Glass Fiber Reinforced Polymer Samples Research Articles

Efficient algorithm for thermal nondestructive testing and evaluation by considering the heteroscedastic nature of noise sources in infrared thermography

Abstract Thermal Imaging is a promising Non Destructive Testing & Evaluation (NDT & E) approach to monitor the health
of composite materials. Among various post processing approaches adopted in thermal imaging for NDT & E, statistical
analysis schemes gained importance due to their reliability and data reduction capabilities. This paper provides an insight
to a factor analysis-based statistical approach to detect the hidden defects in the Glass Fiber Reinforced Polymer (GFRP)
sample. The proposed approach models the observed data covariance into combination of temporal signal covariance
and noise covariance matrices. The modeling of the diagonal covariance matrix (with different elements) is motivated by
the presence of heterogeneity in the experimental data obtained from GFRP sample. This novel method is based on the
coordinate descent technique, which estimates the covariance matrix of the noise variances iteratively by minimizing the
negative log likelihood function. The obtained results from the chosen GFRP samples compared with the widely used
statistical Principal Component Thermography (PCT) technique illustrate the improved performance in terms of defect
detection with the proposed technique.

Reusing Egyptian in-situ construction &demolition waste(CDW)to produce sustainable concrete.Investigation of compressive &bond strengths using steel & glass fiber reinforced polymer(GFRP)rebars

ABSTRACT This research paper presents the effect of using recycled concrete aggregate (RCA), from the Egyptian construction waste, in different proportions on the bond performance between concrete and its internal reinforcement, where two types of reinforcement were used: a- steel reinforcing bars, and b- glass fiber reinforced polymer (GFRP) bars. 72 samples of concrete were prepared and tested with replacement ratios for natural aggregate (NA) with RCA: 0, 20, 40, 60, 80, and 100 %. The samples were divided into 150 mm cubes to conduct axial compression tests, and pull-out samples to investigate the effect of replacement ratios on the bond between concrete with steel and GFRP rebars. Results showed that concrete loses 26% of its compressive strength after replacing NA with 100% RCA. The results of the pull-out test for steel samples showed 30% higher bonding than GFRP samples. A slight decrease in bond was observed between the control and the tested samples, where the steel and GFRP samples showed a decrease of 5% and 9%, respectively, over the control samples for each, at a replacement percentage of 100%. It was also clear that the steel reinforcement bars had a stronger bond with the concrete, and this can be attributed to the stronger mechanical interlocking of the steel ribs than their counterparts in the GFRP. Failure patterns showed the crushing of concrete with steel bars, in contrast to the GFRP bars, which showed slipping from within the concrete with failure of the ribs in many samples.

Mechanothermal assessment of outdoors aged hybrid glass fibre reinforced polymer composite filled with fly ash as industrial waste

Industrial fly ash impregnated glass fibre reinforced polymer (GFRP) composites were examined to assess their overall characteristics in adverse ambient conditions, keeping in mind that use of filler materials in FRP composites was expected to enhance the strength properties of the material. GFRP composite specimens with 2–10 wt % industrial fly ash were fabricated in the laboratory. These were exposed to open ambient ageing for 120 days. The samples impregnated with 8 wt % fly ash and aged for 120 days were seen to absorb the minimum moisture and exhibited the maximum ILSS of 35.19 MPa and flexural strength of 690 MPa, respectively. These samples also exhibited the highest Tg of 101.53°C as revealed through low temperature DSC. It was also observed that the 8 wt% fly ash containing samples aged in the open for 120 days showed the ILSS to be 5.83% higher and Tg to be 21.95% higher as compared to the unaged GFRP samples without fly ash. FTIR spectra confirm the trend of thermo-mechanical properties. Both optical and scanning electron microscopy of the fractured surfaces of the test samples revealed the modes of mechanical failure of the hybrid GFRP composite with their indicative properties at optimized extent of fly ash dispersion.

Nondestructive Evaluation of Tensile Stress-loaded GFRPs Using the Magnetic Recording Method.

This paper presents the results of inspecting tensile stress-loaded GFRP (glass fiber-reinforced polymer) samples using the Magnetic Recording Method (MRM). The MRM can be utilized solely to examine ferromagnetic materials. The modification was proposed in order to examine nonmagnetic composites. Ferromagnetic strips made of low-carbon steel DC01 were bonded to the surface using an adhesive composed of epoxy resin with the addition of triethylenetetramine. The modified method's feasibility was tested on six samples made of GFRP. The research procedure consisted of three steps. In the first step, a metal strip is glued at the top surface of each sample, and an array of 100 cylindrical permanent magnets is used to record a sinusoidal magnetic pattern on the strip. The initial residual magnetization is measured in the second step, and the samples are subjected to static stress. In the third step, the residual magnetization is measured one more time. Ultimately, the measurement results from the second and third steps are compared. Generally, the applied stress causes changes in the amplitude and frequency of the sinusoidal magnetization pattern. In the case of GFRP, the frequency changes have not been used for evaluation due to minimal variations. The statistical parameters (mean, median, max, and mode) of the RMS (root mean square) value of the sinusoidal pattern were calculated and analyzed. The analysis demonstrates that the modified method is suitable for providing unequivocal and exact information on the load applied to a nonmagnetic composite material. For the presented results, the applied load can be assessed unambiguously for the samples elongated up to 0.6%.

Cutting Parameter Optimization in Milling of Glass Fiber and Carbon Fiber Reinforced Composites to Reduce Cutting Force

Abstract Composite materials are often preferred in many engineering applications because of their lightness and high-strength properties. However, in addition to the advantages of composite materials, processing problems exist. The processing of fiber-reinforced composite materials is more complex than other metal materials. The forces that arise during milling cause undesirable results, such as tool wear and energy loss. In this study, the cutting parameter optimization was made by measuring the force during carbon fiber and glass fiber-reinforced epoxy matrix composites milling. Cutting velocity (90, 120, and 150 m/min) and feed per tooth (0.1, 0.15, and 0.2 mm/tooth) were selected as cutting parameters. Experiments were planned according to the Taguchi method with two parameters and three levels. Optimization of the parameters was evaluated using the signal/noise ratio approach. The effectiveness of parameters on results was determined by analysis of variance (ANOVA). Optimum levels were found as 120 m/min for cutting velocity and 0.1 mm/tooth for feed per tooth both glass fiber-reinforced polymer (GFRP) and carbon fiber-reinforced polymer (CFRP) composites. According to the ANOVA results, the contribution rates of the force parameters were 81.05 % for cutting velocity and 15.03 % for feed per tooth in GFRP composites. The contribution rates of the force parameters were 43.69 % for cutting velocity and 46.18 % for feed per tooth in CFRP composites. The surfaces of the milled samples were examined with an optical microscope to investigate the damage. It was observed that the surfaces of CFRP samples had a better surface quality than GFRP samples.

Low Frequency GFRP Imaging with Variable Aperture TFM

Graphite or glass fiber reinforced polymer (GFRP) inspections are usually made with a zero-degree linear scan (L-scan) (1). Total focusing may not be applicable for some anisotropic materials, but in some cases, TFM may increase the ultrasound imaging quality. The imaging potential of 1D linear probes has been broadened by TFM by many aspects so there is an opportunity to apply the total focusing concepts to exotic composite assessment. The ultrasonic limitations associated with testing thicker sections of GFRP restrict inspections to lower frequency phased array probes. When a low frequency probe uses the L-Scan technique, the performance and sensitivity is negatively affected once compared to a typical phased array probe of a higher frequency due to the element size and pitch restrictions associated with low frequency probes. Therefore, new testing techniques should be found. The alternative imaging acquisition method presented here involves variable aperture receivers that improve the usual LScan characteristics. This study explicitly covers the differences over the L-scan type, the TFM image and its variable aperture TFM feature. In addition of the theory concepts, ultrasound metrics have been recorded on a real GFRP sample and using as reference reflector side drilled holes (SDH).

Mechanical properties and microstructure of glass fiber reinforced polymer (GFRP) rebars embedded in carbonated reactive MgO-based concrete (RMC)

Reactive MgO cement (RMC) is an emerging low-carbon material which can be a sustainable alternative to ordinary Portland cement (OPC) due to its capability to sequester CO2. However, the application of carbonated RMC concrete in conventional steel reinforced concrete infrastructure is restricted by its inherent low alkalinity (∼10.4), which cannot protect steel from rusting. To address this issue, glass fiber reinforced polymer (GFRP) rebars can be employed. In this study, the mechanical properties and microstructures of RMC-GFRP exposed to various temperatures (23, 40 or 60 °C) for 21, 45 and 90 days were investigated. Experimental results showed that the low-alkalinity carbonated RMC concrete was less aggressive than the normal concrete (NC) to GFRP rebars, remarkably enhancing the tensile strength retention of GFRP rebars from 73.6% to 90.4% after 90 days of exposure at 40 °C. Micromorphology observation indicated that the higher tensile strength retention was ascribed to the reduced GFRP degradation (i.e., matrix cracking, fiber erosion and fiber/matrix interfacial debonding) under the low-alkalinity condition. Additionally, the flexural strength of 90-day aged RMC- and NC-GFRP samples at 60 °C were close to that of unconditioned GFRP samples (791 MPa and 788 MPa vs. 791 MPa), which was attributed to the unchanged glass transition temperature and stable elastic modulus of GFRP. The RMC- and NC-GFRP samples also showed very little change in inter-laminar shear strength as compared to the unconditioned GFRP samples, as the middle layer of GFRP rebars was well-protected from external water and chemicals. Based on the empirical degradation model and activation energy derived from test results at different temperatures, the predicted tensile strength retention of RMC-GFRP after an extended period (∼100 years) under Canadian climate is still sufficient. This study will shed light on the understanding of degradation mechanism and practical application of GFRP reinforced RMC in infrastructure construction.

Wavelet-Based Output-Only Damage Detection of Composite Structures.

Health monitoring of structures operating in ambient environments is performed through operational modal analysis, where the identified modal parameters, such as resonant frequencies, damping ratios and operation deflection shapes, characterize the state of structural integrity. The current study shows that, first, time-frequency methods, such as continuous wavelet transform, can be used to identify these parameters and may even provide a large amount of such data, increasing the reliability of structural health monitoring systems. Second, the identified resonant frequencies and damping ratios are used as features in a damage-detection scheme, utilizing the kernel density estimate (KDE) of an underlying probability distribution of features. The Euclidean distance between the centroids of the KDEs, at reference and in various other cases of structural integrity, is used as an indicator of deviation from reference. Validation of the algorithm was carried out in a vast experimental campaign on glass fibre-reinforced polymer samples with a cylindrical shell structure subjected to varying degrees of damage. The proposed damage indicator, when compared with the well-known Mahalanobis distance metric, yielded comparable damage detection accuracy, while at the same time being not only simpler to calculate but also able to capture the severity of damage.

Bond behaviour between CFRP, GFRP, and hybrid C-GFRP tubes and seawater sea sand concrete after exposure to elevated temperatures

The paper provides an overview of the current research on bond performance between fibre-reinforced polymer (FRP) tubes and SWSSC and investigates the bond strength of SWSSC-filled FRP tubes after exposure to different temperatures using push-out tests. A total of 27 samples, including glass FRP (GFRP), carbon FRP (CFRP), and glass-carbon hybrid FRP (HFRP) tubes, were tested to determine the effect of elevated up to 250 °C temperatures on the bond performance of the samples. It was observed that exposure to elevated temperatures increased the bond strength of all the tubes. The GFRP and CFRP tubes exhibited the strongest and weakest bond strengths, respectively. Since the inner layer of the HFRP tube was composed of CFRP, the bond strength of HFRP was found to be more similar to that of CFRP samples than GFRP samples. The study concluded that the bond strength of all the samples was predominantly influenced by the surface roughness of the inner tube rather than the fibre type.

Low-Velocity Impact Resistance of Glass Laminate Aluminium Reinforced Epoxy (GLARE) Composite

This study intends to determine the behavior of glass laminate aluminum-reinforced epoxy (GLARE) and glass fiber-reinforced polymer (GFRP) composites under a low-velocity impact test. Experimental tests and numerical simulations are considered for this investigation. All samples are made by the hand lay-up method. Moreover, specimens are produced with a 7075-T6 aluminium sheet with a 0.5 mm thickness, resin 3001, and E-glass fiber. The drop weight test performs the low-velocity impact at 6.7 J and 10 J impact energy levels and the heights of 1.0 m and 1.5 m. Numerical simulation is also conducted by ANSYS software to compare the results obtained by the experimental tests. Generally, results show that maximum deflections of the GLARE samples are significantly lower as compared to GFRP ones by 87% and 83.5% for 1.0 m and 1.5 m drop heights, respectively. Experimental results demonstrate that although aluminum sheets prevent damage to the fibers in GFRP, delamination and fractures between layers are observed in GFRP samples. An appropriate agreement is also obtained between the FE results and experimental data.

Quasi-brittle fracture performance of glass fiber composites based on boundary effect model

Light surface scratch around 50 μm or less in-depth was a formidable engineering challenge for the application of quasi-brittle materials. In the present study, a boundary effect model (BEM) was used to analyze the fracture performance of glass fiber reinforced polymer (GFRP) composites with shallow surface scratch. The ply thickness of single-layer prepreg was introduced as the controlling GFRP microstructure parameter or characteristic composite unit Cch of the model, and the parameter of ply thickness was set to 1.2 mm. Quasi-brittle transverse fracture parameters of the tensile strength ft and the fracture toughness KIC of GFRP were discussed. Finally, the average values of the fracture parameters uf and uk of the GFRP were obtained by normal distribution analysis. The means, and upper and lower bounds of the fracture parameters were within the range of 95% reliability covering almost all test discrete points. The laboratory samples of conventional sizes were used to predict the fracture parameters of GFRP. The result reflected that the error between the test results of all samples is only 5.7%. In addition, as the ratio α of the gap height of the GFRP sample increases, the fracture parameters showed a trend of increase firstly and then declining. The average fracture parameter estimated value has decreased by 14.5%, which was in good agreement with the test results.

Experimental Study and Artificial Neural Network Simulation of Cutting Forces and Delamination Analysis in GFRP Drilling

This paper reports the results of measurements of cutting forces and delamination in drilling of Glass-Fiber-Reinforced Polymer (GFRP) composites. Four different types of GFRP composites were tested, made by a different manufacturing method and had a different fiber type, weight fraction (wf) ratio, number of layers, but the same stacking sequence. GFRP samples were made using two technologies: a novel method based on the use of a specially designed pressing device and hand lay-up and vacuum bag technology process. The study was conducted with variable technological parameters: cutting speed vc and feed per tooth fz. The two-edge carbide diamond-coated drill produced by Seco Company was used in the experiments. Cutting-force components and delamination factor were measured in the experiments, and photos of the holes were taken to determine the delamination. In addition, modeling of cause-and-effect relationships between the technological drilling parameters vc and fz was simulated with the use of artificial neural network modeling. For all tested GFRP materials, an increase in fz led to an increase in the amplitude of cutting-force component Fz. The lowest values of the amplitude of cutting-force component Fz were obtained with the lowest tested feed per tooth value of 0.04 mm/tooth for all tested materials. It was observed that materials produced with the use of the specially designed pressing device were characterized by lower values of the cutting-force component Fz. It was also found that the delamination factor increased with an increase in fz for all tested GFRP materials. A comparison of the lowest and the highest values of fz revealed that the lowest delamination factor increase was archived by the B1 material and amounted to about 12.5%. The error margin of the obtained numerical modeling results does not exceed 15%, so it can be concluded that artificial neural networks are a suitable tool for modeling cutting force amplitudes as a function of vc and fz. The study has shown that the use of the special pressing device during the manufacturing of composite materials has a positive effect on delamination.

The resistance to UV radiation for GFRP pultruded bridge panels

Depending on the type of the load which affects the durability and design life glass fibre reinforced polymer (GFRP) structures should be designed so as to take into account as first of all the chemical-physical conditions in which the structure is used including: ultraviolet (UV) radiation, temperature influences, humidity, water and chemicals. The results presented herein provide a predictions regarding of the mechanisms involved in the ageing of GFRP pultruded bridge profiles and predicting the property micro scale changes with time and remaining service life of GFRP under real environmental degradation impacts and during simulation laboratory conditions. The outermost layers of FRP (fibre reinforced polymer) composites are damaged mostly because of UV radiation. Radiation also induces remarkable microstructural changes depending on wavelength and intensity, and oxygen availability, eventually leading to polymer chain scission. A scanning electron microscope (SEM) was used to investigate the degradation mechanism of the GFRP samples subjected among others to UV radiation and water vapor condensation. Glass fibre-reinforced polymer GFRP pultruded profiles have great potential in the construction industry, presenting several advantages comparing with traditional materials, among which, the potentially improved durability under environmental influents.

Influence of Loading Direction on Impact Strength and Small Span Length Variation on Flexural Strength in GFRP Laminate

Abstract This experimental work focuses on the two aspects of glass fiber–reinforced polymer (GFRP) laminate. The first is the effect of GFRP plate thickness and impact loading direction on impact strength, and the second is the effect of GFRP panel thickness and small span length variation on flexure strength. The GFRP laminate thicknesses considered for experimentation were 2, 3, and 4 mm. Both Izod and Charpy impact tests were conducted by considering edgewise (lateral impact) and flatwise (normal impact) impact loading directions. The quasistatic flexural tests were conducted using a three-point bend method for two small span length variations, viz., 50.8 and 56 mm. Impact strength of 2-mm-thick GFRP laminate under the edgewise loading condition showed 13 and 134.1 % more for Charpy and Izod, respectively, compared with 4-mm GFRP samples. In contrast, under the flatwise loading condition, 2-mm-thick specimens showed 14 and 54 % more impact strength when compared with 4-mm-thick GFRP specimens. GFRP samples with 2-mm thickness showed 59.85 and 59.6 % enhancement in flexural strength for span lengths of 56 and 50.8 mm, respectively, when compared with 4-mm-thick GFRP panels. Improvement in flexural strength between span lengths 50.8 and 56 mm for thicknesses 4 and 2 mm is 26 and 42 MPa, respectively, which is a 10 % improvement. Analysis of variance statistical analysis showed that the loading conditions and thickness of GFRP panels in the case of impact and small span length variation, and thicknesses in the case of flexural strength, is the significantly influencing parameters.

Electrical characterization of glass fiber reinforced polymer (GFRP) composites for future metasurface antenna applications

In this paper, the Glass Fiber Reinforced Polymer (GFRP) composite samples are explored in order to evaluate their feasibility and adaptability for use in future metasurface antenna application. Multi-layer GFRP composite samples are fabricated with a proportionate ratio of resins and fiber using Vacuum Assisted Resin Transfer Molding (VARTM) technique. N type to waveguide (WR-187) adapter specially designed for electrical characterization of these GFRP composite samples is used. Thru-Reflect-Line (TRL) calibration technique is used for the test setup, and scattering parameters of these GFRP samples is measured by using the manufactured adapter along with the sample holder on a two port Vector Network Analyzer (VNA). Relative permittivity and dielectric loss tangent of GFRP composite samples are computed using Nicholson-Ross-Weir (NRW) and New Non-Iterative conversion methods. The comparative analyses of both methods showed a very good agreement between them. The GFRP sample with the lowest relative permittivity is short listed for its possible application in future metasurface antenna.

High-Speed Time- and Frequency-Domain Terahertz Tomography of Glass-Fiber-Reinforced Polymer Laminates with Internal Defects

Composite materials are increasingly being utilized in many products, such as aircrafts, wind blades, etc. Accordingly, the need for nondestructive inspection of composite materials is increasing and technologies that allow nondestructive inspection are being studied. Existing ultrasound methods are limited in their ability to detect defects due to high attenuation in composite materials, and radiographic examination methods could pose a danger to human health. Terahertz (THz) wave technology is an emerging approach that is useful for imaging of concealed objects or internal structures due to high transmittance in non-conductive materials, straightness, and safety to human health. Using high-speed THz tomography systems that we developed, we have obtained THz tomographic images of glass-fiber-reinforced polymer (GFRP) laminates with artificial internal defects such as delamination and inclusion. The defects have various thicknesses and sizes, and lie at different depths. We present THz tomographic images of GFRP samples to demonstrate the extent to which the defects can be detected with the THz tomography systems.

Degradation of Glass Fiber Reinforced Polymer (GFRP) Material Exposed to Tropical Atmospheric Condition

Fibre reinforced polymers (FRPs) have emerged as popular materials for structural application in recent decades due to numerous of advantages. Despite the growing body of research on the use of glass fibre reinforced polymers (GFRP) composites in repairing and retrofitting the important structures such as oil and gas pipelines, the lack of comprehensive data on the long-term degradation mechanism for these materials is still impeding their widespread use in open-air structures repairs particularly in tropical climate locations such as Malaysia. Therefore, this paper presents an experimental investigation to determine the influence of tropical atmospheric condition on tensile properties of the GFRP. In this study, a set of GFRP samples were fabricated using epoxy resin as polymer matrix and woven E-glass fibres as reinforcing materials. These samples were exposed to tropical atmospheric condition in Malaysia for a period of four months. Tensile test was carried out for each sample before and after four-months period of exposure. The experimental tensile test results recorded a 15% reduction in tensile strength after 4 months of exposure as compared to its original strength. Further, the dominant failure mode of the exposed sample was characterized with longitudinal splitting of the fibres without completely breaking out. Overall, the tropical atmospheric condition has a noticeable impact on the GFRPs tensile strength degradations over the exposure duration.

X-ray imaging techniques for inspection of composite pipelines

The literature has shown that the application of laminography provides advantages as 3D radiographic imaging with depth information for in house and mobile testing. This permits to distinguish between overlapping indications, measure the extension along radiation direction and classify indications as surface open or subsurface ones as required in critical engineering assessment. This work provides a comparative study and measurements of the three techniques Digital Radiography (DR) with Digital Detector Arrays (DDA), Coplanar Translational Laminography (CTL) and Computed Tomography (CT), applied for composite pipeline inspection. It is demonstrated that CTL and CT provide advantages for the evaluation of pipe-to-pipe connections and the evaluation of adhesive applications. They show indications of discontinuities with higher contrast sensitivity than radiography. Beyond it, two specimen, namely Phantom 1 and Phantom 2, were developed and manufactured by additive manufacturing to analyze the preferential detection sensitivity and the direction of features and depth information for laminographic measurements. Another goal was to show the laminographic capabilities to distinguish between overlapping discontinuities. CTL is especially suitable for mobile inspection. Special glass fiber reinforced polymer samples (GRP) were manufactured for further analysis and comparisons between the abovementioned techniques. Finally, Phantoms 1 and 2 show the capability of laminography to detect overlapping indications and also show that discontinuities oriented perpendicular to the scan direction have the highest contrast sensitivity for laminographic measurements.

Tensile after impact response on the scarf repaired glass fiber reinforced polymer samples – Experimental approach

Abstract The effect of impact energy on the scarf repaired/primary and the resultant residual tensile strength is discussed in this research paper. The low-velocity impact test was carried out at the center of the repaired and primary samples by varying impact energies and subsequently the damage assessment was done through damage dent, damage area and visual inspection of each impacted sample. Moreover, in order to visualize the microscopic damage and to get a clear picture of internal damage, SEM (Scanning electron microscopy) was employed on impacted samples. Additionally, the impact reaction performance of the impacted sample was compared with the primary samples by taking into account the contact force, displacement, absorbed energy, impact dent, impact time, and the damaged area. It was observed that the contact force was highest for the sample that was impacted with maximum impact energy. On calculation of the residual strength for both types of impacted samples it is found that the scarf repair sample is capable to recover 81.23% of the tensile strength. The tensile load percentage variations of the repaired samples when compared to primary samples are found to be 16.76%, 15.66%, 15.40%, 14.31%, and 13.51%. On application of this method, samples attained the least impact response (High impact energy) and during the process revealed the least tensile strength during impact after tensile test.

Depth resolved pulse compression favourable frequency modulated thermal wave imaging for quantitative characterization of glass fibre reinforced polymer

InfraRed Thermography is an expeditiously growing non-destructive testing and evaluation technique widely used to estimate the subsurface defect details in fibre reinforced polymer materials due to its remote, fast, easy, safe and wide area monitoring abilities. In order to explore the details of subsurface anomalies in active infrared thermography, a modulated heat flux with a predefined intensity and bandwidth is employed to excite the test sample. Among various widely used active infrared thermographic techniques for non-destructive testing and evaluation applications, pulse based and modulated lock-in thermography are predominantly in use. However, these conventional techniques limits their applicability due to high peak power heat source requirements (pulse based techniques) and poor resolution (modulated lock-in thermography) for detecting the defects located at various depths of different lateral dimensions. The present paper highlights a pulse compression favourable frequency modulated thermal wave imaging which can be carried out with moderate peak power heat sources in a limited span of time with improved test resolution and sensitivity. Further the proposed experimentation is validated with the analytical as well as numerical simulations on glass fibre reinforced polymer samples having flat bottom hole defects.