Identifying interfacial failure mode in aerospace adhesive bonds by broadband dielectric spectroscopy

The most widely accepted test for bond durability analysis in aerospace metal-bonded structures is the bondline corrosion test introduced in the late 90s. Little progress has been made however on non-destructive testing methods that allow determining the bond quality after years of use. Here, a non-destructive method based on dielectric spectroscopy is introduced to evaluate the state of a metal-adhesive-metal bond exposed to salt fog spray up to 180 days. Several samples were evaluated with broadband dielectric spectroscopy (BDS), floating roller peel (FRP) tests and bondline corrosion (BLC) after exposure to salt fog spray test for different times. Relaxation processes and conductivity phenomena extracted from the BDS data (e.g. apparent conductivity relaxation time (τmax) using electric modulus) are found to correlate well with the bond strength measured in peel test and BLC progression. The BDS-based protocol was able to identify the local interfacial degradation stages in a non-destructive mode and with high resolution. The protocol has the potential to be further developed into a test method for durability on coupon level.Facility Aerospace Structures & Materials Laboratory(OLD) MSE-1Novel Aerospace Material

Testing non-destructive spectrometric methods for the identification and distinction of archaeological pine wood tar and birch bark tar

Archaeological findings prove the appearance and use of birch bark tar since the Middle Palaeolithic. The production and use of birch bark tar and pine wood tar has overlapped since at least the late Neolithic, but probably for much longer. The reliable chemical identification of such archaeological tar residues can offer valuable insights into, for example, ancient technical complexity, trade and culture. In this context, the scarcity of these mainly organic residue findings in the archaeological record bears the need for non-destructive analytical tools. However, there is currently no systematic proposed way for this purpose. We aim here to verify the organic nature and test the reliability of the identification of archaeological pine wood tar and birch bark tar with a combination of SEM-EDS, FTIR microspectroscopy in reflectance mode and XRD. We examined a set of experimental adhesive replicas of pine tar and birch tar in pristine form, but also after a three-year-long weathering experiment. Additionally, we studied a set of archaeological samples, consisting of Mesolithic bone/antler points with adhering hafting residues, form the Dutch North Sea. This research shows that degradation negatively influences the reliable verification and identification of the organic residue constituents significantly. SEM-EDS as a starting point of analysis verifies the residue's organic nature, but it cannot be used to identify birch or pine tar. XRD can identify crystalline additives in the adhesive mixture, like ochre and wax, as well as phases related to the artefact's environment of burial and provenance. Micro-FTIR is also capable of verifying the organic matter of the residue constituents. The differentiation of birch and pine tars is hindered by vibrational modes occurring in neighbouring wavenumbers for both tars, and by the limited research on degradation markers indicative of thermal treatment to prove tar production. Until reference collections also account for degradation and include a wide variety of adhesives, results of FTIR collected in reflectance mode are best treated with some caution.Team Joris DikTeam Amarante BottgerDelft Aerospace Structures and Materials LaboratoryGroup Alderlieste

Point contact abrasive wear behavior of MAX phase materials

The room temperature abrasive wear behavior of three selected MAX phases, Ti3SiC2, solution strengthened Ti2.7Zr0.3SiC2 and Cr2AlC, is investigated by low velocity scratch testing using a diamond conical indentor with a final radius of 100 μm and a cone angle of 120° and applied loads of up to 20 N. All three materials showed a relatively low wear resistance in comparison to most engineering ceramics such as Al2O3, Si3N4 and SiC. For all three materials, the wear rate scaled more or less linearly with the applied load. The softer Ti3SiC2 with a hardness of 2.8 GPa showed the lowest wear resistance with extensive ploughing and grain breakout damage, both within and outside the direct wear track, in particular at the highest load. The hardest material, Ti2.7Zr0.3SiC2, with a hardness of 7.3 GPa, showed a 5 times better wear resistance. The Cr2AlC with a hardness of 4.8 GPa showed a wear resistance equal to or even better than that of the Ti2.7Zr0.3SiC2. The wear mechanism depends on the applied load and the microstructure of the MAX phase materials tested. For the Ti3SiC2 sample, a quasi-plastic deformation behavior occurs below a point load of 10 N, resulting in grain bending, kink band formation and delamination, grain de-cohesion, as well as trans-and intra-granular fracture near the scratch groove. At this load, the Ti2.7Zr0.3SiC2 and Cr2AlC MAX samples display plastic ploughing, grain boundary cracks and material dislodgments.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.(OLD) MSE-1Facility Aerospace Structures & Materials LaboratoryNovel Aerospace Material

The Structure–Property Correlations in Dry Gelatin Adhesive Films

Gelatins are proteinaceous natural materials that are widely used in areas such as conservation and restoration of artifacts as adhesives and consolidants, in pharmaceutics as drug delivery carriers, and in the food industry as structurants. Herein, type A porcine gelatin adhesive films are prepared via solution casting method and their physical and mechanical properties are investigated using X-ray diffraction (XRD), differential scanning calorimetry, contact angle measurement, dynamic mechanical analysis, and uniaxial tensile tests. The results demonstrate a linear correlation between microstructure of gelatin films in terms of their triple-helix content and their macroscopic mechanical properties such as tensile strength and gel (Bloom) strength. Moreover, the findings of this study can help the scientists, in, e.g., art conservation and restoration, to predict the mechanical performance of these adhesives by performing a less material demanding and nondestructive physical measurement such as XRD.Structural Integrity & CompositesFacility Aerospace Structures & Materials LaboratoryNovel Aerospace MaterialsAdhesion Institut

Quantitative predictions of maximum strain storage in shape memory polymers (SMP)

Shape memory polymers (SMPs) are dynamic materials able to recover previously defined shapes when activated by external stimuli. The most common stimulus is thermal energy applied near thermal transitions in polymers, such as glass transition (Tg) and melting (Tm) temperatures. The magnitude of the geometrical changes as well as the amount of force and energy that a SMP can output are critical properties for many applications. While typically deformation steps in the shape memory cycles (SMC) are performed at temperatures well above thermal transitions used to activate shape changes, significantly greater amounts of strain, stress, and mechanical energy can be stored in Tg-based SMPs when deformed near their Tg. Since maximum shape memory storage capacity can be appraised by evaluating the viscoelastic length transitions (VLTs) in a single dynamic mechanical analysis (DMA) experiment, this study correlates VLTs with the measured storage capacities obtained from stress-strain experiments for a broad range of well-defined crosslinked acrylates, epoxies, and polyurethanes. This systematic approach allows for assessment of crosslink/junction density (νj), viscoelasticity, and chemical composition effects on maximum deformability, and enables predictions of the magnitude of shape memory properties across a wide variety of polymers. These studies demonstrate that the maximum storable strain (ε-storemax) can be accurately predicted using junction density (νj) and shape memory factor (SMF), the latter accounting for the contribution of chemical makeup.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Facility Aerospace Structures & Materials LaboratoryNovel Aerospace Material