What Are Residual Stresses?

Residual Stress Self-phase equilibrium internal stress that remains in the object after eliminating external forces or uneven temperature fields. Both machining and strengthening processes can cause residual stress. Such as cold drawing, bending, cutting, rolling, shot peening, casting, forging, welding and metal heat treatment, etc., residual stress may be caused by uneven plastic deformation or phase change. Residual stress is generally harmful. For example, after improper heat treatment, welding or cutting, the residual stress will cause the part to warp or distort, or even crack. Or cracks will appear on the surface after quenching and grinding. The existence of residual stress sometimes does not immediately appear as a defect, and when the total stress exceeds the strength limit due to the superposition of the working stress and the residual stress during work, cracks and fractures occur. Most of the residual stress of the parts can be eliminated by proper heat treatment. Residual stress is also sometimes beneficial. It can be controlled to increase the fatigue strength and wear resistance of the part.

Residual Stress Self-phase equilibrium internal stress that remains in an object after eliminating external forces or uneven temperature fields. Both machining and strengthening processes can cause residual stress. Such as cold drawing, bending, cutting, rolling, shot peening, casting, forging, welding and metal heat treatment, etc., residual stress may be caused by uneven plastic deformation or phase change. Residual stress is generally harmful. For example, after improper heat treatment, welding or cutting, the residual stress will cause the part to warp or distort, or even crack. Or cracks will appear on the surface after quenching and grinding. The existence of residual stress sometimes does not immediately appear as a defect, and when the total stress exceeds the strength limit due to the superposition of the working stress and the residual stress in the work, cracks and fractures occur. Most of the residual stress of the parts can be eliminated by proper heat treatment. Residual stress is also sometimes beneficial. It can be controlled to increase the fatigue strength and wear resistance of the part.

Chinese name: Residual Stress
Foreign name: Residual Stress

Meaning: Stresses existing in the internal equilibrium without external action
Principle: Based on the famous Bragg equation 2dsin = n
Measurement methods: Residual stress measurement by blind hole method

Brief introduction of residual stress

During the manufacturing process, the workpiece will be affected and influenced by various processes and other factors; after these factors disappear, if the above-mentioned effects and influences on the components cannot be completely disappeared, some of the effects and influences remain on the components Within, the residual role and impact. Also called residual stress.

Residual stress is the stress that exists when the object is kept in balance when there are no external factors acting on it.

Any stress that has no external effect and maintains self-phase equilibrium inside the object is called the intrinsic stress of the object, or the initial stress, also called the internal stress.

Residual stress tester

Residual stress analyzer

The principle is based on the well-known Bragg equation 2dsin = n: that is, X-rays of a certain wavelength are irradiated onto the crystal material, and the difference in the X-ray optical path when the adjacent two atomic planes are diffracted is an integer multiple of the wavelength. By measuring the change in diffraction angle to obtain the change in lattice spacing d, the residual stress of the material is calculated according to Hooke's law and the principles of elasticity.

Stress equation

According to the theory of elasticity, the strain in the directions of angles and (see Figure 1) on macro-isotropic crystal materials can be expressed by the following equation:

(figure 1)

Normal and shear stress

The stress components and are the normal and shear stresses in the direction S :

Stress equations and curves with shear stress

If there is shear stress in a plane perpendicular to the surface of the specimen ( ₁₃ 0 and / or ₂₃ 0), the function of and sin2 is an elliptic curve, at > 0 and <0 It is shown graphically as " bifurcation" (see Figure 3). If _{33 is} not equal to zero, the slope of sin2 is proportional to - ₃₃ . In this case, equation (4a) becomes:

(image 3)

Residual stress measurement method

1.Residual stress measurement by blind hole method

Its principle is to drill holes in the original stress field under equilibrium to remove a part of the metal with stress, and relax the stress in the part of the metal near the circular hole. The hole destroys the original stress equilibrium state and causes the stress to be restored. Distribution, and present a new stress balance, so that the metal near the circular hole is displaced or strained, and the strain value of the original stress field can be calculated by measuring the strain after drilling with a highly sensitive strain gauge.

The residual stress detector mainly uses the blind hole method to analyze and study the residual stress of various materials and structures. It can also be used as a stress analysis instrument for measuring the deformation of structures and materials at any point in the static strength study. If equipped with corresponding sensors, physical quantities such as force, pressure, torque, displacement and temperature can also be measured. It uses a computer as the central microprocessor, and uses high-precision measurement amplifiers, data acquisition, and processors. There is no need to adjust zero during the measurement, and the magnitude and direction of the residual stress value can be directly measured, thereby realizing the automation of residual stress measurement.

2.Residual stress measurement by magnetic method

Residual stress detection method by magnetic method is mainly to measure the change in permeability of ferromagnetic materials under the action of internal stress by magnetic method to determine the magnitude and direction of residual stress. As we all know, ferromagnetic materials have a magnetic domain structure, the direction of which is easy to magnetize the axial direction, and also has magnetostrictive effect, and the magnetostrictive coefficient is anisotropic, under the action of a magnetic field, stress generates magnetic anisotropy . Permeability as a tensor is similar to stress tensor. Through precise sensors and high-precision measurement circuits, the change in magnetic permeability is converted into electrical signals, and the output current (or voltage) value reflects the change in stress value, and is calculated by a computer equipped with specific residual stress computer software to obtain the residual The magnitude, direction, and trend of stress.

3.Residual stress measurement by X-ray diffraction

Among various methods for non-destructive determination of residual stress, X-ray diffraction method is recognized as the most reliable and practical. Its principle is mature and its method is perfect. After more than 70 years of process, it is widely used in mechanical engineering and materials science at home and abroad, and has achieved outstanding results.
X-ray stress tester is a simplified and practical X-ray diffraction device, so it also has an additional function-determination of residual austenite content in steel. Since it is suitable for various solid workpieces, and can be tested at the same point with different and angles to detect the effect of texture, this function has an important and unique purpose.

Residual stress test procedure

The steps for measuring residual stress are as follows:

1. Paste the strain flower exactly according to the general method of pasting strain gages, and weld the measuring lead accurately on the measurement point of the sample. The surface of the sample should be polished before the paste, but the original residual stress field cannot be destroyed during the polishing.

2. Connect the strain gauge. Connect the working piece and the compensation piece to the ports of the strain gauge (the strain flower to be measured can also be used as the compensation piece), and check the resistance of each strain piece.

3. Install drilling tools:

Place the drill with the observation mirror on the surface of the specimen, turn on the light if necessary, observe in the observation mirror, and initially align the center of the strain flower.

Then, glue is dripped on the contact point between the drill leg and the sample. After the glue is solidified, the lock cap on the drill leg is tightened to fix the drill on the surface of the sample.

Then loosen and lock the four adjustment screws 3 in the X-Y direction (must be loosened and tightened first), so that the center of the crosshair of the observation mirror always coincides with the center of the strain flower when rotating and observing.

Finally, the gland is locked and the strain gauge is reset to zero.

4. Drilling:

Remove the observation mirror, wipe the drill rod of the special end face milling cutter, apply lubricant, (requires sewing machine oil, not general oil), insert it into the sleeve of the drill, and turn it by hand to remove the drilling After taking the strain on the base, take out the drill pipe. At this time, the strain reading of each strain gauge should not change much, and the zero point of the resistance strain gauge is tuned again.

Wipe the drill rod of the 1.5 drill bit in the configuration, drip the lubricant (requires sewing machine oil, not ordinary oil) and insert it into the drill sleeve, loosen the positioning collar on the drill rod, Insert a drilling depth control pad with a thickness of 2 mm between the drill sleeve and the drill sleeve to fix the collar after the drill is in contact with the workpiece. Remove the 2 mm spacer, connect a hand drill, and adjust the voltage regulator to 60-70V to start drilling. Maintain the proper pressure, drill until the snap ring fits with the clamp sleeve, that is, the predetermined hole depth, and pull out the drill pipe. After the drill pipe is pulled out for 3 to 5 minutes, when the strain gauge indicates stability, the displayed values of stress, strain and residual stress can be measured.

Classification of residual stress

According to the causes of stress, there are:

(1) Thermal stress

The thickness of each part of the casting is different. For example, the guide rail part of the machine bed is very thick, and the side ribs are thin. The lateral end faces are shown in Figure 1. After casting, the cooling rate of the thin-walled part is fast and the shrinkage is large, while that of the thick-walled part is slow and the shrinkage is small. The shrinkage of the thin-walled part is hindered by the thick-walled part, so the thin-walled part receives tension and the thick-walled part receives pressure. Due to the large difference in longitudinal shrinkage, the tension and compression generated is also large. At this time, the temperature of the casting is high, and the thin and thick walls are in a plastic state. The compressive stress makes the thick part thick, the tensile stress makes the thin part thin, and the tensile and compressive stress disappears with plastic deformation. The casting is gradually cooled. When the thin-walled portion enters the elastic state and the thick-walled portion is still plastic, the compressive stress causes the thick-walled portion to plastically deform and continue to thicken, while the thin-walled portion is only elastically elongated. The wall part thickened and disappeared. The casting continues to cool. When the thin-thick-walled part enters the elastic zone, due to the high temperature of the thick-walled part, the shrinkage is large. But the thin wall part prevents the thick wall part from shrinking, so the thin wall is subjected to compressive stress and the thick wall is subjected to tensile stress. The direction of stress has changed. This effect continued until room temperature, and as a result, the thick-walled part was under tensile stress and the thin-walled part was under compressive stress at normal temperature. This stress is due to the different thicknesses of the parts. Different cooling rates and uneven plastic deformation are called thermal stress.

In the same cross section of the guide rail or the side wall, the surface layer and the inner core, due to different cooling speeds, also generate mutually balanced tensile and compressive stresses. Using similar and similar methods to the above analysis, we can know that the compressive stress of the lower layer and the core are tensile at room temperature. Stress, and the larger the section, the greater the stress. This stress is also called thermal stress.

(2) Phase transition stress

Commonly used cast iron has a carbon content of 2.8-3.5%, which belongs to the hypoeutectic cast iron. It can be known from the crystallization process. : When the eutectic crystal of the thick-walled part is crystallized at 1153 ° C, eutectic graphite precipitates, causing volume expansion, and the thin-walled part hinders its expansion The thick-walled part is under compressive stress and the thin-walled part is under tensile stress. Due to the high temperature of the thick wall, the cooling rate is fast and the shrinkage is fast, so the thick wall gradually becomes a tensile stress. The thin wall is the opposite. During the shrinkage before eutectoid (738 ° C), the thin and thick walls are in a plastic state. Although the stress is constantly generated, it is constantly relaxed by plastic deformation, and the stress is not large. When the temperature drops to 738 , the eutectoid transformation of cast iron occurs, changing from face-centered cubic to body-centered cubic structure (that is, -Fe to a-Fe), and the specific volume increases from 0.124cm ³ / g to 0.127cm ³ / g. At the same time, eutectoid graphite is precipitated, so that the thick-walled part protrudes and generates compressive stress. The two kinds of stress mentioned above are caused by two phase transitions at 1153 and 738 , which are called phase transition stress. The phase transition stress is opposite to the thermal stress generated during the cooling process, and the phase transition stress is offset by the thermal stress. After the eutectoid transformation, some phase change forces are no longer generated, so the castings are mainly affected by the thermal stress caused by the difference from the thin cooling rate.

(3) shrinkage stress (also known as mechanical obstruction stress)

When the casting shrinks in the solid state, the stress caused by the obstruction of the mold, core, and runner is called shrinkage stress. Because each part has a transition from plastic to elastic state, the resistance of the core and the like to shrinkage will cause uneven plastic deformation in the casting, resulting in residual stress. Shrinkage stress is generally not large and mostly disappears after boxing.

According to the range of residual stress balance, it can be divided into three types:

(1) The first type of internal stress, also known as macro residual stress, is caused by the non-uniformity of macro deformation of different parts of the workpiece, so its stress balance range includes the entire workpiece. For example, when a metal rod is subjected to a bending load, the upper side is stretched and the lower side is compressed; when the deformation exceeds the elastic limit and plastic deformation occurs, the external side removes the compressive stress after the external force is removed, and the short side is tensioned. stress. The distortion energy corresponding to this type of residual stress is not large, accounting for only about 0.1% of the total stored energy.

(2) The second type of internal stress, also known as micro residual stress, is caused by the heterogeneity of deformation between grains or sub-grains. Its range of action is comparable to grain size, that is, to maintain a balance between grains or subgrains. This internal stress can sometimes reach very large values, and may even cause microcracks and cause workpiece damage.

(3) The third type of internal stress, also known as lattice distortion. Its action range is tens to hundreds of nanometers, which is caused by a large number of lattice defects (such as vacancies, interstitial atoms, dislocations, etc.) formed in the plastic deformation of the workpiece. Most of the stored energy (80% ~ 90%) in deformed metal is used to form lattice distortion. This part of the energy increases the energy of the deformed crystal, making it in a thermodynamically unstable state, so it has a spontaneous tendency to restore the deformed metal to a stable structural state with the lowest free enthalpy, and causes the plastic deformed metal to recover when heated, and Recrystallization process.

Residual stress related effects

Impact on structure or component Residual stress is the initial stress on the cross section of the component that has not yet undergone the load, and during the service of the component, it is superimposed on the working stress caused by other loads, causing two Redistribution of secondary deformation and residual stress will not only reduce the stiffness and stability of the structure, but also under the combined effect of temperature and medium, it will also seriously affect the structure's fatigue strength, resistance to brittle fracture, resistance to stress corrosion cracking and high temperature creep The ability to crack.

Effect on the stiffness of the structure When the sum of the stress caused by the external load and the residual stress in a certain area of the structure reaches the yield point f _y , the material in this area: the local plastic deformation will occur and is lost The ability to further withstand external loads reduces the effective cross-sectional area of the structure and reduces the stiffness of the structure. When there are longitudinal and transverse welds on the structure (such as rib welds on I-beams) or after flame correction, residual tensile stress may be generated on larger sections, although the distribution range of the length of the component is not the same. Not too big, but they can still have a big impact on stiffness. In particular, a large number of flame-corrected welded beams may have a significant decrease in stiffness during loading and rebound during unloading. Structures with high requirements for dimensional accuracy and stability cannot be ignored.

Effect on the stability of members < br When the sum of the compressive stress caused by external load and the compressive stress in the residual stress reaches f _y, the section will lose the ability to withstand the external load further, and the effect of continuing to carry the member is effective The cross-sectional area is reduced, the stiffness of the rod is reduced, and the stable bearing capacity is reduced. The effect of residual stress on the stable bearing capacity of the compressive member is related to the distribution of residual stress.

Residual stress is an unstable stress state. When components are subject to external forces, temperature and other factors, due to the interaction of these stresses and residual stresses, some parts of the component are plastically deformed, and the residual stress in the section is redistributed. When external factors are removed, the entire component will deform. During the use of the component, the residual stress will relax, so the residual stress affects the stability of the component. This is also one of the issues that the engineering department is most concerned about.

The influence of residual stress on the deformation of the component includes two aspects: on the one hand, the deformation resistance of the component against static and dynamic loads; on the other hand, the ability to recover from the deformation after the load is unloaded. The effect of residual stress on components in these two aspects is very large, so people have been studying effective methods to eliminate this effect.

However, recent research indicates that residual stress will also affect the fracture toughness of the specimen. For example, in the sample tensile, the effect of tensile stress will cause residual stress in the sample, which may increase the fracture toughness of the material (see Luke's strain rate article)