As I know, there is one qualitative relationship. The more residual stresses, the less strength of joints. All quantitative relationships depend on materials, technologies, welding materials, joint configurations, thickness of work-pieces and many other parameters.

thank you so much Mikhail Slobodyan

• in what ratio residual stress and strength of joints various
• i have got 150 MPa - residual stress
• thickness 5mm
• what is strength of weld joint?

I shall restrict my answer to plastics since these materials are within my field. When talking about strength then tensile strength is usually meant & tensile strength is measured by the maximum stress that a material can withstand while being stretched or pulled before breaking. In polymer technology, measurement is preferred over calculation using "Hooke's Law".

For amorphous plastics, the weld can be as strong as the main domain. In welding, the secondary bonds between the polymer chains are broken so that the polymer chains can flow freely across the joint and, when cooled, they bond with chains from the other part.

For semi-crystalline plastics, the weld may be weaker due to not reconstituting crystalline regions during rapid cooling.

However, practically the tensile strength of a plastic gathering is often limited by the part geometry & weld conditions rather than by the micro-structure in the weld area.

If the parts are insufficiently heated, a "cold" weld is formed which is very weak due to low mixing of the polymer chains across the interface. If there is a stress concentration point in the part design (such as a sharp corner) that will generally fail before the weld. Also, typically, the width of the weld area is far narrower than the wall width elsewhere, so that the joint area will fail due to simple reduced cross-section.

Welds & joints are always weaker in peel than in tensile, so if the parts can deform in any way to initiate a peel failure, the strength will be greatly reduced.

thank you Nizar Matar

i am expecting a equation or formula to find the welding strength with the help stress

i have found stress and displacement

now i needed to find welding strength

Instron's website gives an idea about a variety of tests together with the required equations:

http://www.instron.us/en-us/our-company/library/test-types/tensile-test

Please note that "strength" & "hardness" are not synonyms in polymer technology.

In order to calculate the tensile strength of materials after welding process , you can make several tests by using tensile machines such as INSTRON machine, after that you can modify mathematical or Statistical model to predict an equation. You can use several techniques such as Response Surface Methodology (RSM) with Analysis of Variance (ANOVA) or Neural networks.

You can also using ready to use data form other researchers if these data compatible with your materials and your experimental conditions.

thank you so much Raheem_Al-Sabur

Stress is the force per unit area, it depends on area of  structure, while  Strength is the resistance to maximum stress at the time of  failure, it does not depend on area Strength is an extrinsic property of the material. It is  derived outcome of heat treatment processes. It is a  function of grain size, No of slip planes within a grain  etc. Therefore strength defines capacity of a material to  carry loads.  Stress is an internal resistance/force per unit area in the  material, generated due to application of external force.  Incidently, both have same units as strength is also  measured by force/area.

 strength is the maximum stress the structure can withstand.