In thermoplastic composite joints with a circular ultrasonic spot weld, the damage growth is located at the interface between the joined components. This means that any damage in these joints is invisible from the outside. This experimental study compares three different in-situ methods to measure the damage growth indirectly. The motivation lies in the potential benefits of using multi-spot welded joints for increased damage tolerance and the need to prove the damage arresting and damage progression behaviour for certification. The current study focused on measuring the damage in single-spot welded Single Lap Shear (SLS) joints during fatigue test with the global specimen compliance, the local out-of-plane displacement and the local strain on the surface. Digital Image Correlation (DIC) measurements were used to obtain the local quantities. The results showed that the local quantities are better suited to obtain detailed information on the damage state. The local surface strain showed more distinct features that facilitate the recognition of damage locations. The benefits and challenges of all methods are discussed as well as the difference in the level of detail, generalization and the prior knowledge necessary to obtain the damage state.

Thermoplastic composite spot welded joints are more well-suited for carrying shear load rather than peel load. However, peel load is difficult to be eliminated in single-lap joint configuration. In this paper, a series of mechanical tests were carried out to study the effects of the spot spacing and the number of welded spots on the secondary bending and subsequently on the strength of the multi-spot welded joints. Based on that, a comparative study was performed on the load-carrying capability of multi-spot welded and mechanically fastened joints assembled with multi-row fasteners. It was found that, by increasing the spot spacing and the number of welded spots, the secondary bending of the multi-spot welded joints can be effectively decreased, which eventually results in a comparable load-carrying capability with respect to the mechanically fastened joints.

In previous work, single-spot ultrasonically welded joints were found to feature similar load carrying capability in shear but significantly low capability in peel as joints with a representative single-mechanical fastener. This leads to questioning welding as an appropriate solution for the commonly-used single-lap joint configuration. The present paper investigates the mechanical performance of spot welded single-lap joints in thermoplastic composites in comparison to their mechanically fastened counterparts. Single-row joints, double-row joints with varying inter-row distance and multi-row joints with varying number of rows were investigated in this study. The results showed that, owing to higher joint stiffness and hence lower secondary bending and peel stresses, the load carrying capability of the spot welded joints was comparable to that of the mechanically fastened joints in all considered cases. Likewise, the effects of increasing the inter-row distance and of increasing the number of rows were similar for both types of the joints.

Additive manufacturing (AM) techniques enable fabrication of bone-mimicking meta-biomaterials with unprecedented combinations of topological, mechanical, and mass transport properties. The mechanical performance of AM meta-biomaterials is a direct function of their topological design. It is, however, not clear to what extent the material type is important in determining the fatigue behavior of such biomaterials. We therefore aimed to determine the isolated and modulated effects of topological design and material type on the fatigue response of metallic meta-biomaterials fabricated with selective laser melting. Towards that end, we designed and additively manufactured Co-Cr meta-biomaterials with three types of repeating unit cells and three to four porosities per type of repeating unit cell. The AM meta-biomaterials were then mechanically tested to obtain their normalized S-N curves. The obtained S-N curves of Co-Cr meta-biomaterials were compared to those of meta-biomaterials with same topological designs but made from other materials, i.e. Ti-6Al-4V, tantalum, and pure titanium, available from our previous studies. We found the material type to be far more important than the topological design in determining the normalized fatigue strength of our AM metallic meta-biomaterials. This is the opposite of what we have found for the quasi-static mechanical properties of the same meta-biomaterials. The effects of material type, manufacturing imperfections, and topological design were different in the high and low cycle fatigue regions. That is likely because the cyclic response of meta-biomaterials depends not only on the static and fatigue strengths of the bulk material but also on other factors that may include strut roughness, distribution of the micro-pores created inside the struts during the AM process, and plasticity. Statement of significance: Meta-biomaterials are a special class of metamaterials with unusual or unprecedented combinations of mechanical, physical (e.g. mass transport), and biological properties. Topologically complex and additively manufactured meta-biomaterials have been shown to improve bone regeneration and osseointegration. The mechanical properties of such biomaterials are directly related to their topological design and material type. However, previous studies of such biomaterials have largely neglected the effects of material type, instead focusing on topological design. We show here that neglecting the effects of material type is unjustified. We studied the isolated and combined effects of topological design and material type on the normalized S-N curves of metallic bone-mimicking biomaterials and found them to be more strongly dependent on the material type than topological design.