The joining of materials is the fundamental component in a product, part, or process creation alongside forming. Most physical things are formed, assembled, fastened, glued, or somehow else attached by two or more particles. Depending on the nature of the product or a process – there should be a special type of joining dedicated to its parts – to ensure characteristics such as sturdiness, longevity, flexibility, etc.

In this blog, we will classify the joining methods, their pros and cons, failure occurrence, and especially how to monitor the integrity of the joint, and prevent its failure.

Materials Joining Methods and Types

First, let’s identify the main joining methods and their specifications. By the method, a joint can be

  • 1. mechanical
  • 2. welded
  • 3. adhesively bonded

With the continuing advances in various industries, device parts and processes are getting increasingly complicated in design. A variety of factors influence the choices in creating a highly reliable joining process such as the economics of the production, mechanical properties (strength, vibration, durability, corrosion, etc.), ability to correct defects on the item, and so on.

To give a quick sub-categorization of the joining techniques, we can classify them as follows.

Mechanical Joining

These capitalise on fasteners, nuts & bolts, screws, clamps, shackles, and use mechanical energy for joining such as Riveting, Caulking, Bolting, Shrink fitting or Folding

The Advantages of mechanical joining are many, such as their:

  • high strength capacity
  • variety of choice, in type and size of components to use
  • relatively easy quality control, etc

And they, of course, come with their disadvantages as well, such as:

  • loosening of screws and bolts, especially causing chain effect
  • difficult or impossible to repair joints
  • patented and high-cost types

However, mechanical joining is one of the most widely used for a good reason, that being the relatively high trustworthiness and predictability of their medium-term operation. For more overview on the topic, including market trends and structural health monitoring of bolted structures, read our dedicated article: Fatigue Failure in Bolts: How to prevent it?

Fusion and Welding:

This is another important joining method, which has proven its usefulness, especially in metallurgical and composite material joining.

Welding includes fusion welding, brazing and soldering, and solid-state welding.

Fusion welding is the melting and solidification of the specific zone which is being joined, however, in metals/plastics, both the workpieces and the filtering compúonentare exposed to melting and solidification.

On the other hand, brazing and soldering is a joining technique which uses added melted filter between the surfaces.

Solid-state welding uses the deformation and diffusion of plastic materials as a joining technique, thus it needs no melting filters or components.

Another option is Pressure Welding which may use the electrical chemical or light (laser) energy to join parts together.

Fusion and Welding process control, and especially their structural integrity during the life-cycle is another important component of consideration for the end-user, which makes this a topic of utmost interest for RVmagnetics.

You can read more about the topic and our solution for welding and composite structures in this article

What are Adhesives?

To define them, we can say that adhesives are substances that, when applied in between the surfaces of two or more objects, are used to hold, bond, or fasten them to each other. These are used for permanent, semi-permanent, and temporary attachment purposes, and depending on the application of use, as well as the industry – these are characterised for residential, commercial, or industry-specific applications.

To go through a quick definition of adhesives we can specify them as follows:

  • Anaerobic adhesives: Acrylic-based, designed to cure in the absence of air.
  • Cyanoacrylates adhesives: These are also known as “instant glues and”, are designed to cure in the presence of moisture and UV.
  • Epoxy adhesives: These are useful for applications where the extreme temperature is present and an adhesive can be exposed to high shear and peel (i.e., gap filling).
  • Hot glue: A melted material (i.e., thermoplastic-based) which solidifies by cooling.
  • White glue: These require contact and pressure when solidifying, and can be used for wood, cloth, paper, porous substrates, and so on.

Adhesives are often used in combination with mechanical joining methods to increase joint integrity such as in combination with bolts as well as in composite manufacturing processes such as autoclave moulding or filament winding where resin is used with mechanical pressure and stacking.

Factors to Consider When Selecting a Joining Process

Joining methods are used by almost all industrial applications, to create devices, structures, and processes of utmost importance as well as for everyday simple service. Joining methods are used not only to create but also to prepare and fix up a broken object.

Choosing the right method will determine the longevity, and thus also the security of a joint piece – so it is of course important to consider several factors before choosing your joining method. These factors are for example.

1) Materials of the joint piece

Will there be the same, similar, or two different materials of the object to join? One of the materials might withstand a weld, and the other might not. Similarly one of them might be easy to glue on. Is one of them stiff and brittle and the other one flexible?

2) Durability of the joint piece

The user might need a joint to be removable such as a sticky note, or sturdy like a welded metal piece. In case the user wants to remove and reposition the item, how many times can this potentially happen before the adhesive starts losing properties? In the case of a sturdy object – how sturdy should it be (as when exposed to some flexing it might break instead of bending and flexing).

3) Exposition to Stresses in the joint part

If it is expected for the joint to be exposed to tension, compression, shear, etc. – what are the levels of these? Is it important for the joint to be just as strong as the material it is joining?

4) Environmental Condition around the joint piece

Are there dusty, high/low fluctuating thermal, the greasy environment at or in the vicinity of the joint? Is there a defined period of time the joint should survive the set environment?

5) Surface preparation for joint

Are the surfaces of the materials necessary to prepare, and how difficult it is to prepare them for the joint process? This can include grinding, sanding, drilling and other actions to be performed on the surface of the material.

6) Final Look of the Joint Piece

Should the joint part be visible (i.e. for inspections)? How important is it for the end-user to have a seamless joint part?

7) Maintenance and repair needs of the joint piece

How often can the joint go through maintenance and repair, how often will it need to undergo maintenance repair and eventually replacement? How consequential can a failed maintenance be for the joint piece?

8) Finance, time and other resources involved

This includes the budget, ROI, and the opportunity cost of the material replacement (if the joining is especially for repair purposes). Are there pressing deadlines and schedules in place for the joining to take place?

When a joining method is finally chosen, it gives birth to other issues to be considered, such as the sub-types of the joining methods, and as already mentioned above a few times – the monitoring of the integrity of the joint, let us expand on this further.

Identifying the Integrity of Structural Joints

The above-mentioned challenges primarily raise the question of joint integrity. The different applications can have different expectations from joint integrity and not any less important, different tolerance of joint failure: if a sticky note’s reusability is inconvenient for some, a failure of a weld on a thermoplastic composite in an aerospace application can have tragic results.

Joint integrity is affected by additional factors even after choosing the most fitting joining method out there. We have already mentioned above the environmental factors, add to that the joining process failures, human factor, failure chain effects, and last but not least the defect or shock hidden, internal affect that goes unnoticed on the appearance of the joint – and we have ourselves a clear need for Joint Integrity Monitoring Strategy!

Structural Joints and their common, relative issues can be described as follows.#

  • Mechanical joints: For a bolted/riveted structure often occurring failures are due to fatigue, cracks, loosening, fibrations, and overall stress concentration in unfitting points due to flaws of design, clamping force and fretting.
  • Welded joints: the welded part is naturally the weaker point of a finally joined piece, and the effects on geometrical stress concentration or irregular distribution of the stress, as well as coarse grains at the heating process can end up with fatigue and yield of the joint, as more often errors of the welded parts.
  • Adhesive joints: Depending on the adhesive, the wetness, solidification i.e., the surfaces of laminates, insufficient pressure applied for bondage are common process errors, and operational eros such as exposure to environmental harsh conditions or levels of wear and tear which the adhesive isn’t designed to withstand, are other common fatigue components for adhesively joined components.

Let us go through a few examples of Identifying of State of integrity of structural joints, that have been subjected to fatigue loads.

Fatigue Simulation of Welds Using the Total-Life Method (Abstract from Andrew Halfpenny, HBM Prenscia).

Fatigue failure is described as a combination of crack initiation and growth to failure. The simulation was focused on both of the two-stage processes of fatigue occurrence. Typical examples in the fundament of this simulation are welded structures and lightweight jointed structures.

This method is a simulation, which is carried out on a continuous weld. The method offers benefits over traditional weld fatigue analysis techniques , which include:

  • Essential point is to identify and recognize the fatigue cycle as a two-stage process (crack initiation and growth to failure).
  • The influence of weld geometry, residual stress distributions, and the dressing of welds, are all important factors to incorporate into fatigue cycle estimates within this method.
  • The capacity of the weld to include complicated multiaxial loading into simulation.
  • Modeling of crack tip plasticity, as well as crack growth (fracture crack-based).

Structural Health Monitoring of Adhesively Bonded Joints: Proposing a new Method by use of Polymer Optical Fibers.

Integrating a Polymer Optical Fibre (POF) into the adhesive layer, this paper explains the monitoring of structural integrity of adhesively bonded joints. This concept uses a sensor and gets accurate results in a non-destructive way.

The method is based on the pressure or deformation affecting the POF from the adhesive which changes the cross-sectional shape of the fibre; a simple optical measuring device can detect the change in the detected signature.

This or similar methods have a potential disadvantage for an end user of having the fibres left out of the material, and needing precise positioning of the fibre and optical measuring device to get detections.This makes it hard for in-line operation, however, keeps it an option for structural integrity identification for adhesively bonded joints.

One size fits all? MicroWire Sensor for Joint Structural Integrity Monitoring from within

Overall, a system able to assess the structural health state of i.e. bolted joints, especially in situ, is dedicated to saving money and time on maintenance activities and allowing comparatively quick identification of the residual life and current degradation state of structures. Compromised joint integrity (i.e due to corrosion) can end up in leaking or other structural failures, eventually leading to production delays, security and safety concerns, fines and penalties, and of course, unplanned operational maintenance outages.

As we have identified some of the Factors to consider when selecting a joining process – we can now identify what are the main characteristics an inspection system should have for a final user to ensure the long-term safety of their joint, monitor the structural integrity with a method fit for their applications needs AND limitations.

Developing a Structural Joint Inspection System needs:

  • Clear information on the joint (environment, materials, type of the joint, etc.)
  • Identified Inspection Needs:
    • What to Inspect (i.e. pressure distribution, bending in the joint, etc.)
    • Non-destructive Testing methods identification
    • How to measure (i.e. what simulation method or sensor to use)
    • How often to measure
    • How accurate should the measurements be
    • How to process the inspected data
  • A central control system to process, collect, analyse the data
  • Data Modelling and Prediction (for special applications of high structural importance)
  • Predictive Maintenance Strategy setting and Implementation

At RVmagnetics, we are dedicated to satisfying some of the most crucial needs for developing an Inspection Method for Structural Joint Inspection system. Structural Health Monitoring is one of our main areas of expertise, as an R&D service provider in the field of custom measurements for our clients and partners.

We do this through developing our own MicroWire sensors. These are the smallest passive sensors in the world, fit for almost all environmentally harsh conditions ( datasheet ), and ready to process signals through almost all materials with no contact or power wiring necessary.

The MicroWIre works through magnetic principles, is thin and elastic like human hair, making it possible to place it in otherwise inaccessible locations and receive data from outside of the material.

Once RVmagnetics knows the factors affecting the joining process and what is necessary to inspect (i.e. environmental conditions, temperature/pres­sure distribution, the clamping force of a bolt, bending of a composite material in a joint part, etc.), the process is likely to follow up by:

The MicroWIre sensor and sensing system is able to serve the Joint Inspection needs of Bolted, Adhesive and even Welded (specifically, Composite Welding) joint structures, by overcoming the limitations of size, power wiring, high manufacturing cost, environmental limitations, longevity, and more.

Contact us for more information and free consultation on your applications’ measurement needs.