Milled Carbon Fibers Research Articles

Scratch load and indenter type dependent scratch evaluation of milled carbon fiber filled basalt fiber/epoxy composites with factorial design

AbstractThis work investigates the addition of milled carbon fiber reinforcement and its effect on scratch resistance and mechanical durability for basalt fiber/epoxy composites, assessed under varied loading conditions with the aid of different indenter types according to a factorial design. Composites with MCF content from 0 to 10 wt% were prepared. Scratch behaviors under Vickers and Rockwell indenters were studied for loads of 5, 10, and 15 N. The addition of 10 wt% MCF has resulted in scratch hardness increase from 13.255 N/mm2 up to 47.952 N/mm2, depending on the type of indenter used. Precisely, in conditions of a 5 N load and when a Rockwell‐type indenter was used, a scratch hardness maximum of 47.952 N/mm2 was achieved, while for a Vickers‐type indenter under the same conditions, the maximum value of its equivalent was 23.327 N/mm2. Profilometric and SEM analyses evidence that matrix cracks and surface deformation have been reduced with higher MCF content, at least for the application of lower scratch test loads. The mechanical and surface performances of the basalt fiber composites were found to be considerably improved by the reinforcement with MCF.Highlights Scratch resistance for milled carbon fibers increases and peaks at 10 wt% reinforcement. The Rockwell indenter indicates a steady increase in hardness up to 47,952 N/mm2. Peak hardness of 10 wt% is observed under the Vickers indenter for the 5 N load. Higher milled carbon fiber content reduces matrix cracking and surface deformation. There is a clear correlation between scratch performance, milled carbon fiber content, and indenter type.

Thermal and mechanical behaviour of recycled short milled carbon fibre reinforced polypropylene and recycled polypropylene composites: A comparative study

Environmental and economic factors have driven research into the recycling and applications of recycled carbon fibres (rCF). This paper presents a comparative study characterizing and comparing the mechanical and thermal performance of recycled short milled carbon fibre (rSMCF) on virgin and recycled polypropylene composites. The effects of rSMCF on VPP and RPP on mechanical performance were analysed and compared. At 5 wt% rSMCF, recycled polypropylene achieved 52.3% and 47.3% improvement on tensile and flexural modulus, while at the same rSMCF loading, virgin polypropylene only improved 37.7% and 17.5%, respectively. The un-notched impact strength of RPP-based composites reduced from 83.2 kJ/m2 to 60.1 kJ/m2 when rSMCF content increased from 1 wt% to 5 wt%, indicating future work should enhance the fibre/polymer interface performance. Crystal contents (χ0) of the PP/rSMCF composites were investigated by differential scanning calorimetry (DSC), and the results were analysed and mapped to the mechanical performance. The results of this study propose a novel scalable method for the production of high-performance VPP/RPP composite materials using rSMCF.

Comparative evaluation of solid particle erosion resistance in milled carbon fiber‐reinforced basalt fiber epoxy composites: Impact of abrasive shape variations

AbstractThe solid particle erosive wear performance of basalt fiber‐reinforced epoxy composites with the incorporation of different percentages of milled carbon fiber fillers (0–10 wt%) is evaluated under various abrasive materials (garnet and ceramic beads) and impingement angles (30° and 90°). Erosion tests have been carried out following ASTM G76 standards using a factorial design method. The data indicate that the erosion rates, crater depths, and areas of craters are all strongly dependent on erodent material type and milled carbon fiber filler ratio. Compared to ceramic beads, garnet abrasives caused severe erosion, with erosion rates reaching approximately 500 mg/g at a 90° impingement angle without milled carbon fiber fillers. Erosion by both abrasives was significantly reduced by the introduction of milled carbon fiber fillers into the basalt fiber composites, with the erosion rate decreasing to around 100 mg/g with 3 wt% milled carbon fiber fillers. The maximum crater depth was also influenced; garnet abrasives produced craters as deep as 1000 μm without fillers at 90°, which was reduced to 451 μm with 3 wt% milled carbon fiber fillers when ceramic beads were used. Of the three factors for the erosion rate and crater depth, inorganic erodent material and ratio of carbon fiber filler were prominent, but the impingement angle effect was much smaller. Among the three factors for the erosion rate and crater depth, erodent material and carbon fiber filler ratio were prominent, while the impingement angle effect was significantly less.Highlights The garnet abrasives cause more severe erosion than ceramic beads at 90°. Composites exhibit optimal erosion resistance with 3–5 wt% milled carbon fiber fillers. Filler ratio and erodent material type significantly impact erosion rates and depths. Factors affecting wear performance identified using factorial design and ANOVA.

High‐performance PPS/PEEK blend and its composites with milled carbon fiber: Study on their mechanical, thermal and dielectric properties

AbstractThe high‐performance PPS/PEEK blends/composites were prepared with different mass ratios using polyphenylene sulphide (Solvey, Ryton®‐R‐4‐230 NA) and polyetherether ketone (Victrex, PEEK450 GL 30). All the blend samples had a single Tg, which revealed good compatibility between PPS and PEEK. The Tm was increased from 279.6 to 339.9°C due to addition of PEEK into PPS. Also, addition of PEEK into the blend increased the impact and tensile strength of the PPS material. The impact strength of the blend, PPS/PEEK (20/80) exhibited the maximum value of 113.64 J/m which was higher than that of PPS (70.66 J/m). The tensile strength of the blend was increased continuously with increasing the content of PEEK from 20 to 80 wt. % and reached the maximum value of 160.64 MPa. Additionally, milled carbon fiber (MCF) was also incorporated in order to obtain high strength PPS/PEEK blends/composites with good dielectric property. The dielectric performance of the composites was observed to increase with MCF content (in the frequency range 5.8 to 18 GHz).Highlights This investigation relates to high‐performance PPS/PEEK blends and composites. Incorporation of milled carbon fiber resulted promising dielectric properties. Such composites may have Advanced structural EMI shielding electronics and/or aerospace applications.

Influence of Milled carbon fiber MCF on mechanical and dynamic mechanical behaviour of carbon fiber fabric reinforced polymer composites

Carbon fiber reinforced polymer (CFRP) composites are widely used in engineering applications. Better understanding about the thermo mechanical properties of CFRP composites are essential for their development. Static and dynamic mechanical properties of CFRP composites can be modified by the filler materials loaded on it. This study investigates the effect of Milled Carbon Fiber (MCF) filler reinforcement on flexural and thermo-mechanical properties of CFRP composites. Fabricated composites are quasi isotropic laminates with unidirectional carbon fibers. The composite laminates were fabricated by hand lay-up technique. XRD analysis illustrates that all the processed specimens are in non-crystalline nature. Three-point bending mode is followed to evaluate the flexural and Dynamic Mechanical Analysis (DMA) tests. MCF fillers are loaded at 6 wt%, 8 wt%, 10 wt%, 12 wt% to evaluate above mentioned properties, i.e, Flexural Strength and Break strain were found from flexural analysis. Storage Modulus (E'), Loss Modulus(E’'), under varying temperature and glass transition temperature (Tg) were determined from DMA. The test results indicated that, 8 wt% of MCF filler loading improves the static and dynamic mechanical properties of CFRP composites when compared to 10 wt% and 12 wt% of MCF content.

High-conductivity cementitious composites suited for flooring applications: Effect of mixing methods and utilization rates of conductive materials

The objective of this study is to develop cementitious electrostatic discharge composites suited for flooring applications with the lowest amount of conductive material using the most appropriate mixing method. For electrical conductivity, milled carbon fiber (MCF), carbon fiber (CF), and brass-coated steel fiber (SF) were incorporated into the cementitious matrices with ratios increasing from 0 to 1% at 0.1% intervals, by the total weight of the binder. To address the homogenous distribution of conductive materials, first (F), synchronous (S), and latter (L) admixing methods were used. Reaching high early strength was another aim of the study. Flow diameter, electrical resistance (ER), compressive strength measurements, and SEM/EDX analyses were performed. According to the experimental results, the ER has a decreasing trend with the increasing fiber ratio in the matrices. CF- and MCF-based mixtures are of adequate electrical performance (10–1000 kΩ) according to related standards after 180-day measurements. No clear percolation thresholds are available for SF- and MCF-based composites up to a 1% utilization rate. Adequate electrical performance can be obtained from CF-based composites at the percolation threshold of 0.6% when prepared by the synchronous admixing method and 0.3% by the latter admixing method. Considering the CF-based mixture incorporating 0.3% CF and prepared by the latter admixing method, the flow diameter is 39.75 cm, 1-day compressive strength is 38.1 MPa, and 180-day ER is 104.15 kΩ in the long term. In conclusion, it is possible to manufacture cementitious composites with significantly improved electrical performance suitable for flooring applications with low cost, high early strength, and good fluidity ready for on-site use.

A data-driven model on the thermal transfer mechanism of composite phase change materials

Phase change materials (PCMs) that are incorporated with highly conductive nanomaterials to fabricate composite phase change materials (CPCMs), received much focus as a promising energy strategy for latent heat storage and conversion systems, due to their excellent thermophysical properties such as oxidation resistance and large enthalpies of fusion. However, the correct prediction of the thermal conductivity of these CPCMs remains deficient, mainly due to the lack of knowledge on the microscopic heat transfer mechanisms between the nanofiller and matrix interphase. Herein, a data-driven, modified Maxwell model is proposed to determine the thermal conductivity of these CPCMs, using milled carbon fiber (MCF)-reinforced PCMs as validation. This new model incorporates the aspect ratio and morphology smoothness of MCFs and introduces compatibility factors for different types of PCM matrices, which are paraffin and polyethylene glycol (PEG) respectively. At filler loadings above 15 wt%, the theoretical model gave poorer forecasts (with an average prediction error of 0.075) due to the random agglomeration of MCF nanoparticles, which can obstruct the phonon pathway. Regardless, this model accurately estimated the thermal conductivities of MCF/PCMs up to 9 wt% and 11 wt% MCF loading, with percentage fit values being 0.983 and 0.996 for PEG and paraffin systems, respectively. This model also eliminates the limitations of existing models, that were only suitable for composites with low filler loadings (<5 wt%). Hence, this work provides a vital prediction guide for the thermal conductivity of commercial CPCMs.

Phenolic carbon fiber composite inks for the additive manufacturing of carbon/carbon (C/C)

Additive manufacturing (AM) is a crucial development area for high temperature, inorganic and ceramic materials which, using conventional methods, are difficult to process into complex shapes. In particular, carbon/carbon (C/C) composites produced using AM techniques are underexplored compared to other ceramics and ceramic matrix composites. This work investigated and optimized the development of phenolic resin/carbon fiber inks for the material extrusion technique of direct ink writing (DIW) to form C/C composites. Utilizing recent advances in the material extrusion of ceramics, namely the DIW preceramic polymers and preceramic polymer-based suspensions or slurries, we were able to create C/C composites. AM processes can be used to obtain complex geometries and material extrusion processes also facilitate the alignment of high aspect ratio fillers, like carbon fiber, which affects material properties like strength or stiffness. Formulation of material extrusion inks from phenolic resole resin, pitch-based milled carbon fiber as a reinforcement, and a low-density carbon black filler is reported herein. The effects of carbon fiber content and filler, which was used to obtain a printable rheology and appreciable yield stress are discussed in the context of printability.

Investigation of Performance Properties of Milled Carbon Fiber Reinforced Hot Mix Asphalt

In this study, the mechanical behavior and resistance to moisture damage of hot mix asphalt (HMA) concrete with the addition of Milled Carbon Fiber (MCF) were experimentally investigated. For this purpose, the gradation curve within the boundaries of the Turkish highway construction specifications (HTS) has been determined. By keeping the determined gradation constant, MCF was added at different rates (1 %, 1.5 %, 2 %, 2.5 %, 3 %) by weight of the mixture. In the study, first, optimum bitumen ratios (OBR) of pure control samples (0 %-Control) without MCF and mixtures with MCF additives were determined by using the Marshall design method. To determine the OBR, samples were prepared with bitumen content of 3.5 %, 4 %, 4.5 %, 5 %, 5.5 %, and 6 % at each carbon additive ratio. The mixture samples prepared using the specified OBRs were subjected to Marshall stability (MS) and flow, as well as to retained Marshall stability (RMS), indirect tensile strength (ITS), and moisture damage resistance tests. According to the test results, it was observed that the MS values of the asphalt concrete with MCF additives increased at certain carbon additive ratios, while the flow values decreased compared to the witness sample. It was determined that the RMS and indirect tensile strength ratio (TSR) values of hot mixes with MCF-added bitumen increased and the moisture damage resistance of the mixes increased. As a result, when the optimum MCF ratio determined for the wearing course is used, it is thought that the engineering properties of HMA will improve.

Research on EMP Shielding Concrete Product Development and Improvement

In this study, the research was conducted for the purpose of developing Electro Magnetic Pulse (EMP) shield concrete using electric furnace oxidizing slag and carbon material and improving the quality of EMP shield concrete mixed with fly ash(FA) and blast furnace slag(BS). As a result of the study, it was confirmed that compressive strength and Shielding Effectiveness (SE) increased when electric furnace oxidized slag(EOS) was used as aggregate, and that SE increased when Carbon Fiber(CF) was used rather than Milled Carbon Fiber (MCF) in the form of carbon materials. In the case of CF, when the mixing ratio increased, the SE increased, but the fluidity significantly decreased, so the Electric Furnace Oxidized Slag CF 0.2 was suitable as an EMP shielding concrete. Fluidity and SE were increased when FA and BS were incorporated, and EMP BS20 was confirmed to be effective in improving quality.

Free-Standing 3D Printing of Epoxy-Vinyl Ether Structures Using Radical-Induced Cationic Frontal Polymerization.

The application of frontal polymerization to additive manufacturing has advantages in energy consumption and speed of printing. Additionally, with frontal polymerization, it is possible to print free-standing structures that require no supports. A resin was developed using a mixture of epoxies and vinyl ether with an iodonium salt and peroxide initiating system that frontally polymerizes through radical-induced cationic frontal polymerization. The formulation, which was optimized for reactivity, physical properties, and rheology, allowed the printing of free-standing structures. Increasing ratios of vinyl ether and reactive cycloaliphatic epoxide were found to increase the front velocity. Addition of carbon nanofibers increased the front velocity more than the addition of milled carbon fibers. The resin filled with carbon nanofibers and fumed silica exhibited shear-thinning behavior and was suitable for extrusion-based printing at a weight fraction of 4 wt %. A desktop 3D printer was modified to control resin extrusion and deposition with a digital syringe dispenser. Flexural properties of molded and 3D-printed specimens showed that specimens printed in the transverse direction exhibited the lowest strength, likely due to the presence of voids, adhesion issues between filaments, and preferential carbon nanofiber alignment along the filaments. Finally, free-standing printing of single, angled filaments and helical geometries was successfully demonstrated by coordinating ultraviolet-based reaction initiation, low air pressure for resin extrusion, and printing speed to match front velocity.

Enhancing the thermal performance of polyethylene glycol phase change material with carbon-based fillers

New implementation of phase change materials (PCMs), such as Polyethylene Glycols (PEGs), can alleviate the overconsumption of natural resources and can also be applied in solar-thermal energy conversion applications due to their distinctive semi-crystalline structure. To further enhance the thermal properties of PCMs, carbon-based nanomaterials such as graphene nanoplatelets (GNPs) and milled carbon fibre (MCF) can be incorporated to fabricate PEG-based composites due to their high thermal conductivity. One of the possible drawbacks of the addition of carbon-based fillers is their aggregation at high loadings, leading to a possible reduction in the latent heat of PCM composites. Hence, this study explored the most appropriate loading level (5 wt.%) of carbon-based fillers with pristine PEGs to achieve optimal thermal performance. Specifically, PEG-based composites were synthesised with three fillers (GNPs, graphite and MCF) via a temperature-assisted ultrasonication method and rapid solidification. Then, Raman Spectroscopy was used to quantify the dispersion of fillers among the PEG matrices. The results showed that despite a 32.6 % reduction in enthalpy of fusion to 155.5 J g–1, the addition of 15 wt.% of MCF increased the thermal conductivity up to 0.67 W m–1 K–1, which was approximately 2.5 times higher than that of pristine PEG. Moreover, explanations were provided on the possible heat transfer mechanisms in different types of carbon-based fillers. PEG/MCF PCMs displayed good properties on latent heat, thermal conductivity, aggregation prohibition, volumetric heat capacity, phase change temperature and thermal stability, while PEG/graphite composites exhibited excellent performance on thermal diffusivity, phase change (melting) temperature, specific heat capacity and cyclability.

Solvent-free surface modification of milled carbon fiber using resonant acoustic mixing

Resonant Acoustic Mixing (RAM) is used to rapidly modify the surface of milled carbon fiber using diazonium salts in solvent free conditions. This novel method allows tuning of the surface properties of this material and reduces the environmental footprint usually associated with surface modification of carbon fiber (discontinuous or otherwise). As a proof of concept, fluorine-containing diazonium salts were successfully grafted as determined by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and an increase in water contact angle (WCA) of the milled carbon fiber samples (+15°). Atomic Force Microscopy (AFM) together with SEM revealed the surface structure and integrity of the milled carbon fibers could be maintained despite vigorous mixing conditions. Using RAM proved more efficient than positive controls produced under thermal conditions in solvent.

Prototype Design for Grading Structures in Powder Bed Fusion Processes.

While targeted alignment in certain additive manufacturing (AM) methods such as material extrusion (MEX) and stereolithography (SLA) has been well documented in the research community, a method for targeted alignment of added fillers or fibrous materials in powder bed fusion (PBF) AM devices has yet to be successfully achieved. Similarly, incorporation of multimaterials does not work easily with any of the AM technologies. This study creates a prototype design that could be integrated into a PBF system to allow for multimaterial layer deposition and alignment of powders and powder blends. The rheological properties of polyamide powder and a range of polyamide composite blends (incorporating milled carbon fibre, graphite flakes, polytetrafluoroethylene, and glass microspheres) in different concentrations were studied, and together with the particle size distribution and particle morphology analysis were applied for the design of a prototype hopper for incorporation in the PBF system to create targeted multimaterial deposition. Different concept designs, multichambered and multi-hopper with hopper angles calculated specifically for the composite blend powders selected, were proposed. Initial deposition trials outside a PBF process were tested, and the deposited layers were measured.

Mechanical and Electrical Properties of Milled Carbon Fiber Reinforced Artificial Graphite Scrap Composites with Phenol Resin as the Binder

Mechanical and Electrical Properties of Milled Carbon Fiber Reinforced Artificial Graphite Scrap Composites with Phenol Resin as the Binder

Self-sensing Sustainable Cementitious Mixtures Incorporating Carbon Fibres

This paper focuses on sustainable cementitious composites in terms of their conductivity, hydration and self-sensing properties, which are key features for smart city infrastructures. Smart cities have massive infrastructures that are interconnected, transmitting data and information for health-monitoring and performance optimization. In this regard, having them made of sustainable building materials (concrete) that are also sufficiently conductive, will be a suitable solution for structures’ performance. The studied sustainable cementitious mixtures are made by sea components (sea water and sea sand), which are abundant resources. The primary goal of this study is to improve the mixes’ electrical conductivity and sensitivity. To achieve this goal, milled carbon fibres (MCFs) and chopped carbon fibres (CCFs) in 6 different proportions were added to the cementitious mixes. The experimental study is divided into impedance spectroscopy to study the conductivity and hydration development, and self-sensing properties, conducted on various mix designs. The results show that incorporating sea components improve the electrical conductivity of the mixes by 40-50%. Further improvements were achieved by adding MCF as it shows a remarkable reduction by 60% compared to the plain ss-sw (sea sand and sea water) samples. Adding CCF improved the conductivity even further and resulted in sample’s resistivity as low as 53 Ωcm after 1 year of curing time.

A high value application of reclaimed carbon fibers: Environmental remediation and redeployment in structural composites

This paper describes the use of reclaimed milled carbon fibers in a non-traditional application of pollutant removal and redeployment in a composite material. Commercially available milled reclaimed carbon fibers were able to remove 78% of perfluorinated and polyfluorinated alkyl substance pollutants from water. Modification of the reclaimed milled carbon fiber surface to model fibers ‘saturated’ with pollutant was undertaken and a comparison made between the modified sample and control materials. Aging these samples in water at 35 °C for 2 months, and periodically determining weight gain, flexural strength, and flexural modulus showed no significant difference between control fibers and those possessing a fluorinated surface. More aggressive aging, by boiling these samples in water for 48 h, again shows no meaningful difference between fiber types. Moreover, analysis of the water used for aggressive aging of samples by 19F NMR shows that no leaching of the fluorinated species occurs.

Experimental and Numerical Analysis of Chlorinated Polyethylene Honeycomb Mechanical Performance as Opposed to an Aluminum Alloy Design.

Manufacturing aircraft components through 3D printing has become a widespread concept with proven applicability for serial production of certain structural parts. The main objective of the research study is to determine whether a chlorinated polyethylene material reinforced with milled carbon fibers has the potential of replacing the current 5052 NIDA aluminum alloy core of the IAR330 helicopter tail rotor blade, under the form of a honeycomb structure with hexagonal cells. Achieving this purpose implied determining the tensile and compression mechanical properties of the material realized by fused deposition modeling. The tensile tests have been conducted on specimens manufactured on three printing directions, so that the orthotropic nature of the material may be taken into account. The bare compression tests were realized on specimens manufactured from both materials, with similar honeycomb characteristics. All the mechanical tests have been performed on the Instron 8872 servo hydraulic testing system and the results have been evaluated with the Dantec Q400 Digital Image Correlation system. The experimental tests have been reproduced as finite element analyses which have been validated by results comparison, in order to determine if the compression model is viable for more complex numerical analysis.

Morphology evolution of self‐same nanocomposites hybridized with jumbo‐sized particles

AbstractThis article reports the production, morphological analyzes, and application of electrospun self‐same nanocomposites with milled carbon fibers (MCFs). The new hybridized structure was also incorporated into conventional fiber reinforced epoxy composites with improved properties. The MCF‐hybridized polymeric nonwoven mats were formed with the simultaneous dual electrospinning of a soften‐able (m‐phase) and a crosslink‐able (x‐phase) variants of poly(styrene‐co‐glycidyl methacrylate). The morphology of the hybrid material was investigated using scanning electron microscopy (SEM). The results showed that electrospinning can successfully deposit reinforcing particles of giant size (MCFs are 7 μm in diameter, 50 μm to 3 mm in length) compared to the diameter of the carrier nanofibers (nanometers). The new hybrid structure preserved the fibrous morphology of the polymer phases up to 250°C. The overall morphology of the hybrid composite was tunable by changing the fractions of the two polymeric phases. The particle‐polymer hybrid structures created morphologies that might find applications in various areas such as the interlayer toughening of laminated composites. It was shown that m‐phase/MCF@x‐phase nonwoven integrated into epoxy matrix composite laminates as interlayer, increased the strain at failure and ultimate strength under tensile loading by 11% and 9%, respectively.

Role of coating add -on and substrate material on microwave and EMI shielding properties of coated fabric

Role of coating add-on and nature of textile substrate have been studied on microwave and EMI shielding properties of coated cotton fabric. Coated fabric samples have been prepared with coating formulations containing carbon black and milled carbon fibre in polyurethane matrix, using cotton (non-conducting) and conducting fabric (warp: SS20%/PET80%, weft: SS 55% / PET 45% ) substrate simultaneously. A high precision lab coating machine (Mathis Lab Coater, UK) has been used to produce coated fabric of uniform thickness (0.31- 0.67 mm). Coated fabrics are studied for microwave and EMI shielding properties in 8.2-18.0 GHz (X & Ku) frequency band in vertical and horizontal polarisation of electromagnetic wave. The microwave properties of coated fabric based on cotton substrate are found due to coating add-on only, as the cotton itself does not have any role to play. Cotton - based coated fabric with add-on of 257.7% exhibits 32-43% reflection, 22-39% transmission, 37-42% absorption and EMI shielding of 5.23-7.93 dB in 8.2- 18.0 GHz. On the other hand, microwave properties of coated fabric prepared on conducting substrates are found to produce synergetic effect of substrate material and coating add-on. Coated fabric on conducting substrate with add-on of 185.5% displays 92-82% reflection, 0.17-0.36% transmission, 7.20 - 26.310% absorption and EMI shielding of 23.98- 21.64 dB. Apart from this, coated fabrics are also prepared with gradual increase of coating add-on in order to understand the effect of ultimate coating add-on loading and to obtain optimum threshold combined effect of coating add-on and substrate materials. Sample with coating add-on of 237.9% acquires high surface conductivity (σ = 46.34 S/m) and low surface resistivity (34.25 Ω/□). Coated fabric offers 93.47-79.47% reflection, 6.39-22.07% absorption, <0.2% transmission, and EMI shielding of 28.86- 26.68 dB.