Ice Shear Strength Research Articles

Size Effect of Micro-Sized Graphene Oxide on Self-Healing and Photothermal Anti-icing Coatings.

Microsized graphene oxide (GO) is promising for the development of efficient anti-icing coatings due to its excellent anti-icing activity, remarkable photothermal property, mature industrialization and easy availability. However, the size effect of microsized GO on the performance of anti-icing coatings remains unknown. Herein, a series of microsized GO with different sizes are incorporated into a supramolecular polymeric matrix to study the GO size effect and develop the self-healable and photothermal anti-icing coatings. Results show that the increase of GO size significantly enhances the anti-icing, deicing, and photothermal performance of the coatings. With the increase of GO size, the freezing delay time is prolonged by ∼124.7%; the ice nucleation temperature and ice shear strength are reduced by ∼22.0% and ∼40.3%, respectively; and the photothermal ability is increased by ∼19.8%. The best-performing coating (LGO-SH) with the largest GO exhibits high stability/durability during 50 icing/deicing cycles and at different low temperatures. Moreover, owing to the multiple dynamic bonds in the supramolecular matrix, LGO-SH can autonomously self-heal at -20 °C and recover its original ice shear strength during 10 damage/healing cycles. Furthermore, LGO-SH achieves fast photothermal deicing due to its excellent photothermal ability. Findings in this study provide a guideline to develop efficient GO-based anti-icing coatings and build a bridge between the GO industry and anti-icing coatings.

Well-Tailored Norbornene-Based Fluorinated Copolymers toward Modulating Icephobicity and Mechanical Robustness.

Well-tailored construction of icephobic surfaces with mechanical robustness and investigation of the structure-property relationships at the molecular level are highly desirable. Herein, a series of norbornene-based fluorinated polyolefin copolymers (FPOR-x) with varying norbornenyl dodecafluoroheptyl ester (NDFHE) molar fractions (0-100 mol %) were well-designed and fabricated via living ring-opening metathesis polymerization (ROMP) employing NDFHE and norbornenyl pentafluorophenyl ester (NPFPE) as the soft and hard segments, respectively. The mechanical and icephobic properties of the fluorinated copolymers can be regulated by adjusting the soft NDFHE contents. As a result, the well-designed norbornene-based copolymers exhibited a wide range of tunable mechanical properties, including tensile strength ranging from 0.2 to 26.4 MPa, elastic modulus ranging from 0.6 to 593.7 MPa, and breaking elongations ranging from 5718.7% to 3.7%, correlating with the proportion of soft NDFHE content. Furthermore, the synergistic interplay between soft and hard segments, particularly the hardness in the majority and softness in the minority or vice versa, could achieve a significant difference in the local modulus and enhance the propagations of cracks within the three-phase regions (soft regions/hard regions/ice), ultimately leading to a significant reduction in ice shear strength. Notably, FPOR-25% with a tensile strength of 12.0 MPa and an elastic modulus of 227.5 MPa exhibited a remarkably low ice shear strength of 57.7 kPa. This study not only highlights the relationship between the polymer molecular structure and surface icephobic properties but also breaks the limitations of icephobic surfaces with a low modulus.

On the sensitivity of sea ice deformation statistics to plastic damage

Abstract. We implement a plastic damage parametrization, distinct from the elastic damage in the elasto-brittle framework, in the standard viscous–plastic (VP) sea ice model to disentangle its effect from resolved model physics (visco-plastic with and without damage) on its ability to reproduce observed scaling laws of deformation. To this end, we compare scaling properties and multifractality of simulated divergence and shear strain rate, as proposed in the Sea Ice Rheology Experiment (SIREx) studies, with those derived from the RADARSAT Geophysical Processor System (RGPS). Results show that including a plastic damage parametrization in the standard viscous–plastic model increases the spatial but decreases the temporal localization of simulated linear kinematic features (LKFs) and brings all spatial deformation rate statistics in line with observations from RGPS without the need to increase the mechanical shear strength of sea ice as recently proposed for lower-resolution viscous–plastic sea ice models. In fact, including damage with a healing timescale of th=30 d and an increased mechanical strength unveils multifractal behavior that does not fit the theory. Therefore, a plastic damage parametrization is a powerful tuning knob affecting the deformation statistics of viscous–plastic sea ice.

Flexible superhydrophobic films with the electrothermal and photothermal response for enhanced passive anti-icing and active de-icing

Surface icing poses a significant threat to various industries, including aircraft, power grid, transportation, communication facilities, and offshore platforms. The inefficiency and high energy consumption associated with conventional de-icing methods necessitate the development of an efficient, energy-saving, stable, and reliable alternative. This study proposes an approach to these challenges by fabricating microstructured MWCNT/TiN/PDMS (MMTP) flexible films using laser processing and functional nanocomposites. The multiresponsive MMTP flexible films allow precise control of the surface temperature by adjusting the voltage or the intensity of near-infrared (NIR) light, with surface temperatures exceeding 150 °C with voltage of 10 V or NIR light of 0.9 W/cm2 intensity. This heating property allows the films to maintain its superhydrophobicity even at temperatures as low as -30 °C. In addition, MMTP flexible films can effectively increase the thermal resistance between the solid-liquid phase and reduce the nucleation sites of the droplet, thereby greatly delaying the formation of ice. By using electrothermal/photothermal effects on the films significantly reduces the ice shear strength, making de-icing easier and reducing energy consumption. Furthermore, the films have excellent processability and durability. The multi-responsive superhydrophobic flexible films demonstrate remarkable anti-icing/de-icing performance and show great potential for a wide range of applications, particularly in complex scenarios where equipment is susceptible to icing.

Experimental investigation and strength model of rough ice-filled joints under tensile and shear loading

Warming softening of ice-filled joints has caused many rockfall disasters in cold regions. A series tensile and shear strength tests were conducted to investigate the thawing softening mechanism of ice-filled joints by considering the effect of the joint opening, temperature and roughness. The joint opening has little influence on the tensile strength of ice-filled joints, but the temperature is a remarkable influencing factor. When the temperature increases from −15 °C to −0.5 °C, the average tensile strength of ice-filled joint reduces from 0.64 MPa to 0.22 MPa, which displays a constant logarithmic decay function against the freezing temperature and it is similar to the change rule of pure ice. The shear strength of the ice-filled joint decreases with increasing the infill thickness, and tends to be the shear strength of pure ice beyond the critical infilling thickness. In addition, all the tensile failures are inside the joint ice, however, the shear failure is mainly caused by the debonding of ice-rock interface when the temperature increases to −0.5 °C. It implies that the tensile bonding strength of ice-rock interface also is larger than that of the solid ice itself even at a high freezing temperature. Based on the experimental results, a new shear strength model of the ice-filled joint is proposed for Barton's joint profiles by considering the effect of temperature and infill thickness. It has been validated that this model is accurate to predict the shear strength of ice-filled joint, which may be used to estimate the stability of freezing joint rock slope in cold regions.

Near-infrared light-responsive functionalised surfaces with shape memory microstructure for droplet manipulation and anti-icing/de-icing

Preparation techniques of functional surfaces by studying the correlations between the macroscopic properties and microscopic morphologies of biological surfaces and by replicating their microstructural features have been widely discussed. However, surfaces with intelligently switched microstructural morphologies, according to the application scenario, have rarely been reported. Herein, laser etching techniques are employed to construct micropillar arrays on a photothermally responsive shape memory polymer (SMP) surface. In situ reversible conversion of surface microstructure morphology is achieved with near-infrared (NIR) light and external forces, further promoting surface wettability conversion. Cycling experiments show that this reversible wettability transition can be repeated many times, demonstrating its applicability as a microdroplet manipulation platform for droplet capture and remote release. By combining the microstructural morphologies and photothermal response properties of the reversible surface transition, the passive anti-icing and active de-icing performance characteristics of different surfaces at various stages are compared by analysing the droplet phase transition processes and variations in ice shear strengths. These functionalized surfaces with NIR light-controlled reversible wettability may allow new possibilities for droplet selection and transfer processes, chemical microreactions and full-process dynamic anti-icing/de-icing applications.

Ice Adhesion Evaluation of PTFE Solid Lubricant Film Applied on TiO2 Coatings

Ice formation affects the performance of many industrial components, including aircraft wings, spacecraft, and power transmission cables. In particular, ice build-up on airplane components increases drag and fuel consumption. A large number of studies have been carried out to reduce ice adhesion by developing passive methods such as icephobic coatings and active ice removal approaches such as mechanical vibrations or chemical-based solutions. Despite remarkable recent breakthroughs in the fabrication of icephobic coatings, passive ice removal solutions require higher durability to resist cyclical mechanical ice detachment treatments. Functionalized TiO2 coatings, applied using the suspension plasma spray (SPS) technique, have been shown to be robust and to have dual-scale characteristics in an ice accretion analysis. In this study, the icephobicity and mechanical durability of a novel duplex coating consisting of polytetrafluoroethylene (PTFE) solid lubricant films on TiO2-coated substrates were evaluated. Notably, various amounts of PTFE were applied on top of the TiO2 coating to identify the ideal quantity required to obtain optimal icephobic properties. Ice was generated in an icing wind tunnel, and the amount of accreted ice was evaluated to assess the anti-icing properties. Wettability parameters, including static water contact angle and contact angle hysteresis, were measured to determine the water mobility and surface energy. Ice shear adhesion to the PTFE-TiO2 duplex coating was measured using a custom-built test rig. The mechanical durability was assessed by measuring the ice shear strength for almost twenty icing–deicing cycles, and after five cycles, the roughness parameters and images taken from the surface of the samples were compared. The combination of PTFE solid lubricant film and TiO2 coating reduced ice adhesion by 70%–90% compared to that of a bare aluminum substrate (reference material). Additionally, the results showed that the application of a uniform layer of PTFE solid lubricant film on dual-scale TiO2 coating significantly reduced ice adhesion and maintained mechanical durability for 25 deicing cycles, making this combination a promising candidate for deicing approaches.

An Extreme-Environment-Resistant Self-Healing Anti-Icing Coating.

Anti-icing coatings on outdoor infrastructures and transportations inevitably suffer from surface injuries, especially in extreme weather events (e.g., freezing weather or acid rain). The coating surface damage can result in anti-icing performance loss or even icing promotion. The development of anti-icing coatings that enables self-healing in extreme conditions is highly desired but still challenging. Herein, an extreme-environment-resistant self-healing anti-icing coating is developed by integrating fluorinated graphene (FG) into a supramolecular polymeric matrix. The coating exhibits both anti-icing and deicing performance (ice nucleation temperature is ≈-30.3 °C; ice shear strength is ≈48.7kPa), mainly attributable to the hydrophobic FG and silicone-based supramolecular material. Notably, owing to the crosslinking polymeric network with various dynamic bonds, this coating can sustain anti-icing/deicing performance after autonomous self-healing under harsh conditions including low temperature (-20 °C), strong acid (pH = 0), and strong alkali (pH = 14) environments. This coating paves the way to meet the anti-icing demand in open air, especially for the infrastructures in polar regions or acid/alkali environments.

Design and preparation of durable anti‐icing polysilazane coatings with abrasion and UV resistance

AbstractThis work developed a method to enhance the durability of anti‐icing performance of a coating to endure abrasion and UV exposure, through grafting functional polydimethylsiloxane (PDMS) to TiO2 particles and further reacting with polysilazane (PSZ) pre‐polymer. This method remarkably improved the anti‐icing properties of PSZ‐based coatings evaluated by freezing delay time and ice removal shear strength. After either surface abrasion for 50 cycles or UV exposure for 100 h, the modified coatings exhibited ice shear strength increase by only 10.44 and 6.44 kPa respectively, significantly lower comparing to that of the coatings containing 30% TiO2 with no PDMS modification (29.69 and 11.88 kPa, respectively). Such strengthened durability of anti‐icing performance is attributed to the PDMS molecules bridging PSZ rigid segments and TiO2 compositions, which formed a hydrophobic cross‐linking network structure. The durable anti‐icing technique developed herein shall appeal to many outdoor applications of facilities and infrastructures.

Influence of temperature and fiber content on direct shear properties of plain ice and fiber-reinforced ice

As a new kind of building material, ice has been paid more and more attention and applied in cold, polar regions and the outer space. The shear mechanical properties of plain ice and fiber-reinforced ice (FRI) were studied by a modified direct shear test. Five types of FRI containing 0%, 1%, 2%, 4% and 6% fiber content of bleached sulfate softwood pulp (BSSP) fiber were prepared and tested under −20, −15, −10 and − 5 °C. The results showed that the brittle failure of plain ice was in contrast to the ductility of FRI. With the decrease of temperature, the shear strength and peak deformation increased. The shear strength and deformation of ice at −20 °C were 2–2.5 times and 1.1–1.4 times higher than those at −5 °C. The study also showed that with the increasing fiber content, the shear strength and peak deformation increased. The shear strength and peak deformation of FRI with 6% fiber content were 3.8–5.9 times and 2–3times higher than those of plain ice. Equations predicting the shear strength and peak deformation of ice considering the temperature and fiber content were proposed, and the relationship between shear strength and compressive strength was investigated. Through the analysis of shear ductility coefficient, the increase of fiber content and decrease of temperature greatly improved the energy absorption capacity and subsequent bearing capacity of ice in the shear process. A shear constitutive model was proposed to predict the relationship between shear strength and deformation.

Slippery coatings with mechanical robustness and self-replenishing properties as potential application on magnesium alloys

Slippery liquid-infused surfaces (SLISs) have the same development potential and application prospects as superhydrophobic surfaces (SHSs) in the fields of corrosion resistance, anti-icing, anti-fouling and drag reduction. However, the durability of SLISs is still a challenge because of the migrating, evaporating, or leaking of the infused liquids. Herein, we proposed a durable polydimethylsiloxane/silicone oil (PDMS-oil) system with mechanical robustness and self-replenishing property. As a demonstration of potential application, the PDMS-oil coating was applied on the magnesium alloy for multi-functions. A layer of PDMS cross-linking network was first cured on the magnesium alloy surface, and then a certain amount of low viscosity dimethyl silicone oil was diffused into the PDMS layer to form the final SLIS coatings. It is found that 50 wt% of silicone oil content is more suitable for the PDMS-oil based slippery coating on Mg alloy, which can maintain a low ice shear strength of less than 30 kPa after 20 icing/deicing cycles and a lower sliding angle of about 30° after wear cycles (under a normal pressure of 10 kPa). Significantly, the proposed PDMS-oil based slippery coating possesses good corrosion resistance and self-replenishing ability, which can also withstand hot water, ice water, and corrosive solutions (i.e. acid and alkaline aqueous solutions). The Mg alloy with the PDMS-oil based slippery coating can remain intact in a 3.5 wt% NaCl aqueous solution even after 20 days at room temperature. It is believed that the Mg alloys with the proposed PDMS-oil based slippery coatings would have more potential applications even in severe environments.

A multifunctional graphene composite coating with switchable wettability

Although many encouraging advances have been made, the further development of the switchable wettability surfaces has been limited by complex preparation process, limited substrate materials, and inconvenient switching wettability methods. Herein, a multifunctional graphene composite coating (MGCC) with switchable wettability was fabricated by simple methods. Attributed to the special properties of nano-TiO2 and graphene, the MGCC can achieve reversible wettability transformation under UV or IR irradiation. The unique wettability and good adhesion make MGCC show potential advantages in many fields. First, spraying MGCC on metal foam or melamine sponge can achieve high-efficiency separation of oil–water layered mixtures (>97.4%) or emulsions (>99.1%). Secondly, due to the light-controlled switchable wettability, MGCC platform can obtain different wettability regions by mask method to manipulate droplets or fluids. Different wetting models (Cassie state, Cassie impregnation wetting state, and Wenzel state) could be realized on the same MGCC surface. Thirdly, MGCC can significantly improve the corrosion resistance of metal surface, providing a new solution for corrosion protection. More specifically, the corrosion inhibition efficiency was 99.996%. Finally, MGCC has good anti-icing and active deicing abilities, which are attributed to the superhydrophobicity and excellent photothermal properties of graphene. In particular, ice adhesion strength (IAS) of MGCC surface was only 117 ± 15 kPa, and the ice shear strength (ISS) was only 183 ± 18 kPa. This study provides a new idea for designing a simple, multifunctional, and switchable wettability coating, which is expected to promote the large-scale preparation and application of functional integrated smart surface.

Summary of the Test Methods for Icing Strength of Freshwater and Seawater

Polar navigation ships, offshore platforms, and amphibious aircraft sometimes need to work in low temperature environments, which will inevitably form varying degrees of icing on the surface of structures such as cabin doors. In order to ensure their normal working conditions, it is advisable to choose reasonable anti-icing and deicing methods to prevent icing or effectively remove icing. Therefore, it is necessary to study the mechanical properties of ice to provide a reference for the selection of anti-deicing methods. This article summarizes the common methods for measuring the shear strength and tensile strength of ice, which can provide some guidance for the freezing strength test of fresh water and sea water.

Low Ice Adhesion Surfaces Based on Flexible Fluorinated Polymers with a Polynorbornene Backbone.

Realizing icephobic surfaces with low ice adhesion and durability continues to be fascinating as well as challenging. Herein, a norbornene-based fluorinated polymer (NFP) with high flexibility and high tensile strength is designed and fabricated using a fluorinated side chain and a norbornene backbone, displaying low ice shear strength less than 20 kPa and excellent durability. Experimental and theoretical analyses show that the flexibility of the polymer chains and the synergistic macromolecular aggregation of the fluorinated side groups and the norbornene backbone play key roles in the excellent surface icephobic properties of the NFP films. Moreover, we also develop a facile approach to the design of durable icephobic slippery surfaces, which possess remarkable icephobic performance. This study not only sheds light on the relationship between the polymer molecular structure and surface icephobic properties but also provides a new avenue to conveniently realize anti-icing coatings.

Coupled thermo-mechanical analysis of stresses generated in impact ice during in-flight de-icing

Before the phase change of ice on the aircraft surface occurs during an electrothermal de-icing process, the stresses developed in impact ice may have certain influence on the shedding of ice. To effectively improve the control accuracy of inflight de-icing, a numerical study on the thermo-mechanical coupling effect caused by aerodynamic force and electric heating is carried out. Based on the fundamental rules of elastic mechanics and heat transfer, this multi-physical process involving fluid-structure interaction and thermal-structure coupling is numerically investigated, with the stress distribution developed across the entire structure predicted. To further evaluate the contribution to ice detachment, the stresses generated under typical conditions are quantitatively compared with the fracture strength of ice. Results show that the effect of the heat flux is much more significant than that of the aerodynamic force on ice failure. Under the action of electric heating, the maximum shear stress is determined at 2.84 MPa, which is 5.6 times the shear strength of ice (0.51 MPa) and will cause ice detachment along the ice-airfoil interface. Besides, the peak principal stress reaches 5.94 MPa after 5 s of heating time and has exceeded the compressive strength of ice (5 MPa), developing local fracture inside the ice. The combined effect of these two aspects weakens the overall adhesion and may eventually lead to ice shedding. Contributions of this study can effectively guide the optimization of aircraft thermo-mechanical de-icing systems.

Enhanced anti-icing properties of branched PDMS coatings with self-regulated surface patterns

Adverse impact of icing, on either infrastructures or facilities, is the great impulsion for seeking anti-icing coatings with high performance. In this work, durable icephobic coatings with self-regulated surface patterns were developed from branched polydimethylsiloxane (PDMS) co-crosslinked with fluorinated polyhedral oligomeric silsesquioxane (POSS) through a breath figure method. The prepared coatings with patterned microstructures on the surfaces exhibited excellent anti-icing performance ascribed to micro-crack generation between ice and coating surface under a small external force. The stable cross-linked network composed of branched PDMS and fluorinated POSS endowed the coatings with durability and abrasion resistance. Low ice shear strength was achieved at approximate 19.7±2.5 kPa on coating surface with self-regulated honeycomb pattern. After experiencing 30 icing/deicing cycles, the ice shear strength of the coating varied slightly. Additionally, owing to dedication of fluorinated POSS, the coating showed merely a little increase in ice shear strength after more than 1600 abrasion cycles, which was still in the icephobic range below 50 kPa. Hence, the prepared coatings with remarkably enhanced anti-icing performance and superior mechanical stability would be promising for the practical applications.

Fabrication of bio-inspired non-fluorinated superhydrophobic surfaces with anti-icing property and its wettability transformation analysis

Ice accumulation leads to several inconvenient and even threatening situations in cold regions. This paper reports an efficient, non-toxic and low-cost method to fabricate hierarchical superhydrophobic surfaces on aluminum alloy with a water contact angle of 155° and a sliding angle of 5°. Tests were performed in an in-house apparatus to evaluate the ice adhesion strength and ice shear strength of the as-prepared surfaces and untreated aluminum surfaces. Results show that ice adhesion strength on different wettability surfaces strongly depends on the efficient specific surface area of the ice-solid interface. The ice shear strength was determined by different ice-solid interface contact shapes. Besides, the freezing delay time was recorded and different mechanisms of the icing process were analyzed by a specific simplified heat transfer model. The de-icing and self-cleaning test shows that the air cushion on the superhydrophobic surface can prevent the melted water from secondary icing and potential contaminations, especially at low temperature. Our results indicate that the as-prepared and recovered superhydrophobic surfaces present excellent anti-/de-icing properties and the special contact conditions and heat transfer process of the ice-substrate interface contribute to the anti-icing property of the superhydrophobic surface.

Low-voltage and -surface energy SWCNT/poly(dimethylsiloxane) (PDMS) nanocomposite film: Surface wettability for passive anti-icing and surface-skin heating for active deicing

Icing is a multiphase/multiscale/multiparameter physical process, and is of frequent occurrence when suitable conditions with temperature, pressure and humidity are met. In the present work, we prepared a series of PDMS-matrix nanocomposite films with different SWCNT contents, which were endowed with hydrophobicity based on the low-surface-energy PDMS matrix and the conductivity on the SWCNT filler. Furthermore, by etching the pillar-textured structure on its surface, the nanocomposite with 5.0 wt% SWCNT was given the superhydrophobicity. These nanocomposites can be easily switched from a hydrophobic anti-icing mode to an electro-thermal deicing mode by supplying a low voltage. Using non-contact infrared thermometry, we presented an analysis of the freezing phase transition process of a single water droplet on cooling surfaces with different wettability, and investigated their ice nucleation rate and macroscopic growth velocity on these surfaces. The ice-retarding capability of superhydrophobic nanocomposite surface subjected to lots of condensed droplets was also confirmed, and understanding in light of weak contact interaction with droplets. Also under consideration is the icephobicity after freezing in terms of ice shear strength. In addition, we performed a statistical analysis about the Joule heat distribution on nanocomposite surface, the results of which demonstrated that the nanocomposite could supply a suitable heating function for active deicing, demonstrating with an energy-input deicing experiment subsequently.

Simulating intersection angles between conjugate faults in sea ice with different viscous–plastic rheologies

Abstract. Recent high-resolution pan-Arctic sea ice simulations show fracture patterns (linear kinematic features or LKFs) that are typical of granular materials but with wider fracture angles than those observed in high-resolution satellite images. Motivated by this, ice fracture is investigated in a simple uni-axial loading test using two different viscous–plastic (VP) rheologies: one with an elliptical yield curve and a normal flow rule and one with a Coulombic yield curve and a normal flow rule that applies only to the elliptical cap. With the standard VP rheology, it is not possible to simulate fracture angles smaller than 30∘. Further, the standard VP model is not consistent with the behavior of granular material such as sea ice because (1) the fracture angle increases with ice shear strength; (2) the divergence along the fracture lines (or LKFs) is uniquely defined by the shear strength of the material with divergence for high shear strength and convergent with low shear strength; (3) the angle of fracture depends on the confining pressure with more convergence as the confining pressure increases. This behavior of the VP model is connected to the convexity of the yield curve together with use of a normal flow rule. In the Coulombic model, the angle of fracture is smaller (θ=23∘) and grossly consistent with observations. The solution, however, is unstable when the compressive stress is too large because of non-differentiable corners between the straight limbs of the Coulombic yield curve and the elliptical cap. The results suggest that, although at first sight the large-scale patterns of LKFs simulated with a VP sea ice model appear to be realistic, the elliptical yield curve with a normal flow rule is not consistent with the notion of sea ice as a pressure-sensitive and dilatant granular material.

A new test apparatus to measure the adhesive shear strength of impact ice on titanium 6Al-4V alloy

We present a new shear test which may be used in an icing environment. Ice is formed on a jig containing the sample material and this is then loaded by a forcing mechanism to effect the adhesive test. It allows impact (atmospheric) ice adhesive shear tests to be undertaken without disturbance or delay, in icing conditions. Finite element analysis is used in order to evaluate the controlling shear stresses in the most highly stressed zone of the ice/substrate interface and some sample experimental data is given for the adhesion of some impact ices to Ti-6Al-4V alloy with different surface finishes. The adhesion forces reported, represent peak values rather than spatially averaged stress values. Therefore values of adhesive shear strength obtained are higher than previous authors (in the range from 2 to 14 MPa instead of 0.05 to 0.5 MPa).