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 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.
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.
Plasticity
is a property shown by many materials, such as polymers, metals and the
majority of foods. It is the ability of a material to undergo permanent
deformation.
If
you press your finger on a spring, it returns to its original shape. If
you made that spring out of cheese, it would not spring back as far –
it would remain squashed. When stretching a sample, plasticity is seen
on the force-distance graph as a change in gradient after the initial
linear section (straight line). The point where the gradient changes is
known as the ‘yield force’. If this point is reached and more force is
applied, the sample will be permanently changed. Before this point, the
behaviour is elastic and spring-like.
Explaining texture preference
Understanding texture preference and the trends that may result in consumer behaviour change is an area of great opportunity. However, remaining agile before a consumer shift occurs can be a challenge.
A new webinar will address this challenge and will offer:
• A view on techniques to forecast upcoming influences on food trends
• A perspective on the linkage of newly monitored issues to texture and the process used to deliver higher value commercial insights
• Expertise on driving rapid product development through localising relevant texture trends from around the world
Speakers: Janet Carver, Culinology Group; Mary Lynne Shafer, Ingredion; Susan Badarroco, Culinary Tides.
Watch this webinar to learn more...
Fish is loved by consumers – it is unique, nutritious and well-balanced. The four quality elements of food are texture, appearance, flavour and nutrition.
Compared with the meat of livestock, tissue of fish is more soft and delicate in its chemical composition.
With the development of people's living standards and of the food processing industry, texture has attracted more and more attention.
A recently published paper describes several texture parameters, analyses various factors affecting fish tissue texture especially the protein composition of fish muscle tissue from 3 aspects – physical factors, chemical factors and post-mortem changes – and mainly reviews the effects of different processing methods such as heating, freezing, salting and smoking on texture of fish meat.
Texture can make or break a product’s perception. How to talk about it, how to target it and how to tap into the ingredient toolkit is a definite requirement for dairy developers.
With everything from Greek-style yoghurt to efforts at fat reduction underscoring texture’s importance, there’s no time like the present to turn texture to your advantage.
“In the past, consumers didn’t really notice texture until it was ‘wrong,’ such as a powdery texture in a smoothie or a slimy, cohesive texture in a sauce,” said Shana Brewer, a marketing manager at Ingredion Inc., Westchester, IL.
“But food manufacturers are now using texture as a key differentiating quality to improve a product’s overall consumer appeal.”
She cites as proof the uptick in front-of-package texture claims, noting that research from Innova Market Insights found the use of texture claims more than doubled globally over the last five years.
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.
The calculation of parameters from the stress-strain graph of a tensile test has already been covered in a previous blog post. 
