CN105092389A - Method for testing bending strength of brittle material - Google Patents

Abstract

The invention provides a method for testing the bending strength of a brittle material. The testing method comprises the following steps: processing the brittle material to form a sample, and forming semi-cylindrical grooves on the surface of the sample in contact with supporting rollers on a test apparatus; allowing the center line of the semi-cylindrical grooves to parallel to the supporting rollers and a pressing part and to be positioned in the middle of the supporting rollers and just below the pressing part; and calculating the bending strength of the brittle material according to a formula shown in the specification. In the formula, a is the radius of the semi-cylindrical grooves, b is the length of the semi-cylindrical grooves and is same to the width of the sample, p is the largest load during fracture, h is the height of the sample, L is a distance between the two supporting rollers on the test apparatus, and k is the stress concentration factor.

Description

A kind of method of testing brittle material bending strength

Technical field

The present invention relates to a kind of method of testing brittle material bending strength, especially test the method for Nd-Fe-B rare earth permanent magnetic material and other brittle alloy bending strength.

Background technology

Rare earth permanent-magnetic material is as a kind of important functional material, and its mechanical property is generally poor, and obdurability official post rare-earth permanent magnet easily ftractures, falls slag in process.This greatly reduces yield rate and the machining precision of magnet, adds the processing cost of magnet, limits the application of magnet at high precision instrument instrument industry.

Meanwhile, because the obdurability of rare earth permanent-magnetic material is poor, antidetonation, impact resistance are corresponding also poor, and material is also being restricted the application of the higher occasion of antidetonation, anti-impact force requirement, such as, and the field such as instrument and high-speed electric expreess locomotive.

The index of the mechanical property of the hard brittle materials such as current evaluation Nd-Fe-B permanent magnet roughly has three kinds, i.e. fracture toughness, toughness and bending strength.The plurality of advantages such as wherein, bending strength is due to its sample handling ease, and computation and measurement is simple, and in test equipment needed thereby is limited, have been widely used in the middle of research practice.

In the method for testing of bending strength, most is representational is exactly three point bending test method, it determines by specific fixture, pressure head or mould and Material Testing Machine the maximum stress that moment that sample is pressed down bears, again according to maximum stress, sample span, the data such as sample is high, sample is wide, utilize mechanics of materials computing formula, obtain the bending strength of material.

But due to hard brittle material itself, there is dispersed larger drawback in measurement result, cross-section morphology is also not ideal enough.So we need to be improved in process of the test and sample processing to reduce experimental error.

Summary of the invention

The object of the present invention is to provide a kind of method of testing brittle material bending strength, be particularly useful for the bending strength of testing Nd-Fe-B rare earth permanent magnetic material and other brittle alloy.

When utilizing the method test of described testing brittle material bending strength, described hard brittle material is processed to sample, and face of carrying out contact excellent with the backing roll on proving installation forms semi-cylindrical groove with described sample.

Preferably, the centerline parallel of described semi-cylindrical groove is in two described backing roll rods and press member, and the position immediately below the middle, described press member of two described backing roll rods.

Preferably, by the bending strength of hard brittle material described in following formulae discovery,

${\mathrm{\σ}}_b=\frac{3\mathrm{pL}}{2b{(h-a)}^2}\·k$

Wherein, a is the radius of described semi-cylindrical groove, and b is the length of described semi-cylindrical groove, it is equal with the width of described sample, maximum load when p is fracture, and h is the height of sample, L is the distance on proving installation between two backing roll rods, and k is factor of stress concentration.

The height h of described sample and the radius of described semi-cylindrical groove preferably meet 0.2h≤a≤0.3h.

Accompanying drawing explanation

Fig. 1 is the schematic diagram of sample in the specific embodiment of the invention.

Fig. 2 is the figure that grooving on sample shown in Fig. 1 is shown.

Fig. 3 is the figure that the section that sample breaks to form in test process is shown, left side is the sample of non-grooving, and right side is the sample of grooving.

Fig. 4 is the maximum relative deviation of the test findings in embodiment corresponding to different technics of slot parameter.

Embodiment

Below in conjunction with accompanying drawing, the specific embodiment of the present invention is described.

First, Nd-Fe-Bo permanent magnet material carried out cut, after the conventional processing such as surface finish, chamfering, be prepared into standard sample, such as, be of a size of 5 × 5 × 35mm ³sample 3 (see Fig. 1), dimensional discrepancy is within ± 0.1mm.

Then such as Wire EDM technique is used to cut out in the bottom surface (that is, the face of the backing roll rod 1 on Contact Test Set) of sample the semi-cylindrical groove (see Fig. 1 and Fig. 2) that a radius is a, length equals specimen width b.The center line of semicolumn and two backing rolls rod 1 and press member 2 (such as, pressure head) parallel, and be in the position immediately below two backing rolls rod 1 middles, press member 2, as shown in Figure 1.

Then sample is put into proving installation, such as model is in the universal testing machine of CMT4503, setting span is L, L is less than the length of sample, for reducing strain rate to the impact of test result, be test low speed (such as 0.1mm/min or less) at press member 2 loading velocity, measure maximum load P during fracture, numerical value is accurate to 0.01N.

Next, the bending strength of sample is calculated by following bending strength formula (1).

${\mathrm{\σ}}_b=\frac{3\mathrm{pL}}{2b{(h-a)}^2}\·k---\left(1\right)$

In above-mentioned formula (1), σ _bfor bending strength, unit is MPa (MPa);

Maximum load when P is sample fracture, unit is newton (N);

B is specimen width, and unit is millimeter (mm);

H is sample thickness, and unit is millimeter (mm);

A is the semi-cylindrical radius of undercut on sample, and unit is millimeter (mm);

K is factor of stress concentration, and it is only relevant with size (a/h) with the shape of grooving, has nothing to do with material itself.

Theory of mechanics of materials computing formula requires stricter to the section of sample, the desirable section formed is preferably parallel to the direction of load p, and the position between two backing roll rods immediately below center, press member, pattern and this cross section, place are similar to, this situation is referred to as just disconnected, as the fracture mode of right side sample in Fig. 3, fracture mode is more close just disconnected, and the credibility of test result is higher.

Compared with general SENB technique, the semi-cylindrical technics of slot selected by the present invention can control better and reduce stress concentration effect to testing the error brought.Through Nd-Fe-Bo permanent magnet material sample just disconnected than obtaining obvious lifting of above-mentioned process, this just significantly decreases the quantity of time needed for test and sample, thus saves manpower and financial resources.

Test of many times data show, as a=1 ~ 1.5mm, namely during a/h=0.2 ~ 0.3, sample is all close to just disconnected, the maximum relative deviation of each group of test result all controls within 2%, is all data available (maximum relative deviation≤10% is data available).

Because the material of factor of stress concentration k and sample has nothing to do, therefore can prepare the sample of same specification by identical technique, but without grooving process, benchmark stress σ can be obtained through For Three Points Bending Specimen test _ave, m is moment of flexure.Following formula (2) is utilized to draw k,

$k=\frac{{\mathrm{\σ}}_{\max }}{{\mathrm{\σ}}_{\mathrm{ave}}}---\left(2\right)$

Wherein σ _maxfor there being the maximum stress of grooving sample, utilize this formula (2) that the curve map relevant with a/h can be drawn.Such as, the curve map utilizing repeatedly coupon results to draw can be found as a/h=0.2, and the value of factor of stress concentration k is 2.69, and as a/h=0.3, the value of k is 2.54.

embodiment

By applicant conveniently explained hereafter, be prepared into 5 × 5 × 35mm without the Nd-Fe-B rare earth permanent magnetic material briquet magnetized ³standard sample, dimensional discrepancy controls within ± 0.1mm.

The radius of prefabricated semi-cylindrical grooving is respectively 0mm, 1mm (0.2h), 1.5mm (0.3h) and 2mm (0.4h).Setting model is the span of the universal testing machine of CMT4503 is 25mm, and loading velocity is 0.1mm/min, tests.

Test result is as shown in table 1.

Table 1

Radius of undercut (mm)	Just disconnected number of times (%)	Bending strength (MPa)
			0	78	297.7352.1
1	95	341.1～348.2

1.5	100	344.9～351.8
			2	100	343.2～351.8

Test findings shows, when the radius of semi-cylindrical grooving is 1mm ~ 1.5mm (0.2h ~ 0.3h), just disconnected number of times is close to 100%, and the maximum relative deviation of test findings is minimum.

Fig. 4 shows the maximum relative deviation of test findings corresponding to different technics of slot parameter.The radius of semi-cylindrical grooving is within the scope of 0.2h ~ 0.3h, and the maximum relative deviation of test findings, all within 2%, is significantly less than the maximum relative deviation 15.42% of non-grooving sample.When radius is increased to 0.4h, the maximum relative deviation of test findings has the trend of increase.Therefore, the radius of semi-cylindrical grooving preferably controls within the scope of 0.2h ~ 0.3h.

Below through the specific embodiment and the embodiment technical scheme of the present invention has been described in detail, but the present invention is not limited to this.Under the prerequisite realizing the object of the invention, those skilled in the art can make various change and distortion to the present invention.

Claims (4)

1. a method for testing brittle material bending strength, is characterized in that, when testing, described hard brittle material is processed to sample, and face of carrying out contact excellent with the backing roll on proving installation forms semi-cylindrical groove with described sample.

2. the method for testing brittle material bending strength according to claim 1, it is characterized in that, the center line of described semi-cylindrical groove is parallel with press member with two described backing roll rods, and is in the position immediately below the middle of two described backing roll rods, described press member.

3. the method for testing brittle material bending strength according to claim 1, is characterized in that, by the bending strength of hard brittle material described in following formulae discovery,

${\mathrm{\σ}}_b=\frac{3\mathrm{pL}}{2b{(h-a)}^2}\·k$

Wherein, a is the radius of described semi-cylindrical groove, and b is the length of described semi-cylindrical groove, it is equal with the width of described sample, maximum load when p is fracture, and h is the height of sample, L is the distance on proving installation between two backing roll rods, and k is factor of stress concentration.

4. the method for testing brittle material bending strength according to claim 3, is characterized in that, the height h of described sample and the radius of described semi-cylindrical groove meet 0.2h≤a≤0.3h.