2 Stiffness requires members to have sufficient deformation capacity.
Deformation here refers to the change of size and shape of solid due to external force.
Stability requires that the components should be completely able to maintain the original equilibrium form.
Therefore, the task of material mechanics is to provide theoretical basis and calculation method for designing economic and safe components that meet the strength, stiffness and stability of materials.
Second, the basic assumption of deformed solids 1 Continuity assumption The substances that make up a solid are filled with the volume of the solid without gaps, that is, the solid is continuous in the whole volume.
Homogeneity assumes that all parts of a solid have the same mechanical properties.
Assuming that the mechanical properties of isotropic solids are the same in all directions, materials with this property are called isotropic materials, and materials with different mechanical properties in anisotropic directions are called anisotropic materials.
Third, the basic concept 1 external force and its classification External force refers to the force acting on the component from the outside.
(1) According to external force, it can be divided into surface force; the force acting on the surface of an object can be divided into distributed force and concentrated force.
Volume force A force, such as its own weight or inertia force, that is continuously distributed at various points inside an object.
) 2) According to the change of load with time, the static load is divided. ) After the load slowly increases from zero to a certain value, it remains unchanged or the fluctuation is not obvious.
Dynamic load (the load changes with time, among which the dynamic load that changes periodically with time is an alternating load, and the dynamic load caused by the instantaneous and sudden change of the object motion is called an impact load.
Internal force and its solution refers to the additional interaction between the internal parts of an object caused by external force, that is, "additional internal force".
Solving internal force by section method is a method to divide the section into two parts and determine internal force by representation.
The specific solution steps are as follows: (1) Divide the bar into two parts along the required cross section, arbitrarily take out one part as the research object, and discard the other parts; (2) Substitution: the internal force as a section replaces the action of the abandoned part on the taken-out part; (3) Balance: establish the balance equation of the extracted part and determine the unknown internal force.
3 Stress and strain (1) The concentrated unit of internal force under external stress is Pa or MPa, 1MPa= 106Pa, 1Pa= 1N/m2.
The concentration of average internal force per unit area is called average stress, which is expressed by pm. That is to say, when pm=F/A and area a is 0, the size and direction of pm reach a certain limit. That is, the stress p at this time, that is
P is a vector, which usually represents the component perpendicular to the section (called normal stress). A component parallel to the cross section).
Strain is the basic quantity to measure the degree of deformation, which is divided into linear strain and angular strain.
The ratio of the length change s to the original length x is the average pincushion distortion, which is expressed by m. That is, the limit value of the average positive distortion of m=s/x is positive distortion, which is expressed by. that is
The right-angle change between adjacent sides of a micro-body is called shear strain, which is expressed by and the unit is rad. When the angle formed by the adjacent edges of the deformed micro-body is represented,
Four, the length is much larger than the cross-sectional size of the basic form of bar deformation is called bar, referred to as bar. Its deformation has four basic forms.
The characteristics of uniaxial tension or compression: a pair of forces with equal magnitude and opposite directions, the line of action is consistent with the axis of the rod; Deformation characteristics: the length of the rod is short.
② Shear force characteristics: equal in magnitude, opposite in direction and parallel to each other; Deformation characteristics: the two parts of the shear bar are relatively offset along the direction of external force.
3. Torsional stress characteristics: two couples with equal size and opposite directions, whose action surfaces are perpendicular to the axis; Deformation characteristics: any two sections of the rod rotate relative to each other around the axis.
4 Bending force characteristics: bear transverse force perpendicular to the rod axis, or a pair of equal and opposite force couples act on the longitudinal plane containing the rod axis; Deformation characteristics: the axis of the bar changes from straight line to curve.
According to the hypothesis of uniform continuity, 1 and 1, the multiple-choice question in the postgraduate entrance examination can be considered as ().
[University of Science and Technology Beijing, 2065438+2002] A. Internal deformation of components is the same everywhere. B. the internal displacement of members is the same everywhere. C. the internal stress of components is the same everywhere. D. the elastic modulus of members is the same everywhere. From the point of view of continuity, the substances that make up a solid are filled with the volume of the solid without gaps.
The composition and structure of each section in a uniform continuous member are the same, and the elastic modulus is the same everywhere.
2 two indicators reflecting the strength of solid materials generally refer to ().
[Study of University of Science and Technology Beijing 20 10] A. Yield limit and proportional limit B. Elastic limit and yield limit C. Strength limit and fracture limit D. Yield limit and strength limit answer D. Analysis shows that the index to measure the strength of plastic materials is yield limit, and the index to measure the strength of brittle materials is strength limit.
3 According to the small deformation hypothesis, it can be considered as ().
[Xi 'an Jiaotong University's research in 2005] A. The component is not deformed B. The component is not damaged C. The component is only elastically deformed D. The deformation of the component is much smaller than the original size of the component. Answer D. According to the analysis, the original size principle is deformation or displacement caused by deformation.
4 for plastic materials without obvious yield stage, the nominal yield limit of the material is usually the stress value corresponding to ().
[Xi Jiaotong University, 2005] A.0.2 strain B.0.2% strain c.0.2% plastic strain D.0.2% plastic strain Answer D is shown in the answer analysis. For plastic materials without yield stage, the stress value corresponding to 0.2% plastic strain after unloading is taken as the yield limit.
When ductile materials are unloaded and reloaded after strain hardening, their mechanical properties change as follows.
[Research on University of Science and Technology Beijing 20 10] A. The elastic modulus decreases with the increase of the proportional limit. B. the proportional limit increases and the toughness decreases. C. the proportional limit remains unchanged and the elastic modulus remains unchanged. D. the proportional limit remains unchanged and the toughness remains unchanged. Answer B After the material is cold cured,
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