Understanding failure

Joining composite together poses a number of very different issues from traditional materials. In automotive and those more use to working with metals the reliance on welding is no longer a turn to design staple. It is also not possible to drill and tap composite and honeycomb panels so standard fastener can be difficult to use too. And while bonding is also a popular option, it doesn’t bode well later in life for disassembly.

Dorset based Bighead Bonding Fasteners thinks it has a solution with its patented BigHead fasteners. These are normally the female insert, which are either bonded or infused on to composite panels. It recently undertook a project with Cranfield University that aimed to investigate the surface preparation and attachment techniques of its steel bonded fasteners for carbon fibre composite automotive panels. Among the goals were to better understand the failure process under different load conditions and the effect that joint design has on the eventual failure mode.

To conduct the tests, five types of steel fasteners and two types of adhesives were used. In addition, two joint designs were investigated, one was embedding the insert fasteners into panel during manufacture so it was infused with the structure of the panel. The second was to bond an insert directly on to the external surface of a sheet moulded composite (SMC) panel.

To prepare the infused and bonded samples, the fasteners were first cleaned three times in acetone. For the infused insert, holes were then made directly in the dry fabric. A six layer 530gsm fabric was used, laid up in ±45°, with the inserts placed through the holes of one, three and five layers of the fabric. The dry fabric was then infused with either methacrylate or epoxy resin and hot pressed to make the consolidated solid composite panel, with the necessary protruding inserts infused in place.

The bonded fasteners were bonded as normal with two types of adhesives used. Five types of fastening were used across the two joint designs that varied the collar length and diameter of the insert, as well as a bonded nut insert with a much shorter, wider collar that appears almost stub like.

A pull test was conducted on the test pieces at 0, 45, and 90° to replicate various in-service loading scenarios.

The results showed that for infused inserts at 90° the nut geometry proved the strongest, as its collar proved the shortest leverage arm. However, at 0 and 45° there was no significant different in the force applied at failure, other than the M8 inserts performed better than M6 inserts, as would be expected. Indeed, for the M6 bolt it was the screw bolted to the insert that failed.

For the bonded joint, the inserts did bend during loading whilst remaining fully attached. The damage growth was reported to be progressive, with failure only being sudden the type 2 laminate where the insert has just a single layer of carbon fibre embedding it.

the panels tended to remain entirely intact, with only superficial delamination of the panel. The failure of the bond line occurred on the methacrylate, with an adhesion failure of the epoxy.

The conclusion of the work is that Infused fasteners proved to provide excellent safety and serviceability as the embedded fastener placed within the composite panel during manufacture provided a stronger joints than simply adhesive bonding. And while acrylic adhesive provided stronger joints, epoxy is expected to provide a higher toughness.

Call to standardise the testing of superhydrophobic materials

Researchers from Aalto University in Finland have called for consistent and standardised testing of superhydrophobic materials. They argue that agreeing on a unified testing method is needed to allow community-wide comparison between published results. This would significantly progress development of superhydrophobic materials and their transfer to commercial products in, for instance, self-cleaning and anti-icing applications.

Currently, research groups around the world use many different kinds of tests to evaluate the durability and wear of superhydrophobic materials. For example, researchers have used linear abrasion, circular abrasion, sandblasting and water jets in testing the surfaces. However, the results obtained through different methods are not comparable, which makes it hard to find the best materials for applications.

Research groups around the world currently use many tests to evaluate the durability and wear of superhydrophobic materials. For example, researchers have used linear abrasion, circular abrasion, sandblasting and water jets to test the surfaces. However, results obtained through different methods are not always comparable, making it hard to find the best materials for applications.

Professor Robin Ras, from the department of applied physics at Aalto University, explains: “We find that linear abrasion of the surface, for example with sandpaper, would be the best general method. It is important to specify the applied pressure, the abrasion distance and the abradant material to make sure the result is reproducible.”

Benefits of the proposed method include, among other things, the availability of the testing materials and the simple test setup. However, Tuukka Verho, a researcher on the team, adds: “The wear test alone is not sufficient to determine the robustness of the superhydrophobic surface. The second step is to perform tests with water droplets to measure the water-repelling properties of the surface after wear.”

The superhydrophobicity of a surface is often measured based on how completely the water droplet beads up in the surface. However, according to the scientists, this so-called static contact angle does not provide enough information about the effects of wear, as water often forms almost spherical droplets even on a damaged surface. Their solution is to measure droplet mobility, for example using an inclination test.

Verho and Prof Ras say they want their suggestions to initiate a discussion in the research community on the evaluation of superhydrophobic surfaces. Even though they propose linear abrasion as a primary test for all superhydrophobic materials, additional application-specific tests may be needed, for instance, to assess the laundering durability of textiles or the weather-durability of outdoor materials.

Justin Cunningham

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