1. What is residual stress? First, the official definition: “Residual Stress”is the effect and influence of factors, such as processes, on an artifact during its manufacture; when these factors disappear, if the above-mentioned actions and effects can not disappear completely, and some of them still remain in the member, then the residual actions and effects are called residual stress. A little dizzy? Well, then tell it to everyone in a more informal way. For example, a person who was thin before and bought a pair of jeans in l size, but after a year or so he became a big fat man and then wore the jeans again, because he gained weight and the pants remained the same, he would feel that the pants were too tight, at this point between the body and the pants have a strong force, if too much force is easy to tear the pants, this destructive force is the effect of residual stress. From the point of view of work done by energy, when an external force causes a plastic deformation of an object, it will lead to the deformation of the inner part of the object, thus accumulating a part of energy, if the Brittleness of an object is low, the object will slowly deform, and if the Brittleness is high, a crack will form.

Residual stress is very common in mechanical manufacturing, and it often occurs in various processes. In essence, however, the causes of residual stress can be classified into three categories: first, non-uniform plastic deformation; Second, non-uniform temperature changes; and third, non-uniform phase transition.

2. The damage of residual stress from the classification of residual stress, it can be seen that the residual stress can cause the object to deform slowly, lead to the change of the object’s size, and lead to the disqualification of the machined workpiece’s size, in the production of the instrument, the whole instrument loses its precision and becomes a scrap, and the casting and forging parts are cracked or even broken. At the same time, the fatigue strength, stress corrosion resistance, dimensional stability and service life of the instrument, etc. , has a very important impact. In the process of casting cooling, the casting fracture is caused by the residual thermal stress due to the uneven cooling caused by the unreasonable process. In the quenching process of heat treatment, it is easy to cause material fracture when undercooled austenite undergoes martensite transformation. Measurement of residual stress for the measurement of residual stress, from the general category can be divided into mechanical method, chemical method and x-ray method. The most common mechanical method is the boring method (also called blind hole method) , in which a length of Bar (or pipe)3 times its diameter is cut from an object and a hole is drilled through the center, then a thin layer of metal is removed from the inside by a bore or drill, each time removing about 5% of the cross-sectional area.

The relationship curves between these values and the borehole section area are drawn, and the derivative of any point on the curve is obtained by using the graph method, and the elongation and the change rate of borehole section are characterized. Chemical Law Chemical Law has two ideas. One idea is to inject the specimen into a suitable solution, measuring the time from the onset of erosion to the discovery of the crack, by which the magnitude of the residual stress is judged, the solution used, for bronze containing tin, Mercury and Mercury Salts are available; for steel, weak bases and nitrates are available; another idea is to suspend the sample in an appropriate solution and weigh it at regular intervals. In this way, a weight reduction versus time curve can be obtained, compared with the standard curve to determine the magnitude of the residual stress, and the higher the position of the resulting curve than the standard curve, the greater the residual stress in the body.

X-ray method x-ray method x-ray method can be used to penetrate metal parts, in which Lloyd’s method determines the residual stress qualitatively by disturbing the change of spot shape.

When there is no residual stress, the interference spots are distributed in a dot shape. When there is residual stress, each interference spot elongates and assumes a “Star-like”shape.

The debye method can be used to measure the residual stress quantitatively according to the changes of the position, width and intensity of the diffraction line on the Debye map.

In summary, mechanical and chemical methods are destructive testing methods that require local sampling of the object to be tested, and the damage is irreversible after testing; x-ray method is a non-destructive testing method that can maintain the integrity of the object. The mechanical method can accurately determine the magnitude and distribution of the residual stress, which is generally applicable to rod-shaped or tube-shaped objects; the chemical method is applicable to objects of wire or sheet type, but the chemical method can only make qualitative judgments, it is difficult to describe quantitatively; the x-ray method is a “Non-destructive”method, but it is only suitable for some materials that can give clear and sharp diffraction lines, and because of the small projection power of X-rays, can only detect objects that are close to the surface. 4. The elimination of residual stress since the damage of residual stress so much, then the effective method to eliminate it is very necessary. The elimination methods include heat treatment, static load pressurization, vibration aging and mechanical treatment.

Heat treatment is to use the thermal relaxation effect of residual stress to eliminate or reduce the residual stress, generally annealing, tempering and other ways to treat. Static load pressurization is to adjust the residual stress of the workpiece by plastic deformation of whole or local or even micro area. For example, large pressure vessels, after welding, in its internal pressure, the so-called “Bulging”, so that a small amount of plastic deformation of welded joints to reduce welding residual stress. Vibration Stress Relief is called Vibration Stress Relief, or VSR. Vibratory stress relief (VSR) is a commonly used method to eliminate the residual internal stress in engineering materials. It is through vibration that the residual internal stress and additional vibration stress vector in the workpiece can reach the maximum of the material yield strength, so that the material has a small amount of plastic deformation, so that the internal stress of the material can be relaxed and reduced. Mechanical treatment is to reduce the residual stress by using the small plastic deformation on the surface of the object, including parts collision, surface rolling, surface drawing and surface shaping in the die. One of the advantages of ironmaking, for example, is the elimination of residual stresses.

Source: Caitong

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