Measurement of residual stresses

Before and after heat treatments it is possible to carry out residual stress measurements directly on the items with diffractometric (X-ray) techniques, with the strain gauge method and with the Barkhausen noise system. Knowing the value of the residual stress present on the surface of a item before putting it into service is of particular interest:

  • for designers, for a correct sizing of the items
  • for those in charge of monitoring structures particularly subject to fatigue and/or corrosion phenomena, to predict the times after which cracking problems may appear, establish the necessary non-destructive investigations (methods and periodicity) and proceed with any repairs.

Residual stress measurement methods

There are three methods for measuring residual stresses used in Trater.

  • Diffractometric method: it is based on the measurement, with great precision, of the angle formed by a ray, generated by an X-ray equipment, incident on the surface of the object to be measured and its refracted beam.
  • Strain gauge method: it is based on the measurement of the deformations produced by the relieving of stresses around a hole drilled in the center of a strain gauge rosette, composed of three triaxially oriented strain gauges.
  • Barkhausen noise: measurement as a function of the magnetic properties of the material.
The diffractometric method is a completely non-destructive method that allows surface measurements. This limitation is easily overcome through the electrolytic attack of the surface, which makes it possible to measure deep into the material, without altering its residual stress state. The development of measurement techniques and equipment has made it possible to carry out residual stress measurements directly on items, even big ones, with extreme precision.
The strain gauge method, regulated by the ASTM E 837 standard, is a semi-destructive process as it is necessary to drill a hole, with a diameter of 1.8 mm and a depth of 2 mm, on the surface of the item to be measured. The results, which are considered valid up to values equal to half the yield value of the material examined, are characterized by excellent accuracy. The method is applicable on site, even on large structures.
When the spins in a ferromagnetic material rotate to align with an induced field, they do so impulsively, producing voltage peaks in the inductor which, if amplified with a speaker, are perceived by the human ear as “noise”. The term “Barkhausen noise” derives from this phenomenon and takes its name from the physicist who discovered it. Barkhausen noise is mainly influenced by the mechanical properties of the material and its tension state (tension or compression). Once the effect of the former has been defined, the difference compared to the perceived signal is the effect of a possible compression (noise reduction compared to the standard) or compression (increase in noise). By digitally processing the signal with special softwares it is possible to define a scale.

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What are residual stresses

All materials are subject to residual stresses, whether crystalline or amorphous. By residual tension we mean the state of stress existing in an elastic solid in balance, once the external loads consisting of applied forces, torques and efforts and other causes of stress such as for example a thermal gradient have been removed. Residual stresses are generally considered a “negative” contribution for the use of an item, as their amount is difficult to estimate and certainly additional to the one known, applied to the component in operation, putting the reliability, safety and in any case the predictability of the behaviour at risk. Residual tensile stresses cause breakages in constructions subject to fatigue; due to the well-known phenomenon of stress corrosion cracking, they are responsible for the onset of cracks in products exposed to even mildly corrosive environments. Tensile and/or compressive stress states, moreover, determine geometric instability during the mechanical processings and operation of the component.
There are three main types of residual stress:
  • Residual stresses of the 1st type (MACROSTRESSES) They affect the entire body, cause a global deformation of the interreticular distances and are homogeneous over large domains of the material.
  • Residual stresses of the 2nd type (MIDSTRESSES) They affect different crystalline grains (therefore small areas of the material) and are caused by the presence, for example, of cracks and inclusions. These include solidification stresses and large-scale precipitates.
  • Residual stresses of the 3rd type (MICROSTRESSES) They affect smaller regions than those of the crystalline grain. They are mainly induced by lattice defects.
Engineering interest is mainly focused on tensions of the first type as they are generally decisive for the operating behaviour of the component.

Origin of residual stresses

Most of the items are affected by the presence of non-zero residual stresses, introduced by welding processes, plastic deformation processes, chip removal processes and heat and surface treatments.
The execution of a weld always causes the onset of residual tensile and compressive stresses; particular measures that can be adopted in the executive phase of the joint only limit its value and distribution. The residual welding stresses are connected to plastic and elastic deformations of the material subjected to rapid heating and cooling. In particular, in proximity of the heat source, the material that is locally liquefied tends to expand. The surrounding, colder base material prevents this expansion. Subsequently, the weld cools quickly and contracts in a not uniform way; being the base material colder and not deformed, a state of stresses is created caused by microscopic deformations, transitional stresses and phase changes which contribute to the formation of a state of residual effort.

Residual stresses develop during a welding process in the longitudinal, transversal and, in the case of joints with high thickness, perpendicular directions. The distribution and extent of residual stresses varies in relation to the geometry of the joints and as a consequence of other factors, such as the constraint conditions, the presence of stress states due to previous processes, the welding parameters, the procedure used and the nature of the filler material. The residual tensile stresses in the heat affected zone are the harmful ones as they contribute to the development of possible cracks during the solidification process, determine a reduction in the fatigue life of the joint and breakages due to stress corrosion cracking phenomena.
Metallic materials are characterized to a greater or lesser extent by the ability to deform plastically. Once the elastic limit has been exceeded, the material deforms permanently, at a constant volume due to the sliding of the planes of atoms, one with respect to the other. This sliding does not occur randomly but, preferably, according to the planes of greatest atomic density and on these in the directions of greatest density.

The combination of a direction and a slip plane is called a slip system. In the presence of plastic deformation of a more or less extensive portion of an elastic solid, the equilibrium condition between the permanently deformed part of the material and the remaining part induces a distribution of elastic stresses (i.e. residual tensions) for the maintenance of volume continuity and structural integrity. In general, all processes with plastic deformation of a material such as rolling, bending, moulding, drawing and calendering generate high levels of residual stress. In metallic solids, plastic deformations are generally caused, at a microscopic level, by defects in the lattice, called dislocations, which facilitate the sliding of the crystalline planes as they move through the material. When the deformation occurs at low temperature, these defects tend to multiply and accumulate, ending up interfering with each other, blocking each other and increasing the residual stress state (work hardening). The extent and distribution of residual stresses in products obtained by plastic deformation depends on the methods with which the process is conducted, the equipment used and the forces used. The rolling of sheets generally determines states of surface compression if small diameter rollers are used; there is traction with large diameter rollers. Metal bending processes, widely used in the industrial field, induce compression on the extrados, following elastic recovery, traction on the intrados, with values close to the yield strength of the material. The cold moulding operation of a three-dimensional object with a shape corresponding to that of a matrix onto which the sheet metal is adapted, by the action of a counter-mould, generates very high tension states of traction and compression with the same methods seen for the bending. In drawing processes, residual stresses may occur due to non-homogeneous deformation. Cold straightening, widely used to correct deformations of items that have undergone distortions during the manufacturing process, also determines a very high state of residual stress. The resulting stresses can be further influenced by the presence of structural instabilities.

Mechanical processing can induce residual stresses in the surface layer of machined surfaces. In particular, in the process for chip removal such as turning, drilling and milling processes, a tearing action is generally determined with the introduction of residual tensile stresses.

The tool stresses the material in front of it until the latter deforms plastically. The deformation goes as far as breaking and consequent separation between the allowance and the item. The allowance gives origin to the chip that slides on the tool.

On lathe-processed hardened steel surfaces we also measured, using the difractometric technique, tensile stresses with very high values, close to the yield point of the material. Furthermore, during the analyses we performed, it emerged, moreover, that the distribution in the thickness and the intensity of the residual stresses of the samples we examined depended on:

  • the type of material;
    the geometry of the tool;
  • the depth of the passes;
  • the cutting speed;
  • the lubrication and cooling conditions;
    tool wear.

Due to the same mechanism seen in turning operations, states of residual tensile stress generally arise on the surface of the items also for milling and drilling operations. Grinding processes generate a residual tensile stress state but, in this case, it is generated by a significant surface heating (chip removal process at high speed), especially on materials with low thermal conductivity. If the heat developed is excessive, it causes an increase in the specific volume of the material on the surface and the subsequent sudden cooling as soon as the interaction with the tool ends. This causes an upsetting at a microscopic level during overheating and a thermal shrinkage that is hindered by the material not affected by the thermal alteration.

Induction hardening and nitriding treatments, widely used to obtain high hardness and toughness of the material, generate a state of surface compression, with improvements in fatigue behaviour that varies depending on the depth of the hardened layer. The compression in leather, due to the balance of forces, is contrasted with a state of traction in the transition areas between the hardened part and the non-hardened core and, in some cases, treated items develop cracks in this region. This is attributable to the fact that the physical and structural properties of the steel in the transition band are lower than those in the hardened state and the tensile residual stresses, in combination with the applied load, develop a notching effect. In general, then, the state of compression causes geometric instability of the shafts during mechanical processing and operation.

Effects of residual stresses

Generally, residual stresses are considered undesirable as they can cause deformations, distortions and promote the initiation and propagation of mechanical cracks or stress corrosion cracking, with premature breaking of components subject to fatigue.
This phenomenon is certainly the one of greatest interest for all those who have to deal with problems related to corrosion. A state of stress with forces of a value lower than those necessary to lead to a purely mechanical fracture can give rise to the formation of local cracks when there is combined action with even a mildly aggressive environment. Stress corrosion cracking manifests itself in breakages with the appearance of cracks, with a trend orthogonal to the direction of stress, branched and of a brittle type so much that, in many cases, they are erroneously attributed to the fragility of the material itself.

The mechanism of stress corrosion cracking and the ways in which cracks progress is often described with a very simple model, at the limitit of scientific rigour, but easily understandable. The material must be imagined as a strip of woven fabric, one side of which, held in tension by an effort, even if not very intense, comes into contact with a very sharp blade that rests perpendicularly on the taut side. The effort applied to the canvas immediately represents the state of surface traction and the blade the action of the aggressive fluid. The contact of the blade with the first fiber under tension cuts the latter, exposing the next one underneath to the cut, which is cut in turn, and so on. Without the action of the tension exerted on the fibers the blade would have no effect on the relaxed tissue and the only action of the tensile stress alone would be harmless to the effects of any breakages.

In stress corrosion cracking, the simultaneous action between chemical and mechanical forces leads to the initiation and propagation of the breakage. The main metallurgical variables in the phenomenon of stress corrosion cracking are:
  • the type of material; its chemical composition;
  • the metallurgical structure (distribution in the microstructure of the precipitates,
  • orientation of the grains, interactions of the distributions,
  • quantity of ferrite in cast iron and for stainless steel or austenitic-ferritic steel);
  • the presence of thermal stresses; the surface condition and cold-formed structure.
The phenomenon of stress corrosion cracking occurs without warning, making this form of attack extremely dangerous. Due to stress corrosion cracking, the explosion of boilers, chemical equipment, the failure of aircraft landing gear and metal ropes have been recorded. In welded structures, residual stresses are the cause of the initiation of stress corrosion cracking. Alloys and not pure metals are subject to this phenomenon, in particular metal/environment combinations. Stainless steels are attacked in hydrochloric and caustic environments but generally resist in ammoniacal and nitric environments; carbon steels, on the other hand, are immune to ammoniacal environments but not to nitric and caustic environments.
Residual stresses have a negative influence on the phenomenon of hydrogen embrittlement which is determined following to the absorption of this element by the material in particular conditions, which can occur in chemical and petroleum plants (high pressure fluids containing hydrogen) or for technological cycles such as pickling, electroplating or electroerosion. Carbon steels, titanium, zirconium and their alloys are subject to embrittlement in the presence of residual stress, surface defects and local hardening situations due to the presence of welded joints. In consideration of this, while a mild steel does not show this problem, high yield steels are particularly sensitive to the phenomenon.

This type of failure originates from the entry of atomic hydrogen into a metal. The hydrogen atom has a small diameter compared to that of other atoms and this allows it to easily occupy empty spaces within the metal lattice. Hydrogen in metals alters their mechanical characteristics. In steels, it causes an increase in brittleness, a decrease in the modulus of elasticity and resilience, and an increase in hardness. In addition to hydrogen embrittlement, the entry of atomic hydrogen into the metal can cause blistering, which consists in the formation of swellings and cracks due to the recombination of hydrogen atoms in correspondence with inclusions or microvoids in the metal matrix.
br> The hydrogen molecules, of such a size that they cannot diffuse into the metal lattice, accumulate and generate extremely high internal pressures, sufficient to locally cause plastic deformation of the metal, which can evolve into cracks.
Compressive residual stresses, on the other hand, have a positive effect on fatigue and the phenomenon of stress corrosion under effort, so much that they are induced on the surface of the items through shot peening processes. See the dedicated page in the “Other activities” section.

Influence of tensions on the mechanical characteristics of a weld.

The influence of the residual stress state on the mechanical characteristics of a welded joint could be considered irrelevant if we imagined welds free of defects and subjected to static loading, in non-critical conditions. Normally, in fact, the steels used have sufficient ductility and plastic deformability to allow the overlapping of the residual stress to the operating stresses. However, in the case of welds with high thicknesses, where it is possible to have a triaxiality of the stresses with limitation of the plasticization of the material, they become non-negligible.

Complex items with many weld seams that intersect in various directions are comparable to thick joints and, even in this case, the dangerousness of high residual stress values constitutes a serious danger. Each weld also has defects; if most of them can be eliminated as in the case of cracks, lack of penetration and incisions, it is difficult or too expensive to make a joint completely free of imperfections. In these conditions, the presence of welding residual stresses can cause breakages, with even catastrophic consequences, especially at low temperatures.
Based on the most recent studies, the influence of residual stresses seems decisive on the appearance and advancement of cracks in the case of joints subjected to fatigue, i.e. periodic loading and unloading cycles. When a mechanical component or an entire structure is loaded with cyclically or randomly variable external forces, repeated over time, fatigue failure can occur, even if none of the applied load cycles could apparently damage the component. The phenomenon of fatigue failure progresses in three stages: the formation of a crack, its growth and finally failure. The life of a component is therefore given by the number of cycles needed to produce and propagate the crack, until the critical dimensions are reached. Crack formation occurs on the surface of the item. Due to the geometry, this area contains the heaviest stress concentrations, surface imperfections due to the technological processes undergone and the influence of residual stresses is maximum. A breakage always occurs starting from a discontinuity oriented unfavourably with respect to the acting forces (operating and residual) and the methods of crack propagation depend on the size and shape of the plasticized area of the material, at the apex of the breaking itself. A breakage can only continue if in the plastic zone critical deformation conditions are reached, well above the yield values of the material. The residual stresses, possibly present at the point of initiation of the breakage, locally increase the state of stress and, at the same time, taking a triaxial type distribution, limit the plastic deformations and favour the propagation of the failure.
At the end of construction, within the metallic mass of an item, the distribution of residual stresses is such as to constitute a system in stable equilibrium.

During the removal of chips, in the mechanical processing of items, there is simultaneously the elimination of a part of the stresses with the removed material and a modification of the geometric section of the surfaces on which the tensions themselves act. At this point the movement of the item is inevitable due to the alteration of the balance that had been established before mechanical processing. Furthermore, in operation, high residual stress states can easily add to the operational stress, much lower than the residual stresses, causing local deformations of the item with certainly negative consequences.
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