Static Flexural Tests Research Articles

Study on the Mechanical Performance of RC Beams under Load Reinforced by a Thin Layer of Reactive Powder Concrete on Four Sides

To repair reinforced concrete beams efficiently in a limited building space, the four-sided application of a reinforcing thin layer of reactive powder concrete (“RPCTL”) was proposed to improve the bending capacity of the members. Static flexural tests of one comparison beam and five reinforced beams were completed on a four-point centralized loading device. Changes in deflection, cracks, stresses, and damage characteristics of the specimens were measured under various levels of loading. The test results showed that the damage patterns of the reinforced specimens were dominated by the yielding of longitudinal tensile reinforcement at the bottoms of the beams and the crushing of the cementitious material in the top compression zones of the beams. The cracking load greatly increased by 1.42 to 7.12 times, and the ultimate bearing capacity increased by 0.29 to 1.41 times. The distribution characteristics and dynamic changes in the displacement, stress, and damage of the specimens were dynamically simulated by finite element software. The effects of reinforcement and initial load-holding level on the reinforcement effect were investigated. A bending capacity calculation formula for RPCTL reinforcement technology is proposed that aligns with the test results and can provide a reference for the design of RPCTL reinforcement.

Experimental study of dynamic mechanical properties and damage mechanism of textile reinforced concrete

The flexural characteristics of alkali-resistant (AR) glass textile reinforced concrete slabs (GTRC) at static-to-dynamic loading rates are investigated in the present study. The fabricated GTRC slabs were manufactured with four and six textile layers as reinforcement. Static flexural testing is conducted at 0.5 mm/min, while dynamic flexural tests are conducted at five loading velocities of 1, 2, 3, 4, and 4.5 m/s. The corresponding strain rates at selected impact velocities are 4, 8, 12, 16, and 18 s-1. Two separate mechanical setups, including (i) the MTS load frame and (ii) Instron drop-weight impact system, are used for distinct experimental testing. Furthermore, the mechanical properties of flexural strength, maximum strain, ultimate strain, flexural modulus, and toughness are calculated by using mathematical relations. In addition, failure patterns and the load-bearing intrinsic mechanisms of GTRC samples are analyzed in detail. Flexural experiments at various loading rates revealed that the loading rate and the number of reinforcing layers significantly influence the bending characteristics and mechanical properties of GTRC samples. In addition, the mechanical response is comparatively reduced at six layers of reinforcement than GTRC with four layers, proving better mechanical properties in flexural load at static and dynamic loading conditions.

Static and Fatigue Mechanical Properties of Polymer Matrix Composite Rods

Tendons (cables or rods) are commonly employed as tension members in civil infrastructure, as well as buildings and offshore engineering structures. This study focuses on reliability evaluation of composite rods consisting of polymer matrix (carbon fiber/glass fiber hybrid thermoplastic composite rods (hybrid composite rods) and basalt fiber/polypropylene composite rod (BF/PP composite rod)). Optical and gravimetric methods were used to characterize the morphologies, including constituent volume fractions, of the composite rods. The hybrid composite rods are braided structures with varying diameters and braid angles. The BF/PP composite rod exhibits slight twisting. The volume fractions of the constituent elements (carbon fiber, glass fiber, basalt fiber, matrix, and void) were evaluated. Tensile and flexural tests were conducted under static and fatigue loadings. During the static tensile test, the stress applied to the composite rods was almost linearly proportional to the strain. The fiber-dominant behaviors of the composite rods were observed. During the static flexural test, the stress-strain relationship was initially linear, but as the stress approached its maximum, deformation became non-linear, and finally, the fibers fractured rapidly. During the fatigue tensile and flexural tests, the regression lines of the full-logarithmic curves showed good agreement with the fatigue test data. In addition, data was collected and statistical analyses were performed to assess the effects of environmental factors, such as temperature, on the static properties of the composite rods.

Analytical and experimental investigation of the flexural and bond adhesive properties of thermally modified rubberwood and hybrid glulam beams with WPVC composites

Thermal modification is a property improvement technique that uses high temperatures to treat wood. For wooden structural members used in construction, in addition to the mechanical properties of thermally modified wood, the wood adhesive bond strength becomes increasingly significant. The present study examined the effects of thermal modification, using various temperatures (180 °C, 190 °C, 210 °C, and 220 °C) on the mechanical properties and adhesive bond performance of rubberwood compared to unmodified rubberwood. The results indicated that increasing the modification temperature negatively affected the mechanical properties of the rubberwood and decreased the wood adhesive bond strength. To improve the durability performance and joint strength reduction, a wood/polyvinyl chloride (WPVC) composite material was combined with thermally modified rubberwood at a temperature of 180 °C, which offered the optimum results in terms of the mechanical properties and wood adhesive bond strength of the various proposed modification temperatures. The study also investigated the mechanical properties of hybrid glulam beams made from thermally modified rubberwood and WPVC composites using static flexural tests, comparing the results to those for the thermally modified rubberwood glulam beams. The findings suggested that the modulus of elasticity of the hybrid glulam beams was not significantly affected because the WPVC composite members were placed in a low load-bearing location. The hybrid glulam beams provided a 38.66 % lower flexural strength than the thermally modified rubberwood glulam beams. Overall, the increase in temperature during the modification process negatively impacted the mechanical properties and wood adhesive bond strength of rubberwood. Moreover, a theoretical analysis was carried out to provide a comparison with the experimental investigation by focusing on the flexural behaviour of glulam beams. The analytical model results showed a good correlation with the observed behaviour of the glulam beams produced by thermal modification at different temperatures. In particular, the behaviour of the glulam beams produced by thermal modification at a temperature of 180 °C could be effectively predicted with high confidence.

MECHANICAL AND DURABILITY STUDY ON STEEL AND GLASS FIBERS BASED CEMENTITIOUS COMPOSITE

The study targets the utilization of steel and glass fibers in developing engineered cementitious composite. Compressive strength (CS) tests, split tensile strength (ST) tests, and static flexure tests were conducted to comment on the mechanical behavior of the composites. The durability properties of the designed mixes are commented on using rapid chloride penetration (RCPT) and sulfate tests. The CS of the conventional mix C-40 is compared with the S-ECC and G-ECC. The tested G-ECC specimens were more ductile than the S-ECC specimens and their respective variants. The S-ECC produced better results for both the CS and ST tests than the S-ECC and its variants. The conventional mixes of C-40 resulted in high penetration values than the composites prepared with steel and glass fiber inclusion. The destruction of the aluminosilicate skeleton, which leads to the liberation of silicic acid and sodium ions, is responsible for the deterioration caused during sulfate attack tests.

Residual Performance of Reinforced Concrete Beams Damaged by Low-Velocity Impact Loading

With the advancement of material science and infrastructure design technology, there is an increased need to reflect various loading scenarios in the design of civil infrastructures, including extreme loads such as impact and blast. However, structural behavior under extreme loads has been observed to be very different from that under static loads due to the high-rate loading condition. Therefore, extensive experimental and analytical studies have been conducted to explain the structural behavior under impact loading. From these efforts, various discussions have been presented on the impact behavior and the evaluation of the impact resistance of structures. However, despite the fact that the repair or reinforcement of structures damaged by impact loads is a critical issue, studies of the residual performance of impact-damaged structures are insufficient. This paper presents an experimental study to improve the understanding of the residual performance of impact-damaged structures. A total of 29 RC beams were tested under static loads to examine the effect of impact-damage on the residual behavior of RC beams. Of these, 8 RC beams were intact, and the other 21 RC beams suffered impact loading through drop weight impact tests and were damaged prior to the static flexural tests. The residual performance was evaluated by comparing the maximum load capacity, flexural stiffness, and displacement ductility of impact-damaged beams and intact beams. The comparison indicated that the flexural stiffness and displacement ductility of the impact-damaged RC beams decreased significantly, whereas their flexural strengths did not exhibit a significant difference. Predictive models for flexural stiffness and displacement ductility of the impact-damaged beams were suggested from the test results, and through these, the residual performance of the impact-damaged structure could be evaluated.

Flexural behavior of different types of steel-concrete composite decks with perfobond rib or head stud shear connectors

With the advent of long-span bridges, steel-concrete composite decks are increasingly concerning structures to address the fatigue problem, wear of the surfacing, and corrosion of the steel decks. The different types of ribs result in the variety of the cross-sections of the composite decks. Thus, the flexural behavior of different types of steel-concrete composite decks with perfobond rib (PBL) or head stud shear connectors was investigated in this study. The different types of composite decks included steel-concrete composite decks (SCCDs), open-rib stiffened steel-concrete composite decks (OSCCDs), and U-rib stiffened steel-concrete composite decks (USCCDs). For this purpose, the static flexural tests of two USCCDs with PBL or head stud shear connectors were conducted. Parametric analyses were also carried out via finite element models to investigate the effects of the number of ribs, the thickness of the ribs, the thickness of the steel plate, the thickness of the concrete slab, and the spacing of the shear connectors on the flexural behavior of the SCCDs, OSCCDs, and USCCDs with PBL or head stud shear connectors. The results indicated that the types of ribs showed significant influence on the flexural behavior of the composite decks. For the same type of shear connector, the flexural behavior of the USCCDs was significantly higher than that of the SCCDs and OSCCDs. Compared with the SCCDs with PBL or head stud shear connectors, the initial stiffness and ultimate load of the USCCDs with PBL or head stud shear connectors were increased by 314.2% and 354.5%, 62.2%, and 89.3%, respectively. The flexural behavior of the composite decks with PBL connections was larger than that of the composite decks with head stud shear connectors for one type of composite deck. The thickness of the concrete slab showed the highest impact on the flexural behavior of the composite decks, followed by the thickness of the rib, the number of ribs, and the spacing of the shear connector. The thickness of the steel plate showed a slight influence on the flexural behavior of the composite decks. The ultimate load of three types of composite decks with PBL or head stud shear connectors was increased by more than 46.5% when the thickness of the concrete slab increased from 80 mm to 120 mm. The reduction stiffness method considering the slip effect was proposed to accurately predict the elastic deflections of three types of composite decks. This study can provide some reference for the design and engineering application of the different types of composite decks.

PENGARUH POSISI SAMBUNGAN KONSTRUKSI KAYU TERHADAP DEDAIN PRODUK MEBEL BERBAHAN DASAR KAYU

Wood furniture products were one of the commodities that have a dominant contribution compared to other furniture materials to the Indonesian economy. In furniture products, the connection system was a weak point in the construction, so a detailed analysis was needed regarding the factors that affect the strength of the construction and the effectiveness of the wood connection system. The purpose of this study was to determine the quality of the wood used as outdoor chair material by carrying out several tests to obtain wood density, moisture content, MOR and MOE values and then carrying out impact tests on actual outdoor chair products. The results showed that the teak wood used in the raw material for making the product had a moisture content of 12% with an average density of 0.68 g/cm3. The results of the static flexural strength test showed that the MOE of the tested teak was 107,108 kg/cm2 and the MOR value was 1,018 kg/cm2. The compressive strength value parallel to the fiber is 536 kg/cm2. The teak sample tested was included in strong class II which is quite strong as a raw material for furniture such as outdoor chairs. Analysis of the results of the construction design found that the type of construction used was tenon mortise, cracks at the joints occurred because the tenon was too close to the head of the wood. Based on the product test results, after the product with a new construction design was made, by lowering the tenon mortise position 3 mm from the initial position. The crack that occurs in the product is caused by an error in product construction design and not from the quality of the product raw materials used.

Rotational flexural strength of cylindrical brittle specimens

AbstractConventional static flexural strength testing of brittle cylindrical rods only subjects a small fraction of the entire specimen's area or volume to the maximum tensile stress. Thus, a nonconservative measured strength likely results since most flaws on the surface or in the bulk are not subjected to a sufficiently high tensile stress that can cause fracture. To mitigate this, a rotational flexural tester and corresponding test method were developed whereby rotation and monotonically increasing three‐point flexure were superimposed to investigate fracture response of solid glass cylinders. This combination of rotation and flexure subjects more area and volume of a cylindrical test specimen to tensile stress than a standard static (nonrotating) flexural test. As anticipated, failure stresses were lower for the rotational flexural test. Expressions for effective area and volume are provided for rotating solid rods and tubes subjected to three‐point, four‐point, uniform, and uniformly distributed load bending configurations.

Mechanical behavior of a novel steel–concrete joint in concrete-composited hybrid continuous bridges

The steel–concrete joint is of great importance to the hybrid girder, while its mechanical behavior has not been understood enough due to the diversity of structural forms and application areas. This paper presented a mechanical performance investigation on a novel steel–concrete joint applied in a hybrid continuous bridge. A static flexural test and numerical simulation were conducted on a 16.6-m-long full-scale segmental joint. Parametric analysis via finite element models with the material damage plasticity on the joint performance was also carried out, in which structural dimensions and pre-stress extent were analyzed. The test results revealed that the border between the concrete girder and steel–concrete joint was the most cracking-prone position, while the joint remained comparatively intact at the ultimate state. The structural safety and internal force transfer mechanism were discussed with the presented strain distributions and deflection along the test girder. The simulation results showed that the effect of joint length on the bending performance of hybrid girder was more significant than back bearing plate thickness and pre-stress extent. The calculated ultimate capacity and bending stiffness increased by 8.3% and 4.0%, respectively, as the joint length increased from 3.6 m to 4.4 m. In addition, the initial cracking mode of the hybrid girder was exhibited to be correlated with the joint length because of changing structural stiffness. The research outcomes could provide the fundamental basis for a rational design of the joint component applied in hybrid continuous bridges.

Flexural Behaviors of Precast Reinforced Concrete -EPSfoam-Steel Deck Hybrid Panel

This paper presented the flexural behavior of the newly developed hybrid panel which included the comparison of the ultimate load, load-deflection behavior, and failure modes. The experimental study was carried out on precast reinforced concrete-EPSfoam-steel deck hybrid panels (CES)� consist of three layers of material : concrete� layer is on the top, the steel deck is located on the bottom layer, and the EPS foam layer as the core. The dimensions of CES are 300 mm x 1200 mm with thickness of concrete layer and EPS foam as variables. The concrete thick were 30 mm and 40mm. The density of EPS foam was 12 kg/m3, 20 kg/m3, and 30 kg/m3. The static flexural test of CES was conducted in accordance with the ASTM C 393-00 standard for determination of flexural strength on concrete, the load was applied at third-point loading. This test was carried out with monotonic static load, deflection control using a loading frame with capacity of 10 kN. The results show that increase the thickness of the concrete layer from 30mm to 40mm with� EPSfoam density of 12 kg /m3, 20 kg/m3, and 30 kg/m3 achieved a maximum load increase of 33.51%; 46,13%; and 37.35%, respectively.

Flexural Strength of Internally Stiffened Tubular Steel Beam Filled with Recycled Concrete Materials.

The flexural strength of Slender steel tube sections is known to achieve significant improvements upon being filled with concrete material; however, this section is more likely to fail due to buckling under compression stresses. This study investigates the flexural behavior of a Slender steel tube beam that was produced by connecting two pieces of C-sections and was filled with recycled-aggregate concrete materials (CFST beam). The C-section’s lips behaved as internal stiffeners for the CFST beam’s cross-section. A static flexural test was conducted on five large scale specimens, including one specimen that was tested without concrete material (hollow specimen). The ABAQUS software was also employed for the simulation and non-linear analysis of an additional 20 CFST models in order to further investigate the effects of varied parameters that were not tested experimentally. The numerical model was able to adequately verify the flexural behavior and failure mode of the corresponding tested specimen, with an overestimation of the flexural strength capacity of about 3.1%. Generally, the study confirmed the validity of using the tubular C-sections in the CFST beam concept, and their lips (internal stiffeners) led to significant improvements in the flexural strength, stiffness, and energy absorption index. Moreover, a new analytical method was developed to specifically predict the bending (flexural) strength capacity of the internally stiffened CFST beams with steel stiffeners, which was well-aligned with the results derived from the current investigation and with those obtained by others.

Influence of conservative endodontic access cavities on instrumentation of oval-shaped straight root canals.

To evaluate the influence of the design of endodontic access cavities on the percentage of unprepared areas of canal walls and flexural fatigue of instruments activated by reciprocating movement in oval-shaped straight root canals of extracted teeth. Forty-two mandibular incisors with oval canals were scanned by a microcomputed tomography (micro-CT) device for homogeneous selection and distribution of the samples. Then, the teeth were divided into two groups (n=21) according to the design of access cavity being tested: ultraconservative endodontic access cavity (UltraAC) and traditional access cavity. The canals were accessed with the aid of a surgical microscope, instrumented with the WaveOne Gold Medium system and irrigated with 2.5% NaOCl and 17% EDTA. The unprepared areas of the canal wall were analysed by overlaying images before and after instrumentation and expressed as percentages. micro-CT data were analysed using t-test, Mann-Whitney and Wilcoxon tests. The endodontic instruments used during instrumentation were subjected to static flexural fatigue testing using an artificial stainless steel canal with a 60° angle of curvature and a radius of 5mm, located 5mm from the tip of the instrument. The instruments were activated until fracture occurred, and the time in seconds for the fracture was recorded using a digital timer. The number of cycles to fracture was calculated and analysed statistically. For flexural fatigue data, an anova test complemented by a Tukey range test was used. The significance level of 5% was used for all analyses. There was no significant difference between the groups related to unprepared areas by the instrument during canal preparation (p>.05). The difference in flexural fatigue resistance between the groups was not significant. The use of UltraAC did not interfere with the canal instrumentation of extracted mandibular incisors with straight and oval canals. There was no difference in the flexural fatigue resistance of the instruments in relation to access cavity design.

Fatigue and static characterization on the behavior of electrospun nano-fiber veil toughened composites

This study focuses on the fatigue and static response of noncrimp fabric laminates modified with electrospun nanofiber veils; thus, the cross-ply was engaged in performing the test program. The results of static flexural tests and short-beam shear tests initially exhibited significant improvement in the flexural and interlaminar shear strength of the nanofiber modified (NM) samples due to different mechanisms. Further, the fatigue tests were conducted under the load-control fatigue condition, and the NM configuration exhibited high deflection and low stiffness decrease rate during the progressive damage. Notably, the fatigue strength of the NM configuration was 1.7 times higher than the virgin configuration, and the run-out point analysis revealed that the nanofiber veil has excellent potential to extend the material fatigue strength range and optimize the fatigue energy absorption capability of composite material. Finally, although the residual strength of the pre-cycling specimens reduced with an increase in load level, the stiffness reduction could not accurately predict the residual strength of the sample.

Study of the physical-mechanical properties of igneous stones from Tlalpujahua Michoacan, Mexico

Research work was carried out on rocks obtained in the municipality of Tlalpujahua Michoacán, located in different quarries of stone material, classifying the name of the quarry with the name of the community where the stones are located: “El moral” stones of bronze (BR) and golden (D) color, “la américa” stones of gray (G) and pink (R) color, “llano grande” stones of dark brown (CO), reddish-brown (CR) and black (N) color. Physical and mechanical tests were carried out, evaluating the percentage of moisture absorption, density, and absolute density. To evaluate the mechanical properties, the following tests were performed: uniaxial compressive strength (UCS), and static flexural test (modulus of rupture). The comparison of the different physical-mechanical tests allows estimating their behavior as a structural and restoration element. The correlation obtained to estimate the UCS has an acceptable correlation coefficient.

Correlation of fatigue behavior and dynamic mechanical properties of hybrid composites of polypropylene/short glass fibers/hollow glass beads

Ternary hybrid composites of Polypropylene (PP)/Short Glass Fibers (GF)/Hollow Glass Beads (HGB) were prepared using untreated and aminosilane-treated HGB, compatibilized with maleated-PP, and with varying total and relative GF/HGB contents. Static/short-term flexural strength properties data revealed, through lower flexural strength values, that the presence of untreated HGB particles induces to fiber-polymer interfacial decoupling at much higher extent than in the presence of aminosilane-treated HGB particles. This phenomenon is also evident when evaluating the data from displacement-controlled three-point bending fatigue tests. Monitored up to 106 cycles, the analyzed hybrid composites presented distinct performance relative to their fatigue stress relaxation rate: the lower the matrix-reinforcements’ interfacial adhesion, more pronounced the stress relaxation rate as a function of the number of fatigue cycles. Dynamic Mechanical Thermal Analysis (DMTA) results could successfully reveal the hybrid composites behavior at the microstructural level when they were submitted to both static flexural test and fatigue, depending on the degree of interfacial interactions between the polymer matrix of PP and the hybrid reinforcements of GF and HGB (with and without aminosilane surface treatment).

Mechanical properties and failure mode of thin-ply fiber metal laminates under out-of-plane loading

In this study, fiber metal laminates (FMLs), which consisted of thin-ply prepreg and stainless steel layers with a ply thickness of 0.04 mm, were fabricated with the aim of suppressing the fiber breakage that decreases the compression-after-impact (CAI) strength. Then, static flexural tests of FMLs were conducted to investigate the effects of the number of metal layers on the failure mode and mechanical properties during static out-of-plane loading, and the results were compared with those of our previous impact and CAI tests. The results confirmed that the plastic deformation of metal layers improved the energy dissipation and thus failure occurred in a limited range of locations. Furthermore, the investigation of the in situ stress during flexural tests by finite element method (FEM) analysis revealed that a hybrid material that both suppresses ply failure and has a high modulus of the metal can be realized owing to the constraining effect of thin-ply laminates.

Mechanical performance of geopolymer pre-stressed railway sleepers

This study is confined to polymerization and the base chemical used is sodium hydroxide 14 molarity and sodium silicate. High Performance Geopolymer Concrete (HPGC) was developed from Alkaline Activator Solutions (AAS) under combination 60% Ground Granulated Blast Furnace Slag (GGBS) and 40% Fly ash (FA) as binder materials produced high compressive strength of 76 N/mm2. Another mixing was made with the same proportion of binder material and filler materials M sand was replaced by Copper Slag (CS). According to the test results the substitution of 50% copper slag with M sand produced at high compressive strength of 80 N/mm2. Geopolymer Concrete was used for the casting of Wide Gauge Prestressed Concrete (PSC) sleepers as per Indian Railway standard specifications. And Pre-tensioning wires of standard High Tensile Strength wire (HTS) were used with a pre-tensioning force of 50kN. Needed Compaction was carried out using table vibrators and steam curing was adopted based on the standard production process of Indian Railways. Under power testing process the hardened concrete sleepers were tested in the laboratory by stimulating the real load transfer condensing the field called Rail Seat Centre Bottom (RSCB), Moment of Resistance (MR) and Moment of Failure (MF). As per the standard static flexural test procedure, the number of sleeper on each side of the sleeper for a load up to 230 kN (MR) and 370 kN (MF) was determined. The result of the tests reveals that the Geopolymer concrete sleeper has a high efficiency load and a high ultimate load.

Impact resistance, flexural and tensile properties of amorphous metallic fiber-reinforced cementitious composites according to fiber length

In this study, we examined the effects of the lengths of amorphous metallic fibers on the flexural and tensile properties and the impact resistance of cementitious composites. Cementitious composites were fabricated following the addition of 1.0 or 2.0 vol% of amorphous metallic fibers (lengths of 15 or 30 mm). After the static flexural and tensile tests, high-velocity impact tests were conducted (impact velocities of 170 and 300 m/s) with a spherical projectile (diameter of 25 mm). Amorphous metallic fibers with lengths of 30 mm inhibited the propagation of fractures based on the absorption and distribution of stress until they fractured without being pulled out, which significantly improved the flexural and tensile properties of cementitious composites; this resulted in better inhibition of scabbing in fibers with lengths of 30 mm compared to fibers with lengths of 15 mm. Meanwhile, the amorphous metallic fibers with lengths of 15 mm were pulled out from the matrix owing to their relatively smaller bonding areas compared with the 30 mm fibers. This caused smaller improvements in flexural and tensile properties as well as impact resistance compared with the 30 mm fibers.

Static flexural behavior of bamboo as a functionally graded material and the effect of heat on dynamic flexural modulus

The objective of the present investigation is to study the static and dynamic flexural behavior of bamboo as a functionally graded material to determine the Static Flexural Modulus (SFM) for untreated bamboo samples and to evaluate the effect of heat on the variation of the Dynamic Flexural Modulus (DFM). For the static flexural test, initially, the image processing has been carried out to establish the fiber distribution equation along the radial direction followed by the use to measure the tensile and compressive modulus of elasticity (MOE). The result of static flexural tests shows four different values of MOE at tensile and compressive state for inner and outer bamboo walls with low and high fiber density respectively. The average of the MOE at the tensile is circa 12% higher than in its compressive state. A non-destructive method, based on the impulse excitation technique is proposed for the heat-treated specimens to establish the DFM. The effects of different temperatures and time exposures as well as moisture content, on the DFM and weight of the specimens, have been considered in this study. Comparing the static and dynamic test results for untreated bamboo does not show a meaningful difference between SFM and DFM. The dynamic flexural test shows the effect of heat treatment on DFM to be less than 6% when heated for 3 h at 200 °C or when heated at 175 °C for 24 h.