Texture Analysis in the 3D Printing Industry

In the 3D printing process, a physical object is produced from a 3D digital model. This generally occurs by the laying down of successive thin layers of material to bring the digital model to life.

The steps involved in this process will vary depending on the type of printer and the raw materials involved, but generally they are as follows:

1: A 3D model is created using a modelling package or 3D scanner, saved as a Computer Aided Design (CAD) file

2: The user slices the CAD file and uploads it to the printer

3: The printer reads and creates each 2D slice to form a 3D object

Why we need controlled failure

We live in a world obsessed by perfection.  Everything is supposed to be better if it’s shiny, defect-free and stronger than a Sherman tank. 

But be careful what you wish for, because a flawless world wouldn’t work nearly as well. Imperfections are tools, and we would be lost without them.  

Of course, flaws are not always convenient.

Imagine bending a long thin object, like a bar of chocolate. If the chocolate has flat surfaces, each side is stretched evenly. It’s hard to break, but if you put a notch in it the section of chocolate just underneath also has to do the job of the bit you’ve taken out. The chocolate will break at the notch. It’s not because it’s thinner at that point but because the sharp point of the notch concentrates the force in a very small area.

In a flawless object the stress you put on a material by pushing, stretching, bending and twisting is relatively evenly spread. Features like notches, voids and tiny cracks make the pattern of stress more complicated, concentrating the stress in some bits and relieving it in others. If you could see stress as colours, every object you pushed on would suddenly light up with bands of colour and the flaws would be tiny pinpoints of dazzlingly bright light. Those pinpoints are vulnerable, overloaded already, they can’t take much more.

Paper and Cardboard Testing with a Texture Analyser – Part 3

Reliable Testing for Packaging and Print

Tensile Test
Example standard: ASTM D828

Tensile testing is applied to paper products for various reasons to assess strength in use; standards such as ASTM D828 cover the methods required to measure the tensile properties of paper and paperboard. Tensile tests are used for combined corrugated board but not as frequently as bending, compression and puncture.

Paper and Cardboard Testing with a Texture Analyser – Part 2

Reliable Testing for Packaging and Print 

Pin Adhesion Test
Example standard: TAPPI T821

Another factor that controls the quality of corrugated board is the strength of adhesion between the corrugating medium and its facings. This adhesion can be measured directly using a ‘pin adhesion test’, where pin adhesion is the force to separate the two components. In this test, pins are inserted between the facing and fluted medium, then attached to rigs connected to the loading arm and base. The rigs are pulled apart at constant speed until the facing is separated from the flute tips. Separation of all bond lines is not necessary. This test assesses the quality of the bond formed when the board was combined, and can be used to detect some manufacturing defects such as poor adhesive penetration and spotty adhesive application.

Paper and Cardboard Testing with a Texture Analyser – Part 1

Reliable Testing for Packaging and Print

 

Corrugated cardboard is used all over the world for strong, cheap and light recyclable packaging.

It is made up of a fluted interior sandwiched between linerboards. Variations include a linerboard on only one face, or several stacked layers. It is generally made into cardboard boxes, although occasionally used in sheet form for strengthening or insulation purposes. Most boxmakers perform a wide range of tests on their products as well as the components used to make them. Many boxes are made to certain customers’ demands, and the aim of these tests is to maximise strength (meeting the customer’s needs) while minimising costs from component materials and weight.

 

Texture Analysis in Action: Vickers and Shore Hardness Probe

Vickers testing has been a very widely-used technique for many materials for decades due to simple data analysis and test preparation. 

Many applications demand a tip made from a very hard material such as diamond or sapphire, as the indenter must be significantly harder than the material under investigation. However, stainless steel is suitable for testing many softer materials such as tablets, soap, fruit and vegetables, cheese, chocolate and some polymers.

Traditionally, Vickers hardness is calculated by inputting the indentation diagonals into a standard formula. However, instrumented indentation on a Texture Analyser allows automatic hardness analysis without the use of a microscope. It also enables other parameters such as stress relaxation and elastic-plastic energy ratios to be calculated. 

Why change from the Standard Method approach?

Tape Unwinding Rig A/TUR

A Standard Test Method is a definitive procedure that produces a test result. In order to ensure accurate and relevant test results, a standard test method is explicit, unambiguous, and experimentally feasible, as well as effective and reproducible.

A standard test method can be considered an experiment that determines one or more characteristics of a given sample and the detail of the test method is usually as a result of creation from a number of experts in that measurement field. Within the materials testing industry, for example, there are an enormous number of standard methods (e.g. ASTM, ISO) that have been developed to measure specific sample characteristics. 

How to measure Engineering Terms with your Texture Analyser

Standard engineering calculations have now been built into Exponent and Exponent Connect software for quick calculation of specific moduli, stresses, strains, strengths and energies which are particularly suited to materials testing applications for our range of Texture Analysers.

Each calculation is designed to be used with a very specific test setup (for example, cuboid three point bend testing) as the specific equations required for analysis depend on the sample and test geometry. A help page is supplied within the software for each quick calculation explaining the parameters that must be entered into the software by the user (such as film thickness) and a derivation and reasoning behind every equation used.

Biaxial Film Testing using a Texture Analyser – Calculating Fundamental Parameters

The use of the Film Support Rig can provide a very useful test for monitoring the quality of irregular objects, such as the toughness of sliced meat. However, this test setup can also provide more fundamental stress-strain data if the sample has a known uniform thickness and homogeneous structure.

Sometimes the properties of a thin film are under investigation, but the film might be unsuitable for any other type of testing. An example would be a film of dried nail polish; depending on its properties, this can be too brittle to clamp in tensile grips, it is too thin to test under compression and it may not be large enough for bend testing. In cases such as this, the Film Support Rig is ideal, as the sample is clamped at the edges and put under “biaxial tension” by a spherical probe pushing into the centre. 

Contained Compression Testing using a Texture Analyser – Calculating Bulk Modulus

Bulk modulus is a defined as the relative change in a sample’s volume when a unit compressive or tensile stress acts uniformly over its surface. 

It is a measure of how resistant a substance is to pure compression. Its measurement is possible by the use of a Texture Analyser, providing care is taken over the sample setup.

The Measurement of Hysteresis using a Texture Analyser

Hysteresis in the context of deforming a material is the loss of energy in the form of heat when a sample is loaded then unloaded. 

This occurs due to internal friction, and so the effect is larger for a material such as rubber, which requires the movement of large molecular chains each time it is stretched or compressed. If a piece of rubber is deformed several times in the hands, this loss of energy can be felt directly as the sample heats up. 

As can be seen on the graph, the deformation of a viscoelastic material follows a different path on the load and unload cycles. The unload cycle shows that the material is slower to recover compared to the load cycle. This difference in recovery is known as hysteresis, which is due to energy dissipation by the generation of heat.

Compression Testing using a Texture Analyser – Calculating Fundamental Parameters

Compression testing involves a self-supporting sample being compressed under a flat probe large enough to cover the whole sample. 

The loading arm (attached to the probe) moves down at a constant speed to deform the sample, first deforming it elastically then plastically (if it is not completely brittle). If the force required to break the sample is within the limit of the load cell, fracture may occur. However, this is not always the case in compression as many samples are stronger (or tougher) in compression than in tension, especially ceramic samples.

Three Point Bend Testing using a Texture Analyser – Calculating Fundamental Parameters

When a customer is presented with an elongated or flat food sample, one of the first things they might do is snap it between their hands. 

During this process, they are subconsciously assessing whether the food product is brittle or ductile, stiff or compliant, and strong or weak. Consequently, bend testing is a technique used very frequently by Texture Analyser users, covering everything from the snap of tablets to the toughness of dog chews. 

The analysis of this type of testing is often limited to looking at a force peak and maybe the distance to fracture. It is a very useful test for monitoring quality of irregular objects. However, this test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample dimensions. “Flexure” and “bending” have the same meaning and are often used interchangeably.

Tensile Testing using a Texture Analyser – Calculating True Stress and True Strain

The calculation of parameters from the stress-strain graph of a tensile test has already been covered in a previous blog post. 

The calculations considered were those most often used when referring to stress and strain, and to give them their full name they would be called “engineering stress” and “engineering strain”. It is usually safe to assume that every time stress and strain are mentioned in the literature, this refers to the engineering values.
 
However, as the load on a sample increases, the cross-section over which the force is applied changes (it gets thinner). If the engineering stress is used (taking into account the initial area), the stress is underestimated. True stress solves this issue by using the instantaneous area over the course of application of load so that as the cross-section changes, the value of stress is calculated using the new cross-sectional area.
Additionally, whereas engineering strain is the amount that a material deforms per unit length, true strain is the natural log of the current length over the original length.

Tensile Testing using a Texture Analyser – Calculating Fundamental Parameters

Tensile testing involves a sample held in two grips a set distance apart. The loading arm (attached to the top grip) moves up at a constant speed to deform the sample, first deforming it elastically then plastically. If the force required to break the sample is within the limit of the load cell, fracture will occur.

It is a very useful test for monitoring quality of irregular objects, such as the toughness of pizza or the texture of fish. However, this test setup can also provide useful stress-strain data if the sample has a uniform cross-section, providing accurate measurements are made of the sample’s dimensions. “Dogbone” shaped specimens are often used in tension, with two wide sections tapering to a narrower central section.

Indentation Testing using a Texture Analyser – Calculating Fundamental Parameters

Although it has long been known that some materials are harder than others, indentation tests to find quantitative hardness values only came about in the 1800s. 

Once it was established as a valuable technique, hardness testing machines started to appear on the market early in the following century. Old fashioned indentation testing involved the application of a weighted probe onto a flat sample surface that was left for a set time period. The hardness of the sample was calculated from the area of the residual dent left in the sample. “Instrumented” (computer controlled) indentation testing has now been in use for many years, and involves the collection of force, displacement and time data, which is why the Texture Analyser is so well-suited to this type of measurement.

Testing the Behaviour of a Keyboard using a Texture Analyser

When performing an objective test on a keyboard, the main components under investigation are the switches, actuated by pressing keys.

From keyboard to keyboard, there is little a given manufacturer can do to affect a switch's performance. The switches come in batches from switch makers, and keyboard manufacturers mount them onto printed circuit boards and do not have the ability to alter them. Seth Colaner at Tom’s Hardware has developed a procedure for testing mechanical keyboard switches using a TA.XTplus Texture Analyser.

Previous attempts at testing switch performance has involved the use of small weights, but this is not an accurate technique. Maintaining the balance of multiple weights placed on top of keys with varying geometry affects the measurement as it is easy for weights to move off-centre. Additionally, the forces required to actuate a switch can be very small (on the order of 50g), so the way even small weights are placed can fluctuate the actual load and throw off the measurement.

Viscoelasticity in the Materials Industry

Many industrial materials have viscoelastic properties depending on their constituents, processing conditions and the conditions during use.

They can be very obviously viscoelastic when held in the hands, or this behaviour may not be apparent until heated up to 1000°C and put under a large stress. The relaxation time also varies a large amount. Whether this viscoelasticity is beneficial or not depends on the material’s type and intended use.

Memory foam mattresses are becoming very popular due to their ability to mould to the shape of the sleeper’s body, relieving pressure on painful joints. They have a layer of temperature-dependent viscoelastic material - when a load is applied (a person), the material relaxes away to take on the load’s shape. They then show slow springback when the load is removed.

Testing packaging pouch performance

Stand-up flexible pouches are in vogue all over the world and according to experts are set to experience high growth in the future, too.

There are many reasons for this. They are attractive to consumers and easy to handle to transport, for instance. They were very much led by the squeezable baby fruit sauce packages but now when we observe the retail food shelves, we see ketchup, mayonnaise, wine, salsa, honey, juice, premixed cocktails, and a host of fluid food products in stand-up flexible pouches.

Hosiery put to the test!

A pair of tights is usually put on with a certain amount of stretching, and fingernails or jewellery often snags on a single thread that will turn into a run or “ladder” once the leg is applying stress to the tights. 

Additionally, a ladder may be caused during the working day, such as a snag occurring when the tights are pushed against the underside of a desk. This can greatly inconvenience the wearer as it appears unprofessional and they may not have a spare pair. It can also be expensive.