Adherend Surface Research Articles

Direct observation of oil-surface adhesion via sum frequency generation spectroscopy

ABSTRACT Adhesive technology can joint dissimilar lightweight materials that are used in the automotive industry to improve fuel efficiency. However, the strength of the adhesive bonding is often reduced by oil contaminants on the adherend surfaces. Adhesives should repel the oil layer and diffuse onto the adherend surface, but there is no general design for reliable adhesion on contaminated adherends because of the poor molecular-level understanding of oil-surface adhesion. Here, we report the direct observation of oil-surface adhesion at a two-part epoxy adhesive/aluminum oxide (AlOx) interface via sum frequency generation (SFG) spectroscopy, a method that can probe buried interfaces. The AlOx surface is intentionally contaminated with commercial silicone oil. The SFG spectra of the cured epoxy adhesive/oil-contaminated AlOx interface suggest that the silicone oil remains after curing, explaining the reduction in adhesion strength. Triethylenetetramine, a hardener used in this study, can repel the oil layer, whereas bisphenol A epoxy resin is much less effective at removing the oil layer upon contact. Thus, the migration of both adhesive components onto the AlOx surface is key to securing sufficient oil-surface adhesion. We also applied SFG to second-generation acryl adhesive (SGAA)/AlOx interface and found that SGAA completely repels the oil from the aluminum interface.

Effects of polybutylene terephthalate molecular weight and nanostructure size of copper surface on replication quality of nano-injection molding under practical condition: A molecular simulation study

Understanding the effects of the molecular weight of the polymers and the pore size of the adherend surface on the mechanical interlocking of polymer/substrate interfaces is central for enhancing the corresponding adhesion strength. In the present study, we comprehensively investigated the effects of the molecular weight of polybutylene terephthalate and the nanostructure size on the replication quality by molecular dynamics simulations. Under practical pressures and temperatures, polymers of various molecular weights infiltrated to the bottom of the nanopores; the filling rate was > 90 %. The high filling rate and infiltration depth were attributable to the large external pressure (which can overcome the increased viscosity) and the semi-spherical nature of the nanopores (beneficial for polymer infiltration). The nanostructure size weakly corresponded to the filling rate and interfacial energy, but substantially affected the infiltration of the polymer chains. The entanglements of long-chain polymers remained during the filling and increased the joint strength, by enhancing the internal friction and the interfacial friction between the polymer and copper substrate. The current findings provide new insights into the effects of the nanostructure size and the molecular weight of the polymer on the replication quality of nanoinjection molding.

Extended non-destructive testing for inline quality control of cleaning and pre-treatment processes in adhesive bonding of polymer blends

This investigation aims to demonstrate relevant pitfalls of exemplary surface contaminations on polymer blends with respect to quality assurance (QA) of adhesive joints, and highlights approaches of Extended Non-Destructive Testing (ENDT) for tackling the challenges inline during manufacture or repair processes. Greasy fingerprints were intentionally applied on surfaces of different acrylonitrile butadiene styrene (ABS) substrates before joining them with a two component (2C) polyurethane (PU) adhesive. Even when a required cleaning process was omitted before performing a pre-treatment with atmospheric-pressure plasma (APP), measured initial strengths of unaged joints did not reveal significant differences, if compared to joints produced following the qualified bonding process, which comprises a wipe-off wet cleaning before the pre-treatment step. However, after accelerated ageing mimicking the application conditions for joints, resulting strengths were more than 30% lower than for joints produced with the qualified bonding process. Thus, applying post-process random inspection of as-manufactured joints based on destructive testing, and merely evaluating initial joint strengths is not sufficient for assuring the reliability of such manufactured joints. Rather, it is recommended to encompass the intended or specified use within the formulation of the quality requirements. Introducing in-process ENDT procedures, and exemplarily monitoring the substrate surfaces with an Aerosol Wetting Test (AWT), is shown to identify latent fingerprints and to ensure a rapid and documented way to validate adherend surface quality before the application of the adhesive.Article HighlightsSurface quality assurance leads the way for process management decisions to safeguard relevant adherend properties before applying adhesive.Using an Aerosol Wetting Test for plastic adherends allows for identifying latent fingerprints that would impair joint strength.Extended Non-Destructive Testing facilitates embracing potential in-process contaminations when scheduling cleaning processes for adherends.

Increasing strength and fracture toughness of carbon fibre-reinforced plastic adhesively bonded joints by combining peel-ply and oxygen plasma treatments

Multifunctional surface treatment combined with mechanical interlocking and chemical bonding is one of the directions to further expand the field of surface treatment of composite materials. In this study, plasma modification was conducted on the surface of carbon fibre reinforced plastic (CFRP) pretreated by peel-ply, and then comparative analyses were conducted. The adherend surface and the bonding interface were analysed from microstructure, element distribution and chemical bonding to study the strengthening mechanism. Experimental results showed that the bonding strength and fracture toughness of the joints significantly improved after oxygen plasma treatment, which removed and transformed the silicon-containing impurity and reorganised the surface groups. Surface properties of the treated specimens were also improved, as indicated by their higher surface energy values and mechanical interlocking between substrate and adhesive. Compared to plasma treatment taken individually, the CFRP bonded joints had better performance after peel ply plus plasma treatments, and there was less strength loss and higher toughness when the surface damage resulted from the long-duration plasma treatment. These findings may provide insights into solving the challenge of simultaneously increasing the strength and toughness of CFRP bonded joints.

Effect of bio-inspired surface texture on the resistance of 3D-printed polycarbonate bonded joints

Surface preparation before adhesive bonding is crucial to improve the resistance and durability of the joint by altering the surface properties of the adherend. The purpose of surface treatment is to clean the surface from contaminants, activate the adherend surface and create an optimal surface structure to promote adhesion mechanisms. In that context, this work aims to investigate the influence of substrate surface texturing on the resistance of adhesive joints. Two bio-inspired surface textures were investigated, Fish scale (FS) and Tree frog (TF). Polycarbonate (PC) specimens with different surface patterns were manufactured using the fused deposition modelling process. Surface morphology, such as pattern dimension (shape and depth), surface roughness (Ra), and wettability, were used to characterise the substrates. The influence of these texture patterns on the shear strength of adhesively bonded joints was evaluated through the standardised block shear test method ASTM D4501-01. Moreover, the shear strength of the structured joints was compared to the results from bonding with polished surfaces (surfaces abraded with 80, 600 and 1000 grit paper), and with as-printed surfaces. The results revealed that the FS and TF surface textures enhanced the shear strength by 242% and 283% compared to the adhesive joints with polished surfaces. It was also shown that the variation in depth of the bio-inspired surface texture has no significant impact on the joint strength. Failure analysis demonstrated that the fracture mode of bonded joints with polished surfaces was the adhesive failure while mixed failure (cohesive and adhesive) characterises the as-printed, TF and FS surfaces. Worthy results are obtained rising the effectiveness of surface texture for the PC's bonded joints. Graphical Abstract [Formula: see text] This is a graphical representation of the abstract.

Effect of ultrasonic vibration on adhesive enhancement of plasma-modified nickel surface

Poor adhesion of nickel surface limits its further application in the aerospace field. In this study, plasma modification was conducted on the surface of the nickel plate pretreated by sandblasting, and then ultrasonic vibration was applied during the adhesively bonding process of the CFRP(Carbon fibre-reinforced polymer)/Ni joints. The bonding strength of the joints was increased by 65%. The adherend surface and the bonding interface were analyzed from microstructure, element distribution and chemical bonding to study the strengthening mechanism. By the sandblasting, irregular pits were formed on the nickel surface, effectively increasing the surface roughness. The plasma modification could introduce active functional groups including hydroxyl, amino and carbonyl on the nickel surface, which improved the surface wettability macroscopically. However, at a microscopic level, the adhesive with high viscosity and poor fluidity did not form a compact interface with the nickel. The ultrasonic application could promote the filling of the adhesive in irregular micro-scale pits on the surface, thereby strengthening the mechanical anchoring effect. Furthermore, the ultrasonic application produced dynamic impingement at the interface, enhancing the contact between the adhesive and the nickel plate. The adhesive molecules could fully collide and react with the active functional groups introduced on the nickel surface to form more chemical bonds, thus effectively improving the bonding strength of the CFRP/Ni joints.

Crack-arresting behavior of adhesively bonded joints with a single rectangular convex/concave shape formed on adherend surfaces

ABSTRACT The effect of a rectangular convex/concave shape formed on adherend surfaces in terms of preventing the crack growth of adhesively bonded joints was analytically investigated. The finite element method was used to evaluate crack growth resistance along the crack path by simulating a double cantilever beam test in which a single convex/concave shape was formed on the adherend material. The obtained crack growth resistance clearly demonstrated that both concave and convex shapes improved the apparent fracture toughness; i.e., the convex/concave shape functioned as a crack arrester. We identified the cause of the improvement as the dispersion of energy allocated to crack propagation due to the accumulation of the strain energy or the reduction of the stress concentration at the crack tip. It is noteworthy that the crack growth was arrested even when the convex shape did not interrupt the crack path.

Superstrong yet water-detachable eutectogel adhesives

Developing superstrong adhesives that can be detached via an environmental friendly trigger under mild operation conditions is highly desirable in industry and it can significantly contribute to sustainability. This work provides a novel design of particular eutectogel adhesives based on poly(acrylic acid)-deep eutectic solvent (PAA-DES). The synthesized adhesives can bond strongly with versatile substrates and be readily detached via simple water soaking. The eutectogel adhesive establishes strong adhesion interactions with the surface of adherends through abundant hydrogen bonds and ion-substrate interactions, with an ultrastrong adhesion strength reaching as high as 12.50 MPa on the glass substrate. The adherends could be easily released by water soaking for several minutes with no damage to the system. Such a novel approach for developing detachable adhesives is believed to have high potential in commercialization that requires temporal joining and assembly processes, including optical lens drilling and wafer dicing.

Tangential bond stiffness evaluation of adhesive lap joints by spectral interference of the low-frequency A0 lamb wave

Transmission spectra of the lowest-order antisymmetric (A0) Lamb wave for adhesively bonded single lap joints are experimentally investigated in a frequency range below 0.4 MHz. Based on the theoretical results of the wave interference, a tangential bond stiffness evaluation method is proposed for the lap joints. Aluminum alloy plates bonded with epoxy adhesive on different conditions were prepared as bonded specimens. An air-coupled ultrasonic transducer was used to generate the A0 mode in the specimens, and the signals of the transmitted waves across the joints were measured with a pin-type piezoelectric transducer. Spectral analysis was performed for the measured waveforms, and the transmission coefficient was calculated for each specimen as a function of frequency. The experimental results showed that the transmission coefficient has local maxima and minima at multiple frequencies, called peak and notch frequencies, respectively. The sets of the peak and notch frequencies were different among the bonded specimens. Based on the peak and notch frequencies, the tangential bond stiffness was estimated for each bonded specimen. As a result, the bond stiffnesses obtained for the specimens with the same nominal bond thickness and adherend pretreatment were almost equal even if the bond lengths were different. Furthermore, the evaluation results showed that pretreatments such as sanding and contamination of adherend surfaces affect the bond stiffness due to the change in the interfacial stiffness.

75 - Bond Strength of Multi-Step Adhesive at Intraoral Repair

In recent years, dental bonding system have been improved its bond strength not only to tooth structure but also to prostheses such as metals and ceramics at intraoral repair. However, in order to provide reliable adhesion to prostheses, repairing procedure is complicated and different depending on product and type of prosthesis and its bonding durability has not been much reported. We have developed a new two-step self-etching adhesive "G2-BOND Universal" that make simplify repairing procedure even multi-step adhesive. In this study, bonding durability of multi-step adhesive at intraoral repair cases was evaluated. Several prostheses, [initial LiSi Block] (GC, LiSi), [initial LRF BLOCK] (GC, LRF), [Ti alloy] (GC, T-ALLOY) and [Aadva Zirconia disk EI] (GC, Zirconia) were used. The adherend were embedded in acrylic resin (GC, Unifast II). The multi-step adhesives used in this study were G2-BOND Universal (GC, G2B), SE BOND2 (kuraray, SE2) and Optibond FL (Kerr, OFL). The flat adherend surface was prepared by 600-grit SiC paper. Each adhesive was applied to the prepared adherend surface according to the manufacturer's instructions, and composite resin (Clearfil AP-X, kuraray) was applied to create an adhesion test specimen referred to ISO29022. After 24 h storage in 37 degree C, the specimens were subjected to a thermocycling (5-55 C, 30 s). Shear bond strength (SBS) was tested by Universal testing equipment, SHIMADZU Autograph, testing speed at 1 mm/min (n=5). Data were analyzed by Tukey test (p<0.05). As results of OFL, its SBS was low and pre-testing failures (PTF) were observed in all prostheses cases. On the other hand, G2B showed high SBS for all prosthesis cases, since G2B contains thiophosphate ester monomer and phosphate ester monomer for precious metal and zirconia. On comparison with G2B and SE2, there was significant difference of SBS on Zirconica which might indicate difference of functional monomer effectiveness and G2B could make functionalize its phosphate ester monomer. G2-BOND Universal has high bond strength at intraoral repair cases, so stable bond strength may be expected in clinical practice.

Molecular Dynamics Study on the Thermal Aspects of the Effect of Water Molecules at the Adhesive Interface on an Adhesive Structure

The presence of adsorbed water on hydrophilic solid surfaces should be taken into account, especially in humid environments. It significantly reduces the adhesive strength between the epoxy resin and the adherend surface. Here, the adhesion structure of interfacial water sandwiched between bisphenol A epoxy resin and a hydroxylated silica (001) surface is investigated with microsecond molecular dynamics simulations. Specifically, interfacial water layers with initial thicknesses of 7.5, 10, and 20 Å are modeled. The density curves of water and the diglycidyl ether of bisphenol A show that at room temperature, the surface of the silica with hydroxyl groups is completely covered with a thick layer of water. For water layers thinner than 10 Å, the density of epoxy resin on the silica surface increases when the system is heated and does not return to the original density when the system is cooled. Furthermore, calculation of the interaction energy revealed that the exclusion of water from the hydroxylated surface by epoxy resin during heating can contribute to the increase in the adhesive interaction between the epoxy resin and the silica surface with hydroxyl groups.

Entropy-driven segregation in epoxy-amine systems at a copper interface.

The composition of an epoxy resin at the interface with the adherend is usually different from that in the bulk due to the enrichment of a specific constituent, a characteristic called interfacial segregation. For better adhesion, it should be precisely understood how epoxy and amine molecules exist on the adherend surface and react with each other to form a three-dimensional network. In this study, the entropic factor of the segregation in a mixture of epoxy and amine at the copper interface before and after the curing reaction is discussed on the basis of a full-atomistic molecular dynamics (MD) simulation. Smaller molecules were preferentially segregated at the interface regardless of the epoxy and amine, and this segregation remained after the curing process. No segregation occurred at the interface for a combination composed of epoxy and amine molecules with a similar size. These findings make it clear that the size disparity between constituents affects the interfacial segregation via the packing and/or translational entropy. The curing reaction was slower near the interface than in the bulk, and a large amount of unreacted molecules remained there. Finally, the effect of molecular shape was also examined. Linear molecules were more likely to segregate than round-shaped ones even though they were similar in volume. We believe that these findings, which are difficult to obtain experimentally, contribute to the understanding of the interfacial adhesion phenomena on a molecular scale.

A tough and self-fusing elastomer tape

A multifunctional polymer self-fusing elastomer tape strengthened by the topological entanglement and H-bonds demonstrates an antibacterial activity, and appealing ability to seal liquid leakage (water, oil) and stop bleeding. • H-bonding and topological entanglements contribute to a self-fusing elastomer. • The tape seals the liquid leakage and bleeding, independent of surface nature. • This self-fusing tape kills harmful bacteria by intrinsic weak acidity. Pressure-sensitive and adhesive tapes have been widely used for many purposes. However, their intrinsically strong sticky nature and weak strength severely limits their applications to vulnerable surfaces and harsh conditions. Herein, a multifunctional tough and self-fusing elastomer tape is fabricated for the first time by simple copolymerization of a model hydrophobic monomer 2-methoxyethyl acrylate (MEA) and a model weak acid-containing monomer acrylic acid (AAc). The PMEA-PAAc copolymer elastomers demonstrate high strengths and toughness with a maximum Young’s modulus, tensile stress, breaking strain, toughness, and recovery stress up to 74.19 MPa, 10.77 MPa, 2260%, 48.95 MJ/m 3 , and 0.60 MPa, respectively, due to hydrogen bonds and hydrophobic interactions. Furthermore, these reversible physical interactions contribute to the room temperature self-fusing ability of the elastomer. Studies on rheological behavior, dynamic mechanical properties and time–temperature equivalence reveal that topological entanglement and hydrogen bonds collectively contribute to the toughness and self-fusing capability. Intriguingly, the weak acidity of PAAc imparts antibacterial activity to the elastomer. Importantly, this elastomer can seal liquid (water and oil) leakage and stop bleeding, which is independent of the adherend surface properties, outperforming the conventional surface-dependent pressure sensitive adhesive and adhesive tape, due to elastic retraction force and self-fusing capability.

Performance and failure assessment of adhesively bonded non-crimp fabric carbon fiber/epoxy composite single lap joints

Carbon fiber-reinforced plastic (CFRP) composite materials have experienced increased use for load-bearing structures in automobiles owing to their high specific mechanical properties. However, critical to widespread adoption of CFRPs are the development of robust joining methods with appropriate surface preparation to maximize joint strength, as well as an improved understanding of failure mechanisms to enable computational modeling for supporting design. In the present study, an adhesively bonded non-crimp fabric CFRP single lap joint (SLJ) was investigated to quantify the effect of the composite adherend surface treatment and stacking sequence on the lap shear strength, and to assess the corresponding failure mechanisms. Single lap shear test results revealed that the highest lap shear strength was achieved by abrading the surface with sandpaper, relative to an unprepared surface or one treated by grit blasting. Similarly, an adherend with a higher effective flexural longitudinal modulus, achieved through stacking sequence, increased the lap shear strength owing to reduced Mode I loading on the adhesive joint. Distinct failure processes were observed for CFRP adherends with different stacking sequences, where the locations of intra-ply and inter-ply cracks varied as a result of variations in the in-plane and out-of-plane stresses. Notwithstanding, the final failure event for all SLJ specimens was failure of the thin layer of matrix in the 0° ply of the adherends. Three-dimensional finite element analyses of the SLJ specimens were employed to determine the local stress components driving the observed failure mechanisms within the plies of the CFRP adherends. The results of this study provide a recommended surface treatment to maximize adhesive joint strength for CFRP adherends, along with a comprehensive assessment of failure mechanisms, to inform the design and application of CFRP materials in lightweight structures.

Adherend surface roughness effect on the mechanical response of silicone-based adhesive joints

The present contribution focuses on the effect of adherend surface roughness on the strength of adhesive joints, which are particularly cost-effective and extensively applied in a wide range of industrial applications. However, the reliability of such solutions is a critical concern for the integrity of commercial products. To gain a deeper understanding on the effect of roughness, an extensive experimental campaign is proposed, where thermoplastic substrates are produced with a specified roughness, whose characterization has been performed using a confocal profilometer. Elastic strips are then bonded onto such substrates using Silicone adhesive while controlling the adhesive thickness. Peeling tests are finally carried out and the effects of joint parameters such as surface roughness, adhesive thickness, and loading rate are discussed in detail. Eventually, it is demonstrated that the surface roughness can increase the adhesion energy of joints depending on the value of a ratio between the adhesive thickness and the root mean square elevation of roughness.

Ultrasonic Vibration-Strengthened Adhesive Bonding of CFRP/Aluminum Alloy Joints with Anodizing Pretreatment

Ultrasonic vibration has been applied to improve the penetration of adhesive into the anodized layer, thereby enhancing the strength of carbon fiber-reinforced plastic/aluminum alloy bonding joints with anodizing pretreatment. The ultrasonic vibration-assisted adhesive bonding process was designed by orthogonal experiments, then verified experimentally. The strengthening mechanism was studied by analyzing the morphology and elemental distribution of the cross-section of the joint. The results show that the ultrasonic vibration-assisted adhesive bonding process can further strengthen the interfacial bonding. For the studied joints, the strength can reach 18.66 MPa, being 55% higher than without ultrasonic strengthening. The ultrasonic vibration creates shock waves in the adhesive layer, causing a high-speed adhesive jet toward the adherend surface, which makes the interfacial bonding tight and promotes penetration of the adhesive into the anodized layer. The bonding strength is thereby remarkably improved by forming a larger interfacial contact area and more mechanically interlocked structures.

A Universal Strategy for Tough Adhesion of Wet Soft Material

AbstractAchieving adhesion between hydrogels and diverse materials in a facile and universal way is challenging. Existing methods rely on special chemical or physical properties of the hydrogel and adherends, which lead to limited applicability and complicated pretreatments. A stitch‐bonding strategy is proposed here by introducing a polymer chain with versatile functional group and triggerable crosslinking property inspired by catechol chemistry. The polymer chain can stitch the hydrogel by forming a network in topological entanglement with the preexisting hydrogel network, and directly bond to the adherend surface by versatile chemical interactions. Through this, the polymer chain solution works as a universal glue for facile adhesion of hydrogels to diverse substrates like metals, glasses, elastomers, plastics, and living tissues, without requiring any chemical design or pretreatment for the hydrogel and adherends. The adhesion energy between polyacrylamide hydrogel and diverse substrates can reach 200–400 J m−2, and it can reach ≈900 J m−2 with a toughened polyacrylic acid polyacrylamide hydrogel. The mechanism of stitch‐bonding strategy is illustrated by studying various influence factors.

Experimental analysis and prediction of strength of adhesive-bonded single-lap composite joints: Taguchi and artificial neural network approaches

Adhesive-bonded joints made up of composite materials offer complex structures with the ease of joining similar or dissimilar materials. The failure behavior of adhesive-bonded joints is influenced largely by geometrical parameters (overlap length and adhesive thickness) and adherend surface preparations. The glass fiber-reinforced epoxy composites are prepared for single-lap adhesive joints to know their strength with different sets of geometrical factors. The adherend surfaces of composites are roughened to 2 ± 0.1 µm, prior to joint preparation. Taguchi L9 experimental matrix representing different combinations of overlap length and adhesive thickness is employed to know the behavior of failure load (FL) and shear strengths (SS) of the adhesive-bonded single-lap composite joints. The results showed that the impact of overlap length of adhesive-bonded joints is more compared to that of adhesive thickness. The interaction effects of geometrical parameters are found negligible toward both the outputs. The optimal factor levels received the highest load to fracture; the joints are found equal to 6096.1 N for FL and 80 MPa for SS, respectively. The empirical relationship based on multiple linear regression (MLR) equations is derived for both the failure load and shear strength. Levenberg–Marquardt algorithm-trained neural networks (NNs) are used for the prediction of both responses. Ten experimental cases are used to check the prediction capabilities of both MLR and NNs. The mean absolute percent error in prediction of both the responses is found equal to 2.27% for NNs and 3.12% for MLR. The NNs and MLR results in accurate prediction might be due to the model development process based on experimental input–output data, rather than assumption-based theoretical and numerical methods.

Strength of scarf-bonded CFRP repairs containing disc-shaped zones of weak bonding considering hot-wet conditioning

A commonly used design for the repair of carbon fibre reinforced polymers (CFRP) is the bonded scarf joint. As contaminations on the adherend surface or bondline flaws can reduce the bonding strength drastically, the effect of such strength reducing phenomena has to be known. This is also the case for bonded joints that had to sustain hot-wet conditions. Therefore, the aim of this study was to investigate the influence of weak bonds and hot-wet conditioning on the mechanical properties of scarf bonded CFRP joints. Circular zones of weak bonding have been artificially produced by application of release agent on the adherend surface. Different sizes and locations were studied. Additionally, two scarf ratios and two lay-ups have been investigated, to account for different stress states in the bondline. The investigation showed that the effect of a weak bond on bond strength is strongly dependent on the location, size and lay-up of the adherend, while hot-wet conditioning reduces the bond strength in all cases.

Improving mode II fracture toughness of secondary bonded joints using laser patterning of adherends

We improve mode II fracture toughness of secondary bonded joints based on the laser treatment of adherend surfaces. We applied CO2 Laser treatment to adherend surface alternatively with low and high energy, resulting in a repeated pattern of rough and smooth surfaces. We used ENF test to characterize mode II fracture toughness, GII, of the bonded CFRP substrates. The results demonstrated that our proposed pattern arrested the crack propagation and triggered new damage mechanisms, such as adhesive cracking, adhesive failure and crack migration to the other interface. These additional mechanisms, some of them were non-local in nature, resulted in higher energy dissipation during propagation and, improved GII compared to joints with uniformly-treated adherends. The improvement in GII was dependent on the pattern morphology and reached up to 60%. The proposed strategy worked well for both thin and thick adhesives, and thus could be applied in aerospace and civil structures.