Composites

Temperature dependent conductivity and structural properties of sol-gel prepared holmium doped Bi2O3 nanoceramic powder

by Prof.Dr. İbrahim USLU

İ. Taşçıoğlu, M. Arı, İ. Uslu, S. Koçyiğit, Y. Dağdemir, V. Çorumlu and Ş. Altındal
Ceramics International, CERI_5314
Accepted paper, Manuscript Number: CERI-D-12-00407R2

Holmium (Ho)-doped Bi2O3 nanoceramic powders derived from sol-gel method have been studied in terms of structural,... more Holmium (Ho)-doped Bi2O3 nanoceramic powders derived from sol-gel method have been studied in terms of structural, morphological, and electrical properties. The morphology of the nanoceramic materials was analyzed by scanning electron microscopy (SEM) and their structure by Xray powder diffraction (XRD). Temperature dependence of DC conductivity measurements of nanoceramic powders were carried out by using DC four-point probe technique (4PPT) in air at temperatures ranging from 429 °C to 896 °C (702-1169 K). Electrical conductivity results demonstrate that there is a sharp increase at around 700 °C, which indicates an existence of order-disorder transition. This result supported by the Differential Thermal Analyzer (DTA) curve and XRD pattern which show that the sample has stable high oxygen ionic conductivity fluorite type face centered cubic -phase. Electrical characteristics also show that the DC conductivity in the studied materials obeys Arrhenius relation with different activation energies and conduction mechanisms: two temperature regions with activation energies Ea1=1.40 eV (702-993 K) and Ea2=0.66 eV (1006-1169 K). The analysis of experimental data revealed that the translation motion of the charge carrier, oxygen vacancies, and space charge polarization are responsible for the change in activation energy as a function of temperature.

Design optimization of laminated composites using a new variant of simulated annealing

by Fazil Sonmez

Akbulut M. ve F.O. Sonmez, Computers and Structures, 2011, 89: 1712–1724

The aim of this study is to minimize the thickness (or weight) of laminated composite plates subject to both in-plane... more The aim of this study is to minimize the thickness (or weight) of laminated composite plates subject to both in-plane and out-of-plane loading. A new variant of the simulated annealing algorithm is proposed to optimize the lay-up design. Fiber orientation angle and number of plies in each lamina are used as design variables. Considering static failure as the critical failure mode, the maximum stress and Tsai–Wu criteria are used together to predict failure. Numerical results show that the optimization methodology proposed in this study can find the globally optimum laminate designs even with a high number of design variables.

Design of composite laminates for optimum frequency response

by Fazil Sonmez

Kayıkçı R. ve F.O. Sonmez, Journal of Sound and Vibration, 2012, 331(8): 1759–1776.

In this study, natural frequency response of symmetrically laminated composite plates was optimized. An analytical... more In this study, natural frequency response of symmetrically laminated composite plates was optimized. An analytical model accounting for bending-twisting effects was used to determine the laminate natural frequency. Two different problems, fundamental frequency maximization and frequency separation maximization, were considered. Fibre orientation angles were chosen as design variables. Because of the existence of numerous local optimums, a global search algorithm, a variant of simulated annealing, was utilized to find the optimal designs. Results were obtained for different plate aspect ratios. Effects of the number of design variables and the range of values they may take on the optimal frequency were investigated. Problems in which fibre angles showed uncertainty were considered. Optimal frequency response of laminates subjected to static loads was also investigated.

Impact damage in polyester-matrix glass fibre-reinforced composites. Part II. Residual load bearing abilities.

by Michał Barcikowski

Co-authored with Bartosz Semczyszyn

Kompozyty 11: 3 (2011) 235-239

In the course of their “life”, fibre-reinforced plastics (FRP) are subjected to impacts which can cause damage. This... more In the course of their “life”, fibre-reinforced plastics (FRP) are subjected to impacts which can cause damage. This damage may lead to a reduction of FRP strength and static load-bearing abilities. In this contribution, new results of three-point flexural tests on glass fibre/polyester composites after non-penetrating ballistic impact are presented. Composite materials were reinforced using a continuous filament mat and a woven roving, and the fibre content varied in the range of 42÷61% wt. The materials were produced using the Resin Transfer Moulding (RTM) method. The impactor was a free-flying 3 g steel ball, and the impact velocities approached 130 m/s. After the impact and evaluation of the extent of damage, the samples were subjected to three-point bending tests under fixed conditions. Reduction in the critical load value was noticed. A novel approach to the evaluation of residual strength has been presented. This approach allows estimation of the actual load-bearing ability of damaged material without removing the undamaged parts of the sample. The said approach involves testing samples including a damaged area as well as undamaged samples. It was considered what effect complete elimination of the damaged field would have on the load-bearing ability of the sample. The load transmitted through the undamaged area surrounding the area of delamination was then subtracted. This allowed for evaluation of the percentage of residual properties in the damaged area. It was found that reinforcement in the form of a continuous-filament mat compares favourably to loose woven roving. Higher-reinforced composites after the impact test seem to lose their properties to a higher extent.

Keywords: polymer composites, laminates, unsaturated polyester resin, glass fibre, ballistic impact, residual properties

Impact damage in polyester-matrix glass fibre-reinforced composites. Part I. Impact damage extent.

by Michał Barcikowski

Co-authored with Bartosz Semczyszyn

Kompozyty 11: 3 (2011) 230-234

Fibre-reinforced composites are susceptible to damage resulting from impacts. This damage may lead to a reduction of... more Fibre-reinforced composites are susceptible to damage resulting from impacts. This damage may lead to a reduction of composite strength and load-bearing abilities, both in static loading as well as during subsequent impact events. Composites with improved tolerance to ballistic impact using inexpensive, common materials i.e. E-glass fibre and unsaturated polyester resin, have been manufactured by means of modern, yet popular moulding technology. Composite materials were reinforced with an E-type glass fibre in the form of a continuous filament mat and woven roving. The fibre content in the composites was varied to evaluate the effect of the reinforcement fraction on impact tolerance. The composites were manufactured using the Resin Transfer Moulding (RTM) method. The extent of damage in glass fibre/polyester composites after non-penetrating ballistic impact has been evaluated. Samples of the manufactured laminates were subjected to impact using a compressed-air gun test assembly. The impactor was a free-flying 3-gram hardened steel sphere, and the impact velocities were up to 125 m/s. After the impact and damage evaluation, the samples were photographed in transmitted light, and the obtained images were digitally processed by software to measure the area of delamination. It was found that the damaged area is directly proportional to the impact energy. Moreover, reinforcement in the form of a continuous-filament mat compares favourably to loose woven roving; such reinforced composites have a much smaller area of delamination after impact of a given energy. The impacted samples were sectioned and imaged microscopically in low magnification. The damage in continuous-filament mat-reinforced composites is visibly less severe than in composites with fabric reinforcement.
Keywords: polymer composites, laminates, unsaturated polyester resin, glass fibre, ballistic impact, damage

Remaining strength in glass/polyester laminated composites subjected to ballistic impact

by Michał Barcikowski

Kompozyty 9: 3 (2009) 271-275

Fibre-reinforced plastics (FRP) are susceptible to damage resulting from impacts, even non-penetrating ones. This... more Fibre-reinforced plastics (FRP) are susceptible to damage resulting from impacts, even non-penetrating ones. This damage, even without outright destruction, may lead to reduction of fibre-reinforced plastics strength and static load-bearing abilities. This paper presents the results of three-point flexural tests on glass fibre/polyester composites after non-penetrating ballistic impact, where the word “ballistic” refers to a high-speed free-flying impactor (projectile). This is a continuation of previous studies, during which the extent of damage in glass/polyester composites after non-penetrating ballistic impact was evaluated. Materials used in the study were laminated composites produced via the Resin Transfer Moulding (RTM) technology. This laminates were produced using Polimal 1094 AWTP-1 unsaturated polyester resin and E-glass reinforcement in the form of multiple perpendicular layers of chopped-strand mats, continuous-filament mats, twisted-yarn fabrics and woven rovings. Composites with varying thickness, number of layers and fibre content were produced. The impactor is a free-flying 3 g steel ball propelled from a gas gun. Two striking velocities were tested - 60 and 70 m/s, producing impact energies of 5.4 and 7.35 J accordingly. After the impact, the extent of damage in samples (100x100 mm square plates) was evaluated through digital image analysis. The as-impacted samples were then subjected to three-point bending under set conditions. The results were compared to the results of identical examination of undamaged samples and the reduction in mechanical properties was determined. In all cases, the reduction in strength and load-at-break value was noticed. Thicker, more reinforced laminates show lower loss of mechanical properties, than thinner ones. Difference between strength and load-at-break approach for load-bearing abilities reduction was discussed, and the conclusion is that the latter is preferred to the former in laminated composites due to high thickness dependence of strength. Comparing reinforcement types, continuous-filament mat is superior to chopped-strand mat and woven-rovings. Tightly woven twisted-yarn fabric compares favourably to the woven rovings.

Wpływ modyfikacji żywicy na statyczne i udarowe właściwości kompozytu poliestrowo-szklanego

by Michał Barcikowski

Effect of resin modificatio on static and impact properties of glass-polyester composite

G. Wróbel (red.) Polimery i kompozyty konstrukcyjne. Monografia, Gliwice 2011, str. 15-23

Coraz szerzej stosowane kompozyty polimerowe narażone są na różnorakie obciążenia statyczne i dynamiczne. Obciążenia... more Coraz szerzej stosowane kompozyty polimerowe narażone są na różnorakie obciążenia statyczne i dynamiczne. Obciążenia udarowe powodują kruche pękanie żywic reaktywnych, takich jak np. poliestrowe. Aby poprawić odporność żywic poliestrowych na kruche pękanie można zastosować do nich dodatki modyfikujące, m.in. żywice elastyczne oraz kauczuki reaktywne. Praca przedstawia wyniki porównawczych badań wytrzymałościowych laminatów poliestrowo-szklanych wytworzonych z użyciem żywicy modyfikowanej żywicą elastyczną, kauczukiem ETBN i VTBN oraz żywicy niemodyfikowanej. Przeprowadzone badania wskazują, że już niewielki dodatek środka modyfikującego może znacząco poprawić odporność na kruche pękanie i udarność. Poprawa ta zachodzi bez istotnego pogorszenia pozostałych badanych parametrów wytrzymałościowych, zwłaszcza wytrzymałości i sztywności przy zginaniu.

Polymer composites, increasingly wider used, are subjected to various static and dynamic loads. Impact loading causes brittle fracture of reactive resins, such as unsaturated polyester (UP). In order to improve toughness of UP resins some addition of modifiers, such as elastic resin or reactive rubbers, may be used. The paper presents the results of comparative mechanical tests of glass/polyester laminates produced using UP resin modified with an addition of elastic resin and ETBN and VTBN rubbers, as well as resin unmodified. Conducted tests show, that even small amount of modifier may significantly improve toughness and impact strength. This improvement occurs without significant deterioration of other mechanical parameters,
especially flexural strength and stiffness.

Glass fibre/polyester composites under ballistic impact

by Michał Barcikowski

Kompozyty 8: 1 (2008) 70-76

This work presents results of ballistic impact tests on glass-fibre/polyester laminates. An effort has been made to... more This work presents results of ballistic impact tests on glass-fibre/polyester laminates. An effort has been made to manufacture composites with improved tolerance to ballistic impact using inexpensive, common materials like fibreglass and unsaturated polyester resin, by means of modern, yet popular moulding technology. Laminates were made using various E-type glass fibre reinforcements: chopped strand mat, continuous filament mat, cloth and two different woven rovings, with varying weight. Varying number of reinforcement layers were tested, as well as varying reinforcement-to-matrix ratio - from 20 to 60% vol. Samples of manufactured laminates were subjected to impact by a 3 g spherical hardened steel impactor moving at the velocity of 60 and 70 m/s (giving 5.4 and 7.35 Joules of kinetic energy, accordingly) using a gas gun test assembly. Samples were subsequently scanned using optical flat-bed scanner and the obtained images were digitally processed by software to measure the extent of delamination. Two ways of interpreting the extent of delamination in composites were tested - through the evaluation of delaminated volume and through the maximal delaminated surface area - to find usefulness of both methods. Impact energies were kept low such that none of manufactured laminates were perforated. The impact testing and image analysis of delaminated zone has shown similar range of damage in low-weight cloth and continuous filament mat, contrasted to inferior performance of chopped strand mat and high-weight, loose-structure woven rovings. Relationship between coherence of structure and delaminated area reduction has been shown. Unexpected three-dimensional shape of delaminated volume that had been found was discussed. Moreover, two methods of delaminated zone evaluation in composites subjected to impact - by means of delaminated volume and by means of delaminated area - has been discussed, suggesting superiority of the latter.
Keywords: composites, laminates, impact testing, RTM, ballistic performance
UDAR

The Effects of Dynamic Vulcanization and Compatibilizer on Properties of Paper Sludge-Filled Polypropylene/Ethylene Propylene Diene Terpolymer Composites

by Salmah Husseinsyah

H. Salmah, H. Ismail, A. Abu Bakar

The effects of dynamic vulcanization (DV)
and dynamic vulcanization plus compatibilizer (DVC) of papermore The effects of dynamic vulcanization (DV)
and dynamic vulcanization plus compatibilizer (DVC) of paper
sludge (PS) filled polypropylene/ethylene propylene
diene terpolymer (PP/EPDM) composites on torque development,
mechanical properties, water absorption, morphology,
and thermal properties were studied. Results show that DV
and DVC composites exhibit higher stabilization torque than
unvulcanized composites (UV). The dynamic vulcanized
(DV) and dynamic vulcanized plus compatibilizer (DVC)
composites exhibit higher tensile strength, elongation at
break, and Young’s modulus but lower water absorption
than unvulcanized composites. The scanning electron microscopy
(SEM) study of tensile fracture surface of DV and DVC
composites shows the improved interfacial interaction
between PS and PP/EPDM matrix. The DV and DVC composites
also exhibit better thermal stability and higher crystallinity
than unvulcanized PP/EPDM/PS composites.

Synthesis and Characterization of NaxCo2O4 Thermoelectric Material Using PVA Polymerized Complex

by Prof.Dr. İbrahim USLU

International Journal of Material Science and Electronics Research, Vol. 1, No. 2, July-Dec. 2010, pp. 55-59
M.O.Erdal, ibrahim uslu, A. Aytimur, M. Koyuncu

For the first time, sodium cobalt oxide (NaxCo2O4) crystalline thermoelectric materials were obtained using polyvinyl... more For the first time, sodium cobalt oxide (NaxCo2O4) crystalline thermoelectric materials were obtained using polyvinyl alcohol (PVA) polymerized complex sol-gel solution. PVA was used as polymer solution with sodium acetate/cobalt acetate, followed by calcination at 800oC. The sodium cobalt oxide calcined crystal structures were characterized by X-ray diffraction (XRD). The sol-gel solution calcined at 800oC and then sintered at the same temperature was single crystal and XRD results have an agreement with the literature values. The results indicated that the polymeric sol-gel method is a promising process for the synthesis of sodium cobalt oxide thermoelectric materials.

Multipoint moment functions of structural properties for polydisperse composites

by Mikhail Tashkinov

The ball mill as a means of investigating the mechanical failure of dental materials

by Noor Hayaty Abu Kasim

Objective: The main purpose of this paper is to present a new method of predicting clinical performance using... more Objective: The main purpose of this paper is to present a new method of predicting clinical performance using mechanical loading in a ball mill.

Methods: A series of four experiments (two involving a hybrid composite and one each on orthodontic brackets and bands) is described in which the ball mill was used to subject specimens to mechanical fatigue.

Results: A reproducibility study using composite beam specimens showed no significant difference between the Mean Survival Time (MST) in all the three experimental runs (P = 0.42). When subjected to thermal cycling, the MST of the cycled group was 155.0 min compared to 247.0 min fur the control group (P < 0.01). The MST of untreated and sandblasted brackets was 7.9 h and 14 h respectively (P < 0.01). There is also a significant difference (P < 0.001) in the MST of sandblasted bands when compared to the untreated bands.

Conclusions: The ball mill proved to be a convenient and reproducible means of producing mechanical fatigue and may be useful in predicting the clinical performance of dental materials.

Demineralization Potential of Qat Extracts at Composite Restoration Interface

by Noor Hayaty Abu Kasim

Objective: To evaluate the demineralization potential of qat extracts at composite restoration interface. Methods:... more Objective: To evaluate the demineralization potential of qat extracts at composite restoration interface. Methods: Class V cavities were prepared on the buccal surface of thirty extracted sound premolars and were restored with a nanohybrid composite (Grandio, VOCO, Germany). Specimens were subsequently coated with nail varnish exposing 2mm of enamel around the restoration margin. The specimens were divided into three equal groups of 10 and were then immersed in acid gel and qat extracts (10% and 20%) for 4 weeks. All specimens were removed and washed using deionized water and later examined by direct vision and stereomicroscope prior to sectioning. Specimens were then immersed in distilled water for 24 hours, following which the sections were examined under polarized light microscope and demineralized area was measured as outer and wall lesion in the coronal and cervical part of restoration using image analyzer software (Image-Pro Version 4.5). Data were subjected to One-way MANOVA. Results: All specimens immersed in acid gel and qat extracts (10% and 20%) exhibit demineralization at the restoration interface. The One-way MANOVA also indicated significant differences between the three groups on the different restoration interface (P<.05) and Tukey multiple comparison test was also performed as illustrated in the table below. Conclusion: 10% and 20% qat caused significantly lower demineralization at restoration interface compared to acid gel. This study was supported by a research grant from University of Malaya, P0218_2007a.

Effect of thermocycling on the fracture toughness of composites

by Noor Hayaty Abu Kasim

Fracture toughness has been recognised to be one of the most important mechanical property for retorative materials.... more Fracture toughness has been recognised to be one of the most important mechanical property for retorative materials. It is the aim of this study to investigate the effect of thermocycling an fracture toughness of a range of dental composites; P50(P)1 , Silux Plus(S)2, Heliomolar(H)3, Clearfil Photo Posterior-light activated(CLA)4 and Clearfil Posterior-chemically activated(CC)3. 110 rectangular pre-notched specimens was prepared for each material utilising a stainless steel mould. The specimens were divided into 11 groups of 10 each. Test groups 1-6 were subjected to 0, 250, 500, 750, 1000 and 10,000 thermal cycles respectively. All specimens were subjected to a 3 point bend test and loaded at a crosshead speed of 1mm/min-1 on an Instron Testing Machine. The fracture toughess of all materials decreases with the number of thermal cycles and length of water storage except for CC where the fracture toughness increases during the earlier stages of thermocycling and water storage. However the lowest fracture toughness values were for groups 6 and 11. One way analysis of variance showed that Group 1 was significantly different from groups 6 and 11 (P<.05), the thermally cyled and the water storage groups were not different from each other for al materials. The decrease in fracture toughness exhibited by all light activated composites could be explained by interfacial stress formation and resin-filler debonding while the increase in fracture toughness during the earlier stages of thermocycling and water storage of chemically-activated composite be explained by continued polymerisation. Thermocycling did not cause any significant in fracture toughness composites over and above by water storage.

Performance of melamine modified urea–formaldehyde microcapsules in a dental host material

by Noor Hayaty Abu Kasim

Urea-formaldehyde (UF) microcapsules filled with dicyclopentadiene (DCPD) show potential for making self-healing... more Urea-formaldehyde (UF) microcapsules filled with dicyclopentadiene (DCPD) show potential for making self-healing dental restorative materials. To enhance the physical properties of the capsules, the urea was partially replaced with 0-5% melamine. The microcapsules were analyzed by different microscopic techniques. DSC was used to examine the capsule shell, and the core content was confirmed by (1)H NMR spectroscopy. Capsules in the range of 50-300 mu m were then embedded in a dental composite matrix consisting of bisphenol-A-glycidyl dimethacrylate (Bis-GMA) and triethylene-glycol dimethacrylate (TEGDMA). Flexural strength, microhardness, and nanoindentation hardness measurements were performed on the light-cured specimens. Optical microscopy (OM) examination showed a random distribution of the microspheres throughout the host material. The incorporation of small amounts of the microcapsules did not affect the performance of the matrix material. Scanning electron microscopy (SEM) analysis revealed excellent bonding of the microcapsules to the host material which is a characteristic of utter importance for maintaining the very good mechanical properties of a dental composite with self-healing ability.

Optimization of Microencapsulation Process for Self-Healing Polymeric Material

by Noor Hayaty Abu Kasim

A series of poly(urea-formaldehyde) (PUF) microcapsules filled with dicyclopentadiene (DCPD) was successfully prepared... more A series of poly(urea-formaldehyde) (PUF) microcapsules filled with dicyclopentadiene (DCPD) was successfully prepared by in situ polymerization. The effect of diverse process parameters and ingredients on the morphology of the microcapsules was observed by SEM, optical microscopy (OM) and digital microscopy. Different techniques for the characterization of the chemical structure and the core content were considered such as FT-IR and (1)H-NMR as well as the characterization of thermal properties by DSC. High yields of free flowing powder of spherical microcapsules were produced. The synthesized microcapsules can be incorporated into another polymeric host material. In the event the host material cracks due to excessive stress or strong impact, the microcapsules would rupture to release the DCPD, which could polymerize to repair the crack.

Vibration frequency of graphene based composites: A multiscale approach

by Rajib Chowdhury

Materials Science and Engineering: B
Volume 177, Issue 3, 25 February 2012, Pages 303–310

This paper presents a multiscale approach for vibration frequency analysis of graphene/polymer composites. The... more This paper presents a multiscale approach for vibration frequency analysis of graphene/polymer composites. The graphene is modelled at the atomistic scale, and the matrix deformation is analysed by the continuum finite element method. Inter-connectivity between graphene and polymer matrix are assumed to be bonded by van der Waals interactions at the interface. The impact of geometrical configuration (armchair and zigzag), boundary conditions and length on the overall stiffness of the graphene reinforced plastics (GRP) is studied. The natural frequency and vibrational mode shapes of GRP studied have displayed dependence on the length and also the boundary conditions. The exceptional vibrational behaviour and large stiffness displayed by GRP makes them a potential replacement for conventional composite fibres such as carbon and glass fibres.

COMPOSICION Y CLASIFICACION DE LOS COMPOSITES DENTALES RESTAURATIVOS.

by Mario Nodarse

Silica Nanotube Multi-Terminal Junctions As a Coating for Carbon Nanotube Junctions

by Vasilii Artyukhov

Physical Review B, Vol. 74, No. 4. (6 July 2006), 045402. Key: 10.1103/PhysRevB.74.045402

The possibility of coating carbon nanotube junctions with single-walled, square-lattice silica nanotubes is... more The possibility of coating carbon nanotube junctions with single-walled, square-lattice silica nanotubes is considered. We describe the topology of junctions of such tubes, based on the generalized Euler formula for a tetragonal lattice with trigonal, pentagonal, and hexagonal topological defects. The types of silica nanotubes best fitting the considered carbon nanotube types are determined using a classical interatomic force field. The influence of the silica coating on the electronic properties of the carbon nanotube is evaluated using semiempirical Paramagnetic Model 3 (PM3) method calculations. It is found that the electronic properties of the carbon nanotube near the Fermi level are not changed considerably. Finally, we propose some examples of SiO2-C composite T-, Y-, and X-junction structures.

Time-variant Reliability Analysis and Flexural Design of GFRP-reinforced Bridge Decks

by Young Hoon Kim, PhD, PE

Kim, Y. H., Trejo, D., and Gardoni, P., In Press 2011, ASCE Journal of Composites for Construction

Glass-fiber reinforced polymer (GFRP) reinforcement is being used in bridge decks as a replacement for steel... more Glass-fiber reinforced polymer (GFRP) reinforcement is being used in bridge decks as a replacement for steel reinforcement. It is thought that since the GFRP reinforcement does not corrode, it can be a more sustainable material for reinforced concrete
structures. However, it is widely reported that GFRP bars do deteriorate when embedded in concrete or when immersed in concrete pore solutions. The American Concrete Institute (ACI) and the American Association of State Highway Transportation
Officials (AASHTO) use environmental exposure factors to reduce the design strength due to this loss of capacity. However, these exposure factors have not been calibrated.
In addition, limited research has been performed to quantify the time-variant flexural moment capacity of GFRP reinforced flexural members. Recently, a Bayesian approach was used to develop a time-variant probabilistic capacity model based on
capacity data of GFRP reinforcement embedded in concrete for a period of up to 7 years. This model is used herein to assess the time-variant flexural moment capacity and the time-variant structural reliability of a bridge deck considering different types
and sizes of GFRP bars. Even though GFRP bridge decks are designed to be over reinforced and the designs are governed by the serviceability limit, the analysis results indicate that the probability of failure of the decks containing both 13M (#4) and 19M (#6) GFRP bars at a reference temperature T23 of 23 oC (73 oF) is higher than the failure probability generally accepted in the AASHTO Load and Resistance Factor Rating (LRFR) Specifications. A lower exposure temperature reduces the probability of failure over the 75 year period. The probability of failure is less than values accepted by AASHTO.