of the weld joints. ep concentration at notch tip of HAZ side is due to the lower yield strength of HAZ. Except for small-scale and large-scale yielding conditions, the influence of ep distribution caused by strength mismatching on fracture strength also needs to be considered.

Bending fatigue characteristics of corroded wire ropes

The corrosion of a wire rope reduces its life expectancy. In this study, repeated bending tests were conducted using a bending fatigue tester by changing the tensile load and corrosion time of wire ropes, which were the same type as those used in elevators. The number of broken wires was studied, and a tensile test was conducted for cases in which the fracture was severe. The effect of corrosion fatigue on life expectancy was considered by comparing fracture strength values and observing fracture surfaces. The results indicate that an increase in accumulated corrosion fatigue, a greater tensile load, and repeated bending cycles may yield a rapid decrease in fracture strength and an increase in the number of broken wires. Therefore, it is concluded that corrosion fatigue is an important factor that decreases the life expectancy of wire rope.

Caspase-2 Maintains Bone Homeostasis by Inducing Apoptosis of Oxidatively-Damaged Osteoclasts

Osteoporosis is a silent disease, characterized by a porous bone micro-structure that enhances risk for fractures and associated disabilities. Senile, or age-related osteoporosis (SO), affects both men and women, resulting in increased morbidity and mortality. However, cellular and molecular mechanisms underlying senile osteoporosis are not fully known. Recent studies implicate the accumulation of reactive oxygen species (ROS) and increased oxidative stress as key factors in SO. Herein, we show that loss of caspase-2, a cysteine aspartate protease involved in oxidative stress-induced apoptosis, results in total body and femoral bone loss in aged mice (20% decrease in bone mineral density), and an increase in bone fragility (30% decrease in fracture strength). Importantly, we demonstrate that genetic ablation or selective inhibition of caspase-2 using zVDVAD-fmk results in increased numbers of bone-resorbing osteoclasts and enhanced tartrate-resistant acid phosphatase (TRAP) activity. Conversely, transfection of osteoclast precursors with wild type caspase-2 but not an enzymatic mutant, results in a decrease in TRAP activity. We demonstrate that caspase-2 expression is induced in osteoclasts treated with oxidants such as hydrogen peroxide and that loss of caspase-2 enhances resistance to oxidants, as measured by TRAP activity, and decreases oxidative stress-induced apoptosis of osteoclasts. Moreover, oxidative stress, quantified by assessment of the lipid peroxidation marker, 4-HNE, is increased in Casp2-/- bone, perhaps due to a decrease in antioxidant enzymes such as SOD2. Taken together, our data point to a critical and novel role for caspase-2 in maintaining bone homeostasis by modulating ROS levels and osteoclast apoptosis during conditions of enhanced oxidative stress that occur during aging.