The approach to cyclic fatigue fracture testing of metals and alloys with the data presentation as "Paris-Graph" developed by Paul Paris and co-workers has proven immensely useful for structural ...engineering. It is, therefore, understandable that the later development of fatigue fracture test methods for fiber-reinforced polymer (FRP) composites followed this route also. However, recently questions have been raised by several researchers that require looking at fatigue fracture of FRP composites in more detail in attempts at using the full potential of this class of materials in structural applications. One of these questions is how the apparent analogy in the shape of the curves between Paris-Graphs of metals, alloys and FRP composites relates to the material specific, physical damage mechanisms. For FRP composites, the investigation of these mechanisms has a long history, but only recently, there seems to be a real advance using sophisticated pattern recognition of acoustic emission waveforms with complementary methods such as multi-physics simulations or high-resolution X-ray computed tomography. Comparing, e.g., crack sizes on the order of tens of micrometers occurring on time-scales of microseconds or less with visually observed delamination propagation, it becomes clear that the data analysis for the Paris-graph is averaging over orders of magnitudes in both, length and time-scales and this inevitably involves some scatter. Another question raised concerns the apparently larger scatter in Paris-Graphs obtained for FRP composites compared to those of metals or alloys. Again, a full understanding of the sources of this scatter has not been reached yet, but likely, the complex morphology and microscopic damage mechanisms of FRP composites play a role. There are further effects from mesoscopic or large-scale fiber bridging in fatigue fracture due to special fiber lay-up used in testing. For structural design with FRP composites, it is not sufficient to select the material performing "best" in the fatigue fracture tests, but it is essential to have safe, reliable, but on the other hand realistic design limits. Scatter in the data and how much of that has to be taken into account first requires understanding the extrinsic (test related) and intrinsic (material related) scatter sources. The concept of a fatigue threshold is also important for fracture-based structural design, since there are indications that threshold values for FRP composites may be quite low.
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•IF-WS2 nanoparticles can toughen certain epoxy systems, but fail to toughen others.•The toughenability depends on the type and amount of curing agent used.•Polyetheramine-cured epoxy ...systems behave entirely differently to others.•Clear correlations were observed for polyetheramine-cured epoxy systems only.
This work deals with the toughening effect of inorganic, fullerene-like WS2 (IF-WS2) nanoparticles (NPs) on epoxy. It has been hypothesized that this toughening effect depends on the epoxy’s cross-link density, its molecular defect fraction or its reference fracture toughness KIc. Seven different epoxy systems were filled with 0.5% laboratory-made IF-WS2 NPs by mass and investigated in order to analyze which material properties are determining the toughening effect. These NPs were similar to commercially available IF-WS2 NPs, but their agglomerates could not be broken up as successfully and they yielded less toughening effect. The cross-link density of the epoxies measured via dynamic-mechanical thermal analysis agreed reasonably well qualitatively with the theoretical estimation. The glass-transition temperature and the compressive yield stress were not affected significantly by the IF-WS2 NPs. The toughening effect of IF-WS2 depended entirely on the curing agent type and quantity. Polyetheramine-cured epoxies behaved differently from the others in their yielding behavior, but also in the IF-WS2 NPs’ toughening effect: While some of the investigated material properties correlate strongly with the toughening effect for polyetheramine-cured epoxies, the correlation for all investigated epoxies is rather low. Thus, none of the mentioned hypotheses could be clearly confirmed.
•We present modified DCB-specimen from GFRP rod.•We discuss comparison of different pre-cracking methods.•We provide evidence for manufacturing effects on fracture toughness.•We investigate effects ...of fiber bridging on data analysis and fracture toughness.
Pultruded glass fiber polymer-matrix composite rods are industrially produced for various applications. The development of a quasi-static mode I delamination test for these rods using a modified Double Cantilever Beam specimen is described. Issues for investigation were size and shape of test specimens, type of load introduction, and different pre-cracking methods. When the test procedure was applied to rods with different lengths, a dependence of the R-curve behavior on the specimen length was revealed. This is attributed to the strong fiber bridging that was observed in the tests. There are also indications for manufacturing effects on scatter.
The feasibility of using the modified Hartman–Schijve (HS) equation to analyze the fatigue fracture performance of adhesively bonded wood specimens under cyclic mode II loading was investigated in ...comparison with the Paris crack growth equation. Wood joints prepared with three different adhesives have been subject to cyclic Mode II testing at room-temperature (
23
∘
C
and 50% relative humidity) in a four-point End-Notched-Flexure configuration, determining the crack length from specimen compliance. It was shown, that the HS-equation can be successfully applied to adhesively bonded wood and that it successfully estimates threshold and maximum energy release rate (ERR) values for three different adhesive systems. Since a limited number of tests were performed for investigating the feasibility, scatter sources and possible scatter reduction methods are analyzed and discussed in detail. Also, a new, automated data reduction method was developed for estimating the maximum and the threshold ERR (
G
thr
)
values. The main advantage of the HS-equation appears to be the application in design standards. However, before the maximum ERR and
G
thr
values derived here can be used in design applications or for drafting a design guideline, additional testing is required for understanding how the number of cycles, the related measurement resolution; the corresponding ERR value influence the threshold value
G
thr
and how and to what extent its scatter can be reduced; and to further explore the link between cyclic ERR and the critical ERR value measured during quasi-static fracture tests.
•Comparison of wood adhesives under quasistatic and cyclic mode II fatigue fracture.•Higher elasticity modulus adhesives perform better in mode II cyclic fatigue fracture.•Short polymer fibers in the ...adhesive improve the mode II fatigue fracture performance.•Compliance-based, visual and Acoustic Emission location yield consistent crack length.•A new algorithm reduces scatter in determining the delamination onset point.
This paper investigates the energy release rate (ERR) in Mode II in-plane shear during delamination propagation under quasi-static and cyclic fatigue fracture loading with the 4-point end notched flexure (4-ENF) fracture test specimen. Wood joints bonded with three different adhesives, one rather brittle phenol resorcinol formaldehyde (PRF) and two different one component polyurethane (1C-PUR) adhesives with relatively low modulus of elasticity were tested in order to investigate the influence of the adhesive properties on the damage propagation under quasi-static and cyclic fatigue loading. A simple reduction method based on the specimens’ compliance was used to calculate the crack growth and the energy release rate during the test. Additionally, an automated analysis method was developed estimating the energy of crack initiation from quasi-static test results. This shall avoid introducing additional scatter due to operator-dependent, manual analysis. It was shown that the three tested adhesives are displaying similar ERR values under quasi-static loading. Under cyclic fatigue fracture loading, the more brittle PRF samples are showing a slower crack growth rate for similar energy release rate in comparison with the 1C-PUR adhesives. The proposed testing method, applied to adhesively bonded wood joints, has been shown to give satisfactory results. This can be used for the development of new adhesives with increased performance regarding fatigue delamination growth. The automated data analysis has potential for application on other materials under cyclic Mode II fatigue fracture loads.
Investigations of quasi-static and fatigue failure in glued wooden joints subjected to tensile shear loading are presented. Lap joints of beech wood (
L.) connected with four different types of ...adhesives, i.e. polyurethane (PUR), melamine urea formaldehyde (MUF), bone glue and fish glue, were experimentally tested until the specimens failed. The average shear strengths obtained from the quasi-static test ranged from 12.2 to 13.4 MPa. These results do not indicate any influence of the different adhesive types. The influence of the adhesives is only visible from the results of the fatigue tests, which were carried out under different stress excitation levels between 45% and 75% of the shear strength. Specimens bound with ductile adhesive (PUR) showed a slightly higher number of cycles to failure (
) at low-stress levels and lower
at high-stress levels in comparison to more brittle adhesives (MUF, fish glue). In general, the performances of animal glues and MUF were similar in both quasi-static and fatigue loading under dry conditions.
Inorganic fullerene-like tungsten disulfide (IF-WS
2
) nanoparticles are useful additives for polymers and lubricating agents, in particular when their surfaces are functionalized by silane ...modifiers. However, both the success of such a silanization reaction and its effect on the final dispersion quality are still doubtful. In this work, IF-WS
2
are functionalized using three different silane modifiers and investigated with X-ray photoelectron spectrometry, infrared spectroscopy, titration, thermogravimetric analysis, and mass spectroscopy. Eventually, they are dispersed within ethanol by sonication to compare the dispersing behavior. The combination of the different analytical techniques revealed that the IF-WS
2
surfaces can be functionalized with two of the used silane modifiers, while the third one was repeatedly unsuccessful. The amount of Si on the particles seems to be a fairly clear indication for the success of the functionalization reaction. The IF-WS
2
seems to oxidize during the functionalization process, probably producing acidic SO
2
or SO
3
, which can fully acidify a basic surface modifier. The executed treatment without any added silane modifier improved the dispersibility of the IF-WS
2
within ethanol to some extent, but added modifiers deteriorated it significantly. TEM images indicate that IF-WS
2
particles form aggregates, which might be the reason for the limited dispersibility.
Summary
Fracture surfaces of stoichiometric and sub‐stoichiometric epoxy, both neat and filled with 0.5 % of inorganic, fullerene‐like WS2 nanoparticles (NPs) by mass are investigated with ...atomic‐force microscopy (AFM). Sub‐stoichiometric epoxy is very brittle, but other than stoichiometric epoxy, it increases significantly in its fracture toughness with NP addition. For sub‐stoichiometric epoxy, AFM investigation revealed a very regular fracture pattern with periodic features normal to the crack propagation direction. Stoichiometric epoxy exhibited ripples several micrometers in size, but nevertheless had a lower fracture surface area. After NP addition, the fracture surfaces were significantly rougher, but the mentioned ripples disappeared. The fracture surface roughness correlated neither to the measured fracture toughness nor to its increase with the nanocomposite's composition.
To better understanding the failure of adhesive joints tensile tests were carried out on miniature test specimens from Norway spruce in the synchrotron. Urea-formaldehyde resin was used as adhesive. ...e. For comparison purposes, tensile tests were carried out on solid wood and on bonded miniature tensile shear samples with acoustic emission. The acoustic emission signals of all the experiments occurred with classified pattern recognition. This resulted in two classes of signals for each two frequency peaks. One class consisted of the low-frequency and the other of the higher-frequency peak of higher intensity, but this was essentially independent from the structure (solid wood or plywood) and size scale of the test specimens. The influence of the adhesive layers was determined on wood test specimens on laboratory scale and on miniature test specimens with an adhesive layer and selected fiber orientations. This gave evidence that the sound emission signals from the failure of the adhesive layer presumably of the class with low frequency signals peak in the range of services can be assigned.
•Hartman-Schijve equation characterizing cyclic fatigue fracture of composites.•Pointing out the limitations of this approach.•Assessing the perspectives for using such data in composites structural ...design.•Suggesting open questions for further research.
Use of fiber-reinforced polymer-matrix (FRP) composites in structural design requires material data, in particular on their cyclic fatigue delamination behavior under various modes of loading, e.g., tensile opening (Mode I) or in-plane shear (Mode II) and the associated threshold values. There are no standard test methods for characterizing delamination propagation yet, except one procedure defining the determination of fatigue delamination onset. Data analysis and interpretation is often based on the expertise obtained from cyclic fatigue testing of metals and alloys and remains empirical. In the present contribution, the applicability of the modified Hartman-Schijve equation (originally developed for metals and alloys) to the characterization of fatigue delamination behavior of FRP under cyclic mode I loading will be discussed. The main focus is on interpreting information on the fatigue threshold and its scatter in FRP composites.
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