Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation

Abstract: After 90 hours of service, the first rivet head at the junction between 15a frame and inlet skin was found broken.

Through appearance observation, macro and micro observation and analysis of fracture surface, metallography, hardness and quantitative deduction of fatigue stress from fracture surface, the result shows that the rivet is fatigue fracture.

There is a deviation in the coaxiality of the mounting hole between the frame and the skin connected by the rivet, which leads to the discrepancy between the abnormal bending stress and the shear stress under normal operation, and the superimposed air flow vibration stress, which makes the initial stress of the rivet larger, which is the cause of the rivet fracture.

The quantitative analysis of fracture surface also shows that the comprehensive fatigue initial stress of rivet is larger and the expansion stress is smaller.

Introduction

Riveting has the advantages of simple process and equipment, vibration resistance, impact resistance, uniform force transmission, firmness and reliability.

Therefore, riveting is an earlier connection form.

After clamping and positioning the connected parts, align the rivet holes with each other, then insert the rivets into the rivet holes of the connected parts, finally rivet out the rivet heads and upset the rivet rods.

The rivet is pulled together by the connector, so as to transfer the load by the friction force on the contact surface of the connector.

Common stress and failure modes of rivets include:

1) The rivet is cut off;

2) The connecting plate edge is sheared;

3) The contact surface of rivet hole is crushed;

4) The connecting plate is damaged along the rivet hole;

5) The connecting plate edge is torn.

The two main failure types to be considered in the design are the shear of rivet rod and the extrusion or collapse of metal at the place where the rivet and the connected parts are compressed.

At home and abroad, the failure of rivets is mainly concentrated in the production process due to improper control of heat treatment (long quenching and heat preservation time), leading to coarse grains or overburning, or the existence of brittle phases in raw materials.

In the subsequent upsetting process, it is found that the shear strength is too high or the rivet head cracks during the riveting process;

When the external force is abnormal, the rivet is often sheared.

Due to the connection mode, stress state and other characteristics of riveting, fatigue failure of rivets is rare.

However, in the process of practical engineering application, due to the influence of abnormal assembly and other stress conditions, it may lead to the fatigue failure of rivets, which may lead to the failure of the fastening connection of riveted parts and increase the opening displacement.

It is possible that other rivets near the broken rivet may suffer from abnormal stress and then lead to the connection fatigue failure.

Therefore, the failure cause analysis of rivet fatigue is of great significance.

This study determines the cause of rivet failure through visual observation, macro and micro observation of fracture surface, metallography, hardness and other detection methods.

Combined with the quantitative analysis and estimation results of fracture surface, it reverses the crack growth characteristics and initial equivalent comprehensive stress of engineering riveted components, providing data support for the stress characteristics and stress size of components, which is conducive to analyzing the real cause of failure and solving practical engineering problems.

1. Test process and results

The first rivet head at the connection between 15a frame and inlet skin was found to fall off after 90 hours of service (130 take offs and landings) (Fig. 1).

The rivet material is LY10 aluminum alloy, and LY10 is medium strength duralumin alloy (tensile strength 390 MPa), with high shear strength (235 MPa).

It has sufficient plasticity for riveting under annealing, quenching, aging and hot conditions, and is commonly used to make medium strength rivets and structural parts.

1.1 Appearance observation

The rivet was broken at the arc transition of the rivet head.

From the side damage of the rivet, the junction of the two connecting plates at the straight section of the rivet was severely deformed, reflecting that there was a deviation in the coaxiality of the mounting holes between the frame and the skin (Fig. 2).

Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 1
Fig. 1 Position of Broken Rivet

1.2 Macro and micro observation of fracture surface

Put the rivet fracture into the scanning electron microscope for microscopic observation.

The rivet fracture source area is located on the outer surface (Fig. 3a);

Steps and extended edges can be seen in the source area, which is a large line source, accounting for about 1/6 of the circumference.

No obvious metallurgical defects and processing traces are found in the source area (Fig. 3b~Fig. 3c);

During the crack growth period, a large number of fatigue small arcs and fatigue strips can be seen (Fig. 3d), and the fatigue growth of the crack is particularly sufficient, accounting for more than 95% of the total cross-sectional area;

Energy spectrum analysis of the rivet fracture source area and expansion area shows that no other foreign elements are found;

The side of the fracture source area was observed, and the surface anodized protective film was complete (Fig. 3e).

Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 2
Fig. 2 Rivet damage and fracture appearance

1.3 Material inspection

The metallographic and hardness inspection of the rivet shows that there is no abnormality in the metallographic of the rivet and the hardness value is relatively uniform.

2. Analysis and discussion

2.1 Analysis of rivet fracture property

The rivet breaks at the arc transition of rivet head, and a large number of fatigue small arcs and fatigue strips can be seen on the fracture surface.

It can be seen that the rivet fracture is fatigue fracture.

The rivet breaks at the first riveting position of the connecting structure (the connection between frame 15a and the air inlet skin).

When the riveting is abnormal, the first rivet is prone to failure, which conforms to the rule of riveting fastener failure.

2.2 Analysis and quantitative estimation of rivet fracture

Generally speaking, as fasteners, the most common failure mode of rivets is shear fracture.

When the rivet has fatigue fracture at the arc of rivet head, it indicates that its riveting function has changed to some extent.

During normal assembly, there is interference fit between the rivet and the rivet hole of the connector, and the stress at the arc transition of the rivet head is very small or almost no stress.

It can be seen from the above observation results that the junction of two connecting plates in the straight section of the rivet is severely deformed, which reflects to some extent that there is a deviation in the coaxiality of the mounting hole between the frame and the skin, which leads to abnormal assembly stress of the rivet.

Under the action of abnormal assembly stress plus air flow vibration stress, the rivet has fatigue cracking.

Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 3

The fracture surface of rivet presents a large line source, and the size of the source area indicates the size of the initial stress to a certain extent, which can be used to preliminarily judge that the initial stress of rivet fracture is large;

The rivet crack is fully expanded, and the fatigue area exceeds 95% of the total fracture area, indicating that the initial stress of the rivet is large, but the expansion stress is relatively small.

Since the rivet fracture location is at the stress concentration point at the arc transition of rivet head, when the alternating stress is small, its fatigue cracking may also show the characteristics of line source, so it is necessary to quantitatively back deduce the fracture surface of the initial comprehensive stress borne by the rivet.

During normal operation, the rivet is interference fit, and the stress at the arc fracture position is small or not under force.

However, the rivet is subject to fatigue fracture, mainly because it bears the bending alternating stress at the arc of the rivet due to the up and down displacement of the connecting plate, forming a semi-elliptical surface crack.

With reference to the stress intensity factor model of “semi-elliptical surface crack under uniform tension and bending” in the literature, that is, the round rod sample with diameter D contains semi-elliptical surface cracks, the major axis of the crack is 2a, the semi-minor axis of the crack depth is b, and the stress intensity factor of the front edge of the crack under uniform tension and bending stress σ (Fig. 4), so the bending condition of the center of the stress intensity factor model is used for calculation.

Then the stress intensity factor at each point of the crack front edge is:

Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 4
Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 5
Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 6

Fig.4 Model of round column specimen,half ellipse surface crack and uniform tension

See Table 1 and Figure 5 for relevant data of comprehensive stress of rivet quantitative analysis.

It can be seen from Fig. 5 that with the increase of crack length, the maximum comprehensive equivalent stress decreases continuously, and the reduction amplitude also decreases.

Considering the limited data, but in order to estimate the initial stress of fracture, here we use the linear fitting between the relevant crack length and the stress data at different crack lengths to make a conservative estimate, that is, Y=273-10.5x.

When x=0, Y=273 MPa, that is, the maximum comprehensive equivalent stress at the beginning of the rivet is 273 MPa, accounting for about 70% of the tensile strength (390 MPa) of LY10CZ aluminum alloy.

It should be noted that the rivet material is LY10 aluminum alloy, and the fatigue stress ratio it bears is R=- 1.

The corresponding material constants c and n are not found. In the process of quantitative estimation, the crack growth constant under the condition of LY12 aluminum alloy’s stress ratio (R=0.25) is used for reverse inference.

The value of the initial maximum comprehensive equivalent stress may have errors.

This study focuses on introducing the stress analysis method.

Table 1 quantitative analysis results of fatigue stress of the rivet

No.b/aB/mmYS/μ△σ/MPaσmax/MPa
10.310.530.9870.08200.9267.9
20.711.400.7820.08192.5256.7
30.811.780.7820.10191.5255.4
Rivet Fatigue Fracture: Cause Analysis and Fatigue Stress Estimation 7

Fig.5 Linear fiting of length and stress

With the crack propagation and the vibration displacement of the upper and lower connecting plates unchanged, the stress on the rivet decreases gradually, and the stress at the crack tip decreases gradually, which corresponds to 95% of the full fatigue area of the rivet crack propagation.

Therefore, the rivet fracture is a fatigue fracture with a large initial stress but relatively small expansion stress.

3. Conclusion

1) The rivet fracture is fatigue fracture.

2) The coaxiality of the mounting hole between the riveted frame and the skin has a deviation, and the fracture reason is mainly related to the abnormal assembly plus the air flow vibration stress.

3) Combined with quantitative analysis and estimation of fracture surface, it is quantitatively determined that the comprehensive initial stress of rivet fatigue is larger and the expansion stress is smaller.

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