Measured Strain Values Research Articles

Load Testing and Analysis of a Large Span Through Simply-Supported Steel Box Arch Bridge

To evaluate the true load-bearing capacity and engineering reliability of a large span through a simply supported steel box arch bridge, a load test was conducted on the bridge. The example used in this test is the Jingchu Avenue Bridge located in Jingmen City, Hubei Province. Specifically, the static load test delineated six operational conditions, measuring parameters encompassing strain, hanger cable force, deflection, and potential cracks. The dynamic load test gauged the bridge’s dynamic response and various indicators, including pulse tests, vehicle tests, jump tests, and barrier-free vehicle tests. The findings indicated that the maximum measured strain values during the static load test surpassed the calculated values; nonetheless, the verification factors and relative residual strains adhered to the code requirements, and no cracks were detected. The dynamic load test unveiled that the measured frequency values exceeded the theoretical ones, the damping ratios were within the normal range, and the measured impact coefficients were lower than the values stipulated in the code, all of which were in conformance with the code requirements. The data obtained from this experiment can be utilized to refine the long-term maintenance plan for the bridge, especially as it holds considerable value for structural health monitoring and aging assessment.

Reliability Assessment of Fiber Optic Shape Sensing

Fiber optic shape sensing is a promising technology capable of enriching the field of Structural Health Monitoring (SHM), with many potential applications that expand the uses of distributed optical fiber sensors. The operational principle is based on the simultaneous strain acquisition from several optical fiber cores deployed in a single sensing cable (or multicore fiber). The parallel strain traces of these cores can be combined to extract curvature and torsion distributions along the sensing cable, that can be used for the three-dimensional reconstruction of the underlying shape. This study evaluates the reliability of fiber optic shape sensing for damage detection through the use of a model-assisted probability of detection framework specifically developed to simulate a carbon fiber-reinforced polymers specimen under quasi-static loading monitored by distributed optical fiber sensors based on Rayleigh backscattering. Specifically, we compare the performance of a traditional distributed optical fiber strain sensor with the one of a hypothetical distributed shape sensing cable (or multicore fiber). To perform this comparison, we redefine the damage index definition, which is usually based on the measured strain values, to make it compatible with shape sensing, where it can be defined in terms of curvature and torsion. With these preliminary results, we aim to evaluate whether shape sensing technology can be employed for damage detection to detect damage-related deformation, whether we can obtain better results compared to traditional optical fiber sensor applications, and discuss how damage metrics need to be modified. In future activities, these aspects will be further investigated taking into account other case studies and shape sensing configurations.

Loss of pre-stress in impregnated superconducting magnets, experimental results and numerical analysis

The SMC (Short Model Coil) R&D program was started at CERN around 2007 to develop the Nb3Sn technology. The small magnet structure allowed relatively cheap and fast testing of various superconducting coils. One of the key questions to be answered, was related to the relation between the pre-stress and the magnet’s performance. To measure this dozens of strain gauges were installed on the coils, the axial tie-rods and the external shell. The experimental results of the strain measurements during all stages of the load: room temperature (RT) pre-stress, cool-down, powering, warm-up were analyzed in an extensive report [56]. A repeatable pattern of a decreasing strain after the warm-up, compared to the value before the cool-down, was observed on the external cylinder for all the tested coils. Values from 2 % to 50 % were reported.In this work a viscoelastic model was used to explain this effect. The Nb3Sn coil was treated as a composite material with decreasing stiffness due to mechanical damage. The Generalized Maxwell Solid model (Prony series model) was employed, including one spring and one damper, leading to a relatively simple model characterized by only two parameters. The two constants of the viscoelastic model were found: 1st – the relative relaxation moduli α based on a calibration curve derived from the experimental results of the SMC program and the 2nd one – relaxation time τ – based on minimizing the computational cost, by finding the asymptotic solution in one integration step. The model showed the capability of explaining the strain drop (loss of pre-stress) of more than 80 %. In addition to the viscoelastic effects, the role of friction coefficient was studied revealing the possibility of explaining up to 14 % of the strain drop. Yet, to fit with the experimentally measures strains on the SMC cylinder, especially during the RT pre-load, the most-probable value of the friction coefficient should be μ<0.4. The strong impact of the stiffness of the G-10/G-11 laminate used to spread the load on the coil was found, indicating the need of knowing the elastic properties of this material very precisely. In addition, the experimentally measured strain values showed strong asymmetric, both in plane and along the magnet’s axis, revealing the potential sensitivity to the geometric imperfections and the need for 360° magnet models.

Numerical Simulation and Experimental Study on Detecting Effective Prestress of 1860-Grade Strands Based on the Drilling Method

In this paper, we study the magnitude of the effective prestressing force of steel strands in prestressed reinforced concrete structures. Through the theory of micro-hole release, the functional relationship equation between tensile stress and strain-containing coefficients A and B is established. Then, Midas FEA NX 2022 (v1.1) finite element software is used to establish the stress-release model of strand drilling holes and analyze the influence of parameters such as drilling depth, drilling diameter, hole–edge distance, and tension stress on the amount of stress release. Finally, through a homemade tensioning platform, we verify the reasonableness of the finite element simulation calculation law and determine coefficients A and B. The results of the study show that based on Kirsch’s analytical formula and the theory of microvia release, the axial tension force and axial strain are linearly correlated; the Midas FEA NX finite element software can effectively simulate the force state of strand cross-section; and through the strand-drilled hole model simulation and analysis, it is found that the tension stress value and the stress-release amount are related to the tensile stress value and the tensile stress value. We found that the value of tensile stress and the amount of stress released are positively correlated; with the increase in the hole margin, the amount of stress released gradually decreases; with the increase in the diameter of the hole, the amount of strain released gradually increases; and the greater the depth of the hole, the greater the amount of strain release. Moreover, the use of a hole margin of 3–6 mm, a hole diameter of 1.5 mm and 1.8 mm, and a hole depth of 2.5 mm is more reasonable in the test conditions, as follows. Through the drilling test conditions of 1.5 mm drilling diameter, 2.5 mm drilling depth, and 4 mm hole side distance, we verified the measured strain value of the steel wire and the tensile force value of the linear correlation between the functional relationship and the use of this functional relationship to determine the theoretical derivation of the coefficient to be determined: A is 1.12 and B is 57.84.

Safety Evaluation on a Fastening Device of an Agricultural By-Product Collector for Hard Flat Ground Driving

In this study, the static safety factor and fatigue life of fastening devices of an agricultural by-product collector were evaluated under hard flat ground driving conditions. The strain gage-based measurement system was constructed, and the strain gage was attached on the highest stress spot of the fastening devices derived from structural analysis. The static safety factor and fatigue life of the fastening devices were calculated using the measured strain values and by converting it into stress data. The two operating conditions are considered to be the loading part of the by-product collector, lifted and non-lifted. The results for all fastening devices showed that the static safety factor was larger than 1.0 and the fatigue life was much greater than the expected lifetime under both operating conditions. Therefore, it can be concluded that the fastening devices of the by-product collector can be operated reliably under hard flat ground driving conditions. In future work, we plan to evaluate the safety of the fastening devices in various actual orchard farm environments.

Development of triaxial compressive apparatus for neutron experiments with rocks.

Underground engineering for processes such as geological disposal of high-level nuclear waste, CO2 capture and storage, and mining and drilling for resources requires an understanding of the mechanical behavior of rocks at subsurface stress states, i.e., triaxial compressive stress. Strain measurement using neutron diffraction can be applied to rocks to analyze strain accumulation mechanisms at the microscopic scale. This study reports the development of triaxial compressive apparatus for strain measurement using neutron diffraction. The apparatus can analyze rock specimens (diameter, 25mm; length, 50mm) and apply a maximum confining pressure of 50 MPa. Materials for the components of the apparatus were investigated theoretically based on neutron beam transmission and experimentally using neutron diffraction experiments. The feasibility of the apparatus was verified by measuring strain at hydrostatic pressure under the application of confining pressure and triaxial compression. The theoretical and experimental results show that the apparatus could obtain sufficient neutron statistics from a rock specimen. It was confirmed experimentally that the measured strain values are correlated with the applied confining pressure and stress. The lattice strains of quartz minerals measured by neutron diffraction showed linear deformation behavior, indicating that elastic strain accumulated in the minerals. This apparatus will enable the finding of new insights into the deformation mechanisms of rocks.

Stressing State Analysis of Reinforced Concrete Beam Strengthened by CFRP Sheet with Anchoring Device

This paper investigates the working performance of reinforcement concrete (RC) beams strengthened by Carbon-Fiber-Reinforced Plastic (CFRP) with different anchoring under bending moment, based on the structural stressing state theory. The measured strain values of concrete and Carbon-Fiber-Reinforced Plastic (CFRP) sheet are modeled as generalized strain energy density (GSED), to characterize the RC beams’ stressing state. Then the Mann–Kendall (M–K) criterion is applied to distinguish the characteristic loads of structural stressing state from the curve, updating the definition of structural failure load. In addition, for tested specimens with middle anchorage and end anchorage, the torsion applied on the anchoring device and the deformation width of anchoring device are respectively set parameters to analyze their effects on the reinforcement performance of CFRP sheet through comparing the strain distribution pattern of CFRP. Finally, in order to further explore the strain distribution of the cross-section and analyze the stressing-state characteristics of the RC beam, the numerical shape function (NSF) method is proposed to reasonably expand the limited strain data. The research results provide a new angle of view to conduct structural analysis and a reference to the improvement of reinforcement effect of CFRP.

Coexisting sub-zero temperature and relative humidity influences on sensors and composite material

The paper is focused on coexisting influence of sub-zero temperature and different relative humidity levels on fibre Bragg grating (FBG) sensors and glass composite. The analyses were performed at different environmental (dry/wet) and sensors boundary (‘free’, woven, glued, embedded) conditions. Strain values determined for sub-zero temperatures differ among sensors, even these embedded into the material. The influence of wet conditions on the measured strain values was stronger for −50 °C than −20 °C. The influence of phase change of water (in the container)/moisture (inside the material) from liquid to the solid state was taken into consideration. Additionally, the application of THz spectroscopy allowed to observe the internal composite structure changes due to the coexisting influence of temperature (from sub-zero to room temperature) and different humidity levels. A theoretical model based on the theory of laminates was presented and discussed.

Compression, tension & lifting stability on a meter gauge flat Wagon: an experimental approach

ABSTRACT This paper presents the experimental study of the compression, tension, and lifting stability test on a meter gauge flat wagon. The freight wagon comprises flat body structure and two bogies, where the flat surface is used to transport containers or goods. Two load cells (2000 KN) of HBM Quantum X-MX840 with CATMAN easy software is used for the compression, tension, and lifting stability test. As per EN 12663-2:2010 standard for axial compression test maximum force of 2MN is applied while for diagonal compression test 0.4MN force is applied. The tension test is carried out by applying a maximum tensile load of 1.5 MN while the lifting test is performed with 29T (empty condition) and 57.5T (loaded condition) wagon load. Strain gauges are pasted on identified critical location (based on FEM simulation in ANSYS) of the wagon to capture the strain values. The steady-state strain values are measured and recorded in a data acquisition system. A standard test methodology and the testing setups are established for freight wagon testing. The measured strain values are analyzed for the wagon specific requirements. It is observed that during testing strain values are within the safe zone as per the designed payload for this wagon.

Development of miniature cruciform specimen and testing machine for multiaxial creep investigation

High temperature components such as boiler tubes and jet engine turbine blades undergo multiaxial creep damage due to extreme temperature conditions and their complex shapes. A multiaxial creep investigation is required for the safety of high temperature components in the design phase of the manufacture of such a component. However, there are only a few commercial testing machines that can conduct multiaxial loading at high temperatures. A miniature cruciform specimen having a plane stress condition gauge section that is 5 mm square was designed using finite element (FE) analysis. Further, a biaxial testing machine capable of tensile loading was designed to conduct multiaxial creep tests. The testing machine had a 2 kN loading capacity and a 1 kW furnace. We also developed a non-contact displacement measuring method for the miniature specimen, which had an excessively small gauge section to permit the attachment of a mechanical extensometer. The proposed method uses a conventional optical camera to capture some images of the surface. We obtained the displacement value of the specimen, by tracking the trace of the target marks painted on the surface of the specimen. The measured strain value that was obtained from the non-contact displacement measuring method corresponds to the strain obtained by the mechanical method at room and high temperatures. By using the developed multiaxial creep testing machine and the non-contact observation system, we can investigate not only the deformation of the testing specimen, but also the surface conditions of materials during the creep test.

The usability of Burger body model on determination of oriented strand boards’ creep behavior

In this work, the usability of the Burger body model (BBM) for determining the behavior of oriented strand boards (OSBs) under long-term loads was evaluated. The actual bending strain data and predicted strain data as a function of different stress levels and load durations under constant environmental conditions (25 ± 2°C and 50% relative humidity) were compared. Two test groups, short-term bending tests and long-term creep-rupture bending tests, were performed according to relevant ASTM standards. Specimens were randomly assigned to three groups and loaded at 47% (132.2 kg), 51% (137.4 kg), or 55% (154.9 kg) of the mean static short-term flexural strength. Specimen creep was monitored for 10,000 h using an automated measurement system. The four-parameter BBM parameters were obtained for all specimens at 2000-h time intervals, providing five different estimates. Measured strain values were compared with strain predictions from the BBM and with the goal of evaluating length of experiment on prediction accuracy. Each stress level provided statistical differences based on the error between the actual strain and predicted strain values. Group 3 provided minimum error compared to group 1 and group 2. The 10,000 and 8000 h loading provided the most accurate predictions compared to 6000, 4000, and 2000 h of data. Overall, the longer the actual data is collected the more accurate predictions were obtained. As a result, the BBM was found useful tool for predicting the creep behavior of OSBs under different loads and load durations. It was also shown that the increased duration of practical loading minimizes the error between the prediction. Therefore, the BBM is suggested for use predicting the creep behavior of OSBs over 8000 h load durations.

P1540 Imaging parameters vs. operator dependence of global longitudinal strain values

Abstract Background Speckle tracking based global longitudinal strain (GLS) values have proven useful in the assessment of subtle changes in left ventricular function. From a clinical point of view, robustness and reliability of measured values are critical to ensure a valid patient assessment and follow-up. However, it is still a matter of debate if imaging parameters systematically alter measured strain values and if these changes are relevant as compared with GLS fluctuations that are caused by different operators or different studies by the same operator. Methods In a consecutive everyday patient population (n = 35), we recorded the apical four chamber view several times in each patient with different ultrasound machine settings (modification of gain, frame rate, sector depth, and transducer frequencies) using a commercially available ultrasound imaging system. Furthermore, apical four chamber views with ‘optimized’ imaging settings at the operators’ discretion were recorded by two different observers (obA/obB) in each subject to compute inter- and intra-observer variability. GLS values were calculated offline with a dedicated software. We fitted a linear mixed effects model with random intercept and slope to assess the effect of imaging parameters on GLS and compared the two investigators with Bland-Altman plots. Results Ejection fraction ranged between 10% and 76% and was correlated well with GLS (r = -0.78). Neither gain settings (range: -24 to 24 arbitrary units, p = 0.68) nor frame rate (range: 51-113 sec-1, p = 0.77) systematically changed measured GLS values. Conversely, higher sector depth increased (range: 12 to 24 cm, mean effect: -0.16%/cm; 95% CI -0.24% to -0.07%, p &amp;lt; 0.01), and higher transducer imaging frequencies decreased absolute GLS values (range for harmonic imaging: 1.5/3.1 MHz to 2.0/4.3 MHz, mean effect 1.10%/MHz; 95% CI 0.61% to 1.59%, p &amp;lt; 0.01). According to our data, a 12 cm increase of sector depth would translate into an average change of -1.87% GLS (95% CI: -2.87% to -0.86%), whereas a switch of the second harmonic imaging frequency from 3.1 MHz to 4.3 MHz would cause a 1.32% GLS change (95% CI: 0.73% to 1.91%). Intra- and inter-observer variability showed good correlation and limits of agreement (obA: mean difference [MD]: -0.20%; 95% limits of agreement [LOA]: -2.42% to 2.02%, p = 0.86, obB: MD: -0.10%; 95% LOA: -4.28% to 4.07%, p = 0.12, obA vs. obB: MD: -0.53%; 95% LOA: -3.68% to 2.62%, p = 0.92). Conclusion Overall, GLS values were robust and reproducible in our cohort of patients. In comparison, potential systematic changes of GLS values caused by modification of imaging parameters (sector depth/transducer frequency) were much less in number than GLS variations caused by different operators or different studies by the same operator.

A Theoretical Model of Strain Measurement System of Membrane Materials in Biaxial Tensile Tests

This paper presents a new method and toolset to measure strain values of the fabrics under biaxial tensile test. The proposed model is based on the reverse engineering of Delta 3D printing system. A comparison is made to the existing widely accepted models of strain measurements. The designed toolset was developed by parameter based modelling and genetic optimization to ensure that the equipment can function within the given strain domains. The equipment design model can be adapted to any biaxial tension frame.

High-temperature strain monitoring of stainless steel using fiber optics embedded in ultrasonically consolidated nickel layers∗∗Notice: This manuscript has been authored by UT-Battelle, LLC, under contract DE-AC05-00OR22725 with the US Department of Energy (DOE). The US government retains and the publisher, by accepting the article for publication, acknowledges that the US government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish

Fiber optic sensors have long been considered for use in structural health monitoring of components because of their small size, high accuracy, and their ability to perform spatially distributed strain measurements when embedded within a component or on its surface. The typical method for transferring strain from the component to the fiber is to use an epoxy, which may not survive extended exposure to high temperatures, thus necessitating a more elaborate technique to embed the fibers. This work represents a first step towards using additive manufacturing techniques to embed fiber optic sensors on stainless steel components for high-temperature strain monitoring, including quantification of the differential thermal strains that develop between the fiber and the steel substrate. Copper-coated optical fibers were embedded in nickel layers on top of a stainless steel substrate using an ultrasonic additive manufacturing technique. The embedded fibers showed minimal signal attenuation and clear compressive strain after embedding. Heating the embedded fibers in steps to temperatures of 300 °C–400 °C resulted in measured strains with values between the expected thermal strain in stainless steel and nickel. Finite element simulations confirm the measured strain values and show that the thermal strain depends on the thickness of the nickel layers deposited on top of the stainless steel substrate. While the fibers failed before reaching temperatures of 500 °C, it is suspected that these failures occurred due to a combination of (1) the lack of strain relief, (2) the high-temperature oxidation of the fiber’s copper coating, and (3) improper sizing of the machined channel in which the fiber is placed prior to embedding. If proper coating selection and sizing of the channel can prevent the failures observed in this work, the next step would be to monitor strain during mechanical loading at high temperatures.

Hybrid testing for evaluation of seismic performance of highway bridge with pier made of HyFRC

The present research is focused on Hybrid testing of a highway Reinforced Concrete (RC) bridge, wherein fibres available in India were used to improve the seismic performance of bridge pier. In order to arrive at best possible fibre proportions to be used, prisms made of conventional concrete and Hybrid Fibre Reinforced Concrete (HyFRC) were first tested to account for variability in fibre proportions. Two types of steel and polypropylene fibres available in India were used, where prisms made of HyFRC exhibited enhancement in energy dissipation capacity. HyFRC was used by previous researchers to investigate their performance in isolated structural elements, but no such research papers are found, wherein structural elements made of HyFRC were investigated using Hybrid testing. Scaled bridge piers of an existing RC bridge designed for most active seismic zones in India and made of conventional concrete as well as HyFRC were considered as experimental elements in the hybrid testing under the action of recorded earthquake excitation of different intensities. After completion of the hybrid testing, cyclic tests were carried out to find the ultimate capacity of these test specimens. Pier specimens made of HyFRC outperformed specimens made of conventional concrete in terms of all the seismic performance parameters, such as improved damage tolerance, enhanced energy-dissipation capacity, higher stiffness at any drift ratio, improved load-carrying capacity and ductility. Detailed strain measurements in concrete and reinforcing bars revealed the merits of HyFRC over conventional concrete as measured strain values were appreciably lesser at any load level.

Modifying Tunnel’s Hazard Warning Levels based on the Laboratory Studies on Different Rock Types

Sakurai proposed hazard warning levels (HWLs) on the basis of the critical strain concept to evaluate the stability of tunnels. When the measured strain values remain smaller than HWL III, the stability of the tunnels is confirmed. The collapses occurred in a number of tunnels existing in Iran have questioned the accuracy of the Sakurai’s criterion in spite of the smaller values of tunnel strains comparing to the HWL III. This study renders an account of the HWLs modified on the basis of the results of 162 uniaxial compression strength (UCS) tests conducted on ten types of rocks and soils. To this end, the UCS, critical strain and initial tangent modulus were first determined. Then, the upper and lower bounds of the critical strain were considered in UCS – critical strain and modified tangent modulus – critical strain graphs as the modified HWLs. According to the results, the modified HWLs are more accurate results regarding the tunnel stability conditions.

Load Prediction of Space Deployable Structure Based on FBG and LSTM

To predict the dynamic load of space deployable structure, a novel prediction method based on fiber Bragg grating (FBG) sensors and long short-term memory (LSTM) neural network is proposed. The FBG sensors can measure strain values of the structure. The LSTM establishes the relationship between the strain values and load without a complex dynamic model. The LSTM first works in training mode, its input is strain values and output is the load. Then, the LSTM works in the prediction mode, which takes the current strain values as input and uses the trained network model to predict load. The network of LSTM is simple, which can achieve good prediction effect and shorten the running time. To evaluate the effectiveness of the proposed method, numerical simulations and experiments of space deployable structure with single-point input load are employed. The results demonstrate that compared with the method based on traditional neural network, the max error, root mean square error, and consuming the time of the proposed method are reduced by more than 43%, 21%, and 67%, respectively.

Strain measurement on four-dimensional dynamic-ventilation CT: quantitative analysis of abnormal respiratory deformation of the lung in COPD.

PurposeStrain measurement is frequently used to assess myocardial motion in cardiac imaging. This study aimed to apply strain measurement to pulmonary motion observed by four-dimensional dynamic-ventilation computed tomography (CT) and to clarify motion abnormality in COPD.Materials and methodsThirty-two smokers, including ten with COPD, underwent dynamic-ventilation CT during spontaneous breathing. CT data were continuously reconstructed every 0.5 seconds. In the series of images obtained by dynamic-ventilation CT, five expiratory frames were identified starting from the peak inspiratory frame (first expiratory frame) and ending with the fifth expiratory frame. Strain measurement of the scanned lung was performed using research software that was originally developed for cardiac strain measurement and modified for assessing deformation of the lung. The measured strain values were divided by the change in mean lung density to adjust for the degree of expiration. Spearman’s rank correlation analysis was used to evaluate associations between the adjusted strain measurements and various spirometric values.ResultsThe adjusted strain measurement was negatively correlated with FEV1/FVC (ρ=−0.52, P<0.01), maximum mid-expiratory flow (ρ=−0.59, P<0.001), and peak expiratory flow (ρ=−0.48, P<0.01), suggesting that abnormal deformation of lung motion is related to various patterns of expiratory airflow limitation.ConclusionAbnormal deformation of lung motion exists in COPD patients and can be quantitatively assessed by strain measurement using dynamic-ventilation CT. This technique can be expanded to dynamic-ventilation CT in patients with various lung and airway diseases that cause abnormal pulmonary motion.

The Research of Low-profile Load Cell Design Sensitivity

This low profile load cells are referred to the current injection molding machine manufacturers commonly used specifications. This study is about the low-profile load cell design by the finite element analysis and experimental strain measurement verification. In the study, two low-profile load cell were designed and analyzed. The A-type cross-section of the sensor is a generally designed cross-section and has sufficient reliability for commercial use. The B-type curve cross-section load cell from the previous study found that can carry a more significant load. In the study, compare the finite element analysis results with the actual strain measurements to verify the correctness of the analysis. In the experimental verification, the two type sections loadcells were calibrated. The load cells were loading from 0kN to 60kN to measure strain values. The strain values were recorded in experiment processes. When the two types of load cell under loading, the loading force, and strain values regression linearity was 99.99 %. The measured values of the A-type and B-type load cells at 60 kN were 298 με and 170 με, and the results matched finite element analysis strain values. It was shown that the setup of boundary conditions and loading method in finite element analysis were acceptable.

Study of random fatigue behavior of C/SiC composite thin-wall plates

As an alternative method of acoustic experiment, vibration tests of C/SiC thin-wall plates were conducted to study the random fatigue behavior. The vibration specimen was clamped with cantilever boundary and subjected to limited-bandwidth vibration excitation. The strain history and fundamental frequency of the specimens were monitored during the vibration test. It was found that the measured strain value and strain amplitude value accorded with Gauss and Rayleigh distribution, respectively. The fundamental frequency decreased greatly when the vibration specimen failed, which could be recognized as failure criterion for the specimen under random loading. The microscopic fracture morphologies showed that the majority failure mode of fibers in the longitudinal and transverse directions were fiber pull-out and fiber splitting, respectively. The tensile force induced by reversed bending load is the main driving force for damage in specimen.