Carbon Fiber Reinforced Polymer Research Articles

Utilizing a combination of experimental and machine learning methods to predict and correlate between accelerated and natural aging of CFRP/AL adhesive joints under hygrothermal conditions

AbstractThis study investigates how carbon fiber reinforced polymer (CFRP)‐to‐aluminum adhesive joints behave under accelerated aging conditions with hygrothermal exposure and compares these findings against naturally aged samples to evaluate material reliability in challenging environments. The CFRP‐to‐aluminum adhesive joints were manufactured and subjected to natural aging for durations ranging from 1 to 3 years with 6‐month intervals, as well as accelerated aging (hygrothermal) for periods ranging from 100 to 1200 h, with intervals of 50 h. Subsequently, the mechanical properties of the joints were evaluated using a three‐point bending test. To forecast natural aging times from accelerated aging data, five machine learning models were utilized: artificial neural network, support vector regression, linear regression, polynomial regression, and random forest regression. Hygrothermal aging significantly degraded the matrix, causing a shift in failure modes from cohesive to mixed types (cohesive, adhesive, and fiber tear failures), leading to a notable decline in bending strength. The study observed a 23.13% strength reduction in samples aged naturally for 3 years and a 24.33% decrease in those subjected to 1000 h of accelerated aging. The random forest regressor demonstrated superior accuracy in predicting natural aging times across different accelerated aging periods. Through the application of machine learning models, this study introduces a novel approach to forecast natural aging durations using data from accelerated aging experiments. This method shows potential for optimizing joints and composite structures, ultimately improving their durability and minimizing the likelihood of failures during operational use.Highlights Studied hygrothermal effects on accelerated aging of carbon fiber reinforced polymer/Aluminum (AL) adhesive joints. Noted strength reduction from hygrothermal aging. Used five machine learning models; random forest regression had the highest accuracy. Analyzed correlation between natural and accelerated aging of dissimilar adhesive joints.

Strength Retention of Carbon Fiber/Epoxy Vitrimer Composite Material for Primary Structures: Towards Recyclable and Reusable Carbon Fiber Composites

Recently, the growth of the recyclability of carbon fiber reinforced polymer (CFRP) composites has been driven by environmental and circular economic aspects. The main aim of this research work is to investigate the strength retention of a bio-based vitrimer composite reinforced with carbon fibers, which offers both recyclability and material reusability. The composite formulation consisted of an epoxy resin composed of diglycidyl ether of bioshpenol A (DGEBA) combined with tricarboxylic acid (citric acid, CA) and cardanol, which was then reinforced with carbon fibers to enhance its performance. Differential scanning calorimetry (DSC) and Fourier transform infrared (FTIR) spectroscopy were performed to analyze the chemical composition and curing behavior of the vitrimer. Mechanical testing under tensile loading at room temperature was carried out on epoxy, vitrimer, and associated carbon fiber reinforced composite materials. The results demonstrated that the DGEBA/CA/cardanol vitrimer exhibited thermomechanical properties comparable to those of an epoxy cured with petroleum-based curing agents. It was observed that the maximum tensile strength of vitrimer is about 50 MPa, which is very close to the range of epoxy resins cured with petroleum-based curing agents. Notably, the ability of the vitrimer composite to be effectively dissolved in a dimethylformamide (DMF) solvent is a significant advantage, as it enables the recovery of the fibers. The recovered carbon fiber retained comparable tensile strength to that of the fresh carbon composites. More than 95% strength was retained after the first recovery, which confirms the use of fibers for primary and secondary applications. These research results open up new avenues for efficient recycling and contribute to the overall sustainability of the composite material at an economic level.

Use of bore-epoxy anchorage system with CFRP laminates for shear strengthening of RC beams: An experimental and analytical investigation

Shear strengthening of reinforced concrete (RC) beams with externally bonded reinforcement (EBR) using carbon fiber reinforced polymer (CFRP) plates and sheets have become a widely accepted practice. To avoid the associated premature debonding failure, this study presents an experimental and analytical investigation on the use of bore-epoxy as an anchorage system for CFRP plates and sheets bonded on both sides of shear deficient RC beams of rectangular cross-sections. Nine (9) RC beams were prepared and strengthened with CFRP plates and sheets as EBR with bore-epoxy anchorage system considering different bore diameters and tested under four point bending. The results indicate that RC beams strengthened with bore-epoxy anchorage system have high shear strength as compared with the unstrengthened control beam and those strenghtended with the conventional EBR approach (without boring). The increase in the shear strength over the control beam was found to be up to 67 % and 121 % for cases of CFRP plates and sheets, respectively. Moreover, the increase of shear strength in comparison with the conventional EBR method ranged from 9.33 % to 20.36 % for CFRP plates and from 12.97 % to 36.10 % for CFRP sheets. Consistently, the contribution of bore-epoxy on the shear strength increased with the increase of bore diameter. Consequently, the existing formulas in the ACI440.2 R for predicting the shear strength of FRP strengthened RC beams were modified to incorporate the improvement made by the proposed bore-epoxy approach. The revised models predicted the experimental shear strength of RC beams, strengthened with bore-epoxy, with a good level of accuracy with average mean absolute percentage error (MAPE) = 6.07 %, 14.71 %, normalized mean square error (NMSE) = 0.097, 0.16, and coefficient of determination R2 = 0.912, 0.974 for plates and sheets, respectively.

Machining-Induced Burr Suppression in Edge Trimming of Carbon Fibre-Reinforced Polymer (CFRP) Composites by Tool Tilting

Several challenges arise during edge trimming of carbon fibre-reinforced polymer (CFRP) composites, such as the formation of machining-induced burrs and delamination. In a recent development, appropriate-quality geometric features in CFRPs can be machined using special cutting tools and optimised machining parameters. However, these suitable technologies quickly become inappropriate due to the accelerated tool wear. Therefore, the main aim of our research was to find a novel solution for maintaining the machined edge quality even if the tool condition changed significantly. We developed a novel mechanical edge-trimming technology inspired by wobble milling, i.e., the composite plate compression is governed by the proper tool tilting. The effectiveness of the novel technology was tested through mechanical machining experiments and compared with that of conventional edge-trimming technology. Furthermore, the influences of the tool tilting angle and the permanent chamfer size on the burr characteristics were also investigated. A one-fluted solid carbide end mill with a helix angle of 0° was applied for the experiments. The machined edges were examined trough stereomicroscopy and scanning electron microscopy. The images were evaluated through digital image processing. Our results show that multi-axis edge-trimming technology produces less extensive machining-induced burrs than conventional edge trimming by an average of 50%. Furthermore, we found that the tool tilting angle has a significant impact on burr size, while permanent chamfer does not influence it. These findings suggest that multi-axis edge trimming offers a strong alternative to conventional methods, especially when using end-of-life cutting tools, and highlight the importance of selecting the optimal tool tilting angle to minimize machining-induced burrs.

Internal shear damage evolution of CFRP laminates ranging from −100 °C to 100 °C using in-situ X-ray computed tomography

Carbon Fiber Reinforced Polymer (CFRP) composites have been widely used in aerospace due to their high specific stiffness, strength, and fatigue properties. However, the ambient temperature significantly influences CFRP's mechanical properties and damage evolution, deriving from the temperature effect on the microstructural behavior and the mesoscopic damage evolution. In this study, the temperature-dependent in-plane shear failure behavior of CFRP composites was investigated. In-situ X-ray Computed tomography (CT) tensile experiments of laminates ([45°/-45°]2s) at RT, −100 °C, and 100 °C were carried out to study the in-plane shear failure mechanisms. The 3D fracture morphology was extracted with internal damage evolution process estimated and quantified. The in-situ 3D deformation fields of critical regions were acquired using the Digital Volume Correlation (DVC) method. The effect of temperature on strain field and the correlation between the high-strain region and the fracture location were analyzed. The results revealed the temperature correlations and failure mechanisms of CFRP's mechanical characteristics and internal damage evolution process. Compared to room temperature (RT), the delamination damage area of the sample increased by 80 % at 100 °C. Meanwhile, the shear modulus of CFRP decreases by 78.4 % from −100 °C to 100 °C, and the fracture strain increases by 95 % from RT to 100 °C. The DVC results indicated a dispersion of high-strain regions at −100 °C, reflecting the brittle damage characteristics, while an extensive ductile deformation region was captured at 100 °C. Fiber-matrix debonding is the dominant failure mode of composites under shear loading, whereas significant matrix cracking was observed at −100 °C and partial fiber pullout occurred at 100 °C.

Local wavenumber imaging in anisotropic structures

Local wavenumber imaging has been proven effective for damage detection in thin-walled structures. Such structures are, however, often inherently anisotropic, posing serious challenges to the known algorithms of local wavenumber mapping. The use of local wavenumber mapping algorithm designed for isotropic, rather than anisotropic, structures results in processing artifacts that may prohibit correct detection and characterization of defects. Therefore, in this work, we propose an improved implementation of the local wavenumber estimation algorithm for anisotropic media (ALWE). The working principle and efficiency of the proposed algorithm are illustrated on datasets coming from numerical simulations and experimental testing on anisotropic carbon fiber reinforced polymer (CFRP) laminates. We demonstrate that the proposed algorithm successfully compensates for the processing artifacts and enhances the overall quality of the wavenumber maps facilitating unequivocal damage detection.

Structure design and ultimate strength envelope of a modular carbon fiber-reinforced polymer (CFRP) laminate tube box on a 5000 ft deep-sea ultra large unmanned underwater vehicle

Deep-sea tube box is a crucial and basic structural component of various deep-sea equipment such as deep-sea submarine and unmanned underwater vehicles. The deep-sea ultra large unmanned underwater vehicle (ULUUV) is the most representative one of them. Tube boxes are the basic modular members on the ULUUV, which are widely used as the load-bearing hull in torpedo launch, load transportation, pipeline protection and etc. Due to the extremely harsh operation condition, deep-sea tube box simultaneously suffers extremely high lateral water pressure and tremendous axial compression induced by water pressure. Considering the strict requirements for lightweight, high strength, and corrosion resistance of deep-sea tube boxes, carbon fiber-reinforced polymers (CFRP) have become the most potential choice for the construction materials of future deep-sea tube box. Thus, the aim of this paper is to make a structure design of a deep-sea CFRP tube box which can operate at the water depth of 5000 ft. In this paper, a detailed design process of the deep-sea CFRP tube box was introduced, including geometrical configuration, critical buckling load prediction method, FE modeling technique and experimental validation. Corresponding to 5 ply schemes, 495 cases of FE analyses were performed to obtain the ultimate strength envelopes of the CFRP tube box subjected to combined axial compression and external lateral water pressure. Based on the obtained results, the best ply scheme for the deep-sea CFRP tube box was figured out. This paper could provide design method and data references for deep-sea CFRP structures which are subjected to combined loads.

Equivalent thickness method of CFRP in chloride ion diffusion equation for CFRP–strengthened RC members and its load capacity assessment method

Carbon fiber reinforced polymer (CFRP)–strengthened reinforced concrete (RC) structures exhibit resistance to chloride ion penetration. However, accurately quantifying this resistance and predicting the load capacity of CFRP–strengthened RC beams subjected to chloride ion erosion requires further investigation. This paper introduces two equivalent thickness methods for non-fully bonded CFRP within the chloride ion diffusion equation: first method assumes CFRP having a fixed equivalent thickness, while the second method allows for CFRP having a linearly decreasing equivalent thickness depending on the depth within the concrete. A model is established between the reduction factor of load capacity in CFRP–strengthened RC beams and the concentration of diffusing chloride ions. Based on experimental results from our research group regarding chloride ion erosion in CFRP–strengthened RC members, both the equivalent thickness method and the load capacity model for CFRP were evaluated. The findings indicate that the chloride ion concentration predicted by the second equivalent thickness method outperforms the first, with an error of 13.1%. Furthermore, this method accurately predicts the chloride ion concentration after 60 days of exposure, exhibiting an error of 7.0%. Additionally, employing the proposed load capacity model, derived from the second equivalent thickness method, the calculated load capacity of CFRP–strengthened RC members subjected to chloride ion erosion shows only a 5.2% deviation from experimental results. This study concludes that the equivalent thickness of non-fully bonded CFRP decreases linearly with depth in concrete and establishes a direct correlation between load capacity and chloride ion concentration, thereby offering more targeted characterization methods.

Lightning Damage Evaluation in Carbon Fiber Composite Polymer Structure

While metallic aircraft structures are largely unaffected by lightning strikes, not the same statement could be made about carbon fibre reinforced polymer (CFRP) ones. CFRPs have low electrical conductivity and high anisotropy, and for this reason they are covered with metallic meshes, generally of aluminium or copper, to redistribute the electrical charge when the plane encounters lightning events. During these phenomena, electrical discharges could lead to physical damage, called direct effects, such as: ablation, erosion, explosion, structural weakening and degradation. Although relatively rare, when aircraft is affected by lightning strikes, structural inspections are necessary to determine the need and magnitude of repairs. Since the extent of lighting strike damage on laminated composite is difficult to determine and quantify visually, other non-destructive evaluation (NDE) techniques must be used. In this work, we complement established techniques, such as pulse-echo ultrasonics, X-ray radiography, eddy currents, and thermography with a novel investigative approach, based on planar capacitive sensing and imaging. This has the advantage of being non-contact, and requiring only single-side access to the part. The technique’s capabilities to detect dielectric changes and breakages in conductive layers make it an ideal candidate for the investigation of CFRP aircraft structure for lightning effects. The outcomes of capacitive imaging inspections on laboratory introduced lightning damage on CFRP panels are discussed along the advantages and disadvantages of the technique compared to more established ones.

Highly protective and functional strengthening strategies for 3D printed continuous carbon fiber reinforced polymer composites: manufacturing and properties

Carbon fiber-reinforced polymer (CFRP) composites have gradually emerged as a crucial material in the aerospace industry. However, inadequate impact resistance and thermal stability limit survival in extreme environments. Inspired by the specific functions of multiple layers of tissue, from bird feathers to the musculoskeletal system. We propose low-cost composite strengthening strategies and efficient novel three-dimensional graphene aerogel manufacturing methods. A novel graphene aerogel/carbon fiber reinforced polymer (GAC) composite prepared by in-situ laser-induced graphene (LIG) layers on the surface of 3D-printed CFRP. GAC composites enhance CFRP's impact protection, thermal insulation, and electromagnetic shielding capabilities. For protection, GAC composites reduce low-velocity impact damage by 26.2%. The graphene layer can increase the thermal buffering capacity of the material four times, and the long-term service temperature can be increased by 40% compared to traditional CFRP. For functionality, the composites enable electromagnetic interference (EMI) shielding effectiveness of up to 50.2 dB. Furthermore, it can achieve surface heating above 400 °C and rapid de-icing through electrothermal effects. The simple and efficient processing method of GAC composites and tunable functionalities holds promise for the development of highly protective and intelligent aircraft skins.

Research on fatigue life of carbon fiber reinforced polymer strengthened single-sided butt welded joints utilizing resin pre-coating for strong adhesive bonding

Single-sided butt welding is a common connection method used in box beam structures such as excavator booms. Single-sided butt weld joints are prone to fatigue failure since the structures are subjected to cyclic loads over extended periods. The purpose of this study is to improve the fatigue performance of single-sided butt welded joints. Carbon fiber reinforced polymer (CFRP) strengthened single-sided butt welded joint specimens with permanent backing plates were prepared by pre-coating method, and constant-amplitude tension–tension fatigue tests were carried out on the specimens. The effects of welding groove parameters and CFRP reinforcement on fatigue life were studied. The test results show that the CFRP-strengthened specimens without blunt edges and with a groove angle of 40° show the highest average fatigue life. The single-sided bonding of 10 layers of CFRP sheets effectively inhibits the propagation of fatigue cracks and significantly improves the fatigue life of welded joints. In the CFRP/welded joint composite structure, the strain monitoring results show that the peak strain of the interface is the largest in the middle position, and the peak strain rises sharply in the rapid crack propagation stage, which indicates that the real-time interface strain peak can reflect the fatigue crack propagation. It provides an early warning method for real-time monitoring of fatigue life failure of welded structures in practical engineering.

The axial compression mechanical performance of CFRP-confined fully replaced nonnatural coal gangue aggregate columns after freeze[sbnd]thaw cycles

With the continuous mining of coal worldwide, the accumulation of coal gangue, a byproduct of coal production, has increasingly posed severe environmental challenges. To mitigate the environmental pollution caused by coal gangue, this paper proposes the use of processed coal gangue as a replacement for coarse aggregates in concrete. Additionally, carbon fiber-reinforced polymers (CFRP) are employed to confine these square concrete columns, with the aim of expanding the application of nonnatural coal gangue concrete in freezethaw environments. Taking the number of CFRP layers, number of freezethaw cycles, and concrete strength grade as variables, a total of 54 samples were designed. Experimental and theoretical analyses were conducted to investigate the mechanical behavior of CFRP-confined fully replaced nonnatural coal gangue aggregate (FNCGA) columns. The specific conclusions are as follows: (1) Under a freezethaw environment, the stressstrain behavior of the CFRP-confined FNCGA columns undergoes three stages: an elastic ascending stage, a plastic descending stage, and a plastic ascending stage. (2) When the number of freezethaw cycles increases from 100 to 300, the Poisson's ratio of the elastic stage of the sample exhibiting an upward trend. (3) For the three-layer CFRP confined sample, the volumetric strain is positive (in compression), and the bearing capacity of the sample exceeds that of the unconfined concrete by approximately 10%. (4) Compared with existing models and experimental results, the bearing capacity and stressstrain model proposed in this study have greater accuracy, enabling better simulation of the mechanical performance of CFRP-confined FNCGA columns.

Electrical energy and overpressure characterization of aeronautical fasteners submitted to a lightning current waveform

Understanding and controlling sparking in fasteners and jointed structures is crucial for flight safety, particularly in fuel tanks. This study investigates the relationship between dissipated electrical energy and pressure buildup within fastener cavities during lightning strikes. Experiments were performed on fasteners installed in aluminum and Carbon Fiber Reinforced Polymer (CFRP) samples, with lightning current waveforms ranging from 1 kA to 10 kA. A sensitivity analysis evaluated the influence of key parameters, such as current peak, clearance fit, sample material, polarity, and fastener coating on pressure rise and energy dissipation. Electrical energy up to 80J and pressure levels reaching 600 bar– unprecedented compared to prior studies – were observed. The pressure-energy relationship showed an approximately linear trend, while pressure exhibited a non-linear dependence on clearance fit. Fastener coating and sample material were found to significantly influence the results, with pressure variations reaching up to a factor of 10 in some cases.

A New Method for Detecting Surface and Subsurface Defects in Carbon Fiber- Reinforced Polymer (CFRP) Unidirectional Laminates

A New Method for Detecting Surface and Subsurface Defects in Carbon Fiber- Reinforced Polymer (CFRP) Unidirectional Laminates

Experimental and numerical study of corrugated anchors for CFRP plates

Carbon fibre reinforced polymer (CFRP) plates are promising materials for cables and prestress tendons in structural engineering for the excellent tensile performance, light in weight and anti-corrosion character. Anchoring has always been the main challenge of this high-performance material due to its anisotropy and brittleness. Corrugated anchors are high-efficiency friction-based anchorage devices. Nevertheless, premature failure of the CFRP plates can still be observed with this type of anchors, and the anchor efficiency for the corrugated anchors needs further improvement. This paper proposes a novel configuration of corrugated anchors. Experimental tests are then carried out to investigate the influence of the pre-tensioning bolt load and adhesive layer. Consequently, numerical analysis is conducted to reveal the anchoring mechanism. The results indicate the traditional corrugated anchors lead to non-uniform stress distribution in the CFRP plate, and with the proposed configuration the anchor efficiency can be increased up to 100%.

Reliability-based multi-objective optimization design of composite patch repair structure using artificial neural networks

Due to the parameter differences and the existence of random factors, the performance of composite repair structure shows a large dispersion. In order to obtain stronger, lighter and more reliable repair structure, multi-objective optimization design of carbon fiber reinforced polymers (CFRP) laminate repair structure is investigated by combining the reliability theory, artificial neural networks and the genetic algorithm. First, a 3D simulation model of the composite laminate patch repair structures is established using the 3D Hashin criterion and the cohesive region model. Then, the Latin Hypercube sampling (LHS) method is used to realize the random sampling, and the strength proxy model of the repair structure is established by using Back-propagation artificial neural network, further a multi-objective optimization model with tensile strength, reliability and weight as objective functions is built considering the design parameters and the random parameters. Finally, NSGAⅡalgorithm is used to solve the multi-objective optimization problem, and a set of solutions on the Pareto front surface are obtained, also the optimal design parameters of the composite repair structure meets the requirements is obtained.

Tailoring magnesium potassium phosphate cement-based adhesive for the improved bonding performance of CFRP/concrete interface

Magnesium potassium phosphate cement (MKPC) with rapid hardening and early high strength can be used as inorganic adhesive for externally bonded carbon fiber reinforced polymer (CFRP). This study aims to tailor MKPC as inorganic adhesive with good infiltration and bonding strength for CFRP-reinforced concrete. The influences of water-to-binder (w/b) ratios and particle sizes of potassium dihydrogen phosphate (PDP) on the fresh properties, hydration kinetics, pore structure, and strength development of MKPC were studied. The bonding strength of CFRP/concrete with MKPC-based adhesive was evaluated by double-shear tests. Results showed that a lower w/b ratio and a finer PDP enhanced the strength of MKPC due to a refined microstructure. The bonding strength of CFRP/concrete was dependent on the rheological characteristics and strength of MKPC. The use of a coarse PDP in MKPC was beneficial to enhance the bonding strength of CFRP/concrete. MKPC with coarse PDP and a w/b ratio of 0.2 resulted in the greatest bonding strength, corresponding to a 20% enhancement compared to epoxy resin. The superior bonding performance of MKPC was ascribed to its higher strength, a better infiltration for CFRP, and a larger MPC/concrete interfacial transition zone (ITZ).

An ultrasonic Lamb wave-based non-linear exponential RAPID method for delamination detection in composites

Accurate detection of defects, particularly delamination, in carbon-fiber reinforced polymer (CFRP) composites is crucial but challenging. This study proposes a baseline-free Lamb wave damage imaging framework that incorporates an adaptive time-reversal technique, and a nonlinear exponential reconstruction algorithm for probabilistic inspection of defects (NE-RAPID) in composites. The framework combines two image fusion strategies: full-summation and full-multiplication. NE-RAPID enhances the traditional RAPID algorithm by replacing linear weights with faster-decaying exponential weights, which improves the localization of delamination and other defect regions with higher resolution. A nonlinear exponential weight is introduced to address uneven probability distributions caused by the non-uniform density of the sensor network, thereby improving the accuracy and reliability of defect detection, including delamination. Experimental validation on CFRP composite plates demonstrates that NE-RAPID significantly outperforms RAPID. NE-RAPID achieves a maximum detection error of only 5.1 mm across different frequencies, while RAPID shows a much higher error of 34.41 mm. Furthermore, NE-RAPID produces sharper damage images with fewer artifacts, significantly reducing the risk of false positives. These findings indicate that NE-RAPID is a highly promising method for precise and reliable delamination detection in composite materials.

Laser-engraved holograms as entropy source for random number generators

Our study introduces a novel approach to true random number generation (TRNG) using speckle patterns generated by laser-engraved holograms on carbon fiber-reinforced polymer (CFRP) composite substrates. Unlike previous methods, our approach simplifies the process by generating the necessary image dataset from a single microscope image of the engraved hologram. We achieve a high extraction ratio of 76 %, demonstrating the effectiveness of our TRNG. Moreover, our method successfully passes rigorous statistical tests proposed by the National Institute of Standards and Technology (NIST), indicating its suitability for cryptographic and secure system applications. This work offers promising implications for enhancing security in various domains, from secure communication networks to IoT devices.

Image enhancement methods for inspection of planar and non-planar FRP structures using a noise-based microwave NDT inspection system

Microwave nondestructive testing (MNDT) includes inspection techniques that assess a particular material’s health status using low-power and contactless inspection systems. In near-field microwave inspections, imaging results are heavily influenced by the standoff distance parameter, i.e., the physical separation between the microwave sensor and the sample under test (SUT). Variations in the standoff distance during an inspection tend to cause defect masking of disbonds and delaminations in fiber-reinforced polymer (FRP) materials, causing defects to go undetected frequently. An MNDT near-field inspection system using noise waveforms is used to identify engineered internal defects within carbon fiber-reinforced polymer (CFRP) samples. Tactics utilizing Principal Component Analysis (PCA), Stacked Sparse Autoencoders (SSAEs), and an autoencoder network trained in a manner for anomaly detection are used to minimize the effects of standoff distance, reduce defect masking, and increase the ability to identify hidden defects. The samples tested are constructed to possess planar and non-planar geometries, such that the viability of the data-driven image enhancement and standoff distance correction methods are demonstrated with respect to a wide variety of in-situ inspection applications.