Flexing

When subjected to flexing, rubber products frequently fail due to the development and propagation of cracks. The cracks reduce other properties, which in turn reduce the service life of the rubber. Cracks can grow through mechanical means or by oxidative and ozone attack.

ASTM D-430, Method B, is a test designed to produce cracking by bending. The time or number of flexes to crack initiation are used as the measure of performance. It employs a DeMattia flexing machine which flexes a 6" x 1" x ¼" specimen having a 0.094" round groove molded transversely in the center of the strip. This machine operates at 300 cycles per minute.

An adaptation of the bend flex method of ASTM D-430 is ASTM D-813 which requires the deliberate cutting of the bottom of the grooved specimen to initiate the crack. The number of flexing cycles needed to attain a specified crack length is then observed.

ASTM D-1052 (Ross Flexer) is another method of determining the resistance of elastomers to cut growth from repeated bending. The equipment is illustrated in Figure 2.



Softer vulcanizates like urethane have excellent flex life. In the Ross notched test, no cut growth occurred during 420,000 flexes (70 hrs) at a rate of 100 cycles/minute. The more vigorous DeMattia test, run at 300 cycles/minute, caused failure in 24 hours using notched specimens; but unnotched samples ran for 100 hours (1,800,000 flexes) with only slight cracking occurring.

Design of the part to reduce localized concentration of the stress or heat build-up will improve flex life. When an elastomeric part is flexed, very high stresses are developed in thick cross sections. Under repeated flexing, any cut in the surface of the part will grow larger because of the high local stresses concentrated at the cut. As with any elastomer, the rate of cut growth under flexing may be reduced (Figure 5) by decreasing the thickness of the part.



Unlike other elastomers, urethane can be utilized practically in very thin sections because of its exceptional strength and toughness.

Internal heat build up

As mentioned in the section on resilience, heat build-up in urethane parts, resulting from internal friction under high frequency flexing, exceeds that of conventional elastomers and is the usual cause of premature failure of urethane parts operating under flexing or high speed rotary motion under load. Because of the low thermal conductivity of urethane elastomers, heat developed by internal friction cannot be readily dissipated. Heat build-up is, therefore, a very important consideration when designing with urethanes. Its adverse effects can be minimized by using thin cross sections from which heat is more easily dissipated. The high strength and load bearing capacity of urethane elastomers makes possible the use of sections which are thin enough to dissipate heat at the same rate at which it is developed so the piece is not harmed.

As example in which thinner sections actually increased the service life of a urethane elastomer part is offered by experience with industrial truck wheels made of urethane. Early test wheels were made to the same dimensions normally used with conventional elastomers. In service, abrasion resistance was excellent but many premature failures occurred as a result of internal fracture and reduction in adhesive bond strength at the hub. Both types of failure were traced to excessive heat build-up under very high loads. The problem was solved by increasing the hub size and reducing the thickness of elastomer in the tire. This change provided a thinner tire section which dissipated internal heat more effectively. It also increased the shape factor of the area over which the load was distributed, thus decreasing the deflection for a given load. With the new design, urethane fork truck wheels are giving outstanding performance.