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
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
To give a quick sub-categorization of the joining techniques, we can
classify them as follows.
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
relatively easy quality control, etc
And they, of course, come with their disadvantages as well, such as:
loosening of screws and bolts, especially causing
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
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
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
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.
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
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
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
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
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
Structural Joints and their common, relative issues can be described as
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 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.
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
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
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
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
Identified Inspection Needs:
What to Inspect (i.e. pressure distribution, bending in the
Non-destructive Testing methods identification
How to measure (i.e. what simulation method or sensor
How often to measure
How accurate should the measurements be
How to process the inspected data
A central control system to process, collect, analyse
Data Modelling and Prediction (for special applications of high
Predictive Maintenance Strategy setting and
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
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
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.